Triethylphosphine oxide

The mode of solvation for aprotic solvents is largely dipole-dipole in nature and is very strongly controlled by steric factors. Otherwise, solvent is simply present because of packing considerations. Generally dimethylsulphoxide, 

Mej S -O, which is pyramidal and hence has an exposed dipole, produces the largest shift whereas non-dipolar solvents such as hexane or tetra-chloromethane (CCI4) are treated as causing zero shifts for condensed systems such as these. Often, the shifts are to lower frequencies when hydrogen bonds form, as in this case. The shift from zero is proportional to the total H-bond strength, but the individual shifts decrease as the number of bound water molecules increases. This is another example of the anti- cooperativity principle. 

So far as I know, this is one of the best methods for estimating primary solvation numbers experimentally. In principle, neutron diffraction should be superior, although it gives somewhat different information, but it has not yet been used very successfully for systems such as those under consideration here. Using these procedures, solvation numbers for a range of probe solutes have been obtained (Table 3.2). It is noteworthy that the solvation number for water 

---

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

Isab, A.A., Shaw, C.E. Ill and Locke, J. (1988) GC-MS and oxygen-17 NMR tracer studies of triethylphosphine oxide formation from auranofin and water- O in the presence of bovine serum albumin an in vitro model for auranofin metabolism. Inorganic Chemistry, 27, 3406-3409. 

The acceptor number, AN, of a solvent is a measure of the power of the solvent to accept a pair of electrons [18], Experimental evaluation of AN involves observing the frequency changes induced by a solvent on the 31P NMR spectrum when triethylphosphine oxide, Et3P=0, is dissolved in the solvent. Donation of an electron pair from the oxygen atom of Et3P=0, as shown in Scheme 1.2, reduces the electron density around the phosphorus, causing a deshielding effect which leads to an increase in chemical shift. Hexane (AN = 0) and SbCls (AN = 100) were used as fixed points to define this scale. 

The electrophilic properties of molecules were derived from 31P NMR chemical shifts9 produced by electrophilic solvents on triethylphosphine oxide as equilibrium 2 illustrates. 

The acceptor number (AN) [10, 11] of solvent A (Lewis acid) is obtained by measuring the 31P-NMR chemical shift (AS, ppm) of triethylphosphine oxide (Et3P=0,... 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

Two common phosphine oxides are triethylphosphine oxide (each R is a C2H5 group) and tribu-tylphosphine oxide (each R is a C4H9 group). The former is a colorless, deliquescent, crystalline solid (mp, 52.9°C bp, 243°C). The latter is a crystalline solid (mp, 94°C). Both compounds probably have high toxicities when ingested. 

Triethylphosphine oxide contains a highly basic oxygen atom, which is easily accessible to solvent electrophilic attack. This causes a polarization of the P=0 bond and a downfield shift of the 31P NMR signal. The observed chemical shifts (5) referred to the reference solvent n-hexane and extrapolated to infinite dilution may be taken as a measure of the acceptor properties of the solvents. Hence, the acceptor number (AN) is defined as follows ... 

Many physical chemists have embraced the concepts of donicity (donor numbers, DN) and acceptor numbers (AN) as developed by Gutmann and his co-workers [12], The DN is measured by the heat of reaction of the donor solvent and antimony pentachloride in a 1 1 ratio as a dilute solution in 1,2-dichloro-ethane. It is taken to be a measure of the strength of the Lewis base. The AN is measured as the relative shift of the 31P NMR peak in triethylphosphine oxide dissolved in the sample solvent. Hexane is given the value of zero on the scale, and antimony pentachloride is given the value of 100. The AN is taken to be a measure of the strength of the Lewis acid. The applications of the concepts have... 

Acceptor number (or acceptivity), AN — is an empirical quantity for characterizing the electrophilic properties (-> Lewis acid-base theory) of a solvent A that expresses the solvent ability to accepting an electron pair of a donor atom from a solute molecule. AN is defined as the limiting value of the NMR shift, S, of the 31P atom in triethylphosphine oxide, Et3P=0, at infinite dilution in the solvent, relative to n-hexane, corrected for the diamagnetic susceptibility of the solvent, and normalized ... 

