Electron-Rich Trialkyl-Type Dihydro-KITPHOS Monophosphines: Efficient Ligands for Palladium-Catalyzed Suzuki–Miyaura Cross-Coupling. Comparison with Their Biaryl-Like KITPHOS Monophosphine Counterparts

The Diels–Alder cycloaddition
between dicyclohexylvinylphosphine
oxide and anthracene or 9-methylanthracene affords the bulky electron-rich
trialkyl-type dihydro-KITPHOS monophosphines 11-(dicyclohexylphosphinoyl)-12-phenyl-9,10-dihydro-9,10-ethenoanthracene
and 11-(dicyclohexylphosphinoyl)-9-methyl-12-phenyl-9,10-dihydro-9,10-ethenoanthracene,
respectively, after reduction of the corresponding oxide. Both phosphines
are highly air-sensitive and rapidly oxidize on silica gel during
purification but have been isolated as air-stable cyclometalated palladium
precatalysts of the type [Pd­{κ2N2′,C1-2-(2′-NH2C6H4)­C6H4}­Cl­(L)]. Both palladium precatalysts
form highly active systems for the Suzuki–Miyaura cross-coupling
of a range of aryl chlorides with aryl boronic acids, giving the desired
products in good to excellent yield under mild conditions and a catalyst
loading of 0.25 mol %. A comparison of the performance of catalysts
based on dihydro-KITPHOS monophosphines against their first-generation
biaryl-like KITPHOS counterparts revealed that the latter are consistently
more efficient for the vast majority of substrate combinations examined,
albeit by only a relatively small margin in some cases. This, together
with the greater air stability and ease of handling of biaryl-like
KITPHOS monophosphines, renders them more practical ligands for palladium-based
cross-coupling. The steric parameters of both classes of KITPHOS monophosphine
and a selection of electron-rich biaryl monophosphines have been quantified
using a combination of Solid-G to determine the percentage of the
metal coordination sphere shielded by the phosphine (the G parameter), and Salerno molecular buried volume calculations (SambVca) to determine the percent buried volume (%Vbur); the corresponding Tolman cone angles have also been
determined from correlations and the relative merits of the two approaches
discussed. The electronic properties of these phosphines have also
been investigated using DFT to calculate the A1 ν­(CO)
frequency in LNi­(CO)3 (B3LYP/6-31G­(2d,p)­[LanL2DZ on Ni]),
and the resulting computed electronic parameters (CEP) were used to
estimate the corresponding experimental Tolman electronic parameters
(TEP).