Partial reduction of ester to aldehydes is an economical and important chemical transformation in organic synthesis. In general preparation of aldehydes from esters is a two-step process (over reduction, re-oxidation). Under cryogenic conditions (-78 0C) aldehydes can be synthesized from ester group with DIBAL-H. Other than DIBAL, only few methods reported at ambient temperature. On the other hand, number of reports were described for the reduction of ester to alcohol such as metal catalyzed hydrogenation (or) hydrosilylation (or) with stoichiometric hydride addition (LiAlH4 or LiBH4). From our lab, we have reported convenient protocols for the aldehyde preparation via partial reduction of esters under mild reaction conditions.
In recent days catalyzed hydroboration for unsaturated (C=C, C=O, and C=N) bonds became a quite rapid and hot topic in reduction chemistry as evidenced by recent publications.1-2 However, catalyzed hydroboration of ester are limited, and few examples are reported with Mg-based catalyst.3-4
Given our interest in the partial, selective reductions and the above situation was prompted us to carried out the catalyzed hydroboration for partial reduction of esters. Accordingly, conditions were optimized with various catalysts. Based on the experimental results, we could synthesize aldehydes from the partial reduction of isopropyl esters with pinacolborane and alkyl lithium (Scheme).
1. Chong, C. C.; Kinjo, R. ACS Catal. 2015, 5, 3238− 3259.
2. Obligacion, J. V.; Chirik, P. J. Nat. Rev. Chem. 2018, 2, 15-34.
3. Mukherjee, D.; Ellern, A.; Sadow, A. D. Chem. Sci., 2014, 5, 959-964.
4. Mukherjee, D.; Shirase, S.; Spaniol, T. P.; Mashima, K.; Okuda, J. Chem. Commun., 2016, 52, 13155—13158.