A Novel Synthesis of Alkylboranes Using Addition Reaction of Alkyl Radicals

本文研究了一种不能通过烯烃氢硼化反应得到区域选择性烷基硼烷的新合成方法。众所周知，在三丁基氢化锡的介导下，烷基自由基与缺电子烯烃的加成反应进行区域选择性地进行碳-碳键形成。据此，应用加成反应合成烷基硼烷。将苯乙基单氢硼化法制备的2-苯乙基硼烷作为缺电子烯烃，进行了三丁基氢化锡介导的卤代烷与缺电子烯烃的偶联反应。在少量AIBN存在下，在苯回流或室温汞灯照射下，研究了一些烷基卤化物的反应。结果表明，在这两种反应条件下，烷基自由基被选择性地引入α -烯基碳原子，生成相应的1-苯基-2-烷基硼烷。所得的烷基硼烷不能通过通常的硼氢化反应进行区域选择性制备。在回流条件下，烷基卤化物与2-苯基硼酸反应得到了良好的效果，因为2-苯基硼酸的反应不会导致烷基自由基与硼基之间的位阻。将3-溴-1-丙烯氢化制得的3-溴-1-丙基硼烷作为卤化烷基，并以类似的方法考察了与缺电子烯烃的反应。3-溴-1-丙基硼烷与氢化三丁基锡通过自由基反应还原为丙基硼烷，而与缺电子烯烃反应困难。因此，需要用其他卤代烃来检验反应。

A novel synthesis of alkylboranes, which can not be obtained regioselectively by hydroboration of alkenes, was investigated in this research. It is well known that the addition reaction of an alkyl radical to an electron deficient alkene, mediated with tributyltin hydride, proceeds regioselectively to carry out carbon-carbon bond formation. Accordingly, the addition reaction was applied to synthesize alkylboranes.2-Phenylethenylborane, prepared by monohydroboration of phenylethyne, was regarded as an electron deficient alkene, and it was subjected to the tributyltin hydride mediated coupling of alkyl halides with electron deficient alkenes. The reaction of some alkyl halides was examined in the presence of a small amount of AIBN under reflux in benzene or under irradiation by a mercury lamp at room temperature. It was found that the reaction proceeded under both reaction conditions to give the corresponding 1-phenyl-2-alkylboranes where the alkyl radical was introduced regioselectively into the alpha-alkenyl carbon atom. The resulting alkylboranes were unable to be prepared regioselectively by usual hydroboration. The reaction of alkyl halides with 2-phenylethenylboronic acid under reflux gave good results, because the reaction using 2-phenylethenylboronic acid did not result in steric hindrance between alkyl radical and boryl group.Next 3-bromo-1-propylborane, prepared by hydroboration of 3-bromo-1-propene, was regarded as an alkyl halide, and the reaction with electron deficient alkenes was examined in a similar manner. 3-Bromo-1-propylborane was reduced to propylborane with tributyltin hydride via radical process, whereas it was difficult to undergo the reaction with electron deficient alkenes. Accordingly, it is needed to examine the reaction using other haloalkenes.