Solvolyses of 3-Oxo- and 3-Methylenebicyclooct-1-yl Triflates: Through-Bond Interaction of a β-Carbonyl Lone Pair with a Cationic p Orbital

Article abstract of DOI:10.1021/jo00061a036

The rates of solvolysis of 3,3-dimethyl-2-oxobicyclo<2.2.2>oct-1-yl triflate (1b), 2-methylenebicyclo<2.2.2>oct-1-yl triflate (2b), 5,5-dimethyl-3-oxobicyclo<2.2.2>oct-1-yl triflate (3b), 3-oxobicyclo<2.2.2>-oct-1-yl triflate (4b), 3-methylenebicyclo<2.2.2>oct-1-yl triflate (5b), 3,3-dimethylbicyclo<2.2.2>oct-1-yl triflate (6b), and bicyclo<2.2.2>oct-1-yl triflate (7b) have been determined in 80percent ethanol at 25 deg C.The 18O- and deuterium-labeling studies exclude both carbonyl addition and the enolization processes in the solvolyses of 3b and 4b.The rate ratios 1b/6b and 2b/7b are 10^-9.1 and 10^-3.9, respectively, indicating that the oxo substituent at the C(2) position is more electron withdrawing than the methylene substituent at the same position.In contrast, the rate ratios 3b/6b and 5b/7b, 10^-2.3 and 10^-2.2, respectively, are comparable with each other.Moreover, the rate ratio 4b/5b is 2.3, suggesting that the 3-oxo substituent is less electron withdrawing than the 3-methylene substituent with respect to the cationic center.Evaluation of the inductive effect of the oxo substituent at the C(3) position by means of the reported rates of solvolysis of various 2- or 3-substituted and unsubstituted bicyclo<2.2.2>oct-1-yl p-nitrobenzenesulfonates (14, 13, and 15, respectively) indicates a rate enhancement of 10³-10⁴ for 3b and 4b.This marked enhancement is interpreted as evidence of the stabilization of the 3-oxo carbocation by a through-bond interaction between the β-carbonyl lone pair and the cationic p orbital.The mY relationships and the formation of fragmentation products 11 (9percent) and 12 (20percent) in the methanolyses of 3-oxo compounds 3b and 4b, respectively, support the through-bond interaction.