DOI: 10.1002/chem.201504959
Full Paper
& Inorganic Chemistry
Exploring Regioselective Bond Cleavage and Cross-Coupling Reactions using a Low-Valent Nickel Complex Addison N. Desnoyer,[a] Florian W. Friese,[b] Weiling Chiu,[a] Marcus W. Drover,[a] Brian O. Patrick,[a] and Jennifer A. Love*[a] Abstract: Recently, esters have received much attention as transmetalation partners for cross-coupling reactions. Herein, we report a systematic study of the reactivity of a series of esters and thioesters with [{(dtbpe)Ni}2(m-h2 :h2-C6H6)] (dtbpe = 1,2-bis(di-tert-butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form h2-carbonyl complexes. In contrast, acetylthioesters underwent rapid Cacyl¢S bond cleavage followed by decarbonylation to generate methylnickel complexes. This decarbonylation could be pushed backwards by the addition of CO, allowing for regeneration of the thioester. Most of the thioester complexes were found to undergo stoichiometric cross-coupling with phenylboronic acid to yield
Introduction In the development of new synthetic pathways, modern chemistry strives towards the activation and functionalization of kinetically inert bonds. Recently, much work has been done in the area of C¢H and C¢O bond activation, allowing for the development of new synthetic protocols that are both shorter and more atom-economical than traditional approaches.[1] While the catalytic reactivity of palladium and platinum towards otherwise unreactive C¢X bonds (X = heteroatom) have been well-documented for decades,[2] there is growing concern about the cost and low natural abundance of these noble metals for their continued use in industrial applications. Our group is one of many currently interested in exploring the fundamental reactivity of nickel.[3] The unique redox reactivity of nickel relative to its heavier congeners enables single-electron transfer (SET) processes, which could result in reactivity complementary to that of palladium and platinum. These characteristics and the growing understanding of them have enabled
[a] A. N. Desnoyer, W. Chiu, M. W. Drover, Dr. B. O. Patrick, Prof. Dr. J. A. Love Department of Chemistry, The University of British Columbia 2036 Main Mall, Vancouver V6T 1Z1 (Canada) E-mail: [email protected] [b] F. W. Friese Organisch-Chemisches Institut, Westflische Wilhelms-Universitt Mìnster Corrensstrabe 40, 48149 Mìnster (Germany) Supporting information for this article can be found under http:// dx.doi.org/10.1002/chem.201504959. Chem. Eur. J. 2016, 22, 4070 – 4077
sulfides. While ethyl trifluoroacetate was also found to form an h2-carbonyl complex, phenyl esters were found to predominantly undergo Caryl¢O bond cleavage to yield arylnickel complexes. These could also undergo transmetalation to yield biaryls. Attempts to render the reactions catalytic were hindered by ligand scrambling to yield nickel bis(acetate) complexes, the formation of which was supported by independent syntheses. Finally, 2-naphthyl acetate was also found to undergo clean Caryl¢O bond cleavage, and although stoichiometric cross-coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive.
numerous transformations long considered to be particularly challenging.[4] Among carbonyl compounds, esters show relatively low reactivity towards nickel complexes, although there are several early reports of C¢O bond cleavage using nickel(0).[5] Since esters are a cheap and naturally abundant feedstock, the direct activation of C¢O bonds of esters is of great interest.[6] The utility of esters as the electrophilic source in Suzuki-type crosscoupling reactions has recently been realized by Garg[7] and Shi[8] using aryl pivalates with aryl boronic acids and aryl boroxines, respectively. Subsequently, Itami succeeded in the isolation and structural elucidation of an arylnickel(II) pivalate as an intermediate in catalytic C¢H/C¢O biaryl coupling.[9] Our group has also recently demonstrated decarbonylative cross-coupling of esters with arylboronic acids by nickel catalysis as an alternative to aryl halide-based Suzuki coupling.[10] Interestingly, thioesters often show different activation pathways than esters. Jones’ recent report of platinum(0)-mediated C¢S bond activation of aliphatic thioesters displayed an initial Cacyl¢S bond cleavage yielding a platinum acyl-thiolate complex.[11] Upon heating, CO is released and activation of a second equivalent of thioester led to a platinum dithiolate complex. In addition, Riordan has reported that the use of a comparable nickel(0) complex led to the cleavage of the Cacyl¢S bond of thioesters containing an electron-deficient C6F5 substituent.[12] We recently reported the synthesis of 2-nickelaoxetanes,[13] prepared from the oxidative addition of [{(dtbpe)Ni}2(m-h2 :h2C6H6)][14] (1) (dtbpe = 1,2-bis(di-tert-butyl)phosphinoethane)
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Full Paper onto epoxides that bear a ketone group. Our mechanistic studies indicated that the precoordination of the metal centre to the carbonyl of the epoxide was crucial for the reaction to occur. We thus sought to explore what other transformations could be enabled by precoordination of a reactive, low-valent nickel centre to a carbonyl fragment of organic molecules. More broadly, we were also interested in exploring the fundamental organometallic reactivity of low-valent nickel complexes with esters and thioesters to gain mechanistic insight that could be useful for catalysis. Inspired by Sanford’s recent work on decarbonylation of anhydrides,[15] we envisioned a new route to the synthesis of RXCF3 (X = O, S) via the decarbonylation of esters and thioesters, respectively. A potential mechanism for this transformation is shown in Scheme 1.
to coupling to both 31P and 19F nuclei. The solid-state structure is shown in Figure 1, and demonstrates a C¢O bond length of 1.345(2) æ, which is elongated relative to most ketones. We found that heating 3 at 70 8C in C6D6 in a sealed tube for three days results in no observable reaction by 31P, 19F, and 1 H NMR spectroscopic experiments. Even subsequent heating at 110 8C for a further 24 h, results in no change to the NMR spectra of 3. Thus, we conclude that the pendant CF3 group is inactive towards thermally induced C¢F activation. Concomitant with our studies on this system, Ogoshi and co-workers published an elegant report on the C¢F activation of related complexes using Lewis acids.[19]
Scheme 1. Potential synthesis of CF3XR.
A side reaction in the outlined catalytic cycle would be C¢F activation of the pendant CF3 moiety, which is well known for low-valent nickel complexes.[16] Indeed, Pçrschke has demonstrated that 1 can activate C6F6 under thermolytic conditions.[14] Thus, we sought to first prepare a trifluoromethyl ketone complex derived from 1 and examine its reactivity.
Results and Discussion Addition of ketone 2 to a solution of 1 in benzene results in an immediate color change from red/orange to yellow. Recrystallization of the crude product from Et2O yields X-ray quality crystals of the h2-ketone complex 3 (see Scheme 2) in 44 % isolated yield, which was characterized by multinuclear NMR spectroscopic experiments, electron-impact mass spectrometry (EI-MS), elemental analysis (EA), and X-ray diffraction studies.[17]
Figure 1. POV-Ray (50 % probability ellipsoids) diagram of 3.
