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Imine hydrogenation by alkylaluminum catalysts† Cite this: Chem. Commun., 2014, 50, 301
Jillian A. Hatnean,a Jordan W. Thomson,b Preston A. Chaseb and Douglas W. Stephan*a
Received 14th October 2013, Accepted 8th November 2013 DOI: 10.1039/c3cc47889k www.rsc.org/chemcomm
Di-isobutylaluminum hydride and tri-iso-butylaluminum (DIBAL 1, TIBAL 2) are shown to be efficient hydrogenation catalysts for a variety of imines at 100 8C and 100 atm of H2, operating via a hydroalumination/hydrogenolysis mechanism.
The reduction of unsaturated molecules by H2 is undoubtedly the most important and widely utilized transformation in industry. This process is facilitated by both homogeneous and heterogeneous catalysts, well established technologies. While this is generally the case for a variety of substrates, the production of amines from imine reduction is of significant interest to the chemical industry.1 The majority of atom efficient approaches involve the use of expensive precious metals including Ir,2 Ru,3 Rh,4 and Pd.5,6 An alternative approach exploiting more abundant, but non-precious metal complexes for the catalytic hydrogenation of imines with H2 has garnered some recent attention. Cantat et al. reported the use of NHC adducts of Zn salts to catalytically deoxygenate reactions of CO2 with methylanilines.7 while Blechert and Roesky et al. exploited ZnCp*2 and Zn2Cp*2 in catalytic hydroamination reactions.8 Beller et al. reported the first example of imine hydrogenation catalyzed by Zn(OTf)2,9 while we recently have used ZnCp*2 in the presence of N-heterocyclic carbenes (NHCs) to effect the catalytic hydrogenation of imines with H2.10 A further alternative to precious metals that has garnered attention in the last few years have been the use of ‘‘frustrated Lewis pairs’’ (FLPs) as metal-free hydrogenation catalysts.11 While this class of catalysts was initially shown to be efficient for bulky imine and nitrile reduction, this concept has been applied to a broadening range of substrates, effecting reductions
a
Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, Canada M5S 3H6. E-mail: [email protected] b GreenCentre of Canada, 945 Princess Kingston, Ontario, Canada K7L 5L9 † Electronic supplementary information (ESI) available: Synthetic, spectroscopic and crystallographic data are deposited. CCDC 966185. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47889k
This journal is © The Royal Society of Chemistry 2014
for sterically unhindered imines, nitriles, enamines, silylenol ethers,11 substituted olefins12 and alkynes.13 In addition, FLP reductions have also been shown to effect the reduction of aniline derivatives to the corresponding cyclohexylammonium salts.14 Such aromatic reductions have also been extended to the reductions of N-heterocycles.15 At the same time, the nature of FLP catalysts has also evolved. Erker11a,16 has developed several highly effective intramolecular P/B systems, while N/B systems have been explored by Repo and co-workers.17 Klankermayer18 has designed chiral boranes for stereoselective reductions while Wang and coworkers12a have recently shown that (C6F5)2BH catalyses the reduction of olefins. This latter finding prompts us to report the ability of readily available di- and tri-isobutylaluminum (DIBAL 1; TIBAL 2) to catalyse imine reductions. In contrast to FLP catalysts, which operate via heterolytic cleavage of hydrogen, the Al-catalyzed reactions are shown to proceed via a mechanism involving hydroalumination of the imine double bond and hydrogenolysis of the resulting Al–N bond. The treatment of imine, RNQC(R 0 )R00 , with catalytic amounts of either 1 (5 mol%) or 2 (10 mol%) under H2 (102 atm) in toluene at 100 1C for 24 h resulted in reduction to the corresponding amine (Table 1). Both catalysts were effective in reducing imines with small donating groups on N (R = Me or SiMe3) in near quantitative yields. These substrates typically are not hydrogenated by FLP catalysts due to their ability to form strong adducts with boron. In general, substrates bearing bulky NtBu groups or electron withdrawing substituents were reduced in poor yield, although the imines tBuNQCHPh and tBuNQCH( p-tBuPh) were reduced in up to 79–100% yield. The quantitative reduction of a ketamine, (2,6-Me2Ph)NQC(Me)Ph by 1 affords a racemic mixture of the chiral amine product. Trials to use catalysts 1 and 2 to hydrogenate enamines or indolines failed, again in contrast to FLP catalysts.11 Efforts to probe the mechanism of action prompted investigation of stoichiometric combinations of these aluminium reagents with imine substrates. Reaction of 1 with the imine, PhNQCH( p-BrC6H4),
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Hydrogenation by alkylaluminum catalysts
Published on 11 November 2013. Downloaded by University of Windsor on 29/10/2014 11:44:38.
