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A New Designed Hydrazine Group-contained Ruthenium Complex Used for Catalytic Hydrogenation of Esters Xuefeng Tan, Qingli Wang, Yuanhua Liu, Fangyuan Wang, Hui Lv*, and Xumu Zhang*
DOI: 10.1039/x0xx00000x www.rsc.org/
A h ydrazine grou p-containe d nitrogen-ph os phine ligan d an d correlate d ruthenium complexes we re s ynthesize d. When these complexes were use d for hydrogen ation of esters, excellent perfo rman ce was observe d (TON u p to 17200). A wi de su bstrate s cope was suit able for this catalytic system. The reduction of esters to their corresponding alcohols is one of the most important chemical transformations in organic synthesis. Traditionally, this reaction was commonly performed with LiAlH 4 and NaBH 4,1 which are hazardous to operate and have tedious workups. Due to the poor reactivity of esters relative to ketones, 2 catalytic hydrogenation of esters mainly proceed with heterogeneous catalysts under harsh conditions (200-300 º C, 200-300 atm),3 which is an industrially important process to obtain long-chain alcohols. In order to reduce the high energy consumption and high cost associated with heterogeneous catalyst, developing highly active homogeneous catalytic systems to replace the traditional way has become one of the most important targets for chemists. Giving the earlier stages of homogeneous catalytic hydrogenation of esters to alcohols, many more effective catalysts with novel ligand structures are discovered other than the few patented systems and introduction of new catalysts can promote broad usages in organic chemistry. Although great efforts have been devoted to this transformation, progresses have been made until recent years.4-5 Representative ester reduction catalysts are listed in Figure 1. In 2006, M ilstein and co-workers developed the PNN pincer-type ruthenium complex I (Figure 1),6 which undergoes an aromatizationdearomatizaiton process when used to reduce esters under a relatively mild condition (5 atm H 2, 115 º C). In 2007, Saudan and co-workers (in Firmenich SA) reported and patented RuCl2(P-N)2 (II) and a tetradentate RuCl2(PNNP) catalyst which could efficiently catalyze the hydrogenation of aromatic and aliphatic esters in the presence of a base.7 A good chemo-selectivity can be obtained when it used in hydrogenation of esters and alkenes. In 2011, Kuriyama and co-workers from Takasago company reported a simple tridentate ruthenium complex III,8 which can be used in hydrogenation of a wide substrate scope of esters, especially for methyl (R)-lactate (TON ≈ 2000 by a 2200 kg scale).8d Subsequently, Gusev developed an electron-rich ruthenium complex IV which greatly improved the
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efficiency of catalytic ester hydrogenation.9 Very recently, Zhou and our group respectively reported the bipyridyl-containing tetradentate ligands 5l and a pyridin-2-ylmethanamine-containing tetradentate ligand,5m both of them exhibited excellent catalytic activities in ester reduction. However, many of the research works centered on the electron and stereo-properties of ligands or other additives, and scare new motifs have been investigated for ligand designing. Therefore, to develop new and efficient ester reduction catalyst is highly desirable.
We notice that most of homogenous ester reduction catalysts bearing the N-H groups, which can activate carbonyl groups by hydrogen bond and provide a proton to carbonyl oxygen atom. Based on these facts and mechanism studies, 10 we envision that replacing the N-H group to a NH-NH group (hydrazine group) whether can make sense for ester reduction as the N-H bond density in per unit increased greatly and electronic property on N-H group changed a lot in hydrazine group. Hence we designed a hydrazine group contained ligand L based on the model of Takasago’s Ru-PNP catalyst III. The novel ligand L can be obtained by a short synthetic route described in Scheme 1. Several ruthenium precursors were used for the coordination reaction under different conditions as the table in Scheme 1 illustrated. All the preparations of A-G were handled with a commonly used method, firstly precipitated the complexes with ethyl ether and then washed with ether for several times. M ost of the directly obtained crystalline products were miscellaneous and structures were difficult to be confirmed by NM R or other spectra analysis, which may be due to the coordinating diversity of the hydrazine group. In order to evaluate the catalytic hydrogenation efficiency of complex A-G, methyl benzoate was used as standard substrate, with 2.5 mol% KO tBu as additive and under 50 atm of H 2 at 80 º C. We set the substrate/catalyst ratio as per milligram catalyst react with 5 mmol
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substrate (Table 1). Several ruthenium precursors, such as RuCl2(PPh3)3, [RuCl2(p-cymene)]2 and RuCl2(DM SO)4 were capable of obtaining high efficient catalysts for methyl benzoate reduction, while CO containing precursor RuHCl(CO)(PPh3)3 gave unsatisfied result. Interestingly, for the same ruthenium precursor, e.g. RuCl2(PPh3)3, [RuCl2(p-cymene)]2, the higher reaction temperature for catalyst preparation, the better catalytic activity performed (Table 1, entries 1-8). In order to finding out the optimal catalyst precursor, we maximized the S/C ratio of catalysts C, D and E (Table 1, entries 4, 6 and 7), and found that catalyst E gave the best result.
