Are copper(I) carbenes capable intermediates for cyclopropanations? The case for ylide intermediates.

A novel approach is used to synthesize a stable, ligated copper(I) carbene in the gas phase that is capable of typical metal carbenoid chemistry. However, it is shown that copper(I) carbenes generally undergo rapid unimolecular rearrangements including insertions into copper-ligand bonds and Wolff rearrangements. The results indicate that most copper(I) carbenes are inherently unstable and would not be viable intermediates in condensed-phase applications; an alternative intermediate that is less prone to rearrangements is required. Computational data suggest that ylides formed by the complexation of the carbene with solvent or other weak nucleophiles are viable intermediates in the reactions of copper(I) carbenes.

Intermolecular C-H Bond Activation by a Cationic Iridium(III) Dichloride Phenanthroline Complex.

It is demonstrated that a cationic iridium(III) dichloride phenanthroline complex is capable of C-H activation and H/D exchange. It can cleave benzylic and unactivated secondary C-H bonds, but exhibits unique selectivity when compared to similar systems that have been studied in the condensed phase. Gas-phase rate constants and kinetic isotope effects are reported for a variety of substrates and the analysis is supported by DFT calculations at the M06/QZVP level.

The impact of substituents on the transition states of SN2 and E2 reactions in aliphatic and vinylic systems: remarkably facile vinylic eliminations.

For a series of α and ß substituted haloethanes and haloethenes, gas-phase experiments and computational modeling have been used to characterize their nucleophilic substitution and elimination reactions. Despite being less thermodynamically favorable, the vinylic eliminations have rate constants and computed barriers that are similar to those of analogous aliphatic eliminations. This is the result of the vinylic systems shifting to more E1(cb)-like transition states and exploiting the inherent greater acidity of vinylic hydrogens. In general, the α-substituents have a greater impact on the S(N)2 pathways and stabilize the transition states via field and polarizability effects. Substantial stabilization is also provided to the E2 transition states by the α-substituents, but they have surprisingly little impact on the geometries of the transition states of either pathway. The ß-substituents generally lead to a strong bias toward elimination and greatly affect the synchronicity of the elimination (more E1(cb)-like) as well as its location on the reaction coordinate (early). The experimental and computational data are in good accord, and the full data set provides a comprehensive picture of substituent effects on solvent-free S(N)2 and E2 processes.