[3.2]Paracyclophane‐10‐enes and [3.2.3.2]Paracyclophane‐10,27‐dienes: A convenient synthesis by the McMurry reaction and dynamic stereochemistry

The 1,3‐bis(4‐acylphenyl)propanes 2 , 3 , 9 , 12 , and 15 were subjected to a cyclization by reductive coupling with low‐valent titanium (McMurry reaction). 2 and 3 are converted into the corresponding 10,11‐dialkyl[3.2]paracyclophane‐10‐enes 4 and 5 , respectively, under carefully controlled conditions in good yields. By the same method the dialdehydes 12 and 15 give rise to [3.2]paracyclophane‐10‐ene 13 and [3.2]metacyclophane‐10‐ene 16 . However, the ditoluoyl derivative 9 gives rise only to the macrocyclic cyclophane 10 by dimerization. The corresponding macrocyclic cyclophanes 6 and 7 were also obtained from 2 and 3 , respectively. The conformational mobility of the cyclophanes 4 , 5 , and 13 was studied by variable‐temperature 1 H‐NMR, and ΔG ≠ = 48–51 KJ/mol was determined for the wobbling motion of the propano bridges.