Elaboration of α-substituted benzyl alkyl ethers and sulphides by suppression of the Wittig and related rearrangements via complexation to tricarbonylchromium

Abstract

Co-ordination of benzyl alkyl ethers and sulphides to tricarbonylchromium allows α-substitution via the corresponding α-carbanions to be achieved by suppression of the Wittig and related rearrangements. Little stereoselectivity was observed in the α-methylation of tricarbonylchromium complexes of benzyl ethers derived from a variety of chiral alcohols. Excellent stereoselectivities were observed in the mono- and di-methylation reactions of tricarbonyl[o-xylene-α,α′-diyl bis(methyl ether)]chromium where single diastereoisomers of the α-methyl and α,α′-dimethyl complexes were formed. Deprotonation and methylation of tricarbonyl-syn-1-methoxytetralinchromium occurred stereospecifically with complete retention of configuration under conditions where the corresponding anti-complex was inert. On treatment with base tricarbonyl-(3-chloropropyl benzyl ether)chromium cyclises to give tricarbonyl-(2-phenyltetrahydrofuran)chromium. Exposure of arenetricarbonylchromium complexes to air and sunlight quantitatively liberates the free arene.