Through Allylic-Hydride Abstraction

As we have already mentioned, it was Kennedy who proposed that monomers possessing an allylic hydrogen can be converted into the corresponding allylic cart nium ions through a hydride-ion abstraction promoted by Levns acids. This reaction would not require a cocatalyst and could therefore explain in principle the occurrence of direct initiation. A similar mechanism had previously been claimed by Holmes and Pettit to rationalise the formation of carbenium ions from certain hydrocarbons and antimony pentachloride, e.g.  

Althou the evidence presented in support of such reaction is not entirely convincing, in its favour is the existence of a lower oxidation state of antimony which can be readily reached by smooth reduction. In the case of other Lewis acids a lower oxidation state is either unavailable or less easily obtained and the likelyhood of hydride abstraction seems therefore more remote, at least from saturated hydrocarbons. Kennedy s scheme implies however the removal of the allylic hydride ion from an olefin. Although plausible in certain cases (but never proved), this mechanism is obviously impossible with styrene and 1,1 -diphenylethylene With 3-phenylindene it would yield an aryl-substituted allylic carbenium ion which would not be expected to be in equilibrium with its precursor yet, this equilibrium was observed With 2,3-dimethylindene in the same conditions initiation did not take place yet Kennedy s mechanism shouldhave operated without impediments. Finally, with 1,1-diphenylpropene hydride abstraction would have produced an allylic ion incapable of giving back the precursor by reacting with methanol yet Bywater and Worsfold showed that this reversible reaction takes place. 

On the basis of the above experimental findings, we can therefore conclude that Kennedy s mechanisms of hydride abstraction does not apply to the q stems reported in Tables 8 and 9. 

Having ruled out other reasonable possibilities, we are left with two mechanisms vdiich can account for direct initiation, viz., the Hunter-Yohe zwitterion reactiwi and the selfionisation of the Lewis acid followed by the addition of the cation onto the monomer double bond. Before discussing the relevance of each one of them, some remarks must be made on the nature of the interactions between Lewis acids and olefins and on the possible conjugation of Lewis acids with anions. 

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This reaction describes the entrance of a nucleophile into the allylic position of an olefin. In aqueous medium this reaction is of minor importance but in nonaqueous medium, particularly under the conditions of acetoxylation, it attracts broad interest. As already mentioned above and outlined later (see Section 3.3.14.6), higher and cyclic olefins give exclusively allylic esters. Two mechanisms have been proposed. One possibility is according to eq. (17) hydride abstraction through the palladium of an oxypalladation moiety by -elimination from the adjacent C-atom whieh had not been added to the nucleophile [9]. 

Finally, the use of stoichiometric amounts of transition metal complexes can play an important role in the synthesis of functionalized piperidines. <01H14.39> Liebeskind and coworkers have developed a chiral transition metal complex and have used it in the synthesis of (-)-indolizidine 209B <01JA12477>. A lipase mediated allylic alcohol resolution provides access to both antipodes of enantiomerically pure allyl acetates (115) which can be used to form an ri -allyl molybdenum complex (116), Hydride abstraction followed by methoxide quench yields a reactive species 117 which may be further functionalized through reactions with Grignard reagents. The eventual products 119 arc 2,3,6-trisubstituted piperidines in enantiomerically pure form. 

Cationic olefin complexes of dicarbonyl(> -cyclopentadienyl) iron have been of wide interest in syntheses for a number of years. These complexes, generally isolated as their tetrafluoroborate or hexafluorophosphate salts, have been prepared by the reaction of Fe(f -CsHs)(CO)2Br with simple olefins in the presence of Lewis acid catalysts, by protonation of allyl ligands in Fe(t/ -CsHjXCO)2[(allyl)KC ] complexes, or by treatment of these with cationic electrophiles, by hydride abstraction from Fe(f) -CsHjXCO)2(alkyI) complexes, through reaction of epoxides with Fe(fi -CsHjXCO)2 anion followed by protonation, or by thermally induced ligand exchange between [Fe( i -CjHsXCO)2(ij -2-methyH-propene)][BF4] " or [Fe( -C,HsXCO)2(tetrahydrofuran)][BF4] and excess olefin. 

Let us start with a consideration of the metal ion-promoted migration of an allylic C=C bond (Scheme 9.3). There are various mechanisms involved in this process, dependent on the electronic properties of the metal ion [12]. In the first case (a), the reactive metal hydride undergoes addition to ti-electrons of the double bond, forming a covalent bond with the more electronegative carbon atom. Abstraction of the hydride ion from the second terminal carbon completes the migration process. More soft metal ions form co-ordinative bonds through their d-electrons and promote migration of the C=C bond via the delocalized n-allylic system (b). 

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