Supercritical carbon dioxide as a reaction medium for silane-mediated free-radical carbonylation of alkyl halides
Supercritical carbon dioxide as a reaction medium for silane-mediated free-radical carbonylation of alkyl halides
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DOI：
10.1021/jo001135q
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发表时间：
2000-11-03
影响因子：
3.6
通讯作者：
Ikariya, T
中科院分区：
文献类型：
--
作者：
Kishimoto, Y;Ikariya, T
自由基羰基化因其在有机合成化学中的潜在广泛应用而被广泛研究，作为引入羰基的有前途的工具。1 Ryu和Sonoda报道了一项开创性的研究，即在苯中用三正丁基锡二氧化物2或三-（三甲基硅基）硅烷（（TMS）3SiH）3对有机卤化物进行自由基还原羰基化，以中等至高产率得到醛或酮。已知这种羰基化的结果取决于烷基或酰基自由基中间体的反应性和稳定性，预期其受到溶剂笼效应的强烈影响。苯是这种自由基反应最常用的非极性溶剂之一，但它是一种对环境有害的化合物。Ryu和Curran提出了有机/氟两相体系作为苯溶剂的替代品，用于烷基卤化物的自由基羰基化，其中氟锡氢化物试剂用作链载体。4超临界二氧化碳（scCO 2）是一种新兴的自由基反应反应介质，原因有很多，例如压力可调的笼效应，不存在自由基链转移到介质中，5以及独特的性质，例如反应气体的高扩散性和高溶解性。6这些独特的性能使得在与气态反应物的反应中获得高生产率并避免潜在的传质限制成为可能。7事实上，DeSimone已经报道了在scCO 2中由2，2′-偶氮二（异丁腈）（AIBN）引发的自由基聚合有效地进行，因为在超临界介质中的笼效应较弱，引发效率比在苯中高。8我们最近通过在这种介质中进行Ru（II）催化的CO2氢化7和Pd（II）催化的芳基碘羰基化9证明了scCO 2的明显优势。因此，采用scCO 2作为反应介质应允许高效的自由基羰基化。在这里，我们报告的第一个硅烷介导的有效的自由基羰基化的有机卤化物酮scCO 2。1-碘辛烷（1）和丙烯腈（2a）与可溶于CO2的（TMS）3SiH在scCO 2中的还原羰基化反应（CO 20大气压，305大气压总压），含有AIBN作为自由基引发剂（1/2a/（TMS）3SiH/AIBN）1：1.2：1.5：0.3）在80 ℃下顺利进行，以良好的产率（80%）得到1-氰基-3-十一烷酮（3a）（当量1）。10表1总结了一些有代表性的结果。3a的产率明显高于苯中获得的产率（条目1和2）。有机卤化物1在这些条件下完全消耗。已知该反应经由正辛基自由基的形成，随后烷基自由基羰基化成酰基自由基并将其亲核加成至丙烯腈而进行。因此，生成的
Free-radical carbonylation has been extensively investigated as a promising tool for the introduction of a carbonyl group because of its potentially wide scope in organic synthetic chemistry. 1 Ryu and Sonoda reported a pioneering study of free-radical reductive carbonylation of organic halides with tri-n-butyltin hydride2 or tris-(trimethylsilyl) silane ((TMS) 3SiH) 3 in benzene, giving aldehydes or ketones in moderate to high yields. The outcome of this carbonylation is known to be dependent on the reactivity and stability of alkyl or acyl radical intermediates, which are expected to be strongly influenced by the solvent cage effect. Benzene is one of most useful nonpolar solvents for this radical reaction, but it is an environmentally hazardous compound. Ryu and Curran proposed organic/fluorous biphase systems as an alternative to benzene solvent for the free-radical carbonylation of alkyl halides, where a fluorous tin hydride reagent was used as a chain carrier. 4 Supercritical carbon dioxide (scCO2) is an emerging reaction medium for the free-radical reactions for a number of reasons, such as the pressure-tunable cage effect, the absence of radical chain transfer to the medium, 5 and unique properties such as high diffusivity and high miscibility of reactant gases. 6 These unique properties make it possible to attain high productivity in reactions with gaseous reactants and to avoid potential mass-transfer limitations. 7 In fact, DeSimone has reported that free-radical polymerization initiated by 2, 2′-azobis (isobutyronitrile)(AIBN) in scCO2 effectively proceeds with higher initiation efficiency than in benzene because of the weak cage effect in the supercritical medium. 8 We have recently demonstrated a discernible advantage of scCO2 by performing the Ru (II)-catalyzed hydrogenation of CO2 7 and the Pd (II)-catalyzed carbonylation of aryl iodides9 in this medium. Thus, adopting scCO2 as a reaction medium should allow for a highly efficient radical carbonylation. Herein, we report the first silane-mediated efficient free-radical carbonylation of organic halides to ketones in scCO2. The reductive carbonylation of 1-iodooctane (1) and acrylonitrile (2a) with CO2-soluble (TMS) 3SiH in scCO2 (CO 20 atm, 305 atm total pressure) containing AIBN as a radical initiator (1/2a/(TMS) 3SiH/AIBN) 1: 1.2: 1.5: 0.3) at 80 C proceeded smoothly to give 1-cyano-3-undecanone (3a) in a good yield (80%)(eq 1). 10 Table 1 summarizes some representative results. The yield of 3a was noticeably higher than that attained in benzene (entry 1 and 2). Organic halide 1 was completely consumed under these conditions. The reaction is known to proceed via formation of n-octyl radical followed by carbonylation of alkyl radical to acyl radical and its nucleophilic addition to acrylonitrile. Thus, the generated