A photochemical reactor was tested in an undergraduate teaching laboratory through the environmentally friendly aromatization of a 1,4-dihydropyridine, successfully using air as an oxidant, organic dye fluorescein as a photosensitizer, and sodium carbonate as a base. This experiment allowed students to be taught key concepts of photochemistry, organic photoredox catalysis, and green chemistry for the first time. Herein, we describe the making of the simple, yet robust, high-intensity blue LED photoreactor. The durable device was constructed with materials acquired at a hardware store and electronics shop.
The asymmetric ring opening of azlactones via dynamic kinetic resolution (DKR) is investigated by contrasting thioureas incorporating 1-arylethyl substituents against their more acidic trifluoromethylated analogs. All the catalysts under study outperform Takemoto’s thiourea because of the inclusion of an additional chiral center. However, the difference in yield and selectivity between the fluorinated and non-fluorinated catalysts is minimal. We explain this observation by analysis of calculated transition states. Our findings show that the hydrogen bond (HB) between the NH linked to the 1-arylethyl and the negatively charged oxygen in the benzyloxy ion is the longest in the HB network, whereas the HB between the ammonium group and the same oxygen atom is the shortest. Thus, the substituents and the HB donor ability of this chiral fragment attached to the thiourea are not important in the reaction.
We report a one-pot synthesis of chiral β-amino acids by the Mannich addition of bistrimethylsilyl ketene acetals to N-tert-butanesulfinyl imines followed by the removal of the chiral auxiliary. The synthesis and isolation of pure β-amino acid hydrochlorides were conducted under mild conditions, without strong bases and this method is operationally simple. The stereoselective reaction was promoted by two different activation methods that lead to different stereoisomers: (1) Lewis Acid (LA) catalysis with boron trifluoride diethyl etherate and (2) Lewis Base (LB) catalysis with tetrabutylammonium difluorotriphenylsilicate. The reaction presented good diastereoselectivity with LB activation and moderate to good dr with LA catalysis. The exceptions in both protocols were imines with electron donating groups in the aromatic ring.
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