Elucidation of Intramolecular Reactions of Acyl Radicals

本文对酰基自由基分子内行为的几个新方面进行了研究。发现酰基自由基在硫上的分子内均溶取代是一个有效的过程。以t-丁基为离去基，通过分子内S_H2反应制备了多种γ -硫代内酯。在25℃时，环化速率约为7.5 × 10^3s^<-1>。从头算MO计算表明，反应发生在t型过渡态，进入的酰基自由基和离开的烷基自由基呈线性结构关系。该体系可成功推广到-硒内酯的合成。还考察了酰基自由基在O-C双键上的环化反应。采用慢速介质，观察了-内酯环的形成。这种形式的自由基5-内环产物是通过碘原子转移到酰基自由基，然后再进行离子5-内环形成的。采用从头算MO方法研究了5-己烯基自由基分子内环化行为的机理。认为该体系中六元自由基的形成是由于最初形成的五元自由基异构化所致。研究了酰基在N-C双键上的环化反应。与相应的烷基自由基环化不同，这种环化是以完全亲氮的方式发生的，从而形成内酰胺环。从理论和合成两方面研究了α、β不饱和酰基自由基的异构化行为。从头算MO计算表明，e型和z型化合物可以很容易地通过烯烯基自由基相互转化。这与几个实验结果一致，包括α、β -不饱和酰基自由基的5-外显子环化得到均匀的环化产物，而与自由基的起始几何形状无关。

In this work, several new aspects of intramolecular behaviors of acyl radicals have been studied. Intramolecular homolytic substitution of an acyl radical at sulfur was found to be an efficient process. By using t-butyl group as a leaving radical, a variety of gamma-thiolactones were prepared based on this intramolecular S_H2 reaction. The rate of cyclization is approximately 7.5 x 10^3s^<-1> at 25ﾟC.Ab initio MO calculations suggest that the reaction occurs via T-shaped transition state where the incoming acyl radical and the leaving alkyl radical have a linear structural relationship. The system can be successfully extended to the synthesis of gamma-selenolactones. Acyl radical cyclization onto O-C double bond was also examined. Using a slow mediator, the formation of gamma-lactone ring was observed. This formal radical 5-endo cyclization product was turned out to be formed via iodine atom transfer to acyl radical followed by ionic 5-endo cyclization. Ab initio MO studies were carried out to know the mechanism of intramolecular cyclization behavior of 5-hexenoyl radical. It was suggested that the formation of six-membered redical in this system is due to the isomerization of the initially formed five-membered radical.Acyl radical cyclizations onto N-C double bonds were investigated. Unlike the corresponding alkyl radical cyclizations, the cyclizations took place in complete nitrogen-philic manner to give lactam rings.The isomerization behavior of alpha, beta-unsaturated acyl radicals was studied in terms of theory and synthesis. Ab initio MO calculations suggest that E-form and Z-form can interconvert very easily via a ketennyl radical. This is in good accord with several experimental results which include that 5-exo cyclization of alpha, beta-unsaturated acyl radicals gave uniform cyclized product irrespective of the starting geometries of the radical.