Carbanions elimination reaction intermediates

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 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)... 

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... 

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. 

Obviously, tetrachloroethylene (146) which is sufficiently active owing to the four chlorine atoms, could react only via the addition-elimination route, and the fraras-disubstituted product (149) is formed by two such consecutive steps (Truce and Kassinger, 1958b Truce et al., 1965). It is interesting that the monothioaryl derivative (147) reacts only in the presence of base. Since elimination-addition is impossible, this was taken as indication that the base is required for the formation of the carbanion (148) which should be the reaction intermediate in this case. The tetrasubstituted product is obtained under drastic conditions only. 

From this, it can be seen that the amount of KOH within the hydroxide mixture would probably be critical in removing organo-sulfur from coal. While the particular role of KOH has not been determined, evidence from the literature has shown that the size of the cation may be important in stabilizing intermediate carbanions. Wallace et al. (J ) conducted a series of base- catalyzed, beta-elimination reactions with isopropyl sulfide and measured the amount of olefin production. The proposed mechanism involved initial abstraction of a proton by the t-butoxide base, and formation of a carbanion, with subsequent elimination of the sulfur moiety (which can be considered a good leaving group) to form the olefin (Equation 5). 

Carbanions are involved as intermediates in ElcB elimination reactions. 

In the bimolecular concerted beta elimination reaction, E2, heterolytic cleavage of the C—X and C—H bonds takes place within the same reaction step, without formation of an intermediate (see Vol. 9). It appears that the energy barrier for the concerted process is lower than each of the barriers for the separate steps involving either a carbanion or a carbonium ion intermediate. 

We studied both carbanions and carbocations using the stereochemical fate of stereogenic centers as a probe of reaction mechanism and solvation phenomena. These studies involved substitution, elimination, and rearrangement reactions, most of which involved carbanions and carbocations as short-lived reaction intermediates. The chiral systems were designed and synthesized, the kinetics of the reactions were examined, and the... 

As mentioned in the previous section, the Peterson reaction proceeds by an irreversible addition of the silyl-substituted carbanion to a carbonyl. It has generally been assumed that an intermediate p-oxidosi-lane is formed and then eliminated. In support of this mechanistic hypothesis, if an anion-stabilizing group is not present in the silyl anion, the p-hydroxysilanes can be isolated fixrm the reaction, and elimination to the alkene carried out in a separate step. Recent studies by Hudrlik indicate that, in analogy to the Wittig reaction, an oxasiletane (304) may be formed directly by simultaneous C—C and Si—O bond formation (Scheme 43). The p-hyd xysilanes were synthesized by addition to the silyl epoxide. When the base-induced elimination was carried out, dramatically different ratios of cis- to rranr-alkenes were obtained than from the direct Peterson alkenation. While conclusions of the mechanism in general await further study, the Peterson alkenation may prove to be more closely allied with the Wittig reaction than with -elimination reactions. 

The intermediate of the ElcB mechanism is a carbanion, and thus any factors that stabilise such an ion should favour this mechanism. We have already noted above that on the face of it, elimination reactions are the reverse of addition reactions. However, we also noted that the actual mechanistic pathways involved in elimination reactions were more similar to substitution reactions than addition reactions. This is because normally elimination reactions proceed via a carbonium ion or in a single step that has certain similarities to an SN2 substitution reaction. However, there are also addition reactions that proceed via a carbanion intermediate, for example the Michael-type reaction, in which a carbanion adds to an a,(3-unsaturated carbonyl compound. Indicate the Michael-type addition between the anion formed from the diester of propandioic acid (or malonic acid) and 2-butenal. 

Table 7-2 summarizes kinetic data for the reaction of O2 with esters, diketones, and carbon dioxide.35,37-39 Esters react with superoxide ion to form diacyl peroxides or the carboxylate and the alcohol. Initial reaction occurs via a reversible addition-elimination reaction at the carbonyl carbon (Scheme 7-9). This conclusion is supported by the products that are observed in the gas-phase reaction of O2 with phenyl acetate and phenyl benzoate, which has been studied by Fourier-transform mass spectrometry.40 in effect, there is a competition between loss of O2 and loss of the leaving group. Carbanions are poor leaving groups, so that simple ketones without acidic a-hydrogen atoms are unreactive. The KC(O)OO- radical should be a reactive intermediate for the initiation of the autoxidation of allylic hydrogens (see Chapter 5). 

The metallation of diethyl 3-(trimethylsilyl)-l-propenylphosphonate with LDA at low temperature gives the delocalized allylphosphonate carbanion, which reacts regioselectively in the a-posi-tion with aliphatic or aromatic aldehydes to afford the corresponding P-alcohol as a mixture of diastereomers. " Subsequent dehydration with DCC in CH2CI2 at reflux in the presence of catalytic CUCI2 stereospeciflcally produces the desired 2-phosphonylated 1,3-dienes. A one-pot approach is based on the P-elimination reaction of an intermediate enol phosphate using z-BuOK. ... 

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