Farbstoffsensibilisierte Photoreaktionen von vinylogen Epoxycarbonylverbindungen

On irradiation in the presence of dye sensitizers, vinylogous epoxy carbonyl compounds undergo a novel photofragmentation reaction, to give furyl ketones in good yields. The fragmentation process is probably preceded by trans-cis isomerization of the double bond; in one case the cis-isomer could be isolated. In a side reaction epoxyenone 10 gives the cyclopentanone derivatives 13 and 14 by a [1, 2] shift of a methyl group. Similar dye sensitized photolysis of the endo-peroxide 2 also gives the corresponding furyl ketone 4 , and the corresponding vinylogous epoxyketone C is probably an intermediate in this transformation. The finding that dyes of relatively low triplet energies (～ 30 kcal/mol) efficiently sensitize these reactions, and a certain wave length dependence suggest that the usual triplet mechanism cannot be involved.     

Photochemie von tricyclischen β, γ-γ′, δ′-ungesättigten Ketonen. 50. Mitteilung über Photoreaktionen

Photochemistry of tricyclic β, γ-γ′, δ′-unsaturated ketones The easily available tricyclic ketone 1 (cf. Scheme 1) with a homotwistane skeleton yielded upon direct irradiation the cyclobutanone derivative 3 by a 1,3-acyl shift. Further irradiation converted 3 into the tricyclic hydrocarbon 4 . However, acetone sensitized irradiation of 1 gave the tetracyclic ketone 5 by an oxa-di-π-methane rearrangement. Again with acetone as a sensitizer the ketone 5 was quantitatively converted to the pentacyclic ketone 6 . The conversion 5 → 6 represents a novel photochemical 1,4-acyl shift. The possible mechanisms are discussed (see Scheme 7). The tricyclic ketone 2 underwent similar types of photoreactions as 1 (Scheme 2). Unlike 5 the tetracyclic ketone 9 did not undergo a photochemical 1,4-acyl shift. The epoxides 10 and 14 derived from the ketones 1 and 2 , respectively, underwent a 1,3-acyl shift upon irradiation followed by decarbonylation, and the oxa-di-π-methane rearrangement (Schemes 3 and 4). The diketone 18 derived from 1 behaved in the same way (Scheme 5). The tetracyclic diketone 21 cyclized very easily to the internal aldol product 22 under the influence of traces of base (Scheme 5). Upon irradiation the γ, δ-unsaturated ketone 24 underwent only the Norrish type I cleavage to yield the aldehyde 25 (Scheme 6).     

1-acyl-2-cyclopentenes and 5-acylbicyclo[2.1.0]pentanes: photochemical and thermal isomerizations

The photochemistry of 1-acyl-2-cyclopentenes varies with the nature of the acyl group. On direct irradiation the aldehyde eliminates carbon monoxide in the singlet excited state, and the aroyl compounds cleave to allyl-aroyl radical pairs both from the singlet and triplet states. In competition to α-cleavage the methyl ketones isomerize in an allylic 1,3-acetyl shift. The lowest-lying reactive triplet of these methyl ketones, characterized as a ³(π, π*) state in the case of the 3-phenyl homologue, undergo oxadi-π-methane rearrangement to a mixture of endo- and exo-5-acetylbicyclo[2.1.0]pentanes.The ground state-acetylbicyclopentanes react in two ways at elevated temperatures: endo-exo stereomutation by selective cleavage of the central cyclopropane bond and reclosure of the 1,3-cyclopentane diradical intermediate, and a rearrangement of the endo isomer to 1-acetyl-2-cyclopentenes on a separate potential energy surface involving a formal 1,2-acetyl shift. The unusually large negative entropy of activation for this latter reaction is suggestive of a concerted electrocyclic process in which the electrons of the internal cyclopropane and the C(5)-acetyl bonds participate.     

