Tetrabutyl phosphonium chloride is used as a phase-transfer catalyst when working at temperatures at which the much less expensive tetrabutyl ammonium bromide is not stable, as long as no strong base is present (such as NaOH). These temperatures are often in the range of about 100 deg C to about 150 deg C, more often 100 deg C to 120 deg C.
PTC esterifications using alkyl halides (not benzyl halides) are often performed at temperatures in the range of 80 deg to 120 deg C, so tetraalkyl phosphonium quats are considered for these applications. When PTC applications are performed at higher temperatures at which tetraalkyl phosphonium quats are not stable due to decomposition by nucleophilic displacement, tetraaryl phosphonium quats (e.g., tetraphenyl phosphonium chloride) are often considered.
Tetraalkyl phosphonium quats such as tetrabutyl phosphonium are not at all stable in the presence of NaOH. PTC-NaOH conditions are used in the most common PTC applications. For these reasons, tetrabutyl phosphonium chloride has a rather narrow range of utility for non-NaOH PTC applications mostly performed at about 100 deg C to 120 deg C.
In the reaction shown in the diagram, tetrabutyl phosphonium was an appropriate choice. In another example described in this publication, the inventors performed an esterification of methacrylic acid at lower temperature (80 deg to 90 deg C) using a more reactive alkylating agent and they successfully used tetrabutyl ammonium bromide for that lower temperature esterification.
Another interesting and very important detail is that PTC is often used in reactions of water-sensitive compounds and that includes alkoxysilanes. In these cases, careful attention is paid to the water content. In this publication the potassium methacrylate salt was dried by azeotropic distillation before adding the water-sensitive 3-chloropropyldimethylethoxysilane reactant.
PTC Organics has helped companies develop and patent PTC applications using water-sensitive reactants and products, including alkoxysilanes. See for example: http://phasetransfercatalysis.com/ptc_tip/important-effect-of-water-on-profit-in-solid-liquid-ptc-esterification/.
Now contact Marc Halpern of PTC Organics when you need to achieve low-cost high-performance green chemistry for water-sensitive applications.
