Sensitized butene - 2 isomerizations

Base catalysts are extremely sensitive to the atmosphere in which they have been stored and generally require some sort of activation procedure, usually heating, prior to application. For materials stored in air, the formation of surface carbonates via reaction with atmospheric carbon dioxide is problematic. Figure 21.1 illustrates the effect of pre-treatment upon base-catalyzed n-butene isomerization [9]. It can be seen that activity increases up to a maximum of circa 650 °C. 

The butene method is the most straightforward and it has been the more thoroughly explored of the two methods. In essence, triplet benzene sensitizes cis- or rrc H.9-butene-2, and in the subsequent relaxation of triplet butene, isomerization occurs. The quantum yield of this isomerization can then be traced back to the quantum yield of triplet benzene formation. 

Cundall and Davies have measured = 0.31 when c -butene isomerization is sensitized by triplet CsDj formed after excitation of benzene with the 2536 A line. Equation (1) gives the yield of triplet C D as... 

The most radiation-stable poly(olefin sulfone) is polyethylene sulfone) and the most radiation-sensitive is poly(cyclohexene sulfone). In the case of poly(3-methyl-l-butene sulfone) there is very much isomerization of the olefin formed by radiolysis and only 58.5% of the olefin formed is 3-methyl-l-butene. The main isomerization product is 2-methyl-2-butene (37.3% of the olefin). Similar isomerization, though to a smaller extent, occurs in poly(l-butene sulfone) where about 10% of 2-butene is formed. The formation of the olefin isomer may occur partly by radiation-induced isomerization of the initial olefin, but studies with added scavengers73 do not support this as the major source of the isomers. The presence of a cation scavenger, triethylamine, eliminates the formation of the isomer of the parent olefin in both cases of poly(l-butene sulfone) and poly(3-methyl-1-butene sulfone)73 indicating that the isomerization of the olefin occurred mainly by a cationic mechanism, as suggested previously72. 

An interesting gas-phase study of the photolysis of ftmy-l-phenyl-2-butene has been published by Comtet.(39,40) He has found that (a) it was not possible to quench the formation of the cyclopropane product under conditions that reduced the fluorescence quantum yield, (b) sensitization by acetophenone only gave cis-trans isomerization, and (c) the quantum yield of cyclopropane formation in the direct photolysis decreases as n-butane is added to the reaction mixture. Comtet suggests that the data are consistent with a reaction from the second triplet state. 

The preparation of acyclic allylic hydroperoxides has been described before (3, 7, 9), but it is not clear how the reactivities differ from the better known saturated hydroperoxides and cyclic allylic hydroperoxides. Dykstra and Mosher prepared allyl hydroperoxide by the reaction of allyl methanesulfonate with hydrogen peroxide and alcpholic potassium hydroxide and purified the hydroperoxide by gas chromatography. It detonated on heating and decomposed on exposure to light but was relatively stable in the cold and dark. The isomeric allylic hydroperoxides formed from the autoxidation of the branched olefin, 4-methyl-2-pentene, have also been isolated and were not abnormally reactive (3). In the present study, cis- and trans-2-butene were photooxidized in the presence of methylene blue as a sensitizer (14), and the product, l-butene-3-hydro-peroxide, was isolated by preparative chromatography. 1-Butene proved unreactive and 2-butene-l-hydroperoxide could be formed only by isomerization of the secondary hydroperoxide. 

Other systems for which the results cannot be explained by energy transfer include the ketone sensitized isomerization of / -methylstyrene, 1,2-dichloro-ethylene and 2-butene studied by Caldwell,147,148 deuterium exchange between acetone-d8 and tetramethylethylene reported by Japar, Pomerantz, and Abrahamson,149 and the acetone sensitized isomerization of 1-methoxy-l-butene examined by Turro and Wriede.71... 

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. 

