An efficient chiral Brønsted acid-catalyzed enantioselective dehydrative Nazarov-type electrocyclization (DNE) of electron-rich aryl- and 2-thienyl-β-amino-2-en-1-ols is described. The 4π conrotatory electrocyclization reaction affords access to a wide variety of the corresponding 1 H-indenes and 4 H-cyclopenta[ b]thiophenes in excellent yields of up to 99% and enantiomeric excess (ee) values of up to 99%. Experimental and computational studies based on a proposed intimate contact ion-pair species that is further assisted by hydrogen bonding between the amino group of the substrate cation and chiral catalyst anion provide insight into the observed product enantioselectivities.
Density functional theory (DFT) was utilized to explore the reduction of gold(III) complexes by the amino acid glycine (Gly). Interestingly, when the nitrogen atom of Gly coordinates to the gold(III) center, its C -hydrogen atom becomes so acidic that it can be easily deprotonated by a mild base like water. The deprotonation converts the amino acid into a potent reductant by which gold(III) is reduced to gold(I) with a moderate activation energy. To our knowledge, this is the first contribution suggesting that primary amines are oxidized to imines via direct α-carbon deprotonation. This finding may provide new insights into the mechanistic interpretation of amine oxidations catalyzed/mediated by a center with high cathodic reduction potential. This work also provides a rationalization behind why gold(III) complexes with amine-based polydentate ligands are reluctant to undergo a redox process. Gold(III) reduction occurs most efficiently if the C proton leaves in the plane of the C , N and Au atoms. Chelation prevents this alignment, resulting in the gold(III) complex being unreactive toward reduction. It has been experimentally found that gold(III) is capable of oxidizing Gly to glyoxylic acid (GA) as the initial product. The latter, in the presence of another gold(III) complex, has been reported to undergo oxidative decarboxylation to afford CO and HCOOH. This process is found to be mediated by formation of a geminal diol intermediate produced by reaction of water with the aldehyde functional group of the coordinated GA.
Thermally induced cycloisomerization reactions of 1,6-allenynes gives α-methylene-γ-lactams via intramolecular Alder-ene reactions. The mechanism is supported by computational and deuterium labelling studies. This thermal, non-radical method enables the discovery of a hitherto unknown route that proceeds via a divergent mechanism distinct from the previous [2 + 2] cycloisomerization manifold.
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