THE BAYLIS—HILLMAN REACTION: TiCl4 MEDIATED COUPLING OF ALKYL VINYL KETONES WITH α-KETO ESTERS AND ALDEHYDES

TiCl₄ mediated coupling of alkyl vinyl ketones with α-keto esters and aldehydes provides respectively 2-aryl-2-hydroxy-3-methylene-4-oxoalkanoates and (Z)-keto allyl chlorides in 1 h time at room temperature. Similar coupling of trifluoromethyl phenyl ketone with methyl vinyl ketone produces 1,1,1-trifluoro-2-hydroxy-2-phenyl-3-methylenepentan-4-one.     

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 Reaktionen. 52. Mitteilung [1]. Zur Photochemie von α,β-ungesättigten cyclischen Ketonen: Spezifische Reaktionen der n,π*- und π,π*-Triplettzustände von O-Acetyl-testosteron und 10-Methyl-Δ1,9-octalon-(2)

The photochemistry of the conjugated cyclohexenones O-acetyl testosterone ( 1 ) and 10-methyl-Δ^1,9-octalone-(2) ( 24 ) has been investigated in detail. The choice of reaction paths of both ketones depends strongly on the solvent used. In t-butanol, a photostationary equilibrium 1 ? 3 is reached which is depleted solely by the parallel rearrangement 1 → 5 (Chart 1; for earlier results on these reactions see [2a] [6] [7]). In benzene, double bond shift 1 → 16 (Chart 3) occurs instead, which is due to hydrogen abstraction from a ground-state ketone by the oxygen of an excited ketone as the primary photochemical process. In toluene, the major reaction is solvent incorporation ( 1 → 17 , Chart 4) through hydrogen addition to the β-carbon of the enone, accompanied by double bond shift and formation of saturated dihydroketone as the minor reactions. Contrary in part to an earlier report [19], the photochemical transformation of the bicyclic enoné 24 exhibit a similar solvent dependence. The corresponding products 25 – 29 are summarized in Chart 5 and Table 1. Sensitization and quenching experiments established the triplet nature of the above reactions of 1 and 24 . Based on STERN -VOLMER analyses of the quenching data (cf. Figures 2, 4–8, and Table 3), rearrangement, double bond reduction and toluene addition are attributed to one triplet state of the enones which is assigned tentatively as ³(π, π*) state, and the double bond shift is attributed to another triplet assigned as ³(n, π*) state (cf. Figure 9). The stereospecific rearrangement of the 1α-deuterated ketone 2 to the 4β-deuterio isomer 4 shows the reaction to proceed with retention at C-1 and inversion at C-10. The 4-substituted testosterone derivatives 33 – 36 (Chart 8) were found to be much less reactive in general than 1 . In particular, 4-methyl ketone 33 remains essentially unchanged on irradiation in t-butanol, benzene and toluene.     

Photochemische Reaktionen. 53. Mitteilung [1]. Photochemische Addition von Alkylaromaten an Aryl-Carbonylverbindungen

In the preceding paper [1] a novel primary photochemical process of triplet excited α,β-conjugated cycloalkenones in toluene solution has been reported: the abstraction of a benzylic hydrogen from the solvent by the β-carbon (cf. 1 → 2 + 3 + 4 ). The reaction has been attributed to the π,π* triplet. Aromatic aldehydes and ketones ( 5–11a ), the triplet state reactivity of which is known to be mostly π,π* in nature, have now been examined under the same irradiation conditions. However, a reaction similar to that of cycloalkenones — expected to result in the addition of hydrogen to the ortho and para positions of the aryl moiety and the formation of benzylcyclohexa-1,3-and 1,4-diene derivatives — could not been found. Compounds 5 – 10 remained essentially unchanged. 4-Methoxyacetophenone ( 11a ) reacted slowly to form the same type of products [tert-carbinol 12a , pinacol 13a and dibenzyl ( 4 )] as the aromatic carbonyl compounds 11b-d , benzophenone and cyclopropylphenylketone, which exhibit typical n,π* triplet reactivity (hydrogen abstraction by the carbonyl oxygen).     

