Calcium-Mediated Fulvene Couplings. 1. A Survey of 6,6-Dialkylfulvenes for the Formation of Bridged and Unbridged Calcocenes
journal contributionposted on 23.05.2001 by Piet-Jan Sinnema, Pamela J. Shapiro, Britta Höhn, Thomas E. Bitterwolf, Brendan Twamley
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The degree to which 6,6-dimethylfulvene, 6,6-diethylfulvene, and 6-cyclopentylidenefulvene form unbridged calcocenes in reductive coupling reactions with activated calcium were compared. The identification of alkenyl substituents along with alkyl substituents among the unbridged calcocenes confirms that hydrogen atom or proton transfer between dialkylfulvenes and their radical anions or dianions is responsible for the formation of unbridged calcocenes. The tendency to form unbridged calcocenes instead of ansa-calcocenes is higher for 6,6-diethylfulvene and 6-cyclopentylidenefulvene than for 6,6-dimethylfulvene. 1,1‘-Dicyclopentenylcalcocene, 1, which is formed in the reaction between cyclopentylidenefulvene and calcium, was purified and crystallographically characterized. Photolysis of 1 leads to [2+2] cyclization of the cyclopentenyl substituents to form 2, an ansa-calcocene with a tricyclo[184.108.40.206]decane-1,10-diyl bridge.