RELATIVE HOMOLYTIC STRENGTHS OF C-H BONDS IN MELDRUM ACID AND DIMETHYL MALONATE

With the aid of a thermochemical cycle comprised of acidity and redox data in dimethyl sulfoxide and aqueous solution, relative homolytic bond dissociation energies (DELTA-BDE values) have been determined for 2,2-dimethyl-1,3-dioxane-4,6-dione (Meldrum's acid), dimethyl malonate, 5,5-dimethylcyclohexane-1,3-dione (dimedone), 2,4-pentanedione, acetone, 3-pentanone, and cyclopentanone. The DELTA-BDE data suggest that (a) secondary C-H bonds present in 3-pentanone are ca. 4 kcal/mol weaker (in a homolytic sense) than analogous primary C-H bonds in acetone; (b) C-H bonds located on carbon atoms adjacent to the carbonyl carbons in 3-pentanone and cyclopentanone are of equal homolytic strength, thus indicating a negligible effect due to cyclization; (c) homolytic BDEs for dimedone and 2,4-pentanedione are nearly equal, also indicative of no bond weakening due to cyclization; and (d) the C-H BDE for Meldrum's acid is ca. 3 kcal/mol less than that of the analogous C-H bond present in dimethyl malonate, indicative of a small cyclization effect on homolytic bond strengths. The Meldrum's acid/dimethyl malonate DELTA-BDE data are therefore in sharp contrast to published dimethyl sulfoxide solution pK(a)'s for Meldrum's acid and dimethyl malonate (7.3 and 15.9, respectively: Arnett et al. J. Am. Chem. Soc. 1987, 109, 809-812). The difference in the pK(a)'s for Meldrum's acid and dimethyl malonate is thought to provide additional experimental support for the effects of rotational barriers on neutral closed-shell ester stabilities. The DELTA-BDE data in this article suggest that rotational barriers have substantial effects on the relative stabilities of the radicals derived from Meldrum's acid and dimethyl malonate as well.