Kinetic vs. thermodynamic factors in .alpha.-hydrogen atom abstractions from alkylaromatics. 2. Reactivities of .alpha.-alkylnaphthalenes and several conformationally locked alkylaromatics toward bromine atom
作者:James M. Tanko、N. Kamrudin、Joseph F. Blackert
DOI:10.1021/jo00022a034
日期:1991.10
The relative reactivities of alpha-methyl-, alpha-ethyl-, and alpha-isopropylnaphthalene toward bromine atom have been determined. A partial breakdown in the normal reactivity pattern (tertiary > secondary > primary) is observed: CH3 (1.00) < CH2CH3 (5.7) > CH(CH3)2 (4.7) per hydrogen at 80-degrees-C. These data (in conjunction with earlier results for the phenyl and 9-anthryl series) show that for alkylaromatics in general, the anticipated correlation between log (k(rel)) and DELTA-H-degree for alpha-hydrogen abstractions by bromine atom is poor. This breakdown in the Evans and Polanyi (E&P) relationship and the observed selectivity reversals in the alpha-naphthyl and 9-anthryl series are attributed to stereoelectronic factors, specifically the alignment of the alpha-C-H bond with the adjacent pi-system in the starting alkylaromatic. The reactivities of indane, 1,8-ethylenenaphthalene, and 1,9-ethyleneanthracene (relative to the corresponding ethylarenes) provide further evidence for the role of stereoelectronic factors. These stereoelectronic factors are reconciled with the E&P equation quantitatively: An excellent correlation between log k(rel) and (sin2 theta x DELTA-H-degree is observed (where theta is the angle defined by the alpha-C-H bond with respect to the aromatic plane). The basis for this treatment and its relationship to the E&P equation is discussed.