Radical allylic acetates

Lacking resonance stabilization, the chain radicals doubtless are very reactive, but owing to the corresponding lack of resonance structures in the transition state allyl acetate is a relatively unreactive monomer. These factors are conducive to the occurrence of the competitive reaction... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

An especially interesting case of inhibition is the internal or autoinhibition of allylic monomers (CH2=CH—CH2Y). Allylic monomers such as allyl acetate polymerize at abnormally low rates with the unexpected dependence of the rate on the first power of the initiator concentration. Further, the degree of polymerization, which is independent of the polymerization rate, is very low—only 14 for allyl acetate. These effects are the consequence of degradative chain transfer (case 4 in Table 3-3). The propagating radical in such a polymerization is very reactive, while the allylic C—H (the C—H bond alpha to the double bond) in the monomer is quite weak—resulting in facile chain transfer to monomer... 

Electroreduction of the cobalt(II) salt in a mixture of either dimethylform-amide-pyridine or acetonitrile-pyridine as solvent, often in the presence of bipyridine, produces a catalytically active cobalt(I) complex which is believed to be cobalt(I) bromide with attached bipyridine ligands (or pyridine moieties in the absence of bipyridine). As quickly as it is electrogenerated, the active catalyst reduces an aryl halide, after which the resulting aryl radical can undergo coupling with an acrylate ester [141], a different aryl halide (to form a biaryl compound) [142], an activated olefin [143], an allylic carbonate [144], an allylic acetate [144, 145], or a... 

It is reported that methyl acrylate, allyl acetate, vinyl acetate and dimethyl maleate give only low yields of oligomers with butyllithium under all experimental conditions (31). Furukawa and coworkers (32) confirm that vinyl acetate will not polymerize and that n-butyl-vinyl-ether will not either. High polymers can be formed from isopropyl acrylate (39) in toluene at —70° and from t-butyl acrylate (65). The reported failure of methyl acrylate and butyl acrylate to yield high polymers could reflect a genuine difference in behaviour connected with the side group or. could simply result from failure to choose the most favourable conditions for polymerization. Vinyl acetate can be polymerized by lithium metal (49) but co-polymerization experiments suggest that the polymer is formed by a radical mechanism. 

This is proved by the observation that the rate of decomposition of persulphate drops to the same low value as when allyl acetate is added in the absence of alcohol. On addition of allyl acetate, the sulphate ion radicals react with the allyl monomer rather than with the alcohol, so that no alcohol radicals are formed and reaction (VII) cannot take place. Formaldehyde is only formed in the absence of allyl acetate. It should be noted that Bartlett and Nozaki (24) could not confirm these observations. 

This problem is not so severe when acyl xanthates are used as precursors because these substrates absorb in the visible region, while the products do not (however, the products might still be recycled to the radical pool by radical addition-elimination). Visible light photolysis of benzoyl xanthane (42) and allyl acetate provides (43) in 60% yield. Standard (ionic) 3-elimination of the xanthane is a facile reaction that gives (44). When the tertiary acyl xanthane (45) is irradiated in the presence of W-benzylmaleimide... 

In the polymerization of allyl acetate, transfer to monomer produces an unreactive radical which fails to re-initiate growth of the polymer chain. Bartlett and Tate (31) compared the rates of polymerization for the unlabelled monomer and.the deuterated monomer CH2 CH-CD2 0-CO CH3. The deuterated monomer polymerized more rapidly giving a product of higher molecular weight. These observations suggest that the rate of polymerization and the molecular weight of the polymer are controlled by the reaction... 

A tin-free radical cyclization of the xanthate 272 using dilauroyl peroxide (DLP), as the radical initiator, in chlorobenzene was used to give the 5//-pyrido[2,3-A azepin-8-one 273 (Scheme 35) <20040L3671>. The xanthate 272 was also made by an intermolecular free radical addition to allyl acetate, using the xanthate 271, as the radical precursor. Somewhat surprisingly in this latter case, intramolecular free radical attack on the pyridine ring did not take place. 

