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

Abstract

The structure of phenylcyclopropane radical cation (1 •+ , R ) H) is derived from CIDNP effects observed during the electron transfer reaction of 1 with chloranil. This species is an example of an elusive structure type. The secondary cyclopropane protons show significantly divergent hyperfine coupling constants due to an unprecedented stereoelectronic effect. Incorporation into a redox-active pentasil zeolite (Na-ZSM-5) converts 1 or its p-methoxy derivative (1, R ) OCH 3 ) to trans-propenylarene radical cations (2 •+ , R ) H, OCH 3 ).The structure and reactivity of cyclopropane radical cations have attracted much attention; 1,2 the spin density distributions of many derivatives have been delineated, and many intra-and intermolecular reactions have been studied in gas phase, 3 solution, 1e,2 or solid matrices. Vertical ionization of cyclopropane from a degenerate pair of in-plane e′ orbitals (s and a) generates a doubly degenerate 2 E′ state, which undergoes first-order JahnTeller distortion to two nondegenerate electronic states, 2 A 1 and 2 B 2 (C 2V symmetry). 4 These components relax to structures with one ("trimethylene"; type A) or two lengthened C-C bonds ("π-complex"; type B). 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,7The 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. 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. 9 The research described in this paper was undertaken to provide experimental evidence for the prevailing structure type of phenylcyclopropane radical cation (1 •+ ). This species has not been characterized by either ESR or CIDNP spectrum as of this date. We have studied electron transfer reactions of 1 in polar solvents and upon incorporation into a redoxactive pentasil zeolite (Na-ZSM-5). In solution, we observed CIDNP effects delineating the spin densities and hyperfine coupling pattern of 1 •+ (R ) H). In the zeolite, on the other hand, the EPR results indicated rearrangement of 1 (R ) H, OCH 3 ) to trans-propenylarene radical cations (2 •+ , R ) H, OCH 3 ).Irradiation of chloranil in the presence of 1 (R ) H) 10 generated strong polarization for the donor molecule: the aromatic multiplet near 7 ppm and the benzylic cyclopropane signals (t,t; δ ) 1.9 ppm) showed enhanced absorption; the less shielded secondary cyclopropane resonances (δ ) 0....

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.…”

“…Non‐symmetrically disubstituted cyclopropane radical cations containing a vinyl group have been probed by experiment and calculation . For bicyclic derivatives, in which the functionalities are locked in either the syn or the anti configuration, electron transfer‐induced CIDNP effects suggest a range of structure types with subtle differences.…”

Regioselective Oxidative Ring Opening of Cyclopropyl Silyl Ethers: A Quantum Chemical Study

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Interestingly aliphatic or aromatic substituted cyclopropyl radical cations are-in absence of nucleophiles-structurally and configurationally stable. This has been demonstrated by CIDNP (chemically induced dynamic nuclear polarization) and by other transient spectroscopic experiments based on photoinduced electron transfer (PET).…”

Increasing the sensitivity of time-resolved photo-CIDNP experiments by multiple laser flashes and temporary storage in the rotating frame

“…Because the signal amplification can be quite substantial but occurs only for molecules that have radical pairs as their precursors, those molecules can be observed selectively and discriminated against the background of unreacted molecules or molecules formed by other pathways. There have been numerous applications of CIDNP to the elucidation of reaction mechanisms [6] and of molecular structures, including radicals [7][8][9] and biomolecules [10,11].…”