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Bidentate activation

Use of a Cinchona alkaloid in which the hydroxy group had been acylated resulted in formation of cycloadducts of low enantiomeric excess, leading the authors to conclude that bidentate activation (Fig. 32) was important in providing... [Pg.1158]

Based on the natural occurring amino acid L-histidine, LaRonde and Brook developed a similar anionic activator. Instead of using the amino alcohol, they use the L-histidine dilithium salt 15 prepared in situ from L-histidine and 2 equivalents of butyllithium. In Si-NMR studies, only pentacoordinated silicon species could be observed therefore, a monodentate activated complex I6a and 16b or a bidentate activated complex 16c with the loss of one alkoxy substituent on the silicon was proposed (Scheme 32.13). ... [Pg.999]

Different main-group-, transition- and lanthanide-metal complexes can catalyze the cycloaddition reaction of activated aldehydes with activated and non-activated dienes. The chiral metal complexes which can catalyze these reactions include complexes which enable substrates to coordinate in a mono- or bidentate fashion. [Pg.164]

One of the problems related to the Lewis acid activation of a,/ -unsaturated carbonyl compounds for the reaction with a nitrone is the competitive coordination of the nitrone and the a,/ -unsaturated carbonyl compound to the Lewis acid [30]. Calculations have shown that coordination of the nitrone to the Lewis acid can be more feasible than a monodentate coordination of a carbonyl compound. However, this problem could be circumvented by the application of alkenes which allow a bidentate coordination to the Lewis acid which is favored over the monodentate coordination. [Pg.322]

Pyridine-based N-containing ligands have been tested in order to extend the scope of the copper-catalyzed cyclopropanation reaction of olefins. Chelucci et al. [33] have carefully examined and reviewed [34] the efficiency of a number of chiral pyridine derivatives as bidentate Hgands (mainly 2,2 -bipyridines, 2,2 6, 2 -terpyridines, phenanthrolines and aminopyridine) in the copper-catalyzed cyclopropanation of styrene by ethyl diazoacetate. The corresponding copper complexes proved to be only moderately active and enantios-elective (ee up to 32% for a C2-symmetric bipyridine). The same authors prepared other chiral ligands with nitrogen donors such as 2,2 -bipyridines 21, 5,6-dihydro-1,10-phenanthrolines 22, and 1,10-phenanthrolines 23 (see Scheme 14) [35]. [Pg.104]


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See also in sourсe #XX -- [ Pg.432 , Pg.435 ]




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