A-Halo carbanions

Makosza and co-workers have reported the preparation of epoxides from a-halo carbanions and ketones, according to the Darzens reaction, under PT conditions, using TEBA72,73 or dibenzo-18-crown-6.74 The ratio of isomers depends on the reaction conditions.75,76 Asymmetric induction has been reported in the Darzens reaction using chiral catalysts.77,78 The use of several chloro carbanions as well as K2C03 and Na2C03 in the solid state has also been studied. 

Satoh, T. Takano, K. A method for generation of a-halo carbanions (carbenoids) from aryl a-haloalkyl sulfoxides with alkylmetals. Tetrahedron 1996, 52, 2349-2358. 

Many electrophilic alkenes and stabilized a-halo carbanions (derived from a-halogenated ketones, esters, nitriles or sulphones) are reported to be applicable in this condensation (equation 53). Usually the reaction was carried out in the presence of sodium... 

Cyclopropanes. Cyclopropanes are fcH-med by condrasation of a-halo-carbanions with electrophilic alkenes under phase-transfer conditions (equation... 

B-trans-l-Alkenyls-9-BBN prepared in situ in THF react with a-halo carbanions generated from phenacyl bromide to afford the ( )-p,y-unsaturated ketones in good yields (Eq. 7.7 Table 7.13) [9]. 

B-trans-l-Alkenyl-9-BBN undergoes facile reaction with the a-halo carbanion generated from chloroacetonitrile in the presence of potassium 2,6-di-tert-butyl-... 

In the initial step " the a-halo ketone 1 is deprotonated by the base at the a -carbon to give the carbanion 4, which then undergoes a ring-closure reaction by an intramolecular substitution to give the cyclopropanone derivative 2. The halogen substituent functions as the leaving group ... 

Even with a-halocarboxylic acid esters 466 the attack of a-sulphinyl carbanion 467 takes place at the carbonyl carbon atom and not at the a-carbon atom and the corresponding a-halo-a-sulphinyl ketones 468 are obtained in high yields539-540 (equation 279). 

There are very few precedents for the reaction of cyclic a-halo ethers with carbanions. Zelinski and coworkers114 and Schudel and Rice115 reported the preparation of diethyl DL-tetrahydropyran-2-ylmalonate (137) by treatment of 2-bromo- or 2-chloro-tetrahydropyran (136) with diethyl sodiomalonate. The product was subsequently converted into the malonic and acetic acid derivatives, 138 and 139, respectively. The same sequence has also been reported by other workers.116... 

Arnold, R. T. Kulenovic, S. T. Carbanion halo-genations with carbon tetrahalides. a-Halo esters./. Org. Chem. 1978, 43, 3687-3689. 

Takeda and coworkers showed that the reaction of a silylmethyllithium with a halo-methyloxirane gave the corresponding cyclobutane derivatives via a 1,4-silyl migration of the initial lithium alkoxide followed by an intramolecular nucleophilic substitution of the resulting carbanion (equation 151)370. 

Two possible mechanisms can be envisaged for this reduction, both involving the formation and protonation of Sm(III) enolates. Molander proposed that a-halo ketones react with Sml2 to give a ketyl radical 32 that is then quenched by the cosolvent.29 A second reduction then gives carbanion 33 that undergoes (3-elimination to produce the enol tautomer of the product ketone (Scheme 4.20). 

When chloromethyl p-tolyl sulfoxide (102 R = p-tolyl) is treated with carbonyl compounds (103) and potassium t-butoxide in t-butyl alcohol ether, (p-tolylsulfmyl)oxiranes (104) are directly formed. Chloromethyl methyl sulfoxide (102 R = Me) exhibits the same behavior (Scheme 29). These reactions proceed with high stereoselection at the position a to the sulfmyl group. The stereochemical course proposed for the attack of the carbanion of an a-halo sulfoxide on a carbonyl compound is shown in Figure 4. ... 

Carbanion amidoalkylations provide a versatile means of generating a-amino acids. The ability to oxidize alkenes or hydrolyze trichloromethyl groups of amidoalkylation products (entries 5 and 6, Table 7) to acids provides access to these compounds. a-Halo-A -(t-butoxycarbonyl)glycine esters have been used as electrophilic glycinates for the preparation of broadly substituted amino acids (entry 7, Table 7). Several approaches have been examined as possible enantioselective routes to relatively inaccessible, synthetic amino acids. The reactipns of vinylmagnesium bromide with A -Cbz-t-Phe-a-chloro-Gly-OMe resulted in no asymmetric induction as a 1 1 mixture of diastereomeric 3,y-unsaturated... 

It should be stressed that several cyclopropylideneamines 28 have been generated in situ from a-haloimines 3 under basic Favorskii conditions and that nucleophilic addition of the bases used (e.g. alkoxides, hydroxide) led to adducts 29 which, in most cases, were not isolable. Instead, they readily open to give the most stable carbanions (30 or 31). This feature accounts for the formation of branched amides 33 from aliphatic a-halo ketimines and of linear imidates 32 (Nu = OR) or amides 32 (Nu = OH) from a-chloro-a-phenyl ketimines. ... 

The reaction between a-halo ketones and sodium dialkyl phosphites has been extended to phosphonate carbanions. For example, diisopropyl 1 -lithio-1-fluoromethylphosphonate, generated from diisopropyl fluoromethylphosphonate and LDA, reacts with 3-chloro-2-butanone at low temperature in THF to give diisopropyl l-fluoro-2-nicthyl-2,3-cpoxybutylphosphonate in 46% yield. 33.134... 

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