Water as nucleophile

Step 2 of Figure 29.3 Conjugate Addition of Water The a,(3-unsaturated acyl CoA produced in step 1 reacts with water by a conjugate addition pathway (Section 19.13) to yield a jG-hydroxyacyl CoA in a process catalyzed by enoyl CoA hydratase. Water as nucleophile adds to the 3 carbon of the double bond, yielding an enolate ion intermediate that is protonated on the a position. 

Jacobsen et al. [48], in 1997 for the first time demonstrated KR of racemic terminal epoxides with water as nucleophile for the production of optically pure epoxides and corresponding 1,2-diols. Since then, various other nucleophiles viz., carboxylic acids, phenols, thiols, amines, carbamates and indols were used in KR to produce optically pure epoxides with concomitant production of corresponding enantioenriched l,2-bifimctional moieties [49-52]. 

All carboxylic acid derivatives are hydrolysed to carboxylic acids by the action of water as nucleophile. Acyl halides and anhydrides of low molecular weight are hydrolysed quite vigorously. Esters and amides react much more slowly, and hydrolysis normally requires acid or base catalysis. This is nicely... 

Serine itself would be insufficiently nucleophilic to attack the ester carbonyl, so the reaction is facilitated by participation of the imidazole ring of histidine. The basic nitrogen in this residue is oriented so that it can remove a proton from the serine hydroxyl, increasing nucleophilicity and allowing attack on the ester carbonyl. This leads to formation of the transient acetylated enzyme, and release of choline. Hydrolysis of the acetylated enzyme utilizes water as nucleophile, but again involves the imidazole ring, and regenerates the free enzyme. 

G The carbocation intermediate reacts with water as nucleophile in a fast step to yield protonated alcohol as product. 

In the anodic oxidation of imines, usually addition of a nucleophile to the carbon of the C = N double bond takes place. If the nucleophile is water, hydrolytic cleavage will occur. For example, controlled-potential oxidation of A-benzylidene-p-anisidines in acetonitrile at the first voltammetric wave yielded the protonated anisidine. At the second wave, benzaldehyde and protonated j -benzoquinone imine are formed by attack of residual water as nucleophile [112] [Eq. (15)]. 

Water as nucleophile then displaces mercury by back-side attack at the more highly substituted carbon, breaking the C-Hg bond. 

Substitution of CO by phosphines 145 The Dotz reaction 149 Rearrangement reactions with loss of CO 151 Photochemical reactions 153 Reactions at the carbene carbon 158 General features 158 Amine nucleophiles 159 Phospine and phosphite nucleophiles 167 Alcohols and alkoxide ion nucleophiles 171 Thiol and thiolate ion nucleophiles 179 Intramolecular nucleophilic reactions 191 Hydroxide ion and water as nucleophiles 194 Insertion reactions initiated by nucleophilic attack Acid-base reactions at the a-carbon 207 General features and methods 207 Kinetic and thermodynamic acidities 209 Effect of structure on pKa values 210 Intrinsic rate constants for proton transfer 219 Thermodynamic acidities in organic solvents 223 Hydrolysis of ionizable carbene complexes 228 Acknowledgments 232 References 233... 

The hydrolysis of Fischer carbene complexes such as 11,13,42,87 and 112 has been discussed in the section Hydroxide ion and water as nucleophiles and proceeds by a standard nucleophilic addition-elimination mechanism (equation 74). On the other hand, carbene complexes with ionizable a-carbons are hydrolyzed by a different mechanism which involves the deprotonated carbene complex as the key intermediate. This conclusion is based on a detailed kinetic investigation of the hydrolysis of 66, 68, 144 and 8 in 50% MeCN-50%... 

Fig. (25). Reactions of (3-carotene dication (11) with water as nucleophile.

In this chapter we have discussed the carbonylative transformations of C-X bonds using amines, alcohols and water as nucleophiles. From a reaction mechanism point of view, they all go through a nucleophilic attack on the acylmetal species by nucleophiles. No reductive elimination step was involved, and the active catalyst was regenerated under the assistant of base. 

A soln. of 2,6-di-ferf-butyl-p-cresol in acetonitrile containing LiC104 and water as nucleophile subjected ca. 15 min. to a constant current of 200 mA at a Pt-gauze anode with a carbon rod as cathode -> 2,6-di-rer -butyl-4-hydroxy-4-methylcyclo-hexa-2,5-dienone. Y 86%. - Similarly with methanol as nucleophile > 4-methoxy analog. Y 88% with Na-acetate in acetonitrile-acetic acid -> 4-acetoxy analog. Y 91%. F. e. s. A. Ronlan and V. D. Parker, Soc. (C) 1971, 3214. 

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