Catalyst-Free Rearrangement of Allenyl Aryldiazoacetates into 1,5-Dihydro‑4<i>H</i>‑pyrazol-4-ones

Phenylpropargyl diazoacetates exist in equilibrium with 1-phenyl-1,2-dien-1-yl diazoacetate - allenes that are rapidly formed at room temperature through 1,3-acyloxy migration catalyzed by gold­(I) or gold­(III) compounds, and these catalysts react solely with the π-donor rather than with the diazo group. The product allene of the aryl­diazo­acetates undergoes rearrangement that is not catalyzed by gold in which the terminal nitrogen of the diazo functional group adds to the central carbon of the allene, initiating a sequence of bond-forming reactions, resulting in the production of 1,5-dihydro-4H-pyrazol-4-ones in good yields. These 1,5-dihydro-4H-pyrazol-4-ones undergo intramolecular 1,3-acyl migration to form an equilibrium mixture and can quantitatively transfer the acyl group to an external nucleophile with formation of 4-hydroxy­pyrazoles. Reactions of phenyl­propargyl phenyl­diazo­acetates catalyzed by cationic gold complexes are initiated at the diazo functional group to form a gold carbene whose subsequent cascade process (intramolecular addition, then aromatic substitution) results in the formation of a product that is uniquely characteristic of this pathway.苯丙炔基重氮乙酸酯（Phenylpropargyl diazoacetates）可与1-苯基-1,2-二烯-1-基重氮乙酸酯（即联烯（allenes））处于动态平衡；该类联烯可在室温下通过金(I)或金(III)化合物催化的1,3-酰氧基迁移（1,3-acyloxy migration）快速生成，且此类催化剂仅与π给体（π-donor）发生反应，而非重氮基团（diazo group）。芳基重氮乙酸酯的联烯产物会发生非金催化的重排反应：重氮官能团的末端氮原子进攻联烯的中心碳原子，启动一系列成键反应，最终以优异收率得到1,5-二氢-4H-吡唑-4-酮（1,5-dihydro-4H-pyrazol-4-ones）。此类1,5-二氢-4H-吡唑-4-酮可发生分子内1,3-酰基迁移（intramolecular 1,3-acyl migration）形成平衡混合物，且能将酰基定量转移至外部亲核试剂（external nucleophile），生成4-羟基吡唑（4-hydroxy-pyrazoles）。由阳离子金配合物（cationic gold complexes）催化的苯丙炔基苯基重氮乙酸酯反应，起始于重氮官能团，先生成金卡宾（gold carbene）；后续的级联反应（包括分子内加成与随后的芳香取代反应）最终生成该反应路径独有的特征产物。