Laboratory-scale photoredox catalysis using hydrated electrons sustainably generated with a single green laser

Abstract

A combined photokinetical approach helped develop and optimize a green-light driven photoredox catalytic system that generates a “super-reductant” with simple instrumentation, consumes only a bioavailable donor, and provides very high turnover numbers.

Generating Hydrated Electrons for Chemical Syntheses by Using a Green Light‐Emitting Diode (LED)

“…As in previous systems affording e À aq with pulsed lasers, [1][2][3] we used chloroacetate ClAc for this purpose.T he value of benchmarking different electron generators with the same test compound is obvious;n o other reductant accessible by photoredox catalysis is known to decompose ClAc,hence this standard provides astringent yardstick;and this bulk chemical with amarket approaching 10 6 tonnes per year combines ah igh acute toxicity with high persistence,soits remediation is an environmentally relevant task. As in previous systems affording e À aq with pulsed lasers, [1][2][3] we used chloroacetate ClAc for this purpose.T he value of benchmarking different electron generators with the same test compound is obvious;n o other reductant accessible by photoredox catalysis is known to decompose ClAc,hence this standard provides astringent yardstick;and this bulk chemical with amarket approaching 10 6 tonnes per year combines ah igh acute toxicity with high persistence,soits remediation is an environmentally relevant task.…”

“…This has been successfully exploited for laboratory transformations of otherwise inert substrates [1][2][3] as well as remediations of industrial pollutants, [4][5][6][7] with particular emphasis on recalcitrant halogenated waste. This has been successfully exploited for laboratory transformations of otherwise inert substrates [1][2][3] as well as remediations of industrial pollutants, [4][5][6][7] with particular emphasis on recalcitrant halogenated waste.…”

“…This has been successfully exploited for laboratory transformations of otherwise inert substrates [1][2][3] as well as remediations of industrial pollutants, [4][5][6][7] with particular emphasis on recalcitrant halogenated waste. [1][2][3] Each of these compromises has specific deficiencies.I onizing radiation incurs safety hazards and regulatory problems;U V-Ci s absorbed so strongly by other components of typical samples as to bar its penetration;and lasers are expensive.Herein we present, characterize,and apply the first catalytic system that is free from these weaknesses.I ta ffords laboratory-scale amounts of the super-reductant e À aq ,y et operates with lowintensity green light from aL ED (light-emitting diode); moreover,i tc onsumes only the approved food additive vitamin C. [1][2][3] Each of these compromises has specific deficiencies.I onizing radiation incurs safety hazards and regulatory problems;U V-Ci s absorbed so strongly by other components of typical samples as to bar its penetration;and lasers are expensive.Herein we present, characterize,and apply the first catalytic system that is free from these weaknesses.I ta ffords laboratory-scale amounts of the super-reductant e À aq ,y et operates with lowintensity green light from aL ED (light-emitting diode); moreover,i tc onsumes only the approved food additive vitamin C.…”

“…Hence,photoredox catalysis is inherently well-suited for accommodating photon pooling as an accessory; [3,21,22] and it is expected to perform this function better than two-photon processes without intervening electron transfer, [1] even when they are catalytic. As we recently demonstrated, [3] the[ Ru(bpy] 3 2+ /Asc 2À system in homogeneous aqueous solution can already provide synthetically useful amounts of e À aq at 532 nm, but only when short laser pulses with power densities on the order of 100 MW cm À2 are used. [24] As sacrificial donor we employ the ascorbate dianion Asc 2À ,which necessitates strongly basic medium.…”

“…S3.4 in the Supporting Information establishes that the micellization changes neither the monophotonic nature nor the quantum yield of the actual ionization step.Employing the ejected ground-state species e À aq for syntheses instead of its excited immediate precursor *OER has adecisive advantage:o nly al ittle of the reducing power is lost but the availability of the rest is prolonged by many orders of magnitude.I n contrast to the *OER lifetime of less than 6ps (S3.4), the e À aq lifetime of 180 ns under our conditions (S3.5) amply suffices for converting laboratoryscale concentrations of substrates; [3] and this disparity widens even more because *OER has to react with our hydrophilic substrates across the decelerating micelle-water interface. Figure 1b furnishes experimental proof of e À aq production with agreen LED.The stationary e À aq concentrations attained by this low-intensity illumination are much too small for direct observation, but adding as ubstrate that is only transformed upon attack by e À aq and monitoring the accumulated product(s) yields the same information;and not only is …”