IV-Substituted pyrroles

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

The retro-Paal-Knorr ring opening reaction leading to 1,4-dicarbonyl compounds was accomplished by heating iV-substituted pyrroles in a citrate buffer <06SL1428>. The sequence was coupled with a forward Paal-Knorr reaction enabling the exchange of the N-substituent on pyrroles. 

The thermal reactions of pyrroles include the rearrangement of IV-substituted pyrroles to C-substituted derivatives (Scheme 1). The rearrangement of W-acylpyrroles has also been reported to occur in the vapour phase on irradiation. 

An early example of this strategy is the palladium black catalyzed conversion of (Z)-2-buten-l,4-diol with primary amines (cyclohexyl amine, 2-aminoethanol, n-hexyl amine, aniline) at 120 °C to give iV-substituted pyrroles in 46-93% yield [119]. Trost extended this animation to the synthesis of a series of N-benzyl amines 169 from the readily available a-acetoxy-a-vinylketones 168 [120]. This methodology allowed for the facile preparation of pyrrolo-fused steroids. 

Erosion of 2-selectivity in favor of the 3-position with IV-substituted pyrroles has also been reported by Wang et al. in the Rh-catalyzed arylation of nitrogen heterocycles [63]. 

Intramolecular reactions of nitronic acids, obtained from nitroalkenes and acetoacetate, lead to furans or IV-substituted pyrroles, depending on the organic base which is used (Scheme 54). ... 

SCHEME 5 Synthesis of 2,5-dimethyl-IV-substituted pyrrole using uranyl nitrate hexahydrate as catalyst. 

Poly(iV-substituted dithieno[3,2-3 2, 3 -rf pyrroles (PDTPs) 167a,b) <2005MM4545>, a class of soluble (chiral) conjugated polymers with a stable oxidized state, were prepared by an oxidative coupling by FeClj in chloroform under an argon atmosphere. The crude material was reduced with hydrazine to afford compound 167 (Scheme 19) <2005MM4545>. 

A regiochemical outcome of a palladium-catalyzed direct C-H bond functionalization of the pyrrole ring can be directed by choice of IV-substitution with bulky groups directing to C-3. The oxidative alkenylation of (V-(Boc)pyrrole led selectively to a 2-vinylpyrrole whereas the same reaction with the (V-(TIPS)pyrrole produced a 3-vinylpyrrole <06JACS2528>. 

FIGURE 8.20 Peptides activated at an IV-methylamino-acid residue are postulated to epimer-ize because of the formation of the oxazolonium ion. Evidence for the latter resides in spectroscopic studies,96 and the isolation of a substituted pyrrole that was formed when methyl propiolate was added to a solution of Z-Ala-MeLeu-OH in tetrahydrofuran 10 minutes after dicyclohexylcarbodiimide had been added.95 The acetylenic compound effected a 1,3-dipolar cycloaddition reaction (B), with release of carbon dioxide, with the zwitter-ion that was generated (A) by loss of a proton by the oxazolonium ion. 

The reaction of 5(4H)-oxazolones (32) and miinchnones with triphenylvinylphos-phonium bromide (33) provides a mild synthesis of substituted pyrroles (34) (Scheme 11). The cycloaddition-elimination reactions of 5-imino-l,2,4-thiadiazolidin-3-ones with enamines and ester enolates produce 2-iminothiazolidines. " Chiral isomtinchnone dipoles show jr-facial diastereoselectivity with IV-phenyl- or A -methyl-maleimide in refluxing benzene. ... 

The formylation of methyl furo[2,3-/)]pyrrole-5-carboxylate 31a and its variously iV-substituted derivatives 31b, 31c, and 153 has been studied and 31d-f, and 154 were obtained (Equation 2) <1997MOL69, 1999CCC1135>. 

Although the same theoretical studies indicate very small energy differences between the syn and anti conformers of the 3-carbaldehydes of furan, thiophene and pyrrole with a slight preference for the syn conformer, in chloroform solution the furan- and thiophene-3-carbaldehydes adopt the anti conformers to the extent of 100 and 80% respectively (82T3245). However, iV-substituted 3-(trifluoroacetyl)pyrroles exist in solution as mixtures of rotational isomers (80JCR(s)42). 

The 1,4-addition of heterocycles to aromatic systems has been reported. Photolysis of piperidine in benzene, for example, leads to the formation of the -substituted piperidine (222).204 Pyrrole, on photolysis in benzene, behaves differently and yields the 2-substituted pyrrole (223).208 In both instances, excitation of benzene, probably to the triplet, appears to be the initial step in the photolysis. The photolysis of iV-nitrosopiperidine in the presence of anthracene also results in 1,4-addition and the formation of an anthrone oxime,... 

The effect of iV-substitution on the polymerization of pyrrole derivatives has been studied by Diaz et al.130). Using cyclic voltammetry, they found that the anodic peak current for polymerization of pyrrole on a Pt electrode was at a potential of... 

Bulky substituents show steric effects which change the usual selectivity, e.g., large 1-alkyl groups increase -substitution in trifluoroacetylation (80JCR(S)42) and in the Vilsmeier formylation (70JCS(C)2572 of iV-alkyl-pyrroles. With larger substituents like triphenylmethyl in position-1 of pyrrole, bromination, formylation, and trifluoroacetylation occur selectively... 

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