Nascimento Daniel L, Gygi David, Drummer Matthew C, Gonzalez Miguel I, Zheng Shao-Liang, Nocera Daniel G
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
J Am Chem Soc. 2024 Oct 23;146(42):28612-28617. doi: 10.1021/jacs.4c08377. Epub 2024 Oct 8.
Oxygenation of hydrocarbons offers versatile catalytic routes to more valuable compounds, such as alcohols, aldehydes, and ketones. Despite the importance of monometallic copper-oxygen species as hydroxylating agents in biology, few synthetic model compounds are known to react with hydrocarbons, owing to high C-H bond dissociation energies. To overcome this challenge, the photoredox chemistry of monometallic copper (pyrazolyl)borate complexes coordinated by chlorate has been explored in the presence of C-C alkanes with BDEs ≥ 93 kcal/mol. Ethane is photooxidized at room temperature under N with yields of 15-30%, which increases to 77% for the most oxidizing tris(3,5-trifluoromethyl-pyrazolyl)borate complex (). This complex also promotes the photooxidation of methane to methanol in significant yield (38%) when the photoredox reaction is run under aerobic conditions. Ligand modification alters the reaction selectivity by tuning the redox potential. The ability to activate 1° C-H bonds of C-C alkanes using visible light is consistent with the photogeneration of a powerfully oxidizing copper-oxyl, which is supported by photocrystallographic studies of the tris(3,4,5-tribromopyrazolyl)borate chlorate complex. Mechanistic studies are consistent with the hydrogen atom abstraction of the C-H bond by the copper-oxyl intermediate. We demonstrate for with hexane as an exemplar, that the photoredox chemistry may be achieved under solar conditions of one-sun illumination.
烃的氧化反应为合成更有价值的化合物提供了多种催化途径，如醇、醛和酮。尽管单金属铜-氧物种在生物学中作为羟基化剂很重要，但由于C-H键的高解离能，已知很少有合成模型化合物能与烃发生反应。为了克服这一挑战，在存在键解离能（BDE）≥93千卡/摩尔的C-C烷烃的情况下，研究了由氯酸盐配位的单金属铜（吡唑基）硼酸盐配合物的光氧化还原化学。乙烷在室温下于氮气氛围中发生光氧化反应，产率为15 - 30%，对于氧化性最强的三（3,5 - 三氟甲基 - 吡唑基）硼酸盐配合物，产率可提高到77%。当光氧化还原反应在有氧条件下进行时，这种配合物还能以可观的产率（38%）将甲烷光氧化为甲醇。配体修饰通过调节氧化还原电位来改变反应选择性。利用可见光活化C-C烷烃的一级C-H键的能力与强氧化性铜氧基的光生现象一致，这得到了三（3,4,5 - 三溴吡唑基）硼酸盐氯酸盐配合物的光晶体学研究的支持。机理研究与铜氧基中间体对C-H键的氢原子提取一致。我们以己烷为例证明，在一个太阳光照的太阳能条件下可以实现光氧化还原化学。
