Studies of 2-AZA analogues of 1, 6, 6a λ⁴-trithiapentalenes : introduction of the =N-S unit into four-electron three-centre bonded structures and reactions of 1, 6, 6a λ⁴-trithiapentalenes with arenediazonium tetrafluoroborates

Abstract

There is only one report in the literature of a structure of the trithiapentalene type containing the =N—S unit. We have 4 thionated 3,4-dimethyl - 1 - oxa - 6, 6aλ⁴. -di selena- 2 -azapentalene with phosphorus pentasulphide and obtained two products, 3,4- 4 dimethyl-1-thia-6, 6a λ⁴ -diselena-2-azapentalene and 3, 4-dimethyl-4 1, 6-dithia-6a λ⁴ - selena-2-azapentane, both new systems which contain this moiety. A third product from the same reaction was 3,4- dimethyl-1-oxa-6—thia-6a λ⁴ - selena-2-azapentalene, the first trithiapentalene analogue containing four different heteroatoms. An attempt to introduce the =N—S unit into 4, 5-dihydro-3H-benzo [cd ]-l-oxa-6, 6a λ⁴ -dithia-2-azapentalene by treatment with phosphorus pentasulphide was unsuccessful. We have demonstrated that the very large increase in acidity of isothiazole-5-carboxaldoxime on methylation at N-2, which had been reported in the literature, is due to the formation of a stabilised bicyclic three-centre bonded structure on deprotonation of the resulting salt in solution. This investigation thus led to the isolation of 6-methyl- 1 -oxa- λ⁴ -thia-2, 6-diazapentalene, the first member of a new heterocyclic system. An attempt to introduce the =N—S unit into this product by thionation with phosphorus pentasulphide was precluded by the low yield in which this compound was obtained. Several other related compounds were also studied. It has been suggested that the products from reactions of 4 l,6,6a λ⁴ -trithiapentalenes with arenediazonium electrophiles arise from two distinct mechanisms, one involving attack at C-3(4) and the other involving attack at S-1(6). We have provided further verification of the latter mechanism by obtaining S-arylation products from the reaction of 5-methyl(ene)- 1, 2-dithiole-3-thiones with arenediazonium tetrafluoroborates. We have also trapped a dithiolium salt intermediate from this type of reaction as 3-p-nitrophenylthio-5-t-butyl- 1, 2-dithiolium tetrafluoroborate.