The Aug 2012 PTC Reaction of the Month uses octyl trimethyl ammonium hydrogen sulfate. There are two interesting aspects of this catalyst: the cation and the anion.

Classical phase-transfer catalysts do not have a head-and-tail structure and are often symmetrical like tetrabutyl ammonium. The head-and-tail structure is more characteristic of classical surfactants such as hexadecyl trimethyl ammonium. As the longest chain of an alkyl trimethyl ammonium quat is shorter, its surfactant properties are less pronounced. Good phase-transfer catalysts do not cause emulsions as good surfactants do. So, it is interesting that the inventors of the patent described above use an alkyl trimethyl ammonium quat with an intermediate size chain. We can only speculate that this catalyst had a chain long enough to be effective for PTC but not too long to form stable emulsions that would hinder workup.

The anion used in the patent was hydrogen sulfate. Since about 1996, it has been common to see hydrogen sulfate quats used in conjunction with hydrogen peroxide oxidations, epoxidations and coupling reactions. In many of these reactions the hydrogen sulfate quat salts work much better than the corresponding chloride quat salts.

As always, the choice of phase-transfer catalyst should be customized to meet the needs of the specific application.

Need to learn more about which phase-transfer  catalyst to choose for a specific PTC  application? Contact Marc Halpern of PTC Organics  by E-mail.

The Jul 2012 PTC Reaction of the Month uses tetrabutyl phosphonium chloride. We don’t often encounter this phase-transfer catalyst, but in this case, it is the right catalyst. The reaction is performed at 160 C for 12 hours, so a thermally stable phase-transfer catalyst is needed. The P-quat is more stable than the corresponding N-quat. In addition, the reaction shown is a chloride-fluoride exchange, so it is good to avoid the introduction of a bromide anion. As a result, the inventors appropriately chose Bu4P Cl and not Bu4P Br.

As always, the choice of phase-transfer catalyst should be customized to meet the needs of the specific application.

Phosphonium salts are available from Dishman and from Cytec

Need to learn more about which phase-transfer  catalyst to choose for a specific PTC  application? Contact Marc Halpern of PTC Organics  by E-mail.