Photochemical substitution process

The use of highly enriched CO and C 0 and ir or nmr monitoring has been a powerful combination for studying the photochemical and thermal substitution processes of metal ear-bonyls and derivatives. The site of substitution and the nature of the reactive intermediates (their geometry and flexibility) have been elucidated. ... 

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). 

Photochemical substitution reactions can proceed through high-energy products such as radical ions, the primary process being a dissociation or an ionization of the excited molecule. Such processes do not have to follow the orientation rules dictated by the charge distribution of the excited molecule, and in many instances the product distribution is still little understood. 

A major advantage is the potential to lock (and protect) written information in the photobistable material. A number of chemical gated systems involving mutual regulation of the photochromic event and, for instance, fluorescence, ion binding, or electrochemical properties have been reported.1501 Scheme 19 illustrates a chiral gated response system based on donor-acceptor substituted alkene 17.[511 The photochemical isomerization process of both the M-ds and the P-trans form was effectively blocked by the addition of trifluoroacetic acid. Protonation of the dimethyl-amine donor unit of M-rfs-17a and P-trons-17b resulted in an ineffective acceptor-acceptor (nitro and ammonium) substituted thioxanthene lower half. Since the stereoselective photoisomerization of 17 relies on the presence of both a donor and acceptor unit, photochemical switching could be restored by deprotonation by the addition of triethylamine. 

The ergosterol system mentioned earlier falls in this category. Before the system is discussed, it is worthwhile to point out that, in general, substitution of even a simple group such as an alkyl seems to change the details of the photochemical primary processes in a startling manner among the dienes and trienes. Thus, in solution, 1,3,5-hexatriene cannot be converted to 1,3-cyclohexadiene photochemically, but 2,4,6-octatriene does cyclize photochemically to (rans-5,6-dimethyl-l,3 Cyclohexadiene under the same conditions. ... 

However, for such well-known reactions 45> as positional isomerization and bimolecular substitution in simple square planar coordination compounds, very recently promissing attempts have been reported to derive selection rules on the basis of the orbital symmetry conservation principle 189> and state correlation diagrams 521> e.g. the cis-trans isomerization of square planar complexes has been predicted as a thermally forbidden and photochemically allowed process, in accordance with experiments 28,31,370) [see aiso section FI]. 

As an example, this apply to enols or tautomeric enols such as maleic acid derivatives. While with a chemical reagent (cerium ammonium nitrate) the only process occurring is oxidative dimerization, when aromatic nitriles are used as the photochemical oxidant, selective trapping of the radicals by an electrophilic alkenes or by the nitrile itself occurs. Under these conditions, both the alkylation of alkenes and the oxidative alkylation/dimerization of dienes have been smoothly obtained (see Scheme 8) and side processes such as double alkylation or polymerization often occurring with other methods have been avoided. A three-component (Nucleophile-Olefin Combination, Aromatic Substitution) process is also possible. ... 

The photochemical reactions of trans-[Ir(en)2Cl2] in aqueous solutions containing added nucleophiles were also examined by monitoring spectral changes during the reactions. The results of these observations and other similar studies are shown in Table 3. These are all photochemical reactions because they are too fast to involve thermal substitutions. Thus, for example, the reactions of trans-[lT en)2C 2] in the presence of bromide ion to form trans-[Ir(en)2Br2] must be a primary substitution process and not a photo-induced aquation with subsequent thermal replacements. 

Reactions involving thermal or photochemical substitution of coordinated carbon monoxide in metal carbonyl complexes are well known (see Chapter 3) and related processes may be induced electrochemically. A number of review articles blS have dealt with the electrochemically-induced... 

Methane, chlorine, and recycled chloromethanes are fed to a tubular reactor at a reactor temperature of 490—530°C to yield all four chlorinated methane derivatives (14). Similarly, chlorination of ethane produces ethyl chloride and higher chlorinated ethanes. The process is employed commercially to produce l,l,l-trichloroethane. l,l,l-Trichloroethane is also produced via chlorination of 1,1-dichloroethane with l,l,2-trichloroethane as a coproduct (15). Hexachlorocyclopentadiene is formed by a complex series of chlorination, cyclization, and dechlorination reactions. First, substitutive chlorination of pentanes is carried out by either photochemical or thermal methods to give a product with 6—7 atoms of chlorine per mole of pentane. The polychloropentane product mixed with excess chlorine is then passed through a porous bed of Fuller s earth or silica at 350—500°C to give hexachlorocyclopentadiene. Cyclopentadiene is another possible feedstock for the production of hexachlorocyclopentadiene. 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

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