Photo irradiation

Semiconductor-electrolyte interface, photo generation and loss mechanism, 458 Semiconductor-oxide junctions, 472 Semiconductor-solution interface, and the space charge region, 484 Sensitivity, of electrodes, under photo irradiation, 491 Silicon, n-type... 

The cycloaddition of photoenol of o-methylbenzaldehyde 66 with 5-alkyli-dene-l,3-dioxane-4,6-dione derivatives 67 is an example of a photo-induced Diels Alder reaction in which one component, the diene in this case, is generated by irradiation [48]. The yields of some cycloadducts 68, generated by photo-irradiation of a benzene solution of 66 and 67 at room temperature, are reported in Table 4.14. The first step of the reaction is the formation of (E)-enol 69 and (Z)-enol 70 (Equation 4.7) by an intramolecular hydrogen abstraction of 66 followed by a stereo- and regioselective cycloaddition with... 

Hexatrienes undergo disrotatory ring closure by thermal activation to afford cyclohex-adienes in agreement with the Woodward-Hoffmann rule (delocalization band in Scheme 8) [41 3]. Photo-irradiation of hexatrienes is known to give bicylic products in a stereospecific [4n +2nJ manner (delocalization band in Scheme 8) [40] in contrast to this rule. 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

More recently in 2001, Winkler and Kwak reported methodology designed to access the pyrrolidine core of the hetisine alkaloids via a photochemical [2+2], retro-Mannich, Mannich sequence (Scheme 1.3) [26]. In a representative example of the methodology, vinylogous amide 42 was photo-irradiated to give the [2+2] cycloaddition product 43. Heating cyclobutane 43 in ethanol provided enamine 44 via a retro-Mannich reaction. Exposure of enamine 44 to acidic conditions then effected a Mannich reaction, resulting in pyrrolidine 45. 

The [6 + 3]-cycloaddition between (cycloheptatriene)chromium(O) tricarbonyl 135 and isocyanate or ketene occurs under photo-irradiation conditions, for example, bicyclo[4.2.1]nonane-type adduct 136 was obtained in moderate yield (Equation (22)).237 238... 

Tungsten(O) pentacarbonyl-methylene chloride complex and alkynes form vinylidene complexes by photo-irradiation, which react with an imine or a dialkylcarbodimide to afford /3-lactams after decomplexation of the metal.233... 

The thioselenation is achieved also by a radical mechanism under photo-irradiation. The adduct has the regio-chemistry opposite to that observed in the rhodium-catalyzed reaction (Scheme 39).194 196... 

Even the three-component coupling reaction of diphenyl diselenide, ethyl propiolate, and unsaturated compounds occurs under photo-irradiation (Table 5). Alkenes,198,199 1,3-dienes,198,199 vinylcyclopropanes,200 and isocyanides199,201 serve as the unsaturated coupling partner to afford the corresponding coupling products with high regioselectivity. 

Under photo-irradiation, (PhS)2 reacts with 1,3-dienes (Equation (76))207 208 and vinylcyclopropanes (Equation (77))197 to give the 1,4- and 1,5-adducts, respectively. 

Butylallene, which fails to react with (PhSe)2 under palladium catalysis, undergoes the diselenide addition upon photo-irradiation (Equation (80)).215 Using the (PhS)2/(PhSe)2 binary system, introduction of two different chalcogen elements into allenic C=C bond is also viable (Equation (81)). 

Toshima et al. obtained colloidal dispersions of platinum by hydrogen- and photo-reduction of chloroplatinic acid in an aqueous solution in the presence of various types of surfactants such as dodecyltrimethylammonium (DTAC) and sodium dodecylsulfate (SDS) [60]. The nanoparticles produced by hydrogen reduction are bigger and more widely distributed in size than those resulting from the photo-irradiation method. Hydrogenation of vinylacetate was chosen as a catalytic reaction to test the activity of these surfactant-stabilized colloids. The reaction was performed in water under atmospheric pressure of hydrogen at 30 °C. The photo-reduced colloidal platinum catalysts proved to be best in terms of activity, a fact explained by their higher surface area as a consequence of their smaller size. 

Figure 3. Change in light scattering intensity (Rjo ) upon photo-irradiation of the micelle of 2a. [2] = M. AABS at 450 nm...

Photo-irradiation of the tryptamine (166) produces an intermediate diradical cation that leads to the formation of an azonino[c f]indole. This is the first example of a vinylogous Witkop cyclization. 

XRD patterns of Pt/FSM-16 [25] (and HMM-1 [32]) show no significant change at 26 = 1-10° before and after the incorporation of metal nanowires and nanoparticles (Figure 15.7). This indicates that the pore structures and mesoporous channels of FSM-16 (and HMM-1) remained unchanged in the synthesis of the Pt wires and Pt particles [18-20, 23, 24] by wet photo-irradiation with methanol -i- water vapor and dry H2 reduction, respectively. Furthermore, in the high 26 region, typical peaks assigned to Pt fee crystalline are observed for both samples of Pt nanowire/FSM-16 and Pt nanoparticles/FSM-16 [25]. 

Dark Reactions If a homogeneous catalytic system is studied with the total exclusion of (1) stray light, (2) intense microwave irradiation or photo irradiation, and (3) spectrometer photon sources with hv > NIR radiation, then the reactive system will be considered to be thermally activated. Only dark reactions occur. 

As described in this chapter, vinylidene complexes of Group 6 metals have been utilized for the preparation of various synthetically useful compounds through electrophilic activation or electrocyclization of terminal alkyne derivatives. These intermediates are quite easily generated from terminal alkynes and M(CO)6, mostly by photo-irradiation and will have abundant possibilities for the catalytic activation of terminal alkynes. Furthermore, it should be emphasized that one of the most notable characteristic features of the vinylidene complexes of Group 6 metals is their dynamic equilibrium with the it-alkyne complex. Control of such an equilibrium would bring about new possibilities for unique metal catalysis in synthetic reactions. 

The switching or memory phenomena induced by electric field application or photo irradiation have been studied on Mott insulators, charge ordered insulators, and N-I transition systems and were found to be fast phase transitions in general. For the former two systems, the phase transitions caused a pronounced change in reflectance and conductivity from insulating to metallic features. The third system also exhibited a change in conductivity and dielectric response connected with the transports of solitons and/or domain walls, dynamic dimerization, and... 

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