Abstract
| - The methodology of changing ring flexibility to detect theπ-conjugative stabilization of bridgeheadcarbocations has been applied to eight 2-oxo (X = O) bridgeheadcarbocations. On the basis of thesolvolytic behavior observed in kinetics and product analyses, theeight 2-oxo bridgehead substrateswere classified into three categories: three substrates solvolyzingwithout ion-pair return that leadsto primary isomers (class A), three substrates that form primaryisomers by ion-pair return duringsolvolysis (class B), and two substrates that undergo solvent additionto the carbonyl group to formhemiacetals during solvolysis (class C). It was concluded that thesubstrates of class C could notbe used for the present purpose. Essentially constant ethanolysisrate ratios, k(X = O)/k(X =H2),of 10-8.2−10-8.7 at 25 °C were obtainedbetween four 2-oxo substrates in classes A and B and thecorresponding parent unsubstituted ones. The result wasinterpreted to suggest that theπ-conjugative stabilization of tertiary α-carbonyl carbocations isnegligibly small, if present. Slightlymore negative k(X = O)/k(X = H2)values of 10-9.7 and 10-9.2 for highlyflexible bicyclo[4.2.2]dec-1-yl and bicyclo[4.3.1]dec-1-yl systems, respectively, were attributedto complex conformations inthe ground and incipient carbocations. PM3 calculations on some2-methylene and 2-oxo bridgeheadcarbocations supported the experimental results. Comparison of thesolvolysis rates of 1,1,3,3-tetramethyl-2-oxobutyl mesylate with those of 1,1,3,3-tetramethylbutylmesylate estimated fromthe rates of the corresponding chloride also failed to support theπ-conjugative stabilization ofα-carbonyl carbocations.
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