Conversiona (%) Substrate
Product
MeNQCHPh (Me3Si)NQCHPh PhNQCHPh PhNQCH( p-BrPh) tBuNQCH( p-tBuPh) (Ph2CH)NQCHPh (2,6-Me2Ph)NQC(Me)Ph tBuNQCHPh BnNQCHPh tBuNQCH( p-NMe2) tBuNQCH(m-BrPh) tBuNQCH( p-BrPh) (PhO2S)NQCHPh (p-ClPh)NQCHPh tBuNQCH(m-OMePh)
98 100 (95) MeNHCH2Ph (Me3Si)NHCH2Ph 96 100b PhNHCH2Ph 100 (87) 86 86 96 PhNHCH2( p-BrPh) 99 (97) 99 (94) tBuNHCH2( p-tBuPh) (Ph2CH)NHCH2Ph 36 100 (97) (2,6-Me2Ph)NHCH(Me)Ph 100 (94) 30 79 60 tBuNHCH2Ph 43 56 BnNHCH2Ph tBuNHCH2( p-NMe2) 48c 33c c tBuNHCH2(m-BrPh) 29 36c tBuNHCH2( p-BrPh) 31c 25c c (PhO2S)NHCH2Ph 25 20c ( p-ClPh)NHCH2Ph 15c 38c tBuNHCH2(m-OMePh) 16c 0c
1
2
a Yield based on 1H NMR data, isolated yields in parentheses. bond hydrolysed upon aqueous workup. c Reaction time: 48 h.
b
N–Si
Fig. 1 POV-ray drawing of 3 (C, black; N, blue; Al, aquamarine; Br, brown). H atoms have been omitted for clarity.
at room temperature overnight afforded colourless X-ray quality crystals of 3 in 92% yield. 1H and 13C{1H} NMR spectroscopic data of 3 were consistent with the incorporation of the imine fragment while the broad 27Al NMR signal at 67.6 ppm suggested the formulation as the dimer [iBu2AlN(Ph)CH2( p-BrC6H4)]2. In a similar fashion, reaction of 1 with tBuNQCH(Ph) afforded the isolation of a colorless oil 4 in 95% yield. On the basis of NMR data this species was formulated as the dimer [iBu2Al(N(tBu)Bn)]2. In the case of 3 the formulation was confirmed by X-ray crystallography (Fig. 1). The compound is centrosymmetric with an inversion symmetry relating the two halves of the dimer. The geometry around the N and Al atoms is pseudo-tetrahedral. The Al–N bond lengths of 2.047(2) Å and 2.021(2) Å are comparable to those reported for other Al-amides.19 This small dissymmetric in the Al–N distances about the planar Al2N2 core presumably reflect steric congestion between the amide-and Al-bound substituents.
302 | Chem. Commun., 2014, 50, 301--303
Scheme 1
Proposed hydrogenation pathway of imines by 1 and 2.
This is also reflected in the Al–N–Al and N–Al–N angles of 91.56(7)1 and 88.44(7)1, respectively. It is reasonable to propose that the mechanism of hydrogenation of imines proceeds via initial hydroalumination of imine as evidenced by the isolation of 3 and 4. The subsequent reaction of the Al-amide with H2 proceeds to regenerate the Al–H bond, liberating the amine. This hydrogenolysis of the Al–N bond is thought to be facilitated by elevated temperatures by effecting dissociation of the Al-amide dimer. The monomeric form can then interact with H2 affording the transient formation of the amine–alane adduct. Thermal dissociation liberates 1 and frees the product amine. In support of this view, reaction of 3 with D2 generated PhNDCH2(p-BrBn) as evidenced by 1H and 2D NMR spectra (see ESI†). Interestingly, on prolonged standing under D2 for 1 week, scrambling of the D over the N and alpha-C sites of the amine is observed, suggesting that hydrogen transfer steps are reversible (see ESI,† Scheme 1). Compound 2 is thought to as a precatalyst with the thermal generation of 1 via loss of isobutene and subsequently proceeding in the mechanism described for 1. This metal-free reaction pathway stands in contrast to that of FLP catalysts, which operates via a mechanism involving heterolytic cleavage of H2. The present mechanism of hydroalumination followed by hydrogenolysis or s-bond metathesis is analogous to that described for the hydrogenation of olefins recently reported by Wang et al.12a It is noteworthy that the mechanism is conceptually similar to that inferred for the hydrogenation of olefins under 1000 psi H2 at 220 1C employing BiPr320 or superacid systems,21 and to the described process for the hydrogenation of coal catalysed by BI3 or alkyl boranes under forcing conditions.22 In conclusion, the hydrogenation of imines has been demonstrated to proceed catalysed by the addition of di-isobutylaluminum hydride and tri-iso-butylaluminum at 100 1C and 100 atm of H2. These reactions proceed via hydroalumination followed by hydrogenolysis/s-bond metathesis. This unique reactivity coupled with the ability to hydrogenate sterically unhindered substrates complements that of FLP catalysts.
