Fragmentation to nitriles

The thermal stability of 1,2,5-thiadiazole is remarkable. The parent ring and its alkyl derivatives can withstand heating to at least 250-300 °C. Under UV photolytic conditions, the aromatic thia-diazoles slowly fragment to nitriles and sulfur. 

Heating furoxans with alkenes may also yield 2-isoxazolines as a result of initial fragmentation to nitrile oxides and subsequent 1,3-dipolar cycloaddition (see Section 4.22.3.2.2) ... 

Simply-substituted 1,2,5-thiadiazoles, e.g. the parent ring and alkyl derivatives, withstand heating to at least 250-300 °C. The mass spectrum of 1,2,5-thiadiazole (see Section 4.26.2.6), which displays a major molecular ion, is further indication of the inherent stability of this ring system. Under IJV photolytic conditions the aromatic thiadiazoles slowly fragment to nitriles and sulfur. Attempts to trap the postulated nitrile sulfide intermediate were not successful (68CC977). 

Side reactions at the azidohydrin stage 5 have not been unequivocally identified but the subsequently formed iminodiazonium ion 6 can fragment to nitriles and species derived from a carbonium ion , reaction (34), and such a mode of reacdon accounts for most of the... 

This reaction was first reported by Semmler in 1892 and subsequently studied by Wolff in 1902 and Schroeter in 1911. It is the rearrangement of an oxime of an a,p unsaturated cyclohexenone (or known as the o, )0-unsaturated cyclohexenyl ketoxime) to an aromatic amine in a mixture of acetic anhydride and acetic acid saturated with HCl or HBr. Therefore, this reaction is generally known as the Semmler-Wolff aromatization," Semmler-Wolff rearrangement, or Semmler-Wolff reaction. Occasionally, it is also referred to as the Semmler-Wolff-Schroeter reaction. It should be pointed out that the Beckmann Rearrangement,and the fragmentation to nitriles often compete with the Semmler-Wolff aromatization. In addition, the Semmler-Wolff aromatization is obstructed by the substituents close to the reaction center. ... 

The thermal or photolytic fragmentation of furazans to nitriles and nitrile Af-oxides has been reported (73JOC1054, 75JOC2880). The irradiation of dimethylfurazan (419) in the presence of cyclopentene, and benzofurazan (420) in the presence of dimethyl acety-lenedicarboxylate, gave isoxazoline (421) and isoxazole (422), respectively, in good yields. The thermolysis of acenaphtho[l,2-c]furazan (423) in the presence of phenylacetylene gave isoxazole (424) in 55% yield. 

Certain ketoximes can be converted to nitriles by the action of proton or Lewis acids. Among these are oximes of a-diketones (illustrated above), a-keto acids, a-dialkylamino ketones, a-hydroxy ketones, p-keto ethers, and similar compounds. These are fragmentation reactions, analogous to 17-25 and 17-26. For example, ot-dialkylamino ketoximes also give amines and aldehydes or ketones besides nitriles. 

Photolysis as well as thermolysis (150°C, copper-catalyzed) of (79) results in quantitative loss of CO2 (Scheme 29). The 1,3-dipole (80) cyclizes to the benzimidazole (81) or is intercepted by dipolarophiles like benzonitrile or by an alcohol (Scheme 29) <68TL325>. Other oxadiazolones of type (79) fragment to CO2, a nitrile, a nitrene, and a carbene finally leading to a host of products <79JCS(P1)483>. 

The furoxan ring is more susceptible to nucleophilic attack and reduction than it is to reaction with electrophiles or oxidation. Grignard reagents react with disubstituted furoxans primarily at C-3 and, in most cases, the resulting adduct fragments to a nitrile and a nitronate salt which affords a ketone on workup. 

Oximes derived from p-silyl and P-stannyl ketones undergo unusual Beckmann rearrangement, giving rise to nitriles as predominant products. For example, Beckmann rearrangement of 3-trimethylsilylcyclohexanone oxime 88 (Scheme 15) proceeds to the formation of the acyclic alkenyl nitrile 89 as the major product.90 The nitrile is believed to be formed by fragmentation of the protonated oxime, giving the P-silyl—stabilized cation 90, followed by desilylation. Our... 

The photocatalytic radical generation was also used in additions to methyl cyanoformate 143 [240]. The product distribution was dependent on the reaction temperature. At room temperature ot-imino esters 144 were formed in 51-99% yield, while at 90 °C nitriles 145 were isolated in 59-76% yield. At low temperature, the iminyl radical 140C generated by the radical addition, abstracts a hydrogen atom from the substrate to continue a chain reaction, while its fragmentation to the nitrile prevails at high temperature, generating carbon dioxide and a methyl radical 146, which acts as a chain carrier. 

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