We then turned our attention to other carbonyl-bearing substrates (see Scheme 3). Addition of CF3-containing thioesters 4 and 5 to complex 1 results in the formation of h2-bound thioester complexes 6 and 7. After workup, complexes 6 and 7 can be isolated in 71 and 70 % yield, respectively.
Scheme 3. Synthesis of complexes 6 and 7. Isolated yields in parentheses.
Scheme 2. Synthesis of complex 3. Isolated yield in parentheses.
The NMR spectroscopic features of 3 are comparable to related complexes.[13, 18] The coordination of the ketone to nickel generates a C1-symmetric complex, with each of the four tBu groups rendered inequivalent. The most notable feature is the downfield-shifted resonance (d = 74.5 ppm) of the carbonyl carbon atom, which is broadened significantly, presumably due Chem. Eur. J. 2016, 22, 4070 – 4077
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These complexes were fully characterized,[17] and the solidstate structure of complex 6 is shown in Figure 2. Analogous to complex 3, complex 6 shows a significantly elongated C¢O bond length of 1.347(1) æ. The 31P NMR spectrum of complex 6 (see Figure 3) displays two resonances for the dtbpe ligand; an [AB] doublet (d = 93.1 ppm, 2JP,P = 48 Hz) that we assigned as the resonance for P2 (i.e. the phosphorous atom trans to the oxygen atom of the thioester moiety) and a doublet of quartets (d = 90.1 ppm, 2JP,P = 48, 4JP,F = 11 Hz) that was assigned as the resonance for P1. Dark-red, X-ray quality crystals of complex 7 can be grown by the slow evaporation of a concentrated
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Figure 2. POV-Ray (50 % probability ellipsoids) diagram of complex 6.
Figure 4. POV-Ray (50 % probability ellipsoids) diagram of complex 7.
Scheme 4. Synthesis of complexes 11 and 12. Isolated yields from metathesis reactions in parentheses.
Figure 3. 31P{1H} NMR spectrum (162 MHz, C6D6, 25 8C) of 6.
solution in toluene, and the solid-state structure of 7 is shown in Figure 4. In sharp contrast to the formation of h2-carbonyl complexes, reacting acetylthioesters 8 or 9 with 1 yields a mixture of products as determined by 31P NMR spectroscopy. One product, present in both reactions, is readily identified as [(dtbpe)Ni(CO)2] (10)[20] by its characteristic 31P NMR chemical shift (d = 94.8 ppm). The other main products in each case display two doublets with small coupling constants consistent with oxidation to nickel(II).[21] Following purification by recrystallization, complexes 11 (formed in 29 % yield by 1H NMR spectroscopy) and 12 (formed in 49 % yield by 1H NMR spectroscopy) were identified as methylnickel complexes derived from decarbonylation of the thioester moiety. Complexes 11 and 12 could be prepared independently by a metathesis reaction of a sodium thiolate with [(dtbpe)Ni(Me)I] (13) in 58 and 67 % isolated yield, respectively (see Scheme 4). The solid-state structure of complex 11 was determined by single-crystal X-ray diffraction studies (see Figure 5). Although analytically pure, dark-red crystals of complex 11 could be grown by recrystallization from pentanes at ¢30 8C, complex 11 was found to be thermally sensitive, as it decomposes in Chem. Eur. J. 2016, 22, 4070 – 4077
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Figure 5. POV-Ray diagram of complex 11. Thermal ellipsoids shown at 50 % probability.
the solid-state at room temperature over the course of two weeks and in solution over a few days. In contrast to complex 11, complex 12 was found to be thermally robust; heating solutions of complex 12 in C6D6 at 50 8C for one week resulted in no change to the 1H and 31 P NMR spectra. Red/orange crystals of complex 12 could be grown by cooling a saturated Et2O solution to ¢30 8C, and the solid-state structure of complex 12 is shown in Figure 6. Intrigued by the rapid decarbonylation of acetylthioesters, we sought to explore whether the reaction could be pushed in the reverse direction. Indeed, exposure of a solution of 12 to
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Full Paper Table 1. Stoichiometric cross-coupling of thioesters with PhB(OH)2.
R
R’
Yield [%][a]
CF3 CF3 CH3 CH3
Et Ph Et Ph
35 73 0 12
[a] Yields determined by GC-FID analysis.
Figure 6. POV-Ray diagram of complex 12. Thermal ellipsoids shown at 50 % probability.
Scheme 6. Synthesis of complexes 15 and 16. [a] = Isolated yield. [b] = 1H NMR yield.
1 atm. of CO gas results in the formation of 10 as the major organometallic species. In addition, thioester 9 is also formed, as determined by GCMS and 1H NMR spectroscopy (84 % yield, see Scheme 5). Although at this point CO insertion into the nickel–thiolate bond cannot be excluded, we propose that 9 is formed by insertion of CO into the nickel–methyl bond of 12,[22 ]followed by reductive elimination.
Scheme 5. Reaction of 12 with CO to yield 9 and 10. [a] = 31P NMR yield. [b] = 1H NMR yield.
Figure 7. POV-Ray diagram of complex 15. Thermal ellipsoids shown at 50 % probability.
While heating samples of 6 and 7 results in decarbonylation leading to the formation of 10, we were unable to detect any formation of RSCF3 (R = Et, Ph) by 1H and 19F NMR spectroscopy or GCMS analysis. Given the success our group and others have had using boronic acids as transmetalation reagents,[10, 23] we sought to explore the feasibility of C¢S bond formation by combining complex 1, thioesters 4, 5, 8, or 9, and PhB(OH)2 in one pot (see Table 1). Although we are able to form substituted sulfides in appreciable yields, we found that attempts to render the cross-coupling reactions catalytic are hampered by the formation of complex 10, which is unreactive to further productive chemistry under the conditions employed. Performing GCMS analyses confirmed the presence of disulfides (RSSR, R = Et or Ph) during the cross-coupling reactions. Given that ligand-exchange reactions have previously been reported for nickel(II) complexes,[24] we wondered if the disulfide could be formed by reductive elimination from [(dtbpe)Ni(SR)2].[25] To probe this possibility, we prepared dithiolate 15 by reaction of two equivalents of NaSEt with [(dtbpe)NiCl2] (14, Chem. Eur. J. 2016, 22, 4070 – 4077
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see Scheme 6). Complex 15 was fully characterized,[17]and the solid-state structure is shown in Figure 7. Heating complex 15 in C6D6 at 60 8C results not in the formation of EtSSEt, but rather the thioacetaldehyde complex 16 in 54 % yield by 1H NMR spectroscopy. Concomitant formation of EtSH is also observed in 46 % yield by 1H NMR spectroscopy. Eliminations of this type from zirconium dithiolates has been reported previously by Buchwald.[26] Based on this, it seems unlikely that the disulfides observed during the cross-coupling are derived from 15. We currently hypothesize that the presence of the disulfides is indicative of a radical process, and further mechanistic studies are currently underway in our laboratory. We then sought to examine the corresponding chemistry of 1 with esters. While 1 was found to be unreactive with 10 equivalents of EtOAc, addition of trifluoromethyl derivative 17 to an orange/red solution of 1 results in a gradual color change to yellow, and the h2-carbonyl complex 18 could be
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Scheme 9. Synthesis of complexes 22 and 23. 1H NMR yields in parentheses.
