Acid/pyridyl complexes

Figure 3.13 Schematic representation and phase diagram of 1 1 (acid/pyridyl) complexes of polymer (4b) with varying proportions of mono- and bifunctional monomers (6b) (n = 6) + (6d) (permission being sought from [19]).

Kwiatek and Seyler were the first to report that many organopenta-cyanides, when treated first with acid and then with alkali, liberate nitriles 110). This reaction occurs with unsubstituted primary and secondary alkyl, benzyl, vinyl, and phenyl complexes, while allyl, 2-oxo-, 2-hydroxy-, and 2-alkoxy complexes simply release the organo-ligand on treatment with acid, and 1-cyanoalkyl and a-pyridyl complexes are stable 105) (see also Table IV). The yield of nitrile is usually far from quantitative and is... 

Considering that the activity of a Lewis acid depends strongly on the stability of the acid-base complex and that the complexation is notoriously hampered by chemically hard solvents like water, it is clear that reactions of bidentate dienophiles can be catalysed very efficiently36. Prototypical are the derivatives of 3-phenyl-l-(2-pyridyl)-2-propen-l-ones (vide infra). Their Diels-Alder reactions (Table 24) clearly show that the accelerating solvent effect of water is still present in the Lewis acid catalysed reactions, and that the Lewis acid activity is not necessarily hindered by the solvent301. While... 

Benzenemethanethiol,a-(selenylmethylene)-metal complexes, 809 Benzeneselenic acid metal complexes, 807 Benzenetetrathiols metal complexes, 606 Benzenethiol, 2-amino-metal complexes, 799 Benzenethiol, o-hydroseleno-metal complexes, 809 Benzimidazole, 2-guanidinium-metal complexes, 283 Benzimidazole, 2-mercapto metal complexes, 802 Benzimidazole, 2-(2-pyridyl)-metal complexes, 93 Benzimidazoles metal complexes, 78, 85 P a, 11... 

The cryptand with an electron-rich pyridyl unit on the third arm can form threaded structures with paraquat derivatives in solution. When the nitrogen atom was proto-nated by adding trifluoroacetic acid, the complex disassembled. After the same equivalents of triethylamine were added, the complex was regenerated. This pH-controllable cryptand/paraquat association is important for its potential... 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

In accordance with the electropositive nature of the bridgehead atoms, all di(pyridyl) substituted anions behave like amides with the electron density accumulated at the ring nitrogen atoms rather than carbanions, phosphides or arsenides. The divalent bridging atoms (N, P, As) in the related complexes should in principle be able to coordinate either one or even two further Lewis acidic metals to form heterobimetallic derivatives. According to the mesomeric structures, (Scheme 7), it can act as a 2e- or even a 4e-donor. However, theoretical calculations, supported by experiments, have shown that while in the amides (E = N) the amido nitrogen does function as... 

The reduction of persulphate by tris-[a-(2-pyridyl)-benzylideneaniline] iron(II) is, by contrast, independent of persulphate ion concentration , and the rates of reaction of several ring-substituted complexes of this type correspond exactly to the rates of acid-catalysed separation of one ligand. Clearly oxidation of the ligand... 

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