In order to further optimize the reaction conditions, a series of solvents, bases and catalyst loading (in situ form, e.g., 1 mmol [RuCl2(p-cymene)]2 and 1.1 mmol L were used for catalyst preparation according to Scheme 1, if we need 0.001 mmol catalyst, then 1/1000 weight of the total catalyst would be weighed) were investigated. As shown in Table 2, different solvents have great influence on this reaction and dioxane gave excellent performance (Table 2, entries 1-9). Subsequently, we screened a number of bases as additives to activate the catalyst. Results showed that inorganic bases (e.g. NaOH, Cs 2CO 3, Table 2, entries 19-20) gave unsatisfied results, while alkali metal alkoxides were capable of this reaction. Interestingly, the alkali metal sodium performed better than lithium and potassium bases. At last, we chose NaOEt with a 5 mol% loading as the standard base additive (Table 2, entries 10-18). Under the optimized reaction conditions, a turnover number of 17200 (Table 2, entry 15) was obtained for methyl benzoate hydrogenation, compared with Takasago’s catalyst III of a 1000 turnover number ever reported,8d which greatly demonstrated the hydrazine group has positive effects on ester reduction. Then we investigated the substrate scope under the optimized reaction conditions in the presence of E (Table 3). It’s obvious that aromatic esters gave more satisfied results than aliphatic esters, except for methyl 4-fluorobenzoate (Table 3, entries 1-3). M ost of the aliphatic esters catalyzed by E with moderate turnover numbers, except for ethyl acetate obtained a TON of 9100 (Table 3, entries 69). A comparison of dimethyl esters, dimethyl o-phthalate and dimethyl terephthalate (Table 3, entries 10-11), we can see that ortho-substituted diesters hardly converted to diols with equal activities as other substrates, which may due to the product inhibition because ortho-position substitution in favor of the chelating with the metal center. For alkenyl group containing esters, such as methyl 3-cyclohexenecarboxylate (Table 3, entry 12), good
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Journal Name conversion was observed (96%), accompanied with 9% of alkenyl group was hydrogenated.
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In our attempts to obtain the catalysts structures of A-E, many crystallization experiments had been tried, but failed to obtain a crystal structure. Then we carried out tracking experiments to analyze the reaction mixture of the preparation of catalysts A-E, monitoring the disassociated ligand PPh 3 and p-cymene (GC analysis , n-tridecane as internal standard). For cat alysts A-C, the
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substitution ratio (actually, no 1H NM R signals of p -cymene can be seen at this stage). These experiments showed the exact high activity species are the complexes that PPh 3 and p-cymene disassociated completely (catalysts C and E). 1H and 31P NM R spectra showed the structure of catalysts C - G are extremely complicated, and highresolution mass spectra revealed there existed RuCl2(L) and its dimer and trimer (see Supporting Information). This may be due to the deficient coordinating atoms to fulfill the 18 electron compositions and solvents participated the coordination or even one hydrazine group coordinated with two ruthenium to form the dimer, trimer or oligomer.
In conclusion, we have synthesized a hydrazine group contained nitrogen-phosphine ligand and its ruthenium complexes. These catalysts were successfully used in catalytic hydrogenation of esters to alcohols. A variety of esters have been reduced with high efficiency, TON up to 17200 for reduction of aromatic esters. This new motif can help us to develop more efficient catalysts for ester reduction in the field.
