Photochemically Generated Reactive Intermediates

Model heme systems The mechanisms of heme and hemoprotein reactions with small molecules such as O2, CO and NO has attracted considerable experimental attention owing to the importance of such processes in biological systems. Flash photolysis studies [87] have indicated that the photolabilization of L from simple heme complexes and kinetics of the resulting back reaction (Eq. 6.40) can be modeled by the intermediacy of solvent caged contact pair . Equation (6.41) illustrates this mechanism for the thermal back reaction for the photochemically generated intermediates for a ferrous porphyrin (Por)Fe L (For = porphyrin) 

6 Pressure Effects on the Photoreactions of Transition Metal Complexes 

According to this model, the rate for formation of (Por)Fe -L from solvent separated (Por)Fe and L will have the form 

A model similar to Eq. (6.41) can be applied to the reactions of the diatomic ligands CO and NO with (TPPS)Fe generated by flash photolysis of the respective complexes in aqueous solution (H2TPPS = tetra(4-sulfonatophenyl)porphine). For L = CO, the kon value at 25 °C and ambient pressure was measured to be 3.6 x 10  

The situation is considerably different for the iron(III) analog (TPPS)Fe (H20)2 which is hexacoordinate in aqueous solution. In this case the reaction with NO is considerably slower (kon = 4.5 x 10 s ) and kojr (500 s ) is sufficiently large to be measurable by the flash photolysis method as well [92]. Temperature and hydrostatic pressure effects were probed and AH, AS and AV values of 69 + 3 kj mol , 95 10 J mol K and +9+1 cm mol were determined for the orf reaction and 76 + 6 kJ mol , 60 + 11 ) mol and 18 + 2 cm mol respectively, for the off reaction [93j. The activation parameters for the on reaction compare very favorably with those measured for exchange between coordinated and solvent water for aqueous solutions of (TPPS)Fe (H20)2 [94] and indicate that kinetics for the reaction of NO with this complex are dominated by the lability of the coordinated water. Furthermore, the large and positive values of AVj and ASj point to a dissociative substitution mechanism as described in Eq. (6.43). 

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The second photochemical strategy for the study of reactive intermediates is flash photolysis (see Section 7.6.2). A very brief pulse of light generates the reactive intermediate under more conventional conditions, typically fluid media at ambient temperatures. Then a fast spectroscopic technique is used to directly monitor the ensuing reactions of the photochemically generated reactive intermediate. As laser pulses have gotten progressively shorter in duration, ever faster processes have been probed. 

Photochemical reductive elimination of H2 from dihydrides serves as a route to generating reactive intermediates. Photolysis of Ru(PPh3)3(CO)H2 in benzene yields Ru(PPh3)3CO. Ultrafast measurements found that hydride loss and Ru(PPh3)3CO formation is complete in 6 ps. Photolysis ofboth Ru(CO)2(PPh2)2H2 and Ru(CO)3(PPh3)2 in gas matrices resulted in the formation of the same Ru(CO)2(PPh3)2 intermediate. ... 

In 2011, Popik and co-workers reported photochemical dehydration of 3-hydroxy-2-naphthalenemethanol (o-naphthoquinone precursor, NQMP, 79) derivatives to o-naphthoquinone methides (oNQMs, 80) [78, 79]. The in situ generated reactive intermediate oNQM underwent facile cycloaddition with vinyl ethers (81) to form photostable benzochromans (82) (Scheme 12). NQMP (79) has two major absorption bands at 275 nm (log s = 4.06) and 324 nm (log s — 3.70) thus it can be photoactivated with either low pressure mercury lamp (254 nm) or fluorescent tubes (300 and 350 nm). The quantum yield of the photoactivation was very high (< 300 = 0.17 0.02 for 79). The in situ generated oNQMs was quenched mostly by... 

Photochemical elimination of carbon dioxide from suitable precursors has given a variety of reactive intermediates at low temperatures where they are often stable and can be studied further. This approach has been utilized in attempts to generate new 1,3-dipolar species, and photolysis of (515) gave an azomethine nitrene intermediate (516) (see Section 4.03.6)... 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

In this article we have summarized the use of both photochemical and more classical thermal kinetics techniques to deduce the nature of intermediates in the ambient temperature, fluid solution chemistry of several triruthenium clusters. In some cases the photochemically generated intermediates appear to be the same as those proposed to be formed along thermal reaction coordinates, while in other cases unique pathways are the results of electronic excitation. The use of pulse photolysis methodology allows direct observation, and the measurement of the reaction dynamics of such transients and provides quantitative evaluation of the absolute reactivities of these species. In some cases, detailed complementary information regarding... 

The photochemical cleavage of Si-Si bonds of cyclotetrasilanes has been reported to generate several reactive intermediates. For example, Nagai and co-workers reported that silylene and cyclotrisilane are generated during the photolysis of a cyclotetrasilane with a folded structure.73 Shizuka, Nagai, West, and co-workers reported that the photolysis of planar cyclotetrasilanes gives two molecules of disilene.74... 

Photochemically-generated radicals are encountered as reactive intermediates in many important systems, being a major driving force in the photochemistry of ozone in the upper atmosphere (stratosphere) and the polluted lower atmosphere (troposphere). The photochemistry of organic carbonyl compounds is dominated by radical chemistry (Chapter 9). Photoinitiators are used to form radicals used as intermediates in the chain growth and cross-linking of polymers involved in the production of electronic circuitry and in dental treatment. 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

Oxasiletene, the unsaturated analogue of oxasiletane, was first postulated by Seyferth and coworkers as a reactive intermediate in the reaction of l,l-dimethyl-2,3-bis(trimethylsilyl)-l-silirene with dimethyl sulfoxide25. The photochemical generation of siladienone intermediate 28 from (pentamethyldisilanyl)diazomethyl 1-adamantyl ketone... 

Photochemical reactions of transition metal complexes that contain unsaturated chelates fall into three categories 1) fragmentation of the ligand to yield two reactive functionalities 2) elimination of the ligand to generate two reactive sites at the metal 3) chelate localized excited states can function as photoreceptors to promote photodissociation of other metal-ligand bonds in the complex. These processes can be used as an entry to new reactive intermediates and catalysts. 

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