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

Abstract

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 (RA)‐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.

Widening the Scope of “Inherently Chiral” Electrodes: Enantiodiscrimination of Chiral Electroactive Probes with Planar Stereogenicity

“…All compounds were synthesized according to previously reported procedures, [81][82][83][84][85] starting from ferrocenyl aldehydes, which were (stereo-)selectively lithiated in ortho-position followed by reaction with the appropriate phosphorus electrophile. The various vinyl functionalities were introduced using Horner-Wadsworth-Emmons (HWE) reactions of ferrocenyl aldehydes with phosphonates.…”

“…[35,45] by stirring of the parent compound with elemental sulfur. Substituents in 1'-position were introduced as described recently [82] . For further details see the Experimental Section and the Supporting Information (section SI.2).…”

“…The parent P III phosphines of compounds 1, [75] 2-6,12-13,15,16, [74] 7-9,14,16 [82] and 10,11 [83][84][85] were synthesized according to published procedures. The herein applied P V -sulfides were obtained by stirring of the parent P III phosphines with an excess of elemental sulfur in dichloromethane or tetrahydrofuran according to literature.…”

Rearrangements and Migrations along the Ferrocene Periphery: On the Way to Planar‐Chiral and (Multi)Substitution Patterns

“…Those compounds are easily accessible by ortho ‐directed metalation [5] . The resulting compounds are widely used as chelating ligands in cross‐coupling catalysis since the planar‐chirality of the three‐dimensional backbone generates an asymmetric entity, as reflected by the coupling products [5,6,7] . Compared to a 1,2‐phenylene entity, the different geometry of a ferrocenyl‐based building block also affects the angle and distance between both groups, resulting in unique reaction products [1] …”

“…[5] The resulting compounds are widely used as chelating ligands in cross-coupling catalysis since the planarchirality of the three-dimensional backbone generates an asymmetric entity, as reflected by the coupling products. [5,6,7] Compared to a 1,2-phenylene entity, the different geometry of a ferrocenyl-based building block also affects the angle and distance between both groups, resulting in unique reaction products. [1] The variety of hetero-elements that can be introduced through the above mentioned processes is limited to those where appropriate electrophiles are available.…”