Pyridine, 4-substituted, complexes with

Harman et al. have demonstrated the dearomatization of 2-substituted pyridine via complexation with tungsten <05JA10568>. The dearomatization process renders the boimd... 

Some of the typical benzene electrophihc substitution reactions do not occur at all Friedel-Crafts alkylation and acylation fail because pyridines form complexes with the Lewis-acid catalyst required, involving donation of the nitrogen lone pair to the metal centre. Milder electrophilic species, such as Mannich cations, diazonium ions or nitrous acid, which in any case require activated benzenes for success, naturally fail with pyridines. 

In attempts to synthesize tungsten alkylidyne complexes containing chelating anionic donor ligands we investigated the reactions of the bis-pyridine-substituted complex 1 with dithiocarbamate ligands and with pyrrole-2-carboxaldehyde methylimine in the presence of... 

There is another important factor in the low reactivity of pyridine derivatives toward electrophilic substitution. The —N=CH— unit is basic because the electron pair on nitrogen is not part of the aromatic n system. The nitrogen is protonated or complexed with a Lewis acid under many of the conditions typical of electrophilic substitution reactions. The formal positive charge present at nitrogen in such species further reduces the reactivity toward electrophiles. 

The Cr analogues 6 (Fig. 4.2) of the bis(carbene)pyridine systems were found to be exceptionally active for the oligomerisation of ethylene [10,11]. Activation with MAO led to optimal results, and complexes with Me, Pr and substitution... 

Heterocyclic nitrogen donors and their adducts with zinc chloride have been studied.623,624 A large number of other ligand systems have also been characterized, for example, zinc halide adducts of 2,2-dimethylpropane-1,3-diamine and hexamethylphosphoramide have been studied.625,626 The formation of mixed ligand complexes with chloride and substituted pyridines has been studied.627 The zinc tris(pyridyl) chloride anion has also been structurally characterized.628 Manganese(II) ions have been used to probe the stereochemistry in reactions of zinc halides with pyrazine.629... 

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

Electrophilic substitution of pyrrole can, however, be carried out under specialised conditions (e.g. acylation with (MeC0)20/BF3, sulphonation with a pyridine/S03 complex, C5H5N-S03, cf. (67)) leading to preferential attack at the 2-, rather than the 3-, position. This reflects the slightly greater stabilisation of the Wheland intermediate for the former (70) compared with that for the latter (71) ... 

The earliest NLO studies involving metal pyridyl complexes were reported by Frazier et al. in 1986 who investigated the SHG properties of various group 6 metal pyridyl carbonyls.63 Although most of the complexes tested show little or no activity, (6) and (7) have respective SHG efficiencies of 0.2 and 1.0 times ADP using a 1,064 nm laser.63 Shortly after, Calabrese and Tam reported SHG from the Re1 complex (8).64 Subsequent studies by Eaton and Tam et al.65,66 describe the preparation of inclusion compounds of various metal complexes with thiourea or tris-ort/ o-thymotide. Unfortunately, none of the complexes [W(CO)5L] (L = pyridine, py, or a 4-substituted py) produce SHG-active materials.65,66... 

The ligand diethylenetriamine, H2NCH2CH2NHCH2CH2NH2, forms stable complexes with Pt2+. When the complexes [Pt(dien)X]+ undergo reaction with pyridine, the rate constants for X = Cl-, I-, and N02- are 3.5 X 10-5, 1.0 X 10-s,and5.0 X 10-8 sec-1, respectively. What is the mechanism forthe substitution reactions Provide an explanation for the difference in rates of the reactions. What would be a reasonable estimate for the rate constant when X = Br- Explain your answer. 

The unusual structure 125 displays an acylborane group the latter was formed from carboxyborane as a complex with secondary or tertiary amines, imidazole, or pyridine derivatives. The synthetic pathway (Scheme 31) consisted in an oxidation with NBS, substitution with cyanide, then treatment with 2-aminopyridine <2004JOM3567>. 

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