THERMODYNAMICS OF YLIDE FORMATION FROM CARBENES AND CARBONYL-COMPOUNDS

Thermodynamic parameters for the reaction of CH2 (singlet and triplet) , CF and C(OH)(2) carbenes with formaldehyde, acetaldehyde and acetone , forming carbonyl ylides, were determined using high level nb initio molecular orbital calculations. Singlet methylene forms a stable ylide with the three carbonyl compounds, with a Delta H-o 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 formaldeh yde with a Delta E of -15.7 kcal/mol, resulting in the tripler ylide b eing formed reversibly. For the reaction with formaldehyde, the CF2 an d C(OH)2 carbenes have a positive Delta H-o of 13.0 kcal/mol and 22.0 kcal/mol, and a Delta G(o) of 25.3 kcal/mol and 34.0 kcalimol, respect ively. Due to its unfavorable free energy of formation, and the potent ial competition of direct 1,2 cycloaddition of carbene to the C=O grou p, CF2 and C(OH)(2) should not react with carbonyl compounds forming y lides.