Head-to-tail homocoupling

Scheme 5.2 Head-to-tail homocoupling of CO, promoted by IrCl(C8Hi4)(PMc3)3 (adapted from...

The role of the unsaturated substrate A=B can be played even by CO2 itself [17]. The head-to-tail homocoupling of two CO2 molecules at the Ir(I) center of IrCl(CgHi4)(PMe3)3 was described by Herskovitz [17] in 1976 and gives the metallacyclic bis-C02 adduct IrCl(C204)(PMe3)3 (Scheme 5.2). 

Palladium catalysts enable the dimerization of functional alkynes with selective head-to-tail coupling in the presence of bulky phosphine ligands, for both homocoupling of terminal alkynes or cross-coupling of mono and disubstituted alkynes [4, 6]. On the other hand, a palladium/imidazolium system gives linear (E)-enynes as the predominant products [7]. 

In this homocoupling, two orientations are possible, head-to-head as in cycle A and head-to-tail as in cycle B. Indeed, exposing the propargyl alcohol 17 to the trisacetoni-trile complex 16 [21], whose ease of dissociation of this ligand makes it a functional equivalent of a highly coordinatively unsaturated ruthenium, effects coupling even... 

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