Linear Intermolecular Couplings Involving Ruthenacycle Intermediates

Linear Intermolecular Couplings Involving Ruthenacycle Intermediates 

One of the most reported pathways for C=C and C=C bonds coupling involves the oxidative coupling and the ruthenacycle intermediate formation. The first ruthenium-catalyzed linear codimerization of disubstituted alkynes and alkenes involved acrylates or acrylamides and selectively produced 1,3-dienes [33] (Eq. 23). The proposed mechanism involves a ruthenacyclopentene via oxidative coupling on the Ru(0) catalyst Ru(COD)(COT). The formation of 1,3-diene results from intracyclic jS-hydride elimination, this process taking place only when a favored exocyclic jS-elimination is not possible. 

Using the complex CpRu(CH3CN)3PF6 as a catalyst, reaction can proceed at room temperature in dimethylformamide (DMF) [36] (Eq. 25). 

The regioselective preference for the formation of the branched product can be reversed by an increase of steric hindrance, especially at the propargylic position. Preferential formation of the linear isomer was also observed with 4-hy-droxyalkynoates, allowing the synthesis of butenolides via cyclization [37] (Eq. 26). 

By contrast, the reaction of silylalkynes and terminal alkenes proceeded with complete control of regioselectivityby the silyl substituent to give only one isomer, similar to the branched isomer [38] (Eq. 27). 

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Linear Intermolecular Couplings Involving Ruthenacycle Intermediates... 

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