Elimination reactions carbanion intermediates

By far the most important of all these mechanisms is addition-elimination. The carbanion intermediate must be stabilised by a group Z such as Ar, COR, CN, RSO, or RS02. The reaction then becomes a conjugate substitution as 10 to 12. It appears there that, if the intermediate 11 has a lifetime longer than bond rotation, only the more stable of the two products (E in this case) will be formed. However, there are quite a few cases where the first step is rate-determining and the intermediate has a very short lifetime indeed. These reactions go with retention of configuration. An important example is the replacement of halides by thiolate nucleophiles3 in 2-bromo styrenes 19. 

Because of thetr electron deficient nature, fluoroolefms are often nucleophihcally attacked by alcohols and alkoxides Ethers are commonly produced by these addition and addition-elimination reactions The wide availability of alcohols and fliioroolefins has established the generality of the nucleophilic addition reactions The mechanism of the addition reaction is generally believed to proceed by attack at a vinylic carbon to produce an intermediate fluorocarbanion as the rate-determining slow step The intermediate carbanion may react with a proton source to yield the saturated addition product Alternatively, the intermediate carbanion may, by elimination of P-halogen, lead to an unsaturated ether, often an enol or vinylic ether These addition and addition-elimination reactions have been previously reviewed [1, 2] The intermediate carbanions resulting from nucleophilic attack on fluoroolefins have also been trapped in situ with carbon dioxide, carbonates, and esters of fluorinated acids [3, 4, 5] (equations 1 and 2)... 

The nucleophilic reaction of bromotrifluoroethene with alkoxides yields not only the expected addition and addition-elimination products but also a product from a bromophilic reaction of the carbanion intermediate [6] (equation 3) Similar are the reactions of sodium phenoxide with perfluorovinyl ethers in the presence of hexachloroethane or selected vicinal dibromoperfluoroalkanes The intermediate carbanion is trapped in high yield by these sources of Cl or Br, which suggests a... 

The nucleophilic attack of nitrogen bases leads to a variety of products as the result of addition or addition-elimination reactions The regioselectivity resembles that of attack by alcohols and alkoxides an intermediate carbanion is believed to be involved In the absence of protic reagents, the fluorocarbanion generated by the addition of sodium azide to polyfluonnated olefins can be captured by carbon dioxide or esters of fluonnated acids [J 2, 3] (equation I)... 

On the basis of these findings, the reaction of acyl imines with methanesulfony 1 chloride-triethylamine is not expected to proceed via a sulfene intermediate as previously proposed [99]. Again, a carbanion intermediate accounts nicely for the experimental facts. The electrophihcity of the hetero-l,3-diene is exdemely high, therefore the carbanion, formed on reaction of triethylamme with methanesulfonyl chloride, should undergo nucleophilic attack at C-4 of the hetero-1,3-diene faster than sulfene formabon by chloride elimination. 

We have already seen examples of carbanions involved as intermediates, e.g. (40), in elimination reactions, i.e. those that proceed by the ElcB pathway (p. 251), for example ... 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

Since the basic or carbanion intermediate can continue to go to product by Steps 2 and 3, we have a chain reaction which is consistent with the rapid isomerizations which may be obtained using these catalysts. This mechanistic interpretation was proposed in one of the first papers published on this subject (5) it and similar interpretations have been very helpful in bringing about an understanding of base-catalyzed reactions. The chain-reaction sequence may be terminated by reaction with a formation of a material which is not basic enough to metallate the olefin. Such compounds may be polyunsaturated hydrocarbons which may be formed by elimination of hydride ions from a carbanion. 

When no electrophile is present, reduction of carbon tetrachloride leads to di-chlorocarbene by elimination of chloride ion from the trichloromethyl carbanion intermediate. Dichloromethane is the best solvent for this process [79], The car-bene is trapped by reaction with an alkene to form a dichlorocyclopropane (Table... 

A number of methods can assist in identifying and characterizing enol intermediates (as well as eneamine and carbanion intermediates) in enzyme-catalyzed reactions. These include (1) proton isotope exchange (2) oxidation of the intermediate (3) coupled elimination (4) spectrophotometric methods (5) use of transition-state inhibitors (6) use of suicide inhibitors (7) isolation of the enol and (8) destructive analysis. 

