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)(2)CO (DPK) but not when NN = 2,2'-bipyridine. Similar results are obtained when X = triflate or when NN = (2-C5H4N)(2)NH. Competition between reaction of anisole and anisole-d(8) with [PtXMe(NN)], X = HOB(C6F5)(3) and NN = DPK, in trifluoroethanol gave an isotope effect k(H)/k(D) = 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.