While the field of organocatalysis underwent fast development in the past decades, many organocatalytic systems still suffer from low activity or selectivity. The low activity can be sometimes counterbalanced by increasing the reaction time or the catalyst loading. The impaired selectivity is, however, irreparable.<br/>A new design of organocatalysts, embracing a nucleophilic active site surrounded by long or branched dendritic tails, potentially controlling the access of substrate molecules to the catalytic core, was explored in our group with the aim of achieving site-selectivity in chemical transformations of diol or polyol substrates. Thus, in an acylation of model amphiphilic diol substrates, the use of a nucleophilic catalyst, particularly branched/dendritic catalyst with the imidazole site in the core, enabled a remarkable shift of the site-selectivity from a polar alcohol site, preferred in background non-catalyzed or base-promoted reactions, to an apolar site (Scheme 1). Elucidation of the reasons for the site-selectivity, observed in each case, was achieved by performing a complimentary set of substrate-competitive acylations of polar and apolar mono-alcohol substrates.<br/>In a related phosphorylation reaction, a similar design of the catalysts, though with long oligoether tails instead of branched aliphatic appendages, led to highly active catalytic systems. Remarkably, when the imidazole active center was replaced by an aminopyridine moiety, the activity of the formed systems surpassed that of DMAP, a benchmark phosphorylating catalyst (Scheme 2).<br/>Expansion of these results to additional catalytic systems and substrates will be presented at the talk.

Inquest for better selectivity and activity: Nucleophilic organocatalysts based on branched/dendritic design

2019 Fall National Meeting

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