Indian Institute of Technology Delhi
Mechanistic studies on ketone hydrogenation and transfer hydrogenation in the absence of deprotonatable ligands: going beyond the Noyori mechanism
Dr. Rinaldo Poli
Laboratoire de Chimie de Coordination, 205 Route de Narbonne, 31077 Toulouse, France.
Date: March 23rd 2018 (Friday)
Time: 4 PM
Venue: Committee Room, Chemistry Department, 6th Floor
The catalyzed reduction of carbonyl compounds, particularly its enantioselective version, attracts considerable interest. Stimulated by Noyori’s work on a Ru system,1 a prevailing mechanistic view calls from the simultaneous presence of a hydride and a deprotonatable donor function (typically an amine) for an outer-sphere transfer of H+ and H- from a neutral transition metal complex to the substrate.2 The presence of a strong base is typically needed for activity. However, ligands that do not contain deprotonatable functions efficiently support the catalyzed ketone hydrogenation in combination with a strong base co-catalyst. As an alternative, the classical coordination-insertion (inner sphere) mechanism is a possibility in these cases. Systems of this kind include an iridium complex with a non-deprotonatable phosphine-thioether ligand, which necessitates a strong base for activity.3
More recent work in our laboratory4 has shown that use of a pre-catalyst with an internal base ([Ir(OMe)(COD)]2 in place of [IrCl(COD)]2) is not sufficient to insure activity and an external strong base is still necessary, suggesting that the active catalyst is an anionic hydride complex. Computational investigations that include solvent effects have demonstrated the thermodynamically accessible generation of the tetrahydrido complex [IrH4(PS)]- and have suggested an operating cycle via a Na+(MeOH)3∙∙∙[IrH4(PS)]- contact ion pair with an energy span of 18.2 kcal/mol. The cycle involves an outer sphere stepwise H-/H+ transfer, the proton originating from H2 after coordination and bifunctional heterolytic activation. The base plays the dual role of generating the anionic complex and providing the Lewis acid co-catalyst for ketone activation. The best cycle for the neutral system, on the other hand, requires an energy span of 26.3 kcal/mol. This work highlights, for the first time, the possibility of outer sphere hydrogenation in the presence of non deprotonatable ligands and the role of the strong base in the activation of catalytic systems with such type of ligands. Another investigated system based on the (p-cymene)ruthenium platform with a non-deprotonatable bidentate iminophosphonamide ligand has revealed a variant of the outer-sphere Noyori mechanism for transfer hydrogenation.5
- (a) T. Ohkuma, H. Ooka, S. Hashiguchi, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1995, 117, 2675; (b) H. Doucet, T. Ohkuma, K. Murata, T. Yokozawa, M. Kozawa, E. Katayama, A. F. England, T. Ikariya, R. Noyori, Angew. Chem. Engl. 1998, 37, 1703; (c) T. Ohkuma, M. Koizumi, H. Doucet, T. Pham, M. Kozawa, K. Murata, E. Katayama, T. Yokozawa, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1998, 120, 13529; (d) K. Matsumura, N. Arai, K. Hori, T. Saito, N. Sayo, T. Ohkuma, J. Am. Chem. Soc. 2011, 133, 10696.
- (a) K. Abdur-Rashid, S. E. Clapham, A. Hadzovic, J. N. Harvey, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2002, 124, 15104. (b) C. A. Sandoval, T. Ohkuma, K. Muniz, R. Noyori, J. Am. Chem. Soc. 2003, 125, 13490.
- E. Le Roux, R. Malacea, E. Manoury, R. Poli, L. Gonsalvi and M. Peruzzini, Adv. Synth. Catal. 2007, 349, 309.
- J. M. Hayes, E. Deydier, G. Ujaque, A. Lledós, R. Malacea, E. Manoury, S. Vincendeau, R. Poli, ACS Catal., 2015, 5, 4368.
- I. S. Sinopalnikova, T. A. Peganova, N. V. Belkova, E. Deydier, J.-C. Daran, E. S. Shubina, A. M. Kalsin, R. Poli, Eur. J. Inorg. Chem. 2018, http://dx.doi.org/10.1002/ejic.201701344.
All are cordially invited to attend.