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 chlorphosphines Cy2PCl (L1) and Cy(FluMe)PCl (L2, with FluMe = 9-methylfluorenyl) and were characterized in both solution and solid state. The smaller PCy2-substituted ligand L1 readily formed the dicoordinate L12Pd when treated with Pd2(dba)3 and showed no activity in palladium-catalysed amination reactions even when applied as defined palladium(II) η3-allyl, tBu-indenyl and cinnamyl precursors. The bulkier fluorenyl ligand L2 similarly was inactive, despite its ability to form stable mono-ligated LPd(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 as a result of the small P-C-S bond angle, which is sterically enforced due to the cyclic nature of the backbone. This small angle was shown through calculations to lead to an increased s-character of the lone pair at the ylidic carbon atom, which leads to a polarizing of the C-P bond strongly towards the carbon and consequently a lowering of the electron richness of the ligand system. The results described herein will have implications for the future of ligand design, with a careful balance of many factors to be considered to achieve catalytic activity.

Ylide-substituted phosphines with a cyclic backbone: Impact on the donor properties

ACS Fall 2021

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