New heteroorganic betaines containing the (+)E15-C-E14-X(-) and (+)E15-C-E14(-) structural fragments (E15 = P, As; E14 = Si, Ge, Sn; X = C, S, O, NR)

The review surveys the data on the reactions of phosphorus and arsenic ylides with compounds containing E=X bonds (E = C, Si, Ge, or Sn; X = C or S), cyclic oligomers (R2ES)(n) (n = 2 or 3), and heavier analogs of carbenes. These reactions give rise to two new classes of heteroorganic betaines containing the E-(+)(15)-C-E-14-X(-) (I) and E-(+)(15)-C-E14(-) (II) (E-15 = P or As; E-14 = Si, Ge, or Sn; X = C or S) structural fragments. Procedures for the synthesis of these compounds, their reactivities, the X-ray diffraction structures, and the electronic structures established by high-level quantum-chemical calculations are considered in detail. The carbon analogs of betaines of type 1, viz., compounds bearing the P(+)-C-C-X(-) fragment (III), are also discussed. The latter were long considered as possible intermediates in the reactions of compounds containing the polar C=X bond (X = C, O, S, NR, etc.) with phosphorus ylides (classical Wittig and Corey-Chaykovsky reactions and related processes).