XL-Xantphos: Design and Synthesis of a Mechanistic Probe of Xantphos O-Coordination in Catalytic Reactions

Abstract

The synthesis and characterization of an analog of the Xantphos ligand that is geometrically incapable of coordination of the xanthene bridging oxygen atom is reported. This new ligand, XL-Xantphos, ((9,9-dimethyl-9H-xanthene-4,5-diyl)­bis­(4,1-phenylene))­bis­(diphenylphosphane), was studied in homogeneous, catalytic reactions for comparison with Xantphos. The XL-Xantphos ligand performed essentially identically to Xantphos in Rh-catalyzed hydroformylation of 1-octene, which suggests that the high regioselectivity for linear aldehyde is due to the large bite angle of these ligands and is not influenced by oxygen coordination to the metal. The Pd-catalyzed amidocarbonylation of 4-bromoanisole with dimethylhydroxylamine hydrochloride similarly showed no difference between Xantphos and XL-Xantphos. Computations on Pd­(II) phosphine complexes at the DLPNO–CCSD­(T) level of theory indicated that these ligands have different preferences for cis and trans coordination modes. The XL-Xantphos ligand has a thermodynamic preference for trans-chelated structures, whereas the cis-[(Xantphos)­PdCl2] isomer was calculated to be thermodynamically more stable than its trans isomer. Given the key role of d8 square planar Pd intermediates in many catalytic cycles, the greater preference of Xantphos to form cis chelates may indeed be a factor which has made this ligand particularly effective.

Ruthenium(II) Complexes of a Xanthene-Spanned Dicarbene Ligand

“…The rich transition metal chemistry of wide-angle diphosphine ether (POP) ligands, particularly those spanned by the flexible xanthene scaffold, such as in xantphos and close derivatives, has underpinned the discovery and development of new, efficacious homogeneous catalytic processes. − An additional advantage of xantphos and other POP ancillary ligands is that the ethereal O donor may be hemilabile, switching on and off a metal ion as needed during a catalytic cycle. , However, diphosphines are not without drawbacks as ancillary ligands in metal catalysts. These drawbacks include difficult syntheses, especially for chiral derivatives, and a proclivity to decompose, leading to catalyst stability and catalysis lifetime problems. − …”