Relative stability and proton transfer reactions of unsaturated isocyanides and cyanides

The typical Gibbs free energy difference between hydrocarbon substituted isocyanides and the corresponding cyanides is 25 to 28 kcal/mol in favor of the cyanides and is mostly independent of the substituent. Triple bonded species with a ―C ≡ C―RN,C (RN,C = CN, NC) structure can be considered as exceptions. Because isocyanide and cyanide species have very similar structures, the relative energy is independent of the pressure and temperature conditions. Theoretical and experimental gas-phase investigations show that basicity of isocyanides ranges from 182.1 to 198.2 kcal/mol which is 14.0 to 19.7 kcal/mol higher than the basicity of respective cyanides. The most favored protonation centers are located on isocyanide or cyanide group depending on the species. The biggest increase of basicity was caused by bulkier substituents. The substitutions have greater influence on the basicity of cyanides than on the basicity of isocyanides. In regard to deprotonation, the cyanides are more acidic than the corresponding isocyanides. For most of the unsaturated cyanide and isocyanide species the (N,C)-CHR′ hydrogen (the one connected to the carbon next to cyanide/isocyanide group) is the most acidic. Our work suggests that for derivatives bearing unsaturated substituent the favored deprotonation center may be different and some cyanides and isocyanides are unstable towards gas-phase deprotonation equilibrium as the formed anion tends to isomerize. Copyright © 2016 John Wiley & Sons, Ltd.