A density functional study of 2-lithio-1,3-dithiane and 2-lithio-2-phenyl-1,3-dithiane: Conformational preference of the C-Li bond and structural analysis

The energetics and structural properties of 1,3-dithiane (1), 2-phenyl-1,3-dithiane (2), and their corresponding axial and equatorial 2-lithio derivatives have been probed by using density functional theory (DFT) at the Becke3LYP/6-31G(d,p) level. In agreement with the experimental observations, a very high preference for the equatorial orientation of the C-Li bond is calculated, the calculated energy difference between 1-Li-ax and 1-Li-eq being 14.2 kcal . mol(-1) and the calculated energy difference between 2-Li-ax and 2-Li-eq being 4.10 kcal . mol(-1). Population analysis indicates high positive charge at Lithium and small C-Li Wiberg bond indexes in these 1,3-dithianyllithiums, suggesting an ionic rather than covalent nature for the C-Li bond. The collected structural data are in line with the expectation that n(c)-->o*(s-c) hyperconjugation stabilizes the equatorial isomers, whereas n(c)/n(s) repulsive orbital interactions destabilize the axial isomers. Nevertheless, coordination of equatorial lithium by the ring sulfurs is apparent in the computed contact ion pairs for 1-Li-eq and 2-Li-eq. An interesting structure was predicted for the axial form of 2-lithio-2-phenyl-1,3-dithiane wherein the lithium is bonded to C(2), C-ipso, C-ortho, and one of the ring sulfurs, in a highly delocalized system in the case of the minimum energy isomer.