Concerning the Stereochemical Course and Mechanism of the Photochemical 1,3-Acetyl Shift in a β,γ-Unsaturated Ketone. Preliminary Communication

Enantiomerically enriched samples of 1,2-dimethyl-3-(²H₃)methyl-2-cyclopentenyl (1) and 1-(²H₃)methyl-2,3-dimethyl-2-cyclopentenyl methyl ketones ( 2 ) have been irradiated at 313 nm in methanol in the temperature range +50 to ?45°. The 1,3-acetyl shift, which interconverts the two isomeric ketones, occurs with a small change in the enantiomeric composition and independently of temperature in the range studied. This change corresponds to an upper limit of approximately 20% reaction with racemization. It is proposed that reaction occurs from both the S₁(n, π*) and T₂(n, π*) excited states with stereospecific reaction from S₁ (rapid primary geminate recombination of a singlet radical pair, with a possible contribution by a concerted 1,3 shift) dominating throughout the temperature range, but with the proportion of reaction from T₂ increasing as the temperature is lowered. The racemization results from secondary geminate recombination of the singlet and triplet radical pairs and the random recombination of free radicals. Viscosity effects are proposed to explain the independence of the racemization on temperature.     

Thermolysis and Chemiluminescence of Monocyclic 1,2,4-Trioxan-5-ones

The 3,6-substituted 1,2,4-trioxan-5-ones 11 – 14 , on heating to 170–200°, underwent unimolecular thermolysis to generate electronically excited singlet ketones with an efficiency of ca. 0.2%. The chemiluminescence quantum yields (?oS_CL) depended on the nature of the 6-substitutents and increased linearly with temperature. The Arrhenius activation energies were obtained by measuring the rate of decay of luminescence and determined as 22.9, 30.4, 35.6, and 34.2 kcal/mol for 11 – 14 , respectively. Step analysis of the chemiluminescence of 14 afforded an average activation energy of 44.3 kcal/mol. This latter result is explicable in terms of two decomposition paths, higher and lower in energy, leading to excited and ‘dark’ products, respectively. The thermolysis of trioxanones 12 – 14 lacking a H-atom at the 6-position is interpreted as involving successive rupture of the peroxide bond, excision of ketone at the 3-substituted end, and loss of CO₂, to finally produce ketone originating from the 6-position (see Scheme 4).     

EPR/Spin‐trapping study of free radical intermediates in the photolysis of trifluoromethyl ketones with initiators

Photolysis of trifluoromethyl ketones (TFMKs) 1a–1e versus the non‐fluorinated ketones 2a–2b in the presence of radical initiators by electron paramagnetic resonance spectroscopy has been studied for the first time. The transient radicals generated after irradiation of the ketones were identified by trapping with 2‐methyl‐2‐nitrosopropane (MNP) and 2,4,6‐tri‐tert‐butylnitrosobenzene (TTBNB) as spin traps. TTBNB is a powerful, particularly useful spin trap in these kinds of processes producing anilino and nitroxyl spin adducts due to the ambivalent reactivity on the N and O atoms. In the presence of t‐butylperoxide, short‐chain TFMKs, such as 1,1,1‐trifluoroacetone (1d) and hexafluoroacetone (1e), give rise to detection of the elusive trifluoromethyl radical. In contrast, long‐chain TFMKs did not provide clues to prove formation of the trifluoromethyl radical but instead to radicals derived by abstraction of one α‐methylene proton to the carbonyl. Although TFMKs are quite stable to photodegradation in the absence of initiator, methyl ketone 2b and phenyl ketone 3 produce radicals resulting from abstraction of a γ‐hydrogen to the carbonyl group. Copyright © 2010 John Wiley & Sons, Ltd.     

Photochemical α-Cleavage of β, γ-Unsaturated Aryl Ketones: Competition between recombination/disproportionation and dissociation in geminate singlet and triplet radical pairs

The photolysis of (R)-(+)-phenyl and (R)-(+)-p-anisyl 1, 2, 3-trimethylcyclopent-2-enyl ketone ( 1 , 2 ) and the corresponding rac-1- and 3-desmethyl analogs ( 3 , 4 ) led to isomerization due to formal 1, 3 aroyl migration and to formation of aryl aldehydes ( 7 , 8 ), dienes ( 9 , 10 ) and dimers ( 5 , 6 ) of the cyclopentenyl radical. Evidence obtained from a chiroptical and mass spectrometric analysis of a crossing experiment and from photolytic CIDNP measurements including the use of CCl₄ as a free radical scavenger, supports the conclusion (1): that the ketones undergo photochemical α-cleavage predominantly in the triplet state; (2): that recombination and disproportionation reactions within the geminate singlet and triplet aroyl/allyl radical pairs ( 11 ) compete with the dissociation into free radicals ( 12 ): (3): that ketone isomerization by paths not involving polarizable radical intermediates is unimportant; (4): that no triplet oxa-di-π-methane type rearrangement products are formed.     

