Quantum chemical modeling of chiral catalysis. Part 17. On the diboranc derivatives of chiral oxazaborolidines used as catalysts in the enantioselective reduction of ketones

Abstract

Relative stabilities of isomers of borane (H 3B) diadducts of oxazaborolidines ( 1) were investigated by means of ab initio MO (RHF) methods [1,3,2-oxazaborolidine ( 1′) as a model of 1]. Three isomers ( 4′, 5′ and 6′, all hydride-bridged) were found to be (51, 24 and 69 kJ mol −1; MP2/6-31G*//6–31G* more stable than the borane N,O-diadduct ( 3′) of 1′. The most stable isomer ( 6′) could be described as a system in which two bridges (“-H 2B-H-BH 2-” and “-H 2B-”) connect the O- and N-ends of the 2-aminoethoxy moiety. The other two isomers ( 4′ and 5′) could be both considered as (amino)diborane derivatives. In the former ( 4′) the diborane system is fused to the BN bond of borane O-adduct of 1′ (one hydride bridge) whereas in the latter the BH bond of the boron of the oxazaborolidine ring of borane N-adduct of 1′ is involved in the diborane structure (two hydride bridges). A new plausible regeneration pathway for the CBS reduction was proposed.