7. “The Ring Closure of Cyclopenta-1, 3-dien-5-yl-carbene to Benzvalene. A mechanistic study of an unusual carbene reaction”
   U. Burger, G. Gandillon, J. Mareda
   Helv. Chim. Acta 1981, 64, 844-853.

The base-induced α-elimination of hydrogen chloride from 5-chloromethyl-5-methylcyclopenta-1, 3-diene (19) produces 1-methyltricyclo [3.1.0.0^2,6]hexene-3 (1-methylbenzvalene) (21) together with toluene and spiro [4.2]heptadiene (23). A common intermediate, 5-methylcyclopenta-1, 3-dien-5-yl-carbene (20), accounts for these results by intramolecular 1, 4-carbene addition, 1, 2-carbon shift and CH-insertion, respectively. Independent synthesis of 2-methylbenzvalene (24) allows us to show that the classic intramolecular cyclopropanation is completely suppressed by the linear cheletropic ring closure. MINDO/3 predicts the key carbene to have a bisected conformation in its singlet ground state. This ideally fulfills the stereoelectronic conditions for a carbene reaction of least motion. The influence of the methyl substituent upon that process is discussed.

DOI : 10.1002/hlca.19810640323 

archive ouverte unige:6214