Intermolecular Coupling with Cycle Formation

The coupling between alkenes and alkynes can also afford cyclization reactions and leads to strained carbocycles. Most of these reactions are performed via a ruthenacycle intermediate leading to [2+2] cycloaddition. 

One of the first examples of ruthenium-catalyzed C-C bond formation afforded the synthesis of cyclobutenes, from norbornene derivatives with dimethyl acetylenedicarboxylate, and was reported by Mitsudo and coworkers [45, 46] by using various catalysts such as RuH2(CO)[P(p-C6H4F)3]3 or RuH2(PPh3)4. More recently, the complex Cp RuCl(COD) has shown to be an excellent catalyst for the [2+2] cycloaddition of norbornenes with various internal alkynes [45] (Eq. 33) and with a variety of substituted norbornenes and norbornadienes [47]. The ruthenacycle intermediate, formed by oxidative coupling, cannot undergo /1-hydride elimination and leads to cyclobutene via a reductive elimination. 

This method was applied to the synthesis of a new range of rigid linear rods based on the [n]ladderanes [48] (Eq. 34). 

A cyclobutene was recently obtained by a related reaction of dimethyl acetylenedicarboxylate with ethylene in the presence of a cationic ruthenium alkylidene catalyst precursor [43] (Eq. 35). 

Cyclobutanes are produced by using cyclooctadiene as an olefinic substrate via an unusual [4+2] cycloaddition. The mechanism is postulated to proceed 

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The transmetallation with organoboron reagent precedes the carbometallative addition of the arylrhodium intermediates 394 to the unsaturated substrate, and the intermediate alkenylrhodium species 395 then undergo hydrolysis to close the catalytic cycle with regeneration of the catalytically active species 393 and formation of the observed enamide products. An intermolecular carborhodation event was proposed to occur in a three-component coupling of arylboronic acids with disubstituted alkynes in the presence of methyl acrylate (Scheme 10.135) [113]. 

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