Carbanions from unsaturated compounds

The carbanions can be generated from unsaturated compounds either by reduction or by addition. 

In that same year, a rather revolutionary publication appeared [35] on The Scope of the Reaction Between Carhanions or Nitron ions and Unsaturated Electron Acceptors. Many carbanions and nitranions seemed to react with unsaturated molecules, such as nitroaromatics, azobenzene, and diaryl ketones, to form the radical anions derived from the unsaturated compounds. The same effect was observed with n-butylmagnesium bromide and n-butyllithium. The radical anions were observed with the aid of electron spin resonance... 

Unlike the carbanions from 1,3-dithians (see Chapter 3, p. 31 and Chapter 6, p. 90), the sulfinyl carbanions have the ability to undergo Michael additions (conjugate or 1,4-additions) with a,p-unsaturated carbonyl compounds. For instance, the secondary carbanion (44) from the ethyl ethylthiomethyl sulfoxide (45) may be sucessively reacted with ethyl iodide and 3-butene-2-one to give heptan-2,5-dione (46) via the tertiary carbanion (47), as shown in Scheme 23. The carbanion (47) may also be condensed with propyl bromide, and hydrolysis of the product yields ethyl propyl ketone (48) (Scheme 23). 

New routes to heterocycles from unsaturated carbanions, heterocumulenes and thiocarbonyl compounds 01EJO4569. 

The three unsaturated compounds remain to be identified. On adding a silver-ammonia solution to a sample from each of the three 1-pentyne will be observed to form a precipitate. 1-Pentyne (HC=C-CH2CH2CH3) has an acidic hydrogen (H-C=) and in NH3 solution forms the carbanion CH3 (CH2)2C=C . This is fairly nucleophilic and reacts with the silver ion to form a precipitate ... 

The resonant anion can act as a typical carbanion and add to C=0 or C=N bonds, just as a Gtignard reagent does. In a true aldol condensation, the anion after formation proceeds to interact with another molecule of the starting aldehyde or ketone, giving a product that is a beta-hydroxy aldehyde or ketone known as an aldol (Scheme 4.29). Frequently, water is eliminated from this compound to produce an a, P-unsaturated carbonyl compound this process is made easy by the resonance stabilization of such conjugated systems. 

The selenium stabilized carbanion (6), derived from a ketone, has been converted into both a-selenoacids and esters (Scheme 2) which, on oxidative removal of the selenium, afford the corresponding a -unsaturated compounds. Julia has shown that the versatile sulphone stabilized carbanions can be converted into... 

In order to prepare conjugated olefins, either an allylsulfone or an a,) -unsaturated carbonyl compound is needed. In the case of allylic sulfones, the delocalized carbanion can undergo either a- or y-addition, and the use of enals or enones can result in 1,2- or 1,4-addition products. In general, allylsulfonyl anions, derived from non-stabUized sulfones, undergo additions to carbonyls with excellent a-selectivities (reaction 3.17 [59] Scheme 3.16). The situation is more complex with a, -unsaturated compounds, but conditions have been defined that lead with high preference to adducts resulting from 1,2-addition (reaction 3.18) [60]. 

The high reactivity of carbanions with respect to unsaturated compounds results in polymerization. Polymerization occurs under the action of strong bases in aprotic media and begins from the addition of R or NHj to the monomer, for example,... 

Sulfoxides (R1—SO—R2), which are tricoordinate sulfur compounds, are chiral when R1 and R2 are different, and a-sulfmyl carbanions derived from optically active sulfoxides are known to retain the chirality. Therefore, these chiral carbanions usually give products which are rich in one diastereomer upon treatment with some prochiral reagents. Thus, optically active sulfoxides have been used as versatile reagents for asymmetric syntheses of many naturally occurring products116, since optically active a-sulfinyl carbanions can cause asymmetric induction in the C—C bond formation due to their close vicinity. In the following four subsections various reactions of a-sulfinyl carbanions are described (A) alkylation and acylation, (B) addition to unsaturated bonds such as C=0, C=N or C= N, (C) nucleophilic addition to a, /5-unsaturated sulfoxides, and (D) reactions of allylic sulfoxides. 

Reaction of the carbanion of chloromethyl phenyl sulphoxide 409 with carbonyl compounds yields the corresponding 0-hydroxy adducts 410 in 68-79% yield. Each of these compounds appears to be a single isomer (equation 242). Treatment of adducts 410 with dilute potassium hydroxide in methanol at room temperature gives the epoxy sulphoxides 411 (equation 243). The ease of this intramolecular displacement of chloride ion contrasts with a great difficulty in displacing chloride ion from chloromethyl phenyl sulphoxide by external nucleophiles . When chloromethyl methyl sulphoxide 412 is reacted with unsymmetrical ketones in the presence of potassium tcrt-butoxide in tert-butanol oxiranes are directly formed as a mixture of diastereoisomers (equation 244). a-Sulphinyl epoxides 413 rearrange to a-sulphinyl aldehydes 414 or ketones, which can be transformed by elimination of sulphenic acid into a, 8-unsaturated aldehydes or ketones (equation 245). The lithium salts (410a) of a-chloro-/ -hydroxyalkyl... 

Amines, thiols, eOH (p. 226), etc., will also add to the 0-carbon atom of 0-unsaturated carbonyl compounds and esters, but the most important reactions of C=C—C=0 systems are in Michael reactions with carbanions reactions in which carbon-carbon bonds are formed. A good example is the synthesis of l,l-dimethylcyclohexan-3,5-dione (dimedone, 100) starting from 2-methylpent-2-ene-4-one (mesityl oxide, 101) and the carbanion 0CH(CO2Et)2 ... 

The synthesis of oc/S-unsaturated sulphoxides from the anion (122) has been described.113 The reaction is non-stereospecific, but good yields are obtained from cyclopentanones and acetophenones. Good yields of mono-alkylated products are obtained from the reaction of carbanions of diethyl 2-oxophosphonates (123) and reactive halides 114 alkylation of the compounds (124) has also been achieved.115 The... 

Unsaturated fluorinated compounds are fundamentally different from those of hydrocarbon chemistry. Whereas conventional alkenes are electron rich at the double bond, fluoroal-kenes suffer from a deficiency of electrons due to the negative inductive effect. Therefore, fluoroalkenes react smoothly in a very typical way with oxygen, sulfur, nitrogen and carbon nucleophiles.31 Usually, the reaction path of the addition or addition-elimination reaction goes through an intermediate carbanion. The reaction conditions decide whether the product is saturated or unsaturated and if vinylic or allylic substitution is required. Highly branched fluoroalkenes, obtained from the fluoride-initiated ionic oligomerization of tetrafluoroethene or hexafluoropropene, are different and more complex in their reactions and reactivities. 

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