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 I2, 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.