Salen ligand

Fig. 25. General formula for silasesquioxanes 93. Compounds 94 and 95 are cage-like bis(diboratetrasiloxane) derivatives, in which two eight-membered diborasiloxane rings are joined by two salen ligands...

Non-covalently Immobilized Catalysts Based on Chiral Salen Ligands. . 152... 

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-... 

Only a few years after the development of the homogeneous chiral Mn(salen) complexes by Jacobsen and Katsuki, several research groups began to study different immobiUzation methods in both liquid and soUd phases. Fluorinated organic solvents were the first type of Uquid supports studied for this purpose. The main problem in the appUcation of this methodology is the low solubility of the catalytic complex in the fluorous phase. Several papers were pubUshed by Pozzi and coworkers, who prepared a variety of salen ligands with perfluorinated chains in positions 3 and 5 of the saUcyUdene moiety (Fig. 2). 

The limitations of the system with regard to substrates and oxidants was attributed to the strong electron-withdrawing character of the perfluorinated chains and the lower steric hindrance in the position adjacent to phenols, in marked contrast to the ferf-butyl groups present in Jacobsen s catalyst, hi view of this, a second generation of fluorinated salen ligands le and If was... 

Another application of salen ligands is the hydrolytic kinetic resolution of epoxides (Scheme 3). For this purpose cobalt complexes are efficient, and fiu-... 

Fig. 5 Chiral salen ligand with improved solubility in ionic liquids...

In an attempt to increase the ionic liquid/hexane partition coefficient, a new salen ligand appended with an imidazolium salt was developed (Fig. 5) [18]. Unfortunately, modification of the ligand caused a dramatic reduction in the enantioselectivity - down to 57% ee at most - although the reasons for this behavior remain unclear. 

Fig. 8 Symmetrical chiral salen ligands used in solid phase immobilization...

Salen ligands have also been used in the titanium-catalyzed trimethylsilyl-cyanation of benzaldehyde. The complexes were immobilized by substitution of a chloride with a surface silanol from the support. In the first study on this reaction [38], the most efficient ligand was the non-symmetrical salen Im (Fig. 11) (94% ee), whereas the selectivity obtained with the symmetrical ligand la was significantly lower (72% ee). In a recent paper, the immobilization of different titanium species, including monomeric and dimeric systems with... 

A complementary approach has been reported very recently [43]. hi this case negative charges were introduced into the salen ligand Iq (Fig. 14) with the aim of exchanging it on cationic supports, such as a layered double (Zn, Al) hydroxide. The expansion in the basal spacing indicated intercalation, at least partially, of the Ig-Mn complex between the layers of [Zn2,i5Alo,86(OH)6,o2]- The complex was used in the epoxidation of (i )-limonene with molecular oxygen and pivalaldehyde. The use of N-... 

Fig. 14 Chiral salen ligands with charged substituents...

In contrast with salen ligands, ionic liquids were used earlier than fluorinated solvents for biphasic liquid systems with bis(oxazoline)-based complexes. In... 

Figure 4.6 An enantioselec-live MOF epoxidation catalyst. Zn-(l,4 -biphenyldicarboxylate) sheets in the ab direction are connected by functionalized Mn-salen ligands to form a doubly...

The development of catalysts for the efficient oxidation of catechol and its derivatives in water is topic of ongoing work in this laboratory. Towards this end, polyethylene glycol side-chains were incorporated in a pentadentate salen ligand to enhance the water solubility of the complexes derived thereof. A dinuclear copper(II) complex is found to catalyze the oxidation of 3,5-di-tert.-butylcatechol into 3,5-di-tert-butyl-o-benzoquinone more than twice as fast in aqueous organic solution as in purely organic solvents (ly,at/knon= 140,000). Preliminary data are discussed. 

In our ongoing efforts to develop oxidation catalysts that are functional in water as environmentally berrign solvent, we synthesized a water-soluble pentadentate salen ligand with polyethylene glycol side chairts (8). After coordination of copper(II) ions to the salen ligand, a dinuclear copper(II) complex is obtained that is soluble in water, methanol and mixtures of both solvents. The aerobic oxidation of 3,5-di-tert.-butylcatechol (DTBC) into 3,5-di-terr.-butylqitinone (DTBQ) was used as a model reaction to determine the catalytically active species and initial data on its catalytic activity in 80% methanol. 

Synthesis of complex 1. The pentadentate salen catalyst 1 was synthesized as described (9). In short, the tosylated 2-[2-(2-methoxyethoxy)-ethoxy]-ethanol 2 (10) was reacted with 2,4-dihydroxybenzaldehyde 3 to yield 4-alkoxy salicylaldehyde 4 after chromatographic purification (eq. 1). Subsequent condensation of 4 with 1,3-diaminopropanol yielded water-soluble salen ligand 5 in sufficient purity and 89% yield (11). The formation of an azomethine bond is indicated by a shift of the NMR signal for the carbonyl carbon from 194.4 ppm in aldehyde 4 to 166.4 ppm for the imino carbon in 5. The pentadentate ligand 5 was then treated with copper(ll) acetate in methanol to obtain the dinuclear copper(ll) complex 1 as a green solid (eq. 2) (11). 

The chiral diol 17 derived from tartaric acid is exploited in the titanium-catalyzed asymmetric pinacol coupling in the presence of Zn and MesSiCl to give the corresponding diol in 11-71 ee % [44], The chiral salen ligands 18-20 are used in the titanium-catalyzed enantioselective coupling reaction, which achieves the higher selectivity [45-47]. The chromium complex with TBOxH (21) efficiently catalyzes the asymmetric coupling reaction of both aromatic and aliphatic aldehydes [48]. 

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