Solvent effects preequilibrium mechanism

For base catalyzed reactions, the kinetic solvent isotope effect does not supply a simple method of distinction among different mechanisms. The ranges for the different mechanisms partially overlap, and kQ H - /kQ D -may be smaller than 1 for slow proton transfer as well as for preequilibrium proton transfer [99]. 

The mechanism of the deprotonation of dipole-stabilized anions has been studied in detail. It has been shown by IR spectroscopy that a preequilibrium exists between the butyllithium base and the amide or amidine, forming a coordination complex prior to deprotonadon. A recent mechanistic study has shown that, in cyclohexane solvent, this prior coordination is between the amide (or added TMEDA) and aggregated s-butyllithium, and that the effect of the coordination is to increase the reactivity of the complex. The diastereoselectivity of proton removal in chiral benzylic systems has also been examined,but since the anions invert, this selectivity is of little consequence in the alkylation step. 

Alkenes lacking the phenyl group are somewhat less convenient to study by kinetic methods, but such data as observation of general acid catalysis and solvent isotope effects are also consistent with rate-limiting protonation in simple alkenes such as 2-methylpropene and 2,3-dimethyl-2-butene. The observation of general acid catalysis rules out an alternative mechanism for alkene hydration, namely, water attack on an alkene-proton complex. The preequilibrium would be governed by the... 

The dependence of rate on Hammett s hg function and the large solvent isotope effects ( Dao/ Hao) for several JV-nitroamines suggest preequilibrium protonation of substrate, but details of the subsequent migration are disputed. What is clear is that some structural feature denied to other W-substituted amines here permits an intramolecular route. Three mechanisms are extant ... 

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