The Dow Process utilizes an elimination/addition reaction to convert chlorobenzene to phenol. The proposed mechanism for this reaction is shown in Figure 8-3. The high-temperature reaction begins with chlorobenzene and aqueous sodium hydroxide. Note that this mechanism starts with the hydroxide attacking as a base, beginning dehydrohalogenation to form benzyne. The second hydroxide ion attacks as a nucleophile to form a carbanion intermediate, which behaves as a base in the last step to yield the final product. 

Carbanions play critical roles in a wide variety of reaction pathways. As stated in the Introduction, this chapter will not focus on the synthetic utility of carbanions, but will instead focus on their mechanistic significance. In this section, a sample of important reaction mechanisms that involve transient or relatively short-lived car-banion intermediates will be introduced. As you will see, the key element in these mechanisms is the ability to form a carbanion that is reasonably stable, and often the kinetics of the reactions are dominated by carbanion stability. The role of carbanion intermediates in elimination reactions will be presented in some detail as a way to illustrate some of the methods that have been developed to probe for carbanion intermediates in reaction mechanisms. Other processes including additions and rearrangement reactions will be presented in less detail, but the role of carbanion stability in these reactions will be outlined. 

It is well known that base-induced elimination reactions can proceed either by a single, concerted step (E2), or by two steps, proton transfer and leaving group expulsion, with a carbanion intermediate (ElcB) to yield an alkene. " The... 

The proposed mechanism of this reaction is composed by an initial S v2-type nucleophilic substitution reaction of 113 with the nucleophilic a-sulfonyl lithium carbanion to give the alkylmagnesium species (114) having a sulfonyl group at the / -position. Then, a -elimination reaction of magnesium sulfinate from the intermediate (114) occurs... 

A practical synthesis of bicyclo[1.1.0]butane-l-carbonitrile (67) from 3-chlorocyclobutane-l-carbonitrile by treatment with potassium Ze/7-butoxide in ttvh-butyl alcohol has been reported.25 Detailed mechanistic studies have also been carried out on the elimination of hydrogen chloride from bww-3-chlorocyclobutane-l-carbonitrile (66) and cis-3-chlorocyclobutane-l-carbonitrile (68), yielding 67.26 28 It has been reported that the elimination reactions of the two isomers are likely to proceed via a common carbanion intermediate. However, in the presence of a crown ether, the carbanion intermediates derived from the trans-isomer 66 and the ds-isomer 68 are found experimentally not to be identical. These intriguing results are attributed to the presence or absence of the potassium cation in assisting the expulsion of the leaving chloride ion.26... 

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. 

An example of an a-ketol formation that does not involve decarboxylation is provided by the reaction catalyzed by transketolase, an enzyme that plays an essential role in the pentose phosphate pathway and in photosynthesis (equation 21) (B-77MI11001). The mechanism of the reaction of equation (21) is similar to that of acetolactate synthesis (equation 20). The addition of (39) to the carbonyl group of (44) is followed by aldol cleavage to give a TPP-stabilized carbanion (analogous to (41)). The condensation of this carbanionic intermediate with the second substrate, followed by the elimination of (39), accounts for the observed products (B-7IMIHOO1). 

Figure 14-5 Some reactions of Schiff bases of pyridoxal phosphate, (a) Formation of the quinonoid intermediate, (b) elimination of a (3 substituent, and (c) transamination. The quinonoid-carbanionic intermediate can react in four ways (1—4) if enzyme specificity and substrate structure allow.

Cyclopropanations are known for several other carbanionic intermediates of the general type (7), in which the substituent G is ultimately lost as an anionic leaving group in the last step of the ring-forming pathway (see Scheme 3 above). The substituent G is most often a functional group based upon sulfur, selenium or nitrogen. Halide-substituted derivatives probably react via the a-elimination pathway in most cases (see Section 4.6.3.1), but in some reactions with electron deficient alkenes as substrates, the normal order of steps may be altered (e.g. Table 10, ref. 162). 

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