Cundall has done extensive work on benzene231,237 and acetone243 sensitized isomerizations of the 2-butenes, and in every case reported a photostationary or radiostationary trans/cis ratio of 1.27-1.37. Sato, however, has measured a value of unity for the benzene photosensitized isomerization.510 With higher homologs, from 2-pentene to 2-octene, benzene-sensitized isomerizations yield trans/cis ratios of 1.0,238 while acetone-sensitized isomerization of the 2-pentenes in solution yields a ratio of 1.65.244 At present no explanation is possible for the differences between 2-butene and 2-pentene. Until much more information is gathered relating to rates of triplet energy transfer as functions of olefin structure, sensitizer, and medium, the natural decay ratios of each olefin s common triplet cannot be deduced from photostationary trans/cis ratios. 

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... 

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

Sigal studied the benzene d-6 sensitized isomerization of butene-2 at pressures below 0.1 mm., where the interval between collisions is long relative to the fluorescence lifetime. The butene-2 isomer concentration was monitored by long-path infrared spectrometry. The data are consistent with a kinetic scheme based on collisions being rate determining for intersystem crossover at low pressures. 

Kistiakowsky and Parmeter studied the benzene-sensitized isomerization of CTS-butene-2 at one pressure, viz., 0.047 mm. They found isomerization even at this low pressure, thus indicating that triplet-state benzene molecules are present in CeHe irradiated at this low pressure. They did not study the effect of pressure by the Cundall technique. They did, however, study the relative emission from benzene as a function of pressure at low pressures. By making the assumption supported within experimental error by the data of Ishikawa and Noyes that all absorbing molecules either emit or cross over to the triplet state, Kistiakowsky and Parmeter conclude that the rate of crossover is independent of collisions even at these low pressures. 

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... 

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... 

By competitive methods Ishikawa and Noyes (226) (sensitized biacetyl phosphorescence) and Cundall and Davies (159) (butene-2 isomerization) both estimated the triplet lifetime to be of the order of 10 ys. In a reexamination of the butene-2 system, Lee (227) estimated a value of 100 ys, a finding confirmed by Cundall and Dunnicliff (105). An examination of the kinetics of the benzene photosensitized composition of cyclopentanone decomposition allowed a value of longer than 3 ys to be deduced. This type of experiment is far from satisfactory since photochemical processes can Intervene at low pressures, and impurities and the quenching effects of photoproducts can affect the results. These problems can only be overcome by some form of direct measurement. Parmenter and Ring (228) used a flash method in which 20 torrs of benzene and 0.01 torr of biacetyl were submitted to a 20 J,... 

The direct or sensitized cis-trans photoisomerization has been described by many authors (58). The Hg( P,) photoisomerization was explained on the basis of vibrationally excited triplet molecules. In the case of the Cd( P,), the photosensitization of unsaturated hydrocarbons mainly results in the cis-trans process. The main difference between mercury and cadmium sensitization resides in the lower energy content delivered in cadmium experiments 366.4 in comparison with 469.4 kJ/mol (59). A very recent study of the zinc photosensitization of 2-butene concludes with the cis-trans isomerization process as the main process (60). 

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. ... 

Z- and -2-Butenes in liquid neopentane undergo ci5—/ra/is-isomerization and it is competitive with their photochemical [2+2]-cycloadditions [3]. When the alkene solution is sufficiently diluted with an inert solvent, the isomerization becomes the dominant reaction. The triplet energy of sensitizer PhH is higher ( 84 kcal/mol) than that for 2-butene ( 80 kcal/mol) and hence triplet energy transfer is... 

Isomerization of 1-butene was studied over crystalline ReOs and Rc207 which were prepared in several ways, and the presence of two different sites were indicated one was sensitively promoted by the presence of H2 and the other (possibly Lewis sites) not. ... 

The acid catalysis of heteropoly compounds in the solid state is classified into bulk-type and surface-type reations. The former type reactions proceed in the catalyst bulk and the latter only on the surface. Dehydration reactions of alcohols belong to the former and isomerization of butene to the latter. So the classification is closely related to the adsorption property of reactants. The activities for the surface-type reactions are more sensitive to pretreatment. 

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