Nitroso dimers

In the 19-nor series, the reaction with NOF is more complex and there is isolated in addition to the fluoro nitrimine corresponding to (31) a 20% yield of the nitroso dimer (34), which dissolves in methanol-methylene dichloride solution to give the pure blue color characteristic of the monomer (35). The latter then isomerizes to the oxime (36). 

The reactions of A -steroids with nitrosyl fluoride parallel those of their A -isomers. Thus, 17 -acetoxyandrost-4-ene (37) is converted to the nitrimine (38), in 67 % yield and thence to the 4-ketone (39), which can be dehydrofluorinated to the A -4-ketone (40) with lithium bromide in di-methylformamide. In the corresponding 19-nor series the nitroso dimer is also formed. 

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. 

Note 1. The same result is obtained when the corresponding 19-oxime is used as starting material, instead of the nitroso dimer. 

Nitriles from 19-nitroso-dimers, 270 Nitrite photolysis, 239 19-Nitroso-5a-pregnane-3/8,6/3,20/3-triol... 

A choice between mechanisms (b) and (c) was achieved by photolyzing a mixture of a 14N-nitrite in the pregnane series and the 15N-nitrite (29) to half-completion. Rearrangement of the nitroso dimers thus obtained to a mixture of oximes followed by oxidation of the pure oxime from (29) to the ketonitrile and mass spectrometric analysis indicated the N15/N14 ratio to be 1.15 1.00. Analysis of pure unreacted (29) gave a ratio of 1.00 0.00. These results indicate that mechanism (c) is operative in nitrite photolyses. 

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. 

Thorough washing to remove the last traces of alkali is essential, or the nitroso dimer will decompose to volatile products on standing. 

Dave and co-workers have reported a successful synthesis of 2,2,4,4-tetranitroadamantane (117) which uses the mono-protected diketone (113) as a key intermediate. In this synthesis (113) is converted to the oxime (114) and then treated with ammonium nitrate and nitric acid in methylene chloride to yield the em-dinitro derivative (115). This nitration-oxidation step also removes the acetal-protecting group to leave the second ketone group free. Formation of the oxime (116) from ketone (115), followed by a similar nitration-oxidation with nitric acid and ammonium nitrate, yields 2,2,4,4-tetranitroadamantane (117). In this synthesis the protection strategy enables each carbonyl group to be treated separately and thus prevents the problem of internal nitroso dimer formation. 

Aliphatic primary amines are known to be oxidized by dimethyl dioxiranes to various products such as oximes, nitroso dimers, nitroalkanes, nitrones and oxazrridines under various conditions depending upon the oxidation reaction . In contrast, when secondary amines lacking a-hydrogens are allowed to react with Oxone and PTC in buffered acetone solution at 0 °C, nitroxides are obtained in good yields in a few minutes (equation 61) . 

The photolysis of nitrites which do not contain g-hydrogen atoms usually results in the elimination of nitric oxide and the formation of hydroxy and carbonyl compounds. When g-hydrogen atoms are present than the product is an oxime or the corresponding nitroso dimer. This reaction is known as Barton S reaction. 

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... 

A method for the preparation of unsymmetrical azoxy compounds involves the reaction of certain diimide dioxides with Grignard reagents [5]. This reaction has somewhat limited applicability because the diimide dioxides which were used were prepared by alkylation of organonitrosohydroxylamines, a class of compounds of which cupferron is perhaps the best-known example. The reaction is, in effect, a reduction of a diimide dioxide to an azoxy compound by use of a Grignard reagent. The overall process is represented by Procedure 2-3. Since the starting materials are, in effect, unsymmetrically substituted nitroso dimers, extension of the reaction to nitroso dimers would be interesting. 

Azo compounds may be considered to have the carbon-nitrogen framework of dimeric nitroso compounds. The oxidation of one of the nitrogen atoms of an azo compound to an azoxy compound has been discussed in Chapter 15. The oxidation of the second nitrogen would give rise to one of the resonance forms of a nitroso dimer. Indeed, on oxidation of 4-methylcinnoline with hydrogen... 

