Sensitized cis-trans isomerization

For an elegant study of the secondary deuterium isotope effects on a non-Schenck sensitized cis-trans isomerization, see Refs. 85-87. 

Continuous processes if photochemical (and thermal) reactors are installed in series. Such an arrangement could be useful in the production of previtamin D (PD), where tachysterol (T) produced as a secondary product (Figure 21) may be recovered by a subsequent triplet sensitized cis-trans isomerization [2, 3, 75-77]. 

An added complication in the interpretation of long-wavelength ketene photolysis is demonstrated by Cundall s discovery of ketene sensitized cis-trans isomerization of the 2-butenes.33 As the pressure of olefin increases, the rate of ketene decomposition decreases and the rate of olefin isomerization increases. At high olefin concentrations part of the apparent nonstereospecificity of cyclopropane formation can thus result from stereospecific singlet addition to already isomerized olefin. 

It is only very recently that much work has been reported on sensitized cis-trans isomerizations of nonconjugated olefins, probably because the spectroscopic triplet energy of ethylene seemed so much... 

The pyridine-sensitized cis-trans isomerization of cw-2-butene in the gas phase has been reinvestigated recently.75 If the lowest singlet states of the pyridine are assumed not to interact with the cw-2-butene in its ground state, the quantum yields of intersystem crossovers of the pyridine from these lowest singlet states could be measured by Cundall s technique.18,26 1,264 As a matter of fact, there are now some doubts about the validity of Cundall s technique for pyridine since it has been suggested that a complex between excited pyridine and the c/s-2-butene may be formed and provide a path of deactivation of the pyridine molecule. With the assumptions of Cundall, 26a 26a the quantum yield of... 

The pioneering investigations of Hammond et al. (7) in the 1960s on the triplet-sensitized cis-trans isomerization of the stilbenes provided much of the basis for our current understanding of triplet-triplet energy transfer. The detailed... 

Ethoxyisoindolenone (50) sensitized cis-trans isomerization of c/s-piperylene has been used in a triplet counting experiment to measure the intersystem crossing efficiency. A linear triplet counting plot is obtained which gives an intersystem crossing efficiency of 0.96 0.01 and a triplet lifetime of 1.5 x 10 7 sec. for 50 in methylene chloride solvent at ambient temperature in the absence of olefin. 

In 1,2,3-triphenylallyl carbanions, electron transfer photochemistry is found in the presence of acceptors like stilbene as demonstrated by the sensitized cis-trans isomerization of this molecule according to the following pathway [145],... 

Sensitized cis-trans isomerization of 2-butenes or other olefins for triplet yield (59,62). 

Step (7) was suggested by Groh et while Cundall and Davies preferred step (8). Investigating the acetone-sensitized cis-trans isomerization of butene-2, Cundall and Davies determined values for ks+k [A]) T (where t is the yield of the triplet acetone) at various acetone pressures. They found that the change in acetone pressure (in the range 25-100 torr) does not affect this ratio (at 3130 A... 

Triplet-sensitized cis-trans isomerization is frequently more efficient. Thus, the direct excitation of 1,3-pentadiene (piperylene), which has been studied in great detail, results in cis-trans isomerization with very low quantum yields (4>,, = 0.09, = 0.01) and very small quantities of di-... 

Using azulene as a triplet quencher it has been shown that triplet states are not involved in the cis-trans isomerization of stilbene on direct excitation. Triplet-sensitized cis-trans isomerization, however, is observed and proceeds in both directions through a minimum in the triplet potential energy surface at a twisted geometry, often referred to as the triplet phantom state P. ... 

Considerable data are available for triplet yields of benzene in dilute solutions of different solvents (see Table 13). In the main, two techniques have been used sensitized phosphorescence of biacetyl, sensitized cis-trans isomerization of butene-2, octene-2, and stllbene. All yield comparable results. In saturated hydrocarbon solvents at room temperature, the triplet yield for CgHg is found to be about 0.24 0.01. There is a solvent dependence of this quantity, the yield dropping to 0.15 in ethanol, 0.13 in methanol, and 0.09 in acetonitrile (91). In determining the effect of environment on the rate constant controlling intersystem crossing, values for emission lifetimes in the various systems are needed. These are, as mentioned previously, often unreliable. Cundall and Pereira (91) have reported... 

