Alkylation carbanions

For leading references see S. Wolfe, L. A. LaJohn and D. F. Weaver, Tetrahedron Letters, 25, 2863 (1984) Cor study of a-lithiosulfinyl carbanion initiated oligomerization, see M. Buese and T. E. Hogan-Esch, J. Amer. Chem. Soc., 107,4509 (1985) for a study of a-lithiosulfinyl carbanion alkylation in a cyclic system, see Y. Tamura, J. Uenishi and H. Ishibashi, Chem. Pharm. Bull., 32, 891 and 898 (1984). 

In general, the technique of carbanion alkylation is the method of choice for providing a broad series of compounds for the study of electron-transfer reactions. In this way, indeed, it is possible systematically to vary... 

As already discussed (Section 3.1.1) the elimination of, for instance, neopentane from penta(neopentyl)tantalum corresponds to an a-deprotonation of one alkyl ligand by another, the latter being eliminated as neopentane. Hence in the reverse reaction the carbene carbon atom of the (nucleophilic) carbene complex must formally deprotonate the incoming alkane with simultaneous electrophilic attack of the metal at the newly formed, carbanionic alkyl group (Figure 3.36). 

The choice of the base and the solvent is crucial for the yield of a-sulfinyl carbanion alkylation. A high diastereoselection (80%) was observed in the alkylation of an a-sulfinyl carbanion with a-bromoacrylate56. In this ease the choice of the base appears to be decisive the highest asymmetric induction is found when the metalation is carried out using highly hindered bases, e.g., lithium tetramethylpiperidide. An interesting asymmetric synthesis of chiral 5-alkyl(or phenyl)dihydro-3-methylene-2(3f/)-furanoncs is based on this reaction56. 

Replacement of the pyridine ring by a more strongly basic ethylpiperidine moiety leads to the antiarrhythmic dmg disobutamide (55-5). The synthesis of this compound also involves successive carbanion alkylation reactions. Thus, reaction of the anion from ortho-chloroacetonitrile (55-1) with A-(2-chloroethyl) piperidine gives the intermediate (55-3) alkylation of the anion from this leads to (55-4). Hydrolysis with sulfuric acid completes the preparation of disobutamide (55-5) [56]. 

Base-catalyzed carbanionic alkylation, isomerization, polymerization reactions are of major significance. Base-catalyzed alkylation of alkylarenes, in contrast to acid-catalyzed ring alkylation, leads to alkylation of the side chain in the benzylic position [Eq. (1.28) see also Chapter 5] of particular interest is the alkylation of toluene to ethylbenzene (for styrene production). 

An alkyl/aryl ER may, in a few extreme cases, be considered to contain R carbanions alkyls/aryls of clearly covalent character behave chemically as sources of R if E is of lower electronegativity than C. There is a decrease in basicity along the isoelectronic series ... 

Scheme 1.30 Addition of carbanions, alkylation and carboxyla-tion of SWCNTs treatment with sec-butyllithium and subsequent reaction with COz.

The closure of the macrocyclic ring by means of an intramolecular 8 2 reaction is a straightforward approach, and sulfur-stabilized carbanion alkylation has been successfully applied in the synthesis of 14-membered cembranoids. The synthesis of nephthenol (40) and cembrene A (59) is an example of this methodology. In Li and Yue s report, the total synthesis of ( )-sarcophytol M (17) was achieved from ger-aniol (137) through 12 steps and in 8.9% overall yield with an intramolecular nucleophilic addition of a sulfur-stabilized carbanion to a ketone as a key step. This example is the first of the closure of a macrocyclic ring with the intramolecular nucleophihc addition of a sulfur-stabilized carbanion to a ketone (Scheme 6-7). 

Radicals can be generated at the anode by oxidation of carbanions, alkyl borates, alkoxides, and carboxylates (see Chapter 22, Sec. V) and at the cathode by reduction of protonated carbonyl compounds or onium salts (see Chapter 10). Thereby, a wide choice of different radical structures can be mildly and simply obtained from readily available precursors. These radicals are especially suited for coupling and additive coupling reac-... 

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