Phosphite-Pyridine Systems

Direct polycondensations of aromatic diamines with dicarboxylic acids have generally been described as a poor route to high molecular linear polyamides. Recently, high molecular weight polyamides have been obtained with limited success by a melt polymerization of 4,4 -diaminodiphenylmethane (MDA) with aliphatic dicarboxylic acids14.  

Aromatic dicarboxylic acids, even with aromatic diamines and aromatic amino acids, do not form high viscous polymers. Isophthalic acid (IPA) gives higher viscous polymers than terephthalic acid. From this result it may be concluded that polycondensation is favored with higher solubility of polymer. 

It was expected that the polycondensation reaction at high temperatures might favor the solubility of the resulting polymer, but be undesirable for the stability of the complexes of phenol with metal salts. As a consequence, an optimum of the reaction temperature might be required in the polycondensation reaction. 

An optimum of viscosity (17 = 1.71) is observed at a reaction temperature at around 80 °C in the polycondensation of p-ABA. Above this temperature the viscosity decreases gradually with temperature. Only low-viscosity polymers are obtained at 60 °C. 

Of the solvents tested in the polycondensation of p-ABA in NMP/pyridine, NMP has been found to be most effective and iV,iV-dimethylacetamide (DMAc), in which the reaction mixture becomes light yellow gives rise to moderate yields whereas dimethylformamide (DMF) largely retards the reaction, probably because of a side reaction of DMF with LiCl at high temperatures. This is indicated by deep discoloration of the reaction mixture. 

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Raney nickel liberates hydrogen from water in the presence of sodium hypo-phosphite, which becomes oxidized to sodium phosphite. This reducing system in aqueous acetic acid or aqueous acetic acid-pyridine reduces nitriles to aldehydes at room temperature and pressure yields 50-90%. ... 

Following the kinetic and mechanistic study of several steps in the [Fe(NCMe)6] -trimethyl phosphite system some years ago, there has recently been a study of the stepwise replacement of pyridine in [Fe(py)6] by trimethyl phosphite. Although stopped-flow techniques were needed to investigate the early stages, the half-life for conversion of [Fe(py)3(P OMe 3)3] into cis-[Fe(py)2(P OMe 3)4] is a few hours, for further substitution several days. Irradiation of an acetonitrile solution of [Fe(CNMe)4(CN)2] either in the charge-transfer or ligand-field band leads to the successive replacement of two of the four isocyanide ligands by acetonitrile. ... 

The highly functionalized imidazole 3 can be polymerized by active phosphite ester methods [4] to afford the A-B polyamide. The monomer is dissolved in a mixed solvent system of N-methylp3n rolidinone and pyridine containing dissolved lithium chloride. Upon addition of triphenyl phosphite, the reaction immediately takes on a yellow color. During the course of the polymerization, the color increases in intensity until the pol5rmerization mixture appears black. The products are isolated as fine yellow powders by precipitation in methanol followed by a methanol wash. 

A system for the preparation of nucleoside-derived 2-pyridylphosphonates using a preformed pyridinium salt as the source of the pyridine fragment has been developed (Scheme 4.217) [357], The base used for the reaction was DBU, and simply stirring the pyridinium salt with the secondary phosphite and DBU in acetonitrile at room temperature afforded the arylphosphonate in excellent yields. The phosphonylation was selective for the 2-position. The mechanism for this transformation was similar to the one proposed... 

This type of reaction was first applied to the preparation of polyamides, as discussed above, for which it is somewhat more effective in forming high molecular weight polymers, but it has now been used for the synthesis of a wide variety of aromatic polyesters, either by the self-condensation of hydroxyacids or by the co-condensation of dicarboxylic acids and difunctional phenols.A fairly wide variety of phosphorous compounds can be used as reducing or dehydrating agents in these reactions, in addition to phosphines, including phosphites, chlorophosphates, phosphates, polyphosphates and phosphazenes. In most cases, lithium chloride is added and the reaction is run in either pyridine or an amide solvent system. The reaction has also been found to be catalyzed by tertiary amine salts. 

The preparation of aramids by direct polycondensation of aromatic diamines with aromatic dicarboxylic acids using triphenyl phosphite and pyridine as condensing agents has been well documented. This method was used here to polymerize the commercially available diamines, 4,4 -oxydianiline (ODA) and 1,4-phenylenediamine (PDA), with 10,11,15, and 16. Polymers obtained with this method formed fibrous precipitates on pouring the reaction mixtures into stirring methanol. Essentially quantitative yields were obtained for all systems evaluated. High molecular weight... 

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