Nucleophilic attack, ester hydrolysis

A simple, convenient method for the selective acylation of a primary OH in the presence of a secondary OH group is its conversion into the t-butanoyl ester (also known as a pivalate ester, OPv). Moreover, the steric bulk of the t-butyl group makes these esters resistant to nucleophilic attack, including hydrolysis under mild basic conditions. The pivalate ester can be cleaved using metal hydride reagents. 

We should distinguish between the phrases nucleophilic attack and nucleophilic catalysis. Nucleophilic attack means the bond-forming approach by an electron pair of the nucleophile to an electron-deficient site on the substrate. In nucleophilic catalysis this results in an increase in the rate of reaction relative to the rate in the absence of the catalyst. However, nucleophilic attack may not result in catalysis. Thus, if methylamine is reacted with a phenyl acetate, the reaction observed is amide formation, not hydrolysis, because the product of the nucleophilic attack is more stable than is the ester to hydrolysis. 

DNA is not susceptible to alkaline hydrolysis. On the other hand, RNA is alkali labile and is readily hydrolyzed by dilute sodium hydroxide. Cleavage is random in RNA, and the ultimate products are a mixture of nucleoside 2 - and 3 -monophosphates. These products provide a clue to the reaction mechanism (Figure 11.29). Abstraction of the 2 -OH hydrogen by hydroxyl anion leaves a 2 -0 that carries out a nucleophilic attack on the phosphorus atom of the phosphate moiety, resulting in cleavage of the 5 -phosphodiester bond and formation of a cyclic 2, 3 -phosphate. This cyclic 2, 3 -phosphodiester is unstable and decomposes randomly to either a 2 - or 3 -phosphate ester. DNA has no 2 -OH therefore DNA is alkali stable. 

Acid-catalyzed ester hydrolysis can occur by more than one mechanism, depending on the structure of the ester. The usual pathway, however, is just the reverse of a Fischer esterification reaction (Section 21.3). The ester is first activated toward nucleophilic attack by protonation of the carboxyl oxygen atom, and nucleophilic addition of water then occurs. Transfer of a proton and elimination of alcohol yields the carboxylic acid (Figure 21.8). Because this hydrolysis reaction is the reverse of a Fischer esterification reaction, Figure 21.8 is the reverse of Figure 21.4. 

Nucleophilic attack by water generally results in the cleavage of the amide, glycoside, or ester bonds that hold biopolymers together. This process is termed hydrolysis. Conversely, when monomer units are joined together to form biopolymers such as proteins or glycogen, water is a product, as shown below for the formation of a peptide bond between two amino acids. 

The first observation of a significant rate enhancement in the alkaline hydrolysis of an ester by a suitably positioned carbonyl group that was related to prior attack of hydroxide at the carbonyl group was made in 1955 (Djerassi and Lippman, 1955). However, in 1962, a more detailed mechanistic pathway was suggested which involved attack by hydroxide at an o-formyl or benzoyl group, followed by intramolecular nucleophilic attack on a benzoate ester (Newman and Hishida, 1962 Bender and Silver, 1962). 

The mechanism of phosphate ester hydrolysis by hydroxide is shown in Figure 1 for a phosphodiester substrate. A SN2 mechanism with a trigonal-bipyramidal transition state is generally accepted for the uncatalyzed cleavage of phosphodiesters and phosphotriesters by nucleophilic attack at phosphorus. In uncatalyzed phosphate monoester hydrolysis, a SN1 mechanism with formation of a (POj) intermediate competes with the SN2 mechanism. For alkyl phosphates, nucleophilic attack at the carbon atom is also relevant. In contrast, all enzymatic cleavage reactions of mono-, di-, and triesters seem to follow an SN2... 

The term acid catalysis is often taken to mean proton catalysis ( specific acid catalysis ) in contrast to general acid catalysis. In this sense, acid-catalyzed hydrolysis begins with protonation of the carbonyl O-atom, which renders the carbonyl C-atom more susceptible to nucleophilic attack. The reaction continues as depicted in Fig. 7. l.a (Pathway a) with hydration of the car-bonium ion to form a tetrahedral intermediate. This is followed by acyl cleavage (heterolytic cleavage of the acyl-0 bond). Pathway b presents an mechanism that can be observed in the presence of concentrated inorganic acids, but which appears irrelevant to hydrolysis under physiological conditions. The same is true for another mechanism of alkyl cleavage not shown in Fig. 7.Fa. All mechanisms of proton-catalyzed ester hydrolysis are reversible. 

Fig. 7.2. a) The most common mechanism of base-catalyzed ester hydrolysis, namely specific base catalysis (HCT catalysis) with tetrahedral intermediate and acyl cleavage. Not shown here are an W mechanism with alkyl cleavage observed with some tertiary alkyl esters, and an 5n2 mechanism with alkyl cleavage sometimes observed with primary alkyl esters, particularly methyl esters, b) Schematic mechanism of general base catalysis in ester hydrolysis. Intermolecular catalysis (bl) and intramolecular catalysis (b2). c) The base-catalyzed hydrolysis of esters is but a particular case of nucleophilic attack. Intermolecular (cl) and intramolecular (c2). d) Spontaneous (uncatalyzed) hydrolysis. This becomes possible when the R moiety is... 

A scheme depicting general base catalysis is shown in Fig. 7.2,b. Because the HO anion is more nucleophilic than any base-activated H20 molecule, intermolecular general base catalysis (Fig. 7.2,bl) is all but impossible in water, except for highly reactive esters (see below). In contrast, entropy may greatly facilitate intramolecular general base catalysis (Fig. 7.2,b2) under conditions of very low HO anion concentrations. Alkaline ester hydrolysis is a particular case of intermolecular nucleophilic attack (Fig. 7.2,cl). Intramolecular nucleophilic attacks (Fig. 7.2,c2) are reactions of cyclization-elimination to be discussed in Chapt. 8. 

Fig. 10.6. Simplified representation of the postulated catalytic cycle of microsomal and cytosolic epoxide hydrolases, showing the roles played by the catalytic triad (i.e., nucleophile, general base, and charge relay acid) and some other residues, a) Nucleophilic attack of the substrate to form a /3-hydroxyalkyl ester intermediate, b) Nucleophilic attack of the /Thydroxyal-kyl ester by an activated H20 molecule, c) Tetrahedral transition state in the hydrolysis of the /f-hydroxyalkyl ester, d) Product liberation, with the enzyme poised for a further catalytic... 

Hydrolysis of substituted phenyl acetates is catalysed by the Zn(II) complex of 1,5,9-triazacyclododecane (220). The results support the mechanism in which the ester is first complexed to the metal centre, and then water or hydroxide ion makes a nucleophilic attack at the complexed ester. ° ... 

Molecular dynamics free-energy perturbation simulations utilizing the empirical valence bond model have been used to study the catalytic action of -cyclodextrin in ester hydrolysis. Reaction routes for nucleophilic attack on m-f-butylphenyl acetate (225) by the secondary alkoxide ions 0(2) and 0(3) of cyclodextrin giving the R and S stereoisomers of ester tetrahedral intermediate were examined. Only the reaction path leading to the S isomer at 0(2) shows an activation barrier that is lower (by about 3kcal mol ) than the barrier for the corresponding reference reaction in water. The calculated rate acceleration was in excellent agreement with experimental data. ... 

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