Cyclobutadiene, formation from alkynes

Although numerous alkyne insertion products have been reported for Mo(II) and W(II), simple dimerization to form cyclobutadiene or tri-merization to form an arene ligand is rare. One brief report of cyclobutadiene formation from a bisalkyne complex (206) has been followed by a full paper which suggests that an rj2-vinyl complex may be the precursor to CpM(S2CNR2) [ 174-C4(CF3)4] (207). 

Compounds 33 and 34 are readily formed from 31 by direct reaction with CpCo(CO)2. A possible reaction sequence is formation of the triene, 31, from alkyne dimerization followed by reaction with the cobalt species to give the three complexes. Reaction of 34 with alkynes yielded only cyclobutadiene complexes alkyne metathesis was not observed, probably since the carbon-to-metal bonds are too strong. 

Aluminium bromide is known to catalyse formation of cyclobutadiene from alkynes at low temperatures. 

The conversion of (828) into (829) and (830) on heating with tri-iron dodecacarbonyl is the first demonstration of the formation of ferroles from a cyclobutadiene and has implications for the mechanism of ferrole formation from cyclic bis-alkynes. ... 

The cobaltacyclopentadiene is converted on heating into a cyclobutadiene derivative. Treatment with carbon monoxide affords a cyclopentadienone complex. It is therefore probably an intermediate in the formation of these products from CpCo(CO) (L = CO) and alkynes. 

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