Di(1-adamantyl)phosphine oxide (SPO-Ad: Ad 2 P(V)(=O)H), a stable tautomer of di(1-adamantyl)phosphinous acid (PA-Ad: Ad 2 P(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 ( t Bu 2 P(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.
In this study, two (HO)R 2 P!Pd(II)Cl 2 ! NHC precatalysts (1 s) were designed and synthesized. Using these precatalysts, unreactive aryl chlorides, benzyl chlorides, or benzoyl chlorides were highly efficiently converted to the desired products in three to tens of minutes at 35 °C. Upon deprotonation of 1 s, density-functional theory calculations and electrospray ionization mass spectrometry revealed that NHC was dissociated from the deprotonated precatalysts. Although it is difficult to ascertain the NHC dissociated from Pd(II) or Pd(0) species, the proposed anionic (O À )R 2 P!Pd(II)Cl 2 species could be reduced using ethoxide to form Pd(0) catalytic species. The cation and anion functions of salt additives were systematically elucidated. The cation size could be used to modulate the catalytic properties of 1 s through outer-sphere electrostatic interactions between the coordinated R 2 P(O À ) and alkali metal ions. Moreover, the basicity of base was proportional to the catalytic yields. This study introduces newly designed Pd(II) precatalysts with superb catalytic properties that are stable and remarkably efficient.
In this study, two (HO)R2P→Pd(II)Cl2←NHC precatalysts (1s) were designed and synthesized. Using these precatalysts, yields of unreactive aryl chloride, benzyl chloride, and benzoyl chloride were obtained in three to tens of minutes. Upon deprotonation, density-functional theory calculations and electrospray ionization mass spectrometry revealed that NHC was dissociated from the deprotonated precatalysts. Subsequently, the anionic (O−)R2P→Pd(II)Cl2 species could be reduced using ethoxide to form Pd(0) catalytic species. The cation and anion functions of salt additives were elucidated. The cation size could be used to modulate the catalytic properties of 1s through outer-sphere electrostatic interactions between the coordinated R2P(O−) and alkali metal ions. Through this study, we newly designed Pd(II) precatalysts with superb catalytic properties that are stable and highly efficient.
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