Acknowledgements We are grateful for the financial s upport by a grant from Wuhan U nivers ity (203273463), N atural Science Foundation of Hubei Province (2014CFB181), “111” P roject of the M inistry of Education of China and the N ational N atural Science Foundation of China (Grant No. 21372179, 21432007).
Notes and references
substituent ratios of PPh3 after 20 h are 62%, 64% and 78%, respectively, which revealed that one of the PPh 3 still bonded to ruthenium center under low temperature (< 70 º C) and more than two PPh3 were moved under high temperature ( 110 º C). 31P NM R showed catalysts A and B have the cis- conformations (confirmed by coupling constants, see Supporting Information), one of the phosphorus in L existed in trans- position with PPh3, the broad peaks of coordinated PPh3 and trace dissociative PPh3 peaks revealed an equilibrium between the five- and six-coordinated species or the existence of interconversion between those species. Tracking the amounts of p-cymene generated in preparation of D and E showed in Figure 2. This experiment showed that in low temperature (70 º C) the disassociation speed is slow and in an elevated temperature the p-cymene can be substituted in short time (4 h) up to 90%
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Key Laboratory of Biomedical Polymers of Ministry of Education & College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, China Email: [email protected] , [email protected] † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/c000000x/ 1 J. seyden-Penne, Reductions by the Allumino- and Borohydride in Organic Synthesis, 2 nd ed., Wiley-VCH, New York, 1997. 2 S. T akebayashi and S. H. Bergens, Organometallics, 2009, 28, 2349. 3 (a) R. Rieke, D. T hakur, B. Roberts and G. White, J Amer Oil Chem Soc, 1997, 74, 333; (b) Y. Pouilloux, F. Autin and J. Barrault, Catalysis Today, 2000, 63, 87. 4 (a) M. L. Clarke, Catalysis Science & Technology, 2012, 2, 2418; (b) P. A. Dub and T . Ikariya, ACS Catalysis, 2012, 2, 1718. 5 Selected examples on hydrogenation of esters, see: (a) R. A. Grey, G. P. Pez and A. Wallo, J. Am. Chem. Soc., 1981, 103, 7536; (b) U. Matteoli, G. Menchi, M. Bianchi and F. Piacenti, Journal of Molecular Catalysis, 1988, 44, 347; (c) Y. Hara, H. Inagaki, S. Nishimura and K. Wada, Chemistry Letters, 1992, 21, 1983; (d) H. T . T eunissen, Chemical Communications, 1998, 1367; (e) M. C. van Engelen, H. T . T eunissen, J. G. de Vries and C. J. Elsevier, Journal of Molecular Catalysis A: Chemical, 2003, 206, 185; (f) W. W. N. O, A. J. Lough and R. H. Morris, Chemical Communications, 2010, 46, 8240; (g) M. Ito, T . Ootsuka, R. Watari, A. Shiibashi, A. Himizu and T . Ikariya, J. Am. Chem. Soc., 2011, 133, 4240; (h) T . Touge, T . Hakamata, H. Nara, T . Kobayashi, N. Sayo, T . Saito, Y. Kayaki and T . Ikariya, J. Am. Chem. Soc., 2011, 133, 14960; (i) K. Junge, B. Wendt,
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F. A. Westerhaus, A. Spannenberg, H. Jiao and M. Beller, Chemistry – A European Journal, 2012, 18, 9011; (j) T. Otsuka, A. Ishii, P. A. Dub and T . Ikariya, J. Am. Chem. Soc., 2013, 135, 9600; (k) S. Chakraborty, H. Dai, P. Bhattacharya, N. T . Fairweather, M. S. Gibson, J. A. Krause and H. Guan, J. Am. Chem. Soc., 2014, 136, 7869; (l) W. Li, J.-H. Xie, M.-L. Yuan and Q.-L. Zhou, Green Chemistry, 2014, 16, 4081; (m) X. T an, Y. Wang, Y. Liu, F. Wang, L. Shi, K.-H. Lee, Z. Lin, H. Lv, X. Zhang, Org. Lett., 2015, 17, 454. (a) J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, Angew. Chem. Int. Ed. Engl., 2006, 45, 1113; (b) E. Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon and D. Milstein, Nat Chem, 2011, 3, 609; (c) E. Fogler, E. Balaraman, Y. Ben-David, G. Leitus, L. J. W. Shimon and D. Milstein, Organometallics, 2011, 30, 3826; (d) C. Gunanathan and D. Milstein, Accounts of Chemical Research, 2011, 44, 588. (a) L. A. Saudan, C. M. Saudan, C. Debieux and P. Wyss, Angewandte Chemie International Edition, 2007, 46, 7473; (b) L. Sa udan, P. Dupau, J.-J. Riedhauser, P. Wyss (Firmenich SA), WO 2006106483, 2006; (c) L. Saudan, P. Dupau, J.-J. Riedhauser, P. Wyss (Firmenich SA), US 2010280273, 2010; (d) Sa udan, C.; Sa udan, L. Firmenich S. A., Switzerland; Saudan, Michel Alfred Joseph; Saudan, Sylvia Joyeuse Adelaiede Ada.; Patent WO2010061350A1, 2010. (a) W. Kuriyama, Y. Ino, O. Ogata, N. Sayo and T . Saito, Advanced Synthesis & Catalysis, 2010, 352, 92; (b) Y. Ino, W. Kuriyama, O.
