EPR and pulsed ENDOR study of intermediates from reactions of aromatic azides with group 13 metal trichlorides

Giorgio Bencivenni,Daniele Nanni,John C Walton,Hassane El Mkami,Riccardo Cesari

doi:10.3762/bjoc.6.84

Giorgio Bencivenni, Daniele Nanni + Show 3 more

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https://doi.org/10.3762/bjoc.6.84

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Abstract

The reactions of group 13 metal trichlorides with aromatic azides were examined by CW EPR and pulsed ENDOR spectroscopies. Complex EPR spectra were obtained from reactions of aluminium, gallium and indium trichlorides with phenyl azides containing a variety of substituents. Analysis of the spectra showed that 4-methoxy-, 3-methoxy- and 2-methoxyphenyl azides all gave ‘dimer’ radical cations [ArNHC6H4NH2]+• and trimers [ArNHC6H4NHC6H4NH2]+• followed by polymers. 4-Azidobenzonitrile, with its electron-withdrawing substituent, did not react. In general the aromatic azides appeared to react most rapidly with AlCl3 but this reagent tended to generate much polymer. InCl3 was the least reactive group 13 halide. DFT computations of the radical cations provided corroborating evidence and suggested that the unpaired electrons were accommodated in extensive π-delocalised orbitals. A mechanism to account for the reductive conversion of aromatic azides to the corresponding anilines and thence to the dimers and trimers is proposed.

Highlights

The number of applications of indium [1,2,3,4,5,6], gallium [7,8,9,10,11] and other group 13 metal derivatives, as promoters of radical reactions, has been increasing ever since the original work of Baba and co-workers with dichloroindium hydride [12,13,14,15,16]
We report here our findings on the behaviour of aromatic azides when treated with the group 13 trichlorides of gallium, indium and aluminium
Each organic azide was reacted with the metal halide in dichloromethane/pentane or acetonitrile solution at rt, and an aliquot (~0.1 mL) was placed in a quartz capillary tube, purged with nitrogen for 15 min and transferred to the resonant cavity of an X-band EPR spectrometer

Summary

Introduction

The number of applications of indium [1,2,3,4,5,6], gallium [7,8,9,10,11] and other group 13 metal derivatives, as promoters of radical reactions, has been increasing ever since the original work of Baba and co-workers with dichloroindium hydride [12,13,14,15,16]. The resulting deep-coloured solution was transferred to the EPR spectrometer and initially the spectrum, Figure 1a, was obtained at 300 K. We showed previously that treatment of phenyl azide 1 with GaCl3 gave well-resolved spectra of 4-aminodiphenylamine radical cation (11a+, the dimer) and of the trimer under different reaction conditions [31].

Results

Conclusion