Tuning the donor properties of bipyridyl-substituted phosphines by metal ion encapsulation & Photocatalytic synthesis of 6H-benzo[c]chromenes from S-aryl sulfonium salts

Abstract

Lewis acid catalysis vs. Brønsted acid catalysisWith the corresponding costs for ligands and noble metals, the question arises why one cannot simply use proton catalysis instead (Brønsted acid catalysis). A proton is after all isolobal to the LAu + and Hg 2+ fragments. An evident difference is the contrasting hardness according to the Pearson concept. [6] Protons are hard Lewis acids, while mentioned metal fragments are soft Lewis acids. This difference leads to the fact that protons often need harsh reaction conditions and are less selective. The softer metal fragments, however, have higher affinity to soft substrates and therefore require less harsh conditions and react in higher selectivity. Besides this evident difference, the use of metal fragments also allows the possibility of stabilizing carbene-like intermediates and to unlock their reactivity. In the case of gold(I), this topic is highly controversial, and resonances of gold carbenes or gold-stabilized carbocations are discussed (→"carbenoid" [6] or "carbene" [11] , Scheme 2). [11,12] Scheme 2: Gold carbene [11] Scheme 14:Chalk-Harrod mechanism [36] The latter mechanism is composed of oxidative addition (I) followed by ligand association (II), insertion (III) and reductive elimination (IV). As a side reaction, mainly β-hydride elimination is present. Because of its ability to activate inert bonds, platinum(II) is also used in the activation of methane, which represents a classic example of CꟷH activation (Shilov system). [37] Methanol and chloromethane can Scheme 20: Oxidation of phosphines to phosphine selenides [55] Since the J-coupling constant contains information about the electronic environment, bonding situation and hybridization of 31 P (I = ½) and 77 Se (I = ½) via the Fermi contact interaction, it is also strongly influenced by the polarization of the PꟷSe bond. If electron pushing substituents are chosen on the phosphorus, the coupling constant 1 JPSe decreases, while electron withdrawing substituents result in high coupling constants. In other words, the coupling constant decreases as the basicity of the corresponding unoxidized phosphine increases (Scheme 21).