Scheme 7. Synthesis of complex 18. Isolated yield in parentheses.
isolated in 59 % yield after recrystallization of the crude product from pentanes (see Scheme 7).[17] Crystals of 18 suitable for X-ray diffraction analysis could be grown from slow evaporation of a saturated toluene solution. The solid-state structure of 18 (see Figure 8) is analogous to that of 6, including the notably elongated C¢O bond length of 1.317(3) æ.
Figure 8. POV-Ray diagram of complex 18. Thermal ellipsoids shown at 50 % probability.
bond cleavage followed by decarbonylation, analogous to the thioester reactivity already discussed (vide supra). Complex 22 could also be prepared independently from a salt metathesis reaction of NaOPh with 13.[17] The major product, complex 23 (58 % yield by 1H NMR spectroscopy) was characterized as the product of Caryl¢O oxidative addition. Notably, the Houk group has recently published a comprehensive computational work[27d] examining the origins of selectivity in Itami’s crosscoupling of benzoxazole with aryl pivalates by using [(dcype)Ni(cod)] (dcype = 1,2-bis(dicyclohexyl)phosphinoethane, cod = 1,4-cyclooctadiene) as a catalyst.[9] Based on their DFT calculations, Houk et al. propose that although Cacyl¢O bond cleavage is energetically more accessible than Caryl¢O bond cleavage, subsequent decarbonylation would generate a sterically very crowded tert-butylnickel intermediate, which prohibits further reactivity.[27d] It is interesting to note here that although the product of decarbonylation is much less sterically hindered, the main product observed is still the result of Caryl¢O oxidative addition. Although similar solubilities of 22 and 23 rendered purification by recrystallization difficult, X-ray quality crystals of 23 could be grown by cooling a concentrated pentanes solution, and the structure of 23 was confirmed by an X-ray diffraction study (see Figure 9). To date, we have been unable to prepare 23 in pure form, either by purification of the reaction of 1 with 21 or by independent synthesis.
In contrast, 1 was found to react rapidly with phenyl ester 19 to give 20 in 59 % isolated yield, which is the product of Caryl¢O bond cleavage (see Scheme 8).[27] Although we were able to grow red crystals of complex 20, they were too disordered to extract meaningful crystallographic data. Nevertheless, the structure of 20 can be assigned using NMR spectroscopy.[17] In particular, the large 31P coupling constants for the ipso-carbon atom resonance (d = 150.8 ppm, dd, 2JC,P = 80 Hz, 41 Hz) are indicative of a phenyl group bound to nickel.[28]
Scheme 8. Synthesis of complex 20. Isolated yield in parentheses.
Figure 9. POV-Ray diagram of complex 23. Thermal ellipsoids shown at 50 % probability.
To our surprise, reacting phenyl acetate 21 with complex 1 results in a complex mixture of products, as determined by 31 P and 1H NMR spectroscopy[17] (see Scheme 9). One minor product, formed in 18 % yield by 1H NMR spectroscopy, was characterized as complex 22, which is the product of Cacyl¢O
The dependence of which C¢X bond (X = O, S) is cleaved during oxidative addition is outlined in Scheme 10. This type of selectivity change is normally observed when altering the phosphine ligand,[27d] and is unusual for a given chelated nickel(0) system.
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Full Paper Table 2. Cross-coupling of esters with boronic acids.
Scheme 10. Regioselectivity of bond cleavage of esters versus thioesters with complex 1. [Ni] = [(dtbpe)Ni].
Esters have recently received much attention as cross-coupling partners in organic synthesis.[29] Thus, we envisioned that 20 and 23 could serve as intermediates in the cross-coupling of aryl esters with boronic acids (see Scheme 11). While heating 20 at 70 8C for 24 h and reacting 20 with phenylboronic acid at room temperature yielded only trace amounts of biphenyl, refluxing a toluene solution of 1, esters 19 or 21, and PhB(OH)2 in stoichiometric amounts for 24 h results in the formation of biphenyl 24 in 10 and 78 % yield, respectively, as shown by GC-FID analysis of the crude reaction mixture. Of note, replacing PhB(OH)2 with Ph2Zn under identical reaction conditions results in generally poorer yields of 24 (16 and 26 %, respectively).
R
Ar
Yield [%] Ar¢Ph
Yield [%] 24
CH3 CH3 CH3 CF3 CF3
Ph p-OMeC6H4 p-OCF3C6H4 Ph p-OMeC6H4
77 16 6 10 31
– 6 6 – 6
[a] Yields determined by GC-FID analysis.
mixture that were suitable for an X-ray diffraction study (see Figure 10). These acetate complexes could be also be prepared independently by the protonolysis of dimethyl complex 27 with trifluoroacetic acid or acetic acid, as shown in Scheme 12. X-ray quality crystals of 26 could be grown by cooling saturated toluene solutions to ¢30 8C. The solid-state structure of 26 is shown in Figure 11.
Scheme 11. Synthesis of biphenyl 24.
It is possible that the biphenyl observed could be formed by the homocoupling of the boronic acid. Sanford has recently reported on the homocoupling of phenylzinc reagents to generate biphenyl.[15] We thus sought to explore the use of alternate boronic acids to determine whether cross-coupling or homocoupling was occurring in our system. Notably, the yields of biaryl decrease dramatically when using substituted arylboronic acids. Based on our results, shown in Table 2, it seems clear that although both cross-coupling and homocoupling are occurring in our system, the cross-coupling process is the most prevalent. The low yields of our system prompted us to study whether the biaryl product was inhibiting turnover. Indeed, heating complex 1 with one equivalent of biphenyl in C6D6 under our reaction conditions led to the formation of a new complex as observed by 31 P NMR spectroscopy, which we tentatively assign as [(dtbpe)Ni(biphenyl)]. To rule out any loss in yield due to undetected Ni-bound biphenyl, C6F6 was added to the reaction upon completion prior to GC-FID analysis.[17] However, the yields remained unchanged. Attempts to induce catalytic turnover were not met with success, as lowering the loading of complex 1 results in a dramatic decrease in the yield of biaryl product. Indeed, during the course of our cross-coupling studies, we noted the formation of a red precipitate as the reactions progressed. From cross-coupling reactions of ester 19, red crystals of the bis(trifluoroacetate) complex 25 could be isolated from the reaction Chem. Eur. J. 2016, 22, 4070 – 4077
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Figure 10. POV-Ray diagram of complex 25. Thermal ellipsoids shown at 50 % probability.
