Activity base catalysis

Addition of HCN to unsaturated compounds is often the easiest and most economical method of making organonitnles. An early synthesis of acrylonitrile involved the addition of HCN to acetylene. The addition of HCN to aldehydes and ketones is readily accompHshed with simple base catalysis, as is the addition of HCN to activated olefins (Michael addition). However, the addition of HCN to unactivated olefins and the regioselective addition to dienes is best accompHshed with a transition-metal catalyst, as illustrated by DuPont s adiponitrile process (6—9). 

Polymerization to Polyether Polyols. The addition polymerization of propylene oxide to form polyether polyols is very important commercially. Polyols are made by addition of epoxides to initiators, ie, compounds that contain an active hydrogen, such as alcohols or amines. The polymerization occurs with either anionic (base) or cationic (acidic) catalysis. The base catalysis is preferred commercially (25,27). 

En me Mechanism. Staphylococcal nuclease (SNase) accelerates the hydrolysis of phosphodiester bonds in nucleic acids (qv) some 10 -fold over the uncatalyzed rate (r93 and references therein). Mutagenesis studies in which Glu43 has been replaced by Asp or Gin have shown Glu to be important for high catalytic activity. The enzyme mechanism is thought to involve base catalysis in which Glu43 acts as a general base and activates a water molecule that attacks the phosphodiester backbone of DNA. To study this mechanistic possibiUty further, Glu was replaced by two unnatural amino acids. 

The most intensively studied oxidizing system is that developed by Pfitzner and Moflatt in which the oxidation is carried out at room temperature in the presence of dicyclohexylcarbodiimide (DCC) and a weak acid such as pyridinium trifluoroacetate or phosphoric acid. The DCC activates the DMSO which in turn reacts with the carbinol to give an oxysulfonium intermediate. This breaks down under mild base catalysis to give the desired ketone and dimethyl sulfide. 

For exchange of non-labile organic hydrogen atoms, acid-base catalysis (or some other catalytic hydrogen-transfer agent such as palladium or platinum) is required. The method routinely gives tritiated products having a specific activity almost 1000 times that obtained by the Wilzbach method shorter times are required (2-12h) and subsequent purification is easier. 

In non-polar solvents, the reaction with piperidine is best represented by a two-term kinetic form indicating a mixed 2nd- and 3rd-order reaction. Also, base catalysis by tri-ri-butylamine was observed. This kinetic pattern is strongly reminiscent of the results obtained with nitro-activated benzenes.Another interesting result is that stepwise replacement of chlorine atoms by amino groups results in marked... 

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

Acid-base catalysis, 232-238 Brqnsted equation for, 233-236 general, 233, 237 mechanisms for, 237 specific, 232-233, 237 Activated complex (see Transition state) Activation enthalpy, 10, 156-160 for composite rate constants, 161-164 negative, 161 Activation parameters, 10 chemical interpretation of, 168-169 energy of activation, Ea, 10 enthalpy of activation (A// ), 10, 156-160... 

The HIV-1 protease, like other retroviral proteases, is a homodimeric aspartyl protease (see Fig. 1). The active site is formed at the dimer interface, with the two aspartic acids located at the base of the active site. The enzymatic mechanism is thought to be a classic acid-base catalysis involving a water molecule and what is called a push-pull mechanism. The water molecule is thought to transfer a proton to the dyad of the carboxyl groups of the aspartic acids, and then a proton from the dyad is transferred to the peptide bond that is being cleaved. In this mechanism, a tetrahedral intermediate transiently exists, which is nonconvalent and which is mimicked in most of the currently used FDA approved inhibitors. 

The base catalysis and the monoelectronic reductive activation processes have been described by a computational investigation at the R(U)B3LYP/6-31 + G(d,p) level of theory for the model imide NI (Scheme 2.14) 47 both in the gas phase and in aqueous solution, using PCM solvation model.40... 

SCHEME 2.13 Activation of QMPs (quinone methide precursors) by base catalysis and single-electron reduction (reproduced from Ref. [47] with permission from American Chemical Society). 

The attacking anion is already present in solution as such so no base catalysis is required, and SO320 is a sufficiently powerful nucleophile not to require activation (by protonation) of the carbonyl group, so no acid catalysis is required either. This nucleophile is a large one, however, and the K values for product formation are normally... 

Generally, HNLs utilize an acid-base catalysis mechanism. The amino acid residues at active sites of these enzymes differ significantly, but share the common motif for cyanogenesis. 

One approach to promoting the kinetics of hydrogen transfer to bound carbon monoxide is based on maximizing the difference in polarity of the carbon (eg. 6+) and hydrogen (eg. 6-) involved (Jt). This strategy leads naturally to a bimolecular approach, based upon MCO and M H. The additional degree of freedom which follows from employing two different transition metals is noteworthy as an alternative to cluster activation or catalysis. 

Another noncatalytic step proposed by King et al. (18) in iron carbonyl/base catalysis of the WGSR involves the formation of formate ion however, we recently observed that formate formation appears to have little importance in the related rhodium catalysis of hydrohydroxymethylation. We plan to perform studies of the CO + KOH and C02 + KOH reactions independent of catalysis to more fully appreciate the relationship of these reactions to solution pH and thus the catalytic activity. 

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