1,2-Shift of Element-Centered Groups (R_nE) in Carbenoid Anions [R_nECF₂CFCl]⁻and its Relevance for Nucleophilic Vinylic Substitution: a DFT Study

Abstract

The 1,2‐shift of β‐substituent to the carbenic or carbenoid centre is one of the primary reaction pathways for these intermediates. In the present work the 1,2‐shift of various element‐centered groups in carbenoid anions [RnECF2CFCl]– (E=C, N, Si, Ge, Sn, P, As, Sb, S, Se, Te, R=Me) is for the first time examined with the help of density functional theory calculations. The calculations were performed on the models of free carbenoid anions [RnECF2CFCl]– and their Na+ and [NMe4]+ salts. In all cases the 1,2‐shift of the RnE‐groups formed by the elements of the third and subsequent periods was calculated to be much more facile than for carbon‐centred groups. Calculated ΔG≠ are as low as 4–6 kcal/mol for the 5−th period Me3Sn‐ and Me2Sb groups. An even more facile 1,2‐shift precludes the existence of the corresponding β‐element substituted carbenes. Periodicity trends, correlation analysis and NBO analysis of the transition state structures were used to discriminate between different intrinsic mechanisms. In free carbenoid anions the main mechanisms of 1,2‐shift of RnE‐groups are: a) electrophilic shift of R3E‐ (E= Si, Ge, Sn) and Ph‐groups; b) nucleophilic shift of RS‐ group c) dyotropic‐type pathway for RS‐, RSe‐, RO‐, R2N and R3C‐ groups. In carbenoid ion pairs: a) nucleophilic shift of RnE‐ (E=Si, Ge, Sn P, As, Sb) and Re(CO)5‐ groups; b) nucleophilic substitution involving the lone pair of RS‐, RSe‐ and R2N‐ groups.