Renaissance of 1,2,5‐Oxadiazolyl Diazonium Salts: Synthesis and Reactivity

Abstract

A new approach for the synthesis and reactivity of the 1,2,5‐oxadiazolyl diazonium salts is represented. A simple and direct protocol to previously inaccessible furoxanyl and furazanyl diazonium salts was developed for the first time. Azo coupling reactions of thus obtained diazonium salts with a series of structurally diverse CH‐acids have been also successfully performed. Therefore, this study reveals new horizons for the use of furoxanyl and furazanyl diazonium salts in organic chemistry.

Straightforward Access to the Nitric Oxide Donor Azasydnone Scaffold by Cascade Reactions of Amines

“…Diazotization of a substrate 1 a with NaNO 2 in acidic media followed by an azo coupling with KC(NO 2 ) 3 afforded target azasydnone 2 a in a low yield (entry 1), while other substituted nitromethanes were ineffective (entries 2–4). Utilization of t BuONO for diazotization was unsuccessful due to the very low nucleophilicity of the amino group in compound 1 a (entry 5) . NOBF 4 in TFA was a more effective diazotization system (entries 7–10).…”

“…Utilization of tBuONO for diazotization wasu nsuccessful due to the very low nucleophilicityo ft he amino group in compound 1a (entry 5). [23] NOBF 4 in TFAwas am ore effective diazotization system (entries 7-10). Further solventa nd Lewis acid screening (see Ta ble S1 in the Supporting Information) indicated that an addition of AcOH and HCOOHw as required to increase the yield of azasydnone 2a,w hile ZnCl 2 (0.25 equiv) was the best catalystofc hoice (entry 10).…”

“…The distribution of bond lengths of heterocycles is similar with previously reported structures. [9,12,17,23] To gain ad eeper insight into the conjugation between furoxan and azasydnone rings some DFT calculations were performedw ith the Gaussian09 softwarep ackage. [24] According to the geometry optimization of isolated molecules (see Figure S1-S3 in the Supporting Information for more details), the nearly planar structure of 2j molecule (the N5N3C2N2t orsion angle is 8.5(2)8 in crystal)i s only slightly affected by crystal packing effects:t he r.m.s.…”

“…Recently,w eh ave shown that amino-1,2,5-oxadiazolesu ndergo mild diazotization upon treatment with NOBF 4 in TFAa ffording corresponding diazonium salts in nearly quantitative yields. [23] We synthesized diazonium tetrafluoroborate 7a and, to our delight,i ts azo coupling with KC(NO 2 ) 3 followed by ZnCl 2 -promoted rearrangement/cyclization sequence afforded azasydnone 2a in ag ood yield (Scheme 5). Thus, it may be concluded that diazonium salt 7a…”

“…Based on the existing literature [19,23] and relying on the reaction scope and control experiments, we proposed ap lausible mechanism for this cascadeo ne-pot process (Scheme 6). Diazotization of initial amine with NOBF 4 generates ad iazonium tetrafluoroborate A which upon azo coupling with KC(NO 2 ) 3 af-fords azo compound B.I ntermediate B is rather unstable and undergoes fast elimination of NO 2 + .T his process is similar to a known solvolytic dissociation of tetranitromethane.…”