Ligand-free reactions copper-catalysed

When the copper-catalysed reactions (type A and type B) were performed in the cavity of an ESR spectrometer, the presence of phenyl radicals was detected by way of their spin adducts with 2-methyl-2-nitrosopropane and with 2,4,6-tribromonitrosobenzene.98 In view of these observations, Dodonov et al. suggested that a free-radical mechanism was involved in both reaction types, A and B, and explained the formation of the hypervalent copper (HI) intermediate. (Scheme 6.23) The copper (HI) intermediate is formed by two consecutive one-electron oxido-reduction elementary steps. The copper (1) catalytic species is first oxidised to a copper(Il) species which is then oxidised by a phenyl rascal to the active copper (HI) intermediate. This hypervalent species then undergoes a ligand coupling reaction with the substrate, either hydroxylic or an amino derivative. In the type B reaction, the in situ generated phenylcopper (III) diacetate reacts with the substrate to eventually afford the O- or the iV-phenyl derivative. 

The copper-catalysed reactions are not so well developed but aryl bromides combine with primary amines catalysed by Cu(I) using the salicylamide ligand 115. Free OH groups are tolerated in either partner the amine or the aryl halide.33... 

While this ligand-free copper-catalysed Mizoroki-Heck-type reaction required relatively high temperatures of 150 °C [19], the use of DABCO (9) as ligand allowed for significantly milder reaction conditions [20]. Thereby, satisfying isolated yields were even achieved for orf/jo-substituted electron-rich aryl iodides and alkenyl bromides (Scheme 10.4). However, aryl bromides, particularly electron-rich ones, were converted only sluggishly. 

Simple bis(oxazoline) ligands, especially azabis(oxazolines), can catalyse the addition of indoles to benzylidene malonates in up to 99% ee, provided that excess of the chiral ligand is avoided.166 The paradigm followed in many asymmetric catalytic reactions that an excess of the chiral ligand with respect to the metal should improve enantioselectivity because the background reaction catalysed by a free metal is suppressed, was shown not to be applicable here,166 which might call for revisiting some of the many copper(II)-bis(oxazoline)-catalysed processes known. Enantioselective additions of pyrroles and indoles to ,/9-unsaturated 2-acylimidazoles catalysed by the bis(oxazolinyl)pyridine-scandium(III) triflate complex have been accomplished.167... 

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