Conformational control of nonplanar free base porphyrins: towards bifunctional catalysts of tunable basicity

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.

A Comprehensive Study on Tetraaryltetrabenzoporphyrins

“…[41,42] Furthermore, the curvature of the saddle-shaped TBP-skeleton can be reversibly increased by protonation of the pyrrolic inner nitrogen, as depicted in the density functional theory (DFT) geometry optimized structures of 2 (free base) and H 2 2 2 + (dication) in [16] 31 (60 %) [48] a) CH 2 Cl 2 ,a rylaldehyde, 1,c at.B Figure 2b.T his is in sound agreement with the protonation, [43] or the irreversible N-methylation of porphyrins. [44,45] Unfolded, the structure of aT ATBP can be well described in as keletal deviation diagram, as it is depicted for the crystal structure of 2 in Figure 2c.H owever,f or comparatives tudies, the flap-height Dh,w hich is defined as the distance between the d-carbon and the plane generated through all meso-carbons (compare Figure 2d), is more suitable for multiple structures. In Figure 2e,t he Dh values derived from the DFT calculations in the gas-phase show ad eviation of 1.0 between the free-base (Dh = 2.02 )and dication( Dh = 3.02 )f orm.…”

Conformational Re‐engineering of Porphyrins as Receptors with Switchable N−H⋅⋅⋅X‐Type Binding Modes

“…Moreover, the porphyrin inner core imine and amine units of planar analogues are usually not involved in intermolecular interactions due to the “shielding” properties of the macrocycle system . Distortion can cause an increase in the degree of outwards orientation of the inner pyrrolic entities, making these positions more basic and accessible to substrates, thereby creating an “active center” …”

Konformativer Umbau von Porphyrinen als Rezeptoren mit schaltbaren N‐H⋅⋅⋅X‐Bindungsmodi

“…In den vergangenen Jahrzehnten faszinierte der innere Porphyrinkern von konformativ designten, nichtplanaren freien Porphyrinbasen als ausgezeichneter Wirt für verschiedene Metalle mit einstellbarer Basizität . Kürzlich fanden solche Porphyrine erste Anwendungen als Organokatalysatoren und als Promoter für Disauerstoff/Wasserstoffperoxid‐Interkonversion . Dies zeigt, dass spezifische Kern‐N‐H⋅⋅⋅Substrat‐Wechselwirkungen durch konformatives Engineering erreicht werden können .…”