Simultaneous determination of primary and secondary thermodynamic isotope effects in tautomeric equilibria

The effect of deuteration on the keto-enol equilibrium and photostability of the sunscreen agent avobenzone

The remarkable properties of deuterium have led to many exciting and favourable results in enhancing material properties, for applications in the physical, medical, and biological sciences. Deuterated isotopologues of avobenzone, a sunscreen active ingredient, were synthesised to examine for any changes to the equilibrium between the diketone and enol isomers, as well as their UV photostability and photoprotective properties. Prior to UV irradiation, deuteration of the diketone methylene/enol moiety (i.e. avobenzone-d2) led to an increase in the % diketone compared to non-deuterated, determined by 1H NMR experiments in CDCl3 and C6D12. This can be rationalised from two angles; mechanistically by a deuterium kinetic isotope effect for the CH vs. CD abstraction step during tautomerisation from the diketone to the enol, and a weaker chelating hydrogen bond for the enol when deuterated allowing increased equilibration to the diketone. Avobenzone-d2 was further examined by solid state 13C NMR. The higher % diketone for avobenzone-d2 was postulated to favour increased photodegradation by a non-reversible pathway. This was investigated by UV irradiation of the avobenzone isotopologues in C6D12, both in real time in situ within the NMR by fibre optic cable as well as ex situ using sunlight. An increase in the relative amount of photoproducts for avobenzone-d2 compared to non-deuterated was observed by 1H NMR upon UV irradiation ex situ. Overall, the study demonstrates that deuteration can be applied to alter complex equilibria, and has potential to be manifested as changes to the properties and behaviour of materials.

Tautomeric properties and gas-phase structure of 3-chloro-2,4-pentanedione

The tautomeric properties of alpha-chlorinated acetylacetone, 3-chloro-2,4-pentanedione CH3C(O)-CHCl-C(O)CH3, have been investigated by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximations with different basis sets up to cc-pVTZ). Analysis of the GED intensities resulted in the presence of 100(2)% enol tautomer at 269(8) K. The following skeletal geometric parameters (rh1 values) of the molecule, which possesses Cs symmetry, were derived: r(C=C) = 1.378(3) A, r(C-C) = 1.450(3) A, r(C=O) = 1.243(3) A, r(C-O) = 1.319(3) A, r(O-H) = 1.001(4) A, r(C-Cl) = 1.752(4) A, angleC-C=C = 121.3(1.0) degrees , angleC=C-O = 119.9(1.2) degrees , angleC-C=O = 119.1(1.2) degrees . Due to very small contributions of the keto tautomer in alpha-chlorinated acetylacetone and its parent species, the effect of alpha-chlorination on tautomeric properties cannot be derived from experimental data. Quantum chemical calculations (B3LYP/6-31G**, B3LYP/cc-pVTZ, and MP2/cc-pVTZ) predict that alpha-chlorination of acetylacetone has no pronounced effect on the tautomeric properties. On the other hand, similar calculations for 1-chloro-1,3-butanedion, ClC(O)-CH2-C(O)CH3, demonstrate that chlorination in one beta position destabilizes the enol tautomer. In both chlorinated species the enol form is strongly preferred.