Solvolyses of 2-Oxo Bridgehead Compounds:  A Critical Examination of π-Conjugative Stabilization of α-Carbonyl Carbocations

The methodology of changing ring flexibility to detect the π-conjugative stabilization of bridgehead carbocations has been applied to eight 2-oxo (X = O) bridgehead carbocations. On the basis of the solvolytic behavior observed in kinetics and product analyses, the eight 2-oxo bridgehead substrates were classified into three categories:  three substrates solvolyzing without ion-pair return that leads to primary isomers (class A), three substrates that form primary isomers by ion-pair return during solvolysis (class B), and two substrates that undergo solvent addition to the carbonyl group to form hemiacetals during solvolysis (class C). It was concluded that the substrates of class C could not be used for the present purpose. Essentially constant ethanolysis rate ratios, k(X = O)/k(X = H₂), of 10^-8.2−10^-8.7 at 25 °C were obtained between four 2-oxo substrates in classes A and B and the corresponding parent unsubstituted ones. The result was interpreted to suggest that the π-conjugative stabilization of tertiary α-carbonyl carbocations is negligibly small, if present. Slightly more negative k(X = O)/k(X = H₂) values of 10^-9.7 and 10^-9.2 for highly flexible bicyclo[4.2.2]dec-1-yl and bicyclo[4.3.1]dec-1-yl systems, respectively, were attributed to complex conformations in the ground and incipient carbocations. PM3 calculations on some 2-methylene and 2-oxo bridgehead carbocations supported the experimental results. Comparison of the solvolysis rates of 1,1,3,3-tetramethyl-2-oxobutyl mesylate with those of 1,1,3,3-tetramethylbutyl mesylate estimated from the rates of the corresponding chloride also failed to support the π-conjugative stabilization of α-carbonyl carbocations.