Density Functional Study of Organocatalytic Cross-Aldol Reactions between Two Aliphatic Aldehydes: Insight into Their Functional Differentiation and Origins of Chemo- and Stereoselectivities

The chemo-, diastereo-, and enantioselectivities in proline and axially chiral amino sulfonamide-catalyzed direct aldol reactions between two enolizable aldehydes with different electronic nature have been studied with the aid of density functional theory (DFT) method. The potential energy profiles for the enamine formation between each aliphatic aldehyde and the catalyst confirm that two subject catalysts can successfully differentiate between 3-methylbutanal as an enamine component and α-chloroaldehydes as a carbonyl component. Transition states associated with the stereochemistry-determining C–C bond-forming step with the enamine intermediate addition to the aldehyde acceptor for proline and chiral amino sulfonamide-promoted aldol reactions are reported. DFT calculations not only provide a good explanation for the formation of the sole cross-aldol product between two aliphatic aldehydes both bearing α-methylene protons but also well reproduce the opposite syn vs anti diastereoselectivities in the chiral amino sulfonamide and proline-catalyzed aldol reactions

Pt-Enhanced Mesoporous Ti³⁺/TiO₂ with Rapid Bulk to Surface Electron Transfer for Photocatalytic Hydrogen Evolution

Pt-doped mesoporous Ti³⁺ self-doped TiO₂ (Pt–Ti³⁺/TiO₂) is <i>in situ</i> synthesized via an ionothermal route, by treating metallic Ti in an ionic liquid containing LiOAc, HOAc, and a H₂PtCl₆ aqueous solution under mild ionothermal conditions. Such Ti³⁺-enriched environment, as well as oxygen vacancies, is proven to be effective for allowing the <i>in situ</i> reduction of Pt⁴⁺ ions uniformly located in the framework of the TiO₂ bulk. The photocatalytic H₂ evolution of Pt–Ti³⁺/TiO₂ is significantly higher than that of the photoreduced Pt loaded on the original TiO₂ and commercial P25. Such greatly enhanced activity is due to the various valence states of Pt (Pt^<i>n</i>+, <i>n</i> = 0, 2, or 3), forming Pt–O bonds embedded in the framework of TiO₂ and ultrafine Pt metal nanoparticles on the surface of TiO₂. Such Pt^<i>n</i>+–O bonds could act as the bridges for facilitating the photogenerated electron transfer from the bulk to the surface of TiO₂ with a higher electron carrier density (3.11 × 10²⁰ cm^–3), about 2.5 times that (1.25 × 10²⁰ cm^–3) of the photoreduced Pt–Ti³⁺/TiO₂ sample. Thus, more photogenerated electrons could reach the Pt metal for reducing protons to H₂