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The aminium radicals generated from the photolysis of N-nitroso-endo-2-methylamino-methylbicyclo[2.2.1]hept-5-en and N-nitroso-endo-2-methylaminomethylbicyclo[2.2.2]oct-5-ene cyclize at the C-6 position, since the relevant reacting centers can achieve excellent orbital overlap with minimum molecular motion in comparison to the alternative cyclization path at the C-5 position. The photolysis of the former compound gave a C-nitroso compound which underwent a series of unusual intramolecular cleavages, deoximations, and redox reactions during the work-up. Stereoelectronic requirements for these intramolecular reactions are discussed. Thermolysis of N-chloro-endo-2-methylaminomethylbicyclo[2.2.2]oct-5-en in methanol also gave the product with azaisotwistane skeleton.

Novel oxidations attending the photorearrangement of a nitrosamide and the photoaddition of nitrosamine are described and compared with the corresponding non-oxidative photoreactions. With proper filters, disproportionation during photorearrangement of N-nitrosohexylacetamide and tautomerization during nitrosopiperidine photoaddition to cyclohexene can be minimized to give improved yields of the anti-dimers of C-nitroso compounds. These side reactions are shown to be exclusively derived from secondary photoreactions of the C-nitroso compounds. In the presence of oxygen, the courses of the photoreactions are diverted cleanly to form nitrate esters. Oxygen reacts with nitric oxide, both in the free state and when paired with the C-radical thus diverting the course of the reaction. The rate constant of radical combination between a C-radical and nitric oxide is estimated to be in the range of 2 × 107 s−1.

Irradiation of the anti-dimers of C-nitroso compounds under nitrogen caused dissociation to the corresponding C-nitroso monomer which underwent photodisproportionation, tautomerization, and dimerization. Similar irradiation underhoxygen led principally to nitrate and nitro compound formation. The disproportionation of N-(4-nitrosohexyl)acetamide was shown to occur by photodissociation of the C-nitroso monomer to the alkyl radical and nitric oxide. Under an oxygen atmosphere, those C-nitroso compounds which could undergo facile photodissociation were quickly oxidized to nitrates. As a result, mixtures of the corresponding cis- and trans-C-nitrates were obtained from the anti-dimer of trans-1-nitroso-2-chlorocyclohexane. The anti-dimer of trans-1-nitroso-2-piperidinocyclohexane was photolytically tautomerized to the corresponding oxime under nitrogen and was oxidized chiefly to the C-nitro compound under oxygen; the two processes obviously share a common intermediate. Mechanisms for these photoprocesses are suggested.

Irradiation of N-nitrosopiperidine and 3,3-dimethyl-1-butene in a Pyrex apparatus gave 3,3-dimethyl-1-piperidino-2-(N-nitrosohydroxylamino)butane as the major product, in contrast to similar additions to 1-hexene and other olefins. Depending on the conditions, minor amounts of 3,3-dimethyl-1-piperidino-2-(N-hydroxylformamido)butane and 3,3-dimethyl-1-piperidino-2-butanone oxime were also isolated. It was proposed that the butane derivatives were formed by the reactions of the C-nitroso monomer with HNO and formaldehyde respectively which were photolytically generated from N-nitrosopiperidine and methanol. In agreement with this proposal, irradiation of a mixture of trans-dimer of 1-piperidino-2-nitrosohexane and N-nitrosopiperidine gave 1-piperidino-2-(N-nitrosohydroxylamino)hexane. By adjusting the irradiation conditions, this nitrosohydroxylamine could be obtained as the major product from addition of the nitrosamine to 1-hexene.

The photolyses of several N-nitrosamides in acidic media were studied and compared with the photolysis of N-nitrosamines on the one hand, and with the thermolysis of N-nitrosamides on the other. The course of the photolysis was found to be similar to that of the N-nitrosamine photolysis, namely, (i) photo-elimination of [NOH], and (ii) a light-catalyzed denitrosation. In contrast to the exclusive cleavage of the nitrogen–carbonyl bond in the thermal decomposition of the N-nitrosamides, photolysis results in fission of the nitrogen–nitrogen bond.

The initial step in photolysis of alkyl N-nitrosamides 1 and 2 has been shown to be homolysis of the N—N bond, giving amidyl radicals and nitric oxide. Under the reaction conditions nitric oxide disproportionated, in part, to NO2 and NO3 radicals. The amidyl radicals abstracted hydrogen atoms inter-molecularly from the solvent as well as intramolecularly from the δ-carbon of the alkyl chain, but not from that of the acyl chain. The combined pathways lead to the observed products as shown in Schemes 1 and 2. By this photolysis, the functionalization at the unactivated δ-carbon site of the alkyl chain could be achieved in 55% yield.

