Catalytic Asymmetric Transfer Hydrogenation of Acylboronates: BMIDA as the Privileged Directing Group

Developing a general, highly efficient, and enantioselective catalytic method for the synthesis of chiral alcohols is still a formidable challenge. We report in this article the asymmetric transfer hydrogenation (ATH) of N-methyliminodiacetyl (MIDA) acylboronates as a general substrate-independent entry to enantioenriched secondary alcohols. ATH of acyl-MIDA-boronates with (het)aryl, alkyl, alkynyl, alkenyl, and carbonyl substituents delivers a variety of enantioenriched α-boryl alcohols. The latter are used in a range of stereospecific transformations based on the boron moiety, enabling the synthesis of carbinols with two closely related α-substituents, which cannot be obtained with high enantioselectivities using direct asymmetric hydrogenation methods, such as the (R)-cloperastine intermediate. Computational studies illustrate that the BMIDA group is a privileged enantioselectivity-directing group in Noyori–Ikariya ATH compared to the conventionally used aryl and alkynyl groups due to the favorable CH–O attractive electrostatic interaction between the η6-arene-CH of the catalyst and the σ-bonded oxygen atoms in BMIDA. The work expands the domain of conventional ATH and shows its huge potential in addressing challenges in symmetric synthesis.