Hydrogen bond donating catalysis

For organizational reasons, several catalytic systems possessing multiple hydrogen bond donating functionalities have been included in this section which may not be classified as BBAs. For example, the bistrifylamide Mikami catalyst could perhaps be classified as a double-hydrogen bond donor catalyst akin to thiourea catalysis. 

Intermediate 1 could also be stabilised by proton transfer from oxygen to give 3 in Scheme 11.9. The proton acceptor B could be solvent water or a general base catalyst. The reaction will only be catalysed if the rate of breakdown of 1 to regenerate reactants is faster than the rate of proton transfer. In this case, such catalysis would be independent of the base strength of the catalyst B as proton transfer would invariably be thermodynamically favourable and hence occur at the maximum diffusion-controlled rate. If proton transfer to solvent is thermodynamically favourable, such that proton donation to 55.5 M water is faster than to, say, 1 M added base, any observed catalysis by base must represent transition state stabilisation by hydrogen bonding, or a concerted mechanism. 

Catalysis and activation are not the same. In Friedel-Crafts acylations of aromatics, aluminum chloride is used in more than stoichiometric amounts as an activator and coreagent. A similar approach is often used on a stoichiometric basis to activate carbon-hydrogen bonds toward hydride donation. This chemical approach stands in sharp contrast to the enzyme alcohol dehydrogenase, which works catalyti-cally via a complex mechanism wherein activation is achieved but not at the expense of irreversible consumption of a coreagent.""... 

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