Photoinduzierte Cycloadditionen von aliphatischen 2H-Azirinen. 37. Mitteilung über Photoreaktionen

The purely aliphatic 2,3-dipropyl-2H-azirine ( 1 ) reacts on irradiation with a mercury high-pressure lamp through a Vycor filter with methyl trifluoroacetate or acetone to form 3-oxazolines 3a, b (65%) resp. 4 (14%) (Scheme 1). 9-Azabicyclo[6.1.0]non-1(9)-ene ( 5 ) on irradiation in the presence of the dipolarophiles methyl trifluoroacetate, methyl difluoroacetate, 1,1,1-trifluoro-propanone and acetone behaves in a similar way, whereby the corresponding bicyclic 3-oxazolines 7–10 result in yields of 60–20% (Scheme 2). By analogy with the photochemical behaviour of 3-aryl-2H-azirines it is assumed that nitrile-ylides 2 resp. 6 represent intermediates. In fact irradiation of 2,3-dipropyl-2H-azirine ( 1 , λ_max 239 nm, ? 240) at ?196° with light of wavelength 245 nm in a hydrocarbonglass gives rise to a pronounced maximum at 280 nm, for which an ? of ? 15000 can be estimated. The quantum yield for the formation of nitrile-methylide 2 is 0,8. Irradiation of the dipole 2 at ?196° or warming to ?150° causes the maximum at 280 nm to disappear.     

Zur Photochemie von Allylaryläthern. 55. Mitteilung über Photoreaktionen

The photochemical reactions of different allyl aryl ethers (Scheme 3) were investigated in hydrocarbons (Chap. 3.1) and in alcoholic solvents (Chap. 3.2). The composition of the photoproducts depended very much on the nature of the solvent. Irradiation (3–95 h) of different methyl substituted allyl aryl ethers ( 1, 3, 5, 7 and 11 ) with a low pressure mercury lamp (λ_Emiss. = 254 nm; 6 or 15 Watt) under argon (quartz vessel) resulted in the formation of 2-, 3– and 4-substituted phenols, dienones and products of consecutive reactions (Tables 1–4 and 6). The results suggested that all products were formed by homolytic cleavage of the C? O bond in the singlet state of the ethers to intermediate radical-geminates (Scheme 5) followed by radical recombination of the two fragments. No products were formed by concerted processes (Table 5, Schemes 5 and 6). Upon irradiation of allyl aryl ethers lacking alkyl substituents at position 4 ( 1 and 5 ) in protic solvents, mainly 2- and 4-allylated phenols were obtained (Tables 1 and 4); 3-allylated phenols were formed only in small amounts (0.02%). However, in aromatic hydrocarbons or cyclohexane 3-allylated phenols were obtained from 1 , 5 and 11 in significant amounts (3–11%; Tables 1, 4 and 6). E.g., upon irradiation of allyl-2,6-dimethyl-2,4-cyclohexadien-1-one ( 6 ) besides 3- and 4-allyl-2, 6-dimethyl-phenol ( 23 and 24 ). Irradiation of 5 in methanol afforded 23 and 6 only in traces, whereas 24 was the main product.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Triphenyl-vinylphosphoniumbromid. 36. Mitteilung über Photoreaktionen

On irradiation in acetonitrile 3-phenyl-2H-azirines of type 1 react with triphenyl vinyl phosphonium bromide to form in approximative 50% yield 2H-indoles of type 4 (Scheme 1). In analogy to other photochemical reactions with 2H-azirines [2] [3] it is assumed that the photochemically generated dipoles 2 react with the triphenyl vinyl phosphonium salt (Scheme 1). The conversion of 1 to 4 represents a new synthesis for 2H-pyrroles.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen an Carbonsäurechloride. 35. Mitteilung über Photoreaktionen

