A reaction cycle for redox-mediated,
Ni-catalyzed aryl etherification
is proposed under both photoredox and electrochemically mediated conditions.
We demonstrate that a self-sustained Ni(I/III) cycle is operative
in both cases by chemically synthesizing and characterizing a common
paramagnetic Ni intermediate and establishing its catalytic activity.
Furthermore, deleterious pathways leading to off-cycle Ni(II) species
have been identified, allowing us to discover optimized conditions
for achieving self-sustained reactivity at a ∼15-fold increase
in the quantum yield and a ∼3-fold increase in the faradaic
yield. These results highlight the importance of leveraging insight
of complete reaction cycles for increasing the efficiency of redox-mediated
reactions.
The quantum efficiency in photoredox catalysis is the crucial determinant of energy intensity and, thus, is intrinsically tied to the sustainability of the overall process. Here, we track the formation of different transient species of a catalytic photoredox hydroamidation reaction initiated by the reaction of an Ir(III) photoexcited complex with 2-cyclohexen-1-yl(4-bromophenyl)carbamate. We find that the back reaction between the amidyl radical and Ir(II) photoproducts generated from the quenching reaction leads to a low quantum efficiency of the system. Using transient absorption spectroscopy, all of the rate constants for productive and nonproductive pathways of the catalytic cycle have been determined, enabling us to establish a kinetically competent equilibrium involving the crucial amidyl radical intermediate that minimizes its back reaction with the Ir(II) photoproduct. This strategy of using an off-pathway equilibrium allows us to improve the overall quantum efficiency of the reaction by a factor of 4. Our results highlight the benefits from targeting the back-electron transfer reactions of photoredox catalytic cycles to lead to improved energy efficiency and accordingly improved sustainability and cost benefits of photoredox synthetic methods.
The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl)]methyl ligand, [Tism], has been employed to form the magnesium carbatrane compound, [Tism]MgH, which possesses a terminal hydride ligand. Specifically, [Tism]MgH is obtained via the reaction of [Tism]MgMe with PhSiH. The reactivity of [Tism]MgMe and [Tism]MgH allows access to a variety of other structurally characterized carbatrane derivatives, including [Tism]MgX [X = F, Cl, Br, I, SH, N(H)Ph, CH(Me)Ph, OCMe, SCMe]. In addition, [Tism]MgH is a catalyst for (i) hydrosilylation and hydroboration of styrene to afford the Markovnikov products, Ph(Me)C(H)SiHPh and Ph(Me)C(H)Bpin, and (ii) hydroboration of carbodiimides and pyridine to form N-boryl formamidines and N-boryl 1,4- and 1,2-dihydropyridines, respectively.
The octahedral core of 84-electron LCuH hexamers does not dissociate appreciably in solution, although their hydride ligands undergo rapid intramolecular rearrangement. The single-electron transfer proposed as an initial step in the reaction of these hexamers with certain substrates has been observed by stopped-flow techniques when [(Ph3P)CuH]6 is treated with a pyridinium cation. The same radical cation has been prepared by the oxidation of [(Ph3P)CuH]6 with Cp*2Fe(+) and its reversible formation observed by cyclic voltammetry; its UV-vis spectrum has been confirmed by spectroelectrochemistry. The 48-electron trimer [(dppbz)CuH]3 has been prepared by use of the chelating ligand 1,2-bis(diphenylphosphino)benzene (dppbz).
Tris(2-pyridylthio)methyl
zinc hydride, [κ3-Tptm]ZnH,
is an effective catalyst for multiple insertions of carbonyl groups
into the Si–H bonds of Ph
x
SiH4–x
(x = 1, 2). Specifically,
[κ3-Tptm]ZnH catalyzes the insertion of a variety
of aldehydes and ketones into the Si–H bonds of PhSiH3 and Ph2SiH2 to afford PhSi[OCH(R)R′]3 and Ph2Si[OCH(R)R′]2, respectively.
The mechanism for hydrosilylation is proposed to involve insertion
of the carbonyl group into the Zn–H bond to afford an alkoxy
species, followed by metathesis with the silane to release the alkoxysilane
and regenerate the zinc hydride catalyst. Multiple insertion of prochiral
ketones results in the formation of diastereomeric mixtures of alkoxysilanes
that can be identified by NMR spectroscopy.
The enzyme galactose oxidase (GOase)
is a copper radical oxidase
that catalyzes the aerobic oxidation of primary alcohols to the aldehydes
and has been utilized to that end in large-scale pharmaceutical processes.
To maintain its catalytic activity and ensure high substrate conversion,
GOase needs to be continuously (re)activated by 1e– oxidation of the constantly formed out-of-cycle species (GOasesemi) to the catalytically active state (GOaseox). In this work, we report an electrochemical activation method for
GOase that replaces the previously used expensive horseradish peroxidase
activator in a GOase-catalyzed oxidation reaction. First, the formation
of GOaseox of a specifically engineered variant via nonenzymatic
oxidation of GOasesemi was studied by UV–vis spectroscopy.
