Catalysts protonation

The anhydride or acyl chloride and the catalyst (proton acid or Lewis acid) interact leading to the acylating agent [formulated here for brevity as an acyl cation (83)]. ... 

Q Acid catalyst protonates the basic carbonyl oxygen atom, making the aldehyde or ketone a better acceptor for nucleophilic addition. 

The biotinylation of amine-dendrimers may be accomplished using either an organic reaction environment or an aqueous medium. For modification of PAMAM dendrimers with a biotinidase resistant biotin compound, Wilbur et al. (1998) performed the reaction in DMF with triethylamine as catalyst (proton acceptor). The following protocol illustrates this type of procedure using the biotinylation reagent NHS-PEG/pbiotin, which closely compares to the biotinidase insensitive compound used in the published procedure. 

The O from ROH must bond to the C of COOH and the OH of COOH ends up in HjO. The acid catalyst protonates the O of C=0 to enhance nucleophilic attack by ROH. 

Mechanism. The carbonyl group of a carboxylic acid is not sufficiently electrophilic to be attacked by the alcohol. The acid catalyst protonates the carbonyl oxygen, and activates it towards nucleophilic attack. The alcohol attacks the protonated carbonyl carbon, and forms a tetrahedral intermediate. Intramolecular proton transfer converts the hydroxyl to a good leaving group as H2O. A simultaneous deprotonation and loss of H2O gives an ester. 

Countercations influence the rate and selectivity of this reaction. The activity order, as for cations, was found to be Ag > Cu, H > Fe > Al > Pd > La > Zn (190). The distributions of product hydrocarbons were found to be similar to those observed for H3PW12O40 (Table XIX), suggesting similar reaction mechanisms. Ag and Cu salts of H3PW12O40 are much more active than the acid form catalyst. Protons generated by the reaction of Ag+ with H2 are presumed to give the more active catalyst (797). 

The mechanism for this reaction is shown in Key Mechanism 18-6. The first step is a typical acid-catalyzed addition to the carbonyl group. The acid catalyst protonates the carbonyl group, and the alcohol (a weak nucleophile) attacks the protonated, activated... 

In the acidic pathway, the acid catalyst protonates the carbonyl group so that the weak nucleophile can add. In most cases, the leaving group also becomes protonated before it leaves, so that it can leave as a weak base rather than as a strong base. 

Initiation steps The acidic catalyst protonates the monomer, starting the chain. 

Many of the reactions in these three chapters require acid or base catalysis. An acid catalyst, protonates the carbonyl oxygen, making the carbonyl carbon more reactive toward nucleophiles, and/or protonates the tetrahedral intermediate, making loss of a leaving group easier. A base catalyst deprotonates the nucleophile, making it more nucleophilic. The pH optimum for these reactions is a compromise between the two needs. 

Preferably the reaction in accordance with the invention is performed in the presence of a suitable catalyst, proton acids such as for instance haloid acids, sulfuric acid, phosphoric acid, perchloric acid, organic sulfonic acids, such as for instance methanesulfonic acid and p-toluenesulfonic acid, carboxylic acids, such as for instance oxalic acid, trifluoroacetic acid and other Lewis acids, such for instance boron trifluoride, ferric chloride, zinc chloride, zinc bromide, stannic chloride, titanium chloride or iodine having proved to be suitable. Furthermore mixtures of the individual catalysts may be used in certain cases. 

In another case, 1 was used to deprotect the 2-(allyloxy)phenylacetyl group, employed as a protecting functionality in carbohydrate chemistry242. Thus, heating compound 266 with a palladium catalyst/proton sponge system results in an almost quantitative yield of compound 268 (equation 29). In accordance with a postulated relay mechanism, the phenolic allyl ether is cleaved by the transition metal followed by intramolecular ester cleavage by nucleophilic attack of the released hydroxyl. The aforementioned conditions... 

Activation of HCHO by acidic sites of catalyst protonation of HCHO ... 

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