Proton-transfer step, amine-catalyzed mechanism

A controversy currently abroad in the field seems a particularly likely candidate for investigations using model reactions. A number of bacterial alcohol dehydrogenases make use of free-standing PQQ (see Fig. 4.3 for the structure) as a cofactor. Figure 4.8 shows two possible mechanisms for a critical step in the mechanism the question is whether the reaction follows a proton-transfer route or a hydride-transfer route [69, 70]. The question is essentially limited to the alcohol and sugar dehydrogenases, while the enzymes that catalyze amine oxidations tend... 

Briefly, the mechanism for formation of an enamine is very similar to that for the formation of an imine. In the first step, nucleophilic addition of the secondary amine to the carbonyl carbon of the aldehyde or ketone followed by proton transfer from nitrogen to oxygen gives a tetrahedral carbonyl addition compound. Acid-catalyzed dehydration gives the enamine. At this stage, enamine formation differs from imine formation. The nitrogen has no proton to lose. Instead, a proton is lost from the a-carbon of the ketone or aldehyde portion of fhe molecule in an elimination reaction. 

Equation (40) is known as a transimination or a trans-SchifSza-tion reaction. It probably proceeds via a two-step mechanism involving a m-diamine intermediate, analogous to the carbinolamine intermediate observed in imine formation fix>m carbonyl compounds. The transimination reactions of Schiff bases derived from either aliphatic or aromatic amines with semicarbazide, hydroxylamine or methoxyamine, of oximes with semicarbazide and of semicarbazones with hydroxylamine all appear general base catalyzed in terms of one protonated and one free base reactant or general acid catalyzed in terms of two reactants of free base. These results may be accounted for by a nearly symmetrical mechanism for transimination (equations 41 and 42). Simple proton transfers are considered to be rapid and... 

Although more studies have been devoted to the mechanism of the acylation of amines with carboxylic esters than with other reagents, the mechanistic details are not yet entirely clear.898 In its broad outlines, the mechanism appears to be essentially BAC2.899 Under the normal basic conditions, the reaction is general base-catalyzed,900 indicating that a proton is being transferred in the rate-determining step and that two molecules of amine are in-... 

At present, we can say that copolymerization initiated by various salts proceeds by an anionic mechanism, after dissociation of the initiators in the reaction medium. The primary step is the addition of the initiator anion to the epoxide. In the initiation by Lewis bases, i.e. by tertiary amines, initiation involves formation of a primary active centre of an anionic character. This active centre is probably generated by interaction of the tertiary amine with the anhydride and an allyl alcohol. The allyl alcohol can be formed by a base-catalyzed isomerization of the epoxide. In the presence of a proton donor, the formation of active centres is possible through interaction of tertiary amine, anhydride and proton donor without epoxide isomerization. Another way of initiation consists in a direct reaction of epoxide with tertiary amine yielding an anionic primary active centre. We believe that in both kinds of initiation in the strict absence of proton donors, the growing chain end has the character of a living polymer. The presence of proton donors, however, gives rise to transfer reactions. 

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