Rh(II)-Catalyzed Enynal Cycloisomerization for the Generation of Vinyl Carbene: Divergent Access to Polycyclic Heterocycles

Described herein is an unprecedented example of generating the endocyclic donor–donor vinyl carbene achieved by the dirhodium(II)-catalyzed enynal cycloisomerization. The endocyclic vinyl carbene intermediate could be trapped by a diversity of alkenes, including 1,1-disubstituted alkenes, dienes, and α-methyl styrenes, which could undergo [2 + 1], [4 + 3] cycloaddition and an unexpected formal allylation, respectively, giving spiro and fused polycyclic heterocycles. In these cascade transformations, up to three rings and three stereocenters were constructed in one step with good selectivities. The enantioselective versions were also achieved with up to 84% ee. Mechanistic studies revealed that a concerted intramolecular [2 + 3] cycloaddition was the key step for the generation of vinyl Rh(II) carbene. Detailed studies were conducted to elucidate the intriguing mechanism of the formal allylation.