Richter reaction

B (11— 12— 14). Examination of a more recent study suggests the von Richter reaction probably involves simultaneous attack of the triple bond on the diazo group accompanied by attack by a halide ion but not by a water molecule. The halocinnoline 13 is then hydrolyzed under the von Richter reaction conditions to furnish the 4-hydroxycinnoline derivative 14 (11 - 12- 13 14, pathway A). 

The behavior of aminopyrazole 88 (R = 4-NO2—C6H4) under these conditions was quite different diazotization using nitrous acid in concentrated hydrochloric acid afforded an alkynylpyrazole diazonium chloride, which did not participate in the Richter reaction, probably due to the electron-withdrawing effect of the nitro group. Instead, after neutralization of the hydrochloric acid with sodium hydrogen... 

Thus, the Richter reaction of the series of alkynylaminopyrazoles opens up a route to halo derivatives of l//-pyrazolo[3,4-c]pyridazines, 2//-pyrazolo[3,4-c] pyridazines, and lff-pyrazolo[4,3-c]pyridazines. 

The intramolecular cyclization of 2-alkynylaryldiazonium salts (Richter reaction) leads not only to 4-hydroxy- but also to 4-bromo- and 4-chlorocinnolines. The behavior of alkynylpyrazolediazonium chlorides differs from that of their benzene analogs. The Richter reaction of the series of alkynylaminopyrazoles gives only 4-halo derivatives of l//-pyrazolo[3,4-c]pyridazines and l//-pyrazolo[4,3-c] pyridazines, and mainly hydroxy derivatives of 2//-pyrazolo[3,4-c]pyridazines. 

Obviously any mechanism proposed for a reaction must account for all the products obtained and for their relative proportions, including products formed by side reactions. Incorrect mechanisms for the von Richter reaction (13-21) were accepted for many years because it was not realized that nitrogen was a major product. A proposed mechanism cannot be correct if it fails to predict the products in approximately the observed proportions. For example, any mechanism for the reaction... 

This reaction and some variations have been successfully used for the synthesis of heterocycles as indoles [253] or for the preparation of precursors necessary for the synthesis of cinnohnes by Richter reaction [140] as mentioned in Section 3.4.2.1. 

In the first reported solid-phase Richter reaction, cinnolines 143 were prepared through a two step sequence of a Heck-type coupling to give intermediates 142 followed by intramolecular ring formation and release of ciimolines 143 <99TL6201>. 

Addition of a suspected intermediate. If a certain intermediate is suspected, and if it can be obtained by other means, then under the same reaction conditions it should give the same products. This kind of experiment can provide conclusive negative evidence if the correct products are not obtained, the suspected compound is not an intermediate. However, if the correct products are obtained, this is not conclusive since they may arise by coincidence. The von Richter reaction (3-25) provides us with a good example here too. For many years it had been assumed that an aryl cyanide was an intermediate, since cyanides are easily hydrolyzed to carboxylic acids (6-5). In fact, in 1954, p-chlorobenzonitrile was shown to give p-chlorobenzoic acid under normal von Richter conditions.29 However, when the experiment was repeated with 1-cyanonaphthalene, no 1-naphthoic acid was obtained, although... 

Thermal electrocyclic reactions of 1-azahexatriene systems can be used to obtain imidazo[4,5-c]pyridines (25) [95H(41)161] and the thermal cyclisation of 2-alkynylbenzene-diazonium salts (the Richter reaction) has been extended to the synthesis of pyrazolopyridazines (26) (Scheme 17) [95LA775],... 

Particularly useful is a synthetic approach utilizing the diazotization of 0-alkynylanilines 222 followed by cyclization of intermediate diazonium salts 223 (the Richter method) (Scheme 128) . In this reaction a nucleophile attacks the C(l) of the alkyne moiety in the diazonium intermediate 223, allowing substitution in the C(4) position of the cinnoline 224. Classically, water was used as the attacking species in the Richter reaction but more recently chloride and bromide nucleophiles have been successfully utilized <1995LA775, 2004T7983>. 

Mechanism of Richter reaction Nucleophilic substitution in gas phase Reactions of radical ions Radical anions of nitro compounds Free radical reactions... 

In Richter reaction m-bromobenzoic acid may be obtained by reactini potassium cyanide with p-nitrobromobenzene. The original mechanism of tht... 

Some of us (94SC1733) have demonstrated that the classical Richter reaction (79MI1), the intramolecular cyclization of 2-alkynylaryldiazonium salts to give cinnolines, can be applied to the synthesis of not only 4-hydroxy- but also... 

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