α-(Aryithio)imidoyl Radicals: [3 + 2] Radical Annulation of Aryl Isothiocyanates with 2-Cyano-Substituted Aryl Radicals

Journal of Organic Chemistry

Volumn 62, Issue 24, 1997, Pages 8394-8399

  Leardini, Rino ^a   Nanni, Daniele ^a   Pareschi, Patrizia ^a   Tundo, Antonio ^a   Zanardi, Giuseppe ^a  

^a UNIVERSITY OF BOLOGNA   (Italy)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000136228     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo971128a     Document Type: Article

References (81)

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74. All the attempts to perform X-ray crystallographic determinations on the reaction products failed because of the unsuitable crystalline properties. Structure 12 was assigned to the products on the basis of the following arguments. It has been reported (ref 2t) that an iminyl radical very similar to 14 gives an exclusive 1,6-ring closure: in that case, the structure of the reaction product has been confirmed by X-ray diffraction. An analogous, exclusive 6-membered cyclization has been observed by Curran and co-workers (ref 2u) with an iminyl radical generated by addition of an imidoyl radical to a cyano group. Furthermore, to assign structure 18 instead of 12 to the reaction product, we have to postulate a mechanism involving exclusive spirocyclization of iminyl 14 (Scheme 3, path α) and one-way-rearrangement of the resulting spirocyclohexadienyl radical 15 through selective cleavage of the preexisting carbon-nitrogen bond (Scheme 3, path c). In our opinion, this is a very unlikely pathway. Finally, we generated radical 13e by addition of (2-cyanophenyl)sulfanyl radical to 4-methoxyphenyl isocyanide isonitrile. The reaction afforded major amounts of quinoxaline 12e together with traces of a crystalline byproduct derived from addition of another sulfanyl radical to 12e. The structure of this compound was fully established by X-ray diffraction, which confirmed the expected position of the substituent on the quinoxaline ring (full details of this result will be reported elsewhere). In light of this discussion, it seems quite reasonable to assign structure 18o to the minor product of isothiocyanate 19. In fact, there is no reason for supposing that, also in this case, iminyl 14o cannot give mainly 1,6-ring closure affording 12o. The minor quantity of 5-membered cyclization, leading to the isomer 18o, is probably the result of both a statistical factor (one of the two ortho-positions is not accessible for cyclization) and an increased stabilization of the spirocyclohexadienyl by the two chlorine atoms (if we postulate a reversible cyclization of 14).
75. An analogous α-phenylimidoyl radical, generated in the reaction of N-(phenylmethylene)-2-aminobenzonitrile with diisopropyl peroxydicarbonate (see refs 3d-k), did not afford any cyclization product either.
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