Scheme 12. Independent syntheses of complexes 25 and 26. Isolated yields in parentheses.
Naphthyl esters and ethers have been demonstrated to be more reactive than their phenyl counterparts for cross-coupling reactions with nickel[30] since their extended p-systems are more capable of retaining aromaticity and stabilizing potential Meisenheimer-type intermediates.[31] In an attempt to improve our own cross-coupling, we prepared naphthyl ester 28 and reacted it with 1 (see Scheme 13).
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Full Paper Unfortunately, while combining 1, 28, and PhB(OH)2 in THF at 50 8C for 24 h results in the formation of 2-phenylnaphthalene in 89 % yield, as shown by GC-FID analysis, we were unable to achieve catalytic turnover, as once again decreasing the loading of nickel drastically decreased the yield of biaryl. We are currently attempting to render the cross-coupling reactions reported herein catalytic, as well as exploring further mechanistic studies on these transformations.
Conclusion We have examined the reactivity of a low-valent nickel complex with a variety of esters and thioesters. While trifluoromethylthioesters were found to bind to nickel to form h2-carbonyl adducts, acetylthioesters were found to undergo Cacyl¢S bond cleavage, and subsequent decarbonylation to generate methylnickel thiolate complexes. Most of the thioesters could undergo cross-coupling with PhB(OH)2 to yield sulfides. In contrast, aryl esters were found to typically undergo Caryl¢O bond cleavage to yield aryl acetate complexes, which could also undergo cross-coupling with arylboronic acids to yield biaryls.
Figure 11. POV-Ray diagram of complex 26. Thermal ellipsoids shown at 50 % probability.
Scheme 13. Synthesis of complex 29. Nap = naphthyl. Isolated yield in parentheses.
Experimental Section General
Gratifyingly, the reaction produced a single product, complex 29, which was recrystallized from pentanes in 40 % isolated yield and characterized as the naphthyl analogue of 23. The solid-state structure of 29 is shown in Figure 12. Interestingly, the 1H NMR spectrum shows two sets of tBu peaks at room temperature: a sharp doublet at d = 1.38 ppm (3JH,P = 11.9 Hz) and a broad multiplet at d = 1.15 ppm, presumably due to hindered rotation of the bulky naphthyl group. Indeed, performing variable-temperature 1H NMR studies at 67 8C resolved the multiplet into a sharp doublet (d = 1.18 ppm, JH,P = 12.8 Hz).[17]
See the Supporting Information for full experimental details. CCDC 1440224 (3), 1440223 (6), 1440231 (7), 1440228 (11), 1440229 (12), 1440227 (15), 1440232 (18), 1440225 (23), 1440226 (25), 440233 (26) and 1440230 (29) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.
Acknowledgements We thank the University of British Columbia, NSERC (Discovery grant, Research Tools and Instrumentation grants) and the Canada Foundation for Innovation for supporting this research. A.N.D. is grateful to NSERC for a CGS-D and to the Izaak Walton Killam foundation for a doctoral scholarship. W.C. is grateful for support from the CREATE SusSyn program. M.W.D. thanks the Government of Canada for a Vanier scholarship. Keywords: cross-coupling · elimination · nickel · P ligands · sulfur [1] a) W. D. Jones, F. J. Feher, Acc. Chem. Res. 1989, 22, 91 – 100; b) M. Lersch, M. Tilset, Chem. Rev. 2005, 105, 2471 – 2526; c) R. G. Bergman, Nature 2007, 446, 391 – 393; d) M. Tobisu, N. Chatani, Top. Organomet. Chem. 2013, 44, 35 – 54; e) K. Chen, X.-S. Zhang, Z.-J. Shi, Pure Appl. Chem. 2014, 86, 361 – 372. [2] a) C. Ming, F. Y. Kwong, Chem. Soc. Rev. 2011, 40, 4963 – 4972; b) K. C. Nicolaou, P. G. Bulger, D. Sarlah, Angew. Chem. Int. Ed. 2005, 44, 4442 – 4489; Angew. Chem. 2005, 117, 4516 – 4563; c) C. C. C. Johansson Seechurn, M. W. Kitching, T. J. Colacot, V. Snieckus, Angew. Chem. Int. Ed. 2012, 51, 5062 – 5085; Angew. Chem. 2012, 124, 5150 – 5174; d) A. E. Shilov, G. B. Shul’pin, Chem. Rev. 1997, 97, 2879 – 2932.
Figure 12. POV-Ray diagram of complex 29. Thermal ellipsoids shown at 50 % probability. Chem. Eur. J. 2016, 22, 4070 – 4077
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f) D. J. Mindiola, R. Waterman, D. M. Jenkins, G. L. Hillhouse, Inorg. Chim. Acta 2003, 345, 299 – 308. R. Doi, K. Kikushima, M. Ohashi, S. Ogoshi, J. Am. Chem. Soc. 2015, 137, 3276 – 3282. K.-R. Pçrschke, C. Pluta, B. Proft, B. Lutz, C. Krìger, Z. Naturforsch. B 1993, 48, 608 – 626. a) J. J. Garcia, W. D. Jones, Organometallics 2000, 19, 5544 – 5545; b) J. J. Garcia, N. M. Brunkan, W. D. Jones, J. Am. Chem. Soc. 2002, 124, 9547 – 9555. Y.-J. Kim, K. Osakada, K. Sugita, T. Yamamoto, A. Yamamoto, Organometallics 1988, 7, 2182 – 2188. a) A. Dauth, J. A. Love, Angew. Chem. Int. Ed. 2010, 49, 9219 – 9224; Angew. Chem. 2010, 122, 9405 – 9410; b) R. Martin, S. Buchwald, Acc. Chem. Res. 2008, 41, 1461 – 1473; c) K. T. Sylvester, K. Wu, A. G. Doyle, J. Am. Chem. Soc. 2012, 134, 16967 – 16970; d) D. K. Nielsen, A. G. Doyle, Angew. Chem. Int. Ed. 2011, 50, 6056 – 6059; Angew. Chem. 2011, 123, 6180 – 6183. a) L. H. Pignolet, W. D. Horrocks, J. Am. Chem. Soc. 1968, 90, 922 – 926; b) W. G. Dougherty, K. Rangan, M. J. O’Hagan, G. P. A. Yap, C. G. Riordan, J. Am. Chem. Soc. 2008, 130, 13510 – 13511. a) A. Shaver, S. Morris, A. Desjardins, Inorg. Chim. Acta 1989, 161, 11 – 12; b) Y. Yamashina, Y. Kataoka, Y. Ura, Eur. J. Inorg. Chem. 2014, 4073 – 4078; c) J. Fitzpatrick, H. Kalyvas, J. Shearer, E. Kim, Chem. Commun. 2013, 49, 5550 – 5552. S. L. Buchwald, R. B. Nielsen, J. C. Dewan, J. Am. Chem. Soc. 1987, 109, 1590 – 1591. a) D.-G. Yu, B.-J. Li, Z.-J. Shi, Acc. Chem. Res. 2010, 43, 1486 – 1495; b) F.S. Han, Chem. Soc. Rev. 2013, 42, 5270 – 5298; c) N. Yoshikai, H. Matsuda, E. Nakamura, J. Am. Chem. Soc. 2008, 130, 15258 – 15259; d) X. Hong, Y. Liang, K. N. Houk, J. Am. Chem. Soc. 2014, 136, 2017 – 2025. a) Y.-J. Kim, R. Sato, T. Maruyama, K. Osakada, T. Yamamoto, J. Chem. Soc. Dalton Trans. 1994, 943 – 948; b) M. A. Bennett, C. C. R. Hockless, E. Wenger, Organometallics 1995, 14, 2091 – 2101. a) E. Koch, R. Takise, A. Studer, J. Yamaguchi, K. Itami, Chem. Commun. 2015, 51, 855 – 857; b) J. Cornella, E. P. Jackson, R. Martin, Angew. Chem. Int. Ed. 2015, 54, 4075 – 4078; Angew. Chem. 2015, 127, 4147 – 4150. a) M. Tobisu, T. Shimasaki, N. Chatani, Angew. Chem. Int. Ed. 2008, 47, 4866 – 4869; Angew. Chem. 2008, 120, 4944 – 4947; b) P. Alvarez-Bercedo, R. Martin, J. Am. Chem. Soc. 2010, 132, 17352 – 17353; c) J. Cornella, C. Zarate, R. Martin, Chem. Soc. Rev. 2014, 43, 8081 – 8097. D. J. Brauer, C. Krìger, Inorg. Chem. 1977, 16, 884 – 891.
