Cis/trans isomerization, of butene

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

The radiation induced cis-trans isomerization of 2-butene in benzene, benzene-dr, toluene, and pyridine has been studied. The triplet yield in benzene is 4.7 molecules per 100 e.v. benzene-dr, is similar. The yield in toluene is 6 molecules per 100 e.v. Nitrous oxide reduces the isomerization yield by about 40%. This indicates a G value of about 2 for excited states produced by the recombination of benzene ions with electrons. Xenon and krypton (up to 1M) increase the isomerization yield from 2.4 to 3.1 and 2.9 respectively, whereas argon has no effect. From the effect of xenon, which is attributed in part to enhanced intersystem crossing, it is estimated that the yield of singlet states which do not cross to the triplet in unperturbed benzene solutions is 1.3 molecules per 100 e.v. 

HOMO of transition state for cis-trans isomerization in 2-butene shows distortion of the 7t bond. 

There are two forms of these isomers the cis and trans versions. The form in which the substituent hydrogen atoms are on the same side of the bond that doesn t allow rotation is called cis, the form in which the hydrogens are on opposite sides of the bond is called trans. An example of a small hydrocarbon displaying cis-trans isomerism is 2-butene. 

Borrell, P. and Holmes, J. D, Photochemical cis-trans isomerization of 2-carbomethoxy-2-butene, /. Photochem., 1, 433,1972/73. 

There is clear evidence that the addition of a sulfonyl radical to an olefin is a reversible reaction. This has been demonstrated for the case of but-2-ene, where cis-trans isomerization of the olefin accompanies addition63,64. Furthermore, 18a and 18b radicals generated by the reaction of Bu3Sn with erythro- and tfireo-2-bromo-3-(phenylsulfonyl)butane, respectively, eliminate benzenesulfonyl radicals to some extent, even at — 67 °C, to form 2-butenes in a non-stereospecific manner, providing also indication that the rotation about the central C—C bond is faster than the /1-cleavage process65. 

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. 

As already mentioned, the triplet state of benzene was identified by the cis-trans isomerization of the 2-butenes. Nevertheless, Wilzbach and Kaplan in the liquid phase, following work of Srinivasan,42 find a photochemical adduct of benzene to olefins. This adduct they find to be an adduct of benzvalene and not of benzene itself.414 There is also evidence of formation of a small amount of this adduct even in the vapor phase.35 Benzene exposed to ultraviolet radiation in a flowing system diluted with nitrogen and condensed shows compound formation with trifluoroethanol, CF3CH2OH. Again the adduct is one of benzvalene.410... 

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 carbonylation of both cis- and trans-2-butene in methanol in the presence of a palladium catalyst and copper (II) has demonstrated the stereochemistry of this oxymetallation of simple monoolefins (19). In the initial stages of the reaction, stereospecific trans methoxypallada-tion is observed, yielding exclusively the threo- and erythro-/3-methoxy esters, respectively (Table I). In the later stages of the reaction ds-trans isomerization of both butenes becomes increasingly important such that trans methoxypalladation to the cis-trans mixtures yield both erythro and threo products. The rate of methoxypalladation of ds-2-butene is approximately three times faster than that of the trans isomer. 

A comparable study of the Ni-catalyzed reactions of isobutene, 1-butene, and cis-butene-2 in the presence of H2 and D2 has revealed a very complex series of chemical reactions that take place, including induced isomerization of 1-butene to 2-butene, deuterium exchange, induced cis-trans isomerization of butene-2, and finally, addition to the double bond. Below 200 mm Hg pressure, the rates of exchange, addition, and isomerization are about equal for 1-butene and are about % order in olefin and order in H2. With increasing excess of H2 this approaches zero order in olefin and % order in H2, while for large excess of 1-butene, all reactions become inhibited (30 to 150°C). Although the authors have attempted to discuss mechanisms in connection with the data, the lack of information on the isotherms makes that of dubious value. 

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... 

Table I. The Effect of Various Additives on the cis-trans Isomerization of 0.25M cis-2-Butene in Benzene"...

Isomerization of butene-2. Siloxene catalyzes the cis-trans isomerization of butene-2, without isomerization of the double bond to form butene-1. A free-radical mechanism has been suggested. ... 

What will determine the stability of the isomeric alkenes One factor is cis-trans conformations. In 2-butene, for example, the cis conformation... 

The heat of cis trans isomerization of butene-2, in the vapour phase, is also available from Lacher s data on the hydrobromina-tion of these isomers, at 100 and 110°C. The difference between the two heats is again — 1-0 kcal/mole. 

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