Photochemical processes, reaction pathways

In the following two reactions, denoted as (A) and (B), cyclodecapentaene cyclizes into trans- and c/s-9,10-dihydronaphthalene. Apply Woodward-Hofimann rules to determine the reaction condition (thermal or photochemical) and reaction pathway (conrotatory or disrotatory) of each process. 

As will be discussed more fully below, some reaction pathways are more readily available via photochemical processes, leading to products that would be difficult to make by other routes. 

In our laboratory we have utilized multiphoton infrared laser activation of metal ion-hydrocarbon adducts to probe the lowest energy pathways of complex reaction systems (6). Freiser and co-workers have utilized dispersed visible and uv radiation from conventional light sources to examine photochemical processes involving organometallic fragments... 

Both the frontier-orbital and the Mobius-Hlickel methods can also be applied to the cyclohexadiene—1,3,5-triene reaction in either case the predicted result is that for the thermal process, only the disrotatory pathway is allowed, and for the photochemical process, only the conrotatory. For example, for a 1,3,5-triene, the symmetry of the HOMO is... 

In Chapter 15 we address the consequences of the direct interaction of organic compounds with sunlight. This also forces us to evaluate the light regime in natural systems, in particular, in surface waters. Chapter 16 then deals with reactions of organic chemicals with photochemically produced reactive species (photooxidants) in surface waters and in the atmosphere. Note that in Chapters 15 and 16, the focus is on quantification of these processes rather than on a discussion of reaction pathways. 

Two lines of inquiry will be important in future work in photochemistry. First, both the traditional and the new methods for studying photochemical processes will continue to be used to obtain information about the subtle ways in which the character of the excited state and the molecular dynamics defines the course of a reaction. Second, there will be extension and elaboration of recent work that has provided a first stage in the development of methods to control, at the level of the molecular dynamics, the ratio of products formed in a branching chemical reaction. These control methods are based on exploitation of quantum interference effects. One scheme achieves control over the ratio of products by manipulating the phase difference between two excitation pathways between the same initial and final states. Another scheme achieves control over the ratio of products by manipulating the time interval between two pulses that connect various states of the molecule. These schemes are special cases of a general methodology that determines the pulse duration and spectral content that maximizes the yield of a desired product. Experimental verifications of the first two schemes mentioned have been reported. Consequently, it is appropriate to state that control of quantum many-body dynamics is both in principle possible and is... 

Photochemical substitution reactions can however follow other pathways than the concerted one which is the rule in the ground state processes. The orientation effects of electron donor and electron acceptor substituents are based on the model of a transition state of a complex which implies a concerted reaction (Figure 4.65). 

Cis-trans isomerization can take place either photochemically or in the dark, but the reaction pathways are quite different. In the light-induced process the reaction goes through a tetrahedral intermediate formed from the triplet excited state, whereas the dark reaction involves a dissociation of the complex, followed by recombination. In the latter case the presence of free glycine is demonstrated by the use of radioactive tracers no free glycine appears in the photochemical reaction. 

In this case, only a thermal pathway for the formation of the cyclooctatetraene is proposed, in agreement with many other publications (for references, see Sec. II and Table 3). Tinnemans and Neckers [62], however, describe the ring opening of the ortho adduct from methyl phenylpropiolate and benzene as well as the reverse reaction as photochemical processes. The formation of this ortho adduct can also be accomplished in a xanthone-sensitized photoreaction [63], and in that case, the authors consider the ring opening as a thermal or a triplet-sensitized reaction and the reverse reaction as one proceeding via the singlet. 

Solid-state photoreaction in the chiral crystal provides not only a useful synthetic method of optically active materials but also mechanistic information on the photochemical process. Two examples of solid-state photoreactions will be described, in which the correlation on the absolute configurations between those before and after the reaction revealed the reaction pathways. 

The (indirect) photodegradation of chemicals (i.e., reactions with reactive species formed by photochemical processes) has been recognized as the major transformation pathway for chemicals in the troposphere. The electrophilic addition of tropospheric radicals constitutes the principal... 

The presence of a good electrofugal group (L + ), such as H+ or Me3Si +, makes elimination the predominant path (Scheme 10.14, pathways a and b). When R is an aromatic ring, pathway (a) is favorable and a benzylic proton is lost to form styrenes this results in a photochemical process analogous to the Heck reaction (Scheme 10.15) [12],... 

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],... 

As in the other 3 sections, catalytic processes controlled by specific reaction pathways have several similar requirements. In many of the above systems the degree of electron transfer between the metal and support was proposed to be important. Various surface structures were also believed to play an important role in enhancement of selectivity. In this particular section it is clear that the type of input energy such as photochemical, thermal or other energy forms may markedly influence product distributions of catalytic reactions. 

There are several reasons for the interest in controlled photocycloadditions. First, a cycloaddition (the 2+2, for example) allows access to the four-member ring system. Second, investigation of the regio- and stereochemical outcome of the cyclization process allows for a better understanding of the mechanistic pathway the reaction takes. The reaction is studied not only for synthetic exploitation, but for basic understanding of the photochemical process. 

Three types of cycloaddition products are generally obtained from the photochemical reaction between aromatic compounds and alkenes (Scheme 31). While [2 + 2] (ortho) and [3 + 2] (meta) cycloaddition are frequently described, the [4 + 2] (para or photo-Diels-Alder reaction) pathway is rarely observed [81-83]. Starting from rather simple compounds, polycyclic products of high functionality are obtained in one step. With dissymmetric alkenes, several asymmetric carbons are created during the cycloaddition process. Since many of the resulting products are interesting intermediates for organic syntheses, it is particularly attractive to perform these reactions in a diastereoselective way. 

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