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