Schiff macrocycles

Macrocycles have been prepared by formation of macrocyclic imines as well as by using variations of the Williamson ether synthesis ". Typically, a diamine or dialdehyde is treated with its counterpart to yield the Schiff s base. The saturated macrocycle may then be obtained by simple reduction, using sodium borohydride, for example. The cyclization may be metal-ion templated. In the special case of the all-nitrogen macrd-cycle, 15, the condensation of diamine with glyoxal shown in Eq. (4.14), was unsuccess-ful ... 

Quite a number of mixed sulfur-nitrogen macrocycles have been prepared, but these have largely been by the methods outlined in Chaps. 4 and 5 for the respective heteroatoms. An alternative method, involves the formation of a Schiff base, followed by reduction to the fully saturated system, if desired. An interesting example of the Schiff base formation is found in the reaction formulated in (6.12). Dialdehyde 14 is added to ethylenediamine in a solution containing ferrous ions. Although fully characterized, the yield for the reaction is not recorded. To avoid confusion with the original literature, we note the claim that the dialdehyde [14] was readily prepared in good yield by reaction of the disodium salt of 3-thiapentane-l, 5-diol . The latter must be the dithiol rather than the diol. 

Bis(ethylenediamine)gold(III) chloride reacts with a variety of -diketonates in aqueous base, via Schiff base condensation, to form 14-membered tetraaza 12jt macrocyclic species [78[. The parent member of the series [AuL ]" being 22 (where H2L = 5,7,12,14-tetramethy]-l,4,8,ll-tetraazacydotetradeca-4,6,ll,13-tetra-ene) (Figure 2.15). The X-ray structure shows the cation to be nearly planar. 

Macrocyclic Schiff base compartmental ligands (750) (Robson-type ligands) derived from the [2 + 2] condensation of a 2,6-diformyl- or 2,6-diketo-substituted phenol and a diamine are very prominent in dinuclear Ni coordination chemistry.1901-1903 Particular interest lies in magnetic exchange interactions between the adjacent metal ions as well as in bioinorganic chemistry, where such dinuclear complexes have been proposed as synthetic analogues for bimetallosites. 

By judicious choice of reaction conditions an acyclic Ni11 complex (784) could be isolated, which serves as a valuable starting material for the preparation of unsymmetrical and mixed metal complexes by subsequent reaction with various amines. Also, a symmetrical Schiff base macrocycle of larger size has been obtained as a minor byproduct upon condensation of (784) with 1,3-diaminopropane. The resulting Ni11 complex (785) is again bimetallic, although room to bind four metal ions is in principle available.1367... 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

Similarly, by Schiff-base condensation reactions have been used to generate free cryptands from triamines and dicarbonyls in [2+3] condensation mode. These ligands react with silver(I) compounds to give dinuclear or trinuclear macrocyclic compounds where Ag Ag interactions may be present. Thus, with a small azacryptand a dinuclear complex with a short Ag- Ag distance (55) is found.498 With bigger azacryptand ligands also dinuclear complexes as (56) are achieved but without silver-silver interaction. 65,499-501 A heterobinuclear Ag1—Cu1 cryptate has also been... 

Figure 5 Some examples of silver complexes with Schiff base macrocycles.

Several silver(I) complexes of the macrocyclic Schiff base derived from the [2+2] condensation of terephthalaldehyde and 3-azapentane-1,5-diamine or A,A -bis(3-aminopropyl)methylamine have been described.509,510 The reaction of 2,ll-diaza-difluoro-m-[3,3]-cyclophane with 2,6-bis (bromomethyl)pyridine lead to the 3 + 3 addition product, which gives a complex with two silver... 

Bis(ethylenediamine)gold(III) chloride reacts with /3-diketonates in aqueous base via Schiff base condensation to form complexes of gold(III) with a 14-membered macrocyclic tetraaza ligand such as (292).1711-1713 The X-ray structure showed the cation to be nearly planar. Delocalization of -electrons within the six-membered /3-diketonate rings was indicated by the observed pattern of C—C and C—N distances. Open-chain tetraaza ligand complexes in which condensation of only one /3-diketonate has occurred, can be isolated as intermediates in this reaction. They may be used for further condensation with a different /3-diketonate. Oxidation of this complex with trityl tetrafluoroborate introduces a double bond in position C2C3.1712,1714... 

In the process of selective synthesis of macrocyclic Schiff bases, derivatives of chiral 1,4-diamines, the H NMR spectra were taken to confirm the synthetic route without 1H signals assignment.20... 

Chiral Schiff bases are intermediates in the synthesis of peptidomi-metic macrocycles which have been studied in detail by Bru et al.24 and Alfonso et al. [9].25... 

Spontaneous formation of the Schiff base [3 + 6] macrocycle being a derivative of 2,6-diformyl-4-methylphenol and polyamine has been shown by Schmidt et al. [10].26... 

For 20- [11] and 40-membered [12] macrocyclic di- and tetra-Schiff bases being derivatives of (R)-BTNOL, the chemical shift differences (AS) measured for diastereotopic methylene protons of BINOL unit in CDCI3 have suggested the presence of the partial rotation around the Ar—O—CH2 bond.27 The splitting of some 1H signals in acetone-d6 was explained as a result of a dynamic interconversion process on the NMR time-scale. 

Schiff Bases and Macrocyclic Nickel and Cobalt Complexes 487... 

Other aza macrocyclic complexes, such as Schiff bases or tetraazaannulene derivatives, have been also used with some success for the electrocatalytic reduction of C02 in organic solvent in the presence of proton sources.166-168... 

The third class of metal catalysts includes nickel and cobalt complexes of Schiff bases and nitrogen macrocyclic ligands, which can form on electroreduction cobalt(I) and nickel(I) reactive intermediates for the activation of organic halides. 

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