UBC Theses and Dissertations
Stereochemistry of bridgehead free radicals Humphrey, Leonard Blair
Supervisor: Dr. R.E. Pincock In order to attempt to define more rigourously the geometric requirements of bridgehead free radicals, two t-butyl peresters of bridgehead acids are prepared — t-butyl adamantane-1-percarboxylate and t-butyl norbornane-1-percarboxylate. The decompositions of these peresters are examined, the rates determined, and the activation parameters for the decompositions calculated. Evidence is presented which suggests that the decomposition of t-butyl adamantane-1-percarboxylate proceeds via a concerted mechanism, splitting out CO₂ and producing the bridgehead free radical. An enthalpy of activation of 30.5 kcal./mole and an entropy of activation of 13 cal./deg. are determined. Similar evidence is presented which indicates that t-butyl norbornane-1-percarboxylate also decomposes via the concerted mechanism. The enthalpy of activation for this decomposition is found to be 32.2 kcal./mole and the entropy of activation found to be -3.6 cal./deg. The results obtained here are compared to those of other workers, and the inference that the entropy of activation plays a major role in determining rates of such decompositions is made. A synthetic pathway to another bridgehead perester, t-butyl bicyclo[2,2,2]octane-l-percarboxylate is explored, and a new and more practicable synthesis of bicyclo[2,2,2]octane-l-carboxylic acid presented. A comparison of bridgehead free radicals to bridgehead carbonium ions is made, and other evidence pertinent to the stereochemistry of bridgehead free radicals is discussed.
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