Systematic Study of the Stereoelectronic Properties of Trifluoromethylated Triarylphosphines and the Correlation of their Behaviour as Ligands in the Rh‐Catalysed Hydroformylation

Abstract

In loving memory of Dr. Julio Real The stereoelectronic properties of a series of trifluoromethylated aromatic phosphines have been studied using different approaches. The σ-donating capability has been evaluated by nuclear magnetic resonance (NMR) spectroscopy of the selenide derivatives and the protonated form of the different trifluoromethylated phosphines. The coupling constants between phosphorous and selenium (1 J SeP) and phosphorous and hydrogen (1 J HP) can be predicted by empirical equations and correlate the basicity of the phosphines with the number and relative position of trifluoromethyl groups. In contrast, the π-acceptor character of the ligands has been evaluated by measuring the frequency of the CO vibration in the infrared (IR) spectra of the corresponding Vaska type iridium complexes ([IrCl(CO)(PAr 3) 2 ], PAr 3 = triarylphosphine). Moreover, the correlation between the electronic properties and the performance of these phosphines as ligands in the rhodium-catalysed hydroformylation of 1-octene has been established. Phosphines with the lowest basicity, that are those with the highest number of trifluoromethyl groups, gave rise to more active catalytic systems.

Preparation of carboxylic arylphosphines by hydrolysis of the trifluoromethyl group

“…The hydrolysis of each trifluoromethyl group producing a new carboxylic group causes a moderate decrease, between 2 and 5 Hz, in the 1 J PSe of the phosphine selenides, consistent with the decrease in the electron withdrawing power of the substituents. In previous works, 19,33 it has been proven that the introduction of trifluoromethyl groups in triarylphosphines was cumulative and resulted in the increase of 12 Hz in the 1 J PSe , regardless of the position of the group. Same observation was found in the case of the trifluoromethylated-carboxylic phosphines.…”

“…Selenide derivatives have been used as a simple tool to study the σ-donation properties of phosphines through the 1 J PSe coupling of the 77 Se isotopomer, measured by 31 P NMR. 19,32,33 The selenide derivatives of the carboxylic-trifluoromethylated phosphines have been prepared following a previously reported methodology in trifluoromethylated triarylphosphines. 19…”

“…19,32,33 The selenide derivatives of the carboxylic-trifluoromethylated phosphines have been prepared following a previously reported methodology in trifluoromethylated triarylphosphines. 19…”

“…Synthesis of the trifluoromethylated triarylphosphines SP1–SP7 was performed according to the previously reported methodologies in the literature. 19,37…”

“…On the other hand, the trifluoromethyl group confers greater π-acidity to the phosphines, which turns into a higher stability towards their oxidation. 17 c ,19 Phosphines with π-acidic character have proven to increase the activity of certain transition metal-catalysed reactions such as some C–C couplings 20 and carbonylations. 17 c ,21…”