Nitrile AT-oxides

The thermal or photolytic fragmentation of furazans to nitriles and nitrile AT-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. 

However, the thermolysis of diacylfuroxans (429) yielded two types of nitrile iV-oxides. An uncrowded diacylfuroxan such as (429a) rearranged to the a- acyloximino nitrile AT-oxide (430) the diacylfuroxan with bulky substituents such as in (429b) gave rise to the half molecule acyl nitrile A/ -oxide (431). Both types of nitrile A/ -oxides (431) and (430) have been trapped with DMAD and hexafluoro-2-butyne to give isoxazoles in good yield. These reactions are shown in Scheme 97. 

Alkylarylisoxazoles can be obtained from the cycloaddition of nitrile AT-oxides to substituted alkynes or alkenes (Section 4.16.4.1.2(H)), and from the condensation of the 1,4-dilithio oximes (358) with benzonitriles (72JHC183) or amides (78JOC3015). 

Nitroisoxazoles have been synthesized by reaction of l-chloro-2-nitroethylene with nitrile AT-oxides (75C350). 

The rate of dimerization of nitrile A-oxides is strongly influenced by the nature of R. When R = Cl, Br, CO2 alkyl or COR, the nitrile A-oxide cannot be isolated nor obtained in solution for any appreciable time. Table 11 gives the approximate time required for complete dimerization of some nitrile A-oxides (335) to furoxans (336) in benzene solution at 18 °C (70E1169). Evidently, steric and electronic effects dramatically increase the stability... 

Hi) Preparation of isoxazoles from nitrile N-oxides The reaction between a nitrile //-oxide and an alkyne is so facile that it is usually sufficient to leave an ether solution of the reactants at room temperature to obtain the desired isoxazole in good yield. The reaction is in general sensitive to the size of the substituent on the alkyne but not on the nitrile -oxide. In the case of poorly reactive alkynes, the difficulty may be overcome by generating the nitrile -oxide in situ and keeping its concentration low. 

The two major methods of preparation are the cycloaddition of nitrile oxides to alkenes and the reaction of a,/3-unsaturated ketones with hydroxylamines. Additional methods include reaction of /3-haloketones and hydroxylamine, the reaction of ylides with nitrile oxides by activation of alkyl nitro compounds from isoxazoline AT-oxides (methoxides, etc.) and miscellaneous syntheses (62HC(i7)i). 

During an attempt at destroying benzyl cyanide residues with sodium hypochlorite, a detonation was caused that was probabiy due to the formation of nitrogen trichloride. However, it might be asked if it was not due to the nitrile group oxidation by the hypochlorite present. 

Thermochemical parameters of some unstable nitrile oxides were evaluated using corresponding data for stable molecules. Thus, for 2,4,6-trimethylbenzo-nitrile N-oxide and 2,4,6-trimethoxybenzonitrile N-oxide, the standard molar enthalpies of combustion and sublimation at 298.15 K were measured by static-bomb calorimetry and by microcalorimetry, respectively, this made it possible to derive the molar dissociation enthalpies of the N—O bonds, D(N—O) (17). 

Primary amines can be dehydrogenated under mild conditions and in high yields to nitriles at the nickel hydroxide electrode (Eq. (9)). Short chain and reactive amines are already oxidized at 5 °C, whilst long chain amines need somewhat higher temperatures. Diamines are converted smoothly to dinitriles (Table 13). 

Table 13. Oxidation of primary a-unbranched amines to nitriles at the nickel hydroxide electrode...

The lower amines have been oxidized in similar yields to nitriles at silver oxide and copper oxide anodes Activation of the electrode by deposition of a nickel hydroxide oxide layer is less essential than with alcohols due to the higher reactivity... 

