Oxidative cyclization of some aldehyde semicarbazones induced by metallic salts

Abstract

The oxidative cyclization of some aldehyde semicarbazones 10 with four different oxidizing agents has been effected. The structure of the semicarbazones and the nature of cyclizing agent affected the rate and yield of cyclization but they did not show any influence on the regiochemistry of reaction. In fact, 1,2,4‐triazoline 20 was the only heterocyclic ring obtained by the cyclization reaction.

Oxidative cyclization of aldehyde thiosemicarbazones induced by potassium ferricyanide and by tris(p-bromophenyl)amino hexachloroantimoniate. A joint experimental and computational study

“…[19][20][21][22][23] Despite the apparent similarity of these reagents, surprisingly we observed that very different results -both in terms of reaction regiochemistry and yields, and in terms of reaction kinetics -can be achieved by the use of different salts, depending on the hardness/softness of the metal cation (or, more properly, of the solvated metal-counteranion complex), as well as on its intrinsic oxidizing strength. On the grounds of a careful analysis of all available data, we proposed 2a,22,23 (Scheme 2) that the formation of the 1,2,4-triazole ring could be induced by an attack of the oxidizing agent on the tail thioamide-like moiety of the molecule, followed by a single electron transfer (s.e.t.)…”

“…Different reagents -such as bromine (or hypobromite), 15,16 bromate or iodate, 17 lead tetraacetate, 18 and high-valent transition metal salts [19][20][21][22][23] -have been used to perform these reactions. The former examples date up to the beginning of the last century; 24 since the pioneer work by Gibson 15 their synthetic and mechanistic aspects have been systematically studied.…”

“…[2][3][4][5] Frequently, such cyclization reactions are induced by acidic media or electrophilic reagents. In the latter case, this might involve de facto a formal oxidation of the organic substrate, as an implicit consequence of the process (the cyclo-etherification or lactonization of unsaturated alcohols and acids induced by halogens or sulfinyl and selanyl halides provides a typical case).…”

“…In the latter case, this might involve de facto a formal oxidation of the organic substrate, as an implicit consequence of the process (the cyclo-etherification or lactonization of unsaturated alcohols and acids induced by halogens or sulfinyl and selanyl halides provides a typical case). 3,[5][6][7][8][9][10][11][12][13] In other occasions, the cyclization mechanism involves a plain oxidation stepelectron abstraction or dehydrogenation -prior to the actual ring closure step. Examples of these reactions have been recently reviewed.…”

Indazole derivatives from substituted ylidene-N-phenylhydrazinecarbothioamides and benzo- as well as naphthoquinones

“…[1][2][3][4][5][6] Several oxidizing agents can be used for the cyclization of semi-and thiosemicarbazones and it has been investigated how the oxidant as well as the structure of the open chain molecules can affect regiochemistry and yield of the process. 1,7-9 2,3,5,6-Tetrachloro-1,4-benzoquinone (CHL-p, 10) and 2,3-dichloro-1,4-naphthoquinone (14) undergo nucleophilic substitution of one or two chlorine atoms by thioacetamide, [10][11][12] thiourea, 13,14 substituted thiourea, 15 thiocarbazones, 16 hydrazine-1,2-dicarbothioamides, 17 thiosemicarbazides, 18 and thiosemicarbazones.…”