Synthesis and conformational study of 9,18-(ethylenedioxy)-2,11-dithia[3.3]metacyclophane: The propelling behavior of the three bridges in a [3.4.3](1,2,3)cyclophane

Abstract

A general approach which involved first the introduction of the central oxy or dioxy bridge and subsequent intramolecular coupling reactions to form the two thia bridges was employed in the syntheses of the 1,2,3-triply bridged cyclophanes 2, 3, and 4. The attempted routes to potential precursors of 2 were unsuccessful while the tetrabromide 16 could be synthesized. Intramolecular cyclizations of the latter, however, failed to yield 4, possibly due to unfavorable stereochemistry of the intermediate leading to the second intramolecular coupling reaction. The tetrabromide 18, with a central bridge of an appropriate length, successfully led to the formation of 3. The conformational behavior of 3 was analyzed by dynamic 1H NMR spectroscopy. The ethylenedioxy bridge is believed to adopt the symmetrical "boat" conformation. All three bridges, however, undergo unrestricted pseudorotation, represented by free conformational interconversion 3b ⇄ 3c, at room temperature. A relatively frozen conformation 3b (3c) was observed at the low-temperature limit. Flipping processes of the ethylenedioxy bridge and the two thia bridges are found to be interdependent, resulting in an overall propelling process with a common conformational barrier estimated at ca. 45 kJ mol-1. © 1991 American Chemical Society.