Are Copper(I) Carbenes Capable Intermediates for Cyclopropanations? The Case for Ylide Intermediates

Abstract

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.

Bond Activation by Metal–Carbene Complexes in the Gas Phase

“…45 This study also addressed the problem of how to avoid the wellknown tendency of copper(I) carbenes to undergo intramolecular rearrangements, e.g., Wolff rearrangement or insertion into copper−ligand bonds, by forming copper(I) carbene ylides through complexation with solvents or weak nucleophiles. 45 5) have been observed (eq 2), these processes are either not very efficient (n = 1) or do not result in carbon−carbon bond formation upon dehydrogenation of CH 4 (n = 5). 46,47 Similarly, the efficiency of Fischer−Tropsch-type coupling of methylene units (eq 3) has been reported to depend crucially on the nature of the transition metal M. 48,49…”

“…On the basis of energetic considerations, a dissociation−association pathway has been ruled out in favor of a methylene-transfer process for the CH 2 → RCHCH 2 ligand exchange at the copper site. 45 This study also addressed the problem of how to avoid the wellknown tendency of copper(I) carbenes to undergo intramolecular rearrangements, e.g., Wolff rearrangement or insertion into copper−ligand bonds, by forming copper(I) carbene ylides through complexation with solvents or weak nucleophiles. 45 5) have been observed (eq 2), these processes are either not very efficient (n = 1) or do not result in carbon−carbon bond formation upon dehydrogenation of CH 4 (n = 5).…”

Gold(I)-Induced Rearrangements of Propargyl Derivatives: A Gas-Phase Study

“…We have shown previously that gold­(I) and other metal-stabilized carbenes are prone to rearrangements leading to ylide species (i.e., addition of a nucleophilic atom to the carbene) . We have also shown that these ylides can potentially undergo carbene-like reactions.…”

“…33 We have shown previously that gold(I) and other metalstabilized carbenes are prone to rearrangements leading to ylide species (i.e., addition of a nucleophilic atom to the carbene). 19 We have also shown that these ylides can potentially undergo carbene-like reactions. Species I and III are essentially carbenes stabilized by coordination to the acetate carbonyl and are analogues to the ylides we have identified as carbene surrogates (each has an ylide resonance form).…”

“…In recent years, gold­(I) complexes have been widely used to catalyze the rearrangements of propargyl species. − The rearrangement can involve 1,3-migrations along the propargyl backbone to yield allenes or 1,2-migrations to yield metal carbene species. ,− Although the synthetic value of these processes is well established, the intermediates and mechanisms of these reactions have not been clearly identified in all cases, and the factors that drive the selectivity of the reactions have not been fully elucidated. The synthetic applications of these species have been demonstrated in solution, but it is well established that gas-phase studies of organometallic processes can lead to important insights, particularly with respect to the stability and characteristics of reactive intermediates that are short-lived in condensed-phase environments. − In such cases, it is possible to isolate the reactive intermediates and selectively challenge them with substrates or test reagents in the gas phase.…”