Nitroso compounds dimerization

Oxaziranes are rapidly decomposed by the further action of peracids. As shpwn by Emmons and Krimm the oxazirane is converted into a carbonyl compound and a nitroso compound dimer. The reaction can be formulated via an oxazirane A -oxide intermediate [Eq. (30)]. 

A related procedure, which may be of value from the preparative standpoint, involves the preparation of /rans-nitrosomethane dimer by adding a solution of diacetyl peroxide in sec-butyl nitrite to warm sec-butyl nitrite [50]. From the product of the reaction it has been assumed that this preparation involves the generation of free methyl radicals which react with the nitrite to give nitrosomethane and alkoxy radicals. The latter disproportionate to ketones and alcohols, while the nitroso compound dimerizes. 

The pyridazine dioxide derivative (108) was made by intramolecular nitroso compound dimerization as shown (Scheme 23). 1,2-Oxathiin 2,2-dioxides are obtained by the addition of sulfuric acid to a,(3-unsaturated ketones, e.g. (109) — (110) (66HC(21-2)774). 1,2-Dithiins are synthesized from conjugated diynes using benzyl thiol reductive debenzylation of intermediate (111) by sodium in liquid ammonia at - 70°C gives, after aerial oxidation, the 1,2-dithiin (112) (67AG(E)698). 

Cyclopentadiene affords the /i-chloro nitroso compound (dimer), most probably as a mixture of regio- and/or stereoisomers29,30. 

Nitrosochlorination of alkenes, review 23, 537 suppl. 24 Nitroso compound dimers s. Azo N,N -dioxides... 

The formation of insoluble nitroso dimers is only observed in reactions involving fuctionalization at C-19 whereas 18-nitroso compounds apparently rearrange rapidly to the 18-oximes. However in most cases it is recommended that rearrangement be completed by brief treatment of the crude irradiation product in boiling isopropanol. 

The yields of the dimeric nitroso compounds are good. Thus the aliphatic nitroso compounds which previously were difficult to obtain become readily available. A series of dimeric nitroso compounds with primary and secondary alkyl groups are given in Table III. 

Dimeric Nitroso Compound.s from Oxaziranes and Peracids"... 

Dimeric nitroso compounds with tertiary alkyl groups show more tendency toward dissociation into the monomers. For example, 2-methyl-2-nitrosopropane is so volatile in the form of the monomer that it can hardly be isolated from organic solvents. For the prepara-... 

For nitrosyl chloride (Entry 8) and nitrosyl formate (Entry 9), the electrophile is the nitrosonium ion NO+. The initially formed nitroso compounds can dimerize or isomerize to the more stable oximes. 

The results for the nitroso compounds are very similar to those for the methylene dimerization. The different paths for cis- and trans-approach were explored, and optimized reaction paths similar to those shown in Fig. 8 were obtained. No activation barrier was found for any of the reactions studied. Experimental values of a few kcal/mole have been reported for the dimerization of nitroso compounds. One interesting result is that the EHT... 

DFT studies of the intramolecular ene-like (or the so-called 1,3-dipolar ene) reaction between nitrile oxides and alkenes show that this reaction is a three-step process involving a stepwise carbenoid addition of nitrile oxide to form a bicyclic nitroso compound, followed by a retro-ene reaction of the nitrosocyclopropane intermediate. The competitive reactions, either the intramolecular [3 + 2] cycloaddition between nitrile oxides and alkenes or the intermolecular dimerization of nitrile oxides to form furoxans, can overwhelm the intramolecular 1,3-dipolar ene reaction if the tether joining the nitrile oxide and alkene is elongated, or if substituents such as trimethylsilyl are absent (425). 

N-Hydroxy-N-nitrosamines with an aliphatic group at O2 produce a compound stable to aqueous acid and base (Fig. 3.4, 29) [158], whereas all other N-hydroxy-N-nitrosamines are susceptible to hydrolysis and appropriate 02-derivatives also render these materials vulnerable. The hydrolysis endpoint is the formation of nitroxyl (HNO) [which dimerizes to form nitrous oxide (N20)] and a C-nitroso compound. These products are formed from aryl [159] and alkyl bound unsubstituted diazenium-diolates as well as Oralkylated derivatives [160]. Studies of the solvolysis of Oi-alkyl derivatives are complicated by their tendency to decompose via competing radical pathways [161], but the Oi-benzyl derivatives are unique in that they hydrolyze back to the original synthetic precursors (Scheme 3.14) [162]. 

