Site-specific and random degenerate rearrangements in η6- and η4-cycloheptatriene metal complexes

Abstract

Carbon-13- labelled cycloheptatriene, containing ¹³C at the 3- and 7-positions, was synthesised from [¹³C-carboxy]-1,4-dihydrobenzoic acid by successive lithium aluminium hydride reduction, toluene-p-sulphonylation, and solvolysis in acetic acid. This was converted into the corresponding tricarbonylmolybdenum complex, which was subjected to thermolysis in [²H₆]benzene solution. The course of reaction was followed by ¹³C n.m.r. spectroscopy, and the redistribution of isotopic label in the product compared with that predicted from solving the differential equations describing the kinetics for each of the possible rearrangement pathways. It was thus shown that a [1,5] hydride shift is involved, in keeping with earlier conclusions of Pauson but not with the observations of Grimme and Roth, or those of Faller. Isotopically enriched cycloheptatrienerhodium(I) acetylacetonate was prepared similarly, and its thermal degenerate rearrangement shown to occur with random redistribution of the label.