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.