Olefins photochemical isomerization

We have reported here the preparations and treatment conditions that are needed to reduce Iron Ions to metallic Iron In zeolites. Although we have not Isolated highly-dis-spersed superparamagnetic Iron particles In zeolites, we have shown that these iron-containing zeolites are active catalysts in Fischer-Tropsch and in olefin isomerization reactions. The added insight that stems from the use of in-situ Mossbauer experiments has led to the preparation of new active catalysts that can be selectively activated. We presently are studying photochemical reactions of other metal carbonyl complexes in zeolites and believe that increased selectivity is a major benefit in these types of reaction. 

A is exothermic. An additional advantage is that one has a wider margin to select spin-state specific photochemical transformations. Representative examples include ds-trans-olefin isomerization (Eq. 49), olefin dimerization (Eq. 50), oxetane formation (Eq. 51), dienone rearrangement (Eq. 52), di-vr-methane rearrangement (Eq. 53), azoalkane denitrogenation (Eq. 54a, b), and photocyclization (Eq. 55). 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

Aldehyde 1032 has been used as a chiral template for the side chain of (255, 26)-di-hydroxycholecalciferol (1036), a metabolite of natural vitamin D3 (Scheme 153) [225]. The chain is introduced by a Wittig coupling of the stigmasterol-derived phosphorane 1033 with aldehyde 1032 to give olefin 1034 as an E/Z isomeric mixture. This is of no consequence, since the double bond is eventually reduced. Transformation of the stigmasterol steroid unit to the 7,8-didehydrocholesterol derivative 1035 followed by photochemical—thermal isomerization furnishes the vitamin D3 framework. 

We now present an overview of the basic types of organic photochemical reactions. We begin with acid-base reactions, and then turn to reactions of hydrocarbon tt systems, such as olefin isomerizations, cycloaddition reactions, and the di-rr-methane rearrangement. We then study "heteroatom" photochemistry, the photoreactions of carbonyls and nitrogen-containing chromophores. 

Perfluoroolefins isomerize photochemically to yield less substituted olefins [272] Photolysis of polyfluorotriarylmtrones leads to polyfluorotriaryloxaziridines [173] (equation 43)... 

Allyl p-tolyl sulphoxide 535 reacts with sodium methoxide in methanol by initial prototropic isomerization and subsequent addition of methanol to give 536 (equation 333). Protic solvents are photochemically incorporated by the open chain olefinic bond of trans methyl )S-styryl sulphoxide 537 in a Markovnikov regiospecificity (equation 334). Mercaptanes and thiophenols add to vinyl sulphoxides in a similar manner (compare also Reference 604 and Section IV.B.3) to give fi-alkylthio(arylthio)ethyl sulphoxides 538 (equation 335). Addition of deuteriated thio-phenol (PhSD) to optically active p-tolyl vinyl sulphoxide is accompanied by a low asymmetric a-induction not exceeding 10% (equation 336) . Addition of amines to vinyl sulphoxides proceeds in the same way giving )S-aminoethyl sulphoxides in good to quantitative yields depending on the substituents at the vinyl moiety When optically active p-tolyl vinyl sulphoxides are used in this reaction, diastereoisomeric mixtures are always formed and asymmetric induction at the p- and a-carbon atoms is 80 20 (R = H, R = Me) and 1.8 1 (R = Me, R = H), respectively (equation 337) ... 

Bromo- and iodocyclopropanes cannot be prepared by the direct halogenation of cyclopropanes. Substituted chloro- and bromocyclopropanes have been synthesized by the photochemical decomposition of a-halodiazomethanes in the presence of olefins iodocyclopropanes have been prepared from the reaction of an olefin, iodoform and potassium f-butoxide followed by the reduction of diiodocyclopropane formed with tri-w-butyl tin hydride. The method described employs a readily available light source and common laboratory equipment, and is relatively safe to carry out. The method is adaptable for the preparation of bromo- and chlorocyclopropanes as well by using bromodiiodomethane or chlorodiiodomethane instead of iodoform. If the olefin used will give two isomeric halocyclopropanes, the isomers are usually separable by chromatography. ... 

The isomerization takes place because the excited states, both St and T,. of many olefins have a perpendicular instead of a planar geometry (p. 236), so cis-trans isomerism disappears upon excitation. When the excited molecule drops back to the S<, state, either isomer can be formed. A useful example is the photochemical conversion of c/s-cyclooctene to the much less stable trans isomer.45 Another interesting example of this isomerization involves azo crown ethers. The crown ether 5, in which the N=N bond is anti, preferentially binds NH4, Li, and Na, but the syn isomer preferentially binds K and Rb (see p. 83), Thus,... 

Since the migration reaction is always toward the end of a chain, terminal olefins can be produced from internal ones, so the migration is often opposite to that with the other methods. Alternatively, the rearranged borane can be converted directly to the olefin by heating with an alkene of molecular weight higher than that of the product (7-15). Photochemical isomerization can also lead to the thermodynamically less stable isomer.72... 

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 geometric isomerization of olefins via photochemical electron transfer is well known28,29 and can be divided into two categories (a) isomerization via the radical cation, in which case the olefin is the donor in the presence of an excited electron acceptor (b) isomerization via the radical ion pair, which leads to the triplet-excited olefin, and in this mechanism the olefin is the acceptor. This subject is not discussed in this chapter because of space limitations. However, several reviews30 can be consulted in this regard. 

Olefins are also susceptible to photodegradation reactions other than cycloaddition reactions (43). The olefin bond is susceptible to E(trans)-Z(cis) isomerization as well as oxidation (Fig. 107) (154). Photochemical E-Z isomerization has a major role in photobiological systems and has practical applications in vitamin A and vitamin D industrial processes... 

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