Received: December 10, 2015 Published online on February 16, 2016
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& Inorganic Chemistry
Exploring Regioselective Bond Cleavage and Cross-Coupling Reactions using a Low-Valent Nickel Complex Addison N. Desnoyer,[a] Florian W. Friese,[b] Weiling Chiu,[a] Marcus W. Drover,[a] Brian O. Patrick,[a] and Jennifer A. Love*[a] Abstract: Recently, esters have received much attention as transmetalation partners for cross-coupling reactions. Herein, we report a systematic study of the reactivity of a series of esters and thioesters with [{(dtbpe)Ni}2(m-h2 :h2-C6H6)] (dtbpe = 1,2-bis(di-tert-butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form h2-carbonyl complexes. In contrast, acetylthioesters underwent rapid Cacyl¢S bond cleavage followed by decarbonylation to generate methylnickel complexes. This decarbonylation could be pushed backwards by the addition of CO, allowing for regeneration of the thioester. Most of the thioester complexes were found to undergo stoichiometric cross-coupling with phenylboronic acid to yield
Introduction In the development of new synthetic pathways, modern chemistry strives towards the activation and functionalization of kinetically inert bonds. Recently, much work has been done in the area of C¢H and C¢O bond activation, allowing for the development of new synthetic protocols that are both shorter and more atom-economical than traditional approaches.[1] While the catalytic reactivity of palladium and platinum towards otherwise unreactive C¢X bonds (X = heteroatom) have been well-documented for decades,[2] there is growing concern about the cost and low natural abundance of these noble metals for their continued use in industrial applications. Our group is one of many currently interested in exploring the fundamental reactivity of nickel.[3] The unique redox reactivity of nickel relative to its heavier congeners enables single-electron transfer (SET) processes, which could result in reactivity complementary to that of palladium and platinum. These characteristics and the growing understanding of them have enabled
[a] A. N. Desnoyer, W. Chiu, M. W. Drover, Dr. B. O. Patrick, Prof. Dr. J. A. Love Department of Chemistry, The University of British Columbia 2036 Main Mall, Vancouver V6T 1Z1 (Canada) E-mail: [email protected] [b] F. W. Friese Organisch-Chemisches Institut, Westflische Wilhelms-Universitt Mìnster Corrensstrabe 40, 48149 Mìnster (Germany) Supporting information for this article can be found under http:// dx.doi.org/10.1002/chem.201504959. Chem. Eur. J. 2016, 22, 4070 – 4077
sulfides. While ethyl trifluoroacetate was also found to form an h2-carbonyl complex, phenyl esters were found to predominantly undergo Caryl¢O bond cleavage to yield arylnickel complexes. These could also undergo transmetalation to yield biaryls. Attempts to render the reactions catalytic were hindered by ligand scrambling to yield nickel bis(acetate) complexes, the formation of which was supported by independent syntheses. Finally, 2-naphthyl acetate was also found to undergo clean Caryl¢O bond cleavage, and although stoichiometric cross-coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive.
numerous transformations long considered to be particularly challenging.[4] Among carbonyl compounds, esters show relatively low reactivity towards nickel complexes, although there are several early reports of C¢O bond cleavage using nickel(0).[5] Since esters are a cheap and naturally abundant feedstock, the direct activation of C¢O bonds of esters is of great interest.[6] The utility of esters as the electrophilic source in Suzuki-type crosscoupling reactions has recently been realized by Garg[7] and Shi[8] using aryl pivalates with aryl boronic acids and aryl boroxines, respectively. Subsequently, Itami succeeded in the isolation and structural elucidation of an arylnickel(II) pivalate as an intermediate in catalytic C¢H/C¢O biaryl coupling.[9] Our group has also recently demonstrated decarbonylative cross-coupling of esters with arylboronic acids by nickel catalysis as an alternative to aryl halide-based Suzuki coupling.[10] Interestingly, thioesters often show different activation pathways than esters. Jones’ recent report of platinum(0)-mediated C¢S bond activation of aliphatic thioesters displayed an initial Cacyl¢S bond cleavage yielding a platinum acyl-thiolate complex.[11] Upon heating, CO is released and activation of a second equivalent of thioester led to a platinum dithiolate complex. In addition, Riordan has reported that the use of a comparable nickel(0) complex led to the cleavage of the Cacyl¢S bond of thioesters containing an electron-deficient C6F5 substituent.[12] We recently reported the synthesis of 2-nickelaoxetanes,[13] prepared from the oxidative addition of [{(dtbpe)Ni}2(m-h2 :h2C6H6)][14] (1) (dtbpe = 1,2-bis(di-tert-butyl)phosphinoethane)
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Full Paper onto epoxides that bear a ketone group. Our mechanistic studies indicated that the precoordination of the metal centre to the carbonyl of the epoxide was crucial for the reaction to occur. We thus sought to explore what other transformations could be enabled by precoordination of a reactive, low-valent nickel centre to a carbonyl fragment of organic molecules. More broadly, we were also interested in exploring the fundamental organometallic reactivity of low-valent nickel complexes with esters and thioesters to gain mechanistic insight that could be useful for catalysis. Inspired by Sanford’s recent work on decarbonylation of anhydrides,[15] we envisioned a new route to the synthesis of RXCF3 (X = O, S) via the decarbonylation of esters and thioesters, respectively. A potential mechanism for this transformation is shown in Scheme 1.
