Ru(II)-Catalyzed C–H Activation/Alkylation of 3‑Formylbenzofurans with Conjugated Olefins: Product Divergence

Ru-catalyzed alkylation of 3-formylbenzofuran
with acrylates and acrylamides has been described. Branched selectivity
with unsubstituted or β-substituted acrylates/acrylamides and
linear selectivity with α-substituted acrylates have been observed.
However, in all of the cases, the intermediate alkylation products
seem to undergo further reactions, either cycloannulation or deformylation,
depending on the substrate employed. For example, with methyl acrylate,
the intermediate branched alkylation product underwent cycloannulation
with another molecule of methyl acrylate, resulting in a densely functionalized
cyclohexene ring formation. On the other hand, in the case of N-monosubstituted
acrylamides, the branched alkylation proceeded with intramolecular
aldehyde–amide condensation, leading to pyridin-2-one ring
annulation. However, with both methacrylate and crotonate, deformylation
of the initially formed alkylation products was observed.

已有文献报道了钌（Ru）催化3-甲酰基苯并呋喃与丙烯酸酯及丙烯酰胺的烷基化反应。对于未取代或β-取代的丙烯酸酯/丙烯酰胺底物，反应表现出支化选择性；而α-取代丙烯酸酯底物则呈现出线型选择性。然而在所有反应体系中，烷基化中间体产物均会根据所使用的底物发生进一步转化，具体为环化反应或脱甲酰基化反应。例如，以丙烯酸甲酯为底物时，支化型烷基化中间体可与另一分子丙烯酸甲酯发生环化反应，生成高官能团化的环己烯环结构。与之相对，当使用N-单取代丙烯酰胺作为底物时，支化烷基化过程通过分子内醛-酰胺缩合反应，构建得到2-吡啶酮环结构。但对于甲基丙烯酸酯与巴豆酸酯而言，初始生成的烷基化产物均发生了脱甲酰基化反应。