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Calcium-Mediated Fulvene Couplings. 2. Survey of 6-Aryl- and 6-Alkylfulvenes for Their rac Selectivity in the Synthesis of ansa-Calcocenes

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journal contribution
posted on 11.12.2001, 00:00 by Piet-Jan Sinnema, Britta Höhn, Robert L. Hubbard, Pamela J. Shapiro, Brendan Twamley, Alexander Blumenfeld, Ashwani Vij
The reductive coupling of 6-mesitylfulvene, 6-(3,5-di-tert-butylphenyl)fulvene, 6-(1-naphthyl)fulvene, and 6-tert-butylfulvene with HgCl2-activated calcium was examined to determine if increasing the steric bulk of the 6-substituent on the fulvene would enhance selectivity for the rac ansa-calcocene product over the meso isomer. Upon formation, 6-mesitylfulvene readily dimerizes by a Diels−Alder cyclization to form (E,E)-5,10-bis(mesitylmethylene)tricyclo[5.2.1.0]deca-3,8-diene (1), the X-ray crystal structure of which was determined. This fulvene does not react with activated calcium. The other fulvenes are successfully coupled by calcium; however, they afford poor to no selectivity for the rac-calcocene isomers in comparison with less sterically hindered 6-phenylfulvene. A 1:1 ratio of the rac and meso isomers of {1,1‘-(1,2-t-Bu-C2H2)(η5-C5H5)}2Ca (rac-5 and meso-5) was obtained from the coupling of 6-tert-butylfulvene with calcium. The two isomers were separated by selective crystallization, and the X-ray crystal structures of the DME adducts of each isomer were determined. The crystal of rac-5 contained two unique molecules of the complex. One molecule contains a single dicoordinated DME on the calcium. The other molecule contains two DME molecules:  one dicoordinated and the other monocoordinated to the calcium. A variable-temperature 1H NMR study of meso-5 was performed to characterize two concurrent dynamic processes in the molecule. One process involves a rearrangement of the chelating ligand framework between λ and δ conformations. The other involves restricted rotations of the tert-butyl groups in the ethylene bridge.

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