[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 1H‐NMR, and ΔG≠ = 48–51 KJ/mol was determined for the wobbling motion of the propano bridges.