Hydration base-catalyzed

Mechanism of Base-Catalyzed Hydration The base catalyzed mechanism (Figure 17 5)... 

FIGURE 17 6 Potential en ergy diagram for base catalyzed hydration of an aldehyde or ketone... 

The mechanism of this reaction is outlined m Figure 17 8 It is analogous to the mech anism of base catalyzed hydration m that the nucleophile (cyanide ion) attacks the car bonyl carbon m the first step of the reaction followed by proton transfer to the carbonyl oxygen in the second step... 

Mechanism of Base-Catalyzed Hydration The base-catalyzed mechanism (Figure 17.5) is a two-step process in which the first step is rate-detennining. In step 1, the nucleophilic hydroxide ion attacks the carbonyl group, forming a bond to carbon. An alkoxide ion is the product of step 1. This alkoxide ion abstracts a proton from water in step 2, yielding the geminal diol. The second step, like all other proton transfers between oxygen that we have seen, is fast. 

Step from general acid-catalyzed formation of immonium ions to general base-catalyzed hydration of these ions to the amino alcohol [Eq. (6)]. 

For the acid-base catalyzed hydration and dehydration of organic bases, such as pteridine, the equations for and k are ... 

The nucleophilic addition of water to an aldehyde or ketone is slow under neutral conditions but is catalyzed by both base and acid. The base-catalyzed hydration reaction takes place as shown in Figure 19.4. The nucleophile is the... 

Process to view an animation of the base-catalyzed hydration of a carbonyl. 

The base-catalyzed hydration of 2-phenyloxirane involves nucleophilic attack preferentially at C(3) (0-C(3) cleavage), but with only partial regio-selectivity. Acid-catalyzed hydration is mainly by 0-C(2) cleavage. The hydration of 2-phenyloxirane catalyzed by epoxide hydrolase is characterized by its very high regioselectivity for the less-hindered, unsubstituted C(3) [175] [176], involving retention of configuration at C(2). In other words, (R)-and (5)-2-phenyloxirane are metabolized to (/ )- and (S)-l-phenylethane-l,2-diol (10.118), respectively. Substrate enantioselectivity was also character-... 

Fig. 10.27. Mechanistic and stereochemical aspects of the hydration of trans-(R,K)-anethole epoxide in the absence (Reactions a and b) and presence of epoxide hydrolase (Reaction c) [178][179]. The ratio of Reaction a to bis ca. 4 1 in acid- and base-catalyzed hydration to yield the threo- and erythro-diols, respectively. The enzymatic Reaction c also yields preferentially the erythro-diol.

Jarsjo J, Destouni G, Yaron B (1994) Retention and volatilization of kerosene Laboratory experiments on glacial and post glacial sods. J Contam Hydrol 17 167-185 Jarsjo J, Destouni G, Yaron B (1997) On the relation between viscosity and hydraulic conductivity values for volatile organic liquid mixtures in soils. J Contam Hydrol 25 113-127 Jensen JL, Hashtroudi H (1976) Base-catalyzed hydration of a, 3-unsaturated ketones. J Organic Chem 41 3299-3302... 

Grunwald also studied the mechanism of the base-catalyzed formation of the hemiacetal, and found it to be the same as that of base-catalyzed hydration (6-1, mechanism a) Grunwald J. Am. Chem. Soc. 1985,107, 4710. Sec also S0rensen Pedersen Pedersen Kanagasabapathy McClelland J. Am. Chem. Soc. 1988,110, 5118 Leussing J. Org. Chem. 1990, 55. 666. 

Fig. 13 Base-catalyzed hydration of a carbonyl compound. Reproduced with permission from ref. 105. Copyright 2000. American Chemical Society.

Base-catalyzed hydration of conjugated carbonyls, followed by retro-aldol fragmentation has been a common strategy for studying the reaction cascade (1-4). The kinetically important step in the base-catalyzed hydration of an alpha/beta unsaturated carbonyl is similar to a nucleophilic substitution reaction at carbon 3. The reaction cascade proceeds rapidly from the conjugated carbonyl through its hydration and subsequent fragmentation. 

The base-catalyzed hydration is a perfect example of base-catalyzed addition to a carbonyl group. The strong nucleophile adds, then protonation gives the hydrate. 

The base-catalyzed hydration reaction takes place as shown in Figure 19.4. The attacking nucleophile is the negatively charged hydroxide ion. 

Mechanism of base-catalyzed hydration of a cetone or aldehyde. Hydroxide ion is a more reactive nucleophile than neutral water. 

In accordance with the above discussion, general base catalysis is not found in thiol addition reactions to aldehydes and ketones only specific base catalysis is prevalent (Lienhard and Jencks, 1966). This is in contrast to the general base-catalyzed hydration of ketones or aldehydes. The reactions of the carbonyl group at the carboxylic acid level of oxidation have much in common with the reactions of the carbonyl group at the aldehyde or ketone level of oxidation. In an excellent review on simple carbonyl addition reactions Jencks (1964) has discussed in detail the mechanisms of catalyzed additions to the carbonyl group of ketones and aldehydes. For general base-catalj ed additions the mechanism... 

In the case of apparent general acid catalysis of acetylimidazole hydrolysis, the mechanism can be defined as a specific acid-general base process by comparison with the general base catalysis of N-methyl,N -acetylimidazolium ion. The rate of disappearance of N-methyl,N -acetylimidazolium ion in water at 25° is proportional to the concentration of the basic form of buffer components such as acetate, phosphate, N-methylimidazole, etc., (equation 30) (Wolfenden and Jencks, 1961). The buffer terms show a 1 1 correlation with the general acid-catalyzed rate of acetylimidazole disappearance (Jencks and Carriuolo, 1959) in water at 25°, when the rate expression for the latter reaction is written in terms of equation (32) rather than equation (31), that is, in terms of a general base-catalyzed hydration of protonated acetylimidazole (pX= 3-6). 

In the realm of homogeneous catalysis we often encounter examples of acid- and base-catalyzed hydration-dehydration and hydrolysis, metal-catalyzed hydrolysis and autoxidation, photocatalytic oxidation and reduction, metal-catalyzed electron transfer, acid-catalyzed decarboxylation, photocatalytic decarboxylation, metal-catalyzed free-radical chain reactions, acid-catalyzed nucleophilic substitutions, and enzymatic catalysis. 

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