Ligands mesityl

Finally, some orgatiocopper compounds undergo diarge disproporLionation under die influence of ligands dial bind strongly to copper. Treatment of mesityl-copper witli 1,2-bis-idipbenylpbospbinoJediane iDPPE), for example, results in tlie fotniation of bisimesityljcopper anions and a copper cation to wbidi four pbos-pborus atoms of two DPPE molecules ate coordinated [75]. 

The hydrosilylation of acetophenone by diphenylsilane in CH2CI2 at rt was used as a test reaction to compare the selectivity obtained with the carbene ligands (Scheme 36). The reactions were performed in the presence of a sUght excess of AgBp4 (1.2% mol). In these conditions, the N-mesityl-substituted catalyst 57c (1% mol) gave the highest selectivity (65% ee). The in situ formation of square-planar cationic rhodium species 58 as active catalysts appears to be crucial since the same reaction performed without silver salt gave both poor yield (53%) and enantioselectivity (13%). 

These different catalysts were first tested in the desymmetrization of the achiral triene 63 (Scheme 40). The best enantioselectivities (up to 39%) were obtained with complexes 60b and 61c-d bearing carbene ligands derived from 1,2-diphenyl-1,2-diaminoethane (R = Ph). ligands derived from 1,2-diaminocyclohexane gave poor enantioselectivities (< 9% ee). Replacement of the mesityl group in complex 60b by o-methyl- or o-isopropylphenyl groups (complexes 61c-d) slightly increases the enantioselectivity (from 13... 

Subsequently, these catalysts were evaluated in the enantioselective desymmetri-sation of achiral trienes, and three distinct trends in catalyst selectivity were found. Firstly, catalysts 56a-b with two phenyl moieties on the backbone of the A -heterocycle exhibited higher enantioselectivity than those with a fused cyclohexyl group as the backbone 55a-b. Secondly, mono-ort/io-substituted aryl side chains induced greater enantioselectivity than symmetrical mesityl wing tips. Thirdly, changing the halide ligands from Cl to I" increased the enantioselectivity. As a result, catalyst 56b turned out to be the most effective. For example, 56b in the presence of Nal was able to promote the desymmetrisation of 57 to give chiral dihydrofuran 58 in up to 82% conversion and 90% ee (Scheme 3.3). 

The ease of dehalogenation of C H X by Ni(ll)/ IMes HCl 1/NaO Pr decreased in the order 1 > Br > Cl F. Subsequent work showed that a 1 1 combination of Ni and NHC in the presence of NaOCHEt resulted in enhanced reactivity towards aryl fluorides [6], Again, the A-mesityl substituted ligand IMes HCl 1 imparted the highest level of catalytic activity. Table 8.2 illustrates that hydrodefluorination is sensitive to both the nature of the substituents on the aromatic ring and the specific regioisomer. Thus, 2- or 4-fluorotoluene (Table 8.2, entry 2) proceeded to only 30% conversion after 15 h, whereas quantitative conversion of 2-fluoroanisole (Table 8.2, entry 3) and high conversion of 3-fluoropyridine (Table 8.2, entry 5) was achieved in only 2-3.5 h. The reactivity of 2-fluoropyridine was compromised by more efficient nucleophilic aromatic substitution. 

NHC ligands with a pendant group that enforces chelation have also been coordinated to copper centers. The reaction of Cu20 with pyridine fV-functionalized carbene ligand led to the formation of several compounds.91 In the case of mesityl derivatives, a dinuclear complex with a weak metal-metal interaction was isolated 60,91 whereas for the bulkier 2,6-diisopropylphenyl group, a monomeric complex was formed and characterized 61 (Figure 25).91... 

Dimethyl- and diethylzinc reacted with dimethylpyrrole- and mesityl-substituted cyclopentadiene ligands to give monocyclopentadienyl(methyl)zinc and -ethyl(zinc) compounds. These products then formed, Scheme 20, crystalline adducts with tetramethylethylenediamine (TMEDA) such as 24, whose solid-state structure is shown in Figure 11. 

Using a similar strategy, Dias and coworkers were able to prepare classical AgI-CO and AgJ-C2H4 complexes supported by the mesityl-substituted tris(pyrazolyl)borate ligand L9 (Fig. 11) (102). The isolation of such complexes is a challenging task because silver(I) is a poor u-acceptor, a poor 7t-donor, and in... 

The more sterically demanding phosphide KPH(mes ) crystallizes solvent-free, even from solutions containing THF, as the ladder polymer [K PH(mes ) ]x (25) (72). Coordination about each K atom is completed by multihapto interactions (approximately if ) between the cation and the mesityl ring of an adjacent phosphide ligand. A... 

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