Alkyl(quinolin-8-yl)phosphine Oxides as Hemilabile
Preligands for Palladium-Catalyzed Reactions
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Abstract
Preligands of quinolyl-substituted
secondary phosphine oxides (SPOs, <b>2a</b>–<b>d</b>) were prepared and characterized.
The unique palladium complex <b>3</b>, having a distorted-square-pyramidal
structure, was obtained from the reaction of 2 equiv of <b>2c</b> with Pd(COD)Cl<sub>2</sub> or [Pd(μ<sub>2</sub>-Cl)(η<sup>3</sup>-allyl)]<sub>2</sub>. In the crystal structure of <b>3</b>, an apical chloride ligand and a supramolecular tetradentate ligand
composed of a deprotonated <b>2c</b>′ and a neutral <b>2c</b>′ were resolved (<b>2c</b>′: PA form
of <b>2c</b>). Intriguingly, the gas-phase optimized geometry
of <b>3</b> converged to a distorted-square-planar structure,
which was predicted by density functional calculations. The solid-state
distorted-square-pyramidal structure of <b>3</b> can only be
explained with the consideration of environmental effects (i.e., the
electrostatic interactions between the surrounding molecules). As
also evidenced by <sup>31</sup>P NMR experiments performed in different
deuterated solvents, the crystal structure of <b>3</b> is retained
in solution. In the crystal structure of <b>3</b>, 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 <b>2c</b> also
tautomerizes to the competent ligand <b>2c</b>′ for palladium-catalyzed
three-component reactions