高周期典型金属の中程度ルイス酸性とπ電子親和性を活かした有機金属種の新規合成法

抄録

Carbometalation of a carbon-carbon multiple bond is the most powerful synthetic tool for organometallic compounds because the corresponding organometallics are produced with the formation of a new carbon-carbon bond. Many transition metal-catalyzed carbometalations have been established. Recently, transition metal catalyst-free carbometalation has attracted much attention because potentially toxic and expensive transition metals are not required. Generally, highly reactive organometallic compounds such as organolithiums and Grignard reagents are needed for the addition to non-activated alkynes and alkenes. However, the high nucleophilicity of the organometallics led to a lack of functional group tolerance. In this study, group-13 heavy metal Lewis acids such as indium trihalides and gallium trihalides have been focused on developing a novel carbometalation system. The Ga(III) and In(III) centers possess moderate Lewis acidity and high π-electron affinity due to the large ionic radius, which leads to compatibility with functional groups and to the activation of carbon-carbon multiple bonds. Therefore, mild organometallic reagents such as organosilicon compounds can be used to avoid undesired overreactions. In addition, organogalliums and -indiums produced by carbometalation are applicable to sequential reactions due to their high-chemoselectivity. Regio- and stereoselective carbometalation of alkynes and alkenes with mild carbon-nucleophiles such as silyl ketene acetals and silyl ketene imines was accomplished by using heavy main-group metal salts such as indium, gallium, and bismuth halides to afford highly-functionalized organometallic compounds possessing carbonyl and cyano groups, respectively. By extending this concept to oxymetalation, new synthetic methods for organometallic compounds with heterocyclic skeletons were also established. In addition, the catalytic cycle involving the present carbometalation realized the cross-coupling between enol derivatives with organosilicon reagents.