Combining high pressure and catalysis: pinacol- or catecholborane hydroboration of functionalized olefins

Abstract

The hydroboration of three families of functionalized olefins (1-bromo- and 1,3-dibromopropenes, allylamines, 2,3-dihydrofuran) by pinacolborane and catecholborane has been studied under various experimental conditions. For 1-bromo- and 1,3-dibromopropenes, pinacolborane (PBH) is a poor reagent that requires the use of high pressure in ethereal solvents and provides only by-products, resulting from the undesired β-bromoboronate regioisomer. By contrast, catecholborane (CBH) affords mainly the expected α-bromoboronate at atmospheric pressure. With dibenzylallylamine, PBH yields a mixture of the two possible regioisomers (β- and γ-aminoboronates), whatever the pressure, while CBH affords selectively the expected γ-isomer in good yields under atmospheric thermal conditions. For 2,3-dihydrofuran, only PBH gives an efficient access, in THF, to a mixture of the corresponding α- and β-regioisomers when combining the effects of 0.5% Wilkinson’s catalyst and high pressure.