Synthetic Applications of Intramolecular Insertion in Arylcarbenes. I. o-Alkylphenylcarbenes and 1-(o-Alkylphenyl)ethylidenes

Abstract

A series of phenylcarbenes and 1-phenylethylidenes, substituted in the ortho-position by alkyl chains of 1-4 carbon atoms, has been generated in the gas phase and pyrolysed at 250-750°/0.05-0.10 mm. Apart from o-tolylcarbene, the phenylcarbenes show efficient insertion to form five- and six-membered rings, the former predominating. In the phenylethylidenes, Bamford-Stevens insertion into the adjacent methyl group competes with cyclization to an extent which decreases with chain length. At the same time, five-membered ring formation is greatly increased over six-membered ring formation, due to facilitation of out-of-plane twisting of the carbene atom. The mechanistic implications of the results are discussed in terms of an angular approach of the carbene p-orbital to the hydrogen atom under attack.