Electrophilic Aromatic Substitution Reactions of a Tungsten-Coordinated Phosphirenyl Triflate

The phosphirenyl cation complex [W(CO)(5){PC(Ph)C(Ph)}](+) (2) is formed by chloride abstraction from the chlorophosphirene complex [W(CO)(5){P(Cl)C(Ph)C(Ph)}] (1) with excess AlCl3. The phosphirenyl triflate complex [W(CO)(5){P(OSO2CF3)C(Ph)C(Ph)}] (3) is formed by reaction of the chlorophosphirene complex with AgOSO2CF3 and is in equilibrium with and typically reacts in the same fashion as the phosphirenyl cation. Reaction of 3 with diethylaniline or anisole leads to electrophilic aromatic substitution preferentially at the para position. Reaction with N,N-dimethyl-p-toluidine, in which the para position is blocked, leads to exclusive ortho substitution. The resulting 1,2-substituted arene can adopt a P,N bidentate coordination mode if a CO is removed from tungsten via photolysis. Compound 3 reacts with aromatic heterocycles thiophene, furan, and pyrrole, leading exclusively to substitution in the 2 position, with no evidence for P-S, P-O, or P-N bond formation. Reaction with indole led to substitution at the 3 position, also with no evidence for P-N bond formation. However, upon chromatographic purification, the substituted indole product decomposes into a disubstituted product, with W-coordinated phosphirenyl units at the 3 position and at N. Reaction with phenol and diphenyl amine led exclusively to P-O and P-N bond formation, with no evidence for aromatic substitution. Phosphine products can be removed via oxidation of W with I-2, followed by displacement with bipyridine. A computational study shows that coordination to W(CO)(5) greatly enhances electrophilicity at P in phosphenium ions, leading to the observed rapid electrophilic substitution reactions.