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

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(FluMe)­PCl (L2, with FluMe = 9-methylfluorenyl)
and were characterized in both solution and solid states. The smaller
PCy2-substituted ligand, L1, readily formed
the biscoordinate L12Pd 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.