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Hydrogen Isotope Exchange with Superbulky Alkaline Earth Metal Amide Catalysts
ACS Catalysis ( IF 12.9 ) Pub Date : 2020-06-12 , DOI: 10.1021/acscatal.0c01359
Johannes Martin 1 , Jonathan Eyselein 1 , Samuel Grams 1 , Sjoerd Harder 1
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Heavier alkaline earth (Ae) metal amide complexes Ae(NR2)2 (Ae = Ca, Sr, Ba) were found to be highly active catalysts for hydrogen isotope exchange (HIE). The activities for D/H exchange between C6D6 and H2 strongly increase with metal size (Ca < Sr < Ba) and with amide bulk: N(SiMe3)2 < N(DIPP)(SiiPr3)< N(SiiPr3)2, DIPP = 2,6-diisopropylphenyl. At 120 °C and pressures of 10–50 bar, no hydrogenation side-products are produced, and TONs of 205 and TOFs of 268, competitive with those for precious metal catalysts, have been achieved. The reverse H/D exchange between C6H6 and D2 is even faster by a factor 1.5–2. Substrates also include a range of substituted arenes. Alkyl-substituted aromatic rings are preferably deuterated in acidic benzylic positions, and this tendency increases with the number of alkyl-substituents. Although unactivated (sp3)C–H units could not be deuterated, the (sp3)Si–H function in primary, secondary, and tertiary alkylsilanes could be converted. Two different pathways for C6H6/D2 isotope exchange have been evaluated by DFT calculations: (A) a deprotonation/protonation mechanism and (B) direct nucleophilic aromatic substitution. Although the exact nature of the catalyst(s) is unclear, the first step is the conversion of Ae(NR2)2 with D2 into R2NAeD which can aggregate to larger clusters. Energy profiles with model catalysts (iPr3Si)2NAeD and [(Me3Si)2NAeD]2 (Ae = Ca or Ba) show that the direct nucleophilic aromatic substitution is the most likely mechanism for deuteration of arenes. The key to this unusual reaction is the initial formation of a π-arene···Ae complex which is followed by the generation of an intermediate with a Meisenheimer anion. Heavier Ae metal amide complexes are, despite the lack of partially filled d-orbitals for substrate activation, potent catalysts for HIE.

中文翻译:

超级大宗碱土金属酰胺催化剂的氢同位素交换

发现较重的碱土(Ae)金属酰胺络合物Ae(NR 22(Ae = Ca,Sr,Ba)是用于氢同位素交换(HIE)的高活性催化剂。C 6 D 6和H 2之间的D / H交换活性随金属大小(Ca <Sr <Ba)和酰胺基团的增加而强烈增加:N(SiMe 32 <N(DIPP)(Si i Pr 3)< N(Si i Pr 32,DIPP = 2,6-二异丙基苯基。在120°C和10–50 bar的压力下,没有产生氢化副产物,与贵金属催化剂相比,TON值为205,TOF为268。C 6 H 6和D 2之间的反向H / D交换甚至快1.5到2倍。底物还包括一系列取代的芳烃。烷基取代的芳族环优选在酸性苄基位置上氘代,并且这种趋势随着烷基取代基的数目而增加。尽管未活化的(sp 3)C–H单元不能氘化,但可以转化伯,仲和叔烷基硅烷中的(sp 3)Si–H功能。C 6的两种不同途径H 6 / D 2同位素交换已通过DFT计算进行了评估:(A)去质子化/质子化机理和(B)直接亲核芳族取代。虽然催化剂(多个)的确切性质是不清楚的,第一步是AE(NR转化22与d 2成R 2 NAED其可以聚集以更大的集群。模型催化剂(i Pr 3 Si)2 NAeD和[(Me 3 Si)2 NAeD] 2的能量分布(Ae = Ca或Ba)表明直接亲核芳族取代是芳烃氘化的最可能机制。这种异常反应的关键是最初形成π-芳烃·····································································································································································································。尽管缺乏用于底物活化的部分填充的d-轨道,重的Ae金属酰胺络合物仍然是HIE的有效催化剂。
更新日期:2020-07-17
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