Correlations between Carbene and Carbenium Stability: Ab Initio Calculations on Substituted Phenylcarbenes, Nonbenzenoid Arylcarbenes, Heteroatom-Substituted Carbenes, and the Corresponding Carbocations and Hydrogenation Products
journal contributionposted on 07.08.2009, 00:00 by S. Gronert, J. R. Keeffe, R. A. More O’Ferrall
Calculations were completed at the G3MP2 level on a large group of carbon- and heteroatom-substituted carbenes (X-CH, singlets and triplets), carbenium ions (X-CH2+), and their hydrogen addition products (X-CH3). One series includes 11 meta- and 12 para-substituted phenylcarbenes, X = Ar. Gas-phase enthalpies of reaction were calculated for four processes: singlet−triplet enthalpy gaps of the carbenes, ΔHST; enthalpies for deprotonation of the cations yielding singlet carbenes, ΔHACID; hydride ion affinities of the carbenium ions, HIA; and enthalpies of hydrogenation of the singlet carbenes, ΔHHYDROG. A plot of HIA vs ΔHHYDROG values provides a direct comparison of substituent effects on the stabilities of the singlet carbenes and the corresponding benzylic cations. These effects are larger for the cations but are remarkably consistent over a wide range of reactivity: 166 kcal/mol in HIA. All four processes were analyzed according to the relative importance of polarizability, polar, and resonance effects. Polar and resonance effects are large and of similar magnitude for meta compounds. For the para compounds resonance effects are more dominant. Calculations were made on three nonbenzenoid arylcarbenes: Ar = cycloheptatrienyl+, cyclopentadienyl−, and cyclopropenyl+. The cyclopentadienyl−-substituted system fits the HIA vs ΔHHYDROG correlation, but the other two fall well off the line, suggesting markedly different interactions are at play. A set of heteroatom-substituted carbenes and carbocations was also examined. Points for these groups lie well above the correlation line for the HIA vs ΔHHYDROG plot defined by the aryl compounds, confirming that heteroatoms stabilize the singlet carbene proportionally more than the carbocation.