Probing the intermediates of halogen addition to alkynes: bridged halonium versus open halovinyl cation; a theoretical study.
[Reaction: see text]. Intermediates formed in halogen addition (X = Br, Cl, F) to alkynes (ethyne, propyne, 2-butyne, trifluoromethylethyne, trimethylsilylethyne, and 1-trimethylsilylpropyne) were studied computationally by MP2 at the MP2/6-311++G(3df,3pd) level and/or by DFT at the B3LYP/6-31+G(d) level. Structure optimization and frequency calculations were performed to identify the minima and to obtain their relative energies. PCM calculations (with H2O, CH2Cl2, and CCl4 as model solvents) were employed to examine solvation effects on the relative stabilities in the resulting bridged halonium, -halovinyl, or -halovinyl cations. GIAO-MP2 and GIAO-DFT calculations were employed to compute NMR chemical shifts (13C, 19F, and 29Si as appropriate). In selected cases, PCM-GIAO calculations were also performed to investigate the extent of solvent effects on the computed NMR shifts. The NPA-derived charges and the GIAO shifts were examined in comparative cases to shed light on structural features. In several cases, structure optimization starting with the -halovinyl cations resulted in -halovinyl cations (via formal hydride shift or trimethylsilyl shift). With the CF3 derivative (when X = F), a formal F shift results in polyfluoroallyl cation generation from fluorovinyl cation as starting geometry.