Cyclobutadiene ring expansion

Ring expansion of a number of (cyclopropenyl)diazoalkanes gives cyclobutadiene derivatives, e.g. reaction of 7, though in some cases, fragmentation occurs leading to alkynes (see Section 10.A.2.2.). ° There is evidence that the cyclobutadienes are formed through singlets, while the alkynes are derived from triplets. 

Some highly substituted phospholium salts, e.g., (180), and phosphole oxides have been prepared by a ring-expansion reaction of aluminium chloride-cyclobutadiene complexes with dichlorophosphines. The [4 - - 2] dimer (181)... 

Cyclobutene (94) undergoes ring expansion to (95) on reaction with (Me3N)3PO and sodamide palladium chloride complexes of tetrasubstituted cyclobutadienes give octasubstituted cyclo-octatetraenes on reaction with triphenylphosphine. ... 

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