Mechanistic study on the reaction of pinB-BMes2 with alkynes based on experimental investigation and DFT calculations: gradual change of mechanism depending on the substituent.

Linlin Wu,Shogo Morisako,Ka-Ho Lee,Makoto Yamashita,Chiemi Kojima,Zhenyang Lin

doi:10.1039/d1sc02863d

Abstract

Transition metal-free direct and base-catalyzed 1,2-diborations of arylacetylenes using pinB-BMes2 provided a syn/anti-isomeric mixture of diborylalkenes. The kinetic analysis showed that the reaction rate and isomer ratio were affected by reaction conditions and substituents on the aryl ring. DFT calculations indicated that direct addition proceeded via the interaction of acetylene-π with the BMes2 fragment. In contrast, for the base-catalyzed diboration, the previously isolated sp2–sp3 diborane and borataallene were confirmed as stable intermediates by calculations. The whole reaction pathways can be divided into the Bpin-migration and deprotonation steps, where the borataallene should be considered as a common intermediate. It should be noted that the deprotonation step is reversible and affords the kinetically less favoured isomer under the thermodynamic conditions. As a result, the composition of isomeric products, in the base-catalyzed diboration, is attributed to the small difference of activation barriers between direct and base-catalyzed systems.

Highlights

Transition metal-mediated diboration of alkynes to produce diborylalkene is useful to construct tri- and tetra-substituted alkenes in a controlled manner.[1]
Produce bis(dihaloboryl)ethylene in 1959 (Scheme 1),[4] whose four-centered transition state was recently proposed by DFT calculations.[5]
We report a detailed experimental study with kinetic analysis and theoretical study with DFT calculations to understand the mechanism of the diboration reaction using pinB-BMes[2] (1)

Summary

Introduction

Transition metal-mediated diboration of alkynes to produce diborylalkene is useful to construct tri- and tetra-substituted alkenes in a controlled manner.[1]. 2, Table 2) for the formation of isomer 2b00 could be reproduced by DFT calculations with a lower-energy transition state of TS_12b00 versus TS_1-2a00 (with the reaction barrier of 25.1 kcal molÀ1 versus 28.1 kcal molÀ1).

Results

Conclusion