Ligands for copper

We have tecently discovered that pbospbotamidile 18 is also an excellent ligand for copper-catalyzed kinetic resolution of chital 2-cydobexenones fScheme 7.15). Chi-... 

Table 9.5 Structures of Ligands for Copper Based ATRP Catalysts...

Glos and Reiser [23] introduced aza-bis(oxazolines) as new chiral ligands for copper and palladium catalysts. Because of the structural flexibility of these compounds they also prepared an immobilized ligand by covalent grafting to methoxypoly(ethyleneglycol) (structures 14 and 15 in Scheme 9). 

Jacobsen et al. reported that a different type of dintrogen ligand (48), fe[(2,6-dichlorophenyl)-methylideneaminojcyclohexane, was an efficient chiral ligand for copper-mediated asymmetric aziridination (Scheme 35).154 The reactions of conjugated c/.v-olefins show high enantioselectivity with this catalyst, but enantioselectivity of the reactions of simple olefins such as styrene and indene is moderate. 

The gallium and indium complexes of novel bis(thiosemicarbazones) have been investigated.71-73 Initial publications indicated a more complex stoichiometry where X-ray crystal structures suggested either dinuclear or trinuclear complexes. Whereas these complexes are quite interesting, they are less likely to result in useful nuclear medicine radiopharmaceuticals. The bis(thiosemi-carbazones) are quite useful as ligands for copper, and are discussed in more detail vide infra). 

Amino 4,6-dihydroxypyrimidine is a useful ligand for copper-catalysed arylations of other heterocyclic NH <06JOC8324>. 

A number of stable heterobimetallic copper alkyne complexes have been reported, based on the strategy of using another metal bis(alkynyl) complex as a chelating ligand for copper. The 1,4-diyne [(r -CsFGSiMe Ti-(C=GSiMe3)2]180 (or related complex) was found to stabilize the copper units GuX, with X = alkyl,180,181 vinyl,180... 

A technique for the determination of free cupric ions in seawater has been described by Sunda and Hanson [332], The method is based on sorption of copper onto Sep-Pak Cis cartridges and internal free cupric ion calibration. Calibration is accomplished by adding cupric ion buffers and EDTA, which competes with natural organic ligands for copper complexation. The method was used to establish that 0-2% of the copper occurs as inorganic species and 98-100% occurs as organic complexes. 

Helmchen and co-worker investigated the use of phosphinooxazolines as ligands for copper(II) catalyzed Diels-Alder reactions (Scheme 19) (214). Optimal selectivities are found for a-naphthyl-substituted phosphinooxazoline (299). These catalysts require 2.5 h to induce complete conversion to cycloadduct, compared to 18 h using the triflate complex 269c under identical conditions. Helmchen invokes a square-planar metal geometry to explain the stereochemistry of the adducts, similar to the model proposed by Evans. He suggests that the bulky phosphine substituents are required to orient binding of the dienophile in such a way as to place the olefin directly below the terf-butyl substituent on the oxazoline. 

Buono and co-workers (215) suggest that the increased activity of mixed P,N-ligands compared to bis(oxazohnes) is due to the ability of phosphorus to behave as a o donor and n acceptor. With this in mind, Buono and co-workers designed chiral quinoline-phosphine 302 as a ligand for copper-catalyzed Diels-Alder reactions. Acryloylimide and cyclopentadiene afford high selectivity of the cycloadduct in the presence of a catalytic amount of 302 Cu(OTf)2, Scheme 20. Buono invokes a square-planar metal center to account for the observed selectivity. 

Phosphoramidites 13, derived from 2,2 -binaphthol, proved to be versatile ligands for copper-catalyzed 1,4-additions of Et2Zn to chalcone and 2-cyclohexenone (Scheme 7.9) [37]. 

We have recently discovered that phosphoramidite 18 is also an excellent ligand for copper-catalyzed kinetic resolution of chiral 2-cyclohexenones (Scheme 7.15). Chiral 2-cyclohexenones are attractive building blocks for a variety of natural products, but their synthesis usually requires multistep routes from chiral starting materials [65]. The development of the new kinetic resolution was the product of two impor-... 

Likewise, PEG-supported bisoxazoline (40) can be used as a ligand for copper-mediated enantioselective reactions such as cyclopropanations of alkenes, [2-1-4] cycloadditions as well as ene reactions. Best results were obtained in case of the latter reactions as products were formed in yields up to 96% and ee s up to 95% (Scheme 4.25) [117]. 

Anthracene can be selectively oxidized to anthraquinone by molecular oxygen in the presence of copper(II) bromide in an ethylene glycol solution. Ethylene glycol (EG) acts as a bidendate ligand for copper and prevents the formation of bromoanthracene as a by-product (equation 267).596... 

A parallel development during this period was the identification of chiral bis(oxazolines) and bis(benzylidene)diaminocyclohexanes as useful ligands for copper-catalyzed, asymmetric tosylaziridination reactions with Phi = NTs (Scheme 64) [174,175]. Evidence for the likely formation of copper(III)-nitrene intermediates,cCu(III) = NTs , in such reactions was also reported [176]. 

Water was preferred to acetonitrile as the solvent, allowing moderately hindered and anionic amines to self-assemble. Acetonitrile is a much better ligand for copper(I) than water, making it more difficult for hindered ligands (such as the one formed from serinol, third entry in Table 1.1) to form complexes in competition with the solvent. More hindered amines as well as cationic amines were not incorporated in either solvent, which we attribute to steric and Coulombic repulsion, respectively. 

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