The protonation of carbenes: structural effects on α-proton acidity of carbocations

Abstract

Ionisation constants (pK a s) are reported for proton loss from the α-carbon of carbocations forming singlet methyl, dimethyl, phenyl, phenylmethyl, diphenyl, methoxy and methoxymethyl carbenes. Equilibrium constants have been evaluated for isomerisation of the carbenes to alkenes or their reaction with water (hydration) to form alcohols based on G3 calculations and estimates of free energies of transfer from gas to solution. Combining equilibrium constants pK H2O for hydration (R 2 C: + H 2 O = R 2 CHOH) with pK R for hydrolysis of the carbocations (R 2 CH + + H 2 O = R 2 CHOH + H + ) provides the desired pK a s as pK R -pK H2O . For the methyl cation an approximate value of pK R = -43 is extrapolated from a correlation of hydride ion affinities of ethyl, isopropyl and t-butyl cations between the gas phase and solution. Based on pK H2O = -70 for H 2 C: the pK a for the methyl cation is 27. The range of equilibrium constants for different carbenes (50 log units in pK H2O ) points to wide variations in stability, in contrast to kinetic measurements which reflect a uniform lack of selectivity. Remarkably, a comparison of substituent effects on pK H2O and pK R shows that methyl and phenyl substituents stabilise the carbocation and carbene similarly but that methoxy substituents are more stabilizing for the carbene. The latter finding is consistent with calculation of a larger barrier to rotation of a methoxy group in dimethoxy carbene than in the dimethoxymethyl cation and reports that O-substituted carbenes react less rapidly with acids than diarylcarbenes. The behaviour is understandable if inductive effects of electronegative atoms complement donation of an electron pair by resonance in the case of carbenes and oppose it for carbocations. Although errors of 2-3 units are likely for individual pKs, these are mitigated by the large span of values considered and cancellations of systematic errors when structurally related values are compared.

Comment on “Activation of methane to CH 3 + : A selective industrial route to methanesulfonic acid”

“…We describe here why this mechanism is unreasonable in a series of ways based on literature precedent and standard chemical precepts. To add quantitative insight into the energetic infeasibility of the process, computational studies were carried out on the key structures in CCSD(T)/aug-cc-pVTZ//M06-2X/6-31+G(d,p) calculations including an SMD implicit solvent correction, supplemented by CAS (10,10)-NEVPT2 for open-shell structures of 3 (4).…”

“…Olah et al were unable to observe CH 3 + under the most stringently non-nucleophilic superacid conditions (9). From an extrapolated pK R value, CH 3 + is less favorable to form in solution than the cumyl cation by ~30 orders of magnitude (10). If CH 3 + were formed in an oleum solution, its reaction with the sulfuric acid to form CH 3 OS(O)(OH) 2 + (4, Fig.…”

Singlet carbene stability: linear free energy analysis of substituent effects

“…Complete tables of the relative contributions of each of the three terms are given in the Supplementary Materials. The fact that earlier LFE studies 5,6 generated correlations with appropriate algebraic signs for polarizability and field effects is attributable the mitigating effects of the aromatic rings with phenylcarbenes and/or smaller sample size in the latter case.…”

Reaction mechanisms : Part (iii) Polar reactions

“…pK a values have been reported for proton loss from the a-carbon of carbocations to form singlet methyl, dimethyl, phenyl, phenylmethyl, diphenyl, methoxy and methoxymethyl carbenes. 33 Equilibrium constants were evaluated for the isomerisation of the carbenes to alkenes or for their hydration to form alcohols (pK H 2 O ) based on G3 calculations and estimates of free energies of transfer from gas to solution. Substituent effects on equilibrium data were discussed.…”