Phosphorus fluoride: Lewis acidity

Arguably the best known phosphorus containing Lewis acid is PF. It is often used to abstract a fluoride ion from another molecule, thus forming PF -, a popular noncoordinating anion. Its P-NMR resonance is observed at around tJp=-144ppm. 

Fluorides with fluorine-phosphorus bonds also react with Lewis acids tert-Butylpentafluorocyclotriphosphazenes are arylated in the presence of aluminum chlonde [26] (equation 18)... 

Lewis Acid Complexes. Sulfolane complexes with Lewis acids, such as boron trifluoride or phosphorus pentafluoride (17). For example, at room temperature, sulfolane and boron trifluoride combine in a 1 1 mole ratio with the evolution of heat to give a white, hygroscopic solid which melts at 37°C. The reaction of sulfolane with methyl fluoride and antimony pentafluoride in liquid sulfur dioxide gives crystalline tetrahydro-l-methoxythiophenium-1-oxidehexafluoroantimonate, the first example of an alkoxysulfoxonium salt (18). 

Covalent fluorides of group 3 and group 5 elements (boron, tin, phosphorus, antimony, etc) are widely used in organic synthesis as strong Lewis acids Boron trifluoride etherate is one of the most common reagents used to catalyze many organic reactions. A representative example is its recent application as a catalyst in the cycloadditions of 2-aza-l,3-dienes with different dienophiles [14] Boron trifluoride etherate and other fluormated Lewis acids are effective activators of the... 

Grown Ethers. Ethylene oxide forms cyclic oligomers (crown ethers) in the presence of fluorinated Lewis acids such as boron trifluoride, phosphorus pentafluoride, or antimony pentafluoride. Hydrogen fluoride is the preferred catalyst (47). The presence of BF 4, PF y, or SbF 6 salts of alkali, alkaline earth, or transition metals directs the oligomerization to the cyclic tetramer, 1,4,7,10-tetraoxacyclododecane [294-93-9] (12-crown-4), pentamer, 1,4,7,10,13-pentaoxacyclopentadecane [33100-27-6] (15-crown-6), andhexamer, 1,4,7,10,13,16-hexaoxacyclooctadecane [17455-13-9]... 

The same intramolecular Lewis base - Lewis acid interaction can be observed when a chlorophosphane is used instead of a fluorophosphane. However, the chloride is less strongly bonded than fluoride, resulting in the displacement of chloride by the phosphane without the use of an auxiliary Lewis acid. The chemical shift of the tricoordinate phosphorus atom is sensitive to the steric bulk of its carbon substituent. Evidently, sterically demanding substituents like tert-butyl hinder the 2T-bonding interaction from nitrogen, resulting in the observed downfield shift. 

Reaction of PF R (R=Me, Ph, F) with a carbene results in the six-coordinate phosphorus species PF R(carbene). The phosphorus compound acts as a Lewis acid toward the Lewis basic carbene, and the phosphorus resonance is shifted upfield by about Afine structure of this upfield shift is of considerable interest. The electronegativity of the substituent R increases in the order Me < Ph < F in accord with an upfield shift in the phosphorus resonance. Therefore, we again witness a case where the loss in electron density through the donor interaction, in the present case from the fluoride substituents on phosphorus. As the influence of the. -bonding interaction on the phosphorus chemical shift is larger than that of the bonding interaction, we witness a net upfield shift. 

Fluorides. PF5 is easily prepared by the interaction of PC15 with CaF2 at 300-400°. It is a very strong Lewis acid and forms complexes with amines, ethers and other bases as well as with F" in which phosphorus becomes 6-coordinate. However, these organic complexes are less stable than those of BF3 and are rapidly decomposed by water and alcohols. Like BF3, PF5 is a good catalyst, especially for ionic polymerization. 

The relatively weak Lewis acidity of the boron center creates a wealth of synthetic chemistry but also allows boron to act as a receptor for hard anions, particularly fluoride, hydroxide, and cyanide. The use of boron as a Lewis acid extends to formation of coordinate bonds with a wide variety of heteroatoms including oxygen, sulfur, phosphorus, and nitrogen. 

---