It makes sense that if one wants to transfer and react an organic dianion from an aqueous phase to an organic phase using phase-transfer catalysis, one could use a bisquat dication that would match the distance between the charges on each molecule for effective electrostatic interaction.
This phase-transfer catalyst, bis(tributyl)-1,6-hexylenediammonium dibromide, was designed for the reaction of the disodium salt of bisphenol A with nitrophthalimide. The two anionic centers of bisphenoxide are located on opposite ends of the molecule and are separated by two rigid rings and another carbon. One might expect good association between the dianion and dication if the approximate distance between the two positive charges on the bisquat were approximately the same as the two negative charges on the bisphenoxide, accounting for flexibility and rotation of the alkyl chain of the bisquat.
The inventors chose spacers of 4, 6 and 10 carbons between the two quat cation centers of the bisquat. They found that 4 carbon atoms is too short for bisphenoxide. They found that spacers of 6 and 10 carbon atoms were very effective and had similar performance, achieving 98% yield after 2 hours using 1 mole% bisquat as phase-transfer catalyst.
They also found that quarternizing the dibromoalkane with trihexylamine instead of tributyl amine, did not increase reactivity. As a result they chose bis(tributyl)-1,6-hexylenediammonium dibromide as the best choice for combination of spacer, organophilicity/accessibility of the two positive charges and cost for bisquat for the target reaction.
The bisquat is made by simply mixing 1,6-dibromohexane with 2 equivalents of tributyl amine in acetonitrile at reflux for 24 hours, stripping off the acetonitrile and recrystallizing from toluene. This procedure is similar to that used to produce many quats.
In 1983, I was finishing my work on phase-transfer catalysis for my Ph.D. thesis and I synthesized several bisquats for screening but never tested them (my supervisor thought that the 15 PTC publications I had during my graduate research was more than enough to proceed to my post-academic career). I was pleased when I saw this patent and similar publication in 1985 that showed that the bisquat-dianion concept worked well.
Now contact Marc Halpern of PTC Organics if you need help choosing the best phase-transfer catalyst to squeeze the most performance out of your commercial PTC process in development, scale up or production.
