Atom-Economical Syntheses of Dihydropyrroles Using Flavin-Iodine-Catalyzed Aerobic Multistep and Multicomponent Reactions

Visible Light-Driven Metal- and Photocatalyst-Free Synthesis of β-Trifluoromethylated Enamines via Trifluoromethyl Thianthrenium Salts

A novel protocol for the visible-light-driven synthesis of β-trifluoromethylated enamines has been developed, which operates without the use of transition metals or any photocatalysts, utilizing trifluoromethylthiosulfonium salts as the source of trifluoromethyl groups under mild conditions. According to this new protocol, more than 40 products have been prepared in moderate to good yields. In addition to eliminating the need for expensive or toxic transition metals and photocatalysts, this new methodology proves its potential scalability through air-stability, the use of safe and readily available reagents, a two-step one-pot procedure, and effective gram-scale reactions. This innovative approach not only demonstrates promise for green chemical synthesis but also offers a new pathway for the advancement of fluorine chemistry in sustainable organic synthesis.

Highly Diastereoselective Synthesis of 2,3-Dihydropyrroles via Formal [4 + 1] Annulation Reaction of α,β-Unsaturated Imines and in Situ Generated Pyridinium Ylide

We report a cascade approach for the synthesis of 2,3-dihydropyrroles derivatives via a formal [4 + 1] annulation reaction of α,β-unsaturated imines with in situ generated pyridinium ylides. Importantly, this protocol is compatible with diverse substituted imines as well as pyridinium ylides, constructing 2,3-dihydropyrroles with excellent yield and selectivity. Thereafter, the Merrifield resin-supported pyridinium ylide as a potential C1 synthon was also employed in our strategy and reused several times, resulting in products with excellent yield and diastereoselectivity.

Aerobic oxidative C-C bond formation through C-H bond activation catalysed by flavin and iodine

We report a metal/light-free aerobic oxidative C-C bond formation using sp3 C-H bond activation of tetrahydroisoquinolines catalyzed by flavin and iodine. The dual catalytic system enabled the oxidative Mannich and aza-Henry reactions by the cross-dehydrogenative coupling between two sp3 C-H bonds. Furthermore, the flavin-iodine-coupled catalysis was applied to the synthesis of pyrrolo[2,1-a]isoquinolines through the sequential oxidative 1,3-dipolar cycloaddition and dehydrogenative aromatization. The biomimetic flavin catalysis efficiently activates molecular oxygen; thus the non-metal dual catalytic system enables green oxidative transformation using molecular oxygen as an environmentally friendly terminal oxidant which generates benign water.

Catalyst-free reaction of 2-(4H-benzo[d][1,3]oxazin-4-yl)acrylates: synthesis of 1,2-dihydroquinolines and 2,3-dihydropyrroles

Catalyst-free annulations of 2-(4H-benzo[d][1,3]oxazin-4-yl)acrylates have been successfully achieved under mild conditions. Specifically, the reaction of 2-(4H-benzo[d][1,3]oxazin-4-yl)acrylates with sulfur ylides furnished various 1,2-dihydroquinolines in generally high yields. Furthermore, [3+2]-annulations of 2-(4H-benzo[d][1,3]oxazin-4-yl)acrylates with α,β-unsaturated imines afforded a broad scope of polysubstituted 2,3-dihydropyrroles with high efficiency.

Exploring the Reactivity of Flavins with Nucleophiles Using a Theoretical and Experimental Approach

Covalent adducts of flavin cofactors with nucleophiles play an important role in non-canonical function of flavoenzymes as well as in flavin-based catalysis. Herein, the interaction of flavin derivatives including substituted flavins (isoalloxazines), 1,10-ethylene-bridged flavinium salts, and non-substituted alloxazine and deazaflavin with selected nucleophiles was investigated using an experimental and computational approach. Triphenylphosphine or trimethylphosphine, 1-nitroethan-1-ide, and methoxide were selected as representatives of neutral soft, anionic soft, and hard nucleophiles, respectively. The interactions were investigated using UV/Vis and 1H NMR spectroscopy as well as by DFT calculations. The position of nucleophilic attack estimated using the calculated Gibbs free energy values was found to correspond with the experimental data, favouring the addition of phosphine and 1-nitroethan-1-ide into position N(5) and methoxide into position C(10a) of 1,10-ethylene-bridged flavinium salts. The calculated Gibbs free energy values were found to correlate with the experimental redox potentials of the flavin derivatives tested. These findings can be utilized as valuable tools for the design of artificial flavin-based catalytic systems or investigating the mechanism of flavoenzymes.