Ketenimine Formation Catalyzed by a High-Valent Cobalt Carbene in Bulky Alkoxide Ligand Environment

High-valent cobalt carbene Co­(OR)2(CPh2) (OR = OCtBu2Ph) undergoes reaction with various isocyanides CNR′
(CNR′ = 2,6-dimethylphenyl isocyanide, 4-methoxyphenyl isocyanide,
2-chloro-6-methylphenyl isocyanide, adamantyl isocyanide) to yield
the corresponding ketenimine. The reaction is accompanied by the formation
of cobalt bis­(alkoxide) bis­(isocyanide) complexes Co­(OR)2(CNR)2, which were independently synthesized and characterized.
DFT calculations suggest the mechanism proceeds through isocyanide
binding to Co, followed by intramolecular insertion into the Co–carbene
bond to form the ketenimine. We have also conducted an investigation
of the catalytic formation of ketenimines at room temperature using
mixtures of diazoalkanes (diphenyldiazomethane, methyl diazo­(phenyl)­acetate,
and ethyl diazoacetate) and isocyanides (2,6-dimethylphenyl isocyanide,
4-methoxyphenyl isocyanide, adamantyl isocyanide, cyclohexyl isocyanide,
and benzyl isocyanide). While no catalytic reactivity was observed
for diphenyldiazomethane, ester-substituted diazoalkanes (diazoesters)
demonstrate catalytic turnover. Relatively high yields are observed
for the reactions involving bulkier aliphatic substrates adamantyl
and cyclohexyl isocyanides. Mechanistic studies suggest that the lack
of catalytic reactivity involving diphenyldiazomethane results from
the inability of Co­(OR)2(CNR)2 to undergo carbene
formation upon reaction with N2CPh2. In contrast,
facile reaction is observed between Co­(OR)2(CNR)2 and diazoesters, which enables the overall catalytic reactivity.

高价钴卡宾配合物Co(OR)₂(=CPh₂)（其中OR=OCᵗBu₂Ph）可与多种异氰化物（isocyanides）CNR'（CNR'分别为2,6-二甲基苯基异氰、4-甲氧基苯基异氰、2-氯-6-甲基苯基异氰、金刚烷基异氰）发生反应，生成对应产物烯酮亚胺（ketenimine）。该反应同时伴随双(烷氧基)双(异氰)合钴配合物Co(OR)₂(CNR)₂的生成，这类配合物已通过独立合成与表征得以确认。

密度泛函理论（DFT）计算显示，反应机理为异氰化物先与钴中心配位，随后经分子内插入Co-卡宾键的步骤生成烯酮亚胺。

我们还开展了室温下催化合成烯酮亚胺的研究，反应底物为重氮烷烃（diazoalkanes）与异氰化物的混合体系，其中重氮烷烃包括二苯基重氮甲烷（diphenyldiazomethane）、重氮苯基乙酸甲酯（methyl diazo(phenyl)acetate）及重氮乙酸乙酯（ethyl diazoacetate），异氰化物则涵盖2,6-二甲基苯基异氰、4-甲氧基苯基异氰、金刚烷基异氰、环己基异氰（cyclohexyl isocyanide）与苄基异氰（benzyl isocyanide）。

实验结果表明，二苯基重氮甲烷未表现出催化活性，而酯基取代的重氮烷烃（重氮酯类）可实现催化循环。对于位阻较大的脂肪族底物金刚烷基异氰与环己基异氰参与的反应，产物收率相对较高。

机理研究揭示，二苯基重氮甲烷无催化活性的原因在于，Co(OR)₂(CNR)₂无法与N₂CPh₂反应生成卡宾物种。与之相反，Co(OR)₂(CNR)₂与重氮酯类的反应易于进行，这为整体催化反应提供了可行路径。