Isocyanates furoxans

The major fragmentation in mass spectra of 1,2,5-oxadiazoles is attributed to the loss of nitrile and nitrile oxide or expulsion of NO. The conversion of 3,4-dicyano-l,2,5-oxadiazole-2-oxide (3,4-dicyanofuroxan) 10 to cyanogen iV-oxide 11 (Equation 5) was investigated under the conditions of collisional activation (CA) and neutralization-reionization (NR) mass spectrometry. Flash vacuum thermolysis mass-spectrometry (FVT-MS) and flash vacuum thermolysis infra-red (FVT-IR) investigations of furoxans 10, 12, and 13 reveal that small amounts of cyano isocyanate accompany the formation of the main thermolysis product 11 <2000J(P2)473>. 

Arylazo-4-(3-ethoxycarbonylureido)furoxans 62, which were synthesized by the reactions of 4-amino-3-arylazo-furoxans with ethoxycarbonyl isocyanate, were subjected to cascade rearrangements under the action of potassium r/-butoxidc in dimethylformamide or by heating in dimethyl sulfoxide to form 4-amino-2-aryl-5-nitro-2//-l,2,3-triazoles 63 (Scheme 13) <2001MC230, 2003RCB1829>. 

These routes are dimerization to furoxans 2 proceeding at ambient and lower temperatures for all nitrile oxides excluding those, in which the fulmido group is sterically shielded, isomerization to isocyanates 3, which proceeds at elevated temperature, is practically the only reaction of sterically stabilized nitrile oxides. Dimerizations to 1,2,4-oxadiazole 4-oxides 4 in the presence of trimethylamine (4) or BF3 (1 BF3 = 2 1) (24) and to 1,4,2,5-dioxadiazines 5 in excess BF3 (1, 24) or in the presence of pyridine (4) are of lesser importance. Strong reactivity of nitrile oxides is based mainly on their ability to add nucleophiles and particularly enter 1,3-dipolar cycloaddition reactions with various dipolarophiles (see Sections 1.3 and 1.4). 

Some routes of chemical transformations of nitrile oxides connected with the problem of their stability were briefly discussed in Section 1.2. Here only two types of such reactions, proceeding in the absence of other reagents, viz., dimerization to furoxans and isomerization to isocyanates, will be considered. All other reactions of nitrile oxides demand a second reagent (in some cases the component is present in the same molecule, and the reaction takes place intramolecularly) namely, deoxygenation, addition of nucleophiles, and 1,3-dipolar cycloaddition reactions. Also, some other reactions are presented, which differ from those mentioned above. 

The stability of o-sulfonylbenzonitrile oxides and their thiophene analogs probably depends on electronic factors. The same factors do not prevent dimerization, as can be seen from data concerning several differently substituted nitrile oxides of the thiophene series (103). Sterically stabilized 3-thiophenecarbonitrile oxides 18 (R = R1 = R2 = Me R = R2 = Me, R1 = i -Pr), when boiled in benzene or toluene, isomerized to isocyanates (isolated as ureas on reaction with aniline) while nitrile oxides 18 with electron-withdrawing substituents (R1 and/or R2 = SOiMe, Br) dimerized to form furoxans 19. 

Dehydration of primary nitroalkanes with phenyl isocyanate or acetic anhydride in the presence of catalytic triethylamine affords nitrile oxides, which may be trapped as their 1,3-dipolar cycloadducts or allowed to dimerize to the corresponding furoxans. Other dehydrating agents that have been used include diketene, sulfuric acid and, when the a-methylene group is activated by electron-withdrawing groups, boron trifluoride in acetic anhydride, trifluoroacetic anhydride with triethylamine, and nitric acid in acetic acid. 

There are general reviews on heterocyclic syntheses by cycloaddition reactions of isocyanates and on the use of heterocyclic cations in preparative organic chemistry. More specific topics are 5-hydroxymethylfuran-2-carb-aldehyde, isobenzofurans and related ort/io-quinonoid systems, the conversion of 2//-cyclohepta[Zj] furan-2-one (1) into derivatives of azulene, the synthesis of indoles from o-alkylphenyl isocyanides, and abnormal Fischer indolization reactions of o-methoxyphenylhydrazones. Two reviews on isoindoles have appeared and a lecture on highly conducting charge-transfer complexes that are based on heterocyclic selenium and tellurium donors has been reprinted.Recent advances in the chemistry of imidazole and in the use of nitro-imidazoles as radiosensitizers have been summarized. There have been reviews on benzimidazole A -oxides and on dihydrobenzimidazoles, benzimidazolones, benzimidazolethiones, and related compounds. Other topics are synthetic applications of 1,3-dithiolium and 1,3-oxathiolium salts and of isoxazoles, the chemistry of benzisoxazoles, 2-amino-oxazoles, 5-oxazolones (2), furoxans, benzofuroxans, and related systems, the synthesis of five-membered meso-ionic compounds, and tetrazoles. ... 

Strained furoxans are exploited as a source of polymer intermediates, since on mild heating they ring-open to dinitrile dioxides, which undergo spontaneous or catalyzed rearrangement to diisocyanates.76,332 Unstrained furoxans are also useful sources of nitrile oxides, which they provide on flash vacuum pyrolysis,334 or isocyanates, if the nitrile oxides are allowed to rearrange333,51 1 (Section V,A). 

The synthetic utility of the thermolytic formation of alkyl and aryl isocyanates from furoxans has been enhanced by the use of flash vacuum pyrolysis. 

In 1960 a new access to nitrile oxides was provided by dehydration of primary nitro compounds 4, achieved with 2 equiv of phenyl isocyanate, which is converted into diphenylurea, beside the substituted furoxan isoxazole derivatives are obtained in the presence of suitable dipolarophiles (Scheme 8.1) [6,7]. This method for the synthesis of isoxazoles has become popular in turn and both methods have been widely employed over the past 50 years the subject has been repeatedly reviewed [8-15]. 

Furoxans and Zl -isoxazolines from prim, nitroparaffins Sym. ureas from isocyanates... 

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