1.2.4- Triazole 4-arylamino

Oxidative procedures have been utilized for the synthesis of both monocyclic five-membered heterocycles and their ring-fused analogs, although the ease of synthesis of the precursors for the latter ring closures results in wider application of this procedure. A variety of oxidizing agents have been used and the conversion of the benzylidene hydrazidines (221) into the 4-arylamino-l,2,4-triazole (222) was effected with mercury(II) oxide (77BCJ953). 

V,iV -Diarylguanidino)-3,5-bis(methylthio)[l,2,4]triazoles 60 undergo base-induced cyclization to give 7-aryl-6-arylamino-3-methylthio-77/-[l,2,4]triazolo[4,3-7][l,2,4]triazoles 61 (Equation 10) <1988H(27)161>. Similarly, the cyclization of the iV-substituted 4-ureido and 4-thioureido derivatives of 3-phenyl-4,5-di hydro-1//-[ 1,2,4] triazole-5-thiones 62 affords 7-substituted. 5//-[l,2,4]triazolo[4,3- ][l,2,4]triazol-6(7//)-one and -triazole-6(7//)-thiones 63, respectively (Equation 11) <1991RRC619>. 

In the case of 4-(./V,./V -diaryl)guanidines 64, base-induced cyclization occurs at carbon bearing the methylthio to give the corresponding 7-aryl-6-arylamino-2-methyl-777-[l,2,4]triazolo[4,3- ][l,2,4]triazol-3(2//)-thiones 65 (Equation 12) <1986H(24)3363,1987JP(1)2667>. 

Very early in the history of heteroeyelic chemistry it was diseovered by Dimroth (09LA183) that l-aryl-5-amino-l,2,3-triazole (1, R = H) undergoes a faeile, reversible isomerization to 5-(arylamino)-l,2,3-triazole (2, R = H) (Scheme IV.4). 

By contrast, 3-hydrazino-l,2,4-thiadiazoles (163), which are prepared by ring closure methods, are very sensitive to acids and undergo elimination of sulfur with formation of 5-arylamino-l,2,4-triazoles (164) as indicated in Scheme 61 (63JCS4566). The mechanism of this conversion is not known but presumably involves the attack of a hard nucleophile at the 5-position followed by ring opening, loss of sulfur and recyclization as has been observed with the quaternary salts (77JCS(P1)1791) (see Schemes 22, 23, and 24). Hydrazine (163) reacts with aldehydes to produce hydrazones (165) and with dimethyl malonate to yield pyrazoles (166). Diazonium salts of type (153) have been prepared by the methods... 

N,N -diaminoguanidine 578 reacts with carbodiimides in DMF to give 4-aryl-3,5-di(arylamino)-579 and 3-amino-4-aryl-5-arylamino-1,2,4-triazoles 580.55 ... 

Another synthetic pathway recently used is the cyclization of linear triazenes, like (224), which in aprotic media with Lewis base catal) is yield 5-amino-1-ary 1-1,2,3-triazoles (225). However, in protic media Dimroth rearrangement affords the isomeric 5-(arylamino)-l,2,3-triazoles (226) (70JOC2215, 81JOC856). The 2-tetrazene (227) gives methyl l-benzyl-1,2,3-triazolecarboxylate (228) by photochemically induced homol) is of a single N—N bond (81TL227). 

Ultraviolet spectra have proved useful in following Dimroth rearrangements (Section III,D) where a 3-alkyl (or 3-aryl) group becomes a 4-alkylamino (or 4-arylamino) group. Four such pairs can be seen in Table II, e.g., 4-amino-3-benzyl- and 4-benzylamino-1,2,3-triazoles. In each case, a bathochromic shift of about 10 nm occurs. H-NMR spectra are also useful in following such reactions (see Section II,D). 

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