Selective Organic Reactions Based on Novel Reactive Species Generated by Organometallic Reagents

铬（II）试剂是公认的单电子还原剂，已被用于选择性有机合成。我们发现，等量的烷基铬酸盐型试剂，由氯化铬和烷基锂试剂制备，作为还原剂与烯丙基和丙炔磷酸分别反应生成相应的烯丙基和丙炔铬试剂，生成的烯丙基和丙炔试剂作为亲核试剂与多种亲电试剂反应。我们还发现，烯醇酸酯是由α-位上有离去基团的羰基化合物与等摩尔量的铬酸盐型试剂反应生成的，其中铬试剂作为正式的双电子还原剂。先前有报道称，α-溴酮、酯和酰胺在铬（II）试剂作为还原剂的存在下，与醛和酮在barbier型条件下反应生成交叉醛醇产物，而采用所谓的grignard型程序的分步反应并不成功；即，在没有亲电试剂的情况下，溴酮与铬（II）生成烯醇酯后，烯醇酯不会被亲电试剂捕获。我们成功地用铬酸盐试剂还原生成烯醇酯，并阐明了所生成的烯醇酯与多种亲电试剂的格氏型反应的特征。这些烯醇化物不与氧烷反应，而在路易斯酸的存在下，根据所使用的路易斯酸，可以得到纯醛醇产物和醛醇产物。醛醇加合物可能是由烯酸铬与乙醛反应生成的，乙醛是在路易斯酸的存在下通过氧烷的阳离子重排形成的。

Chromium(II) reagents are well recognized to be a one-electron reductant and have been used for selective organic synthesis. We found that an equimolar amount of an alkylchromium ate-type reagent, prepared from chrornium(III) chloride and alkyllithium reagent, reacted as a reductant with allyl and propargyl phosphates to produce the corresponding allyl- and propargylchromium reagents, respectively, and the generated allylic and propargylic reagents reacted as a nucleophile with a variety of electrophiles.We also found that enolates were reductively generated by the reaction of carbonyl compounds bearing a leaving group at the α-position with an equimolar amount of a chromium ate-type reagent, where the chromium reagent serves as a formal two-electron reductant. Previously, it was reported that α-bromo ketones, esters, and amides reacted with aldehydes and ketones in the presence of a chrpmium(II) reagent as a reductant to produce cross-aldol products under the Barbier-type conditions, while a stepwise reaction by the so-called Grignard-type procedure was not successful ; namely, after the generation of enolates from bromo ketone with chromium(II) in the absence of an electrophile, the enolate was not trapped with the electrophile. We succeeded in the reductive generation of enolates by a chromium ate-reagent and elucidated the characteristic feature of the Grignard-type reactions of the generated enolates with a variety of electrophiles.These enolates did not react with oxiranes, while in the presence of a Lewis acid, homoaldol product and aldol product were obtained, depending on the Lewis acid used. The aldol adduct was possibly produced by the reaction of chromium enolate with aldehyde that was formed through cationic rearrangement of oxirane in the presence of the Lewis acid.