Intermediates of halogen addition to phenylethynes and protonation of phenylethynyl halides. Open halovinyl cations, bridged halonium, or phenyl-bridged ions: a substituent effect study by DFT and GIAO-DFT.

Formation of alpha-phenyl-beta-halovinyl cation, beta-phenyl-alpha-halovinyl cation, as well as the halogen-bridged and the spirocyclic phenyl-bridged cations as intermediates of protonation of phenylethynyl halides or by halogen addition to phenylethynes was evaluated by DFT at B3LYP/6-31+G(d) and, for comparison in representative cases, by B3LYP/6-311++G(d,p). Relative stabilities of the resulting minima were gauged as a function of substituents on the phenyl group with p-OH, p-OMe, p-H, p-CF3, p-CN, and p-NO2 and with p-OMeH+, p-NO2H+, and p-N2+. In the majority of cases, the alpha-aryl-beta-halovinyl cations were identified as the most likely intermediates, irrespective of X and for most R groups. For R = p-N2+ (with X = Br and Cl), R = CNH+ (with X = Cl), and R = MeOH+ (with X = Br), the corresponding beta-aryl-alpha-halovinyl cations become more stable than alpha-aryl-beta-halovinyl cations (but in most cases with a relatively small stability difference). Whereas competitive formation of the spirocyclic aryl bridged cations via this route appears remote, with R = N2+ and R = NO2H+ as substituents (with X = Br), cyclic halonium ions could intervene, since their relative stabilities are within approximately 4 kcal/mol of the lowest energy vinyl cations. Geometrical features, GIAO NMR chemical shifts, and NPA-derived charges were used to gain insight into the structural/electronic features in the resulting mono and dications. The study provides a basis for stable ion and solvolytic/kinetic studies on a series of substituted phenylethynyl halides that are being synthesized.