Nucleophilic Aromatic Substitution on Aryl-Amido Ligands Promoted by Oxidizing Osmium(IV) Centers

Addition of amine nucleophiles to acetonitrile solutions of the OsIV anilido complex TpOs(NHPh)Cl2 (1) [Tp =
hydrotris(1-pyrazolyl)borate] gives products with derivatized anilido ligands, i.e., TpOs[NH-p-C6H4(N(CH2)5)]Cl2 (2)
from piperidine and TpOs[NH-p-C6H4N(CH2)4]Cl2 (3) from pyrrolidine. These materials are formed in ∼30% yield
under anaerobic conditions, together with ∼60% yields of the OsIII aniline complex TpOs(NH2Ph)Cl2 (5). Formation
of the para-substituted materials 2 or 3 from 1 involves oxidative removal of two hydrogen atoms (two H+ and two
e-). The oxidation can be accomplished by 1, forming 5, or by O2. Related reactions have been observed with
other amines and with the 2-naphthylamido derivative, which gives an ortho-substituted product. Kinetic studies
indicate an addition−elimination mechanism involving initial attack of the amine nucleophile on the anilido ligand.
These are unusual examples of nucleophilic aromatic substitution of hydrogen. Ab initio calculations on 1 show
that the LUMO has significant density at the ortho and para positions of the anilido ligand, resembling the LUMO
of nitrobenzene. By analogy with nucleophilic aromatic substitution, 2 is quantitatively formed from piperidine and
the p-chloroanilide TpOs(NH-p-C6H4Cl)Cl2 (7). Binding the anilide ligands to an oxidizing OsIV center thus causes
a remarkable umpolung or inversion of chemical character from a typically electron-rich anilido to an electron-deficient aromatic functionality. This occurs because of the coupling of redox changes at the TpOsIV center with
bond formation at the coordinated ligand.

将胺亲核试剂加入Os(IV)苯胺基配合物TpOs(NHPh)Cl₂（1）的乙腈溶液中，其中Tp为氢化三(1-吡唑基)硼酸酯（hydrotris(1-pyrazolyl)borate），可得到带有衍生化苯胺基配体的产物：例如与哌啶反应生成TpOs[NH-p-C₆H₄(N(CH₂)₅)]Cl₂（2），与吡咯烷反应生成TpOs[NH-p-C₆H₄N(CH₂)₄]Cl₂（3）。在无氧条件下，此类产物的产率约为30%，同时伴随产率约60%的Os(III)苯胺配合物TpOs(NH₂Ph)Cl₂（5）生成。由配合物1生成对位取代产物2或3的过程，涉及氧化脱除两个氢原子（两个质子与两个电子），该氧化可由配合物1自身介导并生成5，亦可由氧气（O₂）完成。已有研究在其他胺类以及2-萘酰胺基衍生物的反应中观察到类似过程，后者可生成邻位取代产物。动力学研究表明该反应遵循加成-消除机理，即胺亲核试剂首先进攻苯胺基配体，此类反应属于少见的氢亲核芳香取代反应实例。对配合物1的从头算（ab initio）计算结果显示，其最低未占据分子轨道（LUMO）在苯胺基配体的邻位和对位具有显著电子密度，这一特征与硝基苯的最低未占据分子轨道相似。类比亲核芳香取代反应规律，哌啶与对氯苯胺基配合物TpOs(NH-p-C₆H₄Cl)Cl₂（7）反应可定量生成产物2。将苯胺基配体与氧化性Os(IV)中心结合，可使配体的化学性质发生显著的极性反转（umpolung），即从典型的富电子苯胺基转变为缺电子芳香官能团，该现象源于TpOs(IV)中心的氧化还原变化与配位配体上的成键过程相互耦合。