Photochemical reactions olefin isomerization

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... 

Photochemical reactions have been used for the preparation of various olefin, and acetylene complexes (7). Application to the coordination of dienes as ligands has not been used extensively, so far. In this article the preparative aspects of the photochemistry of carbonyls of the group 6 and group 7 elements and some key derivatives, with the exception of technetium, with conjugated and cumulated dienes will be described. Not only carbonyl substitution reactions by the dienes, but also C—C bond formation, C—H activation, C—H cleavage, and isomerizations due to H shifts, have been observed, thereby leading to various types of complexes. 

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. 

Examples of well-known photochemical reactions which involve electron transfer include the primary step in plant and bacterial photosynthesis [2], the photoreduction of ketones by amines [3], a series of sensitized isomerizations of olefins and small ring compounds such as cyclopropanes or of strained polycyclics such as quadricyclane to norbornadiene or Dewar benzenes to benzenes [4], and the reactions of electron-rich substrates in the presence of oxygen which proceed via superoxide [5]. These reactions and others have proved valuable for synthetic applications in addition to their fundamental interest to photochemists. 

As one of the key types of photochemical reactions, cis-trans isomerization of olefins has well been investigated from mechanistic as well as synthetic aspects of the reaction (3-16). Many olefins seemed to isomerize mutually between their cis- and trans-isomers. A following well accepted mechanism was proposed. In the singlet and triplet excited states, cis (c") and trans (t ) forms are less stable than the perpendicular form (p"), and c and t twist around the C=C bond to p", which deactivates to the ground state giving a mixture of the cis- and trans-isomers (Fig. 1). 

In this chapter, we attempted to show an approach to find out new types of basic photochemical reactions, particularly, the isomerization of olefins. 

Olson [8-10] was first to postulate that photochemical trans-cis isomerization in olefins may be an adiabatic reaction. Hammond and co-workers [121] and others [122-124] reported the quantum chain cis-to-trans isomerization process from the triplet excited state of olefin. They postulated the energy transfer process from the trans isomer adiabatically formed to cis isomer in the ground state. Later Arai and Tokumaru [94] with their extensive investigations showed that cis-to-trans adiabatic photoisomerization resulted in quantum chain process from the triplet excited state in a variety of olefins. Nevertheless, the adiabatic photoisomerization in olefins originating from the singlet excited state is less studied and the same is highlighted. 

Barton has suggested 35> that in photochemical reactions of cyclic conjugated olefins, ring fission will predominate in rings of n annular atoms containing (n/2)-l double bonds and valence tautomerization will occur in other systems. Consequently, it was anticipated that 1,3,5-cyclo-octatriene upon photolysis should give rise to an acyclic tetraene in the same manner that 1,3-cyclohexadiene opens 35> 36> to 1,3,5-hexatriene. When 1,3,5-cyclooctatriene (55) was irradiated in solution (ether or pentane) 37> 39> two isomerization products were isolated, bicyclo[4.2.0]-octa-2,7-diene (56) and tricyclo[5.1.0.0 4> 8]oct-2-ene (57). The formation of 56 is not exceptional. The formation of 57 has been visualized by... 

The reaction between the PtCh ion and alkanes can be induced by irradiation [26]. When a solution of hexachloroplatinic acid and n-hexane in acetic acid is irradiated by light (A > 300 nm) [26a,c,d] or y-quanta [26b], a 7t-coinplex of hex-1-ene with platinum(II) is formed in addition to isomeric chlorohexanes. This complex has been isolated in the form of the pyridine adduct, (hex-l-cne)PtCl2Py. The yield of the it-complex in the y-induced reaction reaches 17% based on Pt. The photochemical reaction is of first order with respect to hexane. It is interesting that the photochemical reaction with hex-2-ene affords the n-complex ofhex-l-ene (Scheme VII.5). This complex can be also prepared by the reaction ofhex-l-ene with PtCL, " under irradiation. It should be noted that the photoinduced reaction of PtCl6 or PtCU with olefins is a convenient method for the synthesis of various n-o efin complexes of platinum(II) [27]. The photostimulated reaction of PtCU with stilbene does not occur, possibly due to steric restrictions. 

Photochemical studies of olefin complexes are more concentrated on diene compounds because they are more stable. Photochemical reactions of hydrogenation, isomerization, and hydrosilation of olefins were carried out. ... 

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. 

Photochemical reactions of compounds containing a P,y-enone chromophore have generated intense interest for more than two decades " and even more so in recent years.Several excellent reviews dealing with various aspects of P,y-enone photochemistry have appeared. " A good historical background on the photoreactions of P,y-enones is described by Schuster. In principle, P,y-enones can undergo photoreactions that are characteristic of carbonyl and olefinic chromophores, such as photoreduction, epimer-ization and cis,trans-isomerization, [2-F2]-cycloaddition, etc. They also undergo two unique reactions as... 

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. ... 

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... 

Demonstration of cis addition with acetylene is unequivocal, in that the stereochemistry of the resulting bis(dihaloboryl)ethylene has been shown directly by spectroscopic evidence (90) and by conversion of the initially produced cis isomer to the trans compound by photochemical isomerization (27). Stereochemical inferences in the olefin addition reactions are based on the assumption that hydrolysis and oxidation of the bis(boryl) compounds to the corresponding diols occurs with retention of configuration, as is the case in closely related systems 19). The NMR spectrum of the addition product of B2CI4 with 1,3-cyclohexadiene is also consistent with cis addition (120). 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes the photocatalyzed valence isomerization of norbomadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially usefulHowever, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

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