Morrison has made a detailed mechanistic study (70) of [77c] and [77d]. On irradiation, compound [77d] leads only to [77c] with a quantum yield of 0.029. Compound [77c], however, forms oxetane [78] ( =0.015) in competition with isomerization to [77d] ( =0.02) no crossed adduct [79] was observed. Triplet quenching did have very little effect on Isomerization or oxetane formation, while on sensitization cis-trans Isomerization but no oxetane formation occurred. The authors concluded that the reaction occurred via that intramolecular complex formation in... 

Only a few studies concerning the temperature dependence of sensitized cis trans isomerization have been reported [2, 108, 184, 196, 236], A small effect of elevated temperature on ([c]/[t])scns for stilbene has been found [2,196] at 59°C the photostationary state is somewhat richer in cis isomer than at 28°C if sensitizers with Er > 222 kJ mol-1 are used. However, Yamashita et al. [184] have obtained activation energies of 2 and 3kJmol 1, respectively, for benzophenone-sensitized trans- cis and cis- trans isomerization (25 to -78°C, liquid state) they observed that both reactions still proceed even at — 196°C (solid state). In contrast Malkin and Fischer [108] found no temperature dependence of the benzophenone-sensitized isomerization down to — 140°C. 

From among further papers dealing with sensitized cis/trans isomerization of 1,2-arylethylenes with larger chromophores, those of Tokumaru... 

Other quenchers that have been used in the benzophenone-sensitized cis trans isomerization of stilbene are /9-carotene, oxygen, and di-tert-butyl nitroxide. /9-Carotene shifts the photostationary state to the trans side similar to the azulene effect [237]. In the presence of di-tert-butyl nitroxide, a radical quencher, the photostationary state is slightly shifted to the cis side Caldwell and Schwerzel [226] have suggested the involvement of the twisted triplet state and a quenching mechanism other than energy transfer, probably vibrational relaxation to the ground state caused by spin exchange. 

The vexed question of the mechanism of the direct and sensitized cis-trans isomerization of stilbene has been the subject of recent papers.546 The studies of the two groups differ in the favoured mechanism for the direct cis-trans isomerization, one proposing the conventional triplet-state mechanism, the other plausibly invoking the intervention of a doubly excited singlet potential surface with a deep potential minimum at a 90° configuration. 

Vibration-vibration energy transfer in SOa has also been discussed.390 The SOa-sensitized cis-trans isomerization of but-2-ene is a complex reaction, which at low pressures can be explained as involving only the ZBX state of S02, but which at higher pressures requires the intervention of some triplet state other than the phosphorescent SBX level.391 The photolysis of S02 in the presence of acetylene and allene also involves these non-phosphorescent triplet states, yielding CO as product, and giving rise to aerosol formation.392 Aerosol formation in S02-N2-02 mixtures has been experimentally investigated,393 and the effects of N02, propylene, and water on SOa reactions have been reported.884 Interactions between S02 and atmospheric particulate matter,395 photochemical removal of SOa from the lower atmosphere,396 the use of SOa to trap 1,4-biradicals generated photo-chemically,397 and the photochlorination of S02 398 have all been discussed in recent publications. 

Quantitative assessment about triplet formation comes principally from two methods. The first is the sensitized cis-trans isomerization of butene-2 introduced by Cundall and Palmer in I960. This method has been used by many workers and the essential elements of the mechanism are well established. " The second method, based on sensitized emission from biacetyl, was introduced by Ishikawa and Noyes, and is discussed in its most recent stage of evolution by Noyes and Harter. ... 

Sensitized cis-trans Isomerization—A Tool in Chemical Spectroscopy... 

A further application of sensitized cis-trans isomerization as a tool in chemical spectroscopy involves the determination of intersystem crossing... 

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