Photochemical sequential reactions

Chain reactions such as those described above, in which atomic species or radicals play a rate-determining part in a series of sequential reactions, are nearly always present in processes for the preparation of thin films by die decomposition of gaseous molecules. This may be achieved by thermal dissociation, by radiation decomposition (photochemical decomposition), or by electron bombardment, either by beams of elecuons or in plasmas. The molecules involved cover a wide range from simple diatomic molecules which dissociate to atoms, to organometallic species with complex dissociation patterns. The... 

Following photochemical production of the initiator moiety from the iodonium salt, sequential reaction with monomer moieties proceeds until exhaustion of available monomer. This living polymerization process should be characterized by the following expressions ... 

It is known that the photochemical reaction E26.I.I is not a straightforward sequential reaction leading to the final product (3). As the photoreaction of (I) progresses, photons are also absorbed by the previtamin (2) giving the undesirable trans isomeric E-triene,... 

The thermal benzannulation of Group 6 carbene complexes with alkynes (the Dotz reaction) is highly developed and has been used extensively in synthesis [90,91]. It is thought to proceed through a chromium vinylketene intermediate generated by sequential insertion of the alkyne followed by carbon monoxide into the chromium-carbene-carbon double bond [92]. The realization that photodriven CO insertion into Z-dienylcarbene complexes should generate the same vinylketene intermediate led to the development of a photochemical variant of the Dotz reaction (Table 14). 

This chapter begins with an introduction to the basic principles that are required to apply radical reactions in synthesis, with references to more detailed treatments. After a discussion of the effect of substituents on the rates of radical addition reactions, a new method to notate radical reactions in retrosynthetic analysis will be introduced. A summary of synthetically useful radical addition reactions will then follow. Emphasis will be placed on how the selection of an available method, either chain or non-chain, may affect the outcome of an addition reaction. The addition reactions of carbon radicals to multiple bonds and aromatic rings will be the major focus of the presentation, with a shorter section on the addition reactions of heteroatom-centered radicals. Intramolecular addition reactions, that is radical cyclizations, will be covered in the following chapter with a similar organizational pattern. This second chapter will also cover the use of sequential radical reactions. Reactions of diradicals (and related reactive intermediates) will not be discussed in either chapter. Photochemical [2 + 2] cycloadditions are covered in Volume 5, Chapter 3.1 and diyl cycloadditions are covered in Volume 5, Chapter 3.1. Related functional group transformations of radicals (that do not involve ir-bond additions) are treated in Volume 8, Chapter 4.2. 

FIGURE 6.14 Potential energy curves of Re(CO)3(4-phenylpyridine)2Cl. They account for the photophysical processes observed when the excited state is produced by the absorption of one photon (left) and the photochemical reaction induced when the excited state is produced by the sequential absorption of two photons (right). 

The photosynthetic process involves photochemical reactions followed by sequential dark chemical transformations (Fig. 3). The photochemical processes occur in two photoactive sites, photosystem I and photosystem II (PS-I and PS-II, respectively), where chlorophyll a and chlorophyll b act as light-active compounds [6, 8]. Photoinduced excitation of photosystem I results in an electron transfer (ET) process to ferredoxin, acting as primary electron acceptor. This ET process converts light energy to chemical potential stored in the reduced ferredoxin and oxidized chlorophyll. Photoexcitation of PS-II results in a similar ET process where plastoquinone acts as electron acceptor. The reduced photoproduct generated in PS-II transfers the electron across a chain of acceptors to the oxidized chlorophyll of PS-I and, consequently, the light harnessing component of PS-I is recycled. Reduced ferredoxin formed in PS-I induces a series of ET processes,... 

Another advantage of IET (and MET) is the matrix formulation of the theory making it applicable to reactions of almost arbitrary complexity. A subject of special attention here will be photochemical reactions composed from sequential geminate and bimolecular stages and accompanied by spin conversion, thermal decay, and light saturation of the excited reactants. The quantum yields of fluorescence as well as the yields of charged and excited... 

While photochemical reactions in solids are somewhat counterintuitive and relatively rare in organic synthesis, recent examples have suggested that reactions in crystals maybe as reliable and efficient as their solution counterparts [57]. As it pertains to the photoelimination of small molecules and C—C bond-forming reactions, it is essential to recognize that product formation relies on the sequential cleavage of two sigma... 

The photoreduction of benzophenone (B) in solution is one of the most extensively investigated photochemical reactions, having been studied using a wide range of reductants, solvents, reactant concentrations, and irradiation conditions. In the presence of H-donating substrates, the triplet excited state of B is able to abstract an H-atom directly (Equation 13.17) or via a sequential electron/H transfer mechanism (Equation 13.18) to produce triplet radical pairs, BHR. 

The photocyclisation of diarylethylenes in which a positively charged nitrogen atom is present yields fused polycyclic azonia arenes further examples of such reactions have appeared. Thus compound (212) upon ultra-violet light irradiation yields (213), presumably by way of sequential photochemical reactions in which (214) is an intermediate. If (213) is phenyl substituted then (215) is obtained also. The initial cyclisation of (212) is regio-selective and none of (216) is formed. Analogous reactions proceed for the systems (217), which yields (218), (219), which yields (220), " and (221), which yields (222). A closely related reaction is reported for (223) which gives (224). ... 

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