Bulky Di(1-adamantyl)phosphinous Acid-Ligated Pd(II) Precatalysts for Suzuki Reactions of Unreactive Aryl Chlorides

Di­(1-adamantyl)­phosphine
oxide (SPO-Ad: Ad2P­(V)­(O)­H),
a stable tautomer of di­(1-adamantyl)­phosphinous acid (PA-Ad: Ad2P­(III)-OH), was employed to synthesize two new PA-Ad-coordinated
complexes, POPd-Ad and POPd2-Ad. POPd-Ad was easily transformed from
POPd2-Ad in acetonitrile, and the [M – H]− ion of the deprotonated POPd-Ad was observed in the electrospray
ionization-mass spectrum of POPd2-Ad. Both complexes are effective
precatalysts for the Suzuki reaction of aryl chlorides. The reduction
of Pd­(II) in POPd-Ad and POPd2-Ad by arylboronic acid was examined,
and the ideal Pd-to-PA ratio in the Suzuki reaction was found to be
1:1. The effect of temperature on the catalytic yields was studied
to examine the possible ligation state of the active species and the
dimer-to-monomer process of POPd2-Ad. Mononuclear and mono-ligated
Pd species was assumed to be catalytically active. The electronic
and steric effects of PA-Ad were slightly better than those reported
for PA-tBu (tBu2P­(III)-OH).
Density functional theory calculations were performed to evaluate
the formation of mono-ligated and mononuclear Pd species from POPd-Ad
and POPd2-Ad. Furthermore, the reaction time and catalyst loading
could be reduced for the reported POPd1-tBu precatalyst using the
optimized reaction conditions for POPd-Ad. The complexes synthesized
in this extensive study will complement the existing SPO-coordinated
POPd series of precatalysts.