Carboboration of Alkynes with Cyclodextrin-Encapsulated N-Heterocyclic Carbene Copper Complexes

The copper-catalyzed carboboration of various alkynes was investigated with a modified N-heterocyclic carbene-capped alpha-cyclodextrin copper(I) complex in which the reactive copper center is deeply encapsulated in the cyclodextrin (CD) cavity. The methylborylation of terminal alkynes was found to give linear (L) (E)-vinyl boron isomers as the major isomers, as expected from the previously proposed "perpendicular" approach of the alkyne to the Cu-B bond, and methylation of the vinyl boron copper intermediate. Under similar conditions, the intramolecular carboboration reaction with terminal alkynes functionalized by alkyl halides, led to exocyclic vinyl boronate species as the major isomer. However, an endocyclic (Z)-isomer was also observed in some cases. This isomer was not previously observed and is unexpected considering the "classical" mechanism. The direct generation of boron functionalized (Z)-alkenes by carboboration of alkynes is unprecedented.