Vinyl carbanions, activated

Heterocyclic tertiary amines as catalysts for the reaction of activated vinyl carbanions with aldehydes 88T4653. 

Beau, J M, Sinay, P, D-Glycopyranosyl phenyl sulfones their use in a stereocontrolled synthesis of cA-2,6-disubstituted tetrahydropyrans ((3-D-C-glycosides), Tetrahedron Lett., 26, 6189-6192, 1985. Schmidt, R R, Preuss, R, Betz, R, Vinyl carbanions. 33. C-1 lithiation of C-2 activated glucals. Tetrahedron Lett., 28, 6591-6594, 1987. 

The related (Z)-lithium dialkenylcuprates (147) derived from acetylene itself also react well with epoxides to provide a useful route to (2)-homoallylic alcohols the lack of reactivity with esters allows an easy access to lactones (148) by condensations between epoxy esters and this type of cuprate (Scheme 29). Likewise, the lower homologs (149) and (151), both of which are relatively easy to prepare in optically active forms, can be readily converted into homoallylic and bishomoallylic alcohols (150) and (152) respectively. An ester unit can also be incorporated into the cuprate functions thus, addition of a mixed lithium cuprate, RCuYLi , to ethyl propiolate gives the cuprates (153), which add to epoxides to give unexpectedly the (Z)-crotonates (154). Such isomerization is not uncommon with vinyl carbanions in general, and is obviously a limitation when isomeric mixtures are produced. 

Amyes TL, Wood BM, Chan K, Gerlt JA, Richard JP. Formation and stability of a vinyl carbanion at the active site of orotidine 5 -monophosphate decarboxylase pKa of the C-6 proton of enzyme-bound UMP. J Am Chem Soc. 2008 130 1574-1575. 

An optically active sulfoxide may often be transformed into another optically active sulfoxide without racemization. This is often accomplished by formation of a new bond to the a-carbon atom, e.g. to the methyl carbon of methyl p-tolyl sulfoxide. To accomplish this, an a-metallated carbanion is first formed at low temperature after which this species may be treated with a large variety of electrophiles to give a structurally modified sulfoxide. Alternatively, nucleophilic reagents may be added to a homochiral vinylic sulfoxide. Structurally more complex compounds formed in these ways may be further modified in subsequent steps. Such transformations are the basis of many asymmetric syntheses and are discussed in the chapter by Posner and in earlier reviews7-11. 

The consecutive reaction of vinyl halides and alkenes with activated methylene systems [42] in the presence of a palladium catalyst and phase-transfer catalyst results from the addition of the methylene carbanion with the initially formed Heck product (Scheme 6.31) an intramolecular version of the reaction leads to the formation of bicycloalk-l-enes (Scheme 6.31) [42], The analogous combined coupling reaction of iodoarenes and activated methylene compounds with non-conjugated dienes under similar conditions forms the monoalkene (Scheme 6.31) [43]. 

Michael-Additions The most important application of this type of reaction is the carbon-carbon bond formation p to a carbonyl function by the addition of carbanion. The asymmetric version of the Michael-addition has been known since 1973. An optically active vinyl sulfoxide served as chiral auxiliary198). 

In the Horner-Emmons reaction (Scheme 3), the sulfonylphosphonate carbanion 5 is formed in the presence of NaH and then reacts with an aldehyde to produce the intermediate 6 that undergoes in situ elimination to yield the vinyl sulfones and phosphonate anion. The sulfonyl group can stabilize the anion in the sulfonylphosphonate 5. The vinyl sulfones that are produced by this method using aldehydes as starting materials are exclusively the E (trans) isomers. The E-isomers of the vinyl sulfones are shown in the NMR spectra based on the coupling constants of the vinylic protons. Although strongly basic conditions are used in the Horner-Emmons reaction and a-amino aldehydes are easily racemized, the amino acid vinyl sulfones prepared by this method still show substantial optical activity. However, the enantiomeric purity of these compounds has not been determined. 5 ... 

Diphenylmethylcarbanions. The carbanions based on diphenyknethane (pKa = 32) (6) are useful initiators for vinyl and heterocyclic monomers, especially alkyl methacrylates at low temperatures (94,95). Addition of lithium chloride or lithium /W -butoxide has been shown to narrow the molecular weight distribution and improve the stability of active centers for anionic polymerization of both alkyl methacrylates and tert-huXyi acrylate (96,97). Surprisingly, these more stable carbanions can also efficiendy initiate the polymerization of styrene and diene monomers (98). 

The carbanionic theory is inadequate for systems with vinylic fluorines. Both vinylic fluorines are replaced in polyfluorocyclobutene (Park et al., 1949) (Table 3) while 1,2-dichlorotetrafluorocyclobutene (24) gives the triether (30). The differences were ascribed to higher reactivity of the vinylic fluorine compared to the chlorine, as a result of the steric and the electronic factors already discussed (Stockel et al., 1964, 1965). Both chlorines of (24) are replaced if an activating group is introduced in the first substitution step, as in the formation of (80) (Frank, 1965). 

The production of the (Z)-haloalkenes is thought to proceed via initial exchange of the tetrafluoroborate and halide ions and collapse of the resulting vinyliodonium halides by the addition-elimination (Ad-E) mechanism (equations 203 and 204)84. As with Ad-E reactions of moderately activated vinyl halides (X = Cl, Br), which typically occur with configurational retention (> 95%)143 145, the intermediate carbanions apparently prefer a least motion rotation of 60° prior to the expulsion of iodobenzene. It has been demonstrated by an NMR study that anion exchange between (Z)-(2)-phenylsulfonyl-l-decenyl)-phenyliodonium tetrafluoroborate and tetrabutylammonium chloride occurs instantaneously in deuteriochloroform84. Furthermore, when authentic halide salts of the... 

The structurally related optically active a-acyl vinyl p-tolyl sulfoxide 269 underwent asymmetric cyclopropanation. Michael addition of the carbanion of bromomalonate to 269 and the subsequent intramolecular alkylation yielded the corresponding optically active a-acyl-cyclopropane 271, with a high degree of diastereoselectivity (Scheme 70).142 It was proposed that the stereochemical outcome of the reaction can be rationalized by transition state 270, in which there is chelation of the oxygen atom of the carbonyl and sulfinyl groups to the metal cation. 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

Formaldehyde possessing a strongly polarised carbonyl bond is very susceptible to either cationic or anionic addition polymerization. With a basic initiator the active centre is an alkoxide ion in contrast to the carbanion of a vinyl monomer. The readiness with which polymerization occurs makes initiation by relatively weak covalent bases possible. 

---