Theoretical Studies on the Structure and Acidity of Meldrum's Acid and Related Compounds

Abstract

The structures and gas-phase ionization energies (ΔG°) of Meldrum's acid (I) and related cyclic (II-VI) and acyclic compounds (VII-IX) are investigated theoretically at the MP2/6-31+<TEX>$G^*$</TEX>, B3LYP/6-31+<TEX>$G^*$</TEX>, B3LYP/6- 311+<TEX>$G^{**}$</TEX>, B3LYP/6-311++G(3df,2p) and G3(+)(MP2) levels. Conformations of three neutral cyclic series vary gradually from boat (Meldrum's acid, I), to twisted chair (II) and to chair (III) as the methylene group is substituted for the ether oxygen successively. The preferred boat form of I can be ascribed to the two strong <TEX>$n_O$</TEX> → σ* c-c antiperiplanar vicinal charge transfer interactions and electrostatic attraction between negatively charged C¹ and positively charged C⁴at the opposite end of the boat. All the deprotonated anionic forms have half-chair forms due to the two strong <TEX>$n_C$</TEX> → π* c=0 vicinal charge transfer interactions. The dipole-dipole interaction theory cannot account for the higher acidity of Meldrum's acid (I) than dimedone (III). The origin of the anomalously high acidity of I is the strong increase in the vicinal charge transfer (<TEX>$n_C$</TEX> → π* c=0) and 1,4-attrative electrostatic interactions (C¹↔C⁴) in the ionization (I → <TEX>$I^-$</TEX> + <TEX>$H^+$</TEX>). In the acyclic series (VII-IX) the positively charged end atom, C⁴, is absent and the attractive electrostatic stabilization (C¹↔C⁴) is missing in the anionic form so that the acidities are much less than the corresponding cyclic series.