Photochemical reaction pathway

Scheme 20.—Potential, Photochemical-Reaction Pathways for Reaction of 1-Deoxy-l-(ethysulfinyl)-D-galactitol (47).

Figure 15.7 Examples of direct photochemical reaction pathways (a) substituted chlorobenzenes, (b) trifluralin, and (c) a ketone (from Mill and Mabey, 1985).

The beginning and end points of a photochemical reaction pathway are the structures of the starting materials (substrates) and the isolated products. Elucidation of product structures can be carried out by conventional methods. Structure determination for products derived from labelled substrates, such as those with isotopic labels or with extra substituents, or from substrates with distinctive stereochemical features, can result in the elimination of certain mechanistic possibilities and provide support for others. Two key questions for photochemical mechanisms, as for thermal mechanisms, are whether or not a reactive intermediate (such as a biradical) lies on the reaction pathway, and if so, what are the rate constants for reaction steps subsequent to its formation. Questions that are peculiar to photochemical mechanisms mav be expressed ... 

Figure 2. Photochemical reaction pathways for enones Refer to Table I for list of substituents.

As a first example, the photochemical synthesis of substituted 1,2-dihydro-[60]fullerenes will be discussed. These compounds can be synthesized by various photochemical reaction pathways. In the first one the radical cation Qo is involved in the reaction. In 1995, Schuster et al. reported the formation of C6o radical cations by photosensitized electron transfer that were trapped by alcohols and hydrocarbons to yield alkoxy or alkyl substituted fullerene monoadducts as major products [211], Whereas Foote et al. used N-methylacridinium hexafluorophos-phate NMA+ as a sensitizer and biphenyl as a cosensitizer [167], Schuster et al. used 1,4-dicyanoanthracene (DCA) as a sensitizer for the generation of C 6o- The... 

One of the most common photochemical reaction pathways of carbonyl compounds is the formation of a diradicaloid excited state which is able to abstract a hydrogen atom at the y (or, more rarely, e) position, followed by either fragmentation or recombination. This process, which is known as the Norrish type II reaction, has a parallel in the photochemistry of nitro groups the intramolecular hydrogen abstraction of excited ortho-nitrotoluene is actually one of the very early synthetic photochemical transformations [9]. It has been exploited in a family of photolabile protecting groups, most prominent among which are derivatives of ortho-nitrobcnzyl alcohol, as introduced in 1966 by Barltrop et al. (Scheme 13.1) [10, 11],... 

The effect of zeolites on the photochemical reaction pathway of organic molecules has been studied recently. Turro et al. (40, 41) have shown that the photochemistry of methyl benzyl ketones (ACOB) in the presence of pentasil zeolites follows a different pathway depending on the location of adsorbed ketones. Figure 5 illustrates the photochemical reaction pathways in the gas phase and on ZSM-5. Para-ACOB is readily adsorbed by the pentasil framework, so that the radicals formed upon... 

Figure 5. Photochemical reaction pathways in the gas phase and on ZSM-5. (Reproduced from reference 1. Copyright 1991 American Chemical Society.)...

With rich luminescent properties, long-lived CT excited states and a variety of bimolecular photochemical reaction pathways, the mixed-ligand square-planar diimine dithiolene complexes of d8 metal ions show great promise for solar-energy conversion, as luminescent probes or in photocatalytic applications. Complexes of this type have also received attention for the nonlinear optical properties, such as second harmonic generation, related to the MMLL CT excited state (14, 129). 

Figure 7 Direct photochemical reaction pathway of Triclosan (5-chloro-2-(2,4-di-chlorophenoxy)phenol) in natural water (3-100 iM of Triclosan in pH-bujfered solutions), irradiated with a Hg-vapour lamp. Only the triclosan phenolate anion (present in water at pH >8) is susceptible to significant photochemical transformation and will also react through indirect photolysis (not shown) with singlet oxygen ( O2) and OH radicals to form 2,4-dichloro-phenol (2,4-DCP). The photoproduct 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) belongs to a notoriously persistent group of chemicals commonly referred to as dioxins ...

The mechanism of a photoreaction should ideally include a detailed characterization of the primary events as outlined by the classification of photochemical reaction pathways in Section 2.3 the lifetimes of the excited states that are involved in the reaction path, the quantum yields and hence the rate constants of all relevant photophysical and photochemical processes, in addition to the information about the structure and fate of any reactive intermediates, their lifetimes and reactivities. 

Scheme 12. Different photochemical reaction pathways of czs-stilbene (1) (i) to phenanthrene (2) by conrotatory [n6] electrocyclization, and (ii) to tetraphenylcyclobutane (33) by a [2+2] photocycloaddition [67a-d, 68]...

Figure 3. Photochemical reaction pathways of oC -amino acetophenones.

Photochemical reaction pathways in cyclobutene. (Adapted from reference 133.)... 

FIGURE 10 The three photochemical reaction pathways of an environmental chemical, EC. S is a sensitizer. 

Of the field and laboratory air studies that have been performed, sunlight-induced chemical oxidations and photochemical reaction pathways usually render pesticide residues less toxic, more polar, and more susceptible to being washed-out of the air mass (J1,12,13,14,15), Field and laboratory atmospheric pesticide fate studies have also reported the formation of photooxidation products that can have equal or higher toxicity and/or greater environmental persistence than the parent pesticide 16,17,18,19), Because of the limited number of atten ted atmospheric fate studies, there remains a substantial degree of uncertainty in regards to the mechanistic behavior and possible fate of many pesticide groups that can reside in the lower atmosphere. 

In the early 1990s, the derivatization of fullerenes was based on thermal reactions, e.g., the Diels-Alder reaction of Cso with substituted dienes leading to cyclohexyl-fused fullerene derivatives, and only a few examples were known, where a fullerene adduct was obtained by photochemical reactions. The pioneering work in the field of photochemical derivatization of fullerenes was done by Wilson, Schuster, and colleagues who, for example, reported the [2 + 2]-photocycloaddition of enones to CgQ. Nowadays, more and more fullerene derivatives are synthesized by photochemical reaction pathways. 

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