Abstract
For the first time, free base and N-methylated porphyrins have been utilized as bifunctional organocatalysts in Michael additions and it was found that distortion of the macrocycle is a vital prerequisite for their catalytic activity. Conformational design has been used to tailor the properties of nonplanar porphyrins with regards to availability of the N-H units for hydrogen bonding (distortion-dependent hydrogen bonding) and the basicity of the heterocyclic groups. NMR spectroscopic- and catalyst screening studies provided insight into the likely mode of catalyst action. This unprecedented use of free base and N-substituted porphyrins as organocatalysts opens a new functional role for porphyrins.
Highlights
Free base and N-methylated porphyrins have been utilized as bifunctional organocatalysts in Michael additions and it was found that distortion of the macrocycle is a vital prerequisite for their catalytic activity
Conformational design has been used to tailor the properties of nonplanar porphyrins with regards to availability of the N–H units for hydrogen bonding and the basicity of the heterocyclic groups
Do porphyrins always require a central metal to be catalytically active? The short answer is ‘No!’ and in the following we detail how conformational design can be used to entice free base porphyrins to act as organocatalysts, revealing a new mode of action for the ubiquitous ‘pigments of life’
Summary
Free base and N-methylated porphyrins have been utilized as bifunctional organocatalysts in Michael additions and it was found that distortion of the macrocycle is a vital prerequisite for their catalytic activity. Rendering free base porphyrins catalytically active would require participation of the pyrrole N–H and N-lone pairs in chemical reactions and/or hydrogen bonding.
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