Copper(II) Triflate-Catalyzed Intramolecular Hydroamination of Homoallylic Amino Alcohols as an Expedient Route to trans-2,5-Dihydro-1H-pyrroles and 1,2-Dihydroquinolines

A new efficient synthetic route to trans-2,5-dihydro-1H-pyrroles and 1,2-dihydroquinolines that relies on copper(II) triflate-catalyzed intramolecular hydroamination of homoallylic amino alcohols under mild and operationally straightforward conditions is described. For reactions leading to the trans-2,5-dihydro-1H-pyrrole product, yields of 52-83% along with trans selectivities up to >99:1 dr and ee values up to 97% were accomplished from enantioenriched 1-(tosylamino)pent-4-en-2-ols ranging from 91-99% ee. Without the need for inert and moisture-free conditions, reactions involving 1-[2(tosylamino)phenyl]but-3-en-1-ols afforded the corresponding 1,2-dihydroquinoline products in excellent yields up to 99% and with complete chemoselectivity. The mechanism is suggested to involve copper(II)-mediated dehydration of the homoallylic amino alcohol. Protonation of the resultant copper(II)-activated aminodiene is then thought to trigger subsequent intramolecular hydroamination to give the partially hydrogenated nitrogen heterocycle.