The Norcaradiene Problem

Abstract

Dedicated to Professor R. Criegee on the occasion of his 65th birthdayCompounds having the norcaradiene structure are stable if one of the double bonds of the norcaradiene also belongs to a benzene ring or if C atoms I and 6 are fixed in position by a bridge. A similar effect that favors the norcaradiene form is produced by the incorporation of nitrogen atoms in positions 3 and 4. Cycloheptatriene and most of the simply substituted tropilidenes exist exclusively in the monocyclic triene form. It has been possible in some cases to detect a valence-tautorneric equilibrium between the norcaradiene and the cycloheptatriene ring system. Non-classical stabilization of compounds having norcaradiene or cycloheptatriene structure in the sense of a pseudoaromatic "homobenzene" is only very slight, if it occurs at all.

Buchner and Beyond: Arene Cyclopropanation as Applied to Natural Product Total Synthesis

“…10 The synthetic utility of this transformation was greatly improved with the advent 4 catalyzes the reaction between ethyl diazoacetate (1) and benzene (13) at room temperature to quantitatively yield cycloheptatriene 3.…”

“…As will be discussed later, substrates with three-atom tethers are prone to cyclopentanone formation via C-H insertion. 18 In an early demonstration of the utility of the Rh-catalyzed intramolecular Buchner reaction, McKervey and Kennedy reported that exposure of a-diazo ketone 16 to catalytic Rh 2 (mandelate) 4 quantitatively provided a mixture of norcaradiene 17 and cycloheptatriene 18.…”

“…4 In simple, unconstrained systems the equilibrium lies toward the cycloheptatriene species. 5,6 However, several elegant studies have determined that a variety of steric 7 and electronic 8 factors may alter this equilibrium to instead favor the norcaradiene.…”

1-Cycloheptatrienylidene-2-Phenylethylene and Its Intriguing Dimerization Reactions

“…10 Cycloheptatrienylacetylene 2 is rather unreactive under the dimer-forming reaction conditions in the absence of DBU, and at elevated temperature it gave tricyclo [3.2.2.02'4] nona-6-ene or 6, 8-diene type adducts by the reactions with dienophiles.…”

Diamagnetic susceptibilities and susceptibility exaltations of some 7-substituted cycloheptatrienes

“…The position of the equilibrium between cycloheptatriene and norcaradiene can be changed by the judicious introduction of substituents (1). Particularly dramatic shifts in the equilibrium position have been observed with substitution at C,, the methylene carbon of these systems (2).…”