Symmetric diimine ligands

On the other hand, a Gu(i)/chiral diimine complex in the cyclic ylide formation/[2,3]-sigmatropic rearrangement has been examined by Clark and co-workers (Equation (3)). Up to 57% ee has been obtained when C2-symmetric ligand 32 is used. 

The complex [Ni(2,3-Me2[14]-l,3-diene-l,4,8,lI-N4)] [ZnCU] is square planar and low-spin. The visible spectra show bands near 21.3 kK (characteristic of square planar nickel(II)), near 26.1 kK (due to the imine functions), and near 35.1 kK. The infrared spectra of all of the nickel complexes prepared show absorptions near 3195 and 1595 cm assignable to the N—H stretching vibration and to the symmetric imine vibration, respectively. A strong sharp band also occurs near 1210 cm and is characteristic of the a-diimine function. The NMR spectrum of the perchlorate complex in nitromethane shows a methyl singlet at 2.33 ppm. The ligand can be hydrogenated on nickel(II) with Raney nickel and hydrogen to produce the fully saturated macrocyclic complex [Ni(2,3-Me2[14]-ane-1,4,8,1 1-N4] ... 

In simplest terms, the incorporation of only two structural properties into the Hgand system is required for attainment of good enantioselectivity in olefin epoxidation by [Mn (salen)] complexes (i) a dissymmetric diimine bridge derived from a C2-symmetric 1,2-diamine and (ii) bulky substituents on the 3-and 3 -positions of the salicylide ligand. This is illustrated in Table 1 for the epoxidation of czs-P-methylstyrene by various [Mn (salen)] catalysts. 

The choice of the most effective catalyst system is highly dependent on the type of olefin under consideration. Polymerization of CO and aliphatic a-olefines is most suitably carried out employing a catalytic system modified with a symmetrical, Cs-bridged aryldiphosphine ligand (2,71). However, these systems are not suitable for copolymerization of CO and styrene (41). For this reaction palladium(II)-based catalysts modified with a conjugated diimine (39,41), a bisoxazoline (43,44), a phosphine-phosphite (43), or a phosphine-imine ligand (43) have been employed, in, under chain-transfer conditions, combination with an oxidant promotor, such as 1,4-benzoquinone or 1,4-naphthoquinone (39-47,72), or a polar, acidic type of solvent (73,74). 

Rose, J.M., Deplace, F., Lynd, N.A. et al. (2008a) C2-Symmetric Ni(II) g-diimines featuring cumyl-derived ligands Synthesis of improved elastomeric regioblock polypropylenes. Macromolecules, 41,9548-9555. 

By comparison of the a-diimine catalysts with the phosphine sulfonate-based palladium catalyst system, the change from a symmetric to an asymmetric ligand stmcture is apparent on first sight. Here (as well as in other ligand systems Section 3.24.4.1.4), it could be shown that this leads to major differences in reactivity. Despite the partially open catalyst stmcture, these complexes are of remarkable stability and activity in... 

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