Anions from Lewis acids, stability

The initiator stability must therefore be balanced with its required reactivity. For example, stable carbenium ions such as tropylium or trityl react very slowly with alkenes and are therefore not optimal initiators. Protonic acids may be sufficiently reactive but often have anions that are basic and cause transfer by j3-proton abstraction from the growing carbenium ion anions resulting from Lewis acid initiators are less basic. Many of the new efficient initiators are prepared by mixing two stable components, such as an alkyl halide and a Lewis acid, which provide reversibly active carbenium ions. 

The ester RO2R can be thought of as the compound formed from the Lewis base, RO2, and the Lewis acid, R " ", and its stability will be determined by the similarity of their cation and anion bonding strengths. 

The use of a Lewis acid to promote the abstraction of a halide anion from a metal halide complex, in the presence of a neutral ligand, has been widely employed to produce both substituted and totally unsubstituted metal carbonyl cations. The halide acceptor most generally used is aluminum trichloride, but other Lewis acids have been employed, as indicated below. The requirements for the halide acceptor include that the anion product should be sufficiently large to stabilize the salt formed (15). 

Rehydrated Mg/Al-OH catalysts have been found to be inactive in the MPV reduction of 4-ferf-butylcyclohexanone by isopropanol, whereas the bifunctional character of the Mg(Al)0 mixed oxides made them highly active in this reaction. The aluminium alkoxidc intermediate of the MPV reaction indeed involves a cooperation between basic and acidic sites. On mixed oxides, the abstraction of a proton from isopropanol on O2 sites gives isopropoxide anions, which are then stabilized on Al3+ and form intermediates with the aldehyde. The high activity of mixed oxide comes from the synergetic effect of strong Lewis basicity and mild acidity. 

Boranes are strong reducing agents and the neutral molecules, inflame spontaneously in air, although the anions [BnHn]2- have remarkable kinetic stability. Diborane itself reacts with Lewis bases to give donor-acceptor complexes with BH3, which is a soft Lewis acid and forms adducts with soft bases such as CO (1). More complex products often result from unsymmetrical cleavage of B2H6, for example,... 

Copolymerization of styrene with diolefins provides further support that monomer coordinates with the cationic site prior to addition. Korotkov (218) showed that in homopolymerizations styrene is more reactive than butadiene, but in copolymerization the butadiene reacted first at its homopolymerization rate and when it was exhausted the styrene reacted at its homopolymerization rate. This interesting result has been duplicated by Kuntz (245) and analogous results have been obtained by Spirin and coworkers (237) for the styrene-isoprene system. Presumably, the diene complexes more strongly than styrene with the lithium and excludes styrene from the site. That the complex occurs at a cationic site, rather than at the anion or the metal-carbon bond, is indicated by the fact that dienes form more stable complexes than styrene with Lewis acids (246). It should be emphasized that selective monomer coordination is not the only factor influencing reactivities in copolymerizations. Of greatest importance are the relative reactivities of the different polymer anions. The more resonance-stabilized anion is more readily formed and is less reactive for polymerizing the co-monomer. 

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

Perhaps the most frequently used example is the HI/ZnI2 system, where the iodine in the HI/12 counterpart is now replaced with the mild Lewis acid, zinc iodide [98,99]. A more detailed discussion of the scope and mechanism of the polymerizations by the HB/MtX systems will be given for respective monomers in the later parts of Section IV. It should be noted here that the suitable nucleophilicity of the initiator s anion B- and the mild Lewis acidity of MtX strongly depend on the nature and stability of the growing carbocations or the monomers from which they are derived. 

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