Catalyst control of selectivity in the C-O bond alumination of biomass derived furans.
Catalyst control of selectivity in the C-O bond alumination of biomass derived furans.
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DOI：
10.1039/d0sc01918f
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发表时间：
2020-07-08
期刊：
影响因子：
8.4
通讯作者：
Crimmin MR
中科院分区：
文献类型：
--
作者：
Hooper TN;Brown RK;Rekhroukh F;Garçon M;White AJP;Costa PJ;Crimmin MR
研究了铝试剂与呋喃、二氢呋喃和二氢吡喃的非催化和催化反应，并由于铝试剂插入杂环的C-O键而导致扩环产物。具体地，[{（ArNCMe）2CH}Al]（Ar = 2，6-二异丙基苯基，1）与呋喃的反应在25 ° C至80 °C之间进行，由于sp2 C-O键净转化为sp2 C-Al键，导致脱芳构化产物。1与呋喃的反应动力学为1级反应，活化参数为ΔH = +19.7（±2.7）kcal mol−1，ΔS = −18.8（±7.8）cal K−1 mol−1和ΔG 298 K = +25.3（±0.5）kcal mol−1，KIE为1.0 ± 0.1。DFT计算支持一个逐步的机制，包括1与呋喃的初始（4 + 1）环加成，形成一个双环中间体，通过α-迁移重排。扩环的选择性受到通过σ*-轨道的布居削弱sp2 C-O键的因素的影响。在这些反应中包含[Pd（PCy 3）2]作为催化剂导致底物范围扩大到包括2，3-二氢呋喃和3，4-二氢吡喃并提高选择性。在催化条件下，断裂的C-O键是与sp 2C-H键相邻的C-O键。二氢化铝（iii）试剂[{（MesNCMe）2CH} AlH 2]（Mes = 2，4，6-三甲基苯基，2）也可以在催化条件下使用以实现呋喃的开环扩环。进一步的机理分析表明，C-O键官能化通过初始C-H键铝化发生。可以分离出衍生自铝试剂在杂环的2-位上的安装的动力学产物。C-H铝化以4.8 ± 0.3的KIE发生，这与涉及C-H键氧化加成到钯催化剂上的转换限制步骤一致。动力学C-H铝化产物到热力学C-O环膨胀产物的异构化是分子内过程，其再次由[Pd（PCy 3）2]催化。DFT计算表明，关键的C-O键断裂步骤涉及基于铝的金属配体对2-钯化杂环的攻击。这一新方法已被应用于来自生物质的重要平台化学品。研究了铝试剂与呋喃、二氢呋喃和二氢吡喃的非催化和催化反应，并由于铝试剂插入杂环的C-O键而导致扩环产物。
Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furans proceeded between 25 and 80 °C leading to dearomatised products due to the net transformation of a sp2 C–O bond into a sp2 C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH‡ = +19.7 (±2.7) kcal mol−1, ΔS‡ = −18.8 (±7.8) cal K−1 mol−1 and ΔG‡298 K = +25.3 (±0.5) kcal mol−1 and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4 + 1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an α-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp2 C–O bond through population of the σ*-orbital. Inclusion of [Pd(PCy3)2] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans and improves selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to the sp2C–H bond. The aluminium(iii) dihydride reagent [{(MesNCMe)2CH}AlH2] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination occurs with a KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to the palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy3)2]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to important platform chemicals from biomass. Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle.
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