Heteroatom stabilized carbenium ions

• Published in: Coordination chemistry reviews Vol. 163; pp. 287 - 344
• Main Authors: Grützmacher, Hansjörg, Marchand, Christina M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.07.1997
• Subjects: Ab initio calculations; Carbenium ions; Carbocations; Heteroatoms; Isodesmic reactions; Laplacians; Preparations; Stabilization energies; Carbenium ions; Carbocations; Preparations; Heteroatoms; Laplacians; Ab initio calculations; Isodesmic reactions; Stabilization energies
• ISSN: 0010-8545; 1873-3840
• DOI: 10.1016/S0010-8545(97)00043-X

A comprehensive overview of α-heteroatom stabilized carbocations is presented, with special emphasis on mono-, di- and tri-substituted acyclic carbocations. The electronic effects of all main group elements on the stability of these compounds were systematically investigated and the calculated data is compared with experimental data. There are three electronic mechanisms by which a formally electron deficient center can be stabilized: (i) positive σ-induction by an electropositive element, (ii) π-donation from non-bonding electron pairs and (iii) positive hyperconjugation. One criterion for the stability of the cation is the stabilization energy calculated according to the isodesmic reaction: H 3− n C(XH m ) n ⊕ + nCH 4→ nH 3CXH m + CH 3 ⊕. Stabilization energies obtained by isodesmic reactions may be problematic because of ground state effects of the species involved in the reactions. Nevertheless it was found that all the α-heteroelements from groups 1, 2, and 13 to 17 stabilize a carbenium ion with respect to the parent methenium ion CH 3 ⊕! For the alkali and alkaline earth metals this stabilization can be explained by positive σ-charge transfer, where the positive charge on the central carbon is reduced. For group 13, the stabilization consists of a mixture between σ-effects and hyperconjugation. It is generally believed that the most efficient stabilizing effect evoked by a heteroatom adjacent to a formally electron deficient carbon center is due to π-charge transfer from a free electron pair on the heteroatom. Remarkably, the CLi 3 + ion and the guanidinium ion, C(NR 2) 3 +, are the most stable carbenium ions with respect to CH 3 + and they demonstrate that σ-donation can be as efficient as π-donation. The decrease in the calculated stability in the series of mono-substituted carbenium ions (H 2X)CH 2 ⊕ where X=N, P, As and Sb, is due to an increase in the energy which is needed for planarization of the H 2X group in order to allow effective π-donation. However, when the planarization energies are taken into account, π-donor centers from the higher periods are not worse π-donors than elements from the second period! Generally, the influence of an α-heteroatom on the stability of a carbenium ion changes strongly along a period but hardly changes within a group. Even very electronegative elements can stabilize a carbocation by σ-attraction due to a large contribution of the Coulomb term to the binding energies. Analysis of the Laplacians of the electron density showed, however, that a minimum of charge transfer by π-donation of the electronegative α-heteroatom is necessary in order to concentrate sufficient charge in the C-X binding region and form a highly polar but localized bond. Furthermore, an overview of various experimental methods used to synthesize α-heteroelement substituted carbenium ions is given and some principles of their reactivity are discussed.