ChemInform Abstract: Excess Substrate Is a Spectator Ligand in a Rhodium‐Catalyzed Asymmetric [2 + 2 + 2] Cycloaddition of Alkenyl Isocyanates with Tolanes.

Abstract

Excess Substrate Is a Spectator Ligand in a Rhodium-Catalyzed Asymmetric [2 + 2 + 2] Cycloaddition of Alkenyl Isocyanates with Tolanes. -Methyl nicotinate proves to be the ideal additive to achieve high yields and enantioselectivities of the products, regardless of the electronic effects in the alkyne part. In the case of (IIa) a higher yield is obtained using 4 equiv. of PhCN. -(OINEN, M. E.; YU, R. T.; ROVIS*, T.; Org. Lett. 11 (2009)

Multi-component cycloaddition approaches in the catalytic asymmetric synthesis of alkaloid targets

“…32 The data suggest that the additive affects the environment of the rhodium(III) intermediate, and thus the enantioselectivity, by replacing the second alkyne as the spectator ligand in the reaction (see M13 in Fig. 4), prior to olefin insertion.…”

“…The advent of Guiphos (( R )- L2 )26 proved to be a turning point for the expansion of the alkyne scope to symmetrical aryl alkynes, the product selectivity favoring the vinylogous amide adducts 32. However, low to moderate yields (15 to 65%) and large variations in enantioselectivities (62 to 91% ee) were observed between different tolanes.…”