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Journal Name Ogata and T . Matsumoto, Topics in Catalysis, 2010, 53, 1019; (c) W. Kuriyama, T . Matsumoto, Y. Ino, O. Ogata, N. Saeki (T akasago Int. Co.), WO 2011048727, 2011; (d) W. Kuriyama, T . Matsumoto, O. Ogata, Y. Ino, K. Aoki, S. T anaka, K. Ishida, T. Kobayashi, N. Sayo and T . Saito, Organic Process Research & Development, 2011, 16, 166. 9 (a) Acosta-Ramirez, M. Bertoli, D. G. Gusev and M. Schlaf, Green Chemistry, 2012, 14, 1178; (b) D. Spasyuk, S. Smith and D. G. Gusev, Angewandte Chemie International Edition, 2012, 51, 2772; (c) D. Spasyuk and D. G. Gusev, Organometallics, 2012, 31, 5239; (d) D. Spasyuk, S. Smith and D. G. Gusev, Angewandte Chemie International Edition, 2013, 52, 2538. 10 (a) K. Abdur-Rashid, S. E. Clapham, A. Hadzovic, J. N. Harvey, A. J. Lough and R. H. Morris, J. Am. Chem. Soc., 2002, 124, 15104; (b) C. A. Sandoval, T . Ohkuma, K. Muñiz and R. Noyori, J. Am. Chem. Soc., 2003, 125, 13490; (c) R. J. Hamilton, C. G. Leong, G. Bigam, M. Miskolzie and S. H. Ber gens, J. Am. Chem. Soc., 2005, 127, 4152; (d) S. T akebayashi and S. H. Bergens, Organometallics, 2009, 28, 2349; (e) J. M. John, S. T akebayashi, N. Dabral, M. Miskolzie and S. H. Bergens, J. Am. Chem. Soc., 2013, 135, 8578; (f) P. A. Dub, N. J. Henson, R. L. Martin and J. C. Gordon, J. Am. Chem. Soc., 2014, 136, 3505.
