Extending Photocatalyst Activity through Choice of Electron Donor

Abstract

Sacrificial additives are commonly employed in photoredox catalysis as a convenient source of electrons, but what occurs after electron transfer is often overlooked. Tertiary alkylamines initially form radical cations following electron transfer, which readily deprotonate to form strongly reducing, neutral α-amino radicals. Similarly, the oxalate radical anion (C2O4 •–) rapidly decomposes to form CO2 •– (E 0 ≈ −2.2 V vs SCE). We show that not only are these reactive intermediates formed under photoredox conditions, but they can also impact the desired photochemistry, both positively and negatively. Photoredox systems using oxalate as an electron donor are able to engage substrates with greater energy demands, extending reactivity past the energy limits of single and multiphoton transition metal catalysts. Furthermore, oxalate offers better chemoselectivity than the commonly employed triethylamine when reducing substrates with moderate energy requirements.

Bidirectional Elongation Strategy Using Ambiphilic Radical Linchpin for Modular Access to 1,4-Dicarbonyls via Sequential Photocatalysis

“…Given the redox potential of 4CzIPN ( E (PC*/PC •– ) = 1.35 V vs SCE; E (PC/PC •– ) = −1.21 V vs SCE), these observations explain the experimental outcome: the use of a conventional acid such as HBF 4 resulted in a low yield of 3a due to a thermodynamically unfavorable SET between 4CzIPN and the corresponding phosphonium salt P + BF 4 – (Table , entry 2). On the other hand, the use of (CO 2 H) 2 ·2H 2 O was effective because the resulting phosphonium oxalate P + OA – could be oxidized by 4CzIPN to generate the potent reductant CO 2 •– via the decomposition of the oxalate radical anion (Table , entry 6) . Thus, we next sought to obtain evidence for the generation of CO 2 •– by capturing it with 1,1-diphenylethylene ( 4 ) as a radical-trapping agent (Figure , 2C).…”

“…•− via the decomposition of the oxalate radical anion (Table 2, entry 6). 24 Thus, we next sought to obtain evidence for the generation of CO 2…”

Bidirectional Elongation Strategy Using Ambiphilic Radical Linchpin for Modular Access to 1,4-Dicarbonyls via Sequential Photocatalysis

“…However, the use of (CO2H)2•2H2O was effective because the resulting phosphonium oxalate P + OAcan be oxidized by 4CzIPN, probably followed by the degradation of the oxalate anion to generate potent reductant CO2 •− (Table 2, entry 6). 21 Thus, we next sought to obtain the evidence for generation of CO2 •− by capturing it with 1,1-diphenylethylene (4) as a radical-trapping agent (Figure 3, 2-C). Indeed, after a reaction of P2' with 4 under optimal conditions, the expected radical adduct 6 could be detected by 1 H NMR analysis and high-resolution mass spectrometry (HRMS) along with 5 as the major product.…”

Degradation Behavior of Donor‐acceptor Cyanoarenes‐based Organic Photocatalysts

“…Based on all the spectroscopic evidence and analysis of structural data, it was demonstrated that the reaction from A to B proceeded smoothly with a loss of aromaticity and partially semi‐saturation of the dtb‐dpy ligand following the initial photoinitiated electron transfer between A and TEA. Furthermore, the scope of ancillary ligands which determined photochemical stability was screened to show effects and reactivity in chemical synthesis [15e,23] …”