Pyrrole ring, hydrogenation

Reduction of isoindoles with dissolving metals or catalytically occurs in the pyrrole ring. Reduction of indolizine with hydrogen and a platinum catalyst gives an octahydro derivative. With a palladium catalyst in neutral solution, reduction occurs in the pyridine ring but in the presence of acid, reduction occurs in the five-membered ring (Scheme 38). Reductive metallation of 1,3-diphenylisobenzofuran results in stereoselective formation of the cw-1,3-dihydro derivative (Scheme 39) (80JOC3982). 

Pyrroles are hydrogenated with more difficulty than are carbocyclic aromatics. In compounds containing both rings, hydrogenation will proceed nonselectively or with preference for the carbocyclic ring (/9), unless reduction of the carbocyclic ring is impeded by substituents. Acidic solvents are frequently used but are not necessary. 

Different spectrosopic investigations and calculations have led to the conclusion5-83-0 that the most stable form of porphin is the 21//,23//-tautomer 1 in which the two hydrogens are attached to the nitrogens in opposite pyrrole rings. 

The 21//,22//-tautomer 2 with hydrogens at adjacent pyrrole rings is less stable because of penetration of each hydrogen into the van der Waals sphere of the other. However, NMR studies with unsymmetrically substituted porphyrins at low temperature have allowed the observation of both tautomers 1 and 2. The kinetic parameters of tautomerism investigated by NMR measurements at different temperatures are consistent with a two-step process forming 3 from 1 via 2 rather than a concerted two-hydrogen shift which could form 3 from 1 directly. 

An approach to isobacteriochlorins1 ln-e makes use of Pd(II) or metal-free bilatrienes 1 as starting materials. Cyclization of the corresponding bilatriene derivatives is induced by base in the presence of palladium(II) or zinc(II) which exercise a template effect. Zinc can be readily removed from the cyclized macrotetracycles by acid whereas palladium forms very stable complexes which cannot be demetalated. Prior to the cyclization reaction, an enamine is formed by elimination of hydrogen cyanide from the 1-position. The nucleophilic enamine then attacks the electrophilic 19-position with loss of the leaving group present at the terminal pyrrole ring. 

The synthesis is straightforward because for the preparation of the tetrapyrranc a bipyrrole can also be used as a central unit to which a benzyl 5-(acetoxymethyl)pyrrole-2-carboxylate can be attached as the terminal pyrrole rings. Debenzylation by catalytic hydrogenation then gives the desired tetrapyrrane building block 68. 

The porphyrins found in nature are compounds in which various side chains are substituted for the eight hydrogen atoms numbered in the porphin nucleus shown in Figure 32-1. As a simple means of showing these substitutions, Fischer proposed a shorthand formula in which the methenyl bridges are omitted and each pyrrole ring is shown as indicated with the eight... 

As expected, under a hydrogen atmosphere in the presence of Pd/C in ethanol, the benzannulated pyrrolizine 48 leads to the dihydropyrrolizine derivative 49. However, semireduction of the pyrrole ring could be performed via the tricarbonyl chromium complex of 49 with various hydrides. Use of cyanoborohydride in trifluoroacetic acid (TFA) gave the best results for compound 50, both in terms of chemical yield (92%) and diastereoselectivity (90% of the trans-isomer) <2000TL1123>. 

In chlorophyll iron as complex-forming metal is replaced by magnesium (Willstatter). The structure of chlorophyll differs from that of haemin as follows. In chlorophyll one propionic acid chain (a) in oxidised form has condensed with a methine carbon atom to form a cyclopentane ring which takes the position at (c) of the vinyl ethyl. Further the two carbonyl groups are esterified and one of the four pyrrole rings is partially hydrogenated... 

In systems consisting of a pyrrole ring fused with a pyridine ring, hydrogen exchange also occurs in the 3-positions in acid solutions (El-Anani et a/., 1973). 

Pyrrole derivatives having double bonds in the side chains are first reduced at the double bonds and then in the pyrrole ring. 2-(2-Butenyl)pyrrole gave 88% yield of 2-butylpyrrole over platinum oxide in ether further hydrogenation in acetic acid gave a 94% yield of 2-butylpyrrolidine [429]. 

Catalytic hydrogenation may selectively reduce the double bond, or reduce the aromatic ring as well depending on the reaction conditions used [430,463, 464]. Only exceptionally has the benzene ring been hydrogenated in preference to the pyrrole ring [463,46S. ... 

In indole s isomer pyrrocoline (l-azabicyclo[4,3,0]nonatetraene) catalytic hydrogenation over palladium in acidic medium reduced the pyrrole ring [466, in neutral medium the pyridine ring [467],... 

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