Last month we explained why tetrabutyl ammonium chloride, TBAC, is more expensive than tetrabutyl ammonium bromide, TBAB. This month we will explain a more important reason why the use of TBAC usually means (though not always) that the PTC process using it is not optimal.
About 80-90% of PTC reactions liberate a leaving group, including C-/N-/O-/S-alkylations and substitutions using inorganic nucleophiles. As the reaction proceeds, more and more leaving group is liberated. That means that any intended effect for the chloride is likely to get swamped out by the ever increasing leaving group anion. This is especially true since most quats usually prefer to pair with most other leaving group anions compared to chloride.
Let’s assume for a moment that the leaving group is bromide. At 50% conversion of the desired reaction, there is obviously 50 mole% bromide in the system. If you are using 5 mole% TBAC as the phase-transfer catalyst, then at 50% conversion, there is 10 times more bromide present than chloride. An example of this is an otherwise excellent patented procedure shown at http://phasetransfer.com/C-alkfluorene.gif.
One reason to choose chloride as the counterion for tetrabutyl ammonium would be to attempt to enhance the extraction of the desired reacting anion due to the relative affinities of quats for chloride versus many useful anions. This is often why PTC chemists often choose hydrogen sulfate. While that may make a difference during the first few percent conversion or in cases in which there is no leaving group (e.g., oxidations with peroxide), the chloride simply gets overwhelmed by the leaving groups at higher conversion.
As a result, the added cost of TBAC vs TBAB is usually not justified when there is a leaving group. TBAC can potentially be optimal in certain cases in which there is no leaving group, but very rarely when there is a leaving group. The use of chloride with tetrabutyl ammonium requires some compelling reason to overcome the cost barrier.
If your company can benefit from achieving higher process performance in a shorter development time by having access to the best PTC expertise available, now contact Marc Halpern by E-mail to inquire about using phase-transfer catalysis to achieve low-cost high-performance green chemistry.
If you’re not sure if PTC can help your reaction, now fill out the form shown at http://phasetransfer.com/projectform.pdf and send it to Marc Halpern by fax at +1 856-222-1124 or by E-mail of a scanned copy. If we do not have a secrecy agreement already in place, please use “R-groups” instead of the exact chemical structures.
