Theoretical Study of Ir-Catalyzed Chemoselective C1–O Reduction of Glucose with Silane

Abstract

Density functional theory (DFT) calculations have been performed to study the mechanism of Ir­(III) pincer complex (POCOP)­Ir­(H)­(acetone)+ (POCOP = 2,6-bis­(dibutylphosphinito)­phenyl) catalyzed chemoselective C1–O hydrosilylative reduction of glucose. The mechanisms for reduction of the external and internal C1–O (i.e., C1–Oext and C1–Oint) on the C1-MeO-substituted glucose (i.e., 1 Me ) and C1–Me2EtSiO-substituted glucose (i.e., 1 Si ) have been investigated. The calculation results show that both mechanisms proceed with the first concerted silyl transfer and the subsequent C1–Oext or C1–Oint bond cleavage and hydride transfer steps. In the hydride transfer step, the Ir-H moiety acts as the hydride source. The C1–O cleavage is the rate-determining step of the overall mechanism. The C1–Oext reduction is more favorable than C1–Oint reduction for the substrate 1 Me , while the C1–Oint reduction is more favorable for 1 Si . These results are consistent with the recent experimental outcomes. Analyzing the origin of chemoselectivity for the C1–Oext or C1–Oint cleavage, we found that the more stable precursor of C1–Oext cleavage and retention of the six-membered-ring structure result in the selective C1–Oext reduction of 1 Me . Meanwhile, the higher basicity of the alkyl ether Oint atom (in comparison to the silyl ether Oext atom) and greater steric hindrance in the precursor favor the C1–Oint bond weakening. Therefore, the C1–Oint reduction occurs selectively for 1 Si .