Geminal bis-borane formation by borane Lewis acid induced cyclopropyl rearrangement and its frustrated Lewis pair reaction with carbon dioxide.

Cyclopropylacetylene reacts with two molar equivalents of Piers' borane [HB(CF)] under mild conditions by an addition/rearrangement sequence with cyclopropyl ring opening to give a mixture of two α-B(CF) substituted tetrahydroboroles. This compound forms an active frustrated Lewis pair with P Bu that heterolytically splits dihydrogen and adds carbon dioxide as a geminal chelate bis-boryl component. The respective reactions of the two-fold HB(CF) addition to Ph-CHCHC[triple bond, length as m-dash]CH were studied as a geminal Lewis acid reference. Most of the products were characterized by X-ray diffraction.