Alkynes oxidative coupling

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

An alkene-alkyne oxidative coupling to give a metalacyclo-pentene, could be followed by (3 elimination and reductive elimination. 

Alkyne-alkyne oxidative coupling leads to a metalacyclopen-tadiene (metallole). Oxidative addition of R2BSnR3 is then... 

The authors postulated that these reactions can follow two different reaction pathways involving either alkyne-alkene or alkyne-alkyne oxidative couplings as the initial steps. The alkyne-alkene coupling is assumed to give high levels of stereocontrol, while the alkyne-alkyne coupling initiates low selectivity pathways. Therefore, it is anticipated that alkyne substituents which favour alkyne-alkyne couplings will afford low enantioselectivity levels. 

The Glaser reaction is an oxidative coupling of terminal alkynes 1 to yield a symmetrical Z -acetylene 2 the coupling step is catalyzed by a copper salt. Closely related is the Eglinton reaction, which differs from the Glaser reaction mainly by the use of stoichiometric amounts of copper salt as oxidizing agent. 

In the previous sections, a number of Ir(I)-catalyzed cycloadditions were described where the oxidative coupling of unsaturated motifs (e.g. alkynes and aUcenes) provides metallacycles as common intermediates. At this point, mention will be made of three examples of Ir(III)-catalyzed cyclizations. 

Eastmond, R. Johnson, T. R. Walton, D. R. M. Silylation as a Protective Method for Terminal Alkynes in Oxidative Couplings, Tetrahedron 1972,28, 4601. 

There have also been several reports of the cyclization of diynes with amines under the influence of copper(I) chloride (equation 73) (65CB98, 70KGS125, 72TL3487). This is a potentially useful reaction for symmetrically substituted pyrroles, since symmetrical diynes can be obtained by oxidative coupling of alkynes. 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

Oxidative coupling of a terminal alkyne is a particularly easily performed carbon-carbon bond forming reaction, which results in a good yield of the symmetrical diacetylene. A widely used procedure involves the oxidation of the alkyne with air or oxygen in aqueous ammonium chloride in the presence of a copper(i) chloride catalyst (Glaser oxidative coupling). 

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