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Alcohols nucleophilic catalysis

If this intermediate, in turn, is more n idly attacked by water or hydroxide ion than the original ester, the overall reaction will be faster in the presence of the nucleophile than in its absence. These are the requisite conditions for nucleophilic catalysis. Esters of relatively acidic alcohols (in particular, phenols) are hydrolyzed by the nucleophilic catalysis mechanism in the presence of imidazole ... [Pg.477]

Pyridine is more nucleophilic than an alcohol toward the carbonyl center of an acyl chloride. The product that results, an acylpyridinium ion, is, in turn, more reactive toward an alcohol than the original acyl chloride. The conditions required for nucleophilic catalysis therefore exist, and acylation of the alcohol by acyl chloride is faster in the presence of pyridine than in its absence. Among the evidence that supports this mechanism is spectroscopic observation of the acetylpyridinium ion. An even more effective catalyst is 4-dimeftiyIaminopyridine (DMAP), which functions in the same wsy but is more reactive because of the electron-donating dimethylamino substituent. ... [Pg.485]

Nucleophilic participation is important only for esters of alcohols that have pK <13. Specifically, phenyl and trifluoroethyl esters show nucleophilic catalysis, but methyl and 2-chloroethyl esters do not. This result reflects the fete of the tetrahedral intermediate that results fi om nucleophilic participation. For relatively acidic alcohols, the alkoxide group can be eliminated, leading to hydrolysis via nucleophilic catalysis ... [Pg.491]

The rate constant /ct, determined by means of Eq. (6-47) or (6-48), may describe either general base or nucleophilic catalysis. To distinguish between these possibilities requires additional information. For example, in Section 3.3, we described a kinetic model for the N-methylimidazole-catalyzed acetylation of alcohols and experimental designs for the measurement of catalytic rate constants. These are summarized in Scheme XVIIl of Section 3.3, which we present here in slightly different form. [Pg.271]

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... [Pg.482]

Another way to esterify a carboxylic acid is to treat it with an alcohol in the presence of a dehydrating agent. One of these is DCC, which is converted in the process to dicyclohexylurea (DHU). The mechanism has much in common with the nucleophilic catalysis mechanism the acid is converted to a compound with a better leaving group. However, the conversion is not by a tetrahedral mechanism (as it is in nucleophilic catalysis), since the C—O bond remains intact during this step ... [Pg.485]

Full details on the phosphorylation of water and alcohols by 4-nitrophenyl dihydrogen phosphate and the NfC H ) - and N(CH3) -salts of its mono- and dianion have been published 146>. Phosphoryl group transfer from the monoanion and dianion is thought to proceed via the monomeric POf ion. Addition of the sterically unhindered amine quinuclidine to an acetonitrile solution containing the phosphate monoanion and tert-butanol produces t-butyl phosphate at a faster rate than does the addition of the more hindered diisopropylethylamine. This nucleophilic catalysis of the phosphorylation reaction is also explained by the intermediacy of the POf ion. [Pg.121]

The hydrolysis of peptides by these proteases represents classic nucleophilic catalysis. The relatively inert peptide is converted to the far more reactive ester or thioester acylenzyme, which is rapidly hydrolyzed. The use of the serine hydroxyl rather than the direct attack of a water molecule on the substrate is favored in several ways alcohols are often better nucleophiles than the water molecule in both general-base-catalyzed and direct nucleophilic attack the serine... [Pg.53]

While our proposed mechanism was interesting, it left some unanswered questions. What was the nature of the catalyst complex and more importantly, why was this not behaving like a classic acid catalysis Boe [11, 12] had postulated protonation of the alkoxy species followed by SN2 attack by the alcohol nucleophile. This is consistent with the negative value that he found for p. This does not agree though with the idea of a catalyst complex, nor does it agree with the findings reported here of a positive value of p. [Pg.175]

In Chapter 12 pyridine was often used as a catalyst in carbonyl substitution reactions. It can act in two ways. In making esters from acid chlorides or anhydrides pyridine can act as a nucleophile as well as a convenient solvent. It is a better nucleophile than the alcohol and this nucleophilic catalysis is discussed in Chapter 12 (p. 282). But nonnucleophilic bases also catalyse these reactions. For example, acetate ion catalyses ester formation from acetic anhydride and alcohols. [Pg.324]

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a Sn2 mechanism.As with 16-57, the mechanism can be S l or tetrahedral. ° Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 16-58). Lewis acids such as lithium perchlorate can be used. [Pg.1411]

Let us return to the reaction we mentioned at the very start of the chapter - the reaction of a chiral alcohol to form an ester. DMAP is used to catalyse the acylation of alcohols reactions by nucleophilic catalysis. [Pg.631]

Vinylsilanes react with chloral in the presence of Lewis acids (Scheme 33), but this type of reaction is little used, probably because the products are allylic alcohols, which are apt to undergo ionization in the presence of Lewis acids to give allyl cations, and hence further reaction. Reactions employing nucleophilic catalysis, although free of this problem, are also limited, only anion-stabilized systems undergoing reaction (Scheme 34). On the other hand, there is less of a problem with 3-elimination of a halide ion, as there would be with most metals 3 to a halogen. ... [Pg.575]

The acylation of alcohols by anhydrides, catalyzed by 4-(dimethylamino)pyridine (DMAP), is one of the most frequently described in the literature examples of nucleophilic catalysis (Figure 5.6). [Pg.159]

In the preceding examples, the asymmetric catalyst is a Lewis acid and hence the catalytic processes reported so far involve electrophilic activation by a metal-centred chiral Lewis acid. There is another strategy, although less explored, which consists of designing chiral Lewis bases for nucleophilic catalysis. It is well known that Lewis bases such as nitrogen heterocycles and tertiary phosphines and amines catalyse a variety of important chemical processes. For instance 4-(dimethylamino)pyridine (DMAP) catalyses the acylation of alcohols by anhydrides the mechanism by which DMAP accelerates this process provides an instmctive illustration of how nucleophiles can... [Pg.91]


See other pages where Alcohols nucleophilic catalysis is mentioned: [Pg.491]    [Pg.387]    [Pg.104]    [Pg.6]    [Pg.19]    [Pg.85]    [Pg.194]    [Pg.123]    [Pg.166]    [Pg.286]    [Pg.175]    [Pg.215]    [Pg.224]    [Pg.215]    [Pg.224]    [Pg.87]    [Pg.350]    [Pg.1090]    [Pg.170]    [Pg.40]    [Pg.671]    [Pg.709]    [Pg.17]    [Pg.215]    [Pg.224]   
See also in sourсe #XX -- [ Pg.665 ]




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Alcohols catalysis

Alcohols nucleophiles

Alcohols nucleophilicity

Nucleophile alcohols

Nucleophile catalysis

Nucleophiles catalysis, nucleophilic

Nucleophilic alcohols

Nucleophilic catalysis

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