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.

本研究针对亲电甲基铂(II)配合物活化苯甲醚C–H键的选择性本质展开报道。苯甲醚与[PtXMe(NN)]在三氟乙醇溶剂中发生反应，生成甲烷与[PtXAr(NN)]；当X=HOB(C6F5)3且NN为二(2-吡啶基)甲酮(DPK, (2-C5H4N)2CO)时，产物中Ar（2-、3-、4-茴香基）的比例为90:8:2，但当NN为2,2'-联吡啶时，未观测到该选择性比例的产物。当X为三氟甲磺酸根(OTf)，或NN为二(2-吡啶基)胺(DPA, (2-C5H4N)2NH)时，可得到类似实验结果。
在三氟乙醇溶剂中，苯甲醚与氘代苯甲醚(anisole-d8)和[PtXMe(NN)]（X=HOB(C6F5)3、NN=DPK）发生竞争反应，测得同位素效应kH/kD=3.6。
本研究制备了多种4-茴香基配合物，包括[PtClAr(NN)]、[PtAr2(NN)]及[PtMeAr(NN)]，其中NN为DPK、DPA或2,2'-联吡啶(bipy)，并将其与质子酸HX（X=Cl、OTf或HOB(C6F5)3）进行反应。其中，[PtMeAr(NN)]（NN为DPK或2,2'-联吡啶）与HX反应时，当X=Cl，可检测到氢化物中间体[PtXHMeAr(NN)]，随后发生甲烷解离得到[PtClAr(NN)]；而当X=OTf时，仅能检测到[Pt(OTf)Ar(NN)]。当使用过量HOTf时，反应产物为苯甲醚与[PtX2(NN)]（X=OTf），且全程未观测到4-茴香基向更热力学稳定的2-茴香基发生异构化的过程。
在氘代甲醇/二氯甲烷(CD3OD/CD2Cl2)混合溶剂中，[PtMeAr(NN)]与HOTf发生类似反应，生成CHnD4-n（n=0~4），且主要产物为4-MeOC6H4D。
研究表明，苯甲醚活化过程中的C–H键断裂步骤，或质子解过程中的茴香基–H键形成步骤，是上述反应中观测到的选择性的核心来源。