It has been established that alkylnitrosamines undergo photoreduction in methanol solutions in the presence of an acid to give the parent amine, formaldehyde, and N-aminoformamide. Where it is possible, an exchange reaction between the δ-hydrogen and the nitroso group is also observed during this photolysis. However, photoelimination also occurs in competition with photoreduction, the extent of which is dependent on the rates of the two photoprocesses. In aqueous solutions, N-nitrosamines predominantly undergo photoelimination. Those nitrosamines that undergo rapid photoelimination or photoreduction do not efficiently add to olefins photolytically. Conversely if photoaddition is retarded, a nitrosamine takes elimination or reduction pathways. Excitation of n → π* or π → π* transition band of N-nitrosamines gives virtually the same photoreaction patterns. Minor differences in the photoproducts are attributed to other secondary reactions.

N-Cyanopiperidines (1 and 2) have been shown to possess a n → π* transition band in the vicinity of 260 mμ and, in the presence of a dilute acid, undergo photolytic reactions leading to hydration products (3 and 4). The fact that the reaction can be sensitized by acetophenone demonstrates that 1 reacts through a triplet state. ET is about 76 kcal/mole. Increasing the water content in the photolysis medium reduced the yield of the hydration product.

A(1,3) type strain was demonstrated directly from the nuclear magnetic resonance studies of the acyl and nitroso derivatives of 2-methylpiperidine (2 series) and 2,6-cis-dimethylpiperidine (1 series). In these two series, the bulky 2-methyl groups are forced to assume the axial conformation due to a severe A(1,3) interaction in the equatorial conformation. The free energy of activation (ΔG*) for the internal rotation around the N—C and N—N bonds in 1a–1g was determined. The comparison of these (ΔG*) values with those of the corresponding derivatives of dimethylamine indicates the decreases are in the range of 3.6–4.7 kcal/mole which is in fair agreement with the conformational energy of 1,3-diaxial CH3/CH3 interaction. An argument was presented to show that this interaction energy corresponds to the decrease of ΔG* in 1a-1g.

The nuclear magnetic resonance spectra of various heterocyclic nitrosamines were studied in solution. A qualitative view of the long-range diamagnetic effects of a nitrosamino group is advanced and correlated with the chemical shifts of α-equatorial protons and the methyl group in those nitrosamines possessing a preferred conformation or a rigid system. The use of this correlation to decide the configuration and conformation of heterocyclic nitrosamines is discussed. A large chemical shift difference (ca. 3 p.p.m.) between a cis α-axial and a cis α-equatorial proton was noted.

N-Nitrosodialkylamines underwent photolysis in the presence of an acid. From the analysis of the end products, the primary photolysis products were shown to be [NOH] and the corresponding alkylideneimine, which underwent further reaction, e.g. (i) recombination of the two species to give amidoximes, (ii) hyponitrous acid formation, and (iii) hydrolysis or polymerization. A pathway is proposed for the reaction. Evidence is presented to show that the formation of the parent amines was not the result of hydrolysis of nitrosamines.

Phenanthraquinone (PQ) photolytically adds to cis- and trans-alkenes to give a mixture of cis- and trans-1,4-dioxenes and two reduction products. The ratios of cis- to trans- 1,4-dioxenes remain the same irrespective of the photoaddition to cis- or trans-alkenes and are 1:1 for 2-butenes, 6:4 for 3-hexenes, and 8:2 for cyclohexene. A triplet excited phenanthraquinone is proposed as the reactive species which initiates the addition leading to the intermediate diradical 10. Depending on the nature of addenda the diradical 10 may be stabilized by a charge transfer structure in different conformations leading to the observed stereochemistry of the adducts. The PQ fails to add to fumaronitrile photolytically or thermally and thermally adds to 2-butenes only inefficiently. Biacetyl undergoes photoreduction readily in the presence of a large excess of cyclohexene.

Rate constants for quenching of triplet excited acetone by primary and tertiary amines in solution have been measured directly by flash and pulsed laser techniques. The rate constants were found to be independent of solvent dielectric constant over the range ε 1.9 to ε 80, but were influenced by amine ionization potential. The results are interpreted as supporting a charge-transfer model for the quenching of triplet ketones by tertiary amines. The intervention of free ions in the quenching process is discussed. Free energy calculations indicate that electron transfer from N,N-dimethylaniline to triplet ketones is exothermic but is slightly more favorable for triplet aromatic ketones than for triplet acetone. Evidence for direct hydrogen atom transfer was obtained for quenching of triplet acetone by n-hexylamine.

The geminal coupling constants between the non-equivalent benzylic protons of a series of para- and meta-substituted N-benzyl-2-methylpiperidines were shown to be proportional to the Hammett σ constants of the substituents with ρ −1.38 in carbon tetrachloride, −1.21 in benzene, and nearly 0 in 1 N DCl solutions. The ρ values were compared with those of other series and were discussed in terms of the possible conformations involved. The chemical shifts of the benzylic protons of the piperidine derivatives did not give a good correlation with the Hammett σ constants in these solvents.

