Insertion of Alkylidene Carbenes into B–H Bonds

We have developed a protocol for
insertion of alkylidene carbenes
into the B–H bonds of amine–borane adducts, enabling,
for the first time, the construction of C­(sp2)–B
bonds by means of carbene-insertion reactions. Various acyclic and
cyclic alkenyl borane–amine adducts were prepared from readily
accessible starting materials in good to high yields and were subsequently
subjected to a diverse array of functional group transformations.
The unprecedented spiro B–N heterocycles prepared in this study
have potential utility as building blocks for the synthesis of pharmaceuticals.
Preliminary mechanistic studies suggest that insertion of the alkylidene
carbenes into the B–H bonds of the amine–borane adducts
proceeds via a concerted process involving a three-membered-ring transition
state.