Steric and Electronic Influences of Buchwald-Type Alkyl-JohnPhos Ligands

The
electron-donating and steric properties of Buchwald-type ligands ([1,1′-biphenyl-2-yl]­dialkylphosphine;
R-JohnPhos, where R = Me, Et, iPr, Cy, tBu)
were determined. The π-acidity and σ-donating properties
of the R-JohnPhos ligands were quantified using a Cotton–Kraihanzel
analysis of the Cr0(CO)5(R-JohnPhos) complexes.
Somewhat surprisingly, the σ-donating abilities of the R-JohnPhos
ligands follow the trend tBu-JohnPhos < Et-JohnPhos
< iPr-JohnPhos < Cy-JohnPhos ≪ Me-JohnPhos.
This ordering is proposed to arise from competition between the intrinsic
electron-donating ability of the R groups (Me < Et < iPr ≈ Cy < tBu) and steric interactions (front
and back strain) that decrease the electron-donating ability of the
phosphine. X-ray crystallographic data of 22 metal complexes (general
forms: trans-Cr0(CO)4(PR3)2, Pd0(PR3)2(η2-dba), and trans-PdII(Cl)2(PR3)2) were also analyzed to help explain
the electronic trends measured for the R-JohnPhos ligands. The R-JohnPhos
ligands are exceptionally sensitive to back strain in comparison to
typical phosphines, and the strong σ-donating ability of the
Methyl-JohnPhos ligand is attributed to its ability to avoid both
front strain and back strain. Consequently, the −PMe2 moiety allows for very short phosphorus–metal bond distances.
Because of the sterically dominating o-biphenyl and
close phosphorus–metal bond distances, MeJPhos maintains a
large overall steric profile that is actually larger than that of
CyJPhos as measured by percent buried volume (%Vbur). Overall, the −PMe2 moiety is a powerful
way to incorporate strong σ-donation into “designer”
phosphines while retaining other advantageous structural and reactivity
properties.