Calcium-mediated fulvene couplings. 1. A survey of 6,6-dialkylfulvenes forthe formation of bridged and unbridged calcocenes

The degree to which 6,6-dimethylfulvene, 6,6-diethylfulvene, and 6-cyclopen tylidenefulvene form unbridged calcocenes in reductive coupling reactions w ith activated calcium were compared. The identification of alkenyl substitu ents along with alkyl substituents among the unbridged calcocenes confirms that hydrogen atom or proton transfer between dialkyl-fulvenes and their ra dical anions or dianions is responsible for the formation of unbridged calc ocenes. The tendency to form unbridged calcocenes instead of ansa-calcocene s is higher for 6,6-diethylfulvene and 6-cyclopentylidenefulvene than for 6 ,6-dimethylfulvene. 1,1 ' -Dicyclopentellylcalcocene, 1, which is formed in the re action between cyclopentylidenefulvene and calcium, was purified an d crystallographically characterized. Photolysis of 1 leads to [2 + 2] cycl ization of the cyclopentenyl substituents to form 2, an ansa-calcocene with a tricyclo[3.3.0.0]decane-1,10-diyl bridge.