Metathesis Reactions of Formaldehyde Acetals – Experimental and Computational Investigation of Isomeric Families of Cyclophanes under Dynamic Conditions

The acid‐catalyzed transacetalation of cyclophane formaldehyde acetals incorporating m ‐xylylene units ( m ‐C i ) generates a well behaved dynamic library of oligomeric macrocycles. Effective Molarities ( EM ) relating to the formation of the lower cyclic oligomers ( i = 2–4) were estimated from equilibrium concentrations measured under conditions closely approaching the critical monomer concentration. Comparison with available EM data relating to the formation of the isomeric paracyclophanes p ‐C i shows that the metacyclophanes m ‐C i are considerably more stable, the stability difference being particularly significant for i = 2. Molecular modeling indicated that incorporation of COCOC moieties in the cyclic structures enforces deviations from the most stable g + g + / g – g – conformation. Consistently with experimental results, such deviations are at a maximum for the dimeric paracyclophane p ‐C 2 , and tend to decrease with increasing ring size in both series. Molecular modeling also showed that the stability of the heterodimer m,p ‐C 2 is comparable to that of m ‐C 2 , which is again in agreement with the results of a cross‐experiment in which a mixture of m ‐C 2 and p ‐C 2 was allowed to equilibrate.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)