Reimer-Tiemann reaction, with pyrroles

The Ciamician-Dennstedt reaction can be thought of as the complement to the Reimer-Tiemann reaction (Scheme 8.3.2). The first step of both reactions is cyclopropanation of one of the carbon-carbon double bonds of a pyrrole with a dichlorocarbene, resulting in intermediate 3. The Ciamician-Dennstedt reaction results from cleavage of the internal C-C bond and elimination of chloride (path a), while the Reimer-Tiemann reaction results from cleavage of the exocyclic bond, and subsequent hydrolysis of the dichloromethyl moiety to furnish aldehyde 5 (path b). 

Under conditions more similar to those of the Reimer-Tiemann reaction 3-bromopyridine was obtained from pyrrole and bromo-form. Treatment of pyrrole with chloroform and aqueous alkali gave pyrrole-2-aldehyde curiously, the formation of 3-chloropyridine under these conditions does not appear to have been reported, in spite of being frequently quoted. However, indole gave both indole-3-aldehyde and 3-chloroquinoline under these conditions [Eq. (10)]. 

A variant of the Reimer-Tiemann reaction, using chloroform or bromoform with ethanohc sodium ethoxide, has been apphed (mainly by Plancher and co-workers) to certain pyrroles and indoles with interesting results. Thus Bocchi has shown that 2,5-dimethylpyrrole gave 3-halogeno-2,6-dimethylpyridine, and 2,4-dimethylpyrrole with bromoform gave two isomeric bromodimethylpyridines [Eq. (11)]. 

In the Reimer-Tiemann reaction, aromatic rings are formylated by reaction with chloroform and hydroxide ion." ° The method is useful only for phenols and certain heterocyclic compounds such as pyrroles and indoles. Unlike the previous formyla-tion methods (11-18), this one is conducted in basic solution. Yields are generally... 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

The contribution of the resonance forms XXI, XXII, XXIII, and XXIV to the structure of the anions is frequently overlooked, yet many base-catalyzed condensation reactions of phenol and pyrrole undoubtedly proceed through these resonance structures at the moment reaction occurs. The condensation of phenol with aqueous formaldehyde, the Kolbc synthesis (p. 197), and the Reimer-Tiemann reaction (p. 202) are striking examples of reactions which occur through the seemingly less important carbanion structure of the resonance hybrid. (See p. 133.)... 

Ring expansion during Reimer-Tiemann reaction conditions on pyrroles and indoles to furnish pyrid-ines and quinolines was observed by Ciamician in 1881 (equations 13 and 14). Although a preparative reaction of little use due to the low yield, this transformation stimulated others to carry out ring expansion attempts on nonphenolic substrates with considerable success. ... 

Methylpyrroles have been converted into pyridines by hydrochloric acid under severe conditions, and also by pyrolysis (p. 109). The formation of a 3-chloropyridine derivative from a pyrrole under Reimer-Tiemann conditions has been mentioned (p. 63). This type of reaction was discovered by Ciamician and Dennstedt treated pyrrole with chloroform in ether and isolated a small yield of 3-chloropyridine. Subsequently, similar reactions were realized with bromoform, carbon tetrachloride, methylene iodide and benzal chloride. Those of several of these reagents with lithium pyrrole in ether and sodium pyrrole under various conditions have been compared. The yields of pyridine derivatives were always low. In submitting 2,5-dimethylpyrrole to the Reimer-Tiemann reaction, Plancher and Ponti23 isolated a pyrrolenine (7). This and its analogues are not intermediates in the conversion of pyrroles into 3-chloropyridines. The idea that dichlorocarbene is the active reagent in reactions using chloroform is supported by recent work 22 ... 

The idea that dichlorocarbene is an intermediate in the basic hydrolysis of chloroform is now one hundred years old. It was first suggested by Geuther in 1862 to explain the formation of carbon monoxide, in addition to formate ions, in the reaction of chloroform (and similarly, bromoform) with alkali. At the end of the last century Nef interpreted several well-known reactions involving chloroform and a base in terms of the intermediate formation of dichlorocarbene. These reactions included the ring expansion of pyrroles to pyridines and of indoles to quinolines, as well as the Hofmann carbylamine test for primary amines and the Reimer-Tiemann formylation of phenols. 

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