Refluxing in THF for the indicated time. One equivalent of DBU is added when allylic acetates are used. b Pd (5%), PPh3 (20%), or DPPE (10%). DPPE is (l,2-bis(diphenylphos-phino)ethane.c Radical R in 35c Me(CH2)4CH=CHCH2—. d Diastereomeric mixture. 

A reductive radical cyclization methodology, which is catalyzed by zirconocene complexes, furnished butyrolactols 91 (Fig. 27) [168, 169]. When 2-allyloxy-3-halotetrahydropyrans or bromoacetaldehyde allyl acetals 90 were treated with 20-30 mol% of zirconocene dichloride, 1 equiv. of BEt3 and an excess Red-Al as the stoichiometric reductant, tetrahydrofurotetrahydropyrans or butyrolactols 91... 

Oshima s group reported the first example of a tandem radical cyclization/intermo-lecular Heck reaction in 2002 (Fig. 60) [289]. Iodoacetaldehyde allyl acetal 242a was treated with styrene 243, catalytic amounts of CoCl2(dpph), and trimethylsi-lylmethylmagnesium chloride 224 as a stoichiometric reducing agent. 4-Cinnamyl-butyrolactol 244 was isolated in 50% yield (cf. Fig. 54). 

Giese and coworkers investigated radical 5-exo cyclization reactions of bromoacetaldehyde allyl acetals 264 (Y=0, R1=OBu) catalyzed by 251 and found that either product 265 or 266 results selectively, depending on the amount of catalyst (1-40 mol%) and the reducing conditions (entry 8) [304]. Mild reducing conditions, such as Zn in DMF or a potential of —0.8 V vs Ag/Ag+, and higher... 

Even the use of Se,N-acetals with stereogenic centers led to the corresponding products without racemization. The N,Se-acetal 141 was prepared from lactaldehyde without racemization. The subsequent radical allylation afforded product 142 in good yields (Scheme 37).243... 

Finally, Molander has shown that the efficiency of 8-endo carbonyl-alkene cyclisations can be significantly enhanced by the presence of a leaving group in the allylic position of the alkene.57 For example, treatment of allylic acetate 47 with Sml2 provides an excellent yield of the cyclooctenol 48 (Scheme 5.36). After the cyclisation step, the resultant secondary alkyl radical is reduced to an... 

Russian workers have proposed that the increased activity of allyl acetate and allyl alcohol in free radical or gamma ray initiated polymerization in the presence of zinc chloride may be connected with the decreased degradative chain transfer with complexed monomer or the activation of the stabilized allyl radical in the complexed monomer—i.e., the conversion of degradative chain transfer to effective transfer (55, 87). However, these explanations have been partially rejected as inadequate. 

The free radical copolymerization of methyl methacrylate or acrylonitrile in the presence of zinc chloride with allylic compounds such as allyl alcohol, allyl acetate, and allyl chloride or butene isomers such as isobutylene, 1-butene, and 2-butene is characterized by the incorporation of greater amounts of comonomer than is noted in the absence of zinc chloride (35). Analogous to the radical homopolymerization of allylic monomers in the presence of zince chloride, the increase in the electron-accepting capability of the methyl methacrylate or acrylonitrile as a result of complexation results in the formation of a charge transfer complex which undergoes homopolymerization and/or copolymerization with a polar monomer-complexed polar monomer complex. 

Hart and Tsai have found that the allylic acetate (18) undergoes directed radical cyclization to afford the pynolizidinone (19) with good stereoselectivity (ScheriK S). The addirct (19) was converted to iso-retronecanol (21) followirtg conversion of the side chain to methyl ketone (20), which was oxidized with TFPAA. 

Deactivating chain transfer to monomer is quite common in polymerization of allyl monomers [40-42], Allyl radicals such as that of allyl acetate are resonance-stabilized, with the result that polymerization rates and molecular weights remain low. Moreover, with chain transfer as the dominant termination mechanism, the termination rate is first order in free radicals. This lets the free-radical population become proportional to the initiator concentration and leads to a polymerization rate that is first order rather half order in initiator and zero order in monomer. 

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