Novel Family of Heterocyclic Strained Helices. Cesium-Assisted Syntheses and Conformational and Chirooptical Properties and Geometries

Article abstract of DOI:10.1021/jo00193a010

The syntheses of 1-oxa-10-thia<2.2>metacyclophane (1) and 1-thia-10-(p-toluolsulfonyl)-10-aza<2.2>metacyclophane (2) have been achieved for the first time by cesium-assisted high-dilution methods.Prior to application of the cesium effect, 1 and 2 were not available.In the NMR spectra, the intraannular hydrogens H_i of both 10-membered rings (Figure 1) are strongly shifted upfield.Both molecular helices 1 and 2 can be resolved (1) or enantiomerically enriched (2) by inclusion chromatography with triacetyl cellulose.The optical rotations are high, i.e. <α>^RT₃₆₅=1542, Δ^ε242=81 for (-)-1 ( in 1,4-dioxane).The absorption spectra and the circular dichroisms of 1 and 2 ( Figures 2 and 3), as well as the comparision with exocyclic-substituted <2.2>metacyclophanes like 6-11, give insight into the dependence between structure and helicity of these 10-membered rings.The absolute configurations of 1 and 2 can be presumed from circular dichroic correlations ( Figure 4).The kinetic parameters for the ring inversions of 1 and 2, accessible by measuring the rates of racemization or interconversion, were determined to be ΔG(excit.)_INT=123.3 kJ mol^-1, k_INT=4.87*10_-5 s^-1 (100 deg C) for 1 and ΔH(excit.)_INT=130.7 kJ mol^-1, k_INT=5.40*10^-6 s^-1 (102 deg C) for 2.The high barrier of racemization of 2 is discussed.X-ray analyses of 1 and 2 disclose for the first time exact data about the C-N, C-O, and C-S bond lenghts as well as the transannular distances d between C(H_i) and C'(H_i) and the bond angles in this new type of two-bladed propellers.The geometries and helicities of 1 and 2 are illustrates and compared by using the torsion angles α-δ (Figure 7).