Ab Initio Study on the Potential Energy Surfaces of B4 Cluster

The singlet and triplet potential energy surfaces (PES) for the isomerization and dissociation reactions of B₄ isomers have been investigated using ab initio methods. Ten B4 isomers have been identified and of these 10 species, 4 have not been reported previously. The singlet rhombic structure ¹1 is found to be the most stable on the B₄ surface, in agreement with the results of previous reports. Several isomerization and dissociation pathways have been found. On the singlet PES, the linear ¹3b can rearrange to rhombus ¹1 directly, while ¹3c rearranges to ¹1 through two‐step reactions involving a cyclic intermediate. On the triplet PES, the capped triangle structure ³2 undergoes ring opening to the linear isomer ³3b with a barrier of 34.8 kcal/mol and 44.9 kcal/mol, and the latter undergoes ring closure to the square structure ³1 with a barrier of 30.4 kcal/mol and 33.0 kcal/mol at the MP4/6–311+G(3df)//MP2/6–311G(d) and CCSD/aug‐cc‐pVTZ//MP2/6–311G(d) levels of theory, respectively. The direct decomposition of singlet B₄ yielding to B₃+B is shown to have a large endothermicity of 87.3 kcal/mol (CCSD), and that producing 2B₂ to have activation energy of 133.4 kcal/mol (CCSD).     

Beiträge zur Chemie der Pyrrolpigmente, 29. Mitt.: Zur anaeroben Photochemie von Gallenpigmenten: Zum Mechanismus der photochemischen Isomerisierung an exocyclischen Doppelbindungen

Studies of heavy atom effects as well as sensitization and quenching experiments with several model compounds suggested that in pyrromethenones and biladienes-ac the photoisomerization at exocyclic double bonds proceeds predominantly via the singlet path on direct irradiation. The energy of the triplet state of pyrromethenones and arylmethylenepyrrolinones was estimated by sensitization experiments to be in the region from 140 to 160 kJ/mol. On sensitization the reaction follows the triplet path. With bilatrienes-abc the situation is complicated by the fact that in homogenous phase there is only unidirectional isomerization but there is evidence that the direct process proceeds also via the singlet path. These experiments are supported by calculations using theFormosinho theory of radiationless deexcitation.

28. Mitt.:H. Falk undK. Grubmayr, Mh. Chem.110, 1237 (1979).     

Photochemical Reactions. 153th communication. Photochemistry of acylsilanes: Photolyses and thermolyses of α,β-epoxy silyl ketones

The photolyses and thermolyses of the α,β-epoxy silyl ketones 5 and 6 are described. On n,π*-excitation, the silyl ketones 5 and 6 were transformed to the ketone 7 and the ketene 8 in quantitative yield. The formation of 8 may be explained by initial cleavage of the C(α)? O bond and subsequent C(1)→C(2) migration of the (t-Bu)Me₂Si group. In contrast to the acylsilanes 5 and 6 , the photolyses of the analogous methyl ketones 11 and 12 gave a very complex mixture of products. On thermolysis, 5 and 6 yielded the ketone 7 and the acetylenic compound 9 , which were probably formed via a siloxycarbene intermediate. In addition, the 1,3-dioxle 10 was formed via an initial C(α)? C(β) bond cleavage leading to the ylide g and subsequent intramolecular addition of the carbonyl group. The analogous 1,3-dioxole 13 was obtained on pyrolysis of the methyl ketones 11 and 12 .     

