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

• Published in: Journal of organic chemistry 2006-12, Vol.71 (26), p.9643-9650
• Main Authors: Okazaki, Takao, Laali, Kenneth K
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Kinetics and mechanisms; Organic chemistry; Reactivity and mechanisms
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo061632s

Formation of α-phenyl-β-halovinyl cation, β-phenyl-α-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 α-aryl-β-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 β-aryl-α-halovinyl cations become more stable than α-aryl-β-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 ∼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.