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Notes and references 1 (a) D. J. Ager, A. H. M. de Vries and J. G. de Vries, Chem. Soc. Rev., 2012, 41, 3340–3380; (b) H.-U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner and M. Studer, Adv. Synth. Catal., 2003, 345, 103–151; (c) F. Spindler, H.-U. Blaser, J. G. d. Vries and C. J. Elsevier, Handbook of Homogeneous Hydrogenation, Wiley-VCH, Weinheim, Germany, 2007. 2 (a) C. Moessner and C. Bolm, Angew. Chem., Int. Ed., 2005, 44, 7564–7567; (b) M. T. Reetz and O. Bondarev, Angew. Chem., Int. Ed., 2007, 46, 8107; (c) N. a. Mrsˇic´, A. J. Minnaard, B. L. Feringa and J. G. d. Vries, J. Am. Chem. Soc., 2009, 131, 8358–8359; (d) W. Li, G. Hou, M. Chang and X. Zhang, Adv. Synth. Catal., 2009, 351, 3123–3127; (e) G. Hou, R. Tao, Y. Sun, X. Zhang and F. Gosselin, J. Am. Chem. Soc., 2010, 132, 2124–2125. 3 (a) N. Uematsu, A. Fujii, S. Hashiguchi, T. Ikariya and R. Noyori, J. Am. Chem. Soc., 1996, 118, 4916–4917; (b) C. J. Cobley and J. P. Henschke, Adv. Synth. Catal., 2003, 345, 195–201; (c) S. E. Clapham, A. Hadzovic and R. H. Morris, Coord. Chem. Rev., 2004, 248, 2201–2237. 4 (a) M. J. Burk and J. E. Feaster, J. Am. Chem. Soc., 1992, 114, 6266–6267; (b) G. Shang, Q. Yang and X. Zhang, Angew. Chem., Int. Ed., 2006, 45, 6360–6362; (c) R. Kadyrov and T. H. Riermeier, Angew. Chem., Int. Ed., 2003, 42, 5472–5474; (d) P. Etayo and A. Vidal-Ferran, Chem. Soc. Rev., 2013, 42, 728–754. 5 (a) Q. Yang, G. Shang, W. Gao, J. Deng and X. Zhang, Angew. Chem., Int. Ed., 2006, 45, 3832–3835; (b) Y.-Q. Wang, S.-M. Lu and Y.-G. Zhou, J. Org. Chem., 2007, 72, 3729–3734; (c) Q.-A. Chen, Z.-S. Ye, Y. Duan and Y.-G. Zhou, Chem. Soc. Rev., 2013, 42, 497–511. 6 (a) J.-H. Xie, S.-F. Zhu and Q.-L. Zhou, Chem. Rev., 2010, 111, 1713–1760; (b) A. Fabrello, A. Bachelier, M. Urrutigoı¨ty and P. Kalck, Coord. Chem. ´geot, V. de la Fuente, Rev., 2010, 254, 273–287; (c) N. Fleury-Bre ´n and C. Claver, ChemCatChem, 2010, 2, 1346–1371. S. Castillo 7 O. Jacquet, X. Frogneux, C. D. N. Gomes and T. Cantat, Chem. Sci., 2013, 4, 2127–2131. ¨hl, L. Hartenstein, S. Blechert and P. W. Roesky, Organometal8 A. Lu lics, 2012, 31, 7109–7116. 9 (a) S. Werkmeister, S. Fleischer, S. Zhou, K. Junge and M. Beller, ChemSusChem, 2012, 5, 777–782; (b) S. Werkmeister, S. Fleischer, K. Junge and M. Beller, Chem.–Asian J., 2012, 7, 2562–2568. 10 P. Jochmann and D. W. Stephan, Angew. Chem., Int. Ed., 2013, 52, 9831–9835. 11 (a) D. W. Stephan and G. Erker, Top. Curr. Chem., 2013, 332, 85–110; (b) D. W. Stephan, Org. Biomol. Chem., 2012, 10, 5740–5746. 12 (a) Y. Wang, W. Chen, Z. Lu, Z. H. Li and H. Wang, Angew. Chem., Int. Ed., 2013, 52, 7496–7499; (b) L. Greb, C. G. Daniliuc, K. Bergander and J. Paradies, Angew. Chem., Int. Ed., 2013, 52, ˜ a-Burgos, B. Schirmer, S. Grimme, 5876–5879; (c) L. Greb, P. On
