Formazans verdazyls

Another method for the preparation of tetrahydro-1,2,4,5-tetrazines is by the cyclization of formazans (248) with an aldehyde in the presence of acid to yield the verdazylium cation (252), which is reduced under basic conditions to give the verdazyl (253) and further to the tetrahydro-1,2,4,5-tetrazines (254) <78HC(33)1075, p. 1174, 74T2841, 80CB2049, 78KGS991, 78KGS1137). 

The most frequently used method of this type is the reaction of hydrazones (247) with diazonium compounds to afford formazans (248). These can be cyclized with carbonyl compounds in the presence of acid to yield verdazylium cations (252) which are reduced under basic conditions to give the verdazyls (253) which can be further reduced to the tetrahydro-l,2,4,5-tetrazines (254). The formazans (248) can be alkylated and the alkylfor-mazans (249) cyclized by heating or treatment with a base to form the tetrahydrotetrazines (250). It is not necessary to isolate the alkylated compounds (249) the complete reaction (248-250) can be carried out at room temperature in the presence of barium oxide or hydroxide in DMF. Oxidation of (250) provides a synthesis of verdazyls (251) (see Section 2.21.3.4.2). 

Heating the hydrazides (398) with thionyl chloride in diglyme affords the chlorohy-drazones (399) which yield formazans (400) on reaction with pentaffuorophenylhydrazine. The formazans (400) undergo cyclocondensation with formaldehyde to give the verdazyls (401) which can be hydrogenated to the tetrahydro-l,2,4,5-tetrazines (402 Scheme 22)... 

Verdazyls 24 are intensely green azaallyl radicals of a tetrahydro-l,2,4,5-tetrazine type. They are obtained by the H -catalysed cyclocondensation of aldehydes, usually formaldehyde, with formazanes 22 and reduction of the initially produced verdazylium ions 23 in a basic medium (Kuhn 1963) ... 

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