摘要/Abstract
报道了一类新颖的Fe(III)介导的形式上的非活性烯烃的氢炔化反应. 该反应利用Fe(acac)3与苯硅烷反应能原位产生氢自由基的特性实现了氢自由基诱导的1,4-烯炔的分子内1,2-炔基迁移, 合成了一系列α-炔酮类化合物, 产率中等到良好. 基于实验结果及文献报道, 提出了可能的反应机理, 其涉及氢自由基加成、3-exo-dig环化(反鲍德温规则)和 C—C键的断裂重组等. 此外, 该反应具有良好的官能团兼容性, 如雌酮衍生的1,4-烯炔也能成功参与该反应.
关键词: 非活性烯烃氢炔化, 鲍德温规则, 炔基迁移, 1,4-烯炔, α-炔酮衍生物
As inexpensive and readily available feedstocks, alkenes possess a unique reactivity profile and thus have been extensively applied in synthetic chemistry. Specifically, radical-triggered difunctionalization of alkenes provides a valuable synthetic strategy for their high utilization by incorporating two functional groups across the C=C π system. Despite the great achievements gained in this field, the vast majority of well-developed methods generally focus on activated alkenes, because its nascent alkyl radical needs to be stabilized by adjacent functional groups (e.g. aryl, carbonyl, heteroatom) via p-π conjugate effect. However, radical induced difunctionalization of unactivated alkenes remains elusive. Herein, a new protocol for Fe(III) mediated hydroalkynylation of unactivated olefins is reported. By using the characteristics of the in-situ-generated hydrogen radical from the interaction of Fe(acac)3 and phenylsilane, hydrogen radical-triggered intramolecular 1,2-alkynyl migration was realized in this reaction, which led to the synthesis of a series of α-alkynyl ketones with moderate to good yields. Based on the experimental results and literature reports, a reasonable reaction mechanism was proposed, which involved hydrogen radical addition, 3-exo-dig cyclization (anti-Baldwin rules) and C—C bond breaking/recombination. Moreover, the reaction features good tolerance of functional groups, in which estrone-derived 1,4-enyne could be accommodated. A typical procedure for hydroalkynylation of unactivated alkenes is as follows: Fe(acac)3 (1.2 equiv., 0.24 mmol) and NaHCO3 (1.0 equiv., 0.2 mmol) are added to the 10-mL pressure tube. Then 1,4-enynes (1.0 equiv., 0.2 mmol) and phenylsilane (2.0 equiv., 0.4 mmol) are dissolved in 1.0 mL ethyl alcohol, respectively. Both of them are injected into this vial. The reaction system was sealed and stirred at 100 ℃ until the 1,4-enynes consumed that is determined by thin layer chromatography (TLC). After the reaction completes, the resulting mixture is extracted with EtOAc for three times, then the organic phase is concentrated and evaporated on a rotary evaporator. The residue was purified by chromatography on silica gel with petroleum ether/ethyl acetate (V∶V=75∶1) as the eluent to afford α-alkynyl ketones.
Key words: hydroalkynylation of unactivated alkene, Baldwin rule, alkynyl migration, 1,4-enyne, α-alkynyl ketone
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