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13 14 15 16
17
18
19 20 21 22
D. W. Stephan and J. Paradies, Angew. Chem., Int. Ed., 2012, 51, 10164–10168. ´. Madara ´sz, I. Pa ´pai, M. Nieger, M. Leskela ¨ and K. Chernichenko, A T. Repo, Nat. Chem., 2013, 5, 718–723. T. Mahdi, Z. M. Heiden, S. Grimme and D. W. Stephan, J. Am. Chem. Soc., 2012, 134, 4088–4091. T. Mahdi, J. N. del Castillo and D. W. Stephan, Organometallics, 2013, 32, 1971–1978. (a) B. H. Xu, G. Kehr, R. Frohlich, B. Wibbeling, B. Schirmer, S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2011, 50, 7183–7186; (b) B. Xu, G. Kehr, R. Frohlich, B. Wibbeling, B. Schirmer, S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2011, 50, 7183–7186; (c) S. Schwendemann, T. A. Tumay, K. V. Axenov, ¨hlich and G. Erker, Organometallics, 2010, I. Peuser, G. Kehr, R. Fro ¨hlich and G. Erker, 29, 1067–1069; (d) K. V. Axenov, G. Kehr, R. Fro J. Am. Chem. Soc., 2009, 131, 3454–3455; (e) P. Spies, ¨hlich and G. Erker, S. Schwendemann, S. Lange, G. Kehr, R. Fro Angew. Chem., Int. Ed., 2008, 47, 7543–7546. ¨, B. Rieger (a) V. Sumerin, K. Chernichenko, M. Nieger, M. Leskela and T. Repo, Adv. Synth. Catal., 2011, 353, 2093–2110; (b) F. Schulz, V. Sumerin, S. Heikkinen, B. Pedersen, C. Wang, M. Atsumi, ¨, T. Repo, P. Pyykko ¨, W. Petry and B. Rieger, J. Am. Chem. M. Leskela Soc., 2011, 133, 20245–20257; (c) V. Sumerin, F. Schulz, M. Nieger, ¨, P. Pyykko ¨, T. Repo and B. Rieger, M. Atsumi, C. Wang, M. Leskela J. Organomet. Chem., 2009, 694, 2654–2660; (d) V. Sumerin, F. Schulz, ¨, T. Repo and B. Rieger, Angew. Chem., Int. Ed., M. Nieger, M. Leskela 2008, 47, 6001–6003; (e) V. Sumerin, F. Schulz, M. Atsumi, C. Wang, ¨, T. Repo, P. Pyykko ¨ and B. Rieger, J. Am. Chem. M. Nieger, M. Leskela Soc., 2008, 130, 14117–14118. (a) D. Chen, V. Leich, F. Pan and J. Klankermayer, Chem.–Eur. J., 2012, 18, 5184–5187; (b) D. J. Chen, B. Sundararaju, R. Krause, J. Klankermayer, P. H. Dixneuf and W. Leitner, ChemCatChem, 2010, 2, 55–57; (c) D. J. Chen and J. Klankermayer, Chem. Commun., 2008, 2130–2131. M. Khandelwal, D. R. Powell and R. J. Wehmschulte, Eur. J. Inorg. Chem., 2011, 521–526. (a) F. L. Ramp, E. J. Dewitt and L. E. Trapasso, J. Org. Chem., 1962, 27, 4368–4372; (b) E. J. Dewitt, L. E. Trapasso and F. L. Ramp, J. Am. Chem. Soc., 1961, 83, 4672. (a) M. Siskin, J. Am. Chem. Soc., 1974, 96, 3641; (b) J. Wristers, J. Am. Chem. Soc., 1975, 97, 4312–4316. ´ ska, (a) M. W. Haenel, J. Narangeral, U.-B. Richter and A. Rufin Angew. Chem., Int. Ed., 2006, 45, 1061–1066; (b) M. Yalpani, R. Koster and M. W. Haenel, Erdoel Kohle, Erdgas, Petrochem., 1990, 43, ¨ster, Chem. Ber., 1989, 344–347; (c) M. Yalpani, T. Lunow and R. Ko 122, 687–693.
