Importance of C-N bond rotation in N-acyl oxazolidinones in their SmI2-promoted coupling to acrylamides.

A detailed mechanistic investigation was undertaken to determine the dominating factors of the postelectron transfer steps in the SmI(2)-promoted carbon-carbon bond forming reaction between N-acyl oxazolidinones and acrylamides. Competition experiments were performed by reacting two N-acyl oxazolidinones with a limiting amount of N-t-butyl acrylamide, and from the product distribution, the relative reactivity values (RV) for a series of N-acyl oxazolidinones were then calculated against N-pivaloyl oxazolidinone as the reference. An almost linear correlation was obtained for the simple alkyl N-acyl oxazolidinones when ln RV was plotted against the activation barriers for C-N bond rotation (s-trans to s-cis) obtained by DFT calculations, implying that C-N bond rotation from the s-trans to s-cis conformation is one of the essential parameters controlling the reactivity. These results were substantiated by other competition experiments carried out for the corresponding imide derivatives, where rotation is not necessary for obtaining bidentate coordination and where no such correlation as described above was observable. The finding that the reactivity of the simple N-acyl oxazolidinones for these SmI(2)-mediated transformations correlates with the activation barriers for C-N bond rotation may have implications for other useful synthetic organic reactions involving similar substrates. Finally, these studies were extrapolated to understanding the poor reactivity of N-acyl oxazolidinones, as those derived from Evans chiral auxiliaries, with N-tert-butyl acrylamide. These couplings appear to be dominated by the activation energy for addition because of arising syn-pentane interactions in the transition-state for C-C bond formation. We demonstrate that the addition of Lewis acids can have a beneficial effect on the coupling yields.