The Ring Closure of Cyclopenta‐1, 3‐dien‐5‐yl‐carbene to Benzvalene. A mechanistic study of an unusual carbene reaction

The base‐induced α‐elimination of hydrogen chloride from 5‐chloromethyl‐5‐methylcyclopenta‐1, 3‐diene (19) produces 1‐methyltricyclo [3.1.0.02,6]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.