Incorporation of an Allene Unit into α-Pinene: Generation of the Cyclic Allene 2,7,7-Trimethylbicyclo[4.1.1]octa-2,3-diene and Its Dimerization

The reaction of etheral methyllithium with 3,3-dibromo-2,7,7-trimethyl-tricyclo[4.1.1.0^2,4]octane (2) was investigated. The generated carbene 12 undergoes intramolecular C-H insertion to provide the tetracyclic hydrocarbon 3 and the bicyclic allene 15, which undergoes [2+2] cyclodimerization. The structures of the formed allene dimers 16, 17, and 18 were elucidated by spectral means. The activation barriers for all possible C-H insertion products 3, 13, and 14 and the allene 15 were investigated by using density functional theory computations at the B3LYP/6-31G(d) level. It was found that the activation barriers for the formation of 3 and 15 (6.2 and 6.3 kcal mol^-1) are much lower than that for the insertion products 13 and 14 (17.5 and 12.6 kcal mol^-1), respectively. This prediction was completely in agreement with our experimental results.