Rhodium complexes oxazoline

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Inspired by the chiral phosphine/oxazoline ligands developed by Helmchen and Pfaltz [131], Crudden and coworkers, have prepared a chiral NHC-oxazoline possessing a rigid backbone (Fig. 14) [ 132 ]. The rhodium complex 74 has been used in the catalytic hydroboration of olefins and the hydrosilylation of prochiral ketones with enantiomeric excesses that did not exceed 10%. 

Using amphiphilic poly(2-oxazoline) derived block co-polymer backbones, immobilized palladium and rhodium carbene NHC complexes have been prepared for use in C-C couplings and hydroformylation, respectively. For the synthesis of the supported rhodium complex, the NHC ligand was attached to prefabricated support This was... 

Oxazolin-2-ones 88 are formed when a-hydroxyamides 85) (R, R = alkyl or aiyl) are treated with the cumulated phosphorus ylide 86. The reactions are thought to proceed via the intermediates 87 <97LA217>. The diazomalonylurea derivative 89 is converted into the betaine 91 in the presence of a catalytic amount of dirhodium tetraacetate, presumably by way of the rhodium complex 90 (L = ligand). The betaine reaiTanges to the isomiinchnone 92, which reacts... 

The imidazolium salts depicted in Scheme 15.12 were subsequently coordinated as carbene-oxazoline ligands to rhodium(I) and palladium(II). The rhodium complex was tested in the hydrosilylation of ketones, giving the secondary alcohols with poor enantioselectivity [49]. 

In view of these previous developments, we directly coupled various N-substituted imidazoles, which display nucleophilic reactivity, with 2-bromo-oxazolines to give the imidazolium precursors (Scheme 15.15) [55]. This direct condensation of an oxazoline and an imidazole salt provided a straightforward and modular route to the development of a new family of stereodirecting ligands. NHC - rhodium complexes could be obtained by reaction of the imidazolium salt with [ Rh(p-OtBu)(nbd))2] (nbd, norbornadiene) generated iw sifn [56]. 

The same group [97] studied the corresponding rhodium oxazoline complexes that led to very similar results in terms of activity (81% yield) and enantioselectivity (68%), however. 

In addition to palladium catalysts, Co(OAc)2 shows a catalytic activity for the arylation of heterocycles, including thiazole, oxazole, imidazole, benzothiazole, benzoxazole, and benzimidazole.78 As shown in Scheme 6, the catalytic system Co(OAc)2/9/Cs2C03 gives G5 phenylated thiazole, while the bimetallic system Co(OAc)2/CuI/9/Cs2C03 furnishes the G2 phenylated thiazole. The rhodium-catalyzed reaction of heterocycles such as benzimidazoles, benzoxazole, dihydroquinazoline, and oxazoline provides the arylation product with the aid of [RhCl(coe)]2/PCy3 catalyst.79 The intermediacy of an isolable A-heterocyle carbene complex is proposed. 

Various X-ray crystal structures of metal-ligand complexes provided evidence of the geometry of the complexes in the solid state, even though the structure of these complexes may differ in solution. The hrst crystal structure of a bis(oxazoline)-metal complex was determined in 1994 by Brown and co-workers. " This group crystallized and elucidated the structure of V,V-bis-[2-((45)-(methyl)-l,3-oxazoli-nyl)]methane-bi(ri ethene)rhodium(I), 18a, as depicted in Figure 9.3. The key features of this crystal structure include the C2-axis of symmetry, the axial positions of the methyl groups and the orientation of the ethene molecules, orthogonal to the complexation square plane. In 1995, Woodward and co-workers were able to crystallize and determine the structure of benzylbis(oxazoline) with ruthenium... 

The tridentate BoxCarb ligand was used to synthesise the corresponding carbene complexes of silver(I), rhodium(III) and palladium(II) [101]. Interestingly, the potentially tridentate BoxCarb ligand coordinates in a bidentate fashion only in the case of the silver(l) complex and it is the linker-free oxazoline ring that remains pendant. The coordination... 

Enantioselective hydrosilylation of acetophenone using either bis[(45)-(l-methylethyl)oxazolin-2-yl]methane or its rhodium(I) complex has been reported, although the enantioselectivity was only 12% (eq 8). ... 

While copper and iron Lewis acids are the most prominent late transition metal Diels-Alder catalysts, there are reports on the use of other chiral complexes derived from ruthenium [97,98],rhodium [99],andzinc [100] in enantioselective cycloaddition reactions, with variable levels of success. As a comparison study, the reactions of a zinc(II)-bis(oxazoline) catalyst 41 and zinc(II)-pyridylbis(ox-azoline) catalyst 42 were evaluated side-by-side with their copper(II) counterparts (Scheme 34) [101]. The study concluded that zinc(II) Lewis acids catalyzed a few cycloadditions selectively, but, in contrast to the [Cu(f-Bubox)](SbFg)2 complex 31b (Sect. 3.2.1), enantioselectivity was not maintained over a range of temperatures or substitution patterns on the dienophile. An X-ray crystal structure of [Zn(Ph-box)] (01)2 revealed a tetrahedral metal center the absolute stereochemistry of the adduct was consistent with the reaction from that geometry and opposite that obtained with Cu(II) complex 31. 

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