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Imine hydrogenation by alkylaluminum catalysts† Cite this: Chem. Commun., 2014, 50, 301
Jillian A. Hatnean,a Jordan W. Thomson,b Preston A. Chaseb and Douglas W. Stephan*a
Received 14th October 2013, Accepted 8th November 2013 DOI: 10.1039/c3cc47889k www.rsc.org/chemcomm
Di-isobutylaluminum hydride and tri-iso-butylaluminum (DIBAL 1, TIBAL 2) are shown to be efficient hydrogenation catalysts for a variety of imines at 100 8C and 100 atm of H2, operating via a hydroalumination/hydrogenolysis mechanism.
The reduction of unsaturated molecules by H2 is undoubtedly the most important and widely utilized transformation in industry. This process is facilitated by both homogeneous and heterogeneous catalysts, well established technologies. While this is generally the case for a variety of substrates, the production of amines from imine reduction is of significant interest to the chemical industry.1 The majority of atom efficient approaches involve the use of expensive precious metals including Ir,2 Ru,3 Rh,4 and Pd.5,6 An alternative approach exploiting more abundant, but non-precious metal complexes for the catalytic hydrogenation of imines with H2 has garnered some recent attention. Cantat et al. reported the use of NHC adducts of Zn salts to catalytically deoxygenate reactions of CO2 with methylanilines.7 while Blechert and Roesky et al. exploited ZnCp*2 and Zn2Cp*2 in catalytic hydroamination reactions.8 Beller et al. reported the first example of imine hydrogenation catalyzed by Zn(OTf)2,9 while we recently have used ZnCp*2 in the presence of N-heterocyclic carbenes (NHCs) to effect the catalytic hydrogenation of imines with H2.10 A further alternative to precious metals that has garnered attention in the last few years have been the use of ‘‘frustrated Lewis pairs’’ (FLPs) as metal-free hydrogenation catalysts.11 While this class of catalysts was initially shown to be efficient for bulky imine and nitrile reduction, this concept has been applied to a broadening range of substrates, effecting reductions
a
Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, Canada M5S 3H6. E-mail: [email protected] b GreenCentre of Canada, 945 Princess Kingston, Ontario, Canada K7L 5L9 † Electronic supplementary information (ESI) available: Synthetic, spectroscopic and crystallographic data are deposited. CCDC 966185. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47889k
This journal is © The Royal Society of Chemistry 2014
for sterically unhindered imines, nitriles, enamines, silylenol ethers,11 substituted olefins12 and alkynes.13 In addition, FLP reductions have also been shown to effect the reduction of aniline derivatives to the corresponding cyclohexylammonium salts.14 Such aromatic reductions have also been extended to the reductions of N-heterocycles.15 At the same time, the nature of FLP catalysts has also evolved. Erker11a,16 has developed several highly effective intramolecular P/B systems, while N/B systems have been explored by Repo and co-workers.17 Klankermayer18 has designed chiral boranes for stereoselective reductions while Wang and coworkers12a have recently shown that (C6F5)2BH catalyses the reduction of olefins. This latter finding prompts us to report the ability of readily available di- and tri-isobutylaluminum (DIBAL 1; TIBAL 2) to catalyse imine reductions. In contrast to FLP catalysts, which operate via heterolytic cleavage of hydrogen, the Al-catalyzed reactions are shown to proceed via a mechanism involving hydroalumination of the imine double bond and hydrogenolysis of the resulting Al–N bond. The treatment of imine, RNQC(R 0 )R00 , with catalytic amounts of either 1 (5 mol%) or 2 (10 mol%) under H2 (102 atm) in toluene at 100 1C for 24 h resulted in reduction to the corresponding amine (Table 1). Both catalysts were effective in reducing imines with small donating groups on N (R = Me or SiMe3) in near quantitative yields. These substrates typically are not hydrogenated by FLP catalysts due to their ability to form strong adducts with boron. In general, substrates bearing bulky NtBu groups or electron withdrawing substituents were reduced in poor yield, although the imines tBuNQCHPh and tBuNQCH( p-tBuPh) were reduced in up to 79–100% yield. The quantitative reduction of a ketamine, (2,6-Me2Ph)NQC(Me)Ph by 1 affords a racemic mixture of the chiral amine product. Trials to use catalysts 1 and 2 to hydrogenate enamines or indolines failed, again in contrast to FLP catalysts.11 Efforts to probe the mechanism of action prompted investigation of stoichiometric combinations of these aluminium reagents with imine substrates. Reaction of 1 with the imine, PhNQCH( p-BrC6H4),
