Isomerization photocatalytic

As shown in Fig. 13, a variety of metal carbonyls upon sonication will catalyze the isomerization of 1-pentene to cis- and tram-2-pentene (186). Initial turnover rates are about 1-100 mol 1-pentene isomerized/mol of precatalyst/hour, and represent rate enhancements of 102 5 over thermal controls (174). The relative sonocatalytic and photocatalytic activities of these carbonyls are in general accord. An exception is Ru3(CO)12, which is... 

Mixtures of isomeric allylic chlorides are formed when 1,1-disubstituted allenes react with HC1.1,3-Disubstituted allenes yield products of both central and terminal carbon attacks. In contrast, selective transformations occur with HBr. The gas-phase, photocatalytic addition of HBr gives selectively vinylic bromides129 [Eq. (6.20)], while hydrobromination in solution phase yields allylic bromides130 [Eq. (6.21)] ... 

At the end of 1970 s we attempted an investigation of the action of semiconductor particles on medium sized organic compounds. In this article, we would like to describe some of the work done in our laboratory on the photocatalytic action of semiconductors on the oxidation of olefins and hydrocarbons, and on the isomerization of unsaturated systems. 

The formation of the latter two can be readily explained by a dimerization of the thiyl radicals [reaction (61)] formed in reaction (60) and a (slow) oxidation by O2 which occurs in competition (no detailed mechanistic study is available at present that accounts for the other major products). The disulfide is not photostable, but slowly isomerizes to its head-to-tail isomer [reactions (62) and (63)], and in subsequent reactions is converted into the betaine [reactions (64) and (65)] which is the photocatalytic agent that causes the oxidation of the guanine moiety (Adam et al. 1999). 

Olefin isomerization has also been mediated by the photolysis of Fe(C0)s.144 Recently, a detailed study of alkene isomerization by photolysis of Fe(CO)5 has shown that the reaction is truly photocatalytic.14S The very high quantum yields ( 1.0), Table 24, and the fact that the pentenes are ultimately equilibrated to the thermodynamic ratio support the notion that the role of the light is to generate a thermally active catalyst. A mechanism similar to that in reactions (53)-(57) involving Fe(CO)3 as the repeating unit can be used to account for the results. 

The photocatalytic isomerization reactions were markedly inhibited by the addition of O2, and the photooxidation of trans-l-C W , occurred instead of isomerization. Analysis of the slope of the Stern-Volmer plots as a function of pressure of added O2 (and trans-2-CnRs) showed the reactivity... 

Fig. 44. Relationship between the yields of the phosphorescence spectrum of the vanadium oxide catalyst anchored to Vycor glass and the yields of the photocatalytic isomerization of tTOra-2-butene on this catalyst. Phosphorescence yields measured at 280 K photocatalytic reaction temperature, 298 K. O, yields of cw-2-butene yields of 1-butene. Initial pressure of rranj-2-buteiie about 399 Pa [reproduced with permission from Anpo et at (69)].

The action spectrum of the photocatalytic isomerization reactions on MgO, defined as the plot of the reaction rate vs the wavelength of the light used, shows a good agreement with the absorption spectrum, i.e., the photoluminescence excitation spectrum of the MgO (96-98, 248). The addition of O2 or CO to MgO led to the quenching of the photoluminescence. Similarly, the rates of the photocatalytic isomerization reactions on MgO were easily inhibited by the addition of CO. its extent increasing with an increase in CO pressure. 

These results indicate that the photocatalytic isomerization reactions on MgO proceed via the same excited state for the catalysis and for the photoluminescence. Thus, the photophysical processes on MgO in the presence of a mixture of 2-C4H8 and CO are described as follows (98) ... 

Figure 60 also show s the Stcrn-Volmer plots for the quenching of the photoluminescence intensity. There is good agreement between the Stern-Volmer plots for the photoluminescence intensity and the yields of the photocatalytic isomerization reactions, indicating that both the photocatalytic reaction and the photoluminescence proceed through the same excited state of MgO, i.e., the charge-transfer excited state on the four-coordinated surface sites on MgO (96-98, 240). 

The irans-cis isomerization of 2-C4Hg proceeds catalytically on the ZnS catalyst under UV irradiation (245). The initial rate of this photocatalytic isomerization on preground ZnS was found to be relatively low, but it increased markedly when the grinding time increased. The effect of the grinding of the catalyst on the efficiency of photocatalytic isomerization was the opposite of the effect on the photoluminescence efficiency of the catalyst. 

The results obtained with vanadium oxide catalysts prepared by the impregnation method show a remarkable contrast with those obtained with anchored vanadium oxide catalysts (63,116). As shown in Fig. 64. the yields of the photocatalytic isomerization as well as the yields of the phosphorescence of the oxide increase with the content of the vanadium ions and then decrease, even when the catalyst contains 0.1 wt% V. When the vanadium content is high, an increase in the efficiency of the radiationless deactivation due to the aggregation of the vanadium oxide species is observed. 

As the photocatalytic carbon-carbon bond is formed, hydrogen evolves when the photocatalytic activation is done on colloidal ZnS [149, 150]. This dehydrodimerization also takes place with saturated ethers, with reactivity related to C H bond strength. Thus, 2,5-dihydrofuran (an allylic ether) is more easily activated than the isomeric 2,3-dihydrofuran (a vinyl ether). With the former substrate, all three dia-stereomeric coupling products are observed. Water is required for the reaction, and the primary photochemical product is thought to be a surface-bound hydroxyl radical. 

A Stem-Volmer plot for photocatalytic stilbene isomerization also has both curved and linear components [183]. This requires at least two separate mechanisms... 

Stimulated by the early work of Bard et al. on the Ti02-catalyzed photo-Kolbe reaction [36], many papers appeared in subsequent years dealing with photocatalytic organic reactions [115] in the presence of colloidal or suspended semiconductor particles. They include cis-trans isomerizations [68, 93, 116-119], valence isomer-izations [120, 121], substitution and cycloaddition reactions [73, 80, 122-125], oxidations [126, 127], and reductions [128-130]. Characteristic of all these reactions is that in almost all cases well known compounds were formed, which were not isolated but only characterized by spectroscopic methods. The nature of the products can be rationalized within the mechanistic scheme of semiconductor photocatalysis type A, which means that at least one reduced and one oxidized compound are... 

The enhanced reactivity of oxiranes due to geometric strain is manifested in their acid- and base-catalyzed reactions, in their rearrangements in the presence of metals and metal compounds, and in their thermal and photocatalytic transformations. These reactions permit the development of new and varied methods of synthesis and the preparation of derivatives that in many cases are difficult to obtain by other routes. The main products of the isomerization reaction are unsaturated alcohols and carbonyl compounds. Great progress has been made in this area of organic chemistry during the past 20 years (The reader is referred to some relevant... 

The photocatalytic isomerization is explained by intramolecular hydrogen migration. The reactive state is an n-n triplet and the products arise from a common biradical intermediate. The cis isomer does not react. Irradiation of oxiranes not containing the phenyl group gives rise to cyclobutanol epimers (Eq. 350). ... 

In 1989 Curran and co-workers reported on a photocatalytically induced free-radical cyclization leading to various cyclic, bi-, or polycyclic carbocycles (fused and spiro) via isomerization of unsaturated iodides (alkenes, alkynes) [63]. This corresponds to the nonreductive variant of the tin hydride method. Under sunlight irradiation and in the presence of 10 mol% hexabutylditin, a-iodo esters, ketones, and malonates are efficiently transformed via an iodide atom transfer chain mechanism (eq. (4)). 

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