Enolates copper-containing

Jorgensen s group44a carried out the reaction using the anhydrous form of chiral bis(oxazoline) coordinated copper complex. Complex 106 containing 83 as the chiral ligand was found to be the most effective. As shown in Scheme 5-32, the asymmetric hetero Diels-Alder reaction of //.y-unsaturated a-keto esters with acyclic enol ethers results in products with excellent yield and enantioselectivity. 

Mukiayama aldol reactions between silyl enol ethers and various carbonyl containing compounds is yet another reaction whose stereochemical outcome can be influenced by the presence of bis(oxazoline)-metal complexes. Evans has carried out a great deal of the work in this area. In 1996, Evans and coworkers reported the copper(II)- and zinc(II)-py-box (la-c) catalyzed aldol condensation between benzyloxyacetaldehyde 146 and the trimethylsilyl enol ether [(l-ferf-butylthio)vinyl]oxy trimethylsilane I47. b82,85 Complete conversion to aldol adduct 148 was achieved with enantiomeric excesses up to 96% [using copper(II) triflate]. The use of zinc as the coordination metal led to consistently lower selectivities and longer reaction times, as shown in Table 9.25 (Eig. 9.46). 

Diketones (8, 126 127). Complete details of the synthesis of 1,4-diketones by oxidative coupling of ketone enolales and trimethylsilyl enol ethers with Cu(OTf)2 are available.1 Use of isobutyronitrile is essential for the coupling it is not only a suitable solvent, but the nitrile group apparently facilitates reduction of the intermediate copper enolate to CuOTf.2 When acetonitrile is used by-products containing a nitrile group are formed. 1,4-Diketones are formed only in traces when DMF, DMSO, or HMPT is used. 

In ammoniacal solutions of copper salts, the oxidation products are likely to contain nitrogen thus, hexoses give oxalic acid, imidazoles, hydrogen cyanide, and urea. Kinetic studies have been reported for the reaction of Cu(II) in the presence of ammonia with maltose, lactose, melibiose, and cellobiose.190 For the oxidation by tetraamminecopper(II) in ammoniacal and buffered media the rate of reaction is first order in disaccharide concentration, order one-half in ammonia concentration, but it is independent of Cu(II) concentration. The reaction rate is decreased by the addition of ammonium chloride, because of the common ion effect. These kinetics suggested mechanisms involving an intermediate enediolate ion, with the rate of reaction being equal to the rate of enolization.191 A similar mechanism has been proposed for the oxidation of D-fructose by a copper-pyridine complex in an excess of pyridine.192... 

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