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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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A New Designed Hydrazine Group-contained Ruthenium Complex Used for Catalytic Hydrogenation of Esters Xuefeng Tan, Qingli Wang, Yuanhua Liu, Fangyuan Wang, Hui Lv*, and Xumu Zhang*
DOI: 10.1039/x0xx00000x www.rsc.org/
A h ydrazine grou p-containe d nitrogen-ph os phine ligan d an d correlate d ruthenium complexes we re s ynthesize d. When these complexes were use d for hydrogen ation of esters, excellent perfo rman ce was observe d (TON u p to 17200). A wi de su bstrate s cope was suit able for this catalytic system. The reduction of esters to their corresponding alcohols is one of the most important chemical transformations in organic synthesis. Traditionally, this reaction was commonly performed with LiAlH 4 and NaBH 4,1 which are hazardous to operate and have tedious workups. Due to the poor reactivity of esters relative to ketones, 2 catalytic hydrogenation of esters mainly proceed with heterogeneous catalysts under harsh conditions (200-300 º C, 200-300 atm),3 which is an industrially important process to obtain long-chain alcohols. In order to reduce the high energy consumption and high cost associated with heterogeneous catalyst, developing highly active homogeneous catalytic systems to replace the traditional way has become one of the most important targets for chemists. Giving the earlier stages of homogeneous catalytic hydrogenation of esters to alcohols, many more effective catalysts with novel ligand structures are discovered other than the few patented systems and introduction of new catalysts can promote broad usages in organic chemistry. Although great efforts have been devoted to this transformation, progresses have been made until recent years.4-5 Representative ester reduction catalysts are listed in Figure 1. In 2006, M ilstein and co-workers developed the PNN pincer-type ruthenium complex I (Figure 1),6 which undergoes an aromatizationdearomatizaiton process when used to reduce esters under a relatively mild condition (5 atm H 2, 115 º C). In 2007, Saudan and co-workers (in Firmenich SA) reported and patented RuCl2(P-N)2 (II) and a tetradentate RuCl2(PNNP) catalyst which could efficiently catalyze the hydrogenation of aromatic and aliphatic esters in the presence of a base.7 A good chemo-selectivity can be obtained when it used in hydrogenation of esters and alkenes. In 2011, Kuriyama and co-workers from Takasago company reported a simple tridentate ruthenium complex III,8 which can be used in hydrogenation of a wide substrate scope of esters, especially for methyl (R)-lactate (TON ≈ 2000 by a 2200 kg scale).8d Subsequently, Gusev developed an electron-rich ruthenium complex IV which greatly improved the
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efficiency of catalytic ester hydrogenation.9 Very recently, Zhou and our group respectively reported the bipyridyl-containing tetradentate ligands 5l and a pyridin-2-ylmethanamine-containing tetradentate ligand,5m both of them exhibited excellent catalytic activities in ester reduction. However, many of the research works centered on the electron and stereo-properties of ligands or other additives, and scare new motifs have been investigated for ligand designing. Therefore, to develop new and efficient ester reduction catalyst is highly desirable.
We notice that most of homogenous ester reduction catalysts bearing the N-H groups, which can activate carbonyl groups by hydrogen bond and provide a proton to carbonyl oxygen atom. Based on these facts and mechanism studies, 10 we envision that replacing the N-H group to a NH-NH group (hydrazine group) whether can make sense for ester reduction as the N-H bond density in per unit increased greatly and electronic property on N-H group changed a lot in hydrazine group. Hence we designed a hydrazine group contained ligand L based on the model of Takasago’s Ru-PNP catalyst III. The novel ligand L can be obtained by a short synthetic route described in Scheme 1. Several ruthenium precursors were used for the coordination reaction under different conditions as the table in Scheme 1 illustrated. All the preparations of A-G were handled with a commonly used method, firstly precipitated the complexes with ethyl ether and then washed with ether for several times. M ost of the directly obtained crystalline products were miscellaneous and structures were difficult to be confirmed by NM R or other spectra analysis, which may be due to the coordinating diversity of the hydrazine group. In order to evaluate the catalytic hydrogenation efficiency of complex A-G, methyl benzoate was used as standard substrate, with 2.5 mol% KO tBu as additive and under 50 atm of H 2 at 80 º C. We set the substrate/catalyst ratio as per milligram catalyst react with 5 mmol
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substrate (Table 1). Several ruthenium precursors, such as RuCl2(PPh3)3, [RuCl2(p-cymene)]2 and RuCl2(DM SO)4 were capable of obtaining high efficient catalysts for methyl benzoate reduction, while CO containing precursor RuHCl(CO)(PPh3)3 gave unsatisfied result. Interestingly, for the same ruthenium precursor, e.g. RuCl2(PPh3)3, [RuCl2(p-cymene)]2, the higher reaction temperature for catalyst preparation, the better catalytic activity performed (Table 1, entries 1-8). In order to finding out the optimal catalyst precursor, we maximized the S/C ratio of catalysts C, D and E (Table 1, entries 4, 6 and 7), and found that catalyst E gave the best result.
