Synthesis of air-stable zwitterionic 2-phosphiniminium-arenesulfonates

Efficient synthetic methodology for preparation of 2-phosphiniminium-5-methylbenzenesulfonate zwitterions is reported. Staudinger reaction between phosphines and n-propyl 2-azido-5-methylbenzenesulfonates followed by sulfonate ester deprotection using pyridinium tetrafluoroborate/pyridine afforded the zwitterions in excellent yields. This new route directly accesses ortho-substituted-arenesulfonate ligands that incorporate a phosphinimine, a strong σ-donor.