Irradiation of 2, 3-diphenyl-2H-azirine ( 1a ) and 1-azido-1-phenyl-propene, the precursor of 2-methyl-3-phenyl-2H-azirine ( 1b ), in benzene, with a high pressure mercury lamp (pyrex filter) in the presence of acid chlorides yields the oxazoles 5a–d (Scheme 2). Photolysis of 2, 2-dimethyl-3-phenyl-2H-azirine ( 1c ) under the same conditions gives after methanolysis the 5-methoxy-2, 2-dimethyl-4-phenyl-3-oxazolines 7a, b, d , while hydrolysis of the reaction mixture leads to the formation of the 1, 2-diketones 8a, c, d (Scheme 4). The suggested reaction path for all these reactions is a 1, 3-dipolar cycloaddition of the photochemically generated benzonitrilemethylides 2 to the carbonyl double bond of the acid chlorides to give the intermediates 4 , followed by either elimination of hydrogen chloride or solvolysis (Schemes 2 and 4). Irradiation of 1c in the presence of acetic acid anhydride leads via the intermediate 9 to the 5-hydroxy-3-oxazoline 10 and the 5-methylidene-3-oxazoline 11 (Scheme 5).     

Thermische und photochemisch induzierte intramolekulare, 1,3-dipolare Cycloadditionen von 4-Phenyl-3-(2-allylphenyl)-sydnon. Vorläufige Mitteilung. 53. Mitteilung über Photoreaktionen

Thermal and photochemically induced intramolecular 1,3-dipolar cycloaddition reactions of 4-phenyl-3-(2-allylphenyl)-sydnone The title compound 9 was synthesised in the usual way, starting from 2-allylaniline and ethyl 2-bromo-2-phenylacetate, via the nitrosaminacid 8 (Scheme 2). 9 reacts at room temperature with its potential azomethinimine-function in an intramolecular [3+2]-cycloaddition to give the tricyclic compound 11 (Scheme 2). On irradiation, 9 yields the dihydro-3H-pyrazolo[2,3-a]indole 10 which probably arises by intramolecular [3+2]-cycloaddition of the corresponding intermediate nitrilimine.     

Photochemische Cycloadditionen von 3-phenyl-2h-azirinen mit Aldehyden. 31. Mitteilung über Photoreaktionen

Experiments concerning the photochemical condensation of 3-phenyl-2H-azirines 1 with aliphatic and aromatic aldehydes to 3-oxazolines 4 are fully described (cf. scheme 1 ). Photochemically nitrile methylides of type 2 are first formed, which then very quickly react thermally with the aldehydes in a regiospecific manner to give the 3-oxazolines 4 . Azirines monosubstituted in position 2 (l b and 1 c ) give mixtures of cis, trans-oxazoline isomers, in which the cis isomer predominates. The stereoselectivity of the cycloaddition reaction can be rationalized by a simple model (scheme 10). The stereoisomeric 3-oxazolines 4 are distinguishable in the NMR. spectra by the large homoallylic coupling constants between the H atoms on C(2) and C(5).     

Thermisch und photochemisch induzierte intramolekulare, 1,3-dipolare Cycloadditionen von 5-(2-Allyloxyphenyl)-2-phenyltetrazol. Vorläufige Mitteilung. 54 Mitteilung über Photoreaktionen

Thermal and Photochemically Induced Interamolecular 1,3-Dipolar Cycloaddition Reactions of 5-(2-Allyloxyphenyl)-2-phenyltetrazole The title compound 5 is easily obtained by a recently described procedure (Scheme 2). The tetrazole 5 reacts at 165–170° or on irradiation at room temperature to yield 2-phenyl-3,3a-dihydrochromano[4,3-c]pyrazole ( 7 , Scheme 3), which probably arises by intramolecular [3+2]-cycloaddition of the intermediate nitrilimine. Dehydrogenation of 7 with chloranil leads to 2-phenylchromano[4,3-c]pyrazole ( 8 , Scheme 3).     

