Dimerization of cyclopropenones

Dimerization of cyclopropenones has also been found to occur under reductive conditions. Tetraphenyl resorcinol is formed in addition to a small amount of tetra-phenyl p-benzoquinone on treatment of diphenyl cyclopropenone with aluminum amalgam200 its formation can be rationalized via dimerization of the cyclopropenone ketyl 266 and subsequent aromatization, possibly according to a prismane mechanism. 

Direct reduction of chloro cation 292 in non-aqueous media to diphenylcyclopro-pene 293 is possible by means of dimethylamine borane211. The chloro cation 292 is easily prepared from the dichlorocyclopropene 154 and Lewis acids like AICI3 or SbCls11S. The reductive dimerization of cyclopropenones has already been mentioned (p. 58). 

Treatment of dialkylcyclopropenones with catalytic amounts of Ru3(CO) 2 and EtgN under CO resulted in carbonylative dimerization of cyclopropenones to give pyranopyrandiones in good yields (Scheme 2.111) [169]. Cross-carbonylation of dipropylcyclopropenone with dec-5-yne resulted in unsymmetrically substituted pyranopyrandione 148. 

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