Rotational Isomerism Involving an Acetylenic Carbon VI. Syntheses, Structures, and Dynamic Stereochemistry of Bis(1-phenyl-9-anthryl)ethynes: Highly Restricted Rotation about Acetylenic Axis in Acyclic Diarylethynes

Abstract

Abstract Some sterically congested di(9-anthryl)ethynes with substituted phenyl groups (4-Me, 4-iPr, 4-tBu, 3-iPr, 3,5-iPr2) at the 1-positions were synthesized by the Stille coupling of the corresponding triflate and bis(tributylstannyl)ethyne to realize the restricted rotation of the acetylenic axis in diarylethyne derivatives. In the X-ray structure of the 4-Me compound, the two 9-anthryl groups were staggered about the acetylenic axis by ca. 60°. The dynamic NMR study of the iPr substituted compounds afforded the kinetic data of the two independent processes: the faster and slower processes were assigned to the rotation about the acetylenic axis (ΔG≠ 45–51 kJ mol−1) and the rotation of the phenyl groups (ΔG≠ ca. 75 kJ mol−1), respectively, from the details of the lineshape changes. The observed barriers to the rotation about the acetylenic axis are rather high for acyclic diarylethynes, being attributed to the severe steric interactions between the 1-phenyl groups and the rigid di(9-anthryl)ethyne framework in the transition state. The mechanism of the dynamic stereochemistry and the substituent effect on the rotational barrier are discussed on the basis of the kinetic data and the molecular structures.