Bimolecular displacement, catalysis

The bimolecular displacement process may often be subject to catalysis, and this may be either inter-or intra-molecular. A well known example of intermolecular catalysis is the... 

Jencks (1969) has pointed out that the role of the enzyme itself is particularly difficult to understand in bimolecular displacement reactions in which general acid-base catalysis would give little advantage, or is not possible. As examples of such reactions Jencks included methyl transfer from S-adenosylmethionine to nicotinic acid to form the N-meth-yl pyridinium, trigonelline (Joshi and Handler, 1960), or to methionine to form the new sulfonium, S-methylmethionine (Greene and Davis,... 

Stoichiometric amounts of tetraethylammonium cyanide react with aliphatic bromides in dichloromethane, acetonitrile or DMSO to give reasonable yields of the corresponding nitriles [13]. These reactions are clearly related to, but not actually examples of, phase transfer catalysis. It is interesting, however, that under these homogeneous conditions, tetraethylammonium cyanide reacts in acetonitrile with neopentyl bromide to give the corresponding nitrile (see Eq. 7.3). Bimolecular displacements on such sterically hindered substrates are usually quite difficult to effect. 

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