Triethylphosphine Oxide may be made in quantity by heating 1 part of phosphorus with 18 parts of ethyl iodide in a sealed tube at 175°— 180° C. The product is distilled, first with ethyl alcohol to remove excess ethyl iodide, and then with concentrated potash, which removes the iodine and oxidises the compound. Thus —... 

N,iV -dimethylaniline AT-oxide, p-Br-N,AT-dimethylaniline AT-oxide, N,N-di-methylaniline AT-oxide, acetonitrile, pyridine N-oxide, and triethylphosphine oxide. 

Adduct formation of triethylphosphine oxide (TEPO) with Ph2BrSn(CH2) SnBrPh2, where n = 6, 10 and 12, and (n-C4H9)3Sn02C(CH2)nC02Sn(n-C4H9)3, where n = 2, 6, 10, 12 and 14, was monitored by P NMR. Equilibrium constants for the former were approximately independent of the chain length from n = 6 to 12, while for the carboxylates the constants for n = 2 and n = 14 were small. Equilibrium constants for the intermediate chains were approximately the same. Solid state NMR shows that the 1 1 TEPO adduct of the n = 12 carboxylate contains two different tin atoms, both of which are five-coordinate, and that the adduct is probably not symmetrically chelated . ... 

A final difference between amine oxides and phosphine oxides lies in the polarity of the molecules. The dipole moment of trimethylamine oxide is 16.7 X 10 C m (5.02 D) compared with 14.6 x 10 C m (4.37 D) for triethylphosphine oxide. A consequence of this polarity is the tendency of the amine oxides to form hydrates, R3NO H2O, and their greater basicity relative to the phosphine oxides. 

Triethylenetetramine, (—CH2NHCH2CH2NH2)2, trien Triethylphosphine, (C2H6)3P Triethylphosphine oxide, (C2H6)3PO Triethylphosphite, (C2H50)gP Trimethylenediamine, see 1,3-propanediamine Urea, OC(NH2)2... 

The acceptor number (AN) was defined as dimensionless number related to the relative chemical P-NMR shift in triethylphosphine oxide (C2H5)3PO in the particular acceptor solvent ... 

Another scale for measuring solvent acidity was formulated by Mayer et al. [43]. It is called the solvent acceptor number (AN) and is based on the relative values of the NMR chemical shifts produced by a given solvent with a strong Lewis base, triethylphosphine oxide (fig. 4.13). The data were normalized so that the acceptor number of hexane is zero and that for the 1 1 adduct with the strong Lewis acid, SbCls, 100 when dissolved in 1-2 dichloroethane. The attractive feature of this scale is that it varies over a wide range for the polar solvents con-... 

Fig. 4.13 Lewis base, triethylphosphine oxide, used to define the AN scale.

The chemical shifts in triethylphosphine oxide measured in polar solvents is used to define the acceptor number scale for solvent acidity [25]. In this case, the oxygen atom in the P=0 bond acts as an electron pair donor to the solvent as a Lewis acid. The resulting inductive effect lowers the electron density at the phosphorus atom and results in a chemical shift which depends on solvent acidity. 

The quaternary phosphonium iodide, R P+I", can be isolated after treatment of the reaction product with aqueous hydrogen sulfide. Thus, white phosphorus heated with excess ethyl iodide at 180° for 22 hr. gave a product from which tetraethylphosphonium iodide was obtained in 49% yield (4). The remainder of the phosphorus was said to be converted to triethylphosphine oxide, although this product was not isolated. When the reaction product is refluxed with ethanol and then distilled over potassium hydroxide, the major product is the trialkyl phosphine oxide (3). Both trialkyldiiodophosphoranes and quaternary phosphonium iodides would be converted to the tertiary phosphine oxide under these conditions. Table III summarizes the... 

---