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 → σ*_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.