Photoinduced Substitution Reactions in Halothiophene Derivatives. Steady State and Laser Flash Photolytic Studies (original) (raw)
Related papers
Gazzetta chimica Italiana
Quantum yields for 14 and 17 were determined in a similar way with 4 as an actinometer: dZMnm(-14) = 0.0.0094 f 0.001; Flash Photolysis Experiments. Triplet absorption spectra were obtained at 25 "C on -3 mL of acetonitrile solutions of 5 (0.0087 M) and 14 (0.010 M) and benzophenone (0.005 M) that were thoroughly purged with argon. Benzophenone triplet decay (2.78 X 105 s-l) was monitored at 480 nm. The absorption spectra were generated by measuring absorbances at 5-nm intervals in the range 290-580 nm. Because of photochemical instability, the triplet absorption spectrum of isoxazole 5 was a composite of measurements taken from different samples. In addition, the spectrum was recorded in both sweep directions in which case ~3000m(-17) = 0.083 f 0.004. small differences in optical densities were observed. Both compounds had a strong absorption peak at ca. 320 nm, and in addition, 5 showed a broad maximum at 480 nm and 14 showed a shoulder a t 410 nm. The transient lifetimes in the absence of biphenyl were 276 and 48 ns, respectively. In the presence of 0.01 M biphenyl a new absorption appeared at 360-365 nm. Quenching rates were calculated from lifetime measurements in the presence and absence of biphenyl: 5, 8.3 X lo8 M-ls-l; 14, 2.0 X lo9 M-' S-1.
J Am Chem Soc, 1968
The photochemical rearrangement of 4,4-diphenylcyclohexenone (1) to cis-and rrans-5,6-diphenylbicyclo[3.1 .O]hexan-2-one (2) and 3,4-diphenylcyclohex-2-en-lone (3) has been studied in further mechanistic detail. Remarkably, over a 100-nm wavelength range the quantum yield of the major reaction product was unchanged and the "excited-state fingerprint" proved insensitive to wavelength. From this it was concluded that initial excess vibrational energy of the excited state is very quickly dissipated to solvent and that intersystem crossing occurs to the same triplet excited state independent of the configuration of the singlet initially produced. In contrast to wavelength insensitivity, a dramatic dependence on reaction temperature was encountered. This was shown not to be due to viscosity effects, for the intramolecular rearrangement rate was noted to be viscosity independent. A 50" temperature increase was found to increase the quantum yield of phenyl migration approximately 6-fold. This temperature increase led to a ca. 16-fold rate enhancement for triplet rearrangement and only a ca. 2-fold increase in rate of triplet decay. It was concluded that an activation energy of about 10 kcalimol is required for rearrangement of the excited triplet of 4,4-diphenylcyclohexenone (1). The frequency factors were 10l3-1O i3. Evidence was uncovered indicating that the stereoselectivity arises only partially from a concerted pathway being energetically favored and mainly from an entropy effect favoring the concerted process. From this study it is concluded that excited-state potential energy surfaces show thermal activation barriers similar to those encountered in ground-state chemistry. Additionally, frequency factors of the same order of magnitude as in ground-state reactions, when encountered, point to excited-state transformations with molecular demands similar to those imposed in ground-state reactions. These aspects and other details of the reaction mechanism are discussed. 0.0006. cis-5,6-Diphenylbicyclo[3.l.0]hexan-2-one (504.2 mg, 2030 pmol) x 10-5. pmol produced, 9 = 6 X x 10-4. Run 20. added to 14C photolysate (46.07 mg, 185.5 pmol): (18), 41 mg, 113-114", 2.930 =t 0.021, 0.0625 pmol produced, 9 = (9.1 f 2.0) x 3,4-Diphenylcyclohex-2-en-lone (500.0 mg, 2013 pmol) added to 14C photolysate (43.88 mg, 176.7 pmol): (lo), 24 mg, 100-102", 0.741-i. 0.015,0.0165 pmol produced, $, = (2.4 &0.3) X 10-5 Control Runs. Run C-1. 4,4-Diphenylcyclohex-2-en-lone -4-14C (171.0 mg, 688.5 pmol) in 40 ml n-dodecane at 75" for 3 hr. The reaction, work-up, and isotope dilution were performed in total darkness. 