Kinetic stability of novel nitrile ylides

Abstract

A series of relatively stable novel trifluoromethyl substituted nitrile ylides have been generated by base promoted elimination of hydrogen chloride from the corresponding imidoyl chlorides in acetonitrile and aqueous dioxane solution at 25 °C. The formation and decay of these sometimes highly coloured species was monitored spectrophotometrically. The rates of formation were first order in base, and the subsequent reaction with solvent was acid catalysed at high pH's in aqueous systems. Studies in aqueous dioxane indicated that the more stable ylides were also the slowest to form. In acetonitrile as solvent, the ylides underwent addition of solvent at rates which were independent of the base concentration and relatively insensitive to the nature of the substituents. In a number of cases, cycloaddition products were also isolated confirming the 1,3-dipolar nature of these compounds. Protonation of the ylides has also been studied both experimentally in aqueous solution and theoretically. While formation of nitrilium ions by protonation of the ylide carbon atoms is unlikely in aqueous solution since the corresponding amides are not observed as products, these ions have been calculated to be the most favoured protonated forms of ylides either in the gas phase or in a continuum of solvent, irrespective of its polarity. Ab initio calculations were carried out using molecular orbital methods (HF, MP2 and CCSD(T)) with basis sets ranging from 6-31G(d) to 6-311++G(3df,2p) and density functional theory (B3LYP) in conjunction with the polarizable continuum model (PCM) for treating non-specific solvent effects. This discrepancy between experiment and theory suggests a probable involvement of specific solvent interaction in inducing a higher thermodynamic stability of either nitrile carbon or NO₂⁻ oxygen protonated forms via hydrogen-bonded complexes.