Thermodynamics of Ylide formation from carbenes and carbonyl compounds

Abstract

Thermodynamic parameters for the reaction of CH2 (singlet and triplet), CF2 and C(OH)2 carbenes with formaldehyde, acetaldehyde and acetone, forming carbonyl ylides, were determined using high level ab initio molecular orbital calculations. Singlet methylene forms a stable ylide with the three carbonyl compounds, with a ΔH0 of -41.6 kcal/mol, -40.0 kcal/mol (E isomer), -44.5 kcal/mol (Z isomer) and -40.8 kcal/mol, respectively. Triplet methylene also forms a ylide with formaldehyde with a ΔE of-15.7 kcal/mol, resulting in the triplet ylide being formed reversibly. For the reaction with formaldehyde, the CF2 and C(OH)2 carbenes have a positive ΔH0 of 13.0 kcal/mol and 22.0 kcal/mol, and a ΔG0 of 25.3 kcal/mol and 34.0 kcal/mol, respectively. Due to its unfavorable free energy of formation, and the potential competition of direct 1,2 cycloaddition of carbene to the C=O group, CF2 and C(OH)2 should not react with carbonyl compounds forming ylides.