Zur Photochemie von 3,5-Diaryl-2-isoxazolinen. 30. Mitteilung über Photoreaktionen

Irradiation of 3,5-diphenyl- or 3-(p-tolyl)-5-phenyl-2-isoxazoline ( 12 and 13 , respectively) in benzene with a high-pressure mercury lamp yields 4,5-diphenyl- or 4-(p-tolyl)-5-phenyl-3-oxazoline ( 17 and 19 , respectively) and the β-amino-chalcones 18 or 20 in addition to benzaldehyde, benzonitrile and p-tolunitrile, respectively (scheme 6 and ‘Anmerkg.’ p. 2600). The 3-oxazolines 17 and 19 are formed by route a (scheme 8) via 3-phenyl- or 3-(p-tolyl)-2H-azirine ( 23 , R = H and CH₃, respectively) and their photochemically rearranged successors, the nitrile methylides 24 , as intermediates. The discovery of this reaction has served as a basis for the quickly developing photochemistry of 3-aryl-2H-azirines [2] [24]. Photolysis of the 2-isoxazoline 13 in methanol leads to the formation of a mixture of syn/anti-p-tolyl trans-styryl ketoximes (syn/anti, trans- 30 ) and anti, cis- 30 , 2-(p-tolyl)-quinoline ( 29 ), the 4-hydroxymethylated derivative 32 of the latter (in small amounts), besides the β-aminochalcone 20 , benzaldehyde, p-tolualdehyde and p-tolunitrile (scheme 9). It could be shown that the stereoisomeric ketoximes 30 are photochemically interconvertible (scheme 12) and that at least one mechanism of formation of 2-(p-tolyl)-quinoline ( 29 ) is the photo-induced cyclisation of p-tolyl-cis-styryl ketoximes (cis- 30 ) (scheme 13). A tentative mechanism for the formation of p-tolual-dehyde is given in scheme 10; the crucial step is the protonation of p-tolunitrile methylide ( 24 , R = CH₃) by methanol at the nitrile carbon atom, after which hydrolysis yields the aldehyde.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Benzoyl-, Äthoxycarbonyl- und Vinylphosphonaten 34. Mitteilung über Photoreaktionen

The irradiation of the 3-phenyl-2H-azirines 1a–c in the presence of diethyl benzoylphosphonate ( 8 ) in cyclonexane solution, using a mercury high pressure lamp (pyrex filter), yields the diethyl (4, 5-diphenyl-3-oxazolin-5-yl)-phosphonates 9a–c (Scheme 3). In the case of 1b a mixture of two diastereomeric 3-oxazolines, resulting from a regiospecific but non-stereospecific cycloaddition of the benzonitrile-benzylide dipole 2b to the carbonyl group of the phosphonate 8 , was isolated. Benzonitrile-isopropylide ( 2a ), generated from 2,2-dimethyl-3-phenyl-2H-azirine ( 1a ), undergoes a cycloaddition reaction to the ester-carbonyl group of diethyl ethoxycarbonylphosphonate ( 15 ) with the same regiospecificity to give the 3-oxazoline derivative 16 (Scheme 5). The azirines 1a–c , on irradiation in benzene in the presence of diethyl vinylphosphonate ( 17 ) give non-regiospecifically the Δ¹-pyrrolines 13a–c and 14a–c (Scheme 6).     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Ketonen,Acylcyaniden und Ketoestern. 33. Mitteilung über Photoreaktionen

Similarly to aldehydes [6] ketones form 3-oxazolines via cyclo-addition to the benzonitrile-methylides 2 that arise photochemically from the 3-phenyl-2H-azirines 1 . With various ketones benzonitrile-isopropylide ( 2a ) gives cyclo-addition products in very good preparative yields (scheme 1). Benzonitrile-ethylide ( 2c ) and benzonitrile-benzylide ( 2b ) [8] react, however, sluggishly with ketones, smooth cyclo-addition being observed in their case only with «activated» ketones (2,2,2-trifluoro-acetophenone, 1,1,1-trifluoro-2-propanone). With 1a acetonyl-acetone forms the bis-adduct 12 While the azirine 1a reacts with cyclohexanone to yield essentially only the spiro-(3-oxazoline) 13 , it gives with cyclopentanone, depending on the reaction conditions, either the spiro-(3-oxazoline) 14 or the butenyl-3-oxazoline 15 (scheme 3). The formation of 15 has to be preceded by the photochemical formation of 4-pentenal from the ketone. Norcamphor and camphor react in a similar way (schemes 4 and 5). The azirines 1a–c react smoothly with the keto groups in acylcyanides and α-keto-esters, giving with the former 5-cyano-3-oxazolines and with the latter 5-ethoxycarbonyl-3-oxazolines (schemes 6 and 7). β-Keto-esters (acetoacetic ester) form with the dipole arising from 1a the expected addition product 31 and, via the protonated dipole d (scheme 8), finally the benzylidene-acetoacetic ester. Analogous results are obtained with malonodinitrile, trifluoro-acetamide and other weak acids such as alcohols [29] [30] (scheme 9). The light-induced rearrangement of the bicyclic isoxazoline 37 into the oxazoline 38 is visualized as an intramolecular cyclo-addition reaction (scheme 10). The cyclo-addition in this case proceeds with the aldehyde group inversed as compared to the related intermolecular benzonitrile–methylide addition to aldehydes.     

