Alkynes, activation hydrocarboxylation

K. Tani and Y. Kataoka, begin their discussion with an overview about the synthesis and isolation of such species. Many of them contain Ru, Os, Rh, Ir, Pd, or Pt and complexes with these metals appear also to be the most active catalysts. Their stoichiometric reactions, as well as the progress made in catalytic hydrations, hydroal-coxylations, and hydrocarboxylations of triple bond systems, i.e. nitriles and alkynes, is reviewed. However, as in catalytic hydroaminations the holy grail", the addition of O-H bonds across non-activated C=C double bonds under mild conditions has not been achieved yet. 

Rhenium A hydrocarboxylation with high selectivity for anti-Markovnikov addition and predominant (Z)-enol ester product is mediated by ReBr(CO)5 (1 mol%, 110°C, 15 h) [168]. The 7i-activation mechanism proposed by the authors does not fit to the observed anti-Markovnikov selectivity. Iridium The precursor complex [ IrCl(cod) 2] (1 mol%) combined with P(OMe)3 (4 mol%) and Na2C03 (2 mol%) produces a catalyst that adds carboxylic acids to terminal alkynes (toluene, 100°C, 15 h) to give a mixture isomers with variable selectivities, although the Markovnikov product is usually formed in excess (ca 5 1) [169]. The complex []IrCl(cod) 2] (1 mol%) in the presence of Na2C03 (0.6 equiv) is also a catalyst for the transvinylation of vinylacetate with diverse alcohols [170]. 

For alkynes (and in part, allenes), synthetically useful protocols for Markovnikov and anti-Markovnikov selective hydrations, hydroalkoxylations (mainly intramolecular), and hydrocarboxylations are available and find increasing applications in organic synthesis. In the past decade, the research focus on cationic gold(l) complexes has led to new additions to the catalysis toolbox. It can be predicted that a further refining of such tools for alkyne functionalization with respect to catalytic activity and functional group tolerance will take place. 

Catalytic carboxylation of internal alkynes with CO2 has been achieved in the presence of Ni [50] or Cu [51] catalysts, and through a hydrocarboxylative pathway, not involving the intermediacy of a metaUaoxacyclopentenone, but, as previously noted for the hydrocarboxylation of alkenes (see Sect. 5.3), implying the intermediate formation of an M—H catalyticaUy active species, which may easily be generated in the reactimi mixture using diethylzinc [50] or hydrosilanes [51] as hydride transfer agent. 

The hydrocarboxylation of activated unsaturated hydrocarbons with 002 is an estabhshed methodology in organic chemistry to yield either a,P- (for alkynes), P,y-unsaturated a-branched (for aUenes and 1,3 alkadienes), or a-branched carboxyhc acid derivatives (for styrenes)/ but requires the stoichiometric use of reductants (i.e., AlEt3, hydrosilanes, RMgX) or... 

---