Photochemically activated catalysts

An indirect photochemical effect on a metathesis reaction was first noticed by Gunther (1970) who found that the activity of WCVj-Bu3A1 as catalyst for the ROMP of cyclopentene could be enhanced if the WClg was first irradiated in C2CI4 solution at 100°C or in toluene solution at about 40 C. Since then it has been found 

Another case of a dramatic aging effect is in the photoassisted ROMP of cyclopentene by WCl /Ziv. Irradiation of a freshly made solution gives all-c/s 

In the metathesis of pent-2-ene catalyzed by W(CO)6/TiCl4/Av the initial rate shows a marked dependence on the pre-irradiation time (/,) (Fig. 2.5). For longer /, the conversion steadily increases reaching equilibrium conversion (50%) at /, = 60 min. For /, 60 min the conversion-time curve remains the same as for /, = 60 min (Vanwijnsberghe 1987). This behaviour should be contrasted with that for the W(CO)6/CCl4// v system (Fig. 2.2). 

The role of the halide (AX ) in the system W(CO)6/AX //iv has been extensively investigated the most active systems are those containing strong Lewis acids, e.g. ZrCU (Szymanska-Buzar 1987), TiCU (Nagasawa 1978 Szymanska-Buzar 1987), BF3 and AICI3 (Borowczak 1984). 

Pent-l-ene metathesis is observed with W(CO)6/ZrCl4//iv and W(CO)6/TiCl4/Av, but is accompanied by double-bond isomerization and cross-metathesis of pent-2-ene with pent-l-ene. In the series of [WL(CO)5]-type complexes (L = CI, CO, py, PPh3), the most active for pent-2-ene metathesis is (Et4N)[WCl(CO)5] (Szymanska-Buzar 1987). 

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Reactor 19 [R 19] Photochemical Active Catalyst Parallel Screening Reactor... 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

The coordination chemistry of the trichalcogenophosphonates is very undeveloped when compared to the analogous metal organophosphonates (RP032), which have been extensively studied owing to their potential and practical applications as ion exchangers, sorbents, sensors, proton conductors, nonlinear optical materials, photochemically active materials, catalysts and hosts for the intercalation of a broad spectrum of guests.145... 

This hypothesis is supported by Chauvin s report (33) on a catalyst derived from (CO)5W=C(OEt)C4H9. This highly stable carbene-W(O) compound does not display catalytic activity for cyclopentene monomer. When mixed in the dark with TiCl4, a slow evolution of 1 equivalent of CO occurs. Subsequent thermal or photochemical activation produces ah extremely efficient catalyst system. Chauvin demonstrated that a high conversion to polypentenamer is obtainable at a W/cyclopentene ratio of 10 li at 5°C. The role of TiCI4 is not well understood nevertheless, it promotes carbonyl displacement which appears to be essential. 

Catalysts. Titania is an active catalyst for different reactions, inorganic and organic, thermal and photochemical. It may be self-supported, or it may be supported on... 

Activation of nucleophiles by coordination, best exemplified by various complexes used as catalysts for hydrogenation, and coordination assistance to photochemical activation, may be similarly demonstrated. 

The range of alkenes that may be used as substrates in these reactions is vast Suitable catalysts may be chosen to permit use of ordinary alkenes, electron deficient alkenes such as a,(3-unsaturated carbonyl compounds, and very electron rich alkenes such as enol ethers. These reactions are generally stereospecific, and they often exhibit syn stereoselectivity, as was also mentioned for the photochemical reactions earlier. Several optically active catalysts and several types of chiral auxiliaries contained in either the al-kene substrates or the diazo compounds have been studied in asymmetric cyclopropanation reactions, but diazocarbonyl compounds, rather than simple diazoalkanes, have been used in most of these studies. When more than one possible site of cyclopropanation exists, reactions of less highly substituted alkenes are often seen, whereas the photochemical reactions often occur predominantly at more highly substituted double bonds. However, the regioselectivity of the metal-catalyzed reactions can be very dependent upon the particular catalyst chosen for the reaction. 

With the semiconductor oxidation catalyst, however, the surface becomes activated only upon photoexcitation. At low light intensities, the possibility that many holes are formed in the valence band is remote, so that the irradiated semiconductor powder becomes an effective one-electron oxidant. Now if the same chemistry ensues on the photochemically activated TiC>2 surface, then the reaction will proceed as in the bottom route of eqn 9. Thus, the carboxy radical is formed, producing an alkyl radical after loss of carbon dioxide. Since the semiconductor cannot continue the oxidation after the first step, the radical persists, eventually recapturing the conduction band electron, either directly or through the intervention of an intermediate relay, perhaps superoxide. The resulting anion would be rapidly protonated to product. 

An improvement in the efficiency of photochemical splitting of water was the incentive for Morris [49]. A parallel optical screening method was developed to select photocatalytically active catalysts by their adsorption spectra (UV and visible light). 

Photoinduced catalysis means the photogeneration of a catalyst that subsequently promotes a catalyzed reaction. Photons are required to generate the catalyst only. Thus, the efficiency of such processes depends only on the activity of the catalyst produced photochemically and, in homogeneous photocatalysis, the turnover number (TON) is the useful tool. The TON is usually expressed as the number of moles of product formed per mole of catalyst and, for photoassisted catalysis, TON <1, whereas for photogenerated catalysis TON >1 and even 1 [135], Therefore, high turnovers of photochemically produced catalysts are one of the main criteria concerning efficient photocatalytic processes. Quantum yields (ratio of moles of product formed to the number of photons absorbed) >1 may occur. The same is true for photoinduced chain reactions. 

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. 

Photochemical Fe(CO)5-induced rearrangement of silylated allyl amine 9 gave N-silylated enamine 1015, which on subsequent Cu-catalyzed cyclopropanation by methyl diazoacetate afforded cyclopropane derivative 11. The use of an optically active catalyst gave an asymmetric induction of 56% ee for the cis isomer and 20% ee for the trans isomer. Further acid-induced ring cleavage afforded the -formyl ester 12, whereas reduction and desilylation produced aminocyclopropane carboxylic acid 13 (equation 2). 

Catalysts without preformed alkylidene moieties may also be used. For example, the compounds (arene)MCl2(L) (M = Ru, Os), where L = sterically demanding trialkylphosphine, are photochemically activated and polymerize norbomene.153... 

Another green alternative is the use of photocat-alytic processes, which involve photochemical activation of semiconductors working as catalysts for redox processes that efficiently transform organic or inorganic pollutants into less dangerous substances. 

Initial reports on the borylation of alkanes using isolated transition-metal-boryl complexes date back to 1995, when Hartwig showed that Cp Re(CO)2(Bpin)2 converts pentane to 1-borylpentane with high regioselectivity. " The catalytic C-H borylation of alkanes with Cp Re(CO)3 using photochemical activation was demonstrated soon thereafter (equation 25). Also, an efficient thermal process that involves the use of rhodium catalysts has since been developed (equation 26). It is interesting to note that this methodology is not restricted to small molecules, but has recently been exploited for the direct side-chain functionalization of polyolefins. ... 

Titania is of great interest as a catalyst or photocatalyst. Matsudo and Kato have reviewed the catalytic (thermal) chemistry of TI02 (2). For photochemical activity, titania has been used to hydrogenate alk-ynes and alkenes (3), to oxidize H2O2 (4), to oxidize ethylene (5), to oxidize 2-propanol (6), for amine production (7) and in water splitting reactions (8). 

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