Crossed Regio‐ and Enantioselective Iron‐Catalyzed [4+2]‐Cycloadditions of Unactivated Dienes
Elena Braconi, Nicolai Cramer
Abstract
The cyclohexene motif is ubiquitous in nature and specialty chemicals. A straightforward selective access to chiral cyclohexenes from unactivated dienes and dienophiles is not feasible by classical Diels-Alder reaction and constitutes an unsolved synthetic challenge. We report a mild and enantioselective iron-catalyzed cross-[4+2]-cycloaddition of unactivated dienes providing access to chiral 1,3-substituted vinyl-cyclohexenes. The development of bis-dihydroisoquinoline ligands was vital to obtain iron complexes that display high reactivities and excellent chemo-, regio- and enantioselectivities towards the targeted cyclohexenes. A range of diene substrates is well accommodated including feedstocks like butadiene, isoprene and myrcene. The structures of different iron complexes are mapped by X-ray crystallographic analysis and linked to their performance.