Enolate anions temperature effects

Alternatively, the use of an amide base, such as LDA or lithium hex-amethyldisilazide (p. 389), in aprotic solvents, such as ether or THF, at low temperatures, generates an enolate anion under conditions where the equilibrium lies more to the right. A second equivalent of the ketone can then be added. Clearly, this technique is effective in reactions of aldehydes. 

Selectivity is more complicated with a methyl or chloro substituent. Again, meta substitution is always significant, but ortho substitution can account for 50-70% of the mixture in some cases [2]. More reactive anions (1,3-dithianyl) and less substituted carbanions (e.g., tert-butyl lithioacetate) tend to favor ortho substitution. Representative examples are shown in Table 3. Entries 2-4 show that variation of reaction temperatures from -100 °C to 0 °C has no significant effect in that highly selective system. The added activating effect of the Cl substituent allows addition of the pinacolone enolate anion (entry 11), whereas no addition to the anisole nor toluene ligand is observed with the same anion. 

Treatment of ll,ll-dichloro-l,6-methano[10]annulene with Bu"Li in an ether solvent yields C22H hydrocarbons of labile and complex nature/ Trapping experiments support the intermediacy of compounds ( )6) and (607) formed by the sequence of rearrangements (605)- (606)- (607). Reaction of cyclo-octa-2,4,6-trien-l-one with the anion of methyl 4-(dimethylphosphinyl)but-2-enoate gave (608) and (609) the predominant isomer (609) resulted from base-catalysed isomerization of (608) under the conditions of reaction. Low-temperature oxygenation of the enolate anion derived from the mixture of (608) and (609), followed by reduction with triethyl phosphite, gave a 1 1 mixture of 8-methoxycarbonylbicyclo[5,3,l]undeca-l,3,5,9-tetraen-8-exo-ol and -1,3,5,8-tetraen-lO-exo-ol. Pyrolysis of the p-nitro-benzoate esters of these alcohols effected their conversion into methyl 1,5-methano-[10]annulenecarboxylate (610). 

The enolate anion on the least substituted carbon is the least stable (more highly substituted being the most stable) and the ratio of (E)-enolate to (Z)-enolate is a function of their method(s) of preparation, the temperature at which their preparation is effected, and the length of time between preparation and use. Thus, it appears that the enolate anions are related as kinetic and thermodynamic products. It is common to find that the (Z)-enolate is the kinetic product. 

The fluoride anion has a pronounced catalytic effect on the aldol reaction between enol silyl ethers and carbonyl compounds [13] This reacbon proceeds at low temperature under the influence of catalytic amounts (5-10 mol %) of tetra-butylammonium fluoride, giving the aldol silyl ethers in high yields (equation 11). 

Although detailed investigations are only known for enolate ions1 2, the effects observed in those cases should also be taken into account for nitroates and carbanions since they may influence the outcome of stereoselective protonations. Principally, all of these anionic species occur as ion pairs and their aggregates, which equilibrate slowly under the usual conditions (low temperature, diethyl ether or THF as solvent). Therefore, the very rapid protonation reaction may result in different stereoselections from each of the aggregates. 

Recently, polydentate dilithium alkoxides (dilithium triethylene glycoxides) (Scheme 12) have been shown to be suitable additives for the polymerization of methyl methacrylates, as they provide high initiator efficiencies and narrow molecular weight distributions (1.1-1.3). The addition of dilithium triethylene glycoxide to the anionic polymerization of MMA (THF, (l,l-diphenylhexyl)lithium as initiator) resulted in the synthesis of well controlled polymers even at relatively high temperatures. This beneficial effect could be assigned to a better coordination with the enolate ion pairs, thus slowing down the polymerization rates (Table 5) [197]. 

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