Carbanion crown ether complexes

In an important new application of crown ethers Cram and Sogah have recently reported that potassium bases complexed to chiral crown ethers catalyze the stereoselective Michael addition of a /3- ketoester to methyl vinyl ketone in high optical yields (81CC625). With chiral crown (46), carbanion (47) gave alkylated products with an optical yield of about 99% enantiomeric excess. These impressive results were rationalized by complex structure (48) in which the crown-complexed K+ and the carbanion form an ion pair. One face of the associated carbanion is shielded from electrophilic attack by the flanking binaphthyl groups and the approach of methyl vinyl ketone occurs in a stereoselective manner. 

Addition of a second crown produces the loose ion pair A, Cr,K, Cr. However, the complexation constant for adding the second crown is 1800 M 1 for the fluorenyl carbanion and only 200 M 1 for the picrate salt. The lower value for picrate may in part be due to less charge delocalization, e.g., the free ion dissociation constant for potassium fluorenyl in TEF is 1.6 x 10 7M (18) as compared to 9.2 x 10 M for potassium picrate (17). The two N02 substituents close to the 0 bond in picrate may also hinder the enlargement of this ionic bond and the insertion of a crown ether molecule because of electronic or sterlc effects. 

Chiral crown ether phosphine-palladium complexes have been used to catalyse the alkylation of carbanions derived from a-nitro ketones and a-nitro esters,63 and proline rubidium salts have been used to catalyse asymmetric Michael addition of nitroalkanes to prochiral acceptors 64 80% enantioselectivity can be achieved in each case. 

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. 

The closely related research on polyether chelates by Michal Szwarc and his co-workers led to a detailed determination of the structure and properties of carbanions in ion pairs and free ions. The fundamental principles which were developed and clarified in their numerous publications contribute to an understanding and interpretation of much of the polyamine chelate work as well. More recently the crown ether chelates, pioneered by Pederson and co-workers at the Dupont Laboratories, have given additional impetus to research on chelated alkali metal compounds. Crown ethers and amines are cyclic variations which can provide greater stability and specificity in complexation of cations, particularly the heavier alkali metal ions. 

Decarboxylation. Hunter et al report that addition of 1 eq. of dibenzo-18-crown-6 to a solution of sodium 3-(fluoren-9-ylidene)-2-phenylacrylate (1) in THF greatly accelerates (>10 ) the decarboxylation to give the orange-red carbanion (2). A mixture of (3) and (4) is obtained on work-up. The catalytic effect of the crown ether is believed to involve formation of a complex with the sodium ion with production of a highly reactive separated ion pair. 

Both the carbanion and carbocations are stable provided they contain [An+ 2) K electrons. For example, cyclopentadienyl anion, cyclopropenium cation, and tropyhum cation exhibit unusual stability. Stable carbanions do, however, exist. In 1984 Ohnstead presented the lithium crown ether salt of the diphenylmethyl carbanion from diphenylmethane, butyllithium, and 12-crown-4 at low temperatures. Addition of n-butyUithium to triphenyhnethane in THF at low temperatures followed by 12-crown-4 resulted in a red solution and the salt complex precipitated at —20°C. The central C-C bond lengths are 145 pm with the phenyl ring propelled at an average angle of 31.2° (Scheme 3.11). 

Carbanions have been oxidized under phase transfer conditions in the presence of both crown ethers and cryptates. The substrate which has been most studied is fluorene which undergoes phase transfer catalyzed air oxidation to yield fluorenone in high yield according to equation 11.11 [10, 18]. Crown complexed f-butoxide in... 

The tetrabenzoliuorenid complexes V and VI represent rare examples of T °-coordinated carbanions employing simple Lewis bases, like THF or DME, instead of crown ethers for coordinative saturation of the cation. 

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