Initiation Reactions Involving Alkyllithiums

The initiation events involving dienes and styrene in hydrocarbon solvents have been thoroughly and accurately studied by the application of UV and visible spectroscopy. The archetype of such studies is the now classic 1960 study of Worsfold and Bywater on the n-butyllithium-styrene system in benzene. The reaction was found to follow the relationship  

The fractional dependency of the initiation process on the total concentration of initiator was rationalized on the basis of the hexameric association state of n-butyllithium (Table 1) and the following equilibrium  

The above process, Eq. (25), is in conflict with the currently available theoretical results (Table 5) regarding the dissociation enthalpies of aggregated organolithiums. A similar conclusion was reached by Brown in 1966 This assessment is fortified by the fact that the measured energy of activation for the reaction of styrene with n-butyllithium, 18 kcal/mole, is a value far lower than that required if the calculated dissociation enthalpy of the n-butyllithium aggregates is included in the overall energetics of the initiation event. Thus, it would seem that any mechanism which involves only unassodated organolithiums as reactive entities is invalid. 

It should also be mentioned that the work of Graham and coworkers apparently demonstrates that the identity of the alkyl substituent on the alpha carbon atom 

Kinetic complications are kown to exist when mixed aggregates are present. When 2,3-dimethyl-1,3-butadiene in heptane is initiated by n-butyllithium, the conversion- 

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Further addition of olefin to the product is impeded by steric factors. Equation (11) represents the initiation step of an anionic polymerization of an olefin (68) the rate of chain propagation in this case is very slow. It is important that the kinetics of Eq. (11) be thoroughly established, since it does form a prototype system for initiation of anionic polymerization. Preliminary work in the writer s laboratory (69) with ethylithium concentrations of about 0.1 M and 1,1-diphenylethylene concentrations of about 0.05 M, employing infrared spectroscopy to follow the disappearance of 1,1-diphenylethylene, have yielded second-order rate plots. The apparent order in alkyllithium, based on the method of initial rates, is in the range 0.7 to 0.95. The experimental details given by Evans and George indicate that meticulous care was taken in their work. It is possible, however, that the spectrophotometric method employed in following the reaction involves some hidden source of error, or that the difference in the experimental results is due to concentration differences. In any case, further work on this important system is desirable. 

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. 

However, the decrease in rate was initially explained by the difference in reactivity of the alkyllithiums present, i.e., whether or not a primary or secondary anion was involved. Yet, the propagation reaction was quite different because the structure of the active species did not vary after the addition of the first ethylene unit. The fact that the constant rate (the rate after 1 mole of ethylene is consumed per mole of BuLi) varied linearly with initiator concentration at TMEDA /-BuLi = 1.0 indicated that a monomeric complex was responsible for the propagation reaction (PELi-TMEDA) [Eqs. (13), (14)]. 

The observed inverse correlation between reaction order dependence for alkyllithium and degree of alkyllithium aggregation is not observed in aliphatic solvents. The use of aliphatic solvents leads to decreased rates of initiation and pronounced induction periods. In fact, a different reaction mechanism involving the direct addition of monomer with aggregated organolithium species has been proposed for aliphatic solvents [3, 56],... 

In aliphatic solvents the inverse correspondence between reaction order dependence for alkyllithium and degree of organolithium aggregation is not observed (49). In addition, the rates of initiation in aliphatic solvents are several orders of magnitude less than in aromatic solvents. Most reaction orders for alkyllithium initiators in aliphatic solvents are close to imity. These results suggest that in aliphatic solvents the initiation process may involve the direct addition of monomer with aggregated organolithium species (eq. 26) to form a cross-associated species. 

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