4,4-Diphenylcyclohex-2-en-lone (505.2 mg, 2034 pmol) added to "C aliquot (10.26 mg, 41.30 pmol): (3), 193 mg, 94-95', 4554 f 23,695.0 pmol recovered. trar1s-5,6-Diphenylbicyclo[3.1.0]hexan-2-one (504.5 mg, 2031 pmol) added to 14C photolysate (20.52 mg, 82.63 pmol): (7), 127 mg, 75-76', no product formed. cis-5,6-Diphenylbicyclo[3.1.0]hexan-2-one (502.3 mg, 2022 pmol) added to 14C aliquot (68.40 mg, 275.4 pmol): (12), 43 mg, 113-114", no product formed. 3,4-Diphenylcyclohex-2-en-lone was not analyzed for in this particular run, since other thermal runs showed no evidence that this product was formed. Run C-2. In this run and run C-3 the minor conversions were found due to room fluorescent light. t~rr~~s-5,6-Diphenylbicyclo[3.l.0]hexan-2-0ne-5-~~C (87.9 mg, 354.0 pmol) in 40 ml of t-butyl alcohol at 69" for 25 hr; activity 12.27 pCi/mmol. rra~~s-5,6-Diphenylbicyclo[3.1.0]hexan-2-one (498.2 mg, 2006 pmol) added to 14C aliquot (10.55 mg, 42.48 pmol): (6), 73 mg, 75-76', 262.8 + 1.4, 366.0 pmol recovered; 0% conversion. 4,4-Diphenylcyclohex-2-en-lone (514.0 mg, 2070 pmol) added to 14C aliquot (35.16 mg, 141.6 pmol): (7), 103 mg, 94-95", 0.036 f 0.016,l.j X 10-2pmol produced, 0 % conversion. cis-5,6-Diphenylbicyclo[3.1.0]hexan-2-one (505.7 mg, 2036 pmol) added to 14Caliquot(5.27 mg, 21.22pmol): (ll), 123 mg, 113-114", 0.021 + 0.007, 5.8 X 3,4-Diphenylcyclohex-2-en-lone (499.3 mg, 201 1 pmol) added to 14C aliquot (36.92 mg, 148.7 pmol): (7), 51 mg, 101-102", 0.025 i 0.003,l X Run C-3. cis-5,6-Diphenylbicyclo[3.1 .O]hexan-2-0ne-5-'~C (78.4 mg, 316.0 pmol) in 40 ml of t-butyl alcohol at 69" for 5 hr; activity 11.97 pCi/mmol. cis-5,6-Diphenylbicyclo[3.1 .O]hexan-2-one (475.4 mg, 1914 pmol) added to 14C aliquot (4.70 mg, 18.92 pmol): (6), 308 mg, 113-114", 119.78 f 0.65, 322.0pmol recovered, 0% conversion. 4,4-Diphenylcyclohex-2-en-lone (480.1 mg, 1933 pmol) added to 14C aliquot (9.41 mg, 37.89 pmol): (5), 200 mg, 94-95", no product formed. trar1s-5,6-Diphenylbicyclo[3.1.0]hexan-2-one (498.0 mg, 2005 pmol) added to 14C aliquot (32.93 mg, 132.6 pmol): (6), 121 mg, 75-76", 0.050 f 0.020,2 X 10-2 pmol produced, 0 % conversion. 3,4-Diphenylcyclohex-2-en-lone (483.4 mg, 1946 pmol) added to 14C aliquot (31.36 mg, 126.3 pmol): (7), 53 mg, 101-102", 0.064 & 0.010, 2.6 X
Mechanism of the Photodissociation of 4-Diphenyl(trimethylsilyl)methyl- N , N -dimethylaniline
The Journal of Organic Chemistry, 2000
On irradiation in hexane (248-and 308-nm laser light) 4-diphenyl(trimethylsilyl)methyl-N,Ndimethylaniline, 2, undergoes photodissociation of the C-Si bond giving 4-N,N-dimethylaminotriphenylmethyl radical, 3 • (λ max at 343 and 403 nm), in very high quantum yield (Φ ) 0.92). The intervention of the triplet state of 2 (λ max at 515 nm) is clearly demonstrated through quenching experiments with 2,3-dimethylbuta-1,3-diene, styrene, and methyl methacrylate using nanosecond laser flash photolysis (LFP). The formation of 3 • is further demonstrated using EPR spectroscopy. The detection of the S 1 state of 2 was achieved using 266-nm picosecond LFP, and its lifetime was found to be 1400 ps, in agreement with the fluorescence lifetime (τ f ) 1500 ps, Φ f ) 0.085). The S 1 state is converted almost exclusively to the T 1 state (Φ T ) 0.92). In polar solvents such as MeCN, 2 undergoes (1) photoionization to its radical cation 2 •+ , and (2) photodissociation of the C-Si bond, giving radical 3 • as before in hexane. The formation of 2 •+ occurs through a two-photon process. Radical cation 2 •+ does not fragment further, as would be expected, to 3 • via a nucleophile(MeCN)assisted C-Si bond cleavage but regenerates the parent compound 2. Obviously, the bulkiness of the triphenylmethyl group prevents interaction of 2 •+ with the solvent (MeCN) and transfer to it of the electrofugal group Me 3 Si + . The above results of the laser flash photolysis are supported by pulse radiolysis, fluorescence measurements, and product analysis. 10.
