Bis-imine ligands

Evans et al. reported that the bis(imine)-copper (II) complex 25, prepared from chiral bis(imine) ligand and Cu(OTf)2, is also an effective chiral Lewis acid catalyst [34] (Scheme 1.44, Table 1.18). By tuning the aryl imine moiety, the bis(2,6-dichlor-ophenylimine) derivative was found to be suitable. Although the endojexo selectivity for 3-alkenoyloxazolidinones is low, significant improvement is achieved with the thiazolidine-2-thione analogs, for which both dienophile reactivity and endojexo selectivity are enhanced. 

Zhang et al. [49] prepared a chiral ruthenium complex coordinated by a pyridine-bis(imine) ligand (structure 43 in Scheme 21). 

Chelating bis(imine) ligands such asA-alkyl(2-pyridyl)methanimine (AlkPMI) have also been successfully utilized in copper-mediated ATRP [74,75,76,77,78]. In... 

Using a similar concept, a square-planar bis(imine)/Cu(OTf)2 complex has been developed, and it has been shown that the 2,6-dichlorophenyl-substituted bis(imine) ligand is, in general, the most effective for the asym metric Diels-Alder reaction of N-acyl oxazolidinones (83-94% ee) [31] (Eq. 16),... 

Cationic copper(II) complex 37 derived from a chiral bis(imine) ligand has also been shown to be an effective catalyst for reactions between cyclopentadiene and acylated thiazolidine-2-thione dienophiles, albeit with slightly lower se-lectivities than for the bis(oxazoline) complex 31 (Scheme 30) [93]. The bis(2,6-dichlorophenylimine) was found to be optimal among a number of electron-rich and -poor aryl imines screened. The reaction exhibits a positive non-linear effect which suggests that the minor ligand enantiomer can be sequestered by the formation of a catalytically less active (l ,l )/(S,S)Cu(II) dimer. 

In addition, preparation of catalysts based on iron and cobalt [21] was also reported. These are complexes of bulky pyridine bis-imine ligands with iron or cobalt that are also activated by methylaluminoxane ... 

The steric environment for iron complexes with tridentate pyridine-bis(imine) ligands is similar to that for the Ni diimine complexes, except that the two metal-bound halogen atoms, which are presumably transformed into a polymerization site upon activation, are located in the plane perpendicular to the Fe-N3 plane (9, Figure 6.2), while they are in the Ni-N2 plane of the square planar Ni complexesIn the iron complex-catalyzed polymerizations, the propylene monomer is inserted in a highly regioregular 2,1 -fashion and exclusively yields 1 -propenyl chain ends. The polypropylene that is produced is prevailingly isotactic (up to 67% mmmm at —20 C 69% mm at 0 °C) irrespective... 

Asymmetric Aziridination of Alkenes. The copper-catalyzed aziridination reaction can be rendered enantioselective by the addition of chiral ligands. The first example of an enantioselective aziridination of an alkene employed the bis(oxazoline) ligand (4) (R = f-Bu) and copper(I) trifluoromethanesulfonate as the metal catalyst (eq 14). This catalyst system affords the aziridine in 97% yield and 61% ee. Other reports have appeared subsequently regarding the extended scope of this reaction. " Important contributions to this area include the copper/bis-(oxazoline)-catalyzed aziridination of aryl acrylate esters (eq 15) and the copper/bis(imine)-catalyzed aziridination of cyclic cis-alkenes with the bis(imine) ligand (5) (eqs 16 and 17). ... 

A number of recent examples have served to expand the scope of chiral Cu(II) catalysts and substrates applicable in 1,3-dipolar cycloadditions of nitrones to electron-deficient alkenes. Saito and coworkers demonstrated Cu(OTf)2/bis(imine) (310) to be an efficient catalyst for highly endo and enantioselective cycloaddition of nitrones (309) with (253) [100]. Interestingly, the substitution of the aromatic imine subunit of the bis(imine) ligands (314a-c) was found to have a dramatic effect on chemical yields and the levels of enantioselectivity observed (Scheme 17.69). 

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