In the presence of an acid, photoexcited N-nitrosopiperidine adds to conjugated acyclic 1,3-dienes to give the 1,4-adducts as the major product in addition to 1,2-adducts. In the photoaddition to 1,3-pentadienes the reverse 4,1-adduct and 4,3-adducts are also identified. These results suggest that a singlet photoexcited N-nitrosopiperidine–acid complex rapidly decays to piperidinium radical which irreversibly attacks the dienes as the first step, followed by either a short propagation step or a termination step to give C-nitroso compounds. The C-nitroso compounds are rapidly removed by irreversible tautomerization to the final oximes. The reactivity of the piperidinium radical toward conjugated double bonds decreases in the order of RCH=CH2 > cis-RCH=CHR > trans-RCH=CHR. This mechanism is in accord with the following stereochemical results. Firstly, the newly-formed double bond in the 1,4-adduct (or the adduct of the reverse orientation) is exclusively trans-oriented. Secondly, the remaining double bonds in the 1,2-adducts maintain their original stereochemistry.The stereochemistry of the photoadducts, enone oximes 1–11, was determined by means of n.m.r. spectroscopy. From the correlation of the n.m.r. chemical shifts of these enone oximes it has been shown that (i) the α-olefinic proton generally resonates at a lower field than the β-olefinic proton (with one exception) and (ii) in changing from CDCl3 to an aromatic solvent the α-olefinic proton and the α′-alkyl protons are de-shielded by 0.3–0.05 p.p.m. while the β-olefinic proton is shifted only slightly either to a higher or a lower field. It is observed that the order of the chemical shifts of the olefinic protons and the aromatic solvent effects on the chemical shifts in these enone oximes are both opposite to those observed in enone systems.

In the presence of an acid, N-nitrosopiperidine photolytically adds to conjugated dienes to give 1:1 adducts in excellent yield. While the major products are the syn- and anti-isomers of piperidino α,β-unsaturated oxime derived from 1,4-addition, similar types of oximes derived from 1,2-addition are also obtained. The ratio of syn- and anti-oximes is determined by the relative configurational and the conformational stabilities of the two forms. The nuclear magnetic resonance (n.m.r.) chemical shifts and coupling constants provide useful information for the determination of the configuration and conformation of the oximes. A comparison of the n.m.r. data from α,β-unsaturated oximes indicates that the α-proton generally resonates at a lower field than the β-proton in contrast to the chemical shifts of α,β-unsaturated carbonyl compounds. The photoaddition of nitrosopiperidine to cycloheptatriene gave tropone oxime, probably through the elimination of piperidine from the 1:1 adduct.

Under the simultaneous influence of light and an acid, the nitrosamines derived from four heterocyclic amines readily form 1:1 photo-adducts with cyclohexene giving excellent yields of 2-tert-aminocyclohexanone oximes. While N-nitrosodi-n-butylamine, under similar conditions, gives a fair yield of the 1:1 photo-adduct, N-nitrosodicyclohexylamine and N-nitrosodiisopropylamine do not. The conformations of these oximes are discussed in conjunction with the n.m.r. data.

Photolysis of 1-pentyl and menthyl nitrites in the presence of oxygen were shown to give mainly the oxidized rearrangement products 4-nitratopentyl and 10-nitratomenthyl derivatives in disagreement with the published report (2). Under the photo-oxidation conditions, menthyl nitrite also gave 10-nitrosomenthyl derivatives which were also shown to be photo-oxidized to 10-nitratomenthyl derivatives. The photo-oxidation products were conveniently reduced to the corresponding diols and were, in turn, identified as bisbenzoate.

The stereochemical course of the photoaddition of nitrosamines to a carbon–carbon double bond was shown to follow a free radical mechanism initiated by aminium radicals. The photoaddition to cyclohexene was highly stereoselective in which the addendum approached in anti-diaxial mode. Similar photoadditions to cis- and trans-2-butenes were less stereoselective, giving mixtures of the erythro- and threo-adducts, owing to the rotation of the carbon-carbon single bond in the intermediate stage.

Summary C-Nitroso-enamines are unstable intermediates generated during the photoaddition of N-nitrosopiperidine to an acetylene.

The photolysis of 2,4-dinitrobenzenesulphenyl esters affords carboxylic acids in high yield. The initial step in this photolysis has been defined. The photolysis of phenolic esters has been further investigated. The esters of fluorene-9-carboxylic acid and xanthen-9-carboxylic acid afford reasonable yields of phenols with concomitant fragmentation of the acid residue. The potential advantages of protecting groups which can be removed photochemically under neutral conditions have been emphasised. A preliminary report of some aspects of our work has appeared.

N-Aralkylacetamido-radicals, generated by photolysis of the corresponding N-nitroso-amides in various solvents, underwent extensive β-scission of C–H and C–C bonds, in benzene solution, shown by isolation of the appropriate fragmentation products. Although the reactions of amido-radicals were similar to those of alkoxy-radicals, β-scission of the C–Ar bond was not detected. Oxygen did not quench the primary photo-processes nor interfere with the reactions of the amido-radical.

The SO moiety of dibenzo[b,f][1,4,6]thiadiazepin 1-oxide is extruded thermally and is trapped by a diene to give a dihydrothiophen 1-oxide.