In all substitution reactions with 2b, the desired product formed concomitantly with an elimination product that was isolated and characterized as the 3-methylene-1-cyclobutene 25 by X-ray crystallography (Scheme 4). We obtained higher yields of 25 and lower yields of 3 when microwaving at higher temperatures for shorter times. We hypothesize the BCP to BCB substitution reaction occurs via an SN1/E1-type mechanism involving the intermediacy of the bicyclo[1.1.0]butyl-1carbinyl carbocation proposed by Wiberg. We cannot rule out a carbanionic or concerted mechanism as proposed by Della and coworkers. In our hands, sulfolane was the optimal solvent for this reaction, and while aprotic solvents are not usually associated with promoting SN1/E1 reactions, it is reported that sulfolane is capable of strongly solvating cations due to its high dipole moment (4.7 D). 26