Nucleophilic Substitution and Ring Fission

Nucleophilic Substitution and Ring Fission. Although 3-chloro-l,2-benzisothiazole (154) is convertible into the corresponding 3-ethoxy- and 3-alkylamino- compounds by nucleophilic substitution, its reaction with sodium cyanide in acetone does not yield the expected 3-nitrile, but produces, with simultaneous ring fission, a mixture of bis-(o-cyanophenyl) disulphide (155) (22%), o-cyanophenyl thiocyanate (156) (62%), and 

2- acetyl-3-aminobenzo[6]thiophen (157) (6%). The last product is obviously formed by participation of the carbanion derived from acetone.  

Action of copper(i) cyanide on (154) yields the disulphide (155) almost exclusively, whereas its successive treatment with ethereal n-butyl-lithium and dimethylformamide at — 70 °C (aimed at the synthesis of the 

3- carbaldehyde) gives merely o-(n-butyIthio)benzonitrile (158). The action of thiophenols yields disulphides of type (159). More detailed experiments employing sodium thiophenoxide showed that a mixture of diphenyl disulphide (46—50%), bis-(o-cyanophenyl) disulphide (155) (50— 

and o-cyanophenyl phenyl disulphide (159 X = H) (5%) is formed. Several possible reaction mechanisms of these ring fissions were discussed. 

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