Equilibrium Lithium Ion Pair Acidities of 2-Phenyl-1,3-dithiane and 2-p-Biphenylyl-1,3-dithiane: A New Approach

Article abstract of DOI:10.1021/jo00110a044

A new technique has been developed to measure the equilibrium ion pair acidities of 2-phenyl-1,3-dithiane (1) and 2-p-biphenyly-1,3-dithiane (2) with lithium as counterion.The lithium ion pair pK values (pK_Li/THF) in tetrahydrofuran (THF) at 25 deg C were determined to be 29.4 (1, contact ion pair (CIP)), 28.2 (2, CIP), and 29.3 (2, solvent separated ion pair (SSIP)), as compared to the values of 30.5 (1, CIP) and 29.1 (2, CIP) measured previously with cesium as counterion.At 25 deg C, the lithium salt of 2-p-biphenylyl-1,3-dithiane (2L) exists as a mixture of CIP, SSIP, and free ion, with the ratio of / = 11.4.The results are consistent with the stronger electrostatic interactions between lithium cation and the dithianyl anion for CIP than for SSIP, resulting in a lower pK_a for CIP.The equilibrium constant of SSIP ->/<- CIP for 2L was measured at 25 to -20 deg C to give ΔH⁰ = 1.43 kcal mol^-1, ΔS⁰ = 9.6 eu.The SSIP of 2L was found to dissociate significantly to the free ion in dilute THF solution.The measured dissociation constants of 2L to the free ion at 25 deg C are (1.22 +/- 0.08)E-6 M and (1.39 +/- 0.09)E-5 M for CIP and SSIP, respectively.The slow transmetalation reactions of 1 and 2 with different lithium indicator anions also allowed the measurements of some kinetic rate constants at 25 deg C, from which Broensted β values were obtained for 1 and 2.