Activation of Anisole by Organoplatinum(II) Complexes: Evidence for Rate-Determining C–H Activation

A study of the basis of selectivity of C–H bond activation of anisole by electrophilic methylplatinum­(II) complexes is reported. Anisole reacts with [PtXMe­(NN)] in trifluoroethanol solvent to give methane and [PtXAr­(NN)], Ar = 2-, 3-, and 4-anisyl, in 90:8:2 ratio when X = HOB­(C6F5)3 and NN = (2-C5H4N)2CO (DPK) but not when NN = 2,2′-bipyridine. Similar results are obtained when X = triflate or when NN = (2-C5H4N)2NH. Competition between reaction of anisole and anisole-d8 with [PtXMe­(NN)], X = HOB­(C6F5)3 and NN = DPK, in trifluoroethanol gave an isotope effect kH/kD = 3.6. Several 4-anisyl complexes, [PtClAr­(NN)], [PtAr2(NN)], and [PtMeAr­(NN)], NN = DPK, DPA, or bipy, were prepared and reacted with HX [X = Cl, OTf, or HOB­(C6F5)3]. Reaction of [PtMeAr­(NN)], NN = DPK or bipy, with HX gave a detectable hydride [PtXHMeAr­(NN)] when X = Cl, followed by loss of methane to give [PtClAr­(NN)], but only [Pt­(OTf)­Ar­(NN)] was detected when X = OTf. Reaction with more HOTf gave anisole and [PtX2(NN)], X = OTf, and no isomerization of the 4-anisyl group to the more favored 2-anisyl group was observed at any stage. The similar reaction of [PtMeAr­(NN)] and HOTf in CD3OD/CD2Cl2 gave CHnD4–n (n = 0–4) and mostly 4-MeOC6H4D. It is argued that the anisole C–H bond cleavage step in anisole activation, or the anisyl–H bond forming step in protonolysis, is responsible for the observed selectivity in these reactions.