Intramolecular Rotations in 2,2-Dimesityl-1-(9-anthryl)- and -1-(9-triptycyl)vinyl Acetates

2,2-Dimesityl-1-(9-anthryl)vinyl acetate (3) and 2,2-dimesityl-1-(9-triptycyl)vinyl acetate (4) were prepared. The rotation of the mesityl and 9-anthryl rings in 3 and of the mesityl rings and triptycyl substituent in 4 around their bonds to the vinylic carbon were investigated by DNMR. For each system an identical rotational barrier was measured for each substituent, indicating a correlated rotation. The rotational barriers were 16.7 ± 0.1 kcal mol^-1 for 3 and 17.0 ± 0.1 kcal mol^-1 for 4. The suggested rotational mechanism is a three-ring flip for 3 and a correlated (geared) rotation for the three rotors of 4. The latter process involves a two-ring flip of the two mesityl rings and a rocking process of the triptycyl moiety in which only one of its rings eclipses the C═C plane in the transition state.