Elimination-addition reactions mechanisms

The synthesis of 4 -thioxy loadcnosine 126, described in the pioneering work by Reist et has been improved" by direct glycosylation of V-benzoyladenine with the 1,4-dithio-D-xylofuranoside 123 in the presence of V-iodosuccinimide and catalytic triflic acid, followed by deprotection. Based on the ratio of isomers obtained (a -.ft 20 34) an iodium ion-promoted elimination-addition reaction mechanism has been proposed, as outlined in Scheme 35. 

Going over the basics and mechanisms of nucleophilic substitution reactions Mastering mechanisms of elimination/addition reactions Determining synthesis strategies for aromatic systems... 

An elimination/addition reaction is another distinct type of reaction mechanism that occurs in aromatic systems. In these mechanisms, the elimination involves the loss of an HX molecule. While this may seem like a dehydrohalo-genation as seen in Organic Chemistry 1, it really is a different reaction. The HX loss leads to the formation of a benzyne intermediate (see Figure 8-2). The mechanism ends with addition to the bond formed by the loss of HX. 

An Sj Ar mechanism is an addition/elimination, not an elimination/addition reaction. 

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. 

Additional Reaction Mechanisms. So far we have confined our discussion to the most common case of ester hydrolysis, that is, the case in which the reaction takes place at the carbonyl carbon. In some cases, however, an ester may also react in water by an SN-type or E-type mechanism (see Section 13.2) with the acid moiety (i.e., "OOC - R,) being the leaving group. The S -type reactions occur primarily with esters exhibiting a tertiary alcohol group. The products of this reaction are the same as the products of the common hydrolysis reaction. In the case of elimination, however, products are different since the ester is converted to the olefin and the corresponding conjugate base of the acid ... 

As exemplified by equation (2), the Perkin condensation of o-hydroxybenzaldehydes is an important method for the synthesis of substituted coumarins. An interesting variation on this procedure has been reported recently. Heating a mixture of o-fluorobenzaldehyde, 2-thiopheneacetic acid, acetic anhydride and triethylamine affords directly the coumarin (20 equation 13) instead of the expected cinnamic acid (21). The reaction proceeds similarly with several arylacetic acids. The reaction presumably proceeds through the cinnamic acids (21). The observed product can conceivably arise by direct nucleophilic displacement involving the carboxylate or by an elimination/addition (benzyne) mechanism. The authors note that when 2-fluorobenzaldehyde is replaced by its 2-bromo analog in this reaction, the substituted cinnamic acid (22) is the major product and the corresponding coumarin (20) is obtained only in low yield. It is suggested that since it is known that fluoride is displaced more rapidly in nucleophilic aromatic substitution reactions, while bromo aromatic compounds form benzynes more rapidly, this result is consistent with a nucleophilic displacement mechanism. 

VII. j3-Elimination-Addition Reactions E2 versus ElcB Mechanisms. 397... 

The elimination-addition reaction for phosphate esters was first proposed to account for the shape of the pH-rate profile of alkyl monoesters (40), which are most reactive at the maximum concentration of the monoanion, that is, pH 4. The explanation for this observation was that the reaction proceeded via a metaphosphate intermediate by the expulsion of an alcohol group with the proton for the alcohol being transferred from the phosphate as the reaction proceeds. Further studies of the reactions of monoesters have implied that the above mechanism operates for all alkyl and aryl monoesters where the p fa of the leaving group exceeds about... 

The benzyne mechanism can be denoted as an Sn(EA) (substitution nucleophilic elimination addition) reaction (reference 7). 

An elimination-addition reaction occurs via a benzyne intermediate. Evidence for this mechanism comes from isotopic labeling experiments as well as a trapping experiment. 

This proposed mechanism is essentially an elimination followed by an addition. So, it makes sense that we call this process an eUmination-addition reaction (as compared with the SNAr mechanism, which was called addition-elimination). This mechanism certainly seems a bit off the wall when you think about. Benzyne It looks like a terrible intermediate. But chemists have been able to show (with other experiments) that benzyne is in fact the intermediate of this reaction. Your textbook or instructor will most likely provide the additional evidence for the short-lived existence of benzyne (we use a trapping technique involving a Diels-Alder reaction). If you are curious about the evidence, you can look in your textbook. For now, let s make sure that we can predict the products of elimination-addition reactions. 

A variety of mechanisms have been show to operate in alkenylation of nucleophiles including ligand coupling, addition-elimination and elimination-addition reactions for anionic nucleophiles and vinylic Sjv2, vinylic Sjvl and elimination-addition reactions for neutral nucleophiles. The mechanisms so far elucidated have been extensively reviewed by Ochiai and Okuyama and herein we provide a summary of these processes. 

Note that for 4.42, in which no intramolecular base catalysis is possible, the elimination side reaction is not observed. This result supports the mechanism suggested in Scheme 4.13. Moreover, at pH 2, where both amine groups of 4.44 are protonated, UV-vis measurements indicate that the elimination reaction is significantly retarded as compared to neutral conditions, where protonation is less extensive. Interestingy, addition of copper(II)nitrate also suppresses the elimination reaction to a significant extent. Unfortunately, elimination is still faster than the Diels-Alder reaction on the internal double bond of 4.44. 

Nucleophilic aromatic substitution (Chapter 23) A reaction m which a nucleophile replaces a leaving group as a sub stituent on an aromatic nng Substitution may proceed by an addition-elimination mechanism or an elimination-addition mechanism... 

When the addition and elimination reactions are mechanically reversible, they proceed by identical mechanistic paths but in opposite directions. In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same in the reverse as in the forward direction. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as... 

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

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)... 

Elimination-addition mechanism (Section 23.8) Two-stage mechanism for nucleophilic aromatic substitution. In the first stage, an aryl halide undergoes elimination to form an aryne intermediate. In the second stage, nucleophilic addition to the aryne yields the product of the reaction. 

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