2,6-Diacetylpyridine, reaction with

Enolizable ketones were converted directly into their a-hydroxy dimethyl acetals upon reaction with DIB and methanolic potassium hydroxide at room temperature. If, during work up, there was acid treatment, then a-hydroxyketones were directly obtained. Numerous examples are known for such transformation, e.g. with 3-pentanone, acetophenones, 2,6-diacetylpyridine [16], tropan-3-one [17], -amino-ketones of great structural variety [18], etc. even a free radical reacted successfully in this way [19]. This conversion for an acetyl-oxazole served for the preparation of pyrimidine derivatives. 

In the presence of scandium perchlorate, 2,6-diacetylpyridine condenses with m-phenylenedia-mine to form a macrocyclic complex ScL(C104)3 4H20 (L is shown as (4)). The structure has not been determined, but the perchlorates are not coordinated. " The reaction probably proceeds via the (isolable) complex Sc(diacetylpyridine)2(C104)3 7H20. 

Reaction of 2,6-diacetylpyridine (203) with two equivalents of A, A -diniethyl-formamide dimethyl acetal (204), under MWI for 6 min, gave 2,6-Z)w(3-dimethyl-amino-l-oxoprop-2-enyl)pyridine 205 in > 95% yield (Scheme 43). This reaction was usually performed by refluxing the reactants for several hours (01S55). 

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. 

Condensation of 2,6-diacetylpyridine with bis(3-aminopropane)amine in the presence of small ions such as Mn(n), Co(n), Ni(n) or Cu(n) readily leads to formation of the corresponding monomeric (14-membered) macrocyclic complexes of ligand (83). However, when the large Ag(i) ion is used as the template, then a dimetallic complex of a 28-membered macrocycle of type (88) is produced. This example illustrates well the importance of metal-ion size in promoting template reactions. 

Unsymmetrical2,6-bis(arylimino)pyridines (Fig. 2), [2-(ArN=CMe)-6-(Ar,N=CMe) C5H3N] (2), are prepared by the successive condensation reactions of 2,6-diacetylpyri-dine with two different anilines [36, 22, 34, 59, 60, 70], For example, the mixed mesityl/m-xylyl derivative is prepared by firstly treating 2,6-diacetylpyridine with... 

The reaction between MoX3 (X = Cl, Br), aniline and 2,6-diacetylpyridine in butanol yields [MoO py(anil)2 X2], where py(anil)2 is a tridentate Schiff base ligand formed in situ.im These complexes are isomorphous with [VO py(anil)2 X2] and, therefore, are considered to have octahedral coordination of the molybdenum. 

Another series of macrocydic complexes has been prepared by Busch and co-workers by the condensation reaction of either pyridine-2,6-dicarboxaldehyde or 2,6-diacetylpyridine with various polyamines (Scheme 38). 24,2628,2640 2645 Other examples of template reactions leading to different types of macrocydic complexes are reported in Schemes 39-44.2628 2646 2650 The selfcondensation of l-amino-2-carboxaldehyde in the presence of nickel(II) gives two types of macrocycle (Scheme 45), M51... 

There are interesting preparative features to be found in this area of coordination chemistry for example, the template condensation of 2,6-diacetylpyridine with 3,6-dioxaoctane-l,8-diamine in the presence of Cd2+ results in the formation of complexes of the 1 + 1 macrocycle (154),1106 The macrocyclic complexes [CdL(NCS)2] (L = 155 R = H or Me) have been prepared by template condensations or by metal exchange reactions,1107 and the compound [CdLI]I (L -156) has also been described.1108... 

An extensive family of tetraaza (17) and pentaaza (18) macrocycles with a range of ring sizes are obtained by reactions of 2,6-diacetylpyridine with tri- or tetr-amines in the presence of suitable metal ions, which include as well as the usual M11 transition metal ions a variety of non-transition metal ions.19 Large rings, such as (19), can be formed by 2 2 condensations in the presence of suitable larger metal ions,20 Reactions of a, to diamines which have other internal heteroatoms with 2,6-diacetylpyridine (or 2,5-diformylfuran) produce a variety or related mixed heteroatom macrocycles. 

Condensation of a monocarbonyl compound with a dihydrazone initially yields a macrocycle with a tetraaza six-membered chelate ring, e.g. (21 Scheme 8) or (23 Scheme 9), but this can isomerize to give a triaza five-membered chelate ring, as for (27 Scheme 10), where cyclization is by a reaction subsequent to the hydrazone/carbonyl condensation,21 or for the isomeric pair of compounds (25) and (26) of Scheme 9. Compounds with triaza (28) and tetraaza (22) seven-membered chelate rings have also been prepared.22 Tetradentate and pentadentate aza macrocycles are formed by condensations of 2,6-diacetylpyridine with hydrazine (29) or with dihydrazines (30).21... 

Pyridine-2,6-dicarbaldehyde and 2,6-diacetylpyridine have been widely used in the template synthesis of imine chelates ranging in complexity from linear tridentates, such as (17),38 39 to macrocyclic structures with a range of ring sizes, such as (18).40-42 The in situ formation of macrocyclic ligands of this type depends upon the ring size and the strength of complexation of the triamine by the metal ion at the pH of the reaction. Related complexes with an additional donor atom attached to R2 have been synthesized also.43 44... 

The reaction of 2,6-diacetylpyridine with hydrazine in the presence of iron(II) salts yields a macrocyclic azine complex (equation 34).192 Similar complexes of cobalt(II), zinc(II), magnesium ) and scandium(III) have been prepared more recently.193... 

A similar complexity of rearrangements is observed in the related compound 6.42, which is derived from the metal-free reaction of 2,6-diacetylpyridine with 1,2-diamino-benzene (Fig. 6-41). In this case, the steric interactions between the methyl groups and the... 

Figure 6-41. The reaction of 2,6-diacetylpyridine with 1,2-diaminobenzene gives a complex product 6.42. Steric interactions prevent 6.43 from being formed.

So far, we have concentrated upon reactions resulting from discrepancies between the size of the metal ion and the size of the macrocyclic cavity. However, it is not only the size of the metal ion that may result in a mismatch what happens if the favoured conformation of the ligand does have an arrangement of donor atoms that matches with the preferred co-ordination geometry of the metal ion This is exactly the situation that we observe with metal complexes of some pentadentate macrocycles. We have previously observed the formation of tetraaza macrocycles from the template condensation of 2,6-diacetylpyridine with diamines in the presence of a transition metal ions. We also noted that if the size of the metal ion were incorrect, it was possible to get [2+2] or other products. Now let us look at this topic in a little more detail. 

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