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Isolation, structural characterization, and synthetic application of oxycyclopentadienyl dianions.

Cyclic dianions are considered to be useful synthons for the construction of a wide variety of cyclic organic compounds and organometallic complexes. However, reported patterns of substituted cyclic dianions are very much limited owing to the lack of synthetic methods. Furthermore, information on the structure of dianion species is very rare, which also to some extent blocks the development of dianion synthesis and applications. The isolation of important dianion species and the investigation of their reactivity are of general interest for both organometallic chemists and synthetic organic chemists. This would not only contribute to the indepth understanding of reaction mechanisms but could also lead to the discovery of new synthetically useful reactions. In this context, we report here the first isolation and the X-ray structure determination of a novel type of cyclic dianion, the fully substituted oxycyclopentadienyllithium compounds 2, which were prepared by the reaction of the linear dianion 1 and CO (Scheme 1). Because of the concomitance of the CpLi moiety, the exocyclic oxy anion, and the multiple reactive sites, these OCp dianions 2 are structurally unique and display novel reaction chemistry towards organic substrates and organometallic compounds. Pure crystalline compounds 1 a–c were isolated in high yield from the lithiation of the corresponding diiodides 3a–c after filtration of LiI and recrystallization from hexane (Scheme 2). The structure of 1 a was determined by X-ray single-crystal structure analysis (see the Supporting Information). Then, treatment of the isolated pure compounds 1 with CO in THF or Et2O under mild conditions resulted