Highly Efficient Production of Heteroarene Phosphonates by Dichromatic Photoredox Catalysis

Abstract

A new strategy to achieve efficient aerobic phosphorylation of five-membered heteraroenes with excellent yields using dichromatic photoredox catalysis in a gel-based nanoreactor is described here. The procedure involves visible aerobic irradiation (cold white LEDs) of a mixture containing the heteroarene halide, trisubstituted phospite, N , N -diisopropylethylamine (DIPEA) as sacrificial agent, and catalytic amounts of 9,10-dicyanoanthracene ( DCA ) in the presence of an adequate gelator, which permits a faster process than at the homogeneous phase. The methodology, which operates by a consecutive photoinduced electron transfer (ConPET) mechanism, has been successfully applied to the straightforward and clean synthesis of a number of different heteroarene (furan, thiophene, selenophene, pyrrole, oxazole, or thioxazole) phosphonates, extending to the late-stage phosphonylation of the anticoagulant rivaroxaban. Strategically, employment of cold white light is critical since it provides both selective wavelengths for exciting first DCA (blue region) and subsequently its corresponding radical anion DCA •– (green region). The resultant strongly reducing excited agent DCA •– * is capable of even activate five-membered heteroarene halides (Br, Cl) with high reduction potentials (∼−2.7 V) to effect the C(sp 2 )–P bond formation. Spectroscopic and thermodynamic studies have supported the proposed reaction mechanism. Interestingly, the rate of product formation has been clearly enhanced in gel media because reactants can be presumably localized not only in the solvent pools but also through to the fibers of the viscoelastic gel network. This has been confirmed by field-emission scanning electron microscopy images where a marked densification of the network has been observed, modifying its fibrillary morphology. Finally, rheological measurements have shown the resistance of the gel network to the incorporation of the reactants and the formation of the desired products.

Sensitizer-controlled photochemical reactivityviaupconversion of red light

“…38,49,116,[154][155][156] Fig. 8 Selected examples of different approaches to oxidative and reductive substrate activation with acridinium dyes and direct (monophotonic) blue/green light (a and b), 4,[140][141][142] direct excitation of DCA in the blue or green spectral range (c and d), 3,9,42,43,119 and two-catalyst systems operating under red light irradiation (e and f). 39 Overall redox-neutral reactions requiring an initial substrate oxidation step have been chosen on the right-hand side of the figure.…”

“…166,167 We hope that the concepts and proof-of-principle reactions presented herein can serve as useful inspiration for future catalytic applications with red and near-infrared light. 9,23,163,[168][169][170]…”

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

“…The mechanistic proposal in Figure 1b involves photoinduced electron transfer (PET) from [Cu(dap) 2 ] + to DCA, and the DCA radical anion as photoexcitable species leading to substrate activation. 35,90,130 However, when considering the photophysical and photochemical characteristics of the overall catalytic system, this is not a priori the only possible mechanistic interpretation of its observable photoreactivity. In the following, we discuss two different plausible mechanisms, one based on the above-mentioned initial photoinduced electron transfer step (PET mechanism in Figure 6a), and the other based on an initial triplet−triplet energy transfer (TTET) step between 3 MLCT-excited [Cu-(dap) 2 ] + and DCA (TTET mechanism in Figure 6b).…”

Recent advances in photochemical construction of aromatic C–P bonds via C–hetero bond cleavage

“…By applying the same strategy, this group recently disclosed an efficient aerobic phosphonation of five-membered heteroaryl compounds including, furan, thiophene, selenophene, pyrrole, oxazole and thioxazol, using DCA as a photocatalyst in a gel-based nanoreactor ( G1 ) (Scheme 6). 21 This kind of supramolecular gel ( G1 ) made from N , N ′-bis(octadecyl)- l -Boc-glutamic diamide in acetonitrile showed perfect performance in separation and reutilization because of its noncovalent nature of the supramolecular network. Various heteroarene phosphonates were obtained in good to excellent yields under mild conditions.…”