Synthesis of NH-Free 3-Substituted Indazoles

Significance

Reported is the synthesis of 3-substituted indazoles 3 from N-tosylhydrazones 2 and o-(trimethylsilyl)aryl triflates 1 through a [3+2] ­dipolar cycloaddition. This reaction requires the presence of a fluoride source which allows the in situ generation of arynes and diazo compounds from the N-tosylhydrazones 2. It has been demonstrated that CsF was the best fluoride source. The addition of phase-transfer catalyst (TEBAC = [Et₃NBn]⁺Cl^-) is required to dissolve the conjugate base of 2. The scope of this reaction has been investigated with different arenes and N-tosylhydrazones [from (hetero)aromatic aldehydes bearing EWG, EDG and sterically hindered]. The unsymmetrical monomethoxybenzyne gave a 4.8:1 mixture of regioisomers, which suggests a [3+2] dipolar cycloaddition through a diazo intermediate. Moreover, N-tosylhydrazones derived from aliphatic aldehydes are not suitable substrates.