Bis(nitrosoalkane), see Nitroso dimers p-Bromoazobenzene, 334 Bromomethylazobenzenes, 309 1 -Bromo-3-methyl-1,2-butadiene, 30... 

N,N-Dinitrosofumardianilide, 438 N,N -Dioxides, reduction of, 377, see also Nitroso dimers and Diimide dioxides 2,2 -Diphenyl-2,2/-azopropane, 323 Diphenylcarbodiimide, 166, 206, 211, 217-218... 

Reduction of (V,(V -di oxides (bis nitrosyl derivatives or intramolecular nitroso dimers) with hydrogen with a palladium-on-carbon catalyst [58] (Eq. 54). 

The structures of nitroso dimers are generally regarded as resonance hybrids of structures such as (I), (II), and (III). 

Further studies of the oxidation of azo, azoxy, and hydrazine derivatives to the nitroso dimer stage would be of considerable interest. In such research the problems of unsymmetrically substituted nitroso dimers might also be considered. 

Photolysis studies of alkyl nitrites in the vapor phase has been limited to the verification of the presence of a nitroso dimer moiety among the products by ultraviolet spectrophotometry. The principal product of photolysis of butyl nitrite in the vapor phase is the trans-isomer of the dimer of nitrosomethane43 86 87 281 286 the reaction has been explained by a mechanism involving fragmentation of alkoxy-radicals. 

Recently, Kabasakalian et al.138-140 have reported the nitroso dimer formation in the photolysis of primary and secondary nitrites. Both this reaction and the Barton reaction16 are explained in terms of reactions of alkoxy radicals. 

Photolysis of primary, secondary, and tertiary alkyl nitrites in benzene indicated that a minimum chain length of five carbon atoms attached to the nitrite grouping is required for appreciable yields of secondary nitroso dimers via the Barton reaction. In the case of primary and secondary nitrites, a chain of length of four carbon atoms or more attached to the nitrite grouping is required for formation of the six-membered intermediate (e.g., A) involved in the Barton reaction.139140... 

The formation of the oxime (2) probably results from the presence of adventitious traces of hydroxylic material which catalyze the isomerization of the nitroso monomer, the primary product of the Barton reaction, to an oxime. More direct evidence for the formation of nitroso compounds in the photochemical rearrangement of nitrites is provided by the isolation of nitroso dimers. ... 

Irradiation of octyl nitrite has already been described Kabasakalian et al. have also studied the behavior of its lower homologs.24 It was observed that primary alkyl nitrites with more than four carbon atoms in a chain produced essentially identical yields (37 to 45%) of nitroso dimer formed by the Barton reaction until the minimum straight-chain length of four carbon atoms was reached. Butyl nitrite underwent internal hydrogen abstraction in poor yield. This is the result of a more difficult abstraction of a primary hydrogen as compared to a secondary hydrogen atom available in compounds with longer chains. Reaction paths 2 and 4 predominated to afford 1-butanol and butanal as the major products. 

Nitro-compounds fRNOj) are isomeric with nitrites, but their electronic structure, excited states and photochemistry are very different. There is no very low-lying (n.jt ) state, and nitroalkanes show n — 3i absorption with a maximum around 275 nm ( —201 mol - cm In cyclohexane solution, nitromethane (CH1NOi) is photoreduced to nitrosomethane(CH,NO, but nitroethane under the same conditions gives rise to a nitroso-dimer derived from the solvent CS.47). The latter process is probably initiated by cleavage of the carbon-nitrogen bond in the nitroalkane. In basic solution (when the nitroalkane is converted to a nitronate anion) irradiation can lead to efficient formation of a hydroxamic acid (S.48), and this reaction most likely proceeds through formation of an intermediate three-mem bered cyclic species. 

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