Radical conditions

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

Under free radical conditions in the presence of peroxides addition takes place with a regioselectivity opposite to that of Markovmkov s rule... 

When propene is polymerized under free radical conditions the polypropylene that results IS atactic Catalysts of the Ziegler-Natta type however permit the preparation of either isotactic or syndiotactic polypropylene We see here an example of how proper choice of experimental conditions can affect the stereochemical course of a chemical reaction to the extent that entirely new materials with unique properties result... 

Polymerization of styrene is carried out under free radical conditions often with benzoyl peroxide as the initiator Figure 1111 illustrates a step m the growth of a poly styrene chain by a mechanism analogous to that of the polymerization of ethylene (Sec tion 6 21)... 

The mutual polymerisation of two or more monomers is called copolymerisation. This topic has been comprehensively reviewed (4,5). Monomers frequentiy show a different reactivity toward copolymerisation than toward homopolymerisation. In fact, some monomers that can be bomopolymerised only with great difficulty, can be readily copolymerised. One such monomer is maleic anhydride. It is rather inert to free-radical homopolymerisation yet can be copolymerised convenientiy with styrene and many other monomers under free-radical conditions. 

Alkyl side chains in both pyrazines and quinoxalines are susceptible to halogenation by elemental halogens (28JCS1960, 68TL5931) and under radical conditions with NBS (72JOC511). Thus, bromination of 2-methylquinoxaline with bromine in the presence of sodium acetate... 

Additions of chlorine or bromine to 2-(4-biphenylyl)pentafIuoropropene take place only under free-radical conditions [5. 6 (Table 1)... 

Analogous reactions under free-radical conditions show a preference for the erythro isomer when X = Cl, however when X = tnfluoromethyl, the reaction shows essentially no stereoselectivity [S] (Table 3)... 

Bromination of the homoconjugated 5-(difluoromethylene)-6 6-difluoro-2-norbornene yields different product mixtures under ionic and free-radical conditions 1771 (equation 6)... 

PenCafluorosulfur chloride adds under free-radical conditions to fluorinated olefins, to acetylene, to ketene, to methyl tnfluorovinyl ether [/] and to perfluoroal lene [8]... 

Dialkylphosphinous acids react with perfiuoroalkenes under free radical conditions to form carbon-phosphorus bonds [10] (equation 7)... 

Methyl groups on pyridopyrimidines can be laterally brominated under radical conditions (79JHC133), but when 204 was treated with NBS in the presence of peroxide very little methyl bromination was observed. Instead, the main product in 88-96% yield was the 3-bromo derivative [84IJC(B)1117]. 3-Chlorination of a derivative of 204 has also been reported [83AHC(33)241]. 

A useful synthesis of allylstannanes from primary alcohols involves conversion of the alcohols into their O-substituted 5-methyl carbonodithioates, thermolysis to effect [3,3] rearrangement to the corresponding 5-substituted 5-methyl carbonodithioates, and treatment with a trialkyl-tin hydride under free-radical conditions to form the allylstannane21. This procedure has been applied to the synthesis of functionalized allylstannanes including (5)-( )-4-(benzyloxy)-2-pen-tenyl(tributyl)stannane22. 

Allylstannanes are also available from allyl sulfoncs24 and sulfides25 by treatment with tri-alkyltin hydrides under free-radical conditions. Of some interest is the stereocontrol exercised by a neighboring hydroxy group, possibly because of interaction with the tin26. 

Treatment of bicyclic lactones 66, derived from Diels-Alder reaction of 3-carboxy-2-pyrone under standard radical conditions using (TMSlsSiH, leads to bridged lactones 67, which can smoothly be converted to bicyclo[3.3.0]-lactones 68 (Scheme 10). For X = CHaOMe, this cascade of rearrangements took place in a 78% overall yield, providing 68 in diastereomerically pure form. Three additional steps provided a novel route toward Corey s lactone 69. 

Examples of the intermolecular C-P bond formation by means of radical phosphonation and phosphination have been achieved by reaction of aryl halides with trialkyl phosphites and chlorodiphenylphosphine, respectively, in the presence of (TMSlsSiH under standard radical conditions. The phosphonation reaction (Reaction 71) worked well either under UV irradiation at room temperature or in refluxing toluene. The radical phosphina-tion (Reaction 72) required pyridine in boiling benzene for 20 h. Phosphinated products were handled as phosphine sulfides. Scheme 15 shows the reaction mechanism for the phosphination procedure that involves in situ formation of tetraphenylbiphosphine. This approach has also been extended to the phosphination of alkyl halides and sequential radical cyclization/phosphination reaction. ... 

Non-activated n-alkylbromides react non-selectively under radical conditions, because the radicals formed at the step of chain growth, actively abstract hydrogen atoms from bromoalkane alkyl chain. 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

RSO2, and so on. Addition of Ph3SiSH to terminal alkenes under radical conditions also leads to the primary thiol.With alkynes it is possible to add 1 or 2 mols of RSH. 

The mechanism is usually electrophilic (see p. 972), but when free-radical initiators (or UV light) are present, addition can occur by a free-radical mechanism. Once Br-or Cl- radicals are formed, however, substitution may compete (14-1 and 14-2). This is espiecially important when the alkene has allylic hydrogens. Under free-radical conditions (UV light) bromine or chlorine adds to the benzene ring to give, respectively, hexabromo- and hexachlorocyclohexane. These are mixtures of stereoisomers (see p. 161). ... 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

The third fact that seemed to argue in favor of the occurrence of radicals 10 was the observation that reactions of a-tocopherol under typical radical conditions, that is, at the presence of radical initiators in inert solvents or under irradiation, provided also large amounts of two-electron oxidation products such as o-QM 3 and its spiro dimerization product 9 (Fig. 6.8).16,25,26 This was taken as support of a disproportionation reaction involving a-tocopheroxyl radical 2 and its hypothetical tautomeric chromanol methide radical 10, affording one molecule of o-QM 3 (oxidation) and regenerating one molecule of 1 (reduction). The term disproportionation was used here to describe a one-electron redox process with concomitant transfer of a proton, that is, basically a H-atom transfer from hypothetical 10 to radical 2. 

Isopropenylferrocene does not homopolymerize under free radical conditions using AIBN as an initiator, but it does copolymerize with styrene.Preliminary results indicate that the IDM monomer also copolymerizes with styrene using AIBN. In benzene solvent at 50°C in 24 h, a 10.6% yield of copolymer (IR vc=0 2020, 1945 cm , vN=0 1675 cm-- -, V q 1601 cm-- -, v 3020 cm-- -) resulted having an M f 5700 and containing 6.8% of IDM as determined by elemental analysis. The initial monomer mixture contained 17% of IDM. 

The [6.5.5]-ring fused tricyclic motif is found in many natural products, and has therefore become an important target in synthesis. A convenient access to this structural framework is offered by a radical domino procedure published by the Nagano group [41]. This reaction of optical pure dibromoacetal 3-85 led to the desired tricycle 3-87 via 3-86 as a single diastereoisomer in a very respectable yield of 94% by applying classical radical conditions (excess tributyltin hydride/AIBN, irradia-... 

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