Alkyl(quinolin-8-yl)phosphine Oxides as Hemilabile Preligands for Palladium-Catalyzed Reactions

Preligands of quinolyl-substituted
secondary phosphine oxides (SPOs, 2a–d) were prepared and characterized.
The unique palladium complex 3, having a distorted-square-pyramidal
structure, was obtained from the reaction of 2 equiv of 2c with Pd­(COD)­Cl2 or [Pd­(μ2-Cl)­(η3-allyl)]2. In the crystal structure of 3, an apical chloride ligand and a supramolecular tetradentate ligand
composed of a deprotonated 2c′ and a neutral 2c′ were resolved (2c′: PA form
of 2c). Intriguingly, the gas-phase optimized geometry
of 3 converged to a distorted-square-planar structure,
which was predicted by density functional calculations. The solid-state
distorted-square-pyramidal structure of 3 can only be
explained with the consideration of environmental effects (i.e., the
electrostatic interactions between the surrounding molecules). As
also evidenced by 31P NMR experiments performed in different
deuterated solvents, the crystal structure of 3 is retained
in solution. In the crystal structure of 3, a long Pd–Cl
bond was analyzed by energy decomposition analysis, showing that the
bond is dominated by electrostatic character. Furthermore, application
of these SPOs using the Heck reaction shows good reactivity toward
common aryl bromides. The hemilabile preligand 2c also
tautomerizes to the competent ligand 2c′ for palladium-catalyzed
three-component reactions.