Diamine/diimine ligands

Chiral salen ligands are diimines of salicylaldehydes with chiral diamines, usually cyclohexane-1,2-diamine (salen Hgands 1) or 1,2-diphenylethylene-diamine (salen ligands 2). The most widely used salen ligand in homogeneous catalysis is probably Jacobsen s ligand (la. Fig. 2), which is commercially available and hence has been used as reference to compare the results of im-... 

Experimental observations of the aziridination of styrene-type alkenes, catalyzed by CuPF6 in the presence of chiral diimine ligands (such as (lR,2R,A i4A i4)-A A -bis(2,6-dichlorobenzylidene)cyclohexane-l,2-diamine 425), have been taken as evidence of the intermediacy of a discrete, monomeric Cu(lll)-nitrene complex, (diimine)Cu=NTs 423. Variation of the steric properties of the aryl group in the oxidant TsN=IAr (Ar = Ph, 2-/-Bu, 5,6-Me3C6H) has no effect on the enantioselectivities in forming the aziridination products 424 (Scheme 108) <1995JA5889>. 

Given the characterization of mononuclear [Fe(CN)5(en)] (98), as well as of other bound diamines (99), their ability toward reactive dehydrogenation could be expected, probably in a more effective way if dinuclear complex formation is allowed for. Some precedent exists for the autoxidation of bound 4-aminomethylpyridine to 4-cyanopyridine in [Fe(CN)5] (100). The preparation of these putative dinuclear complexes, bridged by unsaturated diimine ligands could be an interesting issue, reinforced by the possibility of preparing new mixed-valence species, and stimulated by the renaissance experienced in this field, related to the diverse electronic delocalization patterns (9,10). 

Diamines and Diimines as Ligands C. Other Group-Transfer Reactions... 

Since 1980 the interest in this reaction increased because enantiospecificity was introduced and much more valuable products could be made. A wide variety of ligands was tested, such as chiral dipyridines, phenanthrolines, diphosphines, aminoalcohols, bis-oxazolines, bis-oxazolines with a third donor atom in the centre, bis-thioureas, diamines, etc [33], In 1981 the highest e.e. reported was still only 20%. For many years the best results were obtained with chiral diimines and phenanthrolines but e.e. s were below 70% [34], Pfaltz introduced bis-oxazolines for this reaction and obtained e.e. s as high as 91% [35] in 1991. 

Fig. 20. Proposed mechanism for the oxidation of chelating diamine ligands bound to [Fen(CN)4 2-, with formation of different intermediates involving Fe11, Feln species, and final formation of the [Fen(CN)4(diimine)]2 complexes.

A number of observations show the profound effect of chelation on the stability of Schiff bases and similar unsaturated nitrogen-containing ligands. Eichhorn and coworkers found that the Schiff base of bis(thiaphenal)ethylene-diimine is hydrolyzed in the presence of Cu+2, with the formation of the ethylene-diamine complex (Equation 18) (26, 28). 

Ligands such as aniline (an), 1,2-diaminobenzene (dab, o-phenylenediamine) and 2,2 -dia-minobiphenyl886 887 are classed separately, not because their ability to bind to a central metal is any less than the ligands discussed previously, but because of their potential non-innocent behavior888 with respect to internal redox reactions. Indeed, the dark blue complex isolated from the air oxidation of Con/dab in aqueous ethanol (a conventional route to yellow Co(diamine)3+ systems) has been shown to have structure (117) with five-coordinate Co11.888 Related diimine complexes have been reported for Ni11889 as well as the conventional Ni(dab)(+,890 Co(dab)3+ 891 and Pt(dab) + 892 systems. 

The impact of achiral ligands on the TOP of the catalyst in this system was then examined by following reaction conversions with various catalysts. In the absence of both Ph2-BINOL and diamine ligands, the reaction of diethylzinc with benzaldehyde is very slow, exhibiting about 1% conversion after 8h at 0°C. When Ph2-BINOL was employed in the asymmetric addition to this substrate, the catalyst TOP increased exhibiting 25% conversion after 2 h. To evaluate the TOP of zinc centers bearing diimine derived from ethylene diamine and 2,4,6-trimethylbenzaldehyde (Table 4, entry 5) or the meso diamine (Table 5, entry 7), these ligands were... 

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