Construction of Cyclobutanes by Multicomponent Cascade Reactions in Homogeneous Solution through Visible-Light Catalysis

[2+2] Photocycloaddition of two olefins is a general method to assemble the core scaffold, cyclobutane, found in numerous bioactive molecules. A new approach to synthesize cyclobutanes through multicomponent cascade reactions by merging aldol reaction and Witting reaction with visible-light-induced [2+2] cycloaddition is reported. An array of cyclobutanes with high selectivity has been achieved from commercially available aldehydes, ketones (or phosphorus ylide), and olefins with visible-light irradiation of a catalytic amount of (fac-tris(2-phenylpyridinato-C₂,N)iridium) ([Ir(ppy)₃]) at room temperature. Control experiments and spectroscopic studies revealed that the triplet–triplet energy transfer from the excited [Ir(ppy)₃]* to enones, generated in situ from aldehyde and ketone or aldehyde and phosphorus ylide, is responsible for these simple and efficient muticomponent transformations.     

A theoretical study of electronic effects in alkyne‐linked reactive intermediates: (nitrenoethynyl)methylenes, (nitrenoethynyl)silylenes,and (nitrenoethynyl)germylenes

Acetylene‐linked reactive intermediates of (nitrenoethynyl)‐X‐methylenes, (nitrenoethynyl)‐X‐silylenes, and (nitrenoethynyl)‐X‐germylenes are almost experimentally unreachable (X–M–C≡C–N; X=H ( 1 ), CN ( 2 ), OH ( 3 ), NH₂ ( 4 ), NO₂ ( 5 ), and CHO ( 6 ); M=C, Si, and Ge). The effects of the electron‐donating and electron withdrawing groups were compared and contrasted at seven levels of theory. All singlet species as ground states with one local open‐shell singlet carbene subunit (π¹π¹) and another local open‐shell singlet nitrene subunit (π¹π¹) were found to be more stable than their corresponding triplets including one local open‐shell singlet carbene (δ¹π¹) (or one local closed‐shell singlet carbene [δ²π⁰]) and another local triplet nitrene subunit (π¹π¹) with 45.94–77.996 kcal/mol singlet–triplet energy gap (ΔE_s‐t). Their relative silylenes and germylenes made reduction of ΔE_s‐t, so the triplet ground states were found for species 3 _Si , 4 _Si , 5 _Si , 2 _Ge , 3 _Ge , 4 _{Ge ,} and 5 _Ge . All the singlet silylenes/germylenes formed by one local closed‐shell singlet silylenes/germylenes (δ²π⁰) and one local closed‐shell singlet nitrene subunit (π²π⁰). Also, one local closed‐shell singlet silylene/germylene subunit (δ²π⁰) and one local triplet nitrene subunit (π¹π¹) were observed for triplet silylenes/germylenes. The singlet and triplet species 3 _Si , 4 _Si , 3 _Ge , and 4 _Ge , due to their electrophilic (Si₄/Ge₄) and nucleophilic (X₅) centers, could be identified as intermediates in chemical reactions.     

Photochemical High-yield Preparation of Tricyclo [3.3.0.02,8]octan-3-ones. Potential Synthons for Polycyclopentanoid Terpenes and Prostacyclin Analogs. Preliminary Communication

Three-step syntheses and the resolution into the enantiomers are reported for the tricyclo[3.3.0.0^2,8]octan-3-ones 7–9 , which are destined to serve as synthons for polycyclopentanoid terpenes and prostacyclin analogs. Routine overall yields of ca. 75% for 7 , 40% for 8 , and 46% for 9 are obtained, with 2-chloroacrylonitrile and 1,3-cyclohexadiene (for 7) and 1-methyl-1,4-cyclohexadiene (for 8 and 9 ) as the starting materials. The key step is the triplet-sensitized oxadi-π-methane photorearrangement of the β,γ-unsaturated ketones 1–3 which can be achieved in 80–90% yields of isolated product and quantum yields of 0.5–1.0. The racemates of both ketone 1 and its photoisomer 7 have been resolved via chromatographic separation of suitable diastereoisomeric acetal mixtures. On the other hand, sensitization of 1 with an optically active donor, (-)- 14 , gave only an impractical maximum enantiomeric excess of 10% (-)- 7 .     