to coupling to both 31P and 19F nuclei. The solid-state structure is shown in Figure 1, and demonstrates a C¢O bond length of 1.345(2) æ, which is elongated relative to most ketones. We found that heating 3 at 70 8C in C6D6 in a sealed tube for three days results in no observable reaction by 31P, 19F, and 1 H NMR spectroscopic experiments. Even subsequent heating at 110 8C for a further 24 h, results in no change to the NMR spectra of 3. Thus, we conclude that the pendant CF3 group is inactive towards thermally induced C¢F activation. Concomitant with our studies on this system, Ogoshi and co-workers published an elegant report on the C¢F activation of related complexes using Lewis acids.[19]
Scheme 1. Potential synthesis of CF3XR.
A side reaction in the outlined catalytic cycle would be C¢F activation of the pendant CF3 moiety, which is well known for low-valent nickel complexes.[16] Indeed, Pçrschke has demonstrated that 1 can activate C6F6 under thermolytic conditions.[14] Thus, we sought to first prepare a trifluoromethyl ketone complex derived from 1 and examine its reactivity.
Results and Discussion Addition of ketone 2 to a solution of 1 in benzene results in an immediate color change from red/orange to yellow. Recrystallization of the crude product from Et2O yields X-ray quality crystals of the h2-ketone complex 3 (see Scheme 2) in 44 % isolated yield, which was characterized by multinuclear NMR spectroscopic experiments, electron-impact mass spectrometry (EI-MS), elemental analysis (EA), and X-ray diffraction studies.[17]
Figure 1. POV-Ray (50 % probability ellipsoids) diagram of 3.
We then turned our attention to other carbonyl-bearing substrates (see Scheme 3). Addition of CF3-containing thioesters 4 and 5 to complex 1 results in the formation of h2-bound thioester complexes 6 and 7. After workup, complexes 6 and 7 can be isolated in 71 and 70 % yield, respectively.
Scheme 3. Synthesis of complexes 6 and 7. Isolated yields in parentheses.
Scheme 2. Synthesis of complex 3. Isolated yield in parentheses.
The NMR spectroscopic features of 3 are comparable to related complexes.[13, 18] The coordination of the ketone to nickel generates a C1-symmetric complex, with each of the four tBu groups rendered inequivalent. The most notable feature is the downfield-shifted resonance (d = 74.5 ppm) of the carbonyl carbon atom, which is broadened significantly, presumably due Chem. Eur. J. 2016, 22, 4070 – 4077
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These complexes were fully characterized,[17] and the solidstate structure of complex 6 is shown in Figure 2. Analogous to complex 3, complex 6 shows a significantly elongated C¢O bond length of 1.347(1) æ. The 31P NMR spectrum of complex 6 (see Figure 3) displays two resonances for the dtbpe ligand; an [AB] doublet (d = 93.1 ppm, 2JP,P = 48 Hz) that we assigned as the resonance for P2 (i.e. the phosphorous atom trans to the oxygen atom of the thioester moiety) and a doublet of quartets (d = 90.1 ppm, 2JP,P = 48, 4JP,F = 11 Hz) that was assigned as the resonance for P1. Dark-red, X-ray quality crystals of complex 7 can be grown by the slow evaporation of a concentrated
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Figure 2. POV-Ray (50 % probability ellipsoids) diagram of complex 6.
Figure 4. POV-Ray (50 % probability ellipsoids) diagram of complex 7.
Scheme 4. Synthesis of complexes 11 and 12. Isolated yields from metathesis reactions in parentheses.
Figure 3. 31P{1H} NMR spectrum (162 MHz, C6D6, 25 8C) of 6.
solution in toluene, and the solid-state structure of 7 is shown in Figure 4. In sharp contrast to the formation of h2-carbonyl complexes, reacting acetylthioesters 8 or 9 with 1 yields a mixture of products as determined by 31P NMR spectroscopy. One product, present in both reactions, is readily identified as [(dtbpe)Ni(CO)2] (10)[20] by its characteristic 31P NMR chemical shift (d = 94.8 ppm). The other main products in each case display two doublets with small coupling constants consistent with oxidation to nickel(II).[21] Following purification by recrystallization, complexes 11 (formed in 29 % yield by 1H NMR spectroscopy) and 12 (formed in 49 % yield by 1H NMR spectroscopy) were identified as methylnickel complexes derived from decarbonylation of the thioester moiety. Complexes 11 and 12 could be prepared independently by a metathesis reaction of a sodium thiolate with [(dtbpe)Ni(Me)I] (13) in 58 and 67 % isolated yield, respectively (see Scheme 4). The solid-state structure of complex 11 was determined by single-crystal X-ray diffraction studies (see Figure 5). Although analytically pure, dark-red crystals of complex 11 could be grown by recrystallization from pentanes at ¢30 8C, complex 11 was found to be thermally sensitive, as it decomposes in Chem. Eur. J. 2016, 22, 4070 – 4077
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Figure 5. POV-Ray diagram of complex 11. Thermal ellipsoids shown at 50 % probability.
the solid-state at room temperature over the course of two weeks and in solution over a few days. In contrast to complex 11, complex 12 was found to be thermally robust; heating solutions of complex 12 in C6D6 at 50 8C for one week resulted in no change to the 1H and 31 P NMR spectra. Red/orange crystals of complex 12 could be grown by cooling a saturated Et2O solution to ¢30 8C, and the solid-state structure of complex 12 is shown in Figure 6. Intrigued by the rapid decarbonylation of acetylthioesters, we sought to explore whether the reaction could be pushed in the reverse direction. Indeed, exposure of a solution of 12 to
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Full Paper Table 1. Stoichiometric cross-coupling of thioesters with PhB(OH)2.
R
R’
Yield [%][a]
CF3 CF3 CH3 CH3
Et Ph Et Ph
35 73 0 12
[a] Yields determined by GC-FID analysis.
Figure 6. POV-Ray diagram of complex 12. Thermal ellipsoids shown at 50 % probability.
Scheme 6. Synthesis of complexes 15 and 16. [a] = Isolated yield. [b] = 1H NMR yield.
1 atm. of CO gas results in the formation of 10 as the major organometallic species. In addition, thioester 9 is also formed, as determined by GCMS and 1H NMR spectroscopy (84 % yield, see Scheme 5). Although at this point CO insertion into the nickel–thiolate bond cannot be excluded, we propose that 9 is formed by insertion of CO into the nickel–methyl bond of 12,[22 ]followed by reductive elimination.
