Benzothiadiazoles, reduction

The photo-reduction of 2,l,3-benzothiadiazole-4,7-dicarbonitrile (13) by EDTA in the presence of micelles gave a stable radical anion which could be observed by ESR <84CC1324>. The observed 17 line ESR signal was attributed to an overlapping quintet of quintets from a radical with (NA) = 0.255 mT and fl(NB) = 0.075 mT. The radical appears to be protected within the micelle when electron transfer is inhibited. 

Reductive cleavage of 2,1,3-benzothiadiazoles to afford the corresponding 1,2-diamines requires vigorous conditions. Both Zn, AcOH <2005JOC2754> and SnCl2, aq. HC1 <1997JCM250> afford benzenediamines. 

The radical anions of five isomers of nitrobenzothiazole (4-N02-, 5-N02-, 6-N02-, 7-N02- and 2-N02-) have been obtained by electrochemical reduction in DMSO and easily characterized by ESR spectroscopy [889], To the contrary, the chemical reduction in alkaline solution (t-BuOK in DMSO or glucose and MeOK in MeOH) presented some problems with 6-N02- and 4-N02-benzothiazoles, and 2-N02-benzothiazole did not provide any detectable paramagnetic species [889], Copper(II) complexes of 6-nitrobenzothiazole-A-sulfonamides as protective agents against superoxide anion have been investigated by ESR spectroscopy [890], The ESR spectrum of 5-nitro-2,l,3-benzothiadiazole RA obtained by electrochemical generation in DMF was reported [886], Unfortunately, the HFS constants of this radical anion are absent and we estimated the HFS constants from simulated spectrum (Table 3.40). 

Photosensitized vectorial electron transfer from the (morpholine)ethenesulfonate anion (MES ) to 1,5-anthraquinone disulfonate (AQDS ) mediated by dibutyl (BTDB) or diethyl (BTDE) esters of 2,l,3-benzothiadiazole-4,7-dicarboxylic acid solubilized in CTAB micelles has also been reported [73], The proposed mechanism of BTD-mediated electron transfer is shown in Figure 7, where reductive quenching of photoexcited BTD by interfacially adsorbed MES is followed by electron transfer from reduced BTD to the AQDS , recycling the sensitizer. 

Reduction of 4-nitro-5-amino-2,l,3-benzothiadiazole 1505 with Sn/HCl in the presence of a carboxylic acid having a C(l)-C(3) chain yields the corresponding 2,5-dialkylbenzo[l,2-i7 3,4-,7 ]diimidazoles 1506 (Scheme 389) <1998H(48)113>. 

Friedel-Craft acetylation of385 followed by bromination yields 5-(2-bromoacetyl)-1,3-dibenzyl-1H, 3H-2,1,3,-benzothiadiazole-2,2-dioxide (387). Reaction of 387 with N-benzylated amines followed by reduction with hydrogen produces (388). No significant increase in heart rate, blood pressure, left ventricular pressure and bronchodilator activity was observed when the thiadiazoles 388 were tested in dogs. 

A new and practical method for the reduction of 2,1,3-benzothiadiazole to 1,2-benzenediamines with magnesium and MeOH was described. The method is unique in that it only worked with MeOH. No reduction was observed when EtOH was used as solvent. Sensitive functional groups such as bromo, chloro, cyano and ester were well tolerated under these conditions, giving excellent yields (75-97%) of the 1,2-benzenediamine <01TL2277>. 

Amino-l,2,3-benzothiadiazoles can themselves undergo this rearrangement. Thus, diazotization followed by reduction with hypophosphorous acid converts 7-amino-6-chloro-4-methyl-l,2,3-benzothiadiazole (40) into the rearranged 4-chloro-6-methyl compound (41) a small but significant quantity of non-rearranged product (42) is also obtained. When C-4 is... 

Diazonium salts derived from 7-aminobenzisothiazole (43) similarly yield l,2,3-benzothiadiazole-7-carbaldehyde and analogues (44) on decomposition under Sandmeyer conditions. Reduction of the diazonium group by hypophosphorous acid produces, without rearrangement, the benziso-thiazole (45). ... 

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