Properties of excited singlet states of N-arylurethanes : Photo-fries reactions,fluorescence, quenching and sensitization

The deactivation of the first excited S(ππ^*) states of N-arylurethanes (produced upon irradiation with UV light) by emission (fluorescence), chemical reaction (photo-Fries rearrangement and fragmentation), energy transfer to quenchers, and radiationless transitions to ground and triplet states is investigated. Arylurethanes exhibit fluorescence (λ_f ≈ 295–350 nm, φ_f ≈ 10^?2, τ_f ≈ 1–6 ns) and phosphorescencs (λ_p ≈ 370–410 nm). The variations of the quantum yields of the fluorescence and of the photo-Fries rearrangement of N-arylurethanes by substituents and solvents are essentially due to variations of the rate constants for the radiationless processes. Fluorescence and photo-Fries reactions can be quenched by diffusion-controlled energy transfer to aliphatic ketones. Quenching is accompanied by sensitization of the ketone fluorescence. The urethane fluorescence and photo reactions may be sensitized by aromatic hydrocarbons. The results of all the quenching and sensitization experiments demonstrate that the photo-Fries reactions of N-arylurethanes proceed via the first excited singlet states of the urethanes.     

On the interaction mechanism of radical photoinitiators with monomers

Rate constants of quenching of triplet excited ketones by several monomers were determined through time‐resolved laser spectroscopy or culled from the literature. The semi‐empirical calculation method PM3 allows the quenching mechanisms to be refined and can be used to predict the reactivity of aromatic ketones toward monomers. It is apparent from both experimental results and theoretical calculations that the rate constant (k_q ) measured for the bimolecular quenching between the triplet state of a given aromatic ketone and both electron‐rich as well as electron‐poor monomers, depends linearly on the free enthalpy of formation of the regioselectively favored 1,4‐biradical, which is the primary reaction step of the ketone/monomer interaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1785–1794, 2000     

Regioselectivity and Competition of the Paternò–Büchi Reaction and Triplet–Triplet Energy Transfer between Triplet Benzophenones and Pyrimidines: Control by Triplet Energy Levels

The photochemical reaction of a pyrimidine and a ketone occurs either as a Paternò–Büchi (PB) reaction or as energy transfer (ET) from the triplet ketone to the pyrimidine. It is rare for the two types of reactions to occur concurrently, and their competitive mechanism remains unknown. In this work, two classes of products, regioisomeric oxetane(s) ( 2 , 3 ) from a PB reaction and three isomeric dimers of 5‐fluoro‐1,3‐dimethyl uracil (FDMU) ( 4 – 6 ) from a photosensitized dimerization of FDMU, are obtained through the UV irradiation of FDMU with various benzophenones (BPs). The ratio of the two products (oxetanes to dimers) reveals that the two competitive reactions depend strongly on the triplet energy levels (E_T) of the BPs. The BPs with higher E_T values lead to higher proportions of dimers, whereas those with lower E_T values give higher proportions of oxetane(s), with the generation of just two regioisomeric oxetanes for the BP with the lowest E_T of the eight BPs investigated. The ratio of the two oxetanes ( 2 : 3 ) decreases with the BP E_T value. The competitive mechanism for the two types of photochemical reactions is demonstrated through quenching experiments and investigation of temperature effects. Kinetic analysis shows that the rate constants of the two [2+2] photocycloadditions are comparable. Furthermore, in combination with the results of previous studies, we have gained insight into the dependence of the photochemical type and the regioselectivity in the PB reaction on the triplet energy gaps (ΔE) between the pyrimidines and ketones. For ketones with higher E_T values than the pyrimidines, the photochemical reaction is a photosensitized dimerization of the pyrimidine. In the opposite case, a PB reaction occurs, and the lower the E_T of the ketones, the lower the ratio of oxetanes ( 2 : 3 ). When the E_T of values of the ketones are close to those of the pyrimidines, the two reactions occur concurrently, and the higher the E_T of the ketones, the higher the proportion of the dimers. The ratio of oxetanes ( 2 : 3 ) decreases with the E_T value of the BPs.