The α-Effect in Methyl Transfers from S-Methyldibenzothiophenium Fluoroborate to Substituted N-Methylbenzohydroxamates

• Published in: Journal of organic chemistry Vol. 68; no. 5; pp. 1810 - 1814
• Main Authors: Fountain, K. R, Felkerson, Cassie J, Driskell, Jeremy D, Lamp, Brian D
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 07.03.2003
• Subjects: Chemistry; Exact sciences and technology; Kinetics and mechanisms; Organic chemistry; Reactivity and mechanisms; SET mechanism; Organic cation; SN2 mechanism; Tricyclic compound; Chemical reaction kinetics; Hammett function; Sulfur heterocycle; Reactivity; Brönsted function; Electrochemistry; Alkalinity; Nucleophilicity; Sulfonium compound; Nucleophile
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo0206263

Studies of the α-effect show increased reactivity of nucleophiles having lone pairs of electrons on atoms neighboring the lone pair involved in reactivity when compared to the basicity of the nucleophiles. Hammett-type plots and Brönsted-type plots of substituted methylphenyl sulfates vs hydrogen peroxide anions and substituted N-methylbenzohydroxanates (NMBH) with substituted methylarenesulfonates or substituted arenedimethylsulfonium ions have large ρ or βnuc values, indicating a putative tightening of the usual SN2 transition states (anti-Hammond effect). Electrochemical studies of SN2-SET or reactivity indicate that SET character occurs in looser transition states, whereas SN2 transition states are associated with greater tightness. The α-effects for the series of sulfonium salts in completion reactions for 3-ClNMBH anions and 3-nitrophenolate anions are (log k α/k normal) 1.124 for dimethylphenyl sulfonium, 1.512 for dimethyl-1-naphthyl sulfonium, 1.835 for dimethyl-9-anthracenyl sulfonium, and 1.137 for S-methyldibenzylthiophenium. Correlations of the sizes of α-effects with typical SET (or ET) experimental parameters and the inverse dependence of the size of the α-effect on electron demand indicate inclusion of SET character in these SN2 transition states, vs no (or at least diminished) SET character in normal transition states. This dichotomy of tighter SN2 transition states, but looser SET transition states indicated in the α-effect, is examined in the present work.