NMR study of intramolecular and intermolecular ligand exchange in phenyltrifluorophosphorane

Abstract

Intramolecular axial-equatorial exchange and base-initiated intermolecular fluorine exchange in phenyltrifluorophosphorane have been studied by the dynamic NMR technique. Axial-equatorial exchange appears to be a unimolecular process with *298 = 13.3 ± 0.5 kcal/mol, l\H* = 12.6 ± 0.4 kcal/mol, AS* = -2.1 ± 1.3 eu, and £a = 13.2 ± 0.4 kcal/mol. Addition of triethylamine or pyridine to C6H5PF3H leads to intermolecular fluorine exchange. Rate studies of the intermolecular process support a mechanism involving slow production of C6H5PF4H~f ollowed by rapid fluorine transfer between fiveand six-coordinate species C6H5PF3H and C6H5PF4H~. Although traces of HF and H20 impurities did not interfere with this study, the impurity C6H5PF4 markedly affected the NMR results. The intramolecular process is not correlated with the intermolecular process, and phosphorus-hydrogen coupling is retained throughout the rapid intermolecular fluorine-exchange process.

Nuclear magnetic resonance study of fluorine exchange in the system

“…As seen in Fig. 1, exchange is slower in acetonitrile than in toluene, in agreement with a similar effect found in the PhPF3H-PhPF,H-system (1) and attributed to coordination of the donor solvent to the phosphorane, thus preventing the 'Visiting scientist. Permanent address: Department of Chemistry, Wuhan University, Wuhan, China.…”

“…Addition of a base such as pyridine (or triethylamine) leads to intermolecular fluorine exchange with AG298 = 7 + 2 kcal/mol. The intermolecular process is solvent-dependent and appears to involve slow reaction with base to produce PhPF,H-, followed by rapid fluorine transfer between five-and six-coordinate species, PhPF3H and PhPF4H-, via a fluorine-bridged intermediate (1). In order to establish whether a similar mechanism is responsible for intermolecular fluorine exchange in phenyltetrafluorophosphorane, PhPF,, (2) we decided to cany out a 3'P nmr study of exchange in the PhPF4-PhPF; system.…”

Phenylfluorophosphoranes: axial–equatorial fluorine exchange in RC6H4PF3H and intermolecular exchange in the PhPF2(H)OMe–MeOH–base system

“…In the absence of a base, axial-equatorial fluorine exchange in RC6H4PF3H may be studied by comparing the variabletemperature 3 '~{ '~) nmr spectra with line shapes calculated by the method described previously for PhPF3H (1). A leastsquares analysis of the data gave the activation parameters for axial-equatorial exchange listed in Table 2, with AGig8 values in the range 52.9 to 56.0 kJ mol-', which may be compared to 55.6 W mol-' for PhPF3H (1).…”

“…A leastsquares analysis of the data gave the activation parameters for axial-equatorial exchange listed in Table 2, with AGig8 values in the range 52.9 to 56.0 kJ mol-', which may be compared to 55.6 W mol-' for PhPF3H (1). These results show that trifluoromethyl or methyl substituents have only a slight effect on the activation barriers, and our attempt to slow down axial-equatorial scrambling in five-coordinate species by this approach was not successful, therefore RC6H4PF3H is not a suitable "rigid" probe of site-specific bond cleavage at ambient temperature.…”

“…Stereoselective bond-cleavage is difficult to observe in fiveand six-coordinate compounds if accompanied by axialequatorial scrambling of ligands, as illustrated by the PhPF3H-PhPF4HP or PhPF4-PhPFSP systems (1,2), although the introduction of three phenyl substituents can avert this problem, as demonstrated by the Ph3TeF3-Ph3TeF2+ or Ph3TeF2C1-Ph3~eFCl+ systems (3). In this paper we describe another, albeit unsuccessful, attempt at slowing down axial-equatorial ligand exchange, namely, by the introduction of trifluoromethyl or methyl substituents into the phenyl ring of PhPF3H.…”