Stereoselective electrochemical synthesis and cathodic behaviour of (9- exo-benzyl-η 6-fluorene)chromium tricarbonyl

Abstract

The one-electron electrochemical reductive cleavage of (η 6-fluorene)Cr(CO) 3 gives the (η 6-fluorenyl)Cr(CO) 3anion. This anion reacts at 0 °C in DMF with benzyl chloride present in excess during electrolysis to give (9- exo-benzylfluorene)Cr(CO) 3 stereoselectively in 64% yield. At higher temperatures there is competing formation of the exo and endo isomers. The reaction of Cr(CO) 6 with 9-benzylfluorene is neither stereoselective nor regioselective, and gives moderate yields of the exo and endo isomers (26 and 17%, respectively). The one-electron reduction of the exo isomer generates the conjugated base which initiates the exo-endo transformation. In the presence of benzyl chloride, the base-catalyzed isomerization competes with a slow benzylation of the base, and (9,9′-dibenzylfluorene)Cr(CO) 3 can be isolated.