In order to further optimize the reaction conditions, a series of solvents, bases and catalyst loading (in situ form, e.g., 1 mmol [RuCl2(p-cymene)]2 and 1.1 mmol L were used for catalyst preparation according to Scheme 1, if we need 0.001 mmol catalyst, then 1/1000 weight of the total catalyst would be weighed) were investigated. As shown in Table 2, different solvents have great influence on this reaction and dioxane gave excellent performance (Table 2, entries 1-9). Subsequently, we screened a number of bases as additives to activate the catalyst. Results showed that inorganic bases (e.g. NaOH, Cs 2CO 3, Table 2, entries 19-20) gave unsatisfied results, while alkali metal alkoxides were capable of this reaction. Interestingly, the alkali metal sodium performed better than lithium and potassium bases. At last, we chose NaOEt with a 5 mol% loading as the standard base additive (Table 2, entries 10-18). Under the optimized reaction conditions, a turnover number of 17200 (Table 2, entry 15) was obtained for methyl benzoate hydrogenation, compared with Takasago’s catalyst III of a 1000 turnover number ever reported,8d which greatly demonstrated the hydrazine group has positive effects on ester reduction. Then we investigated the substrate scope under the optimized reaction conditions in the presence of E (Table 3). It’s obvious that aromatic esters gave more satisfied results than aliphatic esters, except for methyl 4-fluorobenzoate (Table 3, entries 1-3). M ost of the aliphatic esters catalyzed by E with moderate turnover numbers, except for ethyl acetate obtained a TON of 9100 (Table 3, entries 69). A comparison of dimethyl esters, dimethyl o-phthalate and dimethyl terephthalate (Table 3, entries 10-11), we can see that ortho-substituted diesters hardly converted to diols with equal activities as other substrates, which may due to the product inhibition because ortho-position substitution in favor of the chelating with the metal center. For alkenyl group containing esters, such as methyl 3-cyclohexenecarboxylate (Table 3, entry 12), good
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Journal Name conversion was observed (96%), accompanied with 9% of alkenyl group was hydrogenated.
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In our attempts to obtain the catalysts structures of A-E, many crystallization experiments had been tried, but failed to obtain a crystal structure. Then we carried out tracking experiments to analyze the reaction mixture of the preparation of catalysts A-E, monitoring the disassociated ligand PPh 3 and p-cymene (GC analysis , n-tridecane as internal standard). For cat alysts A-C, the
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substitution ratio (actually, no 1H NM R signals of p -cymene can be seen at this stage). These experiments showed the exact high activity species are the complexes that PPh 3 and p-cymene disassociated completely (catalysts C and E). 1H and 31P NM R spectra showed the structure of catalysts C - G are extremely complicated, and highresolution mass spectra revealed there existed RuCl2(L) and its dimer and trimer (see Supporting Information). This may be due to the deficient coordinating atoms to fulfill the 18 electron compositions and solvents participated the coordination or even one hydrazine group coordinated with two ruthenium to form the dimer, trimer or oligomer.
In conclusion, we have synthesized a hydrazine group contained nitrogen-phosphine ligand and its ruthenium complexes. These catalysts were successfully used in catalytic hydrogenation of esters to alcohols. A variety of esters have been reduced with high efficiency, TON up to 17200 for reduction of aromatic esters. This new motif can help us to develop more efficient catalysts for ester reduction in the field.
Acknowledgements We are grateful for the financial s upport by a grant from Wuhan U nivers ity (203273463), N atural Science Foundation of Hubei Province (2014CFB181), “111” P roject of the M inistry of Education of China and the N ational N atural Science Foundation of China (Grant No. 21372179, 21432007).
Notes and references
substituent ratios of PPh3 after 20 h are 62%, 64% and 78%, respectively, which revealed that one of the PPh 3 still bonded to ruthenium center under low temperature (< 70 º C) and more than two PPh3 were moved under high temperature ( 110 º C). 31P NM R showed catalysts A and B have the cis- conformations (confirmed by coupling constants, see Supporting Information), one of the phosphorus in L existed in trans- position with PPh3, the broad peaks of coordinated PPh3 and trace dissociative PPh3 peaks revealed an equilibrium between the five- and six-coordinated species or the existence of interconversion between those species. Tracking the amounts of p-cymene generated in preparation of D and E showed in Figure 2. This experiment showed that in low temperature (70 º C) the disassociation speed is slow and in an elevated temperature the p-cymene can be substituted in short time (4 h) up to 90%
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