Lithium enolate with crown ethers

The conformational situation for twisted 2,2-diacyl compounds of type 29 is quite different from that for sodium or lithium 1,3-diketone enolates. In the latter, the ZZ form is stabilized by complexation with the cation75, and only in the presence of crown ethers is the EZ form observed76. The barrier to EZ - ZE exchange in the free carbanion is 12.9 kcal mol-1, as expected quite close to that found for 29. 

Thermodynamic control. Note that it is also possible for the aldolate adduct to revert to aldehyde and enolate, and equilibration to the thermodynamic product may afford a different diastereomer (the anti aldolate is often the more stable). The tendency for aldolates to undergo the retro aldol addition increases with the acidity of the enolate amides < esters < ketones (the more stable enolates are more likely to fragment), and with the steric bulk of the substituents (bulky substituents tend to destabilize the aldolate and promote fragmentation). On the other hand, a highly chelating metal stabilizes the aldolate and retards fragmentation. The slowest equilibration is with boron aldolates, and increases in the series lithium < sodium < potassium, and (with alkali metal enolates) also increases in the presence of crown ethers. ... 

Similar effects are also seen with enolates of simple ketones. For isopropyl phenyl ketone, the inclusion of one equivalent of 12-crown-4 in a DME solution of the lithium enolate changes the C 0 ratio from 1.2 1 to 1 3, using methyl sulfate as the alkylating agent. The crown ether selectively solvates the Li+ ion, leaving the anion in a more... 

Primary alkyl amines. a-Alkoxy azides are reduced to primary amines by lithium aluminum hydride in ether. A side reaction noted occasionally, reduction to an ether, can be suppressed by addition of TMEDA. Sodium bis(2-methoxy-ethoxy)aluminum hydride cannot replace lithium aluminum hydride in this reductive amination. The a-alkoxy azides are available by reaction of aldehydes with azidotrimethylsilane, by addition of hydrazoic acid to an enol ether, or by reaction of ketals with the l8-crown-6 complex of sodium azide. 

Topping and co-workers [19] have shown that lithium battery membranes fabricated from a bistrifluorovinyletherarylamide have excellent electromechanical stability at high electric potentials, making them potential candidates for use in battery membranes. Preliminary molecular modelling studies indicated that lithium imine enolates may play a useful role in lithium ion transport along with the crown ether linkage. 

Monoprotected P-keto-aldehydes may be prepared by the a-dialkoxymethyl-ation of pre-formed enolates or enamines with trimethyl orthoformate and boron trifluoride diethyl etherate. Regiospecificity is maintained when the enolate is released from the silyl enol ether with methyl-lithium. P-Keto-acetals or P-diketones may also be formed by acylation of enol ethers with acid chlorides. High yields depended on the use of activated acid chlorides, such as a-halo- or a-cyano-acetyl chlorides. Enaminosilanes are acylated by a wide range of acid chlorides in the presence of potassium fluoride and a crown ether, giving very high yields of the enaminone [equation (46)]. Under the reaction conditions,... 

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