Solutions of many organic nitroso compounds, R—NO, display blue or blue-green colours owing to a weak absorption in the visible region (X 700 nm, e < 50). In the crystalline state, however, most of these compounds are colourless or at most pale yellow. The reason for this phenomenon is the dimerization reaction124. The dimers have an NN distance of about 131 pm, which indicates a certain degree of double-bond character, and show E/Z isomerism. They may thus be termed diazene-1,2-dioxides. 

PE spectroscopic studies of C-nitroso compounds have sometimes been hampered by these properties, but also the dimer-monomer transformation has been studied by this technique125,126 (vide infra). 

The Hel PE spectrum of nitrosomethane (Figure 13) was first studied by Bergmann and Bock125,127. This compound as well as several other aliphatic and aromatic C-nitroso compounds were investigated by Pfab and coworkers126,128, however several of them were dimers. 

Mass spectrometric investigations of C-nitroso compounds are rarely encountered in the literature2. C-nitroso compounds are difficult to handle due to dimerization and isomerization. Dimerization is a common feature leading to a dimer exhibiting cis/trans isomeric... 

Consequently, the MS investigations of C-nitroso compounds are frequently hampered by superimposed spectra due to dimerization/rearrangement of the initial compound. Further, electron impact spectra of larger non-aromatic C-nitroso compounds in general carry little information due to extensive fragmentation118. 

A study of a series of C-nitroso compounds, including monomers as well as dimers, by field desorption has demonstrated the superiority of this technique to this class of compounds118. All the compounds display intense molecular ions118. The method has a significant potential for studies of the equilibrium between mixed and pure C-nitroso compounds, since the amount of pure and mixed dimers present in a solution apparently can be visualized by the relative abundances of the respective molecular ions see Scheme 43. Determination of the concentrations versus time may resolve the kinetics of the dimer formation118. 

Nitroso derivatives (with the nitroso group bound to a carbon atom) can exist in three molecular forms176177 the monomer 63 and the dimers 64 and 65, Z and E, respectively. Aliphatic C-nitroso compounds are mainly dimers178. Aromatic nitroso derivatives, in solution, may be monomers or dimers, depending on the concentration and temperature, and on the substituent on the aromatic ring. Nitrosobenzene itself is in the E dimer form in the solid state179. 

Irradiation of 1-methyl-2-nitrocyclohexene 200 in benzene in the presence of methyl acrylate showed a dual pathway to give both isoxazoline 201 (54%) and the C-nitroso dimer 202 (22%)118 (equation 96). The isoxazoline 201 arose from an excited-state intramolecular cyclization and scission to give a nitrile N-oxide which is trapped by the acrylate. Concurrently, the photoinduced nitro-nitrite inversion also occurs competitively to give the C-nitroso compound which is isolated as the dimer 202. 

Radiation-induced electron transfer to nitroso compounds has also been studied. This technique, using electron expulsion from trichlorofluoromethane, provided data that the radical cation 54 is formed from nitrosobenzene at 77 K. Analysis of the EPR spectrum indicates that the singly occupied MO lies in the plane of the benzene ring and has high 2s character74. Irradiation of the dimer 55 under the same conditions shows that a trace of the monomeric radical cation 56 is produced74. 

Although electrochemical oxidation of Al-cyclohexyl-Al-hydroxylamine in the presence of pyridine afforded the corresponding dimeric nitroso compound with a low yield, A-hydroxy t-alkylamines were transformed into the corresponding nitroso compounds (equation 8) . Similarly, A-phenylhydroxylamine was transformed into nitrosobenzene under similar reaction conditions. ... 

In contrast to a straightforward and predictable decomposition pattern of photolysis with >400 nm light, irradiation of nitrosamides under nitrogen or helium with a Pyrex filter (>280 nm) is complicated by the formation of oxidized products derived from substrate and solvent, as shown in Table I, such as nitrates XXXIII-XXXV and nitro compound XXXVI, at the expense of the yields of C-nitroso compounds (19,20). Subsequently, it is established that secondary photoreactions occur in which the C-nitroso dimer XIX ( max 280-300 nm) is photolysed to give nitrate XXXIII and N-hexylacetamide in a 1 3 ratio (21). The stoichiometry indicates the disproportionation of C-nitroso monomer XVIII to the redox products. The reaction is believed to occur by a primary photodissociation of XVIII to the C-radical and nitric oxide followed by addition of two nitric oxides on XVIII and rearrangement-decomposition as shown below in analogy... 

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