Nitrile oxides, dimerisation

Several examples of the synthesis of furoxans by dimerisation of nitryl oxides are shown below. The treatment of oximes 302 with iV-bromosuccinimide (NBS) and then with triethylamine leads to the formation of nitrile oxides 303, as shown by the presence of a strong IR absorption band at around 2300 cm 1 typical of the CNO group stretching. Slow dimerization of nitrile oxides 303 took place at room temperature leading to the furoxans 304 in good yields (Scheme 75 and Table 4) <2002S1701>. 

Trifluoromethyl substituted 1 -oxadiazoles 113 were prepared by heating the bis(oximes) 112 with dry silica <99H(51)627>. l -Oxadiazole-2-oxides(furoxans)are often produced by dimerisation of nitrile oxides and some novel examples of the method, which produces 3,4-disubstituted derivatives bearing medium and large rings, have been described <99JOC8428>. Two examples of compounds prepared in this way from bis(nitrile oxides) are the ftiroxans 114 (84%) and 115 (86%). 

Indoles have been prepared from reactions of o-aminophenylketones with reactive , or stable " arsonium ylides. Oxo-stabilized ylides reacted with 2-chloro-oximes to give trans-5-acyl-A -isoxazolines, and isoxazoles have been obtained from reactive arsonium ylides and a-isonitrosoketones, and from triphenylarsonium methylide and nitrile oxides The latter ylide reacts similarly with nitrile imines to give pyrazoles. With triphenylarsonium benzylides and benzoylylides,benzene diazonium salts give 1,3,4,6-substituted 1,4-dihydro-1,2,4,5-tetrazines in a reaction in which initial coupling of the reagents is followed by a dimerisation. ... 

Oxadiazoles," 1,3,4-oxadiazoles" and 1,2,5-oxadiazoles are well known, but the 1,2,3-oxadiazole system, which calculations indicate to be unstable relative to its ring-open diazo-ketone tautomer," is known only as a benzo-fused derivative (in solution) and in mesoionic substances, known as sydnones ," which have been well investigated. Furoxans ," which are formed by the dimerisation of nitrile oxides, have also been extensively studied. 1,2,3-Thiadiazoles, 1,2,4-thiadiazoles, 1,3,4-thiadiazoles" and... 

Finally, when 2,4-dimethoxy-6-methyl-l,3,5-triazine was nitrated with nitric acid alone at 20 °C, the product was 3,4-bis(3, 5 -dimethoxy-1,3,5-triazinyl)-l,2,5-oxadiazole 2-oxide (27). This product was presumably formed by the normal dimerisation of a nitrile oxide to a furazan N-oxide, the nitrile oxide in this case being 3,5-dimethoxy-l,3,5-triazinonitrile oxide (26). The latter would be formed by tautomerism and elimination of water from the mono-nitration product 3,5-dimethoxy-2-nitromethyl-l,3,5-triazine (25) before it could be further nitrated to the product formed imder mixed acid conditions (Scheme 24). 

Cycloadditions were found to be first-order reactions with respect to both 1,3-dipole and dipolarophile, in all cases so far investigated. There are some limits to kinetic studies of these reactions, as many 1,3-dipoles are very reactive substances. While aryl azides, diazoalkanes, some classes of azomethine imines (for instance sydnones), and some classes of azomethine oxides (nitrones) are stable and isolable, azomethine ylides are usually unstable, an exception being represented by a mesoionic oxazolone that has been used for kinetic investigations benzonitrile oxide has a very limited stability, although some substituted derivatives are stable for long periods nitrile imines are not commonly isolable because of their strong tendency to dimerise. 1,3-Dipoles of... 

---