Vinyl acetate abstraction « addition

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

In the vinyl acetate/acrylonitrile system, we have also examined the effect of added Lewis acids as these are known to enhance alternation in alternating copolymerisation. In the presence of zinc chloride, no dimeric products were observed. Nor were the normal addition products 11 and 12 observed. The major products fornied were 5 (55%), 13 (20%) and 14 (25%). Similar results were obtained with ferric chloride. Thus with 4% FeCla, the ratio of 5 13 14 was 84 10 6 with 60% FeCls, the ratio changed to 25 3 72 respectively. Apparently Lewis acids catalyse the p-fragmentation of rerr-butoxyl radicals (or of di-ferr-butyl peroxalate) and/or slow the rate of addition of the (electrophilic) ferr-butoxyl radicals to both vinyl acetate and acrylonitrile. The product 14 is thought to arise via hydrogen-abstraction from vinyl... 

AA sec acrylic acid abstraction sec hydrogen atom transfer abstraction v,v addition and micleophilicity 35 by aikoxy radicals 34-5, 124-5, 392 by alkoxycarbonyloxy radicals 103,127-8 by alkyl radicals 34 5, 113, 116 by f-amyloxy radicals 124 by arenethiyl radicals 132 by aryl radicals 35, 118 by benzovloxy radicals 35, 53, 120, 126 wilh MM a" 53, 120 by /-butovy radicals 35, 53, 55, 124 solvent effects 54, 55. 123 with alkenes 122 3 with ally I acrylates 122 wilh AMS 120, 123 wilh BMA 53, 123 with isopropenvl acetate 121 with MA 120 with MAN 121 with MMA 53, 55, 120.419 with VAc 121 with vinyl ethers 123... 

Addition reactions with Mode B are not popular, but are occasionally useful. Eq. 4.7 indicates the reaction of ethyl bromoacetate and sugar vinyl ether with Bu3SnH initiated by AIBN. The ethyl acetate radical is electrophilic and it reacts with electron-rich sugar vinyl ether through SOMO-HOMO orbital interaction to form a ribosyl anomeric radical, as shown below. Then, the formed ribosyl anomeric radical abstracts... 

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