Redistribution reactions on boron. Exchange of alkoxy-groups on boric and phenylboronic esters

Abstract

The mixed boric esters B(OR¹)₂(OR²) where R¹= Et, Prⁿ, Prⁱ, Buⁿ, or ClCH₂·CH₂ and R²= Me have been prepared. They cannot be isolated by conventional methods, but have been detected by proton n.m.r. (in solution) and mass spectrometric techniques (gas phase). Semiquantitative data on the equilibria and rates of the reversible formation process are given together with the n.m.r. spectra of many of the esters. The systems PhB(OPrⁱ)₂–PhB(OMe)₂, PhB(OMe)₂–B(OMe)₃, PhB(OEt)₂–B(OEt)₃, and PhB(OMe)₂–B(OPrⁱ)₃ were also studied by n.m.r. Systems of two phenylboronate esters exchange more rapidly than systems of two boric esters, and methyl groups exchange more rapidly than ethyl groups. Exchange rates observed are compatible with a four-centre mechanism.