Factors Affecting the Conformational Preference and Magnetic Shielding of Isobutenylene Chains in Macrocyclic Salicylideneaniline Derivatives

The 1H NMR chemical shifts and X-ray crystal structures of various salicylideneaniline derivatives containing an isobutenylene (2-methylenepropane-1,3-diyl) chain were analyzed in detail. In solution, these compounds were present to some extent as syn-syn conformers with respect to the isobutenylene chain; the actual syn-syn population depended on the molecular structure, especially on the size of the macrocycle. Small substituents on the phenyl ring hardly affected the stability of the synsyn form at all, whereas replacement of the phenyl group with a naphthalene moiety considerably destabilized the syn-syn form relative to the syn-skew form. The conformation-chemical shift relationship was validly maintained irrespective of the substituent, provided the conformational preference was correctly taken into account.