One product, two pathways: Initially divergent radical reactions reconverge to form a single product in high yield

The paper describes examples of net diastereotopic-group-selective radical processes having the unusual feature that a single product is formed even though the key reaction of the two diastereotopic radical precursors is nonselective. For example, reaction of (R)-N-(cyclohex-2-en-1-yl)-N-(2,6-diiodo-4- methylphenyl)acetamide with tributyltin hydride produces 1-((4aR,9aR)-6-methyl- 2,3,4,4a-tetrahydro-1H-carbazol-9(9aH)-yl)ethanone with high product selectivity and in high yield. Analysis of the concentration profiles of the closed-shell intermediates at the halfway point of the reaction shows that nonselective abstraction of diastereotopic iodides by tin radicals occurs, leading to diastereomeric aryl radicals. These isomeric intermediates evolve via two nonintersecting reaction pathways, cyclization and bimolecular trapping or vice versa, into the same final product. Origins of the selectivity are suggested on the basis of conformational analysis of the products using both X-ray crystallography and density functional theory calculations. © 2011 American Chemical Society