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

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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.