This patent from Eastman Chemical (with PTC veteran Neil Boaz as an inventor) has some very interesting aspects. The focus of the patent is the identity and synthesis of trialkylamines and quaternary ammonium cations that contain a branched alkyl chain that meet 15 criteria related mostly to performance (such as low foaming, solubility & compatibility characteristics), safety and environmental aspects as surfactants and antimicrobials. As PTC experts we would also be interested to learn if the quat salts that include a branched alkyl chain would have unique characteristics as phase-transfer catalysts (mentioned in “Aspect #24” of 69 aspects).
The branched alkyl chain segments are synthesized from aldehydes using phase-transfer catalyzed aldol condensation such as the formation of the enal shown in the reaction diagram. Since the aldol condensation is performed using a mixture of aldehydes, an excess of one of the aldehydes is required to achieve an acceptable level of selectivity.
This PTC process was performed in a 3 Liter continuous stirred tank reactor (CSTR). Process parameters were studied including residence time, temperature, mole ratios and phase-transfer catalyst.
The catalyst chosen for the continuous process was benzalkonium chloride. Two of the four alkyl groups of the quaternary ammonium cation are methyl, that cannot undergo Hofmann Elimination, one is benzyl that cannot undergo Hofmann Elimination and one alkyl group is large (C8 to C18), a mixture of alkyl groups similar in number of carbons to the aldehydes. Please note that the q-value of this quat is about 2.2-2.5, so it is predicted to be good for T-reactions. The pKa of the CH group alpha to the aldehyde is about 20, so the Halpern pKa Guidelines predict this would likely be a T-reaction.
The reaction was also reported in a batch process using 8.6 mole% tetrabutylammonium bromide, also achieving 60% conversion to the desired product (Example 4).
Importantly, the inventors found that without a phase-transfer catalyst, the conversion to the desired product was 3% (Example 6).
This patent is very worthwhile reading.If you need to achieve low-cost high-performance green chemistry for strong base reactions, such as aldol condensation, now contact Marc Halpern of PTC Organics who started publishing a 15-part series entitled “Hydroxide Ion Initiated Reactions Using Phase-Transfer Catalysis: Mode of Action and Applications” in the 1970’s and has been practicing strong base PTC ever since.
