ChemInform Abstract: Liquid-Crystalline Solvents as Mechanistic Probes. Part 27. Control of Norrish II Reactions of 2- and sym-Alkanones by the Ordered Solid Phases of Heneicosane.

ChemInform Abstract

In order to study if solvent order enhances the population of one conformer of the immediate precursors to the elimination products (II) + (III) and diastereomeric cyclization products (IV) (i.e. the different conformers of the same hydroxy 1,4-biradical) the ketones (I) are irradiated in heneicosane at 45 (isotropic phase), 35 (solid phase II), and 25 °C (solid phase I of the solvent) andthe ratios of elimination/cyclization ((II) + (III)/(IV)) and trans/cis-cyclization (trans-(IV)/cis-(IV)) are determined. None of the trans/cis-ratios for the 2-alkanones (Ia) in any of the solvent phases differs appreciably from what is found in the isotropic phase at 45 °C; very different trans/cis-ratios, however, are found for the sym. ketones (Ib). The elimination/cyclization ratios from (Ia) and (Ib) indicate that phase I does exert greater control over the biradical precursors of (II) (or (III)) and (IV) than does phase II. The results presented provide clear evidence that solvent order that does not rely upon strong attractive forces can control the dynamics of the Norrish II reaction.