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Hydrogenation by alkylaluminum catalysts
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Conversiona (%) Substrate
Product
MeNQCHPh (Me3Si)NQCHPh PhNQCHPh PhNQCH( p-BrPh) tBuNQCH( p-tBuPh) (Ph2CH)NQCHPh (2,6-Me2Ph)NQC(Me)Ph tBuNQCHPh BnNQCHPh tBuNQCH( p-NMe2) tBuNQCH(m-BrPh) tBuNQCH( p-BrPh) (PhO2S)NQCHPh (p-ClPh)NQCHPh tBuNQCH(m-OMePh)
98 100 (95) MeNHCH2Ph (Me3Si)NHCH2Ph 96 100b PhNHCH2Ph 100 (87) 86 86 96 PhNHCH2( p-BrPh) 99 (97) 99 (94) tBuNHCH2( p-tBuPh) (Ph2CH)NHCH2Ph 36 100 (97) (2,6-Me2Ph)NHCH(Me)Ph 100 (94) 30 79 60 tBuNHCH2Ph 43 56 BnNHCH2Ph tBuNHCH2( p-NMe2) 48c 33c c tBuNHCH2(m-BrPh) 29 36c tBuNHCH2( p-BrPh) 31c 25c c (PhO2S)NHCH2Ph 25 20c ( p-ClPh)NHCH2Ph 15c 38c tBuNHCH2(m-OMePh) 16c 0c
1
2
a Yield based on 1H NMR data, isolated yields in parentheses. bond hydrolysed upon aqueous workup. c Reaction time: 48 h.
b
N–Si
Fig. 1 POV-ray drawing of 3 (C, black; N, blue; Al, aquamarine; Br, brown). H atoms have been omitted for clarity.
at room temperature overnight afforded colourless X-ray quality crystals of 3 in 92% yield. 1H and 13C{1H} NMR spectroscopic data of 3 were consistent with the incorporation of the imine fragment while the broad 27Al NMR signal at 67.6 ppm suggested the formulation as the dimer [iBu2AlN(Ph)CH2( p-BrC6H4)]2. In a similar fashion, reaction of 1 with tBuNQCH(Ph) afforded the isolation of a colorless oil 4 in 95% yield. On the basis of NMR data this species was formulated as the dimer [iBu2Al(N(tBu)Bn)]2. In the case of 3 the formulation was confirmed by X-ray crystallography (Fig. 1). The compound is centrosymmetric with an inversion symmetry relating the two halves of the dimer. The geometry around the N and Al atoms is pseudo-tetrahedral. The Al–N bond lengths of 2.047(2) Å and 2.021(2) Å are comparable to those reported for other Al-amides.19 This small dissymmetric in the Al–N distances about the planar Al2N2 core presumably reflect steric congestion between the amide-and Al-bound substituents.
302 | Chem. Commun., 2014, 50, 301--303
Scheme 1
Proposed hydrogenation pathway of imines by 1 and 2.
This is also reflected in the Al–N–Al and N–Al–N angles of 91.56(7)1 and 88.44(7)1, respectively. It is reasonable to propose that the mechanism of hydrogenation of imines proceeds via initial hydroalumination of imine as evidenced by the isolation of 3 and 4. The subsequent reaction of the Al-amide with H2 proceeds to regenerate the Al–H bond, liberating the amine. This hydrogenolysis of the Al–N bond is thought to be facilitated by elevated temperatures by effecting dissociation of the Al-amide dimer. The monomeric form can then interact with H2 affording the transient formation of the amine–alane adduct. Thermal dissociation liberates 1 and frees the product amine. In support of this view, reaction of 3 with D2 generated PhNDCH2(p-BrBn) as evidenced by 1H and 2D NMR spectra (see ESI†). Interestingly, on prolonged standing under D2 for 1 week, scrambling of the D over the N and alpha-C sites of the amine is observed, suggesting that hydrogen transfer steps are reversible (see ESI,† Scheme 1). Compound 2 is thought to as a precatalyst with the thermal generation of 1 via loss of isobutene and subsequently proceeding in the mechanism described for 1. This metal-free reaction pathway stands in contrast to that of FLP catalysts, which operates via a mechanism involving heterolytic cleavage of H2. The present mechanism of hydroalumination followed by hydrogenolysis or s-bond metathesis is analogous to that described for the hydrogenation of olefins recently reported by Wang et al.12a It is noteworthy that the mechanism is conceptually similar to that inferred for the hydrogenation of olefins under 1000 psi H2 at 220 1C employing BiPr320 or superacid systems,21 and to the described process for the hydrogenation of coal catalysed by BI3 or alkyl boranes under forcing conditions.22 In conclusion, the hydrogenation of imines has been demonstrated to proceed catalysed by the addition of di-isobutylaluminum hydride and tri-iso-butylaluminum at 100 1C and 100 atm of H2. These reactions proceed via hydroalumination followed by hydrogenolysis/s-bond metathesis. This unique reactivity coupled with the ability to hydrogenate sterically unhindered substrates complements that of FLP catalysts.
