We have an unusual set of reaction conditions (can't say what for) but basically I have a reaction in acetonitrile to which I add conc H2SO4 (1 equiv). The reaction proceeds but slowly and in moderate yield. When I repeat the same reaction but add the HCl salt of Hunig's base (1 equiv) as well as the conc H2SO4 (1 equiv) the reaction proceeds very fast and in high yield. My first guess is that the Hunig's base.HCl salt is acting as a PTC but I'd like to hear thoughts. We are now running the reaction in the presence of a R4NX PTC. Will let you know what happens! The use of conc HCl instead of conc H2SO4 does not affect reaction rate or yield.
The presence of the auxiliary base is most likely acting as a proton shuttle in your reaction mechanism as the energy requirements for acetonitrile to act as a base are siginificant. The validity of phase transfer catalysis generically requires biphasic media. The impression given is that your system is homogenous.
It is hard to come to further conclusions without more information, which I assume to be commercially sensitive.
Your description suggests homogenious, acidic reaction system.
There is no free amine and ammonium salt could not act as PTC cat.
However other reagents, which you skip, could form separate phase.
Conc. H2SO4 could substitute HCl in DIPEAxHCl.
Maybe conc. H2SO4 alone is too strong and inhibits reaction and presence of HCl influences a protonation of other reagents or AN.
In “acidic PTC” usually onium cat. with lipophilic anions are used.
Just bunch of thoughts.
Check out the use of Pyridinium para toulenesulfonate in place of the strong acids.
What about sulfonic acids polymer-bound, alone or modified with amines (eg. PPTS 82817Aldrich & others) ? Also phosphoric acid or PPA should be weeker than H2SO4 in AN.
The use of ionic liquids illustrates how electrolyte could influence reactions. For example Aldrichimica Acta Last issue 2002
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