Radical cyclization to the imino functional group

Abstract

<p>Aryl radicals, generated by abstraction of Br from ortho-substituted bromoarenes, add to an imino functional group (C = N) in the ortho substituent in a competition involving 6-endo closure to the C and 5-exo closure to the N-atom. Absolute rate constants for the 6-endo closure were measured for two systems; k_(6-endo)>10⁸ s⁻¹ (80℃). Rate constants for the competing 5-exo closure were estimated to be 10-100 fold smaller. The utility of the 6-endo closure for the synthesis of tetrahydroisoquinolines was demonstrated with eight aldimine systems. Moreover, 1,2-asymmetric induction with up to 57% of diastereomeric excess was discovered for the 6-endo closure to the C-atom of the CN double bond derived from glyceraldehyde acetonide. Radical cyclization in the lower homologue, wherein the choice was between 4-exo closure to the N-atom and 5-endo closure to the C-atom, was not competitive with either 1,5 H-atom transfer of the azomethine proton (aldimine) or transfer of H-atom from Bu₃SnH (ketimine). In the isomeric imines, wherein the choice was between 5-exo closure to the C-atom and 6-endo to the N-atom, aryl radical closure was shown to be exclusively in the 5-exo sense. The absolute rate constant for the 5-exo closure to the C-atom was measured to be 5.3x10⁸ s⁻¹ (80℃). Finally, bis(tributylstannyl)benzopinacolate (TBBP) was found to be a novel source of canned tributylstannyl radicals.</p>