Copper catalysts high-activity ligands

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

Rates of degradation observed in the presence of ionic iron and copper systems have been compared with those obtained for the respective acetylacetonate chelate systems in Tables X and XI. Lower relative lifetime and relative stability values are observed for the copper(II) acetylacetonate catalyzed system than those obtained in the presence of higher concentrations of the ionic copper species. A similar increase in the catalytic efficiency of copper upon coordination has been reported by Ericsson et al. (10). However, iron(III) acetylacetonate shows no catalytic effect at all. This observation of contrary effects on the stability of paper with the same chelates of two highly active transition metal catalysts is most interesting. Unlike the relatively stable octahedral iron(III) acetylacetonate molecule, the tetrahedral, tetracoordinate copper(II) chelate could accept two more ligands if it were to assume an... 

The catalytic activity of ruthenium complexes for the cyclopropanation of olefins and diazoacetates has been well investigated and, depending on the ligands utilized, the complexes have a high potential to produce high yields, stereoselectivities and enantioselectivities that are almost comparable to those of rhodium or copper catalysts [35], Moreover, related carbene complexes of ruthenium have been isolated in order to clarify the mechanism of cyclopropanation [32-34], It is likely that further improvements in these reactions will lead to the development of industrial processes utilizing cyclopropanation. 

Since then, a whole host of catalysts have been pubhshed, e.g. by Masamune, Kanemasa, Scott and Itagaki, for the enantioselective [2+l]-cycloadddition, but only a few give good yields for the reaction of 2,5-dimethylhexa-2,4-diene and the readily available t-butyl diazoacetate. [76] In the meantime, systematic optimisation work at Sumitomo has made highly active catalysts available, not only of the copper-salicylaldimine type, but also with bisoxazohne ligands, which enable excellent yields and selectivities. 

More recently, the asymmetric hydrosilylation of aryl ketones and aryl imines has been developed using copper catalysts. " In this case, axially chiral biaryl bisphos-phine ligands boimd to copper generate remarkably active catalysts for tihe hydrosilylation of ketones. These reactions occur with high selectivity using the hydrosilane polymer... 

Several Cu(I) complexes with N-heterocyclic carbene ligands have been described as CuAAC catalysts at elevated temperature in organic solvents, under heterogeneous aqueous conditions (when both reactants are not soluble in water), and under neat conditions [75]. These catalyst show high activity under the solvent-free conditions, achieving turnover numbers as high as 20 000. However, their activity in solution-phase reactions is significantly lower than that of other catalytic systems (for example, a stoichiometric reaction of the isolated copper(I) acetylide/NHC complex with benzhydryl azide required 12 h to obtain 65% yield of the product [76], whereas under standard solution conditions even a catalytic reaction would proceed to completion within 1 h). 

---