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Abstract
Saturated aqueous solutions of various common salts were examined for their effect on aqueous aldol reactions catalysted by a highly active C2-symmetric diprolinamide organocatalyst developed in our laboratory. With respect to the aldol reaction between cyclohexanone and 4-nitrobenzaldehyde, deionised water was always a superior medium to salt solutions though some correlation to increasing anion size and depression in enantiomeric excess could be observed. Additionally, the complete inhibition of catalyst activity observed when employing tap water could be alleviated by the inclusion of ethylenediaminetetraacetate (EDTA) into the aqueous media prior to reaction initiation. Extension of these reaction conditions demonstrated that these ionic effects vary on a case-to-case basis depending on the ketone/aldehyde combination.
Highlights
IntroductionList and Barbas demonstrated the great potential and broad scope of organocatalysed reactions [1,2,3]
The field of organocatalysis has undergone a dramatic increase in interest since the seminal work ofList and Barbas demonstrated the great potential and broad scope of organocatalysed reactions [1,2,3].As the benefits of organocatalysts are realized, such as their benign nature toward air and moisture, the introduction of asymmetric carbon centers via proline based catalysis is becoming a viable tool to the synthetic organic chemist
In conclusion we have shown that the use of saturated ionic solutions as reaction media for organocatalysed aldol reactions can have dramatic effect on both the yield and enantiopurity of reaction products
Summary
List and Barbas demonstrated the great potential and broad scope of organocatalysed reactions [1,2,3]. It’s often observed that once a catalyst has been tailored toward compatibility with aqueous environments, water itself encourages further activity and enantiomeric induction through hydrogen bonding networks participating in theorized transition states It is not uncommon for organocatalysis carried out in aqueous environments to require a polar organic co-solvent (e.g., DMSO or DMF) and/or a rate enhancing additive such as benzoic acid, trifluoroacetic acid among others to compensate for a deficiency in water compatibility [22,23]. Despite the impact afforded by ions in solution, the investigation into ionic solutions has, by and large, been restricted to saturated solutions of sodium chloride and/or tap water In light of this lack of in depth investigation, this study endeavors initially to evaluate the effects of common ionic solutions have on an organocatalysed aldol reaction between p-nitrobenzaldehyde and cyclohexanone, expand this study to other ketone/aldehyde variations
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