Discovery of Deep-Seated Skeletal Rearrangements in the Photocyclizations of Some tert-Butyl-Substituted 1,2-Diarylethylenes.

The in-solution oxidative photocyclization of stilbenes to phenanthrenes is a well-known and synthetically valuable reaction. We report here our discovery that the oxidative photocyclization of several tert-butyl-substituted 1-styrylphenanthrenes resulted not only in the expected formation of tert-butyl-substituted picenes but also in the previously unknown rearrangement leading to the formation of tert-butyl-substituted pentahelicenes.

Phenyl groups versus tert-butyl groups as solubilizing substituents for some [5]phenacenes and [7]phenacenes.

In recent years, we have used the photocyclizations of diarylethylenes to synthesize a number of [n]phenacenes in the hope that they might be useful as the bridging groups for electron transfer processes in donor-bridge-acceptor molecules. Because [n]phenacenes with n > 5 are very insoluble, their synthesis and characterization has required the attachment of solubilizing substituents such as tert-butyl. The studies of Pascal and co-workers of some large polynuclear aromatic compounds having multiple phenyl substituents prompted us to explore the use of phenyls as alternative solubilizing groups for [n]phenacenes. Although phenyl groups turned out to provide significantly less solubilization than tert-butyl groups in these compounds, we found some interesting structural comparisons of the phenyl-substituted and tert-butyl-substituted [n]phenacenes.

Steric and solvation effects in ionic S(N)2 reactions.

This paper explores the contribution of solvation to the overall steric effects of S(N)2 reactions observed in solution. The reactions of chloride ion with a series of alkyl chloronitriles, RCH(CN)Cl (R = methyl, ethyl, isopropyl, tert-butyl) were investigated both experimentally and theoretically. These reactions serve as a model system for the parent reactions, Cl(-) + RCH(2)Cl, which are too slow to measure. Each additional substitution at the beta-carbon lowers the reactivity, clearly demonstrating a steric hindrance effect. The magnitude of the steric effect, however, is not significantly different in the gas phase and in solution. We conclude that the solvation energies of the corresponding S(N)2 transition states must be similar regardless of size of the substituent. This lack of size dependence in the current system is in sharp contrast with many other ionic systems such as ionization of simple alkyl alcohols, where solvation depends strongly on size. We propose that the weak size dependence results from the compensation between a direct shielding effect of the substituent and an indirect ionic solvation effect, which arises from the geometric perturbations introduced by the substitution. The conclusion is further supported by calculations using polarizable continuum models and QM/MM simulations.

Steric effects and solvent effects in ionic reactions.

Rates of SN2 reactions of chloride ion with methyl- and tert-butyl-substituted chloroacetonitrile were measured by using Fourier transform-ion cyclotron resonance spectrometry to follow the isotopic exchange reaction. Barrier heights for these reactions indicate that steric effects in the gas phase are diminished relative to apparent steric effects in solution. We attribute the increased barrier in solution to a solvation effect. Monte Carlo simulations done using statistical perturbation theory confirm that steric hindrance to solvation contributes to SN2 barriers in solution.