NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl-alkyl cross-coupling.
NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl-alkyl cross-coupling.
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DOI：
10.1039/c7sc04750a
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发表时间：
2018-02-21
期刊：
影响因子：
8.4
通讯作者：
Neidig ML
中科院分区：
文献类型：
--
作者：
Fleischauer VE;Muñoz Iii SB;Neate PGN;Brennessel WW;Neidig ML
在催化过程中观察到的铁-NHC的分离和反应性证明了配体对亲核螯合的影响，抑制了烷基-烷基交叉偶联中的β-氢化物消除。虽然铁-NHC催化的交叉偶联已被证明对各种各样的反应（例如芳基-芳基、芳基-烷基、烷基-烷基）是有效的，但有效催化体系中原位形成的和反应性铁物质的性质在很大程度上仍不确定。在目前的研究中，冷冻捕获穆斯堡尔谱，EPR研究结合无机合成和反应研究被用来定义关键的原位形成和反应性铁NHC物种的Kumada烷基-烷基交叉偶联（2-（1，3-二氧六环-2-基）乙基）溴化镁和1-碘-3-苯基丙烷。原位形成的关键活性铁物质被鉴定为（IMes）Fe（（1，3-二氧杂环己烷-2-基）乙基）2，而先前在该化学中鉴定的S = 1/2铁物质被发现仅是非常少的停止循环物质（<所有铁的0.5%）。反应和动力学研究表明，（IMes）Fe（（1，3-二氧杂环己烷-2-基）乙基）2对亲电试剂具有高度反应性，导致相对于铁的两次周转（kobs > 24 min-1），以产生具有类似于催化的总体选择性的交叉偶联产物。该催化体系对烷基亲核试剂的β-氢化物消除的高抗性归因于其通过亲核试剂的缩醛部分的碳和一个氧的连接而与铁螯合。事实上，替代的NHC配体如SIPr在催化中不太有效，因为它们增加的空间体积抑制了烷基配体螯合的能力。总体而言，本研究确定了一种新的烷基螯合方法，以实现有效的烷基-烷基交叉耦合与铁（II）-NHC，提供直接的结构洞察NHC对催化性能的影响，并扩展铁（II）反应物种在铁催化的交叉耦合的重要性。
Isolation and reactivity of iron–NHCs observed during catalysis demonstrate ligand effects on nucleophile chelation, suppressing β-hydride elimination within alkyl–alkyl cross-coupling. While iron–NHC catalysed cross-couplings have been shown to be effective for a wide variety of reactions (e.g. aryl–aryl, aryl–alkyl, alkyl–alkyl), the nature of the in situ formed and reactive iron species in effective catalytic systems remains largely undefined. In the current study, freeze-trapped Mössbauer spectroscopy, and EPR studies combined with inorganic synthesis and reaction studies are utilised to define the key in situ formed and reactive iron–NHC species in the Kumada alkyl–alkyl cross-coupling of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide and 1-iodo-3-phenylpropane. The key reactive iron species formed in situ is identified as (IMes)Fe((1,3-dioxan-2-yl)ethyl)2, whereas the S = 1/2 iron species previously identified in this chemistry is found to be only a very minor off-cycle species (<0.5% of all iron). Reaction and kinetic studies demonstrate that (IMes)Fe((1,3-dioxan-2-yl)ethyl)2 is highly reactive towards the electrophile resulting in two turnovers with respect to iron (kobs > 24 min–1) to generate cross-coupled product with overall selectivity analogous to catalysis. The high resistance of this catalytic system to β-hydride elimination of the alkyl nucleophile is attributed to its chelation to iron through ligation of carbon and one oxygen of the acetal moiety of the nucleophile. In fact, alternative NHC ligands such as SIPr are less effective in catalysis due to their increased steric bulk inhibiting the ability of the alkyl ligands to chelate. Overall, this study identifies a novel alkyl chelation method to achieve effective alkyl–alkyl cross-coupling with iron(ii)–NHCs, provides direct structural insight into NHC effects on catalytic performance and extends the importance of iron(ii) reactive species in iron-catalysed cross-coupling.
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影响因子：
15
作者：
Daifuku SL;Kneebone JL;Snyder BE;Neidig ML
通讯作者：
Neidig ML
影响因子：
15
作者：
Adams, Christopher J.;Bedford, Robin B.;Nunn, Joshua
通讯作者：
Nunn, Joshua
影响因子：
5.2
作者：
Agrawal, Toolika;Cook, Silas P.
通讯作者：
Cook, Silas P.
影响因子：
3.6
作者：
Bedford, RB;Betham, M;Hird, M
通讯作者：
Hird, M
DOI：
10.1007/3418_2015_99
发表时间：
2015-01-01
期刊：
IRON CATALYSIS II
影响因子：
--
作者：
Bedford, Robin B.;Brenner, Peter B.
通讯作者：
Brenner, Peter B.