Carbometallation of alkynes

Trost reported the synthesis of 1,4-dienes with ruthenium catalysis through regioselective carbometallation of alkynes with alkenes.51 Di- and trisubstituted olefins can also be obtained with arylboronic acids through an intermolecular process under rhodium,30 52 55 nickel,56 and palladium catalysis.57 Recently, Larock has reported an efficient palladium-catalyzed route for the preparation of tetrasubstituted olefins.58,59... 

Attempts to use the isobutyl group in the carbometalation of alkynes only give rise to hy-drometalated products, but ethyl and n-propyl groups can be successfully transferred from the corresponding dialkylaluminum chlorides. The regioselectivity in these reactions is lower than that for the methyl transfer. Indeed, the reaction mechanism may be different from that of methylalumination [62]. 

Scheme 2.48. Intermolecular and intramolecular carbometalation of alkynes with copper-zinc reagents.

The carbometallation of alkynes is also unique in its stereochemical pattern, since it is executed as an exclusive cis addition. For example, the stereoisomeric purity of pheromone 104, prepared as shown, is higher than 99.9% This feature is of special importance in the synthesis of natural pheromones, as the biological activity of these compounds is dramatically affected by the presence of an even negligible amount (less than 0.5% ) of the undesirable stereoisomer. Not surprisingly, this reaction has found numerous areas of application, especially in the stereospecific synthesis of the tri- and tetrasubstituted alkenes, a goal difficult to achieve by other methods. To illustrate the effectiveness of this approach, the synthesis of faranal 105, the trail-marking pheromone of the ant species Monomoriumpharaonis, is shown in Scheme 2.33. 

The central problem in the synthesis of acyclic polyunsaturated compounds (for example, many pheromones) is the construction of an aliphatic chain possessing double bonds of the required configuration in specified positions. The strategy of synthesis in this area could be based either at the identification of unsaturated synthons with double bonds in the needed configuration (as was done in the synthesis of the juvenile hormone, see Section 2.8), or the employment of stereoselective reactions in the process of chain construction, such as carbometallation of alkynes (see Section 2.3.3). 

Generation of stereo- and regio-defined alkenylmetals via hydrometallation or carbometallation of alkynes followed by cross-coupling (Scheme 1-11) is a synthetically attractive notion for preparing arylated alkenes, conjugated dienes, and conjugated enynes. Its feasibility was demonstrated in 1976 [14,15] in the prototypical examples shown in Schemes 1-4 and 1-5. In these processes, hydroalumination was employed for generating the required alkenylmetals. 

Carbometalation of alkynes is a widely used method for the synthesis of stereodefmed di- and trisubstituted alkenes. These structural features are often encountered in... 

Carbometallation of alkynes by Cp2TiMe2 affords vinyl complexes which serve as intermediates for the formation of titanacyclobutenes (Scheme 546). Alkyne insertion to form the vinyl species followed by oxidative addition into the 7-C-H bond and reductive elimination of methane is proposed.1437... 

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