1.2.5- Thiadiazole electrochemical reduction

Simple 1,2,3-thiadiazoles show three absorption bands in the ultraviolet (UV) 211-217 (emax 4380-5300), 249-253 (1460-2100), 290-294 (195-245) nm <1996CHEC-II(4)289>. The ESR spectrum for the radical anion generated by the electrochemical reduction of the 1,2,3-thiadiazolium ion 8 has been reported. A number of 5-substituted derivatives were also examined and the splitting constants in the ESR spectrum were analyzed <1998MRC8>. 

Electrochemical reduction of various 3,4-disubstituted-l,2,5-thiadiazole 1,1-dioxides (3,4-diphenyl- 10, phenanthro[9,10]- 51, and acenaphtho[l,2]- 53) gave the corresponding thiadiazoline 1,1-dioxides <1999CJC511>. Voltammetric and bulk electrolysis electroreduction of 3,4-diphenyl-l,2,5-thiadiazole 1-oxide 9 at ca. —1.5 V, in acetonitrile, gave 3,4-diphenyl-l,2,5-thiadiazole 8 (50%) and 2,4,6-triphenyl-l,3,5-triazine 54 (30%) (Equation 3) <2000TL3531>. 

Reversible electrochemical reduction of the [l,2,5]thiadiazolo[3,4-f][l,2,5]thiadiazole 6 led to the long-lived radical anion 7 and further to the dianion 8. The radical anion 7 was also obtained by the chemical reduction of the parent 6 in an ESR tube with 2 equiv of /-BuOK in degassed absolute MeCN at 50 °C for 20 min (Scheme 9). Electrochemical approaches to generation of the radical cation 12 failed <2005IC7194>. 

They can be replaced with hydrogen atoms by catalytic (Pd, Ni, etc.) or chemical reduction (HI or Zn/H2S04). For example, halopyrazoles with HI and red phosphorus at 150C give pyrazoles, and 5-bromo-l,2,4-thiadiazole is reduced by Raney nickel to the parent heterocycle. 2-Bromothiazole can be reduced electrochemically. 

A. Kaminaga, T. Tatsuma, T. Sotomura, N. Oyama, Reactivation and reduction of electrochemically inactivated polyaniline by 2,5-dimercapto-l,3,4-thiadiazole, Journal of the Electrochemical Society 1995, 142, L47. 

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