Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1-Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes(2)PCH(2)CH(2)B(C6F5)(2) undergoes 1,1-carboboration reactions with the Me3Si-substituted enynes to give ring-enlarged functionalized C-3-bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H+/[B]H- products. One such product shows activity as a metal-free catalyst for the hydrogenation of enamines or a bulky imine. The ring-enlarged FLPs contain dienylborane functionalities that undergo "bora-Nazarov"-type ring-closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C6F5 migration is very favorable. Furthermore, substituted 2,5-dihydroborole products are derived from cyclization and C6F5 migration from the photolysis reaction. In the case of the six-membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2-addition reaction with a terminal alkyne with rearrangement.