The Journal of Organic Chemistry, 1990
Quantum yields for 14 and 17 were determined in a similar way with 4 as an actinometer: dZMnm(-14) = 0.0.0094 f 0.001; Flash Photolysis Experiments. Triplet absorption spectra were obtained at 25 "C on -3 mL of acetonitrile solutions of 5 (0.0087 M) and 14 (0.010 M) and benzophenone (0.005 M) that were thoroughly purged with argon. Benzophenone triplet decay (2.78 X 105 s-l) was monitored at 480 nm. The absorption spectra were generated by measuring absorbances at 5-nm intervals in the range 290-580 nm. Because of photochemical instability, the triplet absorption spectrum of isoxazole 5 was a composite of measurements taken from different samples. In addition, the spectrum was recorded in both sweep directions in which case ~3000m(-17) = 0.083 f 0.004. small differences in optical densities were observed. Both compounds had a strong absorption peak at ca. 320 nm, and in addition, 5 showed a broad maximum at 480 nm and 14 showed a shoulder a t 410 nm. The transient lifetimes in the absence of biphenyl were 276 and 48 ns, respectively. In the presence of 0.01 M biphenyl a new absorption appeared at 360-365 nm. Quenching rates were calculated from lifetime measurements in the presence and absence of biphenyl: 5, 8.3 X lo8 M-ls-l; 14, 2.0 X lo9 M-' S-1.
The Transfer and Conversion of Electronic Energy in Some ‘Model’ Photochemical Systems
Photochemistry and Photobiology, 1965
Recent studies of the effects of molecular structure and reaction environment on the mechanism of primary photochemical processes involving transfer and conversion of electronic energy in relatively 'simple' organic molecules are presented and discussed. A quantitative i.r. spectroscopic method for studying intramolecular and intermolecular photoprocesses of U.V. irradiated substrates dispersed in solid alkali halide matrices at room temperature is described. Current data for the substrates ortho-nitrobenzaldehyde, anthracene and benzophenonebenzhydrol are presented. A series of 'model' ketones containing cyclopropyl groups have been synthesized and while their absorption spectra are similar, the efficiency of vapor-phase photodissociation into radicals is shown to be strongly dependent on molecular structure. Butyrophenone and a series of ring substituted derivatives have been photolyzed in the liquid phase using the quantum yield of the photoelimination of ethylene (Type I1 split) as a "probe" to determine the effect of substituents on the internal H atom abstracting power of the excited carbonyl chromophore. @ c~R~ is very sensitive to ring substitution, dropping from 0.24 in butyrophenone to 0.20, 0.058 and 0.00 in the p-CH3, p-OCH3 and p-NHz derivatives respectively, and to 0.00 in both ortho and para hydroxy derivatives. This effect is correlated with their absorption spectra in terms of the lowest states of these alkyl aryl ketones being s(n, n*) rather than 3(n, n*) in character. While several 'classic' photochemical reactions, unimolecular and bimolecular, proceed efficiently in solid KBr matrices giving the same product as in liquid systems, the 'model' cyclopropyl compounds and the alkyl aryl ketones did not undergo their usual intramolecular processes. Implications of this molecular environment effect are pointed out. IN ORDER to obtain a 'complete' understanding of the photochemistry or photobiology of a given system, ideally one has to elucidate the entire 'life history' of the photoprocess, starting with the act of absorption and concluding with a detailed analysis of the system in its final physical and chemical state. Clearly this is a monumental (and perhaps impossible) task even for simple organic molecules, much less for complex molecules of biological significance. However, despite the formidable challenges that face us, in recent years, great advances have been made in the understanding of photoprocesses. In part, these have been *Presented at the Rapporteur Session,
Dihydrophenanthrene-Type Intermediates during Photoreaction of trans-4‘-Benzyl-5-styrylfuran
The Journal of Organic Chemistry, 2005
Photoreaction of trans-4′-benzyl-5-styrylfuran (trans-BSF) has been studied by the 355-nm laser flash photolysis (LFP) in CH 2 Cl 2 using a Nd 3+ :YAG laser (30 ps, 5 mJ pulse-1 or 5 ns, 30 mJ pulse-1). Transient fluorescence and absorption spectra assigned to the singlet excited trans-BSF were observed during the 30-ps LFP, whereas a transient absorption spectrum with two peaks at 400 and 510 nm, assigned to the trans-fused dihydrophenanthrene (DHP)-type intermediate (DP1), was observed during the 5-ns LFP. It is clearly suggested that a two-photon absorption process is involved in the formation of DP1. The first photoreaction is the photoisomerization of trans-BSF, which occurs to give cis-BSF. The second photoreaction process is photocyclization of cis-BSF, which occurs to give DP1 decaying with the half lifetime (τ 1/2) of 2.8-4.0 µs to produce another DHP-type intermediate (DP2) with an absorption peak at 400 nm in the absence of O 2 , through [1,9]-hydrogen shift. DP2 decayed with τ 1/2 > 500 µs to give the product through aromatization. In O 2-saturated CH 2 Cl 2 , DP1 decayed with τ 1/2) 250 ns to give a radical intermediate (X) with two peaks at 410 and 510 nm, through hydrogen abstraction of DP1 by O 2. X decayed with τ 1/2) 150 µs to give the product through successive hydrogen abstraction.
