The aza-Diels−Alder reaction of nonactivated dienes and imines was realized through the action of the ionpaired Lewis acid catalyst [FeCl 2 ] + [FeCl 4 ] − generated by the in situ disproportionation of FeCl 3 . The uniquely high reactivity of [FeCl 2 ] + [FeCl 4 ] − was attributed to both the highly Lewis acidic FeCl 2 + and thermodynamically stable FeCl 4− acting as an ion-paired catalyst. Synchrotron-based Xray absorption fine structure measurements provided fundamental insights into the disproportionation and structure of the resulting ion-paired iron complex. A theoretical study was performed to analyze the catalytic reaction and better understand the "ion-pairing effect" which transforms simple FeCl 3 into a high turnover frequency Lewis acid catalyst in the aza-Diels−Alder reaction of nonactivated dienes and imines.

The development and use of a multiple‐activation catalyst with ion‐paired Lewis acid and Brønsted acid in an asymmetric aza‐Diels–Alder reaction of simple dienes (non‐Danishefsky‐type electron‐rich dienes) was achieved by utilizing the [FeBr2]+[FeBr4]− combination prepared in situ from FeBr3 and chiral phosphoric acid. Synergistic effects of the highly active ion‐paired Lewis acid [FeBr2]+[FeBr4]− and a chiral Brønsted acid are important for promoting the reaction with high turnover frequency and high enantioselectivity. The multiple‐activation catalyst system was confirmed using synchrotron‐based X‐ray absorption fine structure measurements, and theoretical studies. This study reveals that the developed catalyst promoted the reaction not only by the interaction offered by the ion‐paired Lewis acid and the Brønsted acid but also noncovalent interactions.

An efficient σ-Lewis acidic Ag(I) complex has been obtained by complexation with an electron-donating π-conjugated molecule as a side-on π ligand. The σ-Lewis acidity is possibly derived from the controlled linear coordination around Ag(I) due to the π ligand. A combination of UV-Vis absorption spectroscopy and X-ray absorption near-edge structure analyses clearly revealed that the complex is formed by π ligand-to-Ag(I) charge-transfer interaction. The σ-Lewis acidity was evaluated by IR spectroscopy using 2,6-dimethyl-γ-pyrone as the σ-Lewis basic molecule.

Cationic cobalt porphyrin-catalyzed allylation of aldehydes with allyltrimethylsilanes is developed. The formation of the aldehyde–cobalt porphyrin complex, the key intermediate for the addition of allylsilanes, is confirmed by theoretical studies and synchrotron-based X-ray absorption fine-structure measurements. Facile dissociation of the product by allylation from the cobalt complex regenerates the active complex with the aldehyde. The readily obtainable [Co­(TPP)]­SbF6 complex serves as an efficient catalyst for this allylation.

The enantioselective oxa-Diels–Alder reaction of nonactivated substrates by utilizing FeCl3 and a 1,1′-bi-2-naphthol (BINOL) derived chiral phosphoric acid as a multiple activation catalyst is reported. Various oxygen-containing six-membered heterocycles were obtained in high yields and in an enantioselective manner. Density functional theory (DFT) calculations elucidate that both Lewis acidic and Brønsted acidic moieties in the catalyst system synergistically activate two lone pairs of an aldehyde to facilitate enantioselective addition reaction of dienes.