Addition-elimination base-catalyzed

There are several methods for generation of benzyne in addition to base-catalyzed elimination of hydrogen halide from a halobenzene, and some of these are more generally applicable for preparative work. Probably the most convenient method is diazotization of o-aminobenzoic acid. Concerted loss of nitrogen and carbon dioxide follows diazotization and generates benzyne. Benzyne can be formed in this manner in the presence of a variety of compounds with which it reacts rapidly. 

The mechanistic pattern established by study of hydration and alcohol addition reactions of ketones and aldehydes is followed in a number of other reactions of carbonyl compounds. Reactions at carbonyl centers usually involve a series of addition and elimination steps proceeding through tetrahedral intermediates. These steps can be either acid-catalyzed or base-catalyzed. The rate and products of the reaction are determined by the reactivity of these tetrahedral intermediates. 

During the course of base-catalyzed exchange in O-deuterated alcohols, the vinylic hydrogen in the a position to the ketone is replaced by deuterium, in addition to the hydrogens activated by enolization. Thus, under these conditions the exchange of androst-l-en-3-one (16, R = H) gives a trideuterio derivative (18) instead of the expected 4,4-d2 analog (16, R = D). " (For other examples see compounds 13, 19, 21, 23, 26 and 27.) Incorporation of this deuterium is due to rapidly reversible alcohol addition (16 -+17) and elimination (17 18) which competes with the enolization step. " ... 

The mechanism of the Fiesselmann reaction between methylthioglycolate and a,P-acetylenic esters proceeds via consecutive base-catalyzed 1,4-conjugate addition reactions to form thioacetal Enolate formation, as a result of treatment with a stronger base, causes a Dieckmann condensation to occur providing ketone 8. Elimination of methylthioglycolate and tautomerization driven by aromaticity provides the 3-hydroxy thiophene dicarboxylate 9. 

Racemization of some substrates can take place through reversible formation of the substrate via an addition/elimination process. The racemization can be acid or base catalyzed. In this section we vill discuss DKR of cyanohydrins and hemithioacetals. 

Vinyl ethers can also be generated by thermal elimination reactions. For example, base-catalyzed conjugate addition of allyl alcohols to phenyl vinyl sulfone generates 2-(phenylsulfinyl)ethyl ethers that can undergo elimination at 200° C.223 The sigmatropic... 

The details of the base-catalyzed decompositions vary somewhat but the mechanisms involve two essential steps.128 The initial reactants undergo a base-catalyzed addition-elimination to form an alkyl diazoate. This is followed by a deprotonation of the a-carbon and elimination of hydroxide. 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

Before discussmg the mechanism of cleavage of carboxylic acid esters and amides by hydrolases, some chemical principles are worth recalling. The chemical hydrolysis of carboxylic acid derivatives can be catalyzed by acid or base, and, in both cases, the mechanisms involve addition-elimination via a tetrahedral intermediate. A general scheme of ester and amide hydrolysis is presented in Fig. 3. / the chemical mechanisms of ester hydrolysis will be... 

In the present chapter, no explicit discussion or review of the acid-and/or base-catalyzed isomerization of olefins will be included. The discussion will be confined to isomerizations achieved with soluble transition metal complexes. However, it will be seen that addition and elimination reactions and allylic intermediates figure prominently in discussions of the mechanisms. 

The Michael addition of nucleophiles to coumarins catalyzed by solid bases provides an interesting approach to the synthesis of 4-substituted 3,4-dihydrocumarins, because with the conventional Michael catalysts the alkaline hydrolysis of the 8-lactone predominates (Scheme 44). Results were obtained when the Michael addition of diethyl malonate to coumarin was catalyzed by the activated Ba(OH)2 292). An unusual 1,2-addition-elimination process at the C = 0 bond was observed. The mechanism of this reaction was explained on the basis of the microcrystalline structure of the catalyst. It was suggested that the rigid coumarin molecule interacts with the Ba ions through the lone-pair electrons of both oxygen atoms of the... 

Base-catalyzed additions of HCN to s-triazine (1) give 2-cyano-l,2-dihydro-s-triazine (9). Its H-NMR spectrum shows that the hydrogen on the nitrogen is equilibrating rapidly between the 1- and 3- positions. When 9 is heated to 65°C it melts and eliminates HCN (72UP1). 

There are an extremely large number of reactions of 2-oxetanones with nucleophilic reagents, and space will allow inclusion of only representative examples. /3-Lactones show the interesting Bal.2 mechanism for base-catalyzed hydrolysis and the Aal2 mechanism for acid-catalyzed hydrolysis, according to data on kinetics and optical rotation studies of optically active lactones. The mechanistic interpretations are complicated, however, by the possibilities for subsequent elimination and addition reactions to occur, so that both of the two sites for nucleophilic attack on the 0-lactone skeleton, C-2 and C-4, may become involved. In fact 0-lactones are unusually insensitive to base, as well as acid, catalysis, the slow reaction with neutral water predominating between pH 1 and 9 (74JCS(P2)377). 

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