Dipeptide-based ligand

A few vinylzinc reagents have been added to aldehydes in the presence of chiral sulfur-containing ligands. As an example, Seto et al. reported, in 2005, the synthesis of dipeptide A-acylethylenediamine-based ligands by parallel... 

Scheme 3.66 Dipeptide A -acylethylenediamine-based ligand for additions of alke-nylzinc reagents to aldehydes.

The role of alkali metal cations in the [ RuCL(p-cymene) 2l-pseudo-dipepLide-catalysed enantioselective transfer hydrogenation of ketones with propan-2-ol has been examined. Lithium salts were shown to increase the enantioselectivity of the reaction when 2-PrONa or 2-PrOK was used as the base. An alternative reaction mechanism for the pseudo-dipeptide-based systems, in which the alkali metal cation is an important player in the ligand-assisted hydrogen-transfer step, has been proposed.370... 

The last two catalytic systems available are intimately based on the stoichiometric ligands 22 and 23, derived from the dipeptide and the chiral phosphoric acid, respectively. The addition of basic additives to slow down or suppress the background reaction allowed the use of catalytic amounts of the ligand. In his initial report, Shi and coworkers have shown that adding 1 equivalent of ethyl methoxyacetate allowed the catalyst loading to be decreased to 0.25 equiv (equation 96) . Under these conditions, the enantioselectivities are similar to those reported in Figure 7. 

A three-site system for peptide synthesis around a cobalt(III) complex has been studied. Instead of a quadridentate ligand as used in the above experiments, Wu and Busch chose the tridentate ligand diethylenetriamine. The formation of dipeptide and tetrapeptide complexes is shown in Scheme 92.360 The ester carbonyl group in the 0-bonded amide intermediate (127) cannot be activated by coordination because it cannot reach the metal ion. Isomerization to the jV-bonded amide complex (128) occurs with base and enables coordination and therefore activation of the ester carbonyl group. 

Complexes such as (39) are likely intermediates in the reaction of [Co(dien)(OH)(OH2)2]2+ with dipeptides.177,181 Hay and Piplani182 have studied the kinetics of base hydrolysis of a variety of complexes of type (39) and have determined values of kOH for hydrolysis of the peptide bond and the N02 and Cl ligands (Table 17). The rate constant kon for peptide bond hydrolysis falls within the range 0.7 to 7 M-1 s I and is some 104 to 105 times greater than that for the uncomplexed peptide. 

The chemistry already described is reproduced by numerous ligands that have not specifically been addressed in the previous discussion. The V-salicylidenehydrazides (Scheme 4.18a) and related compounds provide a good example. The structure [76] of a typical complex, represented in Scheme 4.18b, is not very different from that proposed for the solution structures of dipeptide complexes (Scheme 4.17). Interestingly, other similar complexes, based on Schiff base-derived ligands, form dimeric [VO]2 core complexes (Scheme 4.1) via two long ( 2.4 A) VO bonds [2], The cyclic core is not necessary for dimer formation, and a dimer can form via a linear VOV bond [77], These complexes otherwise are not significantly different in their vanadium coordination from that depicted in Scheme 4.18b. 

The selected examples by Cole et al. [120] and Shimizu et al. [121] reported the parallel synthesis of a small library of solid supported dipeptide Schiff bases as ligands for the Ti-catalyzed enantioselective addition of trimethylsilyl cyanide to meso epoxides, and the determination of their catalytic activity on different substrates. The catalyzed addition reaction and the general structure of the dipeptide ligands are shown in Figure 7.15. 

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