School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St Andrews, United Kingdom KY16 9ST.
Dalton Trans. 2011 Jan 7;40(1):249-61. doi: 10.1039/c0dt00819b. Epub 2010 Nov 17.
The present mechanistic study comprehensively explores alternative scenarios for activation of the amine-linked allene C=C linkage toward nucleophilic amido attack in the intramolecular hydroamination of a prototypical 1,3-disubstituted aminoallene by a well-characterised samarocene-amido catalyst. Firstly, the non-insertive mechanism by Scott featuring C-N ring closure with concomitant amino proton delivery onto the allene unit has been explored and its key features have been defined. This scenario has been compared and contrasted with the classical stepwise Ln-N σ-bond insertive mechanism that involves rapid substrate association/dissociation equilibria for the Ln-amido-substrate resting state and also for Ln-azacycle intermediates, facile and reversible exocyclic ring closure through migratory allene insertion into the Ln-N amido σ-bond, linked to turnover-limiting Ln-C azacycle aminolysis. The Ln-N σ-bond insertive mechanism prevails for the studied intramolecular hydroamination of 4,5-heptadien-1-ylamine 1 by [Cp*(2)Sm-CH(TMS)(2)] starting material 2. The following aspects are in support of this mechanism: 1) the derived rate law is consistent with the observed empirical rate law; 2) the assessed effective barrier for turnover-limiting aminolysis does agree reasonably well with empirically determined Eyring parameters; 3) the ring-tether double bond selectivity is consistently elucidated. On the other hand, this study reveals that the non-insertive mechanism, which features a prohibitively large barrier, is unachievable. Spatial demands around the lanthanide centre effect the two mechanisms differently. A sufficiently accessible lanthanide is a crucial requirement of the Ln-N σ-bond insertive mechanism and enhanced encumbrance renders the cyclisation step less accessible kinetically. This contrasts with the non-insertive mechanism, where greater lanthanide protection has a rather modest influence. The present study indicates that the non-insertive mechanism would prevail if the lanthanide centre were to be protected effectively against C=C bond approach, whilst ensuring a high polarity of the Ln-N σ-bond together with a sufficiently acidic amino proton.
本机理研究全面探讨了在典型的 1,3-取代氨基烯丙基与经过充分表征的钐酰胺催化剂的分子内环化氨化反应中，胺键连接的烯丙基 C=C 键对亲核酰胺进攻的活化的替代方案。首先，探索了 Scott 的非插入机理，该机理涉及 C-N 环闭合，同时氨基质子传递到烯丙基单元上，并且已经定义了其关键特征。该方案已与经典的逐步 Ln-Nσ-键插入机理进行了比较和对比，该机理涉及 Ln-酰胺-底物的快速基态缔合/解离平衡，以及 Ln-氮杂环中间体，通过易迁移的烯丙基插入到 Ln-N 酰胺σ-键中进行可逆的外环闭合，与周转限制的 Ln-C 氮杂环酰胺水解反应相关联。Ln-Nσ-键插入机理适用于所研究的 4,5-庚二烯-1-基胺 1 通过 [Cp*(2)Sm-CH(TMS)(2)] 起始原料 2 的分子内环化氨化。以下方面支持该机理：1）推导的速率定律与观察到的经验速率定律一致；2）评估的周转限制的酰胺水解的有效势垒与经验确定的 Eyring 参数相当吻合；3）一致阐明了环系双键选择性。另一方面，本研究表明，具有不可逾越的大势垒的非插入机理是无法实现的。镧系元素中心周围的空间需求对两种机理的影响不同。足够可接近的镧系元素是 Ln-Nσ-键插入机理的关键要求，并且增强的阻碍使得环化步骤在动力学上难以接近。这与非插入机理形成对比，在非插入机理中，更大的镧系元素保护对其影响相对较小。本研究表明，如果有效地保护镧系元素中心免受 C=C 键的接近，同时确保 Ln-Nσ-键具有足够的极性和足够酸性的氨基质子，那么非插入机理将占主导地位。
