Pyrrole acid-base properties

The Lewis acid-base properties of zeolites received a growing interest recently. A series of works " suggested that the infrared and XPS spectra of adsorbed pyrrole can characterize both the Lewis basicity and the Lewis acidity of alkali-exchanged zeolites. The zeolite base strength decreases with an increase in Si/AI ratio of the framework, and increases when the electropositivity of the counter alkali cation is raised. The Lewis acid strength of counter cations increases with Si/AI ratio and decreases with electropositivity. In other words, the alkali-exchanged zeolites have both Lewis acid and Lewis base centres and provide an easy way to tune up the relative strengths of these two sites. 

Acid-base properties of zeolites are probed by studying their interactions with basic/acidic molecules by appropriate techniques (IR, NMR, calorimetry, TPD). For the external surface region, analogous methods based on XPS detection were developed following the pioneering work of Defosse and Canesson [65]. The probe molecules used are pyridine [55,56,66-72], ammonia [21,43,44,73], and pyrrole and chloroform for basic sites [70,74,75]. Kaliaguine [59] has published a review of the results. 

Fig. 11. Study of acidity and basicity with adsorbed probe molecules. Left column N Is lines of pyridine chemisorbed on Al-ZSM-22 (A),Fe-ZSM-22 (B),Al-ZSM-5 (C),Fe-ZSM-5 (D),B-ZSM-5 ( ) right column N Is lines of pyrrole chemisorbed on alkali-exchanged X. Taken from Borade RB, Huang M, Adnot A, Sayari A, Kaliaguine S (1993) Acid-base properties of zeolites an XPS approach using pyridine and pyrrole probe molecule. In New frontiers in catalysis, Proc 10th Int Congr Catal, Budapest, Hungary 1992, p 1625, with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netiierlands...

Indole thus appears to be closely related to benzene and to pyrrole. Its chemical properties are in accord with this view of its structure. Indole is a weak base and like pyrrole gives a red coloration with a pine shaving which has been moistened with hydrochloric acid. 

As with any metalloprotein, the chemical and physical properties of the metal ion in cytochromes are determined by the both the primary and secondary coordination spheres (58-60). The primary coordination sphere has two components, the heme macrocycle and the axial ligands, which directly affect the bound metal ion. The pyrrole nitrogen donors of the heme macrocycle that are influenced by the substitutents on the heme periphery establish the base heme properties. These properties are directly modulated by the number and type of axial ligands derived from the protein amino acids. Typical heme proteins utilize histidine, methionine, tyrosinate, and cysteinate ligands to affect five or six coordination at the metal center. 

Sulfonamides (R2NS02R ) are prepared from an amine and sulfonyl chloride in the presence of pyridine or aqueous base.1 The sulfonamide is one of the most stable nitrogen protective groups. Arylsulfonamides are stable to alkaline hydrolysis, and to catalytic reduction they are cleaved by Na/NH3,2 Na/butanol/ sodium naphthalenide,4 or sodium anthracenide,5 and by refluxing in acid (48% HBr/cat. phenol).6 Sulfonamides of less basic amines such as pyrroles and indoles are much easier to cleave than are those of the more basic alkyl amines. In fact, sulfonamides of the less basic amines (pyrroles, indoles, and imidazoles) can be cleaved by basic hydrolysis, which is almost impossible for the alkyl amines. Because of the inherent differences between the aromatic — NH group and simple aliphatic amines, the protection of these compounds (pyrroles, indoles, and imidazoles) will be described in a separate section. One appealing property of sulfonamides is that the derivatives are more crystalline than amides or caibamates. 

Subjecting 2 or 3 to strongly acidic conditions (e.g. 1 M CF3CO2H or concentrated HQ) leads to rapid demetalation and the production of two isomeric free-base macrocycles 10 and 11 [11, 12]. The structure of 10 was conflrmed by its spectral properties and its remetalation to form the nickel complex 5. On the other hand, the structure of 11 was determined solely by its spectral properties. For instance, the NMR spectrum of 11 demonstrated the presence of a fully unsaturated system. Typical also was the presence of pyrrolic proton signals in the 6-7 ppm range of the NMR spectrum [12]. Compound 11 was very unstable... 

Generally speaking, the spectral properties of the heterosapphyrins resemble those of the pentaaza parents . That is to say, each of these species displays intense Soret- and Q-bands that are red-shifted relative to those of the porphyrins. For instance, when one, two, or even three of the pyrrolic subunits of sapphyrin is formally replaced by a 3,4-dialkyl furan (as in 5.74-5.76), the absorption spectrum is relatively unaffected (Xmax for the free-base form is ca. 450 nm as compared to the max value of 445 nm for typical decaalkyl sapphyrins such as 5.21). Treatment of these oxasapphyrins with an acid such as HCl or HBr has only a minimal effect on the location of the Soret band it is also found at ca. 450 nm in the case of these protonated systems. 

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