Pyrrolic nitrogens soluble

Pyrrole is soluble in alcohol, benzene, and diethyl ether, but is only sparingly soluble in water and in aqueous alkaUes. It dissolves with decomposition in dilute acids. Pyrroles with substituents in the -position are usually less soluble in polar solvents than the corresponding a-substituted pyrroles. Pyrroles that have no substituent on nitrogen readily lose a proton to form the resonance-stabilized pyrrolyl anion, and alkaU metals react with it in hquid ammonia to form salts. However, pyrrole pK = ca 17.5) is a weaker acid than methanol (11). The acidity of the pyrrole hydrogen is gready increased by electron-withdrawing groups, eg, the pK of 2,5-dinitropyrrole [32602-96-3] is 3.6 (12,13). 

A mild and effective method for obtaining N- acyl- and N- alkyl-pyrroles and -indoles is to carry out these reactions under phase-transfer conditions (80JOC3172). For example, A-benzenesulfonylpyrrole is best prepared from pyrrole under phase-transfer conditions rather than by intermediate generation of the potassium salt (81TL4901). In this case the softer nature of the tetraalkylammonium cation facilitates reaction on nitrogen. The thallium salts of indoles prepared by reaction with thallium(I) ethoxide, a benzene-soluble liquid. 

Solubility - The oxidized polymer (VIII) has a greater solubility than the original polymer (VII). It was found to be soluble in acetone, chloroform, benzene, DMF and DMSO. Unlike the polymer (VII), (VIII) was not soluble in formic acid or trifluoroacetic acid that was expected since the pyrrole moiety is less basic than pyrrolidine. In the oxidized polymer, the pair of unshared electrons on the nitrogen atom are contributing to the pyrrole ring aromaticity, therefore, unavailable for protonation as in the case of polymer (VII). A comparison of the solubilities is given in Table I. 

Adducts of VO(acac)2 with pyridines have been extensively studied (see Section 33.5.5.4.H). Adducts VOF4-py416 and VOCl4-L with L = py, PhN=NPh, pyrrole, pyrazine, 2,6-dimethylpyrazine, bipy and phen have been discussed.6 A [VCUM complex (L = carbazole) has been reported.4 4 Again, the rule is the formation of 1 2 adducts with monodentate nitrogen donors and 1 1 adducts with bidentate donors. IR and solubility data have been used to decide whether bidentate ligands are chelated or bridging. 

Comparable to thiophene, pyrrole is a five-membered heterocycle, yet the ring nitrogen results in a molecule with distinctly different behavior and a far greater tendency to polymerize oxidatively. The first report of the synthesis of polypyrrole (PPy) 62 that alluded to its electrically conductive nature was published in 1968 [263]. This early material was obtained via electrochemical polymerization and was carried out in 0.1 N sulfuric acid to produce a black film. Since then, a number of improvements, which have resulted from in-depth solvent and electrolyte studies, have made the electrochemical synthesis of PPy the most widely employed method [264-266]. The properties of electrosynthesized PPy are quite sensitive to the electrochemical environment in which it is obtained. The use of various electrolytes yield materials with pronounced differences in conductivity, film morphology, and overall performance [267-270]. Furthermore, the water solubility of pyrrole allows aqueous electrochemistry [271], which is of prime importance for biological applications [272]. 

A,B-Diheteropentalenes have only limited solubility in water. The hydrogen donor properties of the NH site of pyrrole and the hydrogen acceptor property of the oxygen atom of the furan ring can increase the solubility of the compounds containing these heteroatoms relative to the systems with only sulfur or selenium atoms in their structure. Alkylation at the nitrogen of furo[3,2-6]pyrrole derivatives increases the solubility in ethanol and other organic solvents. 

Imidazoles are amphoteric compounds with a basic, pyridine-type nitrogen (they are about 106 times more basic than oxazoles and 104 times more basic than thiazoles173), and (where the NH is unsubstituted) a weakly acidic, pyrrole-type amino nitrogen in the ring. In consequence, imidazoles readily form salts with acids and often form salts (or complexes) with metals. The sparingly soluble silver salts formed by imidazoles have been used by Giesemann et al.174 as intermediates in the synthesis of 1-triphenylmethylimidazoles. Normally, however, the salts formed with acids are more important in isolation and purification procedures. 

In the synthesis of ferrocene-functionalized polypyrrole systems, numerous derivatives are readily obtained by substitution at either the 3-position of the ring or on the nitrogen of the pyrrole prior to polymerization. It was reported that polymers with both metallic properties and enhanced solubility may be synthesized by judicious derivatization and choice of polymerization conditions, such as a variety of processing solvents [67, 68]. 

The first application of natural-abundance solid state 15NNMR was on the complex solids from the reaction of hydrogen cyanide and ammonia. Signals in the solid state spectrum of the cold water soluble fraction are attributed to the NH4 salt, amine-, urea-, peptide-, nitrile-, and pyrrole-like nitrogens. The spectrum of the cold water insoluble fraction shows two broad resonances indicating a diversity of nitrogen functionality, possibly including crosslinked aromatics and purine-like compounds (3). 

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