Radical Reactions Induced by Visible Light in Dichloromethane Solutions of Hünig's Base: Synthetic Applications and Mechanistic Observations

Abstract

β-(3-Iodopropoxy)-substituted α,β-unsaturated lactams, lactones, and cycloalkenones (eight examples) underwent reductive radical reactions in a dichloromethane solution of N,N-diisopropylethylamine (Hünig's base) upon irradiation with visible light (λ=419 nm). Apart from plain reduction reactions (hydro-de-iodination), a significant degree of cyclization was observed in three cases. In parallel to the conversion of the substrates, the formation of intensely colored by-products was observed. Based on mass spectrometric evidence and upon comparison with known compounds, the by-products were identified as cyanine dyes. Their formation supports the hypothesis that irradiation of dichloromethane solutions of Hünig's base leads to the formation of radicals, which in turn can either initiate a radical reaction or combine with cyanine precursors. It was shown by deuterium-labelling experiments, that one equivalent of dichloromethane is incorporated into the cyanine dyes and that the reductive quenching of radical intermediates is at least partially due to hydrogen abstraction from the solvent. As a consequence, a reductive cyclization of the starting materials is favored in CD Cl solutions as shown for two β-(3-iodopropoxy)-substituted tetronates, which underwent in dichloromethane almost exclusive reduction, but gave predominantly the cyclization products in CD Cl .

Oxidation of Trialkylamines by BrCCl3: Scope, Applications and Mechanistic Aspects

“…The formation of chlorostreptocyanine 10 is consistent with a recent publication from the Bach group, in which similar but non‐chlorinated cyanines were detected as the products of the reaction of CH 2 Cl 2 and DIPEA (diisopropylethylamine). These conjugated chromophores resulting from N ‐dealkylation/condensation with the polyhalomethane, and reaction of the resulting iminium salts with enamines (formed through oxidation/deprotonation) were shown to be responsible for the amber color in similar photoinduced reactions …”

Base Catalysis Enables Access to α,α-Difluoroalkylthioethers

“…purified Et 3 N) are competent base catalysts (Table 1, entry 2). Third, when running the reaction in CD 2 Cl 2 (which can transfer •D) 17 D was, not incorporated into the product. Fourth, reactions run in presence of radical traps (e.g.…”

“…Second, although the reaction utilizes TMG (which can have inorganic impurities that can oxidize a thiolate), other amine bases that lack such impurities (e.g., purified Et 3 N) are competent base catalysts (Table , entry 2). Third, when running the reaction in CD 2 Cl 2 (which can transfer ·D) D was not incorporated into the product. Fourth, reactions run in the presence of radical traps (e.g., 1,4-dicyanobenzene and BHT) proceed to full conversion and comparable yields.…”

Defluorodearomatization: A Photocatalytic Birch-Like Reduction That Enables C–C Bond Formation and Provides Access to Unnatural Cannabinoids

“…The inclusion of water may either prevent their formation or hydrolyze them if they do form. 52,53 We had presumed that the amine indirectly (as the radical cation) served as both the terminal reductant and the source of a H atom in the formation of the diene. However, we found that when normal water was replaced with heavy water (15 equiv D 2 O, Scheme 3a) the reaction resulted in 69% deuterium incorporation into the diastereotopic methylene signal.…”