Photochemie von in 4-Stellung substituierten 5-Methyl-3-phenyl-isoxazolen.44. Mitteilung über Photoreaktionen

Photochemistry of 4-substituted 5-Methyl-3-phenyl-isoxazoles. 4-Trideuterioacetyl-5-methyl-3-phenyl-isoxazole ([CD₃CO]- 27 ), upon irradiation with 254 nm light, was converted into a 1:1 mixture of oxazoles [CD₃CO]- 35 and [CD₃]- 35 (Scheme 13). This isomerization is accompagnied by a slower transformation of ([CD₃CO]- 27 ) into [CD₃]- 27 . Irradiation of the isoxazole derivatives 28, 29, 30 and (E)- 31 yielded only oxazoles 36, 37, 38 and (E), (Z)- 39 ; no 4-acetyl-5-alkoxy-2-phenyl-oxazole, 2-acetyl-3-methyl-5-phenyl-pyrrole or 2-acetyl-4-methoxycarbonyl-3-methyl-5-phenyl-pyrrole, respectively, were formed (Scheme 9 and 10). Similarly (E)- 32 gave a mixture of (E), (Z)- 40 only (Scheme 11). Upon shorter irradiation, the intermediate 2H-azirines (E), (Z)- 41 could be isolated (Scheme 11). Photochemical (E)/(Z)-isomerization of the 2-(trifluoro-ethoxycarbonyl)-1-methyl-vinyl side chain in all the compounds 32, 40 and 41 is fast. At 230° the isoxazoles (E)- and (Z)- 32 are converted into oxazoles (E), (Z)- 40 . The same compounds are also obtained by thermal isomerization of the 2H-azirines (E), (Z)- 41 . The most probable mechanism for the photochemical transformations of the isoxazoles, as exemplified in the case of the isoxazole 27 , is shown in Scheme 13. A benzonitrile-methylide intermediate is postulated for the photochemical conversion of the 2H-azirines into oxazoles. 2H-Azirines are also intermediates in the thermal isoxazole-oxazole rearrangement. It is however not yet clear, if the thermal 2H-azirine-oxazole transformation involves the same transient species as the photochemical reaction. A mechanism for the photochemical isomerization of the 2H-azirine 11 to the oxazole 15 is proposed (Scheme 3).     

Photoinduzierte Cycloadditionen von 2,2-Dimethyl-3-phenyl-2H-azirin an Nitrile und «push-pull»-Olefine. 43. Mitteilung über Photoreaktionen

Photoinduced Cycloadditions of 2,2-Dimethyl-3-phenyl-2H-azirine with Nitriles and ‘push-pull’ Olefines. Electron deficient nitriles of the type 5a–e in contrast to nonactivated nitriles undergo regiospecific [2+3]cycloadditions to benzonitrile isopropylide ( 2b ), which was generated in situ by irradiation of 2,2-dimethyl-3-phenyl-2H-azirine ( 1b ), to yield the 2H-imidazole derivatives 6a – e (Scheme 2). The structure of the photoproducts was mainly deduced from ¹³C-NMR. and mass spectrometry. Whereas normal olefins or enolethers do not react with 2b , push-pull olefins of the type 10a – d readily undergo the cycloaddition to give the 3-alkoxy-5,5-dimethyl-2-phenyl-1-pyrrolines 11a – d (Scheme 3 and 4). The structure of the photoproducts 11a – d indicates that the regiospecificity of the cycloaddition corresponds to that of acrylonitriles and acrylesters with 2b .     

