Theoretical study of the BF3-promoted rearrangement of oxiranyl N-methyliminodiacetic acid boronates

PELLEGRINET, S. C.; VALLEJOS, M. M.

JOURNAL OF ORGANIC CHEMISTRY

AMER CHEMICAL SOC

Lugar: Washington; Año: 2017 vol. 82 p. 5917 - 5925

0022-3263

The mechanism of the rearrangement of oxiranyl N-methyliminodiacetyl (MIDA)boronates has been extensively investigated with the density functional theory in dichloromethane. Several reaction pathways were examined. Our results revealed that the most favorable mechanisms for the BF3-promoted rearrangement of 2-phenyl oxiranyl MIDA boronate (1) and 1-phenyl oxiranyl MIDA boronate (24) comprise two steps: ring opening of the epoxide to a carbocation intermediate followed by migration of a MIDA-boryl group (for the reaction of 1) and hydrogen(for the reaction of 24), to give the same BF3-coordinated α-boryl aldehyde in both cases. The first step of ring opening of the epoxide is the rate-determining step of these reactions. In the rearrangement step for the reaction of 1, the MIDA-boryl group migrates easily probably due to its electron-rich sp3-hybridized boron center. For 24, the most favorable pathway involves a rare borylsubstituted carbocation. The course of these reactions is mainly controlled by electronic effects, although steric effects are also significant. The higher energy barrier calculated for the unsubstituted oxiranyl MIDA boronate (31) explains the lack of reactivity in the studied BF3-promoted rearrangement.