Cyclobutadienes from alkyne dimerization

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. 

Insertion and condensation reactions of alkynes have been well known for many years (1,191). Alkyne dimerization to form either cyclobutadiene (192) or metallocyclopentadiene products (193), trimerization to arenes (194), condensation of two alkynes and carbon monoxide to cyclo-pentadienones (195), and condensation of alkyne and isonitriles to cyclo-pentadienimines (196) are common reactions. This section surveys alkyne insertion products originating from monomeric Mo(II) and W(II) reagents. 

Alkvnes do rot dimerize photochemically to give cyclobutadienes, but dimers are formed from arylalkynes under conditions of electron-transfer sensitization (2.105). These dimers arise from a reaction of the alkyne radical cation with ground-slate alkyne, followed by intramolecular electrophilic attack on the benzene ring. 

In contrast to cycloaddition reactions of phosphaalkenes, cycloaddition reactions between phosphaalkynes and other unsaturated systems are comparatively rare. Indeed, there are only a limited number of reports for monophos-phacyclobutadiene) complexes, which are obtained from the corresponding phosphaalkyne. Relatively recently, the reaction of phosphaalkynes with highly electron deficient alkynes was reported <19990M4838>. Treatment of a CF3C=CGF3-coordinated dimeric rhodium complex with phosphaalkynes in hexane at — 20°C followed by warming to room temperature afforded the red air- and moisture-stable phosphete complexes 60 in ca. 50% isolated yields. When phosphaalkynes are allowed to react with a kinetically stabilized cyclobutadiene, 2-Dewar-phosphinines, for example 93 (Equation 30), are obtained <1998S1305>. 

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). 

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