Evaluation of the Transferability of the “Flexible Steric Bulk” Concept from N-Heterocyclic Carbenes to Planar-Chiral Phosphinoferrocenes and their Electronic Modification

The concept of “flexible steric bulk” is discussed at 2-phenylvinyl-1-phosphinoferrocenes. The introduction of freely rotatable 1'-silyl groups increases the catalytic productivity within the synthesis of tri-ortho-substituted biaryls by Suzuki–Miyaura C,C cross-coupling reactions, giving higher yields with 1/4 of catalyst concentration than for the non-silylated derivatives. Electronic modification of the P and the vinyl donor functionalities was investigated by introducing substituents in the para positions of both groups. Therein, electron-withdrawing phosphines increased the yield from 78 to 91 % for a given biaryl, by changing from a diphenylphosphino to the P(p-CN-C₆H₄)₂ unit. Opposite results, obtained from electron-donating and sterically demanding phosphines, were in accordance with the ¹J(³¹P,⁷⁷Se) values. However, the electron density of the ferrocenyl backbone, expressed by the redox potential of the first ferrocenyl-related redox process, cannot be correlated with the donor-properties at the P atom. Changing from a PPh₂-substituted ferrocene to a (R_A)-1,1'-binaphthyl-containing phosphonite, a complex interaction between the axial- and the planar-chiral motifs occurs, resulting a change of the absolute biaryl configuration.