Ketones and nitriles are rather soft bases their coordination onto electron-deficient sites on oxides is, therefore, relatively weak. One may, however, expect an improved specificity of chemisorption due to their softness. Unfortunately, however, these substances very easily undergo chemical transformations at oxide surfaces. Thus, carboxylate structures are formed on adsorption of acetone on alumina (194, 245-247), titanium dioxide (194), and magnesium oxide (219, 248, 249). Besides, acetone is also coordinated onto Lewis acid sites. A surface enolate species has been suggested as an intermediate of the carboxylate formation (248, 249). However, hexafluoroacetone also leads to the formation of trifluoroacetate ions (219). The attack of a basic surface OH ion may, therefore, be envisaged as an alternative or competing reaction path ... 

Oxygen spillover was also advocated for reactions other than simple oxidations. Tascon et al. (151) studied the transformation of formamides to nitriles at 275°C on BiPMo catalysts prepared by a physcial mixture of powders of Mo03 and BiP04 ... 

Anodic oxidation of porphin in alkyl nitriles at a platinum electrode takes place in 2 one-electron steps.203 The loss of electrons occurs from the lone pairs on the nitrogen atoms according to Scheme 22. 

At oxidation level 3, acid chlorides occupy a key position, since they may serve as a nearly universal substrate for an isohypsic transformation into any kind of carboxylic acid derivative. Acid halides are electrophiles that are synthetically equivalent to acyl cations (RCO ). In this capacity they are used for the synthesis of such important compounds as esters, amides (and hence, nitriles), thioesters, etc. (see Scheme 2.57), and for the formation of C-C bonds in the Friedel-Crafts reaction (see above). Acid chlorides may readily lose HCl upon treatment with triethylamine. This isohypsic conversion leads to ketenes, important reagents widely employed in [2 + 2] cycloadditions, as we will see later. 

Nitrile-A -oxides are reducible at rather positive potentials to the nitriles thus 2,4,6-trimethylbenzonitrile-A/ -oxide is reduced at —0.7 V at pH 6 in aqueous ethanol to the corresponding nitrile [254]. 

Nitrogen Is ESCA spectra (at an 18° electron take-off angle with respect to the surface) of polypropylene treated in flames containing 4% added nitrous oxide at four different flame equivalence ratios (equivalence ratio represents the most fuel-lean flame, while the 1.20 equivalence ratio represents the most fuel-rich flame. The constituent peaks are labeled as follows N5 nitrate at 408.1 eV N4 nitro and nitrite at 406.3 eV N3 nitrosoamine and nitroso at 402.4 eV N2 oxime, nitrosoamine, hydroxylamine, and amide at 400.9 eV and Nl amine and nitrile at 399.8 eV. The arbitrary y-axis represents the signal strength. 

Aryl-substituted aldehydes 352 can be cleaved to chain-shortened carbonyl compounds 353 and formaldehyde by iodosylbenzene in the presence of acids or Lewis acids (Scheme 3.140). Formaldehyde is further oxidized to CO and CO2 under the reaction conditions [457]. Oxidative decarboxylation of 2-aryl-substituted carboxylic acids 354 into corresponding aldehydes, ketones (e.g., 355) and nitriles at room temperature can be achieved by treatment with (diacetoxyiodo)benzene and a catalytic amount of sodium azide in acetonitrile (Scheme 3.141) [458]. 

Toward this end, a Fischer indole synthesis employing 4-carbethoxycyclo-hexanone (212) and phenyl hydrazine in warm acetic acid was followed by reduction of the resulting indole with lithium aluminum hydride to fomish hydroxymethyltetrahydrocarbazole 213. Alcohol activation with tosyl chloride and subsequent displacement of the tosylate with cyanide yielded nitrile 214. Oxidation with periodic add in methanol then formed ketone 215. Reduction of both the nitrile and carbonyl moieties was next achieved using lithium aluminum hydride in a mixture of THF and glyme at reflux to furnish aminoalcohol 216. A thermal dehydrative cyclization via heating this product in o-dichlorobenzene at reflux then led to 1,3-(iminoethano)carbazole 83. 

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