Ylide-Substituted Phosphines with a Cyclic Ylide-Backbone: Angle Dependence of the Donor Strength

Abstract

Ylide-substituted phosphines (YPhos) have been shown to be highly electron-rich and efficient ligands in a variety of palladium catalyzed transformations. Here, the synthesis and characterization of novel YPhos ligands containing a cyclic backbone architecture are reported. The ligands are easily synthesized from a cyclic phosphonium salt and the chlorophosphines Cy₂PCl (L1) and Cy(Flu^Me)PCl (L2, with Flu^Me = 9-methylfluorenyl) and were characterized in both solution and solid states. The smaller PCy₂-substituted ligand, L1, readily formed the biscoordinate L1 ₂ Pd species when treated with Pd₂(dba)₃ and showed no activity in palladium-catalyzed amination reactions even when applied as defined palladium(II) η³-allyl, t-Bu-indenyl, or cinnamyl precursors. Bulkier fluorenyl-substituted ligand L2 similarly was inactive, despite its ability to form the stable monophosphine complex L2·Pd(dba). Assessment of the electronic properties by experimental and computational methods revealed that L1 and L2 are considerably less electron-rich than previously synthesized YPhos ligands. This was shown to be the result of the small P-C-S bond angle, which is sterically enforced due to the cyclic nature of the backbone. Density functional theory calculations revealed that the small angle results in an increased s-character of the lone pair at the ylidic carbon atom and leads to a polarization of the C-P bond toward the carbon atom, thus decreasing the electron density at the phosphorus atom. The results demonstrate the tunability of the donor strength of YPhos ligands by modification of the ligand backbone beyond simple changes of the substitution pattern and are thus important for future ligand design, with a careful balance of many factors to be considered to achieve catalytic activity.