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‐Ci) 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‐Ci shows that the metacyclophanes m‐Ci 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‐C2, and tend to decrease with increasing ring size in both series. Molecular modeling also showed that the stability of the heterodimer m,p‐C2 is comparable to that of m‐C2, which is again in agreement with the results of a cross‐experiment in which a mixture of m‐C2 and p‐C2 was allowed to equilibrate.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)