Dimeric boroles as thermal sources of monomers.

The reactions of a monomeric borole and a dimeric borole with 2,3‐dimethyl‐1,3‐butadiene and 1,3‐cyclohexadiene were investigated. The monomeric borole reacted at ambient temperature whereas heat was required to crack the dimer to form the monomer and induce reactivity. 2,3‐Dimethyl‐1,3‐butadiene reacts to give diverse products resulting from a cycloaddition process with the B−C moiety of the boroles acting as a dienophile, followed by rearrangements to furnish bicyclic species. For 1,3‐cyclohexadiene, a [4+2] process is observed in which 1,3‐cyclohexadiene serves as the dienophile and the boroles as the diene partner. The experimental results are corroborated with mechanistic theoretical calculations that indicate boroles can serve as either a diene or dienophile in cycloaddition reactions with dienes.