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‐C 6 H 4 ) 2 unit. Opposite results, obtained from electron‐donating and sterically demanding phosphines, were in accordance with the 1 J ( 31 P, 77 Se) 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 PPh 2 ‐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.