Crystal structure of 1,4,9,10-tetramethyltriptycene and barriers to rotation of bridgehead-methyl groups in 1,4,9,10-tetramethyl- and 1,4-dichloro-9,10-di-methyl-triptycene

Abstract

Barriers to rotation of the bridgehead-methyl groups in 1,4,9,10-tetramethyl-(1) and 1,4-dichloro-9,10-dimethyl-triptycene (2) are investigated both in solution and in the solid. The barrier is larger for (2) than for (1) not only in the solid state but also in solution, the barriers in solution being somewhat higher than those in the solid for both (1) and (2). X-Ray crystallography for (1)[monoclinic, space group P2₁/n, Z= 4, a= 20.525(5), b= 9.776(2), c= 8.499(2)Å, and β= 99.31 (2)°, R= 0.0417 and R_w= 0.0467 (w= 1.0) for 2 181 reflections] reveals planar benzene rings in the triptycene skeleton of (1). Remarkable upfield ¹³C n.m.r. chemical shifts of the methyl carbon atoms for (1) in solution compared with those in the solid suggest deformation of the benzene rings and an increase of the methyl barrier in solution. Molecular mechanics calculations of features of the molecular structure of (1) are critically evaluated.