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

This work is divided into two parts. The first part introduces a novel approach for the synthesis of cationic phosphine ligands by metal ion encapsulation. In this context, multidentate heterophosphines encapsulate a cationic metal ion and change their electronic and structural properties on the uncoordinated phosphorus. The influence of the implemented cationic charge on the donor ability of the phosphines was investigated using Tolman electronic parameters and 1JPSe coupling constants, demonstrating the low net donor ability of the ligand class. Subsequent P-coordination to platinum(II) or gold(I) led to the synthesis of bimetallic complexes. After activation of the corresponding platinum(II) bimetallic complexes, high reactivity was observed in the pi-acid catalyzed hydroarylation of propargyl ethers. The second part focusses on a two-step protocol converting specially designed benzyl phenyl ethers into tricyclic 6H-benzo[c]chromenes. The first step consists of a selective CꟷH sulfenylation, while the second step involves a photoredox-catalyzed cyclization. The photocatalytic reaction proceeds via a one-electron reduction of the corresponding sulfonium salt, selective SꟷAr(exo) bond cleavage and subsequent radical 5-exo-trig cyclization, reoxidation, ring expansion and rearomatization. The proposed mechanism is supported by several studies. Furthermore, the methodology can be used to enable double cyclizations, to build up helical structures and provides a basis for the synthesis of pyrylium salts.