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.