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Notes and references 1 (a) D. J. Ager, A. H. M. de Vries and J. G. de Vries, Chem. Soc. Rev., 2012, 41, 3340–3380; (b) H.-U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner and M. Studer, Adv. Synth. Catal., 2003, 345, 103–151; (c) F. Spindler, H.-U. Blaser, J. G. d. Vries and C. J. Elsevier, Handbook of Homogeneous Hydrogenation, Wiley-VCH, Weinheim, Germany, 2007. 2 (a) C. Moessner and C. Bolm, Angew. Chem., Int. Ed., 2005, 44, 7564–7567; (b) M. T. Reetz and O. Bondarev, Angew. Chem., Int. Ed., 2007, 46, 8107; (c) N. a. Mrsˇic´, A. J. Minnaard, B. L. Feringa and J. G. d. Vries, J. Am. Chem. Soc., 2009, 131, 8358–8359; (d) W. Li, G. Hou, M. Chang and X. Zhang, Adv. Synth. Catal., 2009, 351, 3123–3127; (e) G. Hou, R. Tao, Y. Sun, X. Zhang and F. Gosselin, J. Am. Chem. Soc., 2010, 132, 2124–2125. 3 (a) N. Uematsu, A. Fujii, S. Hashiguchi, T. Ikariya and R. Noyori, J. Am. Chem. Soc., 1996, 118, 4916–4917; (b) C. J. Cobley and J. P. Henschke, Adv. Synth. Catal., 2003, 345, 195–201; (c) S. E. Clapham, A. Hadzovic and R. H. Morris, Coord. Chem. Rev., 2004, 248, 2201–2237. 4 (a) M. J. Burk and J. E. Feaster, J. Am. Chem. Soc., 1992, 114, 6266–6267; (b) G. Shang, Q. Yang and X. Zhang, Angew. Chem., Int. Ed., 2006, 45, 6360–6362; (c) R. Kadyrov and T. H. Riermeier, Angew. Chem., Int. Ed., 2003, 42, 5472–5474; (d) P. Etayo and A. Vidal-Ferran, Chem. Soc. Rev., 2013, 42, 728–754. 5 (a) Q. Yang, G. Shang, W. Gao, J. Deng and X. Zhang, Angew. Chem., Int. Ed., 2006, 45, 3832–3835; (b) Y.-Q. Wang, S.-M. Lu and Y.-G. Zhou, J. Org. Chem., 2007, 72, 3729–3734; (c) Q.-A. Chen, Z.-S. Ye, Y. Duan and Y.-G. Zhou, Chem. Soc. Rev., 2013, 42, 497–511. 6 (a) J.-H. Xie, S.-F. Zhu and Q.-L. Zhou, Chem. Rev., 2010, 111, 1713–1760; (b) A. Fabrello, A. Bachelier, M. Urrutigoı¨ty and P. Kalck, Coord. Chem. ´geot, V. de la Fuente, Rev., 2010, 254, 273–287; (c) N. Fleury-Bre ´n and C. Claver, ChemCatChem, 2010, 2, 1346–1371. S. Castillo 7 O. Jacquet, X. Frogneux, C. D. N. Gomes and T. Cantat, Chem. Sci., 2013, 4, 2127–2131. ¨hl, L. Hartenstein, S. Blechert and P. W. Roesky, Organometal8 A. Lu lics, 2012, 31, 7109–7116. 9 (a) S. Werkmeister, S. Fleischer, S. Zhou, K. Junge and M. Beller, ChemSusChem, 2012, 5, 777–782; (b) S. Werkmeister, S. Fleischer, K. Junge and M. Beller, Chem.–Asian J., 2012, 7, 2562–2568. 10 P. Jochmann and D. W. Stephan, Angew. Chem., Int. Ed., 2013, 52, 9831–9835. 11 (a) D. W. Stephan and G. Erker, Top. Curr. Chem., 2013, 332, 85–110; (b) D. W. Stephan, Org. Biomol. Chem., 2012, 10, 5740–5746. 12 (a) Y. Wang, W. Chen, Z. Lu, Z. H. Li and H. Wang, Angew. Chem., Int. Ed., 2013, 52, 7496–7499; (b) L. Greb, C. G. Daniliuc, K. Bergander and J. Paradies, Angew. Chem., Int. Ed., 2013, 52, ˜ a-Burgos, B. Schirmer, S. Grimme, 5876–5879; (c) L. Greb, P. On
