Selectivity in radical chain reactions

Cases in which allyl radicals display sufficient reactivity to participate successfully in radical chain reactions include the addition of bromotrichloromethane to butadiene the reaction of cyclopentadiene with tosyl cyanide, the addition of thiols , stannanes " and hydrogen halides . All these reactions follow the simple two-step radical chain mechanism depicted in Scheme 1, and the low reactivity of the intermediate allyl radicals can be compensated by using the trapping agent in excess or even as the solvent. In chain reactions with three or more chain-carrying radicals, this compensation is not possible anymore, because the concentration of the reaction partners has to be chosen such that the selectivity requirements for all intermediate radicals are satisfied. Complex radical chain reactions with polyenes as one of the reactants are therefore not known. 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

The ultraviolet lamps used in the photochlorination process serve to dissociate the chlorine into free radicals and start the radical-chain reaction. Other radical sources, such as 2,2 -a2obisisobutyronitrile, have been used (63,64). Primary by-products of the photochlorination process include 1,1,2-trichloroethane (15—20%), tetrachloroethanes, and pentachloroethane. Selectivity to 1,1,1-trichloroethane is higher in vapor-phase chlorination. Various additives, most containing iodine or an aromatic ring in the molecule, have been used to increase the selectivity of the reaction to... 

The present method offers several advantages over earlier methods. The use of carbon tetrachloride instead of diethyl ether as solvent avoids the intrusion of certain radical-chain reactions with solvent which are observed with bromine and to a lesser degree with chlorine. In addition, the potassium bromide has a reduced solubility in carbon tetrachloride compared to diethyl ether, thus providing additional driving force for the reaction and ease of purification of product. The selection of bro-... 

Cyclic ethers were also obtained by cyclization of alkoxyl radicals, generated in a radical chain reaction by reacting the thione 42 with (TMSfsSiH under photochemical conditions at 20 °C (Reaction 46). Regioselectivities of cyclization have been investigated and a progressive increase of the 6-endo-trig selectivity along the series R2 = H[Pg.140]

In the past few years, however, very efficient new methods of cyclisation proceeding via radical intermediates have been developed and several reviews [19a] and a comprehensive book by Giese [19b] have been published. Rather than reactions involving the dimerisation of two radicals -as in the Kolbe electrochemical synthesis [20] or the radical induced dehydrodimerisation developed by Viehe [21]-more important are the reactions between a radical with a non-radical species. The advantage of this type of reaction is that the radical character is not destroyed during the reaction and a chain-reaction may be induced by working with catalytic amounts of a radical initiator. However, in order to be successful two conditions must be met i) The selectivities of the radicals involved in the chain-reaction must differ from each other, and ii) the reaction between radicals and non-radicals must be faster than radical combination reactions. 

Finally it has to be remarked briefly that the reactivity and selectivity of free radicals is certainly not only determined by steric and bond energy effects or by the thermodynamic stability of these transients. Polar effects are also important, in particular in those reactions which have early transition states e.g., the steps of free radical chain reactions They are either due to dipole interactions in the ground state or to charge polarization at transition states. FMO-theory apparently offers a more modern interpretation of many of these effects ... 

Thermal dehydrochlorination of 1,2-dichloroethane188-190 272 273 takes place at temperatures above 450°C and at pressures about 25-30 atm. A gas-phase free-radical chain reaction with chlorine radical as the chain-transfer agent is operative. Careful purification of 1,2-dichloroethane is required to get high-purity vinyl chloride. Numerous byproducts and coke are produced in the process. The amount of these increases with increasing conversion and temperature. Conversion levels, therefore, are kept at about 50-60%. Vinyl chloride selectivities in the range of 93-96% are usually achieved. 

The foregoing discussion adds further to our understanding of the selectivity observed in the halogenation reactions discussed in Chapter 4. When propene is chlorinated in sunlight, the product is 3-chloropropene, and we may explain this on the basis that the radical-chain reaction involves propagation steps in which a chlorine atom attacks the hydrogen corresponding to the weakest C-H bond ... 

The free-radical construction of C—C bonds either inter- or intramolecularly using a hydride as mediator is of great importance in chemical synthesis. The propagation steps for the intermolecular version are shown in Scheme 2. For a successful outcome, it is important (i) that the R sSi radical reacts faster with RZ (the precursor of radical R ) than with the alkene and (ii) that the alkyl radical reacts faster with alkene (to form the adduct radical) than with the silane. In other words, for a synthetically useful radical chain reaction, the intermediates must be disciplined. Therefore, in a synthetic plan one is faced with the task of considering kinetic data or substituent influence on the selectivity of radicals. The reader should note that the hydrogen donation step controls the radical sequence and, often, the concentration of silane provides the variable by which the products distribution can be influenced. 

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