On the stabilization of carbanions by adjacent phenyl, cyano, methoxy-carbonyl, and nitro groups in the gas phase

Abstract

By using the method of Fourier transform ion cyclotron resonance mass spectrometry, substituent stabilization energies of homologous series of cycloalkyl carbanions, X- c-C nH 2n−2 − (n = 3,4,5,6,7) with π-accepting substituents (X = Ph, CN, COOMe, NO 2) have been determined experimentally in the gas phase as the difference between the proton affinity of the substituted and corresponding unsubstituted (X = H) cycloalkyl carbanions. The stabilization energy data have been analyzed in terms of Taft's parametrization of polarizability, field/inductive, and resonance effects. The linear regression analyses show excellent correlations within the XCH − 2, X- c-C nH 2n−2 − (n = 4,5,6,7), and X- c-C 3H − 4 carbanion series, from which it appears that the contributions of polarizability effects are independent of the above type of carbanions and only depend on the nature of the substitutuent. Further, it follows that inductive stabilization is more effective in the substituted methyl, XCH − 2, than in the substituted cycloalkyl, X- c-C nH 2n−2 − (n = 4,5,6,7) carbanions. This result suggests that inductive stabilization is counteracted by the electron releasing effect of alkyl groups. Resonance stabilization is significantly more effective in the substituted cycloalkyl, X- c-C nH 2n−2 − (n = 4,5,6,7), than in the substituted methyl, XCH 2 −, carbanions, which suggests that in contrast to inductive stabilization, resonance stabilization is assisted by the electron releasing effect of alkyl groups. Finally, it appears that substituent stabilization in the geometrically restricted substituted cyclopropyl carbanions, X- c-C 3H 4 −, is dramatically less effective than in the corresponding geometrically unrestricted larger substituted cycloalkyl carbanions, X- c-C nH 2n−2 − (n = 4,5,6,7). The linear regression analyses of the substituted cycloalkyl carbanions indicate that reduction of the stabilization energy is caused not exclusively by a geometrically hindered resonance stabilization, but also to a smaller extent by a less efficient inductive stabilization in the substituted cyclopropyl carbanions.