Extended conjugation in enynones, dienones and related species: a theoretical and experimental study. The molecular structures of 3-ethynyl-2-methylcyclopent-2-enone, 3-ethenyl-2-methylcyclopent-2-enone and 3-ethyl-2-methylcyclopent-2-enone, as studied by gas electron diffraction

The molecular structures of three 3-R substituted conjugated enones have been determined by the gas electron diffraction method. Depending on the nature of the substituents the molecules represent examples of molecules with no additional conjugation, relative to that of the enone system (R = CH3CH2-), or those with extended conjugation involving different pi systems (R = CH2=H-; R = HC=C-). The molecular structures of the three enones have also been calculated by ab initio MP2/6-31G* optimizations. In order to better understand the conjugative effects of ethenyl and ethynyl substituents on a conjugated enone system, a large number of hydrogenation reactions and substituent exchange re;actions have been studied, based on the energies of the various molecules involved in the reactions, as calculated by MP2/6-31G* optimizations. The studies indicate that the conjugative effect of an ethenyl substituent on a conjugated enone system is approximately 4.2 kcal mol(-1), while that of an ethynyl substituent is considerably lower and more variable; on average ca. 2.3 kcal mol(-1).