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

Abstract

The electron-donating and steric properties of Buchwald-type ligands ([1,1'-biphenyl-2-yl]dialkylphosphine; R-JohnPhos, where R = Me, Et, (i)Pr, Cy, (t)Bu) were determined. The π-acidity and σ-donating properties of the R-JohnPhos ligands were quantified using a Cotton-Kraihanzel analysis of the Cr(0)(CO)5(R-JohnPhos) complexes. Somewhat surprisingly, the σ-donating abilities of the R-JohnPhos ligands follow the trend (t)Bu-JohnPhos < Et-JohnPhos < (i)Pr-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 < (i)Pr ≈ Cy < (t)Bu) 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-Cr(0)(CO)4(PR3)2, Pd(0)(PR3)2(η(2)-dba), and trans-Pd(II)(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 (%V(bur)). Overall, the -PMe2 moiety is a powerful way to incorporate strong σ-donation into "designer" phosphines while retaining other advantageous structural and reactivity properties.