Pyrrole hydrogenation

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). 

Vibrational spectroscopy, too, has been used to study supercritical fluid systems. Buback reviewed (59) this area however, much of his discussions are on fluid systems that are well removed from ambient conditions or difficult to handle easily (e.g., H20, HC1). In an early report, Hyatt (21) used IR absorbance spectroscopy to determine the influence of several solvent systems, including COz, on the vibrational frequencies ( ) of solute molecules. Specifically, he studied the vc=o of acetone and cyclohexanone and vs.H of pyrrole. The goal of this work was to determine the suitability of supercritical fluids as reaction solvent. Hyatt concluded that the ketones experienced an environment similar to nonpolar hydrocarbons in COz and that there were no differences between liquid and supercritical CO2. In contrast, the pyrrole studies indicated that the solvent strength of CO2 was between ether and ethyl acetate. This apparent anomalous result was a manifestation of the, albeit weak, degree of pyrrole hydrogen bonding to CO2. 

NMR analysis confirmed the presence of pyrrole hydrogen atoms only. 

The next stage of synthesis was the replacement of pyrrole hydrogen atoms by fluorine atoms. The zinc complex was fluorinated by cobalt fluoride according to the appropriate technique [44, 46], The target product was extracted from reaction products by chromatographic separation in a column filled with neutral aluminum oxide. 

Vacuum sublimation of a Zn(III) complex solution produced a pink-colored solid, which was Zn(III) perfhiorotetraphenylporphyrin. NMR analysis of this product showed the absence of pyrrole hydrogen atoms, which is the evidence for complete replacement. Then bivalent iron chloride (FeCl2) was prepared according to the common technique [17]. This compound was then used for Zn(III) ion replacement in a porphyrin ring. For this purpose, the Zn(III) complex was dissolved in dimethylformamide (DMF) and then, with the addition of an FeCl2 solution, also in DMF. The reaction mixture was heated in a steam bath at water boiling point for 5 h. Finally, a dark-brown sediment was deposited, and the solution was colored blue, which is typical of iron(III) perfhiorotetraphenylporphyrin (perFTPhPFe(III)) dissolved in DMF. In this case, the active weight of the catalyst iron(III) perfhiorotetraphenylporphyrin equaled 47.05 mg. 

Zinc and other metal ions have been found to promote pyrrole hydrogen ionization in 2-(2 -pyridyl)imidazole.269 Complexation studies114 on the systems N-methyl-histamine and NN-dimethylhistamine with bivalent zinc, copper, cobalt, and nickel have shown that the stabilities of the complexes follow the Irving-Williams series. With respect to the variation of a ligand with the same metal ion, the stability decreases in the series histamine, N-methylhistamine, NN-dimethylhistamine, possibly as a result of steric hindrance. The complexes are assigned the structure (12). 

The X-ray crystal structure of 3,4-dichoro-l//-pyrrole-2,5-dicarboxylic acid bis-phenylamide tetrabutylammonium salt reveals the formation of an unusual dimer in the solid state via amide NFI- -N pyrrole hydrogen bonds... 

It has been established by crystallographic data (12) and by H-NMR studies that the inner pyrrole hydrogens are opposite to each other. At room temperature the switch to equivalent positions is fast on the H-NMR time scale, as shown for several porphyrins dissolved in CDCI3/CS2 (22, 23). It is not known whether proton jumps are coupled to out-of-plane vibrations of the porphyrin skeleton. Double protonation to H4P2+ freezes in an out-of-plane conformation of tetraphenyl-porphin as shown in Fig. 1, which is taken from Ref. (12). Another type of distortion has been postulated recently in connection with an intermediate observed in the reaction of PtCll with haematoporphyrin IX (24). It has been argued... 

In counting the number of electrons contributed by Fe, it is helpful to think of the manner in which heme is formed from protoporphyrin (77). Two protons, belonging to pyrrole hydrogens, are displaced leaving behind two electrons. A ferrous ion with six d-electrons is incorporated into the structure to make heme. The original two electrons may be assigned to Fe-orbitals thus a total of eight electrons will be contributed. 

Loeb, Gale and co-workers have used Pt(II) as a structural template for the self-assembly of a series of anion receptors. Receptors 129-132 were found to bind a variety of anions in DMSO-d solution [92,93]. Even the simple tetrapyridine receptor 129 has an affinity for anions due to its electrostatic charge. The addition of pyrrole hydrogen-bond donors increases the stability of the receptor anion complexes by more than five-fold in 130, but in 131... 

Figure 76. Hydrogen-bond dimer between-CN group and /i-pyrrole hydrogen. Reprinted with permission from Kumar, R. K. Balasubra-manian, S. Goldberg, I. Inorg. Chem. 1998, 37, 541. 1998 Cordon and Breach Publishers.

Substitution of pyrrole hydrogens by Br and/or of phenyl hydrogens by Cl and/or F gave an increased activity [117,118]. 

More recently reported systems include compound 8 that contains a fifth pyrrole hydrogen bond donor in the strap (Figure 5). In the presence of substoichiometric quantities of chloride, this compound undergoes slow exchange on Ihe... 

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