Esteves Pierre M, De M Carneiro José Walkimar, Cardoso Sheila P, Barbosa André G H, Laali Kenneth K, Rasul Golam, Prakash G K Surya, Olah George A
Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, California 90089-1661, USA.
J Am Chem Soc. 2003 Apr 23;125(16):4836-49. doi: 10.1021/ja021307w.
The mechanism of electrophilic aromatic nitration was revisited. Based on the available experimental data and new high-level quantum chemical calculations, a modification of the previous reaction mechanism is proposed involving three separate intermediates on the potential energy diagram of the reaction. The first, originally considered an unoriented pi-complex or electron donor acceptor complex (EDA), involves high electrostatic and charge-transfer interactions between the nitronium ion and the pi-aromatics. It explains the observed low substrate selectivity in nitration with nitronium salts while maintaining high positional selectivity, as well as observed oxygen transfer reactions in the gas phase. The subsequent second intermediate originally considered an oriented "pi-complex" is now best represented by an intimate radical cation-molecule pair, C(6)H(6)(+)()()/NO(2), that is, a SET complex, indicative of single-electron transfer from the aromatic pi-system to NO(2)(+). Subsequently, it collapses to afford the final sigma-complex intermediate, that is, an arenium ion. The proposed three discrete intermediates in electrophilic aromatic nitration unify previous mechanistic proposals and also contribute to a better understanding of this fundamentally important reaction. The previously obtained ICR data of oxygen transfer from NO(2)(+) to the aromatic ring are also accommodated by the proposed mechanism. The most stable intermediate of this reaction on its potential energy surface is a complex between phenol and NO(+). The phenol.NO(+) complex decomposes affording C(6)H(6)O(+)()/PhOH(+) and NO, in agreement with the ICR results.
亲电芳香硝化反应的机理被重新审视。基于现有的实验数据和新的高水平量子化学计算，提出了对先前反应机理的修正，该修正涉及反应势能图上的三个独立中间体。第一个中间体，最初被认为是无定向的π-络合物或电子供体-受体络合物（EDA），涉及硝鎓离子与π-芳烃之间的高静电和电荷转移相互作用。它解释了在用硝鎓盐硝化时观察到的低底物选择性，同时保持高位置选择性，以及在气相中观察到的氧转移反应。随后的第二个中间体最初被认为是定向的“π-络合物”，现在最好由紧密的自由基阳离子-分子对C(6)H(6)(+)()()/NO(2)来表示，即一个单电子转移（SET）络合物，表明从芳香π-体系到NO(2)(+)发生了单电子转移。随后它分解生成最终的σ-络合物中间体，即芳鎓离子。亲电芳香硝化反应中提出的这三个离散中间体统一了先前的机理提议，并有助于更好地理解这一至关重要的反应。先前获得的关于从NO(2)(+)到芳香环氧转移的离子回旋共振（ICR）数据也与所提出的机理相符。该反应在其势能面上最稳定的中间体是苯酚与NO(+)之间的络合物。苯酚.NO(+)络合物分解生成C(6)H(6)O(+)()/PhOH(+)和NO，这与ICR结果一致。
