The Pathway of Photochemical Reactions

Since all photochemical reactions require the absorption of a photon, the result is that the reactant molecule is raised to a higher energy level. The outcome of this process depends on the nature of the upper and lower electronic states of the molecule. Four types of absorption behaviour are possible and we will first illustrate these by referring to Morse curves for the simple, diatomic, molecules. Although the potential energy of a complex molecule as a function of its molecular geometry is not a simple two-dimensional curve but a complex multidimensional surface, the conclusions arrived at by the use of Morse curves are instructive. 

Another way of visualising the potential energy of a molecular system is in terms of the potential-energy changes that occur during a chemical reaction. This is represented by means of a reaction profile where the potential energy values of the reactants, products, transition states and intermediates are plotted against the reaction coordinate (the lowest 

Adapted from N. J. Turro, Modern Molecular Photochemistry, (1991). 1991 University Science Books 

Absorption of a photon by an organic molecule, R, leads to formation of an electronically-excited state, R  

The primary photochemical process is concerned with the subsequent reactions of these electronically-excited states, which tend to react in one of two ways  

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Both target compounds discussed in this review, kelsoene (1) and preussin (2), provide a fascinating playground for synthetic organic chemists. The construction of the cyclobutane in kelsoene limits the number of methods and invites the application of photochemical reactions as key steps. Indeed, three out of five completed syntheses are based on an intermolecular enone [2+2]-photocycloaddition and one—our own—is based on an intramolecular Cu-catalyzed [2+2]-photocycloaddition. A unique approach is based on a homo-Favorskii rearrangement as the key step. Contrary to that, the pyrrolidine core of preussin offers a plentitude of synthetic alternatives which is reflected by the large number of syntheses completed to date. The photochemical pathway to preussin has remained unique as it is the only route which does not retrosynthetically disconnect the five-membered heterocycle. The photochemical key step is employed for a stereo- and regioselective carbo-hydroxylation of a dihydropyrrole precursor. 

The photolytic degradations of drugs are complex, and include reactions such as oxidations, reductions, cis-trans isomerizations, structural rearrangements, hydrolyses, dechlorinations, etc. Oxidation is one of the major pathways of photochemical reactions. Table 1 lists the pathways of photochemical decomposition of some therapeutic and nontherapeutic agents. 

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. 

Early on, it was demonstrated that aromatic nitro compounds may form radical anions in alkaline solutions [4], with the possibilities of photochemical reactions [5]. There followed the development of the radical chain mechanism [6]. An interesting early danonstration of reaction by this mechanism was in the reaction of ort/io-halogenoanisoles with potassium amide in liquid ammonia [7]. Reaction by the benzyne mechanism gives predominantly the uiera-substituted product due to the electronic influence of the methoxy group. Howeva, with an access of potassium metal, which promotes electron transfer, the pathway predominates yielding ort/io-anisidine, as shown in Scheme 6.4. The mechanism now forms an important synthetic pathway, and this and other homolytic processes are covered in Chapters 9 and 10. 

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... 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

Cobalt(II) hexacyanoferrate, formally similar to Prussian blue, exhibits a far more complex electrochemistry. Only recently, Lezna etal. [65] succeeded in elucidating this system by a combination of in situ infrared spectroscopy and electrochemistry, and ex situ X-ray photoelectron spectroscopy. Figure 8 shows the pathways of the three different phases involved in the electrochemistry, and their interconversion by electrochemical redox reactions and photochemical reactions. 

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... 

These theories assert that the pathway of a chemical reaction accessible to a compound is controlled by its highest occupied molecular orbital (HOMO). For the thermal reaction of butadiene, which is commonly called ground-state chemistry, the HOMO is 2 and lowest unoccupied molecular orbital (LUMO) is photochemical reaction of butadiene, which is known to be excited-state chemistry, the HOMO is 1//3 (Fig. 3.5.6). 

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],... 

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