Scheme 5. Reaction of 12 with CO to yield 9 and 10. [a] = 31P NMR yield. [b] = 1H NMR yield.
Figure 7. POV-Ray diagram of complex 15. Thermal ellipsoids shown at 50 % probability.
While heating samples of 6 and 7 results in decarbonylation leading to the formation of 10, we were unable to detect any formation of RSCF3 (R = Et, Ph) by 1H and 19F NMR spectroscopy or GCMS analysis. Given the success our group and others have had using boronic acids as transmetalation reagents,[10, 23] we sought to explore the feasibility of C¢S bond formation by combining complex 1, thioesters 4, 5, 8, or 9, and PhB(OH)2 in one pot (see Table 1). Although we are able to form substituted sulfides in appreciable yields, we found that attempts to render the cross-coupling reactions catalytic are hampered by the formation of complex 10, which is unreactive to further productive chemistry under the conditions employed. Performing GCMS analyses confirmed the presence of disulfides (RSSR, R = Et or Ph) during the cross-coupling reactions. Given that ligand-exchange reactions have previously been reported for nickel(II) complexes,[24] we wondered if the disulfide could be formed by reductive elimination from [(dtbpe)Ni(SR)2].[25] To probe this possibility, we prepared dithiolate 15 by reaction of two equivalents of NaSEt with [(dtbpe)NiCl2] (14, Chem. Eur. J. 2016, 22, 4070 – 4077
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see Scheme 6). Complex 15 was fully characterized,[17]and the solid-state structure is shown in Figure 7. Heating complex 15 in C6D6 at 60 8C results not in the formation of EtSSEt, but rather the thioacetaldehyde complex 16 in 54 % yield by 1H NMR spectroscopy. Concomitant formation of EtSH is also observed in 46 % yield by 1H NMR spectroscopy. Eliminations of this type from zirconium dithiolates has been reported previously by Buchwald.[26] Based on this, it seems unlikely that the disulfides observed during the cross-coupling are derived from 15. We currently hypothesize that the presence of the disulfides is indicative of a radical process, and further mechanistic studies are currently underway in our laboratory. We then sought to examine the corresponding chemistry of 1 with esters. While 1 was found to be unreactive with 10 equivalents of EtOAc, addition of trifluoromethyl derivative 17 to an orange/red solution of 1 results in a gradual color change to yellow, and the h2-carbonyl complex 18 could be
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Scheme 9. Synthesis of complexes 22 and 23. 1H NMR yields in parentheses.
Scheme 7. Synthesis of complex 18. Isolated yield in parentheses.
isolated in 59 % yield after recrystallization of the crude product from pentanes (see Scheme 7).[17] Crystals of 18 suitable for X-ray diffraction analysis could be grown from slow evaporation of a saturated toluene solution. The solid-state structure of 18 (see Figure 8) is analogous to that of 6, including the notably elongated C¢O bond length of 1.317(3) æ.
Figure 8. POV-Ray diagram of complex 18. Thermal ellipsoids shown at 50 % probability.
bond cleavage followed by decarbonylation, analogous to the thioester reactivity already discussed (vide supra). Complex 22 could also be prepared independently from a salt metathesis reaction of NaOPh with 13.[17] The major product, complex 23 (58 % yield by 1H NMR spectroscopy) was characterized as the product of Caryl¢O oxidative addition. Notably, the Houk group has recently published a comprehensive computational work[27d] examining the origins of selectivity in Itami’s crosscoupling of benzoxazole with aryl pivalates by using [(dcype)Ni(cod)] (dcype = 1,2-bis(dicyclohexyl)phosphinoethane, cod = 1,4-cyclooctadiene) as a catalyst.[9] Based on their DFT calculations, Houk et al. propose that although Cacyl¢O bond cleavage is energetically more accessible than Caryl¢O bond cleavage, subsequent decarbonylation would generate a sterically very crowded tert-butylnickel intermediate, which prohibits further reactivity.[27d] It is interesting to note here that although the product of decarbonylation is much less sterically hindered, the main product observed is still the result of Caryl¢O oxidative addition. Although similar solubilities of 22 and 23 rendered purification by recrystallization difficult, X-ray quality crystals of 23 could be grown by cooling a concentrated pentanes solution, and the structure of 23 was confirmed by an X-ray diffraction study (see Figure 9). To date, we have been unable to prepare 23 in pure form, either by purification of the reaction of 1 with 21 or by independent synthesis.
In contrast, 1 was found to react rapidly with phenyl ester 19 to give 20 in 59 % isolated yield, which is the product of Caryl¢O bond cleavage (see Scheme 8).[27] Although we were able to grow red crystals of complex 20, they were too disordered to extract meaningful crystallographic data. Nevertheless, the structure of 20 can be assigned using NMR spectroscopy.[17] In particular, the large 31P coupling constants for the ipso-carbon atom resonance (d = 150.8 ppm, dd, 2JC,P = 80 Hz, 41 Hz) are indicative of a phenyl group bound to nickel.[28]
Scheme 8. Synthesis of complex 20. Isolated yield in parentheses.
Figure 9. POV-Ray diagram of complex 23. Thermal ellipsoids shown at 50 % probability.
To our surprise, reacting phenyl acetate 21 with complex 1 results in a complex mixture of products, as determined by 31 P and 1H NMR spectroscopy[17] (see Scheme 9). One minor product, formed in 18 % yield by 1H NMR spectroscopy, was characterized as complex 22, which is the product of Cacyl¢O
The dependence of which C¢X bond (X = O, S) is cleaved during oxidative addition is outlined in Scheme 10. This type of selectivity change is normally observed when altering the phosphine ligand,[27d] and is unusual for a given chelated nickel(0) system.
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Full Paper Table 2. Cross-coupling of esters with boronic acids.
Scheme 10. Regioselectivity of bond cleavage of esters versus thioesters with complex 1. [Ni] = [(dtbpe)Ni].
Esters have recently received much attention as cross-coupling partners in organic synthesis.[29] Thus, we envisioned that 20 and 23 could serve as intermediates in the cross-coupling of aryl esters with boronic acids (see Scheme 11). While heating 20 at 70 8C for 24 h and reacting 20 with phenylboronic acid at room temperature yielded only trace amounts of biphenyl, refluxing a toluene solution of 1, esters 19 or 21, and PhB(OH)2 in stoichiometric amounts for 24 h results in the formation of biphenyl 24 in 10 and 78 % yield, respectively, as shown by GC-FID analysis of the crude reaction mixture. Of note, replacing PhB(OH)2 with Ph2Zn under identical reaction conditions results in generally poorer yields of 24 (16 and 26 %, respectively).
