Carbanion substitution

Intramolecular reactions between donor and acceptor centres in fused ring systems provide a general route to bridged polycyclic systems. The cts-decalone mesylate given below contains two d -centres adjacent to the carbonyl function and one a -centre. Treatment of this compound with base leads to reversible enolate formation, and the C-3 carbanion substitutes the mesylate on C-7 (J. Gauthier, 1967 A. Belanger, 1968). 

With active methylene compounds, the carbanion substitutes for the hydroxyl group of aHyl alcohol (17,20). Reaction of aHyl alcohol with acetylacetone at 85°C for 3 h yields 70% monoaHyl compound and 26% diaHyl compound. Malonic acid ester in which the hydrogen atom of its active methylene is substituted by A/-acetyl, undergoes the same substitution reaction with aHyl alcohol and subsequendy yields a-amino acid by decarboxylation (21). 

Finally, steric effects have an important influence on regioselectivity. This is very clearly demonstrated, for example, in the case of allyl carbanions substituted with silyl groups16. Therefore, despite wide investigation of this topic17, understanding of regiocontrol is still very poor, due to the complexity of the situation. 

For a review of allylic and benzylic carbanions substituted by hetero atoms, see Biellmann Duccp Org. React. 1982, 27. 1-344. 

Scheme 5.70. Carbanion-carbene tautomery of vinylic or alkyne-derived carbanions substituted with electron-withdrawing groups.

Carbanion-electrophile disconnections Disconnection should normally occur adjacent to an electron withdrawing group. In each case, one of the compounds derived from the synthon should be able to form a carbanion whilst the structure of other should contain an electrophilic centre. Reconnection is by means of a suitable carbanion substitution or condensation reaction. 

Closed shell anions were also examined and fluorene carbanions substituted in the 9 positions shown to photoeject electron under UV light in EPA glasses at 77 K this process was demonstrated as biphotonic and strongly dependent on the ion pair status slight modifications in the EPA glasses composition modifies the reaction efficiency [57, 58]. 

Depending upon its structure and the experimental conditions employed, this species may undergo a carbanion addition ( Claisen Condensation, p. 185) or a carbanion substitution reaction ( Alkylation, p. 201). 

The second example refers to a-sulfonyl and a-sulfinyl carbanion substituted cyclobutenes and their facile ring-opening reaction 174). Treatment, e.g., of the cyclobutene sulfone 215 with n-butyllithium in THF/hexane between —78 and —30 °C led after 10 min (protonation with an aqueous solution of ammonium chloride) to the diene sulfone 218 in 90% yield. If the reaction mixture is treated at —78 °C with methyl iodide, the carbanion 216 is trapped before it rearranges to the carbanion 217. If the methylation is performed at —30 °C, 217 alone reacts. 

Marshall reported two examples that differed only in the degree of substitution at the allylic position. In one case, with a quaternary allylic carbon, the enantiomeric purity of the product was only 59% ee (Scheme 6.12) [80]. Apparently there is less preference for the carbanionic substitutent to occupy the convex face in preference to the concave face of the transition structure. When the angular methyl is replaced by hydrogen, the chirality transfer is 100%. 

A. Mendoza-Frias. Carbanions substituted with boron and sulphur and studies of isonitrile chemistry. Diss. Abs. Internat. (B), 1977, 38, 2677. 

Muller JFK, Kulicke KJ, Neuburger M, Spichty M (2001) Carbanions substituted by transition metals synthesis, structure, and configurational restrictions of a lithium titanium phosphonate. Angew Chem Int Ed 12 2890-2893. doi 10.1002/1521-3773(20010803) 40 15<2890 AID-ANIE2890>3.0.CO 2-T... 

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