Elucidation of Intramolecular Reactions of Acyl Radicals

本文对酰基自由基的分子内行为进行了一些新的研究。分子内均裂取代的酰基自由基在硫被认为是一个有效的过程。以叔丁基为离去基团，利用分子内S_H_2反应合成了多种γ-硫代内酯。在25 ℃时，环化速率约为7.5 × 10^3s^<-1>。从头算分子轨道计算表明，反应通过T形过渡态发生，其中进入的酰基和离开的烷基具有线性结构关系。该体系可以成功地推广到γ-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。使用慢介体，观察到γ-内酯环的形成。这种形式自由基5-内环化产物是通过碘原子转移到酰基自由基上，然后进行离子5-内环化而形成的。用从头算方法研究了5-己烯酰基自由基分子内环化反应的机理。结果表明，该体系中六元自由基的形成是由于五元自由基的异构化作用，并研究了酰基自由基在N-C双键上的环化反应。与相应的烷基自由基环化反应不同，环化反应是以完全亲氮的方式进行的，得到内酰胺环。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。从头算分子轨道计算表明，E-型和Z-型可以很容易地通过烯酮自由基相互转化。这与几个实验结果雅阁，这些实验结果包括α，β-不饱和酰基自由基的5-外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.