Salen derivatives

The synthesis of the first polymer-supported chiral Mn-salen derivatives was reported independently by Sivaram171 and Minutolo.171-173 Different monomeric Jacobsen-type units, containing two polymerizable vinyl groups, were copolymerized with styrene and divinylbenzene to yield the corresponding cross-linked polymers as a monolithic compact block.174-176 The less mobile system (Figure 19) with no spacer between the aromatic ring and the polymer backbone is less enantioselective. 

The study of alkynylation of methyl ketones using a terminal alkyne, ZnMe2, and a salen derivative 196 as a chirality inductor provided a new method for the preparation of ct-hydroxyacetylenes (197, Scheme 112).292... 

It is now clear that asymmetric catalytic hydrogenation is rather successful. However, the initial research work of Sharpless [5] in the asymmetric epoxidation, followed by the results of Jacobsen et al. [6] opened large opportunities for liquid-phase asymmetric oxidation. Sharpless epoxidation has been widely applied in bench-scale organic synthesis, and more recently, salene derivatives emerged among the most effective catalysts in this reaction [7,8],... 

Similar results were reported by Kim et al. [45 a] and Che et al., [83] who both employed salen complexes attached to zeolite MCM-41. Similarly, Seebach and coworkers generated dendritic elongated salen derivatives that could be embedded in polystyrene by co-polymerization [80]. 

Seebach and coworkers prepared salen derivatives possessing two to eight st)ryl groups for crosslinking copolymerization with styrene [71]. The salen cores were derived from ( R,Rj-l,2-diphenylethylenediamine or 2-cyclohexane-... 

V,Af -Disalicylideneethylenediamine (Salen) and its analogues are most versatile chelate ligands in inorganic and organometallic chemistry (Chart 2). The synthesis of unsymmetrical salen derivatives which consist of two different salicylideneimine moieties... 

Fig. 15 Cobalt(III)salen derivative containing an anchored organic base (1,5,7-triabicyclo[4,4,0]dec-5-ene) on the salen ligand framework...

Katsuki and coworkers reported that Zr(salen)-based derivatives served as efficient catalysts for asymmetric BV reactions using UHP as terminal oxidant . Complex 117 in chlorobenzene at 0 °C showed the best catalytic activity with enantiomeric excesses higher than 80%, in most cases. Similarly to what was proposed for Ti(salen) derivatives (equation 51), the treatment of Zr(salen) complexes with H2O2 would give a Zr-peroxo (salen) complex 118, prone to ring-opening with formation of a Criegee adduct 119 and evolution to products (equation 81). 

Fig. 10. Example (a) of BII compounds and a Cl compound (b) and the cis (c) and trans (d) conformations for Salen derivatives...

Mn111 and Fem complexes are isomorphous with the CrUI complex. The chromium(III) complexes with X = H, 3-OMe, 5-C1, 5-Br, 5-Me1121 or 3,5-Cl21122 have been prepared by methods similar to those for the salen derivatives. 

Chromium porphyrins constitute a comparatively new and important class of compound. In order to avoid an artificial separation, complexes of various oxidation states starting with chromium(II) are dealt with in this section and some chromium(V) salen derivatives are included. Electronic, ESR and IR spectra are routinely recorded to characterize chromium porphyrin complexes and chromatography is extensively used in their purification for details the original references should be consulted. The chemistry of the porphyrins is covered in a recent series.1241... 

Several novel modified salen derivatives of cobalt(III) have provided convincing evidence for the importance of the propagating copolymer chain staying in the vicinity of the metal center, so as to avoid the formation of cyclic carbonates this procedure is especially relevant to processes involving the PO monomer. Both, computational and experimental studies have shown that cyclic carbonate formation is enhanced relative to monomer enchainment under conditions where the growing polymer chain is outside the influence of the metal catalyst [50, 51]. To circumvent this issue, Nozaki and coworkers prepared a salen complex containing a piperidinium end-capping arm (Scheme 8.4) [52]. 

Many claims have been made for manganese salen derivatives, from the hotly debated report that they extend the lifespan of nematode worms [27, 28] to curative properties in cases of lung damage [29], Unfortunately the compounds suffer from the same problems as other simple enzyme mimics in that they are not shielded from deactivation mechanisms by a protein shell. Furthermore they can also act as prooxidants through mechanisms such as the following ... 

Ru(ON+)(salen) complexes were also applied to ACP [43]. Complex 16 afforded mainly the cis product (9 91 trans-cis ratio) with 91% ee for the cis form under irradiation and it was also applied to intramolecular cyclopropanation giving 94% ee. Several Ru salen derivatives were investigated for ACP [44]. 

Figure 30. This figure is of salen and the connectivity of different salen-derived CPs (a) and MOFs (b).

As a result of this and Eq. 2.4, Hammett constants directly influence the complex formation eqnilibria, although to varying extents depending on whether this is first-order or second-order, the latter due to aromatic substitution next to different donor sites (P (phosphines), N (benzamides, pyridines or pyrazininm ions), S (thiophenolates) or O (phenolates, benzoates, combinations like salicylate derivates) or combinations thereof (see case c. above) (salen derivatives or those of hippnric acid, both linked to metal by N -t- O)). 

Bis-salicylidene (or bis-salen) derivatives of 1,2-diaminocyclo-hexane are prepared by treatment of the diamine (or its tartrate salt) with an o-hydroxybenzaldehyde derivative and are used for asymmetric manganese-, cobalt- or chromium-catalyzed reactions. The most important ligand of this type is A(A -bis(3,5-di-terf-butylsalicylidene)-diaminocyclohexane (12), the detailed synthesis of which has been published. Another important ligand is the atropoisomeric derivative (13) (eq 21). ... 

Other approaches to self-assembling receptors have been reported in recent years. A self-assembling, trimeric palladium complex based on the bis(benzimidazole) ligand (17) was designed by Williams and coworkers [4]. The complex contains a hydrophobic cavity that in the X-ray structure has included a molecule of acetonitrile. In a different context, Schepartz and McDevitt [70] have used the chelation of nickel(n) by A,7V -bis(salicylaldehy-de)ethylenediamine (salen) derivatives to control the position of K -binding glyme chains, and it has been shown that these self-assembled ionophores influence alkali metal transport across liquid membranes [71]. Also, Shinkai and coworkers [72] and Schneider and Ruf [73] have used metal chelation to induce an allosteric effect on binding at a second site. 

One of the most important aspects of those listed above is catalysis. A great deal of research has been carried out in the investigation of catalysts for the epoxidation of alkenes. The oxidation of a-pinene to a-pinene epoxide (Equation 1) is one of the standard trial reactions for catalyst identification. The Jacobsen manganese complex catalyst, based on a salen derivative, shows brilliant enantioselectivity, whereas the use of related indole derivatives provides greater structural variety and powerful catalysis, but so far shows poorer enantioselectivity (Figure 2). Such catalysts can sometimes be effectively embedded in zeolites. ... 

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