Initiators Lewis bases

Aliphatic azo compounds Lewis acids with or without co-initiators Lewis bases Mixed catalysts (Ziegler-Natta catalysts)... 

A more complex reaction sequence may be responsible for the formation of the products of formal insertion of silylenes into the H—C bonds of carbaldimines110 116. As shown in equation 56, an initial Lewis base or TT-bond adduct can undergo rearrangement to both of the products observed from reactions of diarylsilylenes and pyridine-2-carbaldimines. A common intermediate is suggested by the thermal isomerization of the insertion product to the cycloadduct which can be the exclusive product from addition of a silylene to a 2,2/-bipyridyl (see Section ni.B)117. 

The anionic polymerization of dienes is also a subject of long-term, continuous interest. Using a variety of initiators, Lewis base additives, and solvent systems, a wide range of polydiene microstructures can be prepared. Several reports have appeared regarding the relationships between polydiene microstructure, monomer concentration and chain-end concentra-tion. In general, the highest cis-1,4-microstructures for either polybutadiene or polyisoprene can be obtained at high ratios of [monomer] to... 

When carbon forms four covalent bonds with halogen atoms the second quantum level on the carbon is completely filled with electrons. Most of the reactions of the Group IV tetrahalides require initial donation by a Lewis base (p. 91) (e.g. water, ammonia) which attaches initially to the tetrahalide by donation of its electron pair. Hence, although the calculated free energy of a reaction may indicate that the reaction is energetically favourable, the reaction may still not proceed. Thus we find that the tetrahalides of carbon... 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

Lithium tetrafluoroborate yields only poorly conducting solutions with all solvents. It is unstable [61] and also leads to polymerization with cyclic ethers initiated by the corresponding Lewis base BFj. 

In view of the chemical nature of alkylaluminums and methyl halides, complexation is most likely to be rapid and complete, i. e. K is large. Indeed Me3 Al and a variety of Lewis bases were found to complex rapidly2. Initiation, i.e., f-butyl cation attack on monomer, is also rapid since it is an ion molecule reaction which requires very little activation energy. Thus, it appears that Rj t. and hence initiator reactivity are determined by the rate of displacement Ri and ionization R2. 

The Staudinger reaction [92], a [2 + 2]-cycloaddition of a ketene and a nucleophilic imine, usually proceeds by an initial imine attack on the ketene thus forming a zwitterionic enolate which subsequently cyclizes. This reaction is an expedient route to p-lactams, the core of numerous antibiotics (e.g., penicillins) and other biologically active molecules [93]. In contrast, for Lewis-base catalyzed asymmetric reactions, nonnucleophilic imines are required (to suppress a noncatalyzed background reaction), bearing, for example, an N-Ts [94] or -Boc-substituent [95]. 

However, in the presence of a suitable Lewis base the polymerization becomes living, due to the nucleophihc stabilization of the growing cation generated by the added base. (3) Initiator, strong Lewis acid and onium salt as additive The previous method cannot be easily applied in polar media. In this case the living cationic polymerization is promoted by the addition of salts with nucleophihc anions, such as ammonium and phosphonium derivatives. 

In the presence of added Lewis bases, sonochemical ligand substitution also occurs for Fe(C0)5, ancl act or roost metal carbonyls. Sonication of Fe(C0)5 in the presence of phosphines or phosphites produces Fe(C0)5 nLn, n=1, 2, and 3. The ratio of these products is independent of length of sonication the multiply substituted products increase with increasing initial [L] Fe(C0)i L is not sonochemically converted to Fe(C0)3L2 on the time scale of its production from Fe(C0)5. These observations are consistent with the same primary sonochemical event responsible for clusterification ... 

The interaction between catalyst and diazo compound may be initialized by electrophilic attack of the catalyst metal at the diazo carbon, with simultaneous or subsequent loss of N2, whereupon a metal-carbene complex (415) or the product of carbene insertion into a metal/ligand bond (416) or its ionic equivalent (417) are formed. This is outlined in a simplified manner in Scheme 43, which does not speculate on the kinetics of such a sequence, nor on the possible interconversion of 415 and 416/417 or the primarily formed Lewis acid — Lewis base adducts. 

Some organometallic compounds and Lewis bases can also act as initiators. In such cases initiation occurs by a direct attack of these compounds on the double bond of the monomer molecule. 

However before the Lewis base can attack the monomer, it must ionise and only then a carbanion can be produced. The process of initiation is shown below ... 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

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