Photoinduzierte Reaktionen von 3-Phenyl-2H-azirinen mit Carbonsäureestern 40. Mitteilung über Photoreaktionen

Irradiation (280–350 nm light) of a benzene solution of 3-phenyl-2H-azirines 1a – e in the presence of carboxylate esters, whose carbonyl groups are activated by electron withdrawing groups situated in the acyl or alkyl moiety, produces 5-alkoxy-3-oxazolines (Tab. 1 and 4, Scheme 2) isolated in 18–82% yield. These heterocycles undoubtedly originate by regiospecific addition of the ester carbonyl group to the azirine-derived benzonitrile-methylide ‘dipole’ (Scheme 1). The 5-(2,′ 2′, 2′-trifluoroethoxy)-3-oxazolines, derived from 2′, 2′, 2′-trifluoroethyl carboxylic esters, on treatment with methanolic hydrogen chloride at low concentration, are smoothly transformed into the corresponding 5-methoxy-3-oxazolines (e.g. 16 → 17 , Tab. 5). Utilizing this process, various hitherto relatively unknown 9. 5-alkoxy-3-oxazolines become accessible. The constitution of the adducts is based essentially on spectral data. The structure of trans-5-methoxy-2,4-diphenyl-5-trifluoromethyl-3-oxazoline (trans- 14 ), the addition product of methyl trifluoroacetate and the benzonitrile-benzylide from 2,3-diphenyl-2H-azirine ( 1d ), was determined by X-ray crystallography (Section 5). Benzonitrile-isopropylide ( 22 ), resulting from the photochemical transformation of 2,2-dimethyl-3-phenyl-2H-azirine ( 1a ), also reacts with S-methyl thiobenzoate to give 2,2-dimethyl-5-methylthio-4,5-diphenyl-3-oxazoline ( 26 ). Ethyl cyanoacetate protonates predominantly the dipolar species derived from 1a at the nitrile C-atom and yields after work-up ethyl α-cyano-cinnamate ( 29 ) and ethyl isopropylidene-cyanoacetate ( 30 ) (Scheme 4). The relative rate of addition (k_rel) of benzonitrile-isopropylide ( 22 ) to methyl α-haloacetates and dimethyl oxalate was determined by competition experiments (Section 6). Log k_rel correlated satisfactorily (r = 0.97) with the pK_a of the acide derived from the ester reactant: log k_rel = ? 1.72 pK_a + 2.58 or with Taft's substituent constants σ*: log k_rel = 2.06 σ* ? 4.11 [k_rel(methyl dichloroacetate) = 1; Section 7.1]. On the basis of the results obtained, the mode of reaction of the so-called benzonitrile-methylide ‘dipole’ is discussed and a model for the transition state of addition of ester-carbonyl groups is proposed that accounts for the observed regiospecifity and steroselectivity.     

Photocyclisierungen von 1, 1'-Polymethylen-di-2-pyridonen. 46. Mitteilung über Photoreaktionen

Photocyclization of 1, 1′-Polymethylene-di-2-pyridones . Benzophenone sensitized irradiation of the four dipyridones 1-4 gave the internal photocyclization products 6 (64%, Scheme 4), 7 (60%, Scheme 5), 8 (Scheme 6), and 11 (26%, Scheme 7), respectively. The decamethylene compound 5 yielded only polymeric material. The primary [2+2] photoproduct 8 from dipyridone 3 (Scheme 6) is relatively unstable. Further irradiation or heating to 65° induced a Cope rearrangement to give compound 9 which, on heating to 137°, was converted into the isomeric compound 10 . This product, as well as the other photoproducts mentioned, are rearranged back to their respective starting materials upon direct irradiation with 254 nm light or by heating to higher temperatures. The various possibilities for cycloadditions of pyridones are discussed as well as the possible factors which are responsible for the highly regioselective photoreactions of the dipyridones 1–4 .     

