Electron Transfer Photochemistry of a Bridged Norcaradiene:  A Mechanistic Probe for Radical Cation Nucleophilic Capture

Abstract

The radical cation of tricyclo[4.3.1.01,6]deca-2,4-diene (3) can be generated by electron transfer to 1,4-dicyanobenzene (DCB) in acetonitrile/methanol; the species is captured by regiospecific nucleophilic attack of methanol in the 2- (and 5-) position. On the other hand, the reaction shows little stereochemical preference. The resulting allylic radical reacts by aromatic substitution on the radical anion of DCB, the 3-position serving as the principal site of coupling. Remarkably, the cyclopropane function of 3 • + is not a target of nucleophilic attack, even though the secondary cyclopropane carbon has spin and charge density. The observed reaction is compatible with a theoretical model proposed by Shaik and Pross, which correlates radical cation reactivity with the spin density of the corresponding triplet state. The capture of 3 • + occurs in the position where both SOMO and LUMO have significant orbital coefficients.

Radical cations of disubstituted cyclopropanes: stereoelectronic effects on hyperfine coupling

“…Structure type B is less common and, therefore, of special interest . The type B structure can be stabilized by conjugation (as in vinylcyclopropane or phenylcyclopropane, viz., 1 •+ ) or homoconjugation (as in norcaradiene derivatives, viz., 2 •+ ) . Hyperconjugation (e.g.…”

“…[15] The type B structure can be stabilized by conjugation (as in vinylcyclopropane [23] or phenylcyclopropane, viz., 1 •+ ) [24,25] or homoconjugation (as in norcaradiene derivatives, viz., 2 •+ ). [15,26] Hyperconjugation [27,28] (e.g. in 1,1-dimethylcyclopropane radical cation, 3 •+ ) also stabilizes the type B structure: it is lower in energy than the corresponding type A structure; however, the putative type B structure is not a stationary point; it undergoes a secondorder Jahn-Teller type distortion to a lower-energy scalene triangular structure.…”

Oxidation of Arylcyclopropanes in Solution and in a Zeolite: Structure and Rearrangement of the Phenylcyclopropane Radical Cation

“…5c, 7 We found it surprising that this relatively simple species, which can be obtained from a readily accessible precursor, was not characterized earlier and, apparently, has not been studied by EPR. This method would allow direct observation and provide detailed structural information.…”

“…1e,5 The majority of cyclopropane radical cations have structures of type A, 1e,5,6 whereas structure type B has been realized in only very few cases. 5c, 7 The stabilization of structure type B can be envisioned via three different mechanisms involving conjugative, hyperconjugative, or homoconjugative interactions. The homoconjugative approach first led to a structure of type B, as exemplified by the norcaradiene radical cation.…”

“…The homoconjugative approach first led to a structure of type B, as exemplified by the norcaradiene radical cation. 5c, 7 On the other hand, hyperconjugation failed to stabilize the type B structure; thus, ab initio calculations on radical cations of 1-methyl-and 1,1-dimethylcyclopropane showed that their type B structures undergo second-order Jahn-Teller type distortions, resulting in scalene structures. 4i Finally, conjugation with a delocalized π-system also stabilizes structure type B, as indicated by calculations on vinylcyclopropane 8 and phenylcyclopropane radical cations.…”