title: Foiled carbenes revisited: When sigma-stabilization surpasses pi-stabilization.
abstract: The bicyclic alkenylidenes 9 (bicyclo[3.2.1]oct-2-en-8-ylidene) and 17 (bicyclo[3.3.1]non-2-en-9-ylidene) were claimed to be stabilized foiled carbenes. Our B3LYP and MP2 computations confirm previous experimental data. Moreover, they show that these carbenes are very reactive and rearrange rapidly, mainly through a 1,2-vinyl shift by overcoming a low barrier (1.2 to 5.4 kcal/mol). This is in contrast to the high barriers (up to 30 kcal/mol) predicted for the same type of rearrangements in norborn-2-en-7-ylidene derivatives. In 17 and bicyclo[4.1.1]oct-2-en-7-ylidene (23), the divalent carbon atom is even bent away from the double bond!
authors.affiliation: Institut für Organische Chemie, Universität Wien, Währinger Strasse 38, A-1090 Wien, Austria.
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Mieusset
authors.forename: Udo H
authors.initials: UH
authors.lastname: Brinker