Calcium-mediated fulvene couplings. 2. Survey of 6-aryl- and 6-alkylfulvenes for their rac selectivity in the synthesis of ansa-calcocenes

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)(eta(5)-C5H5)}(2)Ca (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 H-1 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 lambda and delta conformations. The other involves restricted rotations of the tert-butyl groups in the ethylene bridge.