OBSERVATIONS OF NONADDITIVE SUBSTITUENT EFFECTS ON THE DIMETHYL-SULFOXIDE SOLUTION HOMOLYTIC BOND STRENGTHS OF ANTHRYLMETHYL C-H BONDS

Acid-base and redox data, when incorporated into a thermochemical cycle, enable accurate determination of the effects of substitutents on the homolytic strengths of chemical bonds. In this study, the effects of substitutents on the dimethyl sulfoxide phase homolytic strengths of labile sp3 C-H bonds present in 9-methylanthracene, seven variously substituted 9-methylanthracenes, phenylacetonitrile, and alpha-methoxyphenylacetonitrile are evaluated. The bond-strength data for these species indicate that, when present individually, 10-cyano and alpha-methoxy substitutents weaken the anthrylmethyl C-H bond in 9-methylanthracene (in a homolytic sense) by 1-2 and 4 kcal/mol, respectively. When both 10-cyano and alpha-methoxy substituents are present in the same molecule, as in 10-cyano-9-(methoxymethyl)anthracene, the anthrylmethyl sp3 C-H bond is weakened by 8 kcal/mol. Therefore, the bond-weakening effect of a 10-cyano substituent on 9-methylanthracene sp3 C-H bond strengths is ca. 2kcal/mol greater when the C-H bond to be cleaved is adjacent to an alpha-methoxy substituent. A similar and somewhat larger synergism (ca. 4 kcal/mol) is observed when an alpha-phenoxy substituent is present in place of alpha-methoxy. In related experiments, an alpha-cyano substituent is shown to weaken the anthrylmethyl C-H bond in 9-methylanthracene by ca. 3 kcal/mol. These data constitute the first nonkinetic experimental observations of synergistic substituent effects on the homolytic strengths of sp3 C-H bonds. While not necessarily indicative of a general phenomenon, the observed changes in 9-methylanthracene sp3 C-H homolytic bond strengths (due to the simultaneous presence of alpha-methoxy (or alpha-phenoxy) and 10-cyano substituents) can be rationalized by invoking a solvent-induced stabilization in the incipient anthrylmethyl radicals that results from the presence of both donor and acceptor substituents in conjugation with the unpaired electron.