R
Ar
Yield [%] Ar¢Ph
Yield [%] 24
CH3 CH3 CH3 CF3 CF3
Ph p-OMeC6H4 p-OCF3C6H4 Ph p-OMeC6H4
77 16 6 10 31
– 6 6 – 6
[a] Yields determined by GC-FID analysis.
mixture that were suitable for an X-ray diffraction study (see Figure 10). These acetate complexes could be also be prepared independently by the protonolysis of dimethyl complex 27 with trifluoroacetic acid or acetic acid, as shown in Scheme 12. X-ray quality crystals of 26 could be grown by cooling saturated toluene solutions to ¢30 8C. The solid-state structure of 26 is shown in Figure 11.
Scheme 11. Synthesis of biphenyl 24.
It is possible that the biphenyl observed could be formed by the homocoupling of the boronic acid. Sanford has recently reported on the homocoupling of phenylzinc reagents to generate biphenyl.[15] We thus sought to explore the use of alternate boronic acids to determine whether cross-coupling or homocoupling was occurring in our system. Notably, the yields of biaryl decrease dramatically when using substituted arylboronic acids. Based on our results, shown in Table 2, it seems clear that although both cross-coupling and homocoupling are occurring in our system, the cross-coupling process is the most prevalent. The low yields of our system prompted us to study whether the biaryl product was inhibiting turnover. Indeed, heating complex 1 with one equivalent of biphenyl in C6D6 under our reaction conditions led to the formation of a new complex as observed by 31 P NMR spectroscopy, which we tentatively assign as [(dtbpe)Ni(biphenyl)]. To rule out any loss in yield due to undetected Ni-bound biphenyl, C6F6 was added to the reaction upon completion prior to GC-FID analysis.[17] However, the yields remained unchanged. Attempts to induce catalytic turnover were not met with success, as lowering the loading of complex 1 results in a dramatic decrease in the yield of biaryl product. Indeed, during the course of our cross-coupling studies, we noted the formation of a red precipitate as the reactions progressed. From cross-coupling reactions of ester 19, red crystals of the bis(trifluoroacetate) complex 25 could be isolated from the reaction Chem. Eur. J. 2016, 22, 4070 – 4077
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Figure 10. POV-Ray diagram of complex 25. Thermal ellipsoids shown at 50 % probability.
Scheme 12. Independent syntheses of complexes 25 and 26. Isolated yields in parentheses.
Naphthyl esters and ethers have been demonstrated to be more reactive than their phenyl counterparts for cross-coupling reactions with nickel[30] since their extended p-systems are more capable of retaining aromaticity and stabilizing potential Meisenheimer-type intermediates.[31] In an attempt to improve our own cross-coupling, we prepared naphthyl ester 28 and reacted it with 1 (see Scheme 13).
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Full Paper Unfortunately, while combining 1, 28, and PhB(OH)2 in THF at 50 8C for 24 h results in the formation of 2-phenylnaphthalene in 89 % yield, as shown by GC-FID analysis, we were unable to achieve catalytic turnover, as once again decreasing the loading of nickel drastically decreased the yield of biaryl. We are currently attempting to render the cross-coupling reactions reported herein catalytic, as well as exploring further mechanistic studies on these transformations.
Conclusion We have examined the reactivity of a low-valent nickel complex with a variety of esters and thioesters. While trifluoromethylthioesters were found to bind to nickel to form h2-carbonyl adducts, acetylthioesters were found to undergo Cacyl¢S bond cleavage, and subsequent decarbonylation to generate methylnickel thiolate complexes. Most of the thioesters could undergo cross-coupling with PhB(OH)2 to yield sulfides. In contrast, aryl esters were found to typically undergo Caryl¢O bond cleavage to yield aryl acetate complexes, which could also undergo cross-coupling with arylboronic acids to yield biaryls.
Figure 11. POV-Ray diagram of complex 26. Thermal ellipsoids shown at 50 % probability.
Scheme 13. Synthesis of complex 29. Nap = naphthyl. Isolated yield in parentheses.
Experimental Section General
Gratifyingly, the reaction produced a single product, complex 29, which was recrystallized from pentanes in 40 % isolated yield and characterized as the naphthyl analogue of 23. The solid-state structure of 29 is shown in Figure 12. Interestingly, the 1H NMR spectrum shows two sets of tBu peaks at room temperature: a sharp doublet at d = 1.38 ppm (3JH,P = 11.9 Hz) and a broad multiplet at d = 1.15 ppm, presumably due to hindered rotation of the bulky naphthyl group. Indeed, performing variable-temperature 1H NMR studies at 67 8C resolved the multiplet into a sharp doublet (d = 1.18 ppm, JH,P = 12.8 Hz).[17]
See the Supporting Information for full experimental details. CCDC 1440224 (3), 1440223 (6), 1440231 (7), 1440228 (11), 1440229 (12), 1440227 (15), 1440232 (18), 1440225 (23), 1440226 (25), 440233 (26) and 1440230 (29) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.
Acknowledgements We thank the University of British Columbia, NSERC (Discovery grant, Research Tools and Instrumentation grants) and the Canada Foundation for Innovation for supporting this research. A.N.D. is grateful to NSERC for a CGS-D and to the Izaak Walton Killam foundation for a doctoral scholarship. W.C. is grateful for support from the CREATE SusSyn program. M.W.D. thanks the Government of Canada for a Vanier scholarship. Keywords: cross-coupling · elimination · nickel · P ligands · sulfur [1] a) W. D. Jones, F. J. Feher, Acc. Chem. Res. 1989, 22, 91 – 100; b) M. Lersch, M. Tilset, Chem. Rev. 2005, 105, 2471 – 2526; c) R. G. Bergman, Nature 2007, 446, 391 – 393; d) M. Tobisu, N. Chatani, Top. Organomet. Chem. 2013, 44, 35 – 54; e) K. Chen, X.-S. Zhang, Z.-J. Shi, Pure Appl. Chem. 2014, 86, 361 – 372. [2] a) C. Ming, F. Y. Kwong, Chem. Soc. Rev. 2011, 40, 4963 – 4972; b) K. C. Nicolaou, P. G. Bulger, D. Sarlah, Angew. Chem. Int. Ed. 2005, 44, 4442 – 4489; Angew. Chem. 2005, 117, 4516 – 4563; c) C. C. C. Johansson Seechurn, M. W. Kitching, T. J. Colacot, V. Snieckus, Angew. Chem. Int. Ed. 2012, 51, 5062 – 5085; Angew. Chem. 2012, 124, 5150 – 5174; d) A. E. Shilov, G. B. Shul’pin, Chem. Rev. 1997, 97, 2879 – 2932.
Figure 12. POV-Ray diagram of complex 29. Thermal ellipsoids shown at 50 % probability. Chem. Eur. J. 2016, 22, 4070 – 4077
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Received: December 10, 2015 Published online on February 16, 2016
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