Three-Electron S_N2 Reactions of Arylcyclopropane Cation Radicals. 2. Steric and Electronic Effects of Substitution¹

Abstract

The nucleophilic substitution reactions on substituted arylcyclopropane cation radicals were studied by a combination of methods including product studies, time-resolved laser flash photolysis, kinetic isotope effects, and quantum chemical calculations. The reactions were found to proceed stereospecifically with inversion of configuration, with high regioselectivity for nucleophilic attack at the more substituted carbon atom, and with very small steric effects. Electronic effects on the nucleophilic substitution regiochemistry and the rate constants were found to be substantial for substituents on the cyclopropane moiety and on the aryl ring.

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

“…Dinnocenzo et al have observed that on ionization 1-phenyl-2-alkylcyclopropanes relax to structures where the PhC À C bond is lengthened to 1.7-1.8 , but is never fully cleaved (in the absence of nucleophiles, arylÀalkyl cyclopropanes do not epimerize upon oxidation). [14] Obviously the OTMS group has a more profound influence on the electronic structure of the cyclopropane radical cation than the phenyl group, as cyclopropyl silyl ethers undergo a multitude of radical cationic fragmentation, addition and rearrangement reactions. [16][17][18][19] Calculations on the model compound, 1-methoxy-1,2-dimethylcyclopropane 3, show that the strong distortions on ionization persist even if the OTMS group is replaced by an OMe group (but 3 a is 6.3 kcal mol À1 more stable than 3 b).…”

“…[1,2,10] The nucleophilic ring opening of cyclopropyl radical cations under PET conditions, which takes place under inversion of the carbon center which is attacked by the nucleophile (Scheme 1), has been studied mainly by the group of Dinnocenzo. [11][12][13][14] In contrast to these results, we found that cyclopropyl silyl ethers undergo spontaneous fragmentation, sometimes followed by re-cyclisation to form new products, in absence of nucleophiles, [17][18][19] that is, under conditions where alkyl-or aryl-substituted cylopropanes do not even epimerize. [10] In cyclic systems intermolecular addition reactions lead predominately to the endo-addition products, which is caused by the selectivity of initial fragmentation (Scheme 2).…”

“…[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).…”

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

“…These conclusions are in full accord with the results of ab initio calculations (B3LYP/6-31G*), which documented the principal structural features and the charge density distribution in the radical cation, 1 •+ . 9 The divergent signal directions for the two pairs of secondary cyclopropane protons document an interesting stereoelectronic effect. To understand the nature of this effect, it is crucial to assign the orientation of these protons relative to the phenyl group.…”

“…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.…”