| Hypervalent iodines as the oxidant and electrophilic reagent, due to the electrophilic nature of the iodine atom associated with the leaving group ability of iodophenyl moieties, have widely development and application in organic chemistry field. It is not only commercial available, non-toxic profile, ease of preparation and handling, and friendly environment, but also possesses merits with novel structure, special function and high selectivity. In many chemical reactions, oxidation of hypervalent iodines are similar to some metals, such as thallium and mercury. It is the best choice for replacing toxic transition metals and also can realize selective oxidation for many complex organic moleculars as the best reagent.In this paper, we have mainly explored the Beckmann-type rearrangement of o-hydroxy and o-aminoaryl N-H ketimines mediated by hypervalent iodines. In experimental process, the optimal reaction conditions have been gotten via testing the loadings and species of hypervalent iodines, solvents and additives, and then the scope of substrates have been investigated. A plausible mechanism is depicted below to explain the influence of the different substituting for rearrangement reaction. We have also developed our method to the application of the sequence to the preparation of two biologically active chlormidazole and clemizole with good yields.The experimental results indicated that the transformation of 1.0 mmol of o-hydroxyaryl N-H ketimines as starting material, in the presence of 1.5 equiv of Ph I(OAc)2 as oxidant, 2 m L of methanol as solvent and 0.5 equiv of NEt3 as additive, was performed at room temperature for 30 min into the various benzoxazoles with yield up to 83%. And two new products 2-methyl-5-cyanobenzoxazole and 2-methyl-5-nitrobenzoxazole are obtained. When to synthesized nitrogen protecting group of benzimidazole, first using commercial available o-aminoarylketones as the starting material react with NH3 in Me OH, and the crude o-aminoaryl N-H ketimines are obtained in quantitative yields after the simple handling to removal of the solvent, and then followed by the Beckmann-type rearrangement in the presence of Ph I(OAc)2. Most of o-hydroxy and o-aminoaryl N-H ketimines with substituted group on the phenyl ring are in favour of rearrangement reaction. But compared to other substrates, o-hydroxy and o-aminodiaryl N-H ketimines are bad for Beckmann-type rearrangement. Nitrogen protecting group of o-aminoaryl N-H ketimines at the para- or ortho-position, due to a higher nucleophilicity of the heteroatom, are also detrimental to get rearrangement products and increase the amount of indazoles. A plausible mechanism depict the procedure of [1,2]-migration of the aryl moiety of o-hydroxy and o-aminoaryl N-H ketimines and unrearrangement procedure. Chlormidazole and clemizole are synthesized through two or three steps with good yield from 2-methyl-N-tosylbenzimidazole and 2-methyl-N-phenylsulfonyl benzimidazole respectively.This work with mild conditions, enriches the synthetic method of benzoxazoles and benzimidazoles. Hypervalent iodines will be extended into new field and promoted the application in organic chemistry. |
