Alkynes, formation from coupling

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

A mechanism based on the intermediate formation of an 7 -benzene complex derived from coupling of the alkyne with the iridacyclopentadiene moiety has been proposed on the basis of the isolation of [(ttiphos)Ir( -C6H6)]PF5 441 from the reaction of [(triphos)IrCI(CFI=CFI-CH=CH)] 442 with acetylene in the presence of TIPF6. In O Connor s system, it is conceivable that the pending ester function in the putative -arene intermediate may drive the reaction toward the C-H activation and the formation of aryl(hydride) species 440 and 441. ... 

The result of zirconium-mediated selective pyridine formation from two different alkynes and a nitrile is summarized in Table 11.2 [18]. This method via azazircona-cyclopentadiene could provide 39a or 39b selectively by changing the addition order of the alkynes (entries 1 and 2). When both alkynes are unsymmetrical, if the first alkyne is a trimethylsilylalkyne, coupling with a nitrile is regioselective. A trialkylsi-lyl group-substituted carbon occupies the a-position in azazirconacycles. This led to selective substituted pyridine formation (entries 3 and 4). 

Six-membered heterocycles were obtained from two different alkynes and other unsaturated organic substrates involving C=0 and C=N moieties. The Reppe-type cyclotrimerization can be also applied for preparation of pyridine derivatives when one of the alkynes is replaced by a nitrile. The pyridine formation from two alkynes and a nitrile with Co complexes was originated by Wakatsuki and Yamazaki [77]. Although this method is effective, there is a critical problem for the selective intermolecular coupling of two different alkynes with a nitrile. As shown in Eq. 67, two isomers of pyridine derivatives are formed when a metallacyclopentadiene reacts with a nitrile, due to the two possible orientations of the nitrile in its coupling with the unsymmetrically substituted metallacyclopentadienes. 

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