Transfer hydrogenation catalysts are important in the synthesis of fine chemicals. Previous work has shown that a base is often necessary to achieve high activities for the asymmetric hydrogenation of polar substrates. New mechanistic views for such systems have emerged to account for the activity of such systems in which the ligand does not actively participate in proton donation which nevertheless need a strong base for activity. In this work, the role of the base in the associated chemistry of [IrCl(COD)(L2)] (L2 = dppe, dppf, (S)-BINAP, P,SiPr, P,SBz, P,SPh, P,SCy, (P,SR = CpFe[1,2-C5H3(PPh2)(CH2SR)]) systems was studied.
In the presence of an alkoxide with a β-hydrogen, two monohydride complexes of the form [IrH(C8H12)(dppe)] resulted from [IrCl(COD)(dppe)], which interconvert and this was supported by complementary DFT studies which gave a similar result. When no β-hydrogen was present on the base, two isomeric monohydride complexes were formed through COD deprotonation, [IrH(1-κ-4,5,6-3-C8H10)(dppe)]. DFT calculations were used to rationalize that the mechanism of interconversion proceeds via partial rotation of the cyclic C8H10 ligand. These model complexes were transformed by heating in the presence of KOtBu (or NaOMe) and isopropanol at 80 °C, to yield M[IrH4(dppe)] (M = K, Na). Similar IrIII products M[Ir(H)4(L2)] (L2 = dppf, (S)-BINAP) were selectively generated from [IrCl(COD)(L2)] and demonstrated that anionic tetrahydrido iridium complexes can be formed under catalytically relevant conditions.
Finally, the alkali metal-dependent hydrogenation activity of these complexes towards benzophenone was examined. The active catalyst, generated in situ from [IrCl(COD)]2 and (P,SR) under H2 in the presence of a strong base was the solvated M[Ir(H)4(P,SR)] salt. Their activity increased, for all R derivatives, in the order Li < Na < K. On the other hand, the nature of the cation did not affect the ee. DFT calculations revealed that the rate-determining barrier corresponds to outer-sphere hydride transfer and that the enantio-discriminating interactions are largely unaffected by the cation but rather through π-π interactions. It was found that the model used to describe the alkali-metal cation coordination sphere in the DFT studies is critical for reproducing the experimental results.
