Unactivated Alkenes
Since 2000, a number of transition metal-catalyzed methods for the enantioselective alkylation of arenes with unactivated (non-Michael acceptor) C=C bonds have been reported. Early examples include the enantioselective ortho-alkylation of benzamide with norbornene catalyzed by iridium(I) bis(phosphine) complexes and the atropselective alkylation of heterobiaryl compounds with ethylene catalyzed by rhodium(I) mono(phosphine) complexes. More recently, a number of effective protocols have been developed for the intramolecular alkylation of arenes with electronically unactivated C=C bonds. For example, Rh(I) mono (phosphoramidite) complexes catalyze the imine-directed 5-endo hydroarylation of 2-propenylarenes. Although high catalyst loadings are required, Rh(I)-catalyzed enantioselective hydroarylation has been employed in complex molecular environments, providing functionalized arenes with up to 96% ee, and has been applied to the synthesis of (+)-lithospermic acid and biologically-active dihydropyrroloindoles. Mechanistically distinct from these Ir(I)- and Rh(I)-catalyzed methods is the 6- and 7-exo hydroarylation of 2-alkenylindoles catalyzed by cationic platinum(II) bis(phosphine) complexes and the 6- and 7-exo hydroarylation of 2-allenylindoles catalyzed by cationic bis(gold) phosphine complexes. These latter transformations likely occur via an outer-sphere mechanism involving attack of indole on a metal-complexed C=C bond and provide tricyclic indole derivatives with up to 92% ee. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.