Ring flips of allenes (C 9H 7X) over triplet carbenes at ab initio and DFT levels (X = H, F, Cl, Br)

Abstract

Ring flips of cyclic C 9H 7X allenes over triplet carbenes are compared and contrasted for X = H, F, Cl and Br. Three series of conjugated non-planar allenes including 4-, 5-, and 6-X-1,2,4,6,8-cyclononapentaenes, turn into their mirror images; through their corresponding triplet planar conjugated carbenic transition states: 3-, 4-, and 5-X-2,4,6,8-cyclononatetraenylidenes, at ab initio and DFT levels of theory. The halogens are deliberately chosen not to be directly attached to the allenic motif. For all three series, the relative ease of racemizations is proportional to the electro-negativity of the employed halogen: F > Cl > Br. All the optimized transition states appear planar, electrophilic, and achiral.