Monomeric and dimeric acyloxydialkylboranes

Abstract

The carboxylic acids RCO₂H [R = Bu^ta, Et b, 3,4,5-(MeO)₃C₆H₂c, 4-MeOC₆H₄d, Ph e, 2,4,6-Me₃C₆H₂f, 2,6-Cl₂C₆H₃g, or 3,5-(CF₃)₂C₆H₃h] were treated with triethylborane or with bis(9-borabicyclo[3.3.1]nonane) to give the acyloxydiethylboranes 1a–1h and the 9-acyloxy-9-borabicyclo[3.3.1]nonanes)2a–2h, respectively. Compounds 1a–1h are largely unassociated in non-polar solvents (¹¹B NMR, IR spectroscopy). As pure liquids, or in the solid state, they form equilibrium mixtures of monomeric and associated (dimeric) molecules (IR spectroscopy). Compounds 2a–2f are completely associated as pure liquids and in the solid state, but only weakly associated in non-polar solvents. Based on the crystal structure of dimeric 2e, it is proposed that, when associated, derivatives of both 1 and 2 form cyclic dimers.