Ring closure nitroso compounds

Trifluoromethyl-substitutedazimines are surprisingly stable compounds. They are accessible by 1,3-dipole metathesis from tnfluoromethyl-substituted azomethine imines and certain nitroso compounds [187, 188] On photolysis, an electrocyclic ring closure first gives the triaziridines, which are stable at room temperature. On heating above 80-100 C, a valence tautomenzation takes place and azimines are formed [189] (equation 43). 

C-nitroso compounds exhibiting an a-H, such as nitrosomethane and nitrosoethane, may isomerize to the corresponding oxime, as evidenced by mass spectrometry116,117. Unsaturated C-nitroso compounds may undergo electrocyclic ring closure as discussed below. 

Reaction between the nitroso group and the anion of a P-diketone is often sufficiently rapid to compete with further reduction of the nitroso compound to the hy-droxylamine. This process is illustrated by reduction of 8. The initial ring closure product undergoes further reduction at the catltode and 9 is isolated in good yield... 

The mechanism of the reductive ring closure by formation of a nitrogen-nitrogen bond is often closely related to the mechanism of the formation of azoxy compounds. This has mostly been formulated as an attack of the nucleophilic hydroxylamino group on the positive center in the nitroso group, followed by loss of water. Inasmuch as the system ArNO + 2 e + 2 H + ArNHOH generally is electrochemically reversible, it is not unlikely... 

Taking into account the vicarious nature of the nucleophile the authors suggested that ring closure occurs inside alkenic aH-adduct 290 with participation of the nitro group. However, in principle, the reaction might proceed via the formation of nitroso compound 291 with subsequent electrocyclization into oxazine 292 and hydrolytic exchange of the p-tosyl group (Scheme 85). 

Phenylhydroxylamines may be reoxidized anodically to the nitroso compound this may be used in ring-closure reactions (Chapter 18) [149]. If the reduction of the nitro compound and the reoxidation of the phenylhydroxylamine is done in a cell with two closely spaced porous electrodes, a condensation to azoxybenzene may be avoided [149]. 

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