Borane-Catalysed Hydroboration of Alkynes and Alkenes

Stephen Thomas,Michael Cowley,Jonathan Carney,Nate Ang,Cornelia Buettner,Scott Docherty,Jamie Docherty,Alessandro Bismuto

doi:10.1055/s-0036-1591719

Abstract

Simple, commercially available borane adducts, H3B·THF and H3B·SMe2, have been used to catalyse the hydroboration of alkynes and alkenes with pinacolborane to give the alkenyl and alkyl boronic esters, respectively. Alkynes and terminal alkenes underwent highly regioselective hydroboration to give the linear boronic ester products. Good functional group tolerance was observed for substrates bearing ester, amine, ether and halide substituents. This catalytic process shows comparable reactivity to transition-metal-catalysed hydroboration protocols.

Highlights

The hydroboration of alkynes and alkenes offers the simplest route to boronic esters, and unlike boranes, boronic esters do not readily undergo olefin hydroboration
Most commonly a 2nd- or 3rd-row transition metal catalyst is needed for the direct hydroboration of olefins with boronic esters.[3]
The active catalyst was generated by ligand metathesis between BArF3 and HBpin to generate a borane which underwent alkene hydroboration and another ligand metathesis with HBpin to give the product boronic ester and regenerate the borane catalyst

Summary

Introduction

Boronic esters are powerful synthetic intermediates which have found widespread use and act as precursors for an array of transformations.[1,2] The hydroboration of alkynes and alkenes offers the simplest route to boronic esters, and unlike boranes, boronic esters do not readily undergo olefin hydroboration. Increasing the reaction temperature to 60 C led to successful catalysis by both borane adducts in only 30 minutes for the hydroboration of phenylacetylene which gave the linear alkenyl boronic ester 2a in good yield, regio- and diastereoselectivity (Entries 2 and 3). Both borane adducts successfully catalysed the hydroboration of 4-tert-butylstyrene to give the alkyl boronic ester in good yield and regioselectivity (Entries 5 and 6).

Results

Conclusion