Cobalt(II) Pincer Catalyst-Mediated Direct Alkenylation of 2-Methylquinolines and 1-Indanones via Cross Dehydrogenative Coupling of Benzylamines

The direct construction of C-C bonds in organic synthesis from amines using a first-row transition-metal catalyst is still an unexplored area. Herein, we report for the first time a phosphine-free Co(II)-N̂N̂O pincer complex catalyzed direct alkenylation of 2-methylquinoline and 1-indanone with aromatic benzylamines. The present approach enables broad substrate compatibility under mild oxidant-free conditions, affording the alkenylated products in 42-95% yields. The catalytic transformation proceeds with ease through a cross-dehydrogenative coupling (CDC) pathway, which has been supported by the identification of key intermediates including phenylmethanimine, aldehyde, and quinolinylaryl ethanol and is scalable to gram quantities. Further, the versatility of our strategy has been demonstrated by the concise synthesis of (E)-2-(3,4-dimethoxystyryl)quinoline, a key intermediate to cuspareine, a natural tetrahydroquinoline alkaloid with reported antiviral and antimalarial activity. This CDC pathway is not only sustainable and atom-economical but also additive-free, phosphine-free, and generating only NH₃ and H₂ as the byproducts, both of which are potential future fuels.