There have been reports in the literature on N-Heterocyclic Carbene - Silicon (NHC·Si) interactions and their involvement in organocatalytic cycles. However, there is little experimental evidence to support this postulation. Indeed, other plausible mechanisms that do not invoke an NHC·Si interaction can be drawn. We show that there is insufficient evidence to support the postulated NHC·Si interaction in the protection of NHCs by silicones reported by Baceiredo and co-workers.
The synthesis and use of 13C-labelled NHCs allowed for the clear monitoring of the carbene moiety by 13C NMR spectroscopy. 1H, 13C and 29Si NMR spectra did not indicate the formation of NHC·Si complexes at temperatures from 298K to 198K in toluene-d8 or THF-d8 for a range of silicones and low molecular weight silicon species of varying Lewis-acidities. DOSY spectra did not show specific or non-specific interactions between NHCs and silicon species in solution, which would have been
expected for an NHC·Si interaction. In situ IR measurements of a model NHC-catalysed reaction of methyl 2-phenylacetate with ethanolamine also failed to provide conclusive evidence of an NHC·Si interaction which would lead to diminished rates of reaction.
Furthermore, computational studies of the above systems do not predict an NHC·Si interaction. Simulations predict that the formation of such NHC·Si interactions is thermodynamically disfavoured at room temperature based on Gibbs free energies calculated at the MP2/aug-cc-pVTZ//B3LYP/6-311+G(d,p) level.
Mass-transfer analysis, as applied to the systems described by Baceiredo and co-workers, show that retardation of decomposition can be explained by steady-state mass-transfer limited by diffusion of oxygen without the need to invoke a NHC·Si interaction. The analytical solution of transient mass transfer shows that protection afforded by diffusion is non-linear and that the evolution of the concentration profile is proportional to
e^(Dt/L^2), where D is the diffusion coefficient, t is time, and L the non-dimensional length.