Tri-insertion with dearomatization of terminal arylalkynes using a carborane based frustrated Lewis pair template

Intramolecular vicinal Frustrated Lewis Pairs (FLPs) have played a significant role in the activation of small molecules, and their stabilities and reactivities are found to strongly depend on the nature of the bridging units. This work reports a new carborane based FLP, 1-PPh (2), which reacts with an equimolar amount of p-R CH (R = Me, Et, Ph) at room temperature to give C C triple bond addition products 1,2-[PPh2C(R2NC6H4)= CHBPh2]-1,2-C2B10H10 (3) in high yields. Compounds 3 react further with two equiv. of p-R2NC6H4C CH (R = Me, Et) at 60-70 degrees C to give unprecedented stereoselective tri-insertion products, 3,3a,6,6a-tetrahydronaphtho[1,8a-b]borole tricycles (4), in which one of the aryl rings from arylacetylene moieties has been dearomatized with the formation of four stereocenters including one quaternary carbon center. It is noted that the phosphine unit functions as a catalyst during the reactions. After trapping and structural characterization of a key intermediate, a reaction mechanism is proposed, involving sequential alkyne insertion and 1,2-boryl migration.

Automated summary

This work reports a new carborane-based Frustrated Lewis Pair which reacts with p-tolylacetylene at room temperature to form addition products in high yields. Further reactions with substituted phenylacetylene at higher temperatures yield unprecedented stereoselective tricyclic compounds with dearomatized aryl rings and multiple stereocenters. The phosphine unit acts as a catalyst in these reactions, with a proposed mechanism involving sequential alkyne insertion and boryl migration.