Dimenthylphosphine P‑Oxide as a Synthetic Platform for Bulky and Chiral Ligands with Dimenthylphosphino Donor Groups

Attaching di­(1R)-menthylphosphino fragments (menthyl = Men = 1R,2S,5R-2-isopropyl-5-methylcyclohex-1-yl) to molecular scaffolds turns them into homochiral, bulky, electron-rich phosphine ligands with proven and potential applications in coordination chemistry and transition-metal catalysis. Dimenthylphosphine P-oxide (Men₂POH; 1) is established as a platform chemical toward dimenthylphosphino-containing targets via transformation to the known ligand precursors dimenthylchlorophosphine (4) and dimenthylphosphine (6). Transformations of 1 to dimenthylphosphinyl chloride (5) and dimenthylphosphinic acid (8) are elaborated. A phospha-Michael type 1,4-addition of 1 to p-benzo- or 1,4-naphthoquinone gives the corresponding o-hydroxyaryl­(dimenthyl)­phosphine oxides. Deprotonation of 1 with n-BuLi provides a phosphinyl nucleophile, whose reactions with alkyl halides or 1,n-dihaloalkanes provide tertiary alkyl dimenthylphosphine oxides or 1,n-bis­(dimenthylphosphino)­alkane bis­(P-oxides) 10a–c, respectively. As an example, oxide 10b was deoxygenated to the diphosphine Men₂P­(CH₂)₃PMen₂ (11) and characterized via the square-planar complex [(Men₂P­(CH₂)₃PMen₂)­PdCl₂] (12). A selection of P-aryl dimenthylphosphines, including PhP­(Men)₂ (19) and 2-ClC₆H₄P­(Men)₂ (22), as well as the menthyl analogues Men-JohnPhos (21) and Men-SPhos (24), of the respective Buchwald ligands have been prepared. The combination of the secondary phosphine oxide (SPO) 1 with PdCl₂ produces halide-bridged [(Men₂POH)₂Pd₂Cl₂] (25), mononuclear [(Men₂POH)₂PdCl₂] (26), or the halide-bridged pseudochelate complex [(Men₂PO···H···OPMen₂)₂Pd₂Cl₂] (27), depending on the reaction stoichiometry and conditions, all of which have been crystallographically characterized. The new ligands 1, 19, 21, 22, and 24 and complexes 25 and 26 have been evaluated in model palladium-catalyzed C–C- and C–N-fragment coupling reactions and found to display specific reactivity profiles due to the presence of the menthyl groups. Ligand 22 in particular catalyzed an asymmetric biaryl-forming coupling to give 2-methoxy-1,1′-binaphthalene with an er of up to 93:7.