Mechanisms by which Alkynes React with CpCr(CO)3H. Application to Radical Cyclization

The reaction of CpCr­(CO)3H with activated alkynes in benzene has been examined. The kinetics of these reactions have been studied with various alkynes, along with the stereochemistry with which the alkynes are hydrogenated. The hydrogenation of phenyl acetylene and diphenyl acetylene with CpCr­(CO)3H has been shown to occur by a hydrogen atom transfer (HAT) mechanism. The reaction of CpCr­(CO)3H with dimethyl acetylenedicarboxylate (DMAD) produced hydrogenated products as well as phenyl substitution from reaction with solvent. On the basis of kinetic data, it is thought that the reaction of DMAD may proceed via a single electron transfer (SET) as the rate-determining step. The radical anion of dimethylfumarate was observed by EPR spectroscopy during the course of the reaction, supporting this claim. The aromatic 1,6 eneyne (8) gave cyclized products in 78% yield under catalytic conditions (35 psi H2), presumably by the 5-exo-trig cyclization of the vinyl radical arising from H• transfer. Using a cobaloxime catalyst (12) hydrogenation was completely eliminated to yield 100% cyclized products.

本研究考察了(η⁵-环戊二烯基)三羰基氢化铬（CpCr(CO)3H）与活化炔烃在苯溶剂中的反应。针对各类炔烃开展了该类反应的动力学研究，并同步探究了炔烃氢化反应的立体化学特征。研究表明，苯乙炔（phenyl acetylene）与二苯乙炔（diphenyl acetylene）经CpCr(CO)3H参与的氢化反应通过氢原子转移（hydrogen atom transfer, HAT）机制进行。CpCr(CO)3H与丁炔二酸二甲酯（dimethyl acetylenedicarboxylate, DMAD）的反应不仅生成了氢化产物，同时还产生了因溶剂参与反应而生成的苯基取代产物。基于动力学数据，学界认为DMAD的反应可能以单电子转移（single electron transfer, SET）作为决速步。反应过程中通过电子顺磁共振波谱（EPR spectroscopy）观测到了富马酸二甲酯的自由基阴离子，为该反应路径提供了实验支撑。芳香族1,6-烯炔（8）在催化条件（35磅每平方英寸（pounds per square inch, psi）氢气）下以78%的收率得到环化产物，该过程推测由氢原子（H•）转移生成的乙烯基自由基经5-exo-trig环化反应完成。当使用钴肟催化剂（cobaloxime catalyst, 12）时，氢化反应被完全抑制，产物仅为100%的环化产物。