Transfer Hydrocyanation of α‐ and α,β‐Substituted Styrenes Catalyzed by Boron Lewis Acids

Acid‐free! Bench‐stable cyclohexa‐1,4‐diene‐based hydrogen cyanide (HCN) surrogates are reported to engage in transfer hydrocyanation of alkenes upon treatment with certain boron Lewis acids. The success also depends on the equilibrium position between the intermediate isocyano‐ and cyanoborate isomers. Tertiary nitriles are obtained with exclusive Markovnikov selectivity.

Abstract

A straightforward gram‐scale preparation of cyclohexa‐1,4‐diene‐based hydrogen cyanide (HCN) surrogates is reported. These are bench‐stable but formally release HCN and rearomatize when treated with Lewis acids. For BCl₃, the formation of the isocyanide adduct [(CN)BCl₃]⁻ and the corresponding Wheland complex was verified by mass spectrometry. In the presence of 1,1‐di‐ and trisubstituted alkenes, transfer of HCN from the surrogate to the C−C double bond occurs, affording highly substituted nitriles with Markovnikov selectivity. The success of this transfer hydrocyanation depends on the Lewis acid employed; catalytic amounts of BCl₃ and (C₆F₅)₂BCl are shown to be effective while B(C₆F₅)₃ and BF₃⋅OEt₂ are not.

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