Photochemische Erzeugung und Reaktionen des Benzonitril-benzylids. 42. Mitteilung über Photoreaktionen

Photochemical Generation and Reactions of Benzonitrile-benzylide The low temperature irradiation of 2,3-diphenyl-2H-azirine ( 1 ) in DMBP-glass at ?196° has been reinvestigated. It was possible to convert 1 nearly quantitatively into the dipolar species benzonitrile-benzylide ( 3 , Φ₃ = 0,78), which exhibits UV.-absorptions at 344 (? = 48000) and 244 nm (? = 28500) (Fig. 1, Tab. 1). Irradiation of 3 with 345 nm light at ?196° resulted in almost complete reconversion to the azirine 1 (Φ = 0,15; Fig. 2). When the solution of 3 in the DMBP-glass was warmed up to about ?160° a quantitative dimerization to 1,3,4, 6-tetraphenyl-2,5-diaza-1,3,5-hexatriene ( 8 ) occurred. This proves that 8 is not only formed by the indirect route 3 + 1 → 7 \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\longrightarrow }\limits^{hv} $\end{document} 11 → 8 known before (Scheme 1), but also by dimerization of 3 either by direct head to head coupling or via the intermediate e (p. 2675), followed by a fast thermal hydrogen transfer reaction. The occurrence of the dipolar intermediate 11 in the photochemical conversion of the bicyclic compound 7 to 8 could also be demonstrated by low temperature experiments: On irradiation at ?196° 7 gave the cherry red dipolar intermediate 11 (λ_max = 520 nm), which at ?120° isomerizes to 8 . It should be noted, that neither 7 nor 11 are formed by dimerization reactions of 3 . Experiments carried out at room temperature demonstrate, that both processes for the formation of 8 may compete: Irradiation of a solution of 1 (DMBP, c = 8 × 10^?4 to 5 × 10^?3M ) with 350 nm light of high intensity (which does not excite the bicyclic compound 7 ) leads to a relative high photostationary concentration of the dipolar species 3 . Under these conditions the formation of 8 is due to dimerization of 3 (Φ₈ = 0,19). With low light intensity only a very low stationary concentration of 3 can be obtained. Therefore the reaction of 3 with 1 , leading to the bicyclic intermediate 7 , becomes now predominant (Φ_?1 = 1,55, which corresponds with the expected value of 2 × 0,8). Irradiation of 1 at ?130° with 350 nm light of high intensity gives 8 with a quantum yield of 0,44. This is in agreement with the theoretical value Φ₈ = 0,4 for an exclusive formation of 8 by dimerization of 3 . The lower quantum yield for the formation of 8 at room temperature makes probable that under these conditions 3 not only dimerizes to 8 , but also to another, so far unidentified dimer, e.g. 2,3,5,6-Tetraphenyl-2,5-dihydropyrazine. By flash photolysis of a solution of 1 (cyclohexane, c = 10^?4M , 25°) the disappearance of 3 could directly be measured by UV.-spectroscopy: At relative high concentrations (c ≥ 10^?7M ) 3 disappeared according to a second order reaction with the rate constant k = 5 × 10⁷M ^?1S ^?1. At lower concentrations (c ≤ 10^?7M) the rate of disappearance of 3 follows first order kinetics. The rate constant of this pseudo first order reaction ( 3 + 1 → 7 ) has been determined to be 1 → 10⁴M^?1S^?1. Using Padwa's table of relative rates for the cycloaddition of the dipolar species 3 to various dipolarophiles, including the azirine 1 , an absolute rate constant of k ≈ 8 × 10⁸M ^?1S ^?1 for the addition of 3 to the most active dipolarophile fumaronitrile could be estimated. In cyclohexane at room temperature, the diffusion controlled rate constant equals 6,6 × 10⁹M ^?1S ^?1. In Table 1 the UV.-maxima of several nitrile-ylides, among them a purely aliphatic one, are given.     

Zur Konfiguration von zwei isomeren rac.-Cassansäure-Derivaten.3. Mitteilung über Cassain-Analoga

Catalytic hydrogenation of cassenic and isocassenic acid derivatives ( 1 resp. 2 ) led to a mixture of the two isomers 3 and 4 , which could be separated. The structures of these isomers were confirmed.