Promoting amine-amine cooperative interactions to increase the catalytic activity of aminosilica materials for the nitroaldol reaction

Cooperative interactions between immobilized amines and surface silanols have been proposed to enhance nitroaldol reactions with aminosilica catalysts. However, current methods to measure cooperative interactions between amines and surface silanols such as capping the surface silanols also alter surface polarity, site mobility, and proximity of adjacent amines. Accordingly, the relative contributions of amine-silanol and amine-amine interactions remain unclear and could be distinct from current understanding of aldol chemistry. Interestingly, we find that amines grafted at high surface density have the highest site time conversion, which is contrary to aldol chemistry. As high surface densities of amines would hinder cooperative amine-silanol interactions, these results are inconsistent with the previously hypothesized beneficial role of cooperative amine-silanol interactions. Since past studies have generally assumed positive contributions of surface silanols through observing a decline in activity when silanols are capped, we attempt to define the role of silanols and conclude that high surface density allows for amine-amine interactions that affect activity more strongly than any influence of amine-silanol interactions. Thus, our results support an imine formation mechanism that involves the simultaneous participation of two amines and propose that high activity is achievable even at the expense of amine-silanol interaction when amine-amine interactions are effectively promoted. These findings establish amine proximity as an important design parameter for primary aminosilica nitroaldol catalysts.