Carbanions addition reaction intermediates

It should be noted that the reaction of [54] with either hydroxide ion on alumina or piperidine yields [56] (Ballester et al., 1967). This is excellent additional evidence in favour of those carbanions being reaction intermediates in dealkylation. As a nucleophile, iodide ion also causes dealkylation. 

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)... 

In the case of carbanion and radical intermediates the solvent is less important but the products are partially determined by the resistance of the medium to proton or hydrogen atom abstraction respectively. The increased stability of these intermediates compared with carbonium ions allows the reaction mechanism to be more readily modified by the addition of trapping agents. For example, carbanions are trapped in high yields by the presence of carbon dioxide in the electrolysis medium (Wawzonek and Wearring, 1959 Wawzonek et al., 1955). 

The decarboxylation reaction usually proceeds from the dissociated form of a carboxyl group. As a result, the primary reaction intermediate is more or less a carbanion-like species. In one case, the carbanion is stabilized by the adjacent carbonyl group to form an enolate intermediate as seen in the case of decarboxylation of malonic acid and tropic acid derivatives. In the other case, the anion is stabilized by the aid of the thiazolium ring of TPP. This is the case of transketolases. The formation of carbanion equivalents is essentially important in the synthetic chemistry no matter what methods one takes, i.e., enzymatic or ordinary chemical. They undergo C—C bond-forming reactions with carbonyl compounds as well as a number of reactions with electrophiles, such as protonation, Michael-type addition, substitution with pyrophosphate and halides and so on. In this context,... 

Two types of intermediates, i.e., radicals or carbanions or their organometallic equivalents, can be used to perform addition reactions to Michael acceptors. The free-radical route has already been investigated with nickel or cobalt complexes as catalysts [62-64]. These studies have been reinvestigated recently with the aim of improving the turn-over of the catalyst and/or using easily prepared cheap complexes. 

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. 

The reactions to be discussed in this section involve carbanion addition to carbonyl centers with a potential leaving group. The tetrahedral intermediate formed in the addition step then reacts by expulsion of the leaving group. The overall transformation results in the... 

The 1,2,5-thiadiazole-l-oxide (335) when treated with a strong base in refluxing xylene gave the 1,2,4-thiadiazole (337) in low yield. The reaction is thought to proceed by carbanion addition to a sulfinylamine intermediate (33(6) (Scheme 74) <86H(24)l 193). 

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. 

Nucleophiles can be added to acceptor-substituted alkenes. In that case, enolates and other stabilized carbanions occur as intermediates. Reactions of this type are discussed in this book only in connection with 1,4-additions of organometallic compounds (Section 10.6), or enolates (Section 13.6) to a,/J-unsaturated carbonyl and carboxyl compounds. 

The mechanism of the thiazolium ion-catalyzed conjugate addition reactions 1s considered to be analogous to the Lapworth mechanism for the cyanide-catalyzed benzoin condensation, the thiazolium yllde playing the role of cyanide. The resulting intermediate carbanion is presumed to be the actual Michael donor. After conjugate addition to the activated olefin, the thiazolium yllde is eliminated to form the product and regenerate the catalyst. 

It is also interesting to mention that in order to arrive at IV the direction of the addition reaction on the C=C bond must be inverted at some point. If the more stable carbanion XVII is formed from the key intermediate XVI, the sequence of events that follows leads inevitably back to this key intermediate (XVI). By contrast, the less stable carbanion XIX, which is equivalent to the also unfavored X, is the only way to reach the target product IV. This less... 

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. 

Carbonyl compounds react via the tetrahedral mechanism. In this case, on the addition of the nucleophile, the negative charge is borne by the oxygen atom, which is an inherently more stable intermediate than the carbanion version. The tetrahedral intermediate may be isolated when a suitable substrate is used. An example is the formation of the cyanohydrin on the addition of a cyanide ion, which we studied in the chapter on addition reactions to a carbon/ oxygen system. 

Although the reaction of hydroxide or alkoxide ions with carbonyl compounds to abstract a proton from a carbon atom adjacent to the carbonyl group is thermodynamically unfavourable, and the equilibria in reactions such as (4.11) and (4.12) lie well over to the left, the high reactivity of the carbanion intermediates formed makes reactions of this type of vital importance in a number of cases, especially in halogenation (reaction 4.14) and in some carbonyl addition reactions, e.g. reactions (4.20) and (4.30). 

Stereospecific synthesis of arylidene and allylidene cyclopentanes and cyclohexanes can be achieved by palladium-catalyzed cyclization of carbanions containing a C — C triple bond in the presence of aryl or vinyl halides52 33". The reaction is stereoselective, occurring with irans selectivity. Thus, a palladacyclic intermediate, resulting from initial aryl or vinyl carbapallada-tion, is ruled out in favor of primary carbanion addition to the alkyne unit. 

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