The Journal of Organic Chemistry, 1995
Parameters useful to predict reactivity and regiochemical control of photocoupling reactions between haloheterocyclic derivatives and arylalkenes or arylalkynes have been studied. Electrochemical properties of arylalkenes and arylalkynes are shown to be useful to predict the reactivity of the substrates. However, oxidation potentials fail as reactivity indices if very fast photochemical processes are in competition with the observed complex formation between substrates and halogen atoms. The regiochemical behavior of the reaction can be estimated on the basis of dipoles of the reagents. In this case the assumptions that a reagent approaches the other on parallel planes and that the prevalent interaction is between the SOMO of the radical and the LUMO of the other reagent have been accepted.
Photorelaxation and Photorepair Processes in Nucleic and Amino Acid Derivatives
Molecules, 2017
Understanding the fundamental interaction between electromagnetic radiation and matter is essential for a large number of phenomena, with significance to civilization. On the most fundamental level, through the molecular origins of life, photosynthesis, and vision, the interaction between sunlight and matter has played an essential role in nature. Many applications of these interactions continue to revolutionize society through advances in medicine, communications, technology, and entertainment. Electromagnetic radiation is also capable of inducing a myriad of chemical transformations, as illustrated by the photodegradation of DNA and proteins [1-4]. These light-induced reactions have been associated with cancer and other diseases in living organisms [5-7], and photochemical investigations of nucleic and amino acids continue to be at the forefront of research. Photochemical investigations of modified nucleobases are also at the center of research because of their potential role as prebiotic materials of the building blocks of life [8-10] and their prospective applications as phototherapeutic agents [11-13]. Studying the basic interactions of these biological molecules with light may hold the key for a complete understanding of the mechanisms responsible for their photostability and photochemistry. It may also provide fundamental insight for a molecular-level understanding of the mechanisms tied to DNA photorepair. Absorption of ultraviolet or visible radiation by the ground state of a molecule populates electronic states, either directly to an excited singlet state or indirectly to a triplet state after intersystem crossing from the singlet manifold [14]. In both singlet and triplet manifolds, ultrafast internal conversion usually leads to the population of the lowest-energy excited state-the S 1 and T 1 states, respectively. These excited states, although relatively short lived (ca. ≤10 −9 s and ≤10 −6 s for singlet and triplet states, respectively), may live long enough that chemical reactions can compete with radiative or nonradiative decay to the ground state. Energy or charge transfer from an excited singlet/triplet state of a molecule (a.k.a., sensitizer) may also populate an excited singlet/triplet state of another molecule by a photochemical process known as photosensitization. In contemporary organic photochemistry, a microscopic description of the electronic relaxation pathways that a photoexcited molecule explores through nuclear coordinate space usually begins with a representation of a reaction coordinate describing the evolution of reactants to products [15-19]. An important distinction between a photochemical and a photophysical relaxation pathway concerns the initial and final states that the reaction coordinate develops. In a photochemical reaction, these states are different, corresponding to the different structures of the reactant(s) and product(s). In a photophysical process, the relaxation pathway ends where it began-in the electronic ground state. In order to return the molecule to its ground state, the absorbed energy can be released radiatively (i.e., through photon emission; usually from the S 1 or T 1 state), or it can be transformed into vibrational energy that can dissipate nonradiatively into the environment. Internal conversion and intersystem crossing can occur at a higher rate than radiative decay when nuclear motions take a molecule into regions of nuclear configuration space where two or more potential energy hypersurfaces cross. The intersection between potential energy hypersurfaces often create crossing seams or conical intersections, where states of equal multiplicity or singlet/triplet