Photochemical addition, radical chain

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. 

Unactivated aryl halides also undergo nucleophilic displacement via electron transfer in the initial step the so-called SRN1 mechanism. It is now clear that in the case of heteroaromatic compounds, nucleophilic substitution by the Srn process often competes with the addition-elimination pathway. The SRN reactions are radical chain processes, and are usually photochemically promoted. For example, ketone (895) is formed by the SRN1 pathway from 2-chloroquinoxaline (894) (82JOC1036). 

Simple homolysis of the C-I bond by heating or by light is the most straightforward approach and was the first used for adding perfluoroalkyl iodides to olefins. One presumes that both the thermal and the photochemically induced addition reactions of perfluoroalkyl radicals proceed via free radical chain reactions as depicted in the Scheme below. However, the conditions of these reactions are rarely ideal for preparative purposes because high temperatures are required for the thermolytic process and long photolysis times are required for the photolytic method [60]. 

The polymer-supported distannane 90 was used as a source of stannyl radicals in several radical cyclization reactions, such as the photochemical radical chain addition of f-butyl iodide to acetylenes yielding the Z/E mixture of alkenes or the photochemical cyclization of citronellyl bromide to give menthane in high yields164 (Scheme 44). 

Not all photochemical additions take place at the carbonyl carbon. Thus photolysis of the ketone 8.215 in isopropanol in the absence of any acid gives the product of attack in the conjugate position 8.218,1185 and this is quite a common and synthetically useful pattern of behaviour.1186 It is consistent with frontier orbital control only if the reaction is not a radical coupling 8.216 + 8.217, but attack of the radical 8.217 on the ground-state a,/3-unsaturated ketone 8.215. This would be a photochemically initiated chain reaction, and should have a high quantum yield the reaction is described as being rapid . 

The mechanism of the secondary radical chain reactions (and their quenching by oxygen) that lead to the formation of the final products is presumably similar to that discussed by other authors in dealing with oxidative addition reactions [3, 128, 130, 133, 135], After the primary photochemical process discussed above, which can be summarized by Eq. (31), the pentacoordinated Pt(III) complex so obtained may undergo a... 

Yoshida reported the oxygenative conjugate addition of perfluoroalkyl radicals to styrene derivatives (Scheme 4) [1 Ij. The reaction is photochemically initiated and used hexabutyldistannane as radical chain mediator and as reducing agent for the intermediate peroxide. 

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115... 

In the absence of catalysts and in the dark, pure benzene does not react with bromine, as is the case also with chlorine. Photochemical addition of bromine, like that of chlorine, is a radical chain reaction.130 131 Bromine has a more powerful substituting action than chlorine, and its rate of addition is slower.132 At low temperatures light and addition of peroxides favor addition of bromine. To date, two isomeric hexabromocyclohexanes have been isolated, by very slow addition of bromine to benzene irradiated at 0° 1% sodium hydroxide solution must be placed under the benzene and frequently renewed even so, yields are poor.126b 133... 

A new radical allylation procedure has been applied to the synthesis of a 3 -C-allyl-2, 3 -dideoxypyrimidine nucleoside, and lactone 121, made by photochemical addition of isopropanol to the corresponding 2,3-ene, has been converted to the nucleoside 122. Branched-chain sulfonates 123 have been made from 3 -ketonucleosides, and the isobutyl group was found to be superior to other possibilities, it being cleavable by iodide ion to give the sulfonic acid, an analogue of a 3 -phosphate. An ingenious route to related phosphonates is outlined in... 

Generation of Acyl Radicals. The generation of acyl radicals from simple thiol esters, either by photochemical methods or in conjunction with silanes and stannanes, is complicated by low quantum yields and lack of reactivity. This problem was circumvented by the inclusion of an additional propagation step, an intramolecular homolytic substitution (Sh2) of an aryl radical on the sulfur atom of a thiol ester carbonyl group (eq 2), in a radical chain sequence. 

Reactions with Alkenes, Alkynes, and Allenes. Pentafluorosulfanyl chloride has been used in a free radical chain addition process to alkenes and alkynes. These free radical chain addition reactions have been accomplished thermally (eq 4) in an autoclave or via a room-temperature gas-phase or low-temperature solution-phase photochemical process (eq 5). A... 

The direct addition of electrophilic radicals such as fluoroalkyl radicals, without previous electron transfer, leads to substitution in positions 4 and 2 [55]. Recently, such a reaction was carried out using the photochemical electron transfer from an electronically excited naphthylamine derivative similar to that one shown in Scheme 29.10 to induce a radical chain reaction [56]. 

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