1-napthyl substituted tetraaryl-1,3-dioxolane-4,5-dimethanol (1nTADDOL) is an excellent diol-based asymmetric organo-catalyst as it provides high product yields and stereoselectivity for various asymmetric catalyses. Conversely, the yield and stereoselectivity of 2-napthyl substituted TADDOL (2nTADDOL), which only differs by isomerism of the naphthyl substituent, are relatively poor. Apparently, the molecular structure – including the hydrogen-bonding structure – are closely related to TADDOL’s catalytic activity. Yet, the differences in the hydrogen-bonding structure and dynamics of both TADDOL isomers have remained elusive.<br/>Here, we study the molecular structure and dynamics of solutions of neat 1nTADDOL and 2nTADDOL as well as in mixtures with benzaldehyde substrates to elucidate the key factors that make a good catalyst. To this end, we use a combination of infrared, nuclear magnetic resonance, and ultrafast infrared spectroscopies for solutions in toluene – the catalytically relevant solvent. We find that in solutions of the neat catalysts TADDOL’s OH group show distinct bonding motifs: (i) intramolecularly hydrogen bonded OH groups, (ii) intermolecularly bonded OH groups that form pi-OH bonds to toluene and (iii) free OH groups (Fig. 1). The good catalyst 1nTADDOL interacts to a lesser extent with the solvent compared to the worse catalyst 2nTADDOL. The good catalyst 1nTADDOL, however, is more efficient in slowing-down the rotational motion of the substrates, which indicates the formation of TADDOL-substrate complex via intermolecular hydrogen-bonding. Our results suggest that the different binding preferences of the TADDOL catalysts to the solvent or the substrate are correlated with the catalytic activity. Thus, the bond dynamics – rather than the bond strengths – are likely the key understand the catalytic efficiency.<br/><br/>Figure 1. Infrared absorption spectra of 1nTADDOL and 2nTADDOL together with the hydrogen-bonding motifs of TADDOL’s OH groups

Hydrogen bond dynamics of TADDOL asymmetric organo-catalyst correlate with catalytic activity

2019 Fall National Meeting

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