Mechanistic Studies of the Hydroamination of Norbornene with Electrophilic Platinum Complexes: The Role of Proton Transfer

Hydroaminations of norbornene with arylsulfonamides and weakly basic anilines were achieved using electrophilic Pt(II) bis(triflate) complexes of the type L2Pt(OTf)(2) (L-2 = (t)Bu(2)bpy, (BuC6H4N)-Bu-t= C(CH3)C(CH3)=(NC6H4Bu)-Bu-t, (C6H5)(2)PCH2CH2P(C6H5)(2), (C6F5)(2)PCH2CH2P(C6F5)(2), S-BINAP). Pseudo-first-order kinetics reveal little to no dependence of the reaction rate on the ancillary ligand. Mechanistic studies do not favor an olefin coordination mechanism but are instead consistent with a mechanism involving sulfonamide coordination and generation of an acidic proton that is transferred to the norbornene. It is postulated that the resulting norbornyl cation is then attacked by free sulfonamide, and loss of proton from this adduct completes the hydroamination. The platinum-sulfonamide complex readily undergoes deprotonation to give a mu-amido platinum-bridged dimer that was isolated from the reaction solution. These studies also involve use of Me3SiPh and Me3SnPh as non-nucleophilic proton traps. Cleavage of the Ph-E bonds was used to detect the acidic, catalytically active species.