Photogeneration of the Xanthyl Cation: .beta.-Cleavage from Excited State Ketones
摘要:
Irradiation of 9-xanthylacetone or 9-xanthylacetophenone in dilute acidic aqueous solution results in facile formation of the xanthyl cation. This cation photogeneration is not subject to acid catalysis over the acid range of 5-30% H2SO4. In contrast, the thermal reaction requires considerably stronger acidic media, and its rate shows a strong dependence on acid concentration. Laser flash photolysis studies demonstrate that the excited-state mechanism proceeds through an initial homolytic carbon-carbon beta-bond cleavage to generate the xanthyl radical. The xanthyl cation is produced from the xanthyl radical in a subsequent step, with oxygen acting as the oxidizing agent. The intermediacy of the xanthyl radical in the mechanism is also supported by the isolation of the radical coupling product bixanthyl from preparative irradiations. Triplet sensitization and quenching experiments demonstrate that the reaction proceeds through an excited triplet state of the ketone substrates.
Reactivity in acid-catalyzed carbon-carbon heterolysis
作者:Weiguo Cao、Ihsan Erden、Richard H. Grow、James R. Keeffe、Jiangao Song、Mary B. Trudell、Teri L. Wadsworth、Fu-Pei Xu、Ji-Bin Zheng
DOI:10.1139/cjc-77-5-6-1009
日期:——
to carbocations plus a carbon-centered nucleofuge. The fragmentation mechanisms are shown to be A1 or A1(ion pair) except for the 2-arylnitrocyclopropanes which cleave in trifluoroacetic acid by a concerted mechanism. Rate comparisons among several unstrained substrate sets indicate that O-centered nucleofuges undergo acid-catalyzed heterolysis ca. 10 3 -10 4 faster than S-centered nucleofuges and ca
平衡和速率常数已经确定了酸催化的两种醇、9-xanthydrol 和对茴香基二苯基甲醇和两种硫化物、(9-xanthyl) 甲基硫化物和 (7-tropyl) 甲基硫化物的杂解。将这些数据与文献信息与酸催化 CC 杂解的速率常数进行了比较,其中包括几种(9-xanthyl)化合物、(7-tropyl)化合物、一组 3-芳基环丁酮和两种 2-芳基硝基环丙烷,所有这些都是碎片到碳正离子加上以碳为中心的离核剂。除了通过协同机制在三氟乙酸中裂解的 2-芳基硝基环丙烷外,裂解机制显示为 A1 或 A1(离子对)。几个无应变底物组之间的速率比较表明,以 O 为中心的核离心剂经历了酸催化的异解作用。10 3 -10 4 比以 S 为中心的离心机快 10 3 -10 4。10 9 -10 14 比此处使用的以 C 为中心的离心机快。有助于 CC 异种分解(及其有效性)的因素包括培养基的酸度(强);离核剂的
Autoxidative Carbon-Carbon Bond Formation from Carbon-Hydrogen Bonds
Only oxygen and acid! The oxidative coupling of xanthene and other activated benzylic compounds with carbon nucleophiles such as ketones, can be performed under ambient conditions without solvent by simply using oxygen and catalytic amounts of methanesulfonic acid. The proposed reaction mechanism involves substrate activation by formation of hydroperoxides; the method can therefore be regarded as an
Copper-catalyzed aerobic asymmetric cross-dehydrogenative coupling of C(sp<sup>3</sup>)–H bonds driven by visible light
作者:Kexu Zhou、Ying Yu、Yu-Mei Lin、Yanjun Li、Lei Gong
DOI:10.1039/d0gc00262c
日期:——
Asymmetric cross-dehydrogenative coupling (CDC) of C(sp3)–H bonds shows great potential for rapid and stereoselective construction of C–Cbonds and use of molecular oxygen as the oxidant for such transformations is appealing in the context of green synthetic methodologies. However, the poor reactivity of oxygen at the ground state and the challenges in controlling the stereochemistry make it extremely
Cascade Nucleophilic Addition−Cyclic Michael Addition of Arynes and Phenols/Anilines Bearing Ortho α,β-Unsaturated Groups: Facile Synthesis of 9-Functionalized Xanthenes/Acridines
作者:Xian Huang、Tiexin Zhang
DOI:10.1021/jo902311a
日期:2010.1.15
A facile synthesis of xanthenes and acridines based on a cascade nucleophilic addition−cyclic Michaeladdition process of arynes and phenols/anilinessubstituted with α,β-unsaturated groups at the ortho positions is described. The reaction has also been successfully extended to the synthesis of 9-spiro-xanthene and acridine derivatives with potential biochemical interest.