This journal is © The Royal Society of Chemistry 2014
13 14 15 16
17
18
19 20 21 22
D. W. Stephan and J. Paradies, Angew. Chem., Int. Ed., 2012, 51, 10164–10168. ´. Madara ´sz, I. Pa ´pai, M. Nieger, M. Leskela ¨ and K. Chernichenko, A T. Repo, Nat. Chem., 2013, 5, 718–723. T. Mahdi, Z. M. Heiden, S. Grimme and D. W. Stephan, J. Am. Chem. Soc., 2012, 134, 4088–4091. T. Mahdi, J. N. del Castillo and D. W. Stephan, Organometallics, 2013, 32, 1971–1978. (a) B. H. Xu, G. Kehr, R. Frohlich, B. Wibbeling, B. Schirmer, S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2011, 50, 7183–7186; (b) B. Xu, G. Kehr, R. Frohlich, B. Wibbeling, B. Schirmer, S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2011, 50, 7183–7186; (c) S. Schwendemann, T. A. Tumay, K. V. Axenov, ¨hlich and G. Erker, Organometallics, 2010, I. Peuser, G. Kehr, R. Fro ¨hlich and G. Erker, 29, 1067–1069; (d) K. V. Axenov, G. Kehr, R. Fro J. Am. Chem. Soc., 2009, 131, 3454–3455; (e) P. Spies, ¨hlich and G. Erker, S. Schwendemann, S. Lange, G. Kehr, R. Fro Angew. Chem., Int. Ed., 2008, 47, 7543–7546. ¨, B. Rieger (a) V. Sumerin, K. Chernichenko, M. Nieger, M. Leskela and T. Repo, Adv. Synth. Catal., 2011, 353, 2093–2110; (b) F. Schulz, V. Sumerin, S. Heikkinen, B. Pedersen, C. Wang, M. Atsumi, ¨, T. Repo, P. Pyykko ¨, W. Petry and B. Rieger, J. Am. Chem. M. Leskela Soc., 2011, 133, 20245–20257; (c) V. Sumerin, F. Schulz, M. Nieger, ¨, P. Pyykko ¨, T. Repo and B. Rieger, M. Atsumi, C. Wang, M. Leskela J. Organomet. Chem., 2009, 694, 2654–2660; (d) V. Sumerin, F. Schulz, ¨, T. Repo and B. Rieger, Angew. Chem., Int. Ed., M. Nieger, M. Leskela 2008, 47, 6001–6003; (e) V. Sumerin, F. Schulz, M. Atsumi, C. Wang, ¨, T. Repo, P. Pyykko ¨ and B. Rieger, J. Am. Chem. M. Nieger, M. Leskela Soc., 2008, 130, 14117–14118. (a) D. Chen, V. Leich, F. Pan and J. Klankermayer, Chem.–Eur. J., 2012, 18, 5184–5187; (b) D. J. Chen, B. Sundararaju, R. Krause, J. Klankermayer, P. H. Dixneuf and W. Leitner, ChemCatChem, 2010, 2, 55–57; (c) D. J. Chen and J. Klankermayer, Chem. Commun., 2008, 2130–2131. M. Khandelwal, D. R. Powell and R. J. Wehmschulte, Eur. J. Inorg. Chem., 2011, 521–526. (a) F. L. Ramp, E. J. Dewitt and L. E. Trapasso, J. Org. Chem., 1962, 27, 4368–4372; (b) E. J. Dewitt, L. E. Trapasso and F. L. Ramp, J. Am. Chem. Soc., 1961, 83, 4672. (a) M. Siskin, J. Am. Chem. Soc., 1974, 96, 3641; (b) J. Wristers, J. Am. Chem. Soc., 1975, 97, 4312–4316. ´ ska, (a) M. W. Haenel, J. Narangeral, U.-B. Richter and A. Rufin Angew. Chem., Int. Ed., 2006, 45, 1061–1066; (b) M. Yalpani, R. Koster and M. W. Haenel, Erdoel Kohle, Erdgas, Petrochem., 1990, 43, ¨ster, Chem. Ber., 1989, 344–347; (c) M. Yalpani, T. Lunow and R. Ko 122, 687–693.
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