Incremental Introduction of Organocatalytic Activity into Conformationally Engineered Porphyrins

Abstract

To study the correlation of macrocycle nonplanarity and catalytic activity of free base porphyrins in detail, a series of six tetraphenylporphyrins with graded degree of β-ethyl substitution (“H2EtxTPPs” 1–6; x = 0, 2, 4, 6, 8) was applied in organocatalyzed reactions. The macrocycles display incrementally increasing nonplanarity due to repulsive peri-interactions. This creates an out-of-plane vector and better accessibility of the core amine and imine groups as the number of alkyl substituents increases. Following such a molecular engineering approach, the inner core system could be used to activate small molecules as a result of significant saddle distortion. The potential organocatalyst “H2EtxTPPs” were used in benchmark sulfa-Michael reactions and we found a distinct relationship between nonplanarity and conversion. These observations were attributed to the combined effect of enhanced basicity and increased H-bonding potential that could facilitate bifunctional organocatalysis. Ultimately, density functional theory (DFT) calculations were performed on 1–6 to monitor some electronic properties of the title compounds.