Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part XI. A kinetic study of the effects of varying the reagent and the nitro-compound in the conversion of o-nitrobenzylideneamines to 2-substituted indazoles

Abstract

A kinetic study of the effects of varying the phosphorus reagent and substituents in the nitro-compound in the reductive cyclisation of o-nitrobenzylideneamines to 2-substituted indazoles has been made. First-order rate constants vary from 8.2 to 52.2 × 10^–5 s^–1 for the triethyl phosphite-induced reduction of N-o-nitrobenzylideneanilines [o-NO₂C₆H₄CHNC₆H₄Y; Y =p-MeO, p-Me, H, p-CI, p-Br, p-CO₂Et, p-CN. p-NO₂, or o-CI] and N-5-chloro-2-nitrobenzylideneaniline, and vary slightly from 5 to 6 × 10^–5 S^–1 for o-nitrobenzylideneamines [o-NO₂·C₆H₄·CHNR; R = Me, Et, Prⁱ, or Buⁿ]. The products of these reactions are the corresponding 2-aryl-(aryl = C₆H₄Y) and 2-alkyl-(alkyl = R)-indazoles. These results point to nucleophilic attack by phosphorus on the nitro-group via an intermediate ArN(O^–)OP(OEt)₃⁺. This is confirmed by a similar kinetic study of the conversion of N-o-nitrobenzylideneaniline into 2-phenylindazole using a series of tervalent phosphorus reagents. Acyclic regents react in the decreasing order (Me₂N)₃P > (EtO)₂PMe ∼ EtOPPh₂ > EtOP(NEt₂)₂∼ EtO-PNC₄H₁₀ > (MeO)₃P ∼(EtO)₃P ∼(PrⁱO)₃P. In accord with the nucleophilic character of the attack by phosphorus, the small ring effect in 2-diethylamino- and 2-ethoxy-1,3,2-dioxaphospholan manifests itself in a rate decrease compared with acyclic analogues. 2-Ethoxy-1,3,2-diazaphospholan, on the other hand, shows much increased activity pointing to stabilisation of the transition state for deoxygenation by hydrogen bonding. These results enable a brief comment to be made on the reagent of choice for preparative reactions.