A Novel Synthesis of Alkylboranes Using Addition Reaction of Alkyl Radicals

本文研究了一种新的合成烷基硼烷的方法，该方法不能通过烯烃的氢硼化反应获得区域选择性的烷基硼烷。众所周知，在三丁基锡氢化物的作用下，烷基自由基与缺电子烯烃的加成反应具有区域选择性地进行，从而形成碳-碳键。因此，将加成反应应用于烷基硼烷的合成。苯乙炔一氢硼化反应得到的苯乙烯基硼烷是一种缺电子的烯烃，它在三丁基锡氢化物的催化下与缺电子的烯烃发生偶联反应。在苯中回流或在室温汞灯照射下，考察了少量AIBN存在下某些卤代烃的反应。结果表明，在两种反应条件下反应均可得到相应的1-苯基-2-烷基硼烷，其中烷基被区域选择性地引入到α-烯基碳原子中。由此得到的烷基硼烷不能通过通常的氢硼化反应进行区域选择性地制备。2-苯基乙基硼酸与卤代烷在回流条件下的反应效果较好，因为2-苯基乙基硼酸的反应不会导致烷基和硼基之间的空间位阻。将3-溴-1-丙烯氢化得到的3-溴-1-丙基硼烷视为卤代烷，并用类似的方法考察了它与缺电子烯烃的反应。3-溴-1-丙基硼烷与三丁基锡氢化物通过自由基反应生成丙基硼烷，而与缺电子烯烃反应困难。因此，需要研究使用其他卤代烯的反应。

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.