Crystal and molecular structures of dimesitylketene and bis(3,5‐dibromo‐2,4,6‐trimethylphenyl)ketene. Intrinsic torsional angles and buttressing effects in 1,1‐dimesitylvinyl propellers

The crystal and molecular structures of dimesitylketene (1) and bis(3,5‐dibromo‐2,4,6‐trimethylphenyl)ketene (2), the first free ketenes to be studied by X‐ray diffraction, have been determined. The molecules of 1 and 2 have crystallographic C 1 and C 2 symmetry, respectively, and exist in a propeller conformation. Molecular mechanics calculations of 1 and 2 using a modification of Allinger's new allene parameters reproduce satisfactorily the structural parameters of the ketenes. The torsional angles of the aryl rings in 1 and 2 are similar ( ca. 50°) and are regarded as ‘intrinsic’ values for a 1,1‐dimesitylvinyl propeller. From the similarity in aryl torsional angles in 1 and 2 it is concluded that the buttressing effects in 2 (if any) are not markedly manifested in the structural parameters of 2 .