Salen-derived ligands

Another interesting approach was taken by Berkessel and co-workers, who synthesized Ni(II) complexes with salen-derivative ligands having a pendant thioether group [101, 108]. They found that intramolecular electron transfer from sulfur to nickel can easily occur. This, combined with other evidence, as described above, led to a proposed catalytic cycle for the final step of methano-genesis, as shown in Fig. 9. The same mechanism was simultaneously proposed by Jaun [83]. 

APPLICATION OF SALEN-DERIVED LIGANDS IN THE ENANTIOSELECTIVE NHK REACTION 349... 

The somewhat different bis(diboratetrasiloxane) derivatives 94 and 95 have been prepared from bimetallic salen B(OEt)2 2 derivatives and diphenylsi-lanediol [131, 138]. In this case the four boron atoms of two diboratetrasilox-ane rings 86 are chelated by salen-type ligands in order to produce cages of cylindrical geometry (Fig. 25). 

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...

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. 

The [4-1-3] cycloaddition between 2-aminofuran and oxyallyl cations, followed by base-induced elimination of the resulting adducts, was used in the synthesis of 3-aminotropones <05TL8475>. A chiral Lewis acid-catalyzed [4t-3] cycloaddition between furans and nitrogen-stabilized oxyallyl cations derived from allenamides was developed. As depicted below, the Cj-symmetric salen-based ligand is the most effective in promoting this asymmetric cycloaddition reaction <05JA50>. 

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