Photochemische Umwandlungen, 41 [1] 3ω → 3 π-Isomerisierungen von Monohetero-trishomobenzol-derivaten. Vorläufige mitteilung

The all-cis-oxa- and azatrishomobenzene diesters 4a and 4b resp. undergo thermally a very clean 3ω → 3π isomerization reaction yielding the heterocyclonona-2, 5, 8-triene derivatives 6a and 6b resp. (E_a = 27.4 and 26.5 kcal/mole). In contrast, the cis, cis, trans-oxatrishomobenzene diester 9 is stable up to 170°. Some applications and limitations of this 3ω → 3π-route to iso- and heterocyclononatriene derivatives are discussed.     

Photochemische Umwandlungen. XV [1] Photochemische Isomerisierung des exo-Tricyclo[4.2.1.02′5]nona-3,7-dien-Systems. Vorläufige Mitteilung

The acetone sensitized isomerization of two exo-tricyclo[4.2.1.0^2,5]nonadiene derivatives, of the corresponding tricyclo[4.3.0.0^2,5]nonadienes, and the photoisomerization of two bicyclo[4.3.0]nonatrienes by direct excitation are described.     

Photochemische Reaktionen 49. Mitteilung [1] Spezifisch π → π*-induzierte Keton-Photoreaktionen: Die Doppelbindungsverschiebung von O-Acetyl-10α-testosteron Protonisierung der Doppelbindung seines δ5-Isomeren

The α,β-unsatured ketone 10α-testosterone has been reported previously [6] to photoisomerize in t-butanol solution to the β,γ-unsaturated ketone. The irradiation had been carried out using a high-pressure mercury lamp in a quartz vessel. For structural reasons this double bond shift cannot proceed through a photoenolization mechanism involving an intramolecular hydrogen transfer from the γ-position to the enone oxygen as has been suggested to operate in several formally analogous cases of aliphatic enone isomerizations. In the present reinvestigation, O-acetyl 10α-testosterone ( 1 ) was used, employing selectively either excitation of its n → π* (with wavelengths > 300 nm) or its π → π* absorption band (with 253,7 nm). In t-butanol solution the doublebond shift 1 → 2 could be effected with π→* excitation only. Experiments in deuterated solvent (t-BuOD) resulted in deuterium in corporation in both the δ5-ketone in the C(4)-position, cf.( 3 ) and in the conjugated ketone. These results indicate that the reactions is initiated either in the, S_π,π* state or in a high vibrational mode of the S₀ or t_ππ*state. n→ π* Excitation of 1 in t-butanol gave essentially no over-all chemical change, while in benzene solution it resulted again in a double bond isomerization ( 1 → 2 ). In analogy to results with similar enones [28] under identical conditions the deconjugation in benzene may be the consequence of an intermolecular hydrogen abstraction of the T_n,π* excited state of the enone. Another specifically π →π* induced photoreaction was observed on irradiation of the β, γ-unsaturated ketone 2 in t-BuOD with 253,7 nm. The olefinic hydrogen at C-6 of 2 was exchanged with deuterium and, to a small extent, isomerization to the conjugated ketone 1 with concomitant deuterium incorporation occurred. It is concluded that from the higher excited state of the β, γ-unsaturated ketone, but not from its S_n,π* state, an activation mode of the double bond is accessible to effect D+ addition at C-6 followed by deprotonation to 4 and to deuterated 1 , respectively.     

Photochemische Reaktionen. 65.(vorläufige) Mitteilung. Spezifisch π → π*-induzierte Photoisomerisierungen von 10-Dimethoxymethyl-Δ1,9-octal-2-on. Ein photochemischer Zugang zu [4.4.3]-12-Oxapropellan-Derivaten

Selective n → π* excitation of the α,β-unsaturated enone 1 in hydrocarbon solvents resulted in a deconjugation reaction to 3 , reminiscent of results previously reported for similar systems [2], whereas the photoreactivity of 1 in alcohol solvents at wavelengths >3400 Å was so small that only product 4 has been identified as yet. Excitation of the π → π* transition of compound 1 at 2537 Å initiated additional phototransformations which could not be effected by irradiation in the first absorption band. The [4.4.3]-12-oxapropellane derivative 2 was identified as one of the two new major photo-isomers. A 6:8 mixture of products 2 and 3 , plus about 1 part of an isomer of still unknown structure (see however, the Addendum), were readily formed in hydrocarbon solvents, and a 1:10 ratio of 2 and the unknown product was obtained in methanol. Abstraction of a methoxyl hydrogen by the ketone oxygen is proposed to account for the primary photochemical step in the cyclization to 2 . A hydrogen-deuterium isotope effect of 2.7 was observed in a competitive experiment using 1 and 1-d _6. 34% of one deuterium atom were exchanged for hydrogen when 1-d ₆ was photolyzed to 2-d ₆ in t-butyl alcohol, which suggests an intermediate of type a in the pathway 1 → 2 possessing a readily exchangeable proton. Steric considerations would require a strongly distorted, non-planar excited-state geometry of the enone group of 1 for the oxygen to approach a methoxyl hydrogen. The transformation 1 → 2 represents a novel reaction type in photochemical processes of conjugated enones which are specifically induced by π → π* excitation only.     

Photochemische Umwandlungen. 30. Mitteilung [1]. Zur Kinetik der Norbornadien-Quadricyclan-Photocycloaddition

Quantum yields for several norbornadiene → quadricyclane conversions have been determined under various experimental conditions (Tab. 1). The data are interpreted in terms of a singlet intermediate of type 15 .     

Photochemische Umwandlungen. XVIII Photochemische Isomerisierung annellierter Oxanorbornadiene Vorläufige Mitteilung

Photochemical cycloaddition reactions in anellated oxanorbornadiene derivatives have been investigated. In the case of 7a the tetracyclic isomer 8a can be isolated which adds dimethylacetylenedicarboxylate in the manner of a bis-homo-furan system. On thermal activation 8a yields the thiepino-oxepin derivative 9a. In contrast, on direct photoexcitation of the benzoxanorbornadiene the primary photoproduct cannot be identified, the reaction leading directly to the benz[d]oxepin.     

Photochemische Umwandlungen. XXVII [1]. Ein «stabiles» Benzoloxid Vorläufige Mitteilung

By thermal isomerization of the 3-oxa-6.7-diphenyl-tetracyclo [3.2.0.0^{2, 7}. 0^{4, 6}]-heptane-1. 5-dicarbomethoxylate (4) besides the oxanorbornadiene 3 and the hydroxy-fulvene 7, the ‘stable’ benzeneoxide 7-oxa-2.5-diphenyl-bicyclo [4.1.0] hepta-2.4-diene-3.4-dicarbomethoxylate (5) has been isolated. In the temperature range ?50 to 170°C no equilibration with the monocyclic oxepine 6 has been detected.     

Specifically (π → π*)-Induced Cyclohexenone Reactions 6-Benzylbicyclo [4.4.0]dec-1-en-3-ones

6-Benzylbicyclo [4.4.0]dec-1-en-3-one ( 9 ) and the 2-methyl homologue ( 10 ) underwent a (γ → α )-1, 3-benzyl shift to the β,γ-unsaturated ketones 21 and 22 , respectively, when excited in the π π* absorption band. The quantum yield was ca. 0.1 at 254 nm for the formation of both products in alkane solvents. These reactions occur specifically from the S₂(π, π*) state in competition with its decay to the S₁(n, π*) and T states. The triplet reaction of 9 , initiated by n → π* irradiation and by sensitization, was a double-bond shift to 20 , whereas no identifiable product was observed from 10 under these conditions. Direct and acetone-sensitized irradiations of 21 and 22 resulted in oxadi-π-methane rearrangements to mixtures of syn- and anti- 30 and syn- and anti- 31 , respectively.     

Photochemische Umwandlungen. XXXIV [1]. Photochemische Isomerisierung substituierter Benzonorbornadiene

The benzonorbornadiene derivatives 4 , 5 , 6 and 19 give upon direct photoexcitation the benzo-tricyclo [3.2.0.0^{2, 7}] heptenes 14 , 17 , 21 and 23 in yields up to 60%. Sensitization by acetone improves the yields considerably. The spectral data of the substrates 4 , 5 , 6 , 7 and 19 , of the photoproducts 14 , 17 , 21 and 23 , as well as of the dihydrobenzonorbornadienes 13 , 16 and 22 , are presented.     

Photochemische Reaktionen. 78. Mitteilung. Anwendung der π → π*-induzierten Cyclisierung von α,β-ungesättigten γ-Dimethoxymethylketonen auf 19-Methoxy- bzw. 19-Dimethoxy-Steroid-enone und -dienone

Irradiation at 254 nm of 19-dimethoxyandrost-4-en-17 β-ol-3-one acetate ( 8 ) afforded the epimeric cyclization products 9 (yield 20%) and 10 (4%). Similar transformations were also achieved with the analogous dimethoxy-enone 24 (→ 25 , 65%), and-dienone 30 (→ 31 , 72%), and with the methoxy-enone 33 (→ 34 , 30%), and-dienone 35 (→ 36 , 17%).     

Photochemische Umwandlungen,XXXVII [1]. Synthese von Methylen-tetracyclo[3.2.0.02,7.04,6]heptan-Derivaten

Several isopropylidene-, benzylidene- and benzhydrylidene-norbornadiene derivatives ( 11 , 16 ) have been synthesized. The [2π+2π] cycloadditions leading to the tetracyclic isomers 12 and 17 resp. upon direct photoexcitation proceed practically quantitatively in the case of the trienes 11 , much less specifically, however, with 16 . Sensitization by acetone or fluorenone allows the isolation of the quadricyclanes 17 in 20–90% yields.     

Photochemische Umwandlungen, 42 [1] Photoisomerisierungen im 5-Isopropyliden-tetracyclo-[4.3.0.02,4.03,7]non-8-en-System. Vorläufige mitteilung

Direct photochemical excitation of the tetracyclo[4.3.0.0.^2,4.0^3,7]non-8-enes 7a – d leads to a mixture of the isomers 8a – d and 10a – d , with the former being the main products. On sensitization by acetone the formation of 10a – d is clearly favored. The cis-bishomobenzenes 9a – d are the postulated intermediates. Spectral data and some thermal reactions are described.     

Photochemische Reaktionen. 39. Mitteilung. Photochemische Umwandlungen von endo-Tricyclo[4.3.12,5.0]-decadien-(3,8)-dion-(7,10) und cis-8, 9-Dihydroindenon

In boiling xylene the cyclopentadienedione 1 is thermally decarbonylated to cis-8,9-dihydroindenone ( 8 ). This method avoids secondary reactions and allows the preparation of 8 in high yield.     

Photochemische Reaktionen. 85. Mitteilung [1]. Zur Photochemie von α, β-Epoxy-eucarvon

Photochemistry of α,β-epoxy-eucarvone . On π,π^*-excitation (λ = 254 nm) 4 isomerizes to the bicyclic ketoaldehyde 5 ; on n,π^*-excitation (λ ? 280 nm) 4 gives 5 , the β,γ-unsaturated ketone 6 , the enone 7 and the cyclobutanone 8 . Scission of the (C—C)-bond of the oxirane 4 would give the dihydrofurane e , which could isomerize to the ketoaldehyde 5 . On the other hand, 4 is assumed to isomerize to the β,γ-unsaturated aldehyde c , which could yield 6 and 7 by photodecarbonylation. The cyclo-butanone 8 is shown to be a photoisomer of the ketone 6 . Furthermore, eucarvol ( 18 ) rearranges by a thermal [1,5]-H-shift to dihydro-eucarvone ( 20 ); on UV.-irradiation 18 gives the bicyclic isomers 27 and 28 .     

Photochemische Reaktionen. 110. Mitteilung [1]. Zur Photochemie γ,δ-Methano-α-enone

Photochemistry of γ,δ-Methano-α-enones Direct excitation (λ = 254 or ≥ 347 nm) converts the γ,δ-methano-α-enone (E)- 10 into the isomeric ether 23 and the isomeric diene-ketone 24 . Furthermore, on ¹π,π*-excitation (λ = 254 nm) (E)- 10 undergoes an 1,3-homosigmatropic rearrangement yielding the enone (E)- 25 . In addition (E → Z)-isomerization of (E)- 10 and conversion of 10 to the isomeric furan 28 is observed. The isomerization (E)- 10 → 23 , 24 and (E)- 25 proceeds by photocleavage of the C(γ), C(δ)-bond, whereas the formation of 28 occurs by photocleavage the C(γ), C(δ)-bond together with that of the C(γ), C(δ′)-bond of 10 . On direct excitation the bicyclic diene-ether 23 yields the methano-enone 10 , the dieneketone 24 and the tricyclic ether 29 . Evidence is given, that the conversion 23 → 10 is a singulet process. On the other hand, the isomerization 23 → 24 and the intramolecular [2 + 2]-photocycloaddition 23 → 29 are shown to be triplet reactions. Irradiation (λ = 254 nm) of the homoconjugated ketone 24 yields the isomeric ketone 27 by an 1,3-acyl shift. The excitation of the (E)-enone 25 induces (E → Z)-isomerization and photoenolization to give the homoconjugated ketone 26 .     

Nachweis von π → σ-Umlagerungen bei Ligandenverdrängungsreaktionen von π-Allyl-π-cyclopentadienyl-Palladiumkomplexen

It has been proved by NMR. measurements at low temperatures that the ligand displacement reactions of (π-all)Pd(π-C₅H₅) and Lewis bases L yielding PdL₄ proceed by a π → σ rearrangement of the allylic group as the primary step. The organic reaction product is the 1-isomer of the corresponding allylcyclopentadiene but in the reactions of (π-1,1,2-Me₃C₃H₂)Pd(π-C₅H₅) with L besides the isomeric allylcyclopentadienes also 2,3-dimethylbutadiene and cyclopentadiene are formed. The reaction mechanism will be discussed.