Copper catalysts bidentate ligands

Hanson, P., Rowell, S. C., Walton, P. H., Timms, A. W. Promotion of Sandmeyer hydroxylation (hemolytic hydroxydediazoniation) and hydrodediazoniation by chelation of the copper catalyst bidentate ligands. Org. Biomol. Chem. 2004, 2, 1838-1855. 

The most common catalysts for ATRP are complexes based on a copper(T) halide and nitrogen based ligand(s). Various ligands have been employed and those most frequently encountered are summarized in Table 9.5. Typically, four nitrogens coordinate to copper. The bidentate bipyridyl (bpy) ligands 132-133 are known to form a 2 1 complex. The tetradentate ligands are expected to form a 1 1 complex. 

It has been found that a number of bidentate ligands greatly expand the scope of copper catalysis. Copper(I) iodide used in conjunction with a chelating diamine is a good catalyst for amidation of aryl bromides. Of several diamines that were examined, rra s-yV,yV -dimethylcyclohexane-l,2-diamine was among the best. These conditions are applicable to aryl bromides and iodides with either ERG or EWG substituents, as well as to relatively hindered halides. The nucleophiles that are reactive under these conditions include acyclic and cyclic amides.149... 

Simple bidentate ligands involving dipyridyl- or dipyrimidylamino fragment (L23) form Pd catalysts of moderate activity for the cross-coupling of terminal acetylenes (copper-free reaction) or arylboronic acids. Supported versions of such ligands were also reported (see Chapter 9.9 for more details about supported catalysts).449,450... 

The use of copper catalysts based on chiral phosphorus ligands to assist 1,4-additions of dialkylzinc reagents has in recent years produced major breakthroughs, with excellent enantioselectivities. A number of monodentate and bidentate phos-phoramidites, phosphites, phosphonites, and phosphines are now available as chiral ligands for alkyl transfer to a variety of cyclic and acyclic enones. So far. 

As for ACP, from 1986 to 1991, several nitrogen ligands of bidentate corrin mimics and bis(oxazoline) were successfully developed as chiral ligands with copper catalysts [23,24]. Pybox families reported by Singh et al. [25] resulted in good-to-excellent activity for ACP in combination with copper. 

Copper salts have become crucial additives for bond forming reactions between carbon and heteroatoms such as N, O, S, or P. While many involve a transmetalation see Transmetalation) step from Pd, Ni, Sn, B, Te, Se, Mg, and so on, several of these cross-coupling see Coupling) reactions occur with copper as the only metal present. Typically, a (bidentate) ligand is used for stabilization of a copper-containing intermediate, while enhancing reactivity of the copper catalyst. 

Many other chiral copper catalysts have been reported, most of them being derived from C2-symmetrical bidentate nitrogen ligands [13, 27]. Some ligands such as the bipyridine derivatives 14 [60, 61, 62], the diamine 15 [63] and the bis(azaferrocene) 16 [64] are capable to induce high ees, but none of them can compete so far with chiral bisoxazolines in terms of high selectivity combined with effectiveness, general applicability and ease of preparation. 

In every case for copper catalyst 31, the absolute stereochemistry of the cycloadducts is accounted for by the intervention of the substrate-catalyst complex depicted in Fig. 23, in which the s-cis configured dienophile is bound to the catalyst in the plane of the ligand in a bidentate fashion. The ferf-butyl group shields the top face and cycloaddition occurs from the exposed si enantioface. Support for this model derives from X-ray structures of aquo complexes of catalysts 31a and 31b which show that the complex possesses a distorted square planar geometry EPR spectroscopy on the binary catalyst-dienophile complex indicates that this geometry carries over from the solid state into solution. Calculations at the PM3 level of theory further favor the indicated reactive assembly [85]. 

The hydrosilylation of carbonyl confounds with Et3SiH (eq 4) has also been the subject of additional research. Owing to these efforts, carbonyls can now be directly converted to their tri-ethylsilyl (TES) ethers with copper catalysts in the company of a bidentate phosphine or Al-heterocycUc carbene ligand. Tri-ethylsilyl ethers can also be made from carbonyl compounds and EtsSiH in the presence of rhenium(V) oxo-complexes. ... 

Importantly, a copper catalyst modified with N,N-bidentate ligand 1,10-phenan-throline (115) enabled the direct arylation of electron-deficient fluoroarenes (Scheme 9.41) [57]. The high catalytic efficacy allowed also for the use of aryl bromides as electrophiles, and K3PO4 as mild base. 

A bidentate N-heterocyclic carbene (NHC) ligand was found to be efficient for the asymmetric allylic alleviation employing diethylzinc (Scheme 15.22). In the reaction, a binuclear silver complex undergoes facile ligand exchange with a copper salt to afford an effective copper catalyst. y-Selective allylic alleviation is realised with allevlboranes as nucleophiles that are easily... 

Catalytic hydroboration of terminal allg nes with B2(pin)2 is achieved by employing a copper catalyst. Products are obtained regioselectively by employing a suitable NHC ligand (Scheme 15.34). A bull bidentate phosphine ligand is effective for regioselective hydroboration of aryl-substituted... 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

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