The photochemical deracemization of 2,4-disubstituted 2,3-butadienamides (allene amides) was investigated both experimentally and theoretically. The reaction was catalyzed by a thioxanthone which is covalently linked to a chiral 1,5,7-trimethyl-3-azabicyclo[3.3.1]­nonan-2-one skeleton providing a U-shaped arrangement of the sensitizing unit relative to a potential hydrogen-bonding site. Upon irradiation at λ = 420 nm in the presence of the sensitizer (2.5 mol %), the amides reached at −10 °C a photostationary state in which one enantiomer prevailed. The enantioenriched allene amides (70–93% ee) were isolated in 74% to quantitative yield (19 examples). Based on luminescence data and DFT calculations, energy transfer from the thioxanthone to the allene amides is thermodynamically feasible, and the achiral triplet allene intermediate was structurally characterized. Hydrogen bonding of the amide enantiomers to the sensitizer was monitored by NMR titration. The experimental association constants (K a) were similar (59.8 vs 25.7 L·mol–1). DFT calculations, however, revealed a significant difference in the binding properties of the two enantiomers. The major product enantiomer exhibits a noncovalent dispersion interaction of its arylmethyl group to the external benzene ring of the thioxanthone, thus moving away the allene from the carbonyl chromophore. The minor enantiomer displays a CH−π interaction of the hydrogen atom at the terminal allene carbon atom to the same benzene ring, thus forcing the allene into close proximity to the chromophore. The binding behavior explains the observed enantioselectivity which, as corroborated by additional calculations, is due to a rapid triplet energy transfer within the substrate-catalyst complex of the minor enantiomer.

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Since the initial reports of the ultraviolet (UV) activity of TiO 2 and SrTiO 3 in the 1970's, researchers have pursued a fundamental understanding of the mechanistic and molecular-level phenomena involved in photo-catalysis. [1][2][3][4][5][6][7] Although it requires UV light, after four decades SrTiO 3 is still the gold standard for splitting water. It is chemically stable and catalyzes both the hydrogen and the oxygen reactions without applied bias. While ultrahigh vacuum (UHV) surface science techniques have provided useful insights, 8 we still know relatively little about the structure of electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a SrTiO 3 electrode during water splitting, before and after training with a positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy (SECM) at open circuit correlates this training with a tripling of the activity toward photoinduced water splitting. A novel first-principles joint density-functional theory (JDFT) simulation constrained to the X-ray data via a generalized penalty function identifies an anatase-like structure for the more active, trained surface.Wide bandgap, n-doped, metal oxide semiconductors such as SrTiO 3 , TiO 2 and WO 3 absorb UV light to form photo-generated (electrons and holes) charge carriers, capable of driving redox reactions at the interface with an electrolyte. 9-11 SrTiO 3 is a prototypic perovskite structure metal oxide ( Figure 1a). The perovskites exhibit a vast range of attractive physico-chemical properties including promising energy conversion activity. 12 In particular, SrTiO 3 has very attractive photocatalytic properties. It is highly stable in base, displays high quantum efficiency for the electro-oxidation of water under UV illumination, and performs the light-driven water splitting reaction (i.e., photo-generation of both O 2 and H 2 from H 2 O) under an applied bias, 2,13 at open circuit aided by an auxiliary metal electrode, 2,13 and on free-standing crystals. 4,5,[13][14][15][16] The fundamental mechanisms underlying surface photochemical reactions, however, remain unclear. While the bulk d-band structure can correlate with activity, 12 surface defects and surface structure are critically important and it is typically difficult to decouple the bulk and surface contributions to observed changes in reactivity. [17][18][19] Here, we illustrate the critical role that surface structure plays by demonstrating under operando conditions that the electrochemical activation (training) of n-doped SrTiO 3 (001) in basic media induces an irreversible surface reordering that enhances (by 260%) its activity for photocatalytic water splitting.The operando structural characterization of SrTiO 3 before and after training wa...

Trisubstituted allenes with a 3‐(1′‐alkenylidene)‐pyrrolidin‐2‐one motif were successfully deracemized (13 examples, 86–98 % ee) employing visible light (λ=420 nm) and a chiral triplet sensitizer as the catalyst (2.5 mol %). The photocatalyst likely operates by selective recognition of one allene enantiomer via hydrogen bonds and by a triplet‐sensitized racemization process. Even a tetrasubstituted allene (45 % ee) and a seven‐membered 3‐(1′‐alkenylidene)‐azepan‐2‐one (62 % ee) could be enantiomerically enriched under the chosen conditions. It was shown that the axial chirality of the allenes can be converted into point chirality by a Diels–Alder (94–97 % ee) or a bromination reaction (91 % ee). Ring opening of the five‐membered pyrrolidin‐2‐one was achieved without significantly compromising the integrity of the chirality axis (92 % ee).

A photochemical deracemization of 5-substituted 3-phenylimidazolidine-2,4-diones (hydantoins) is reported (27 examples, 69%-quant., 80–99% ee). The reaction is catalyzed by a chiral diarylketone which displays a two-point hydrogen bonding site. Mechanistic evidence (DFT calculations, radical clock experiments, H/D labeling) suggests the reaction to occur by selective hydrogen atom transfer (HAT). Upon hydrogen binding, one substrate enantiomer displays the hydrogen atom at the stereogenic center to the photoexcited catalyst allowing for a HAT from the substrate and eventually for its conversion into the product enantiomer. The product enantiomer is not processed by the catalyst and is thus enriched in the photostationary state.

The combination of photochemistry and halogen bonding interactions has risen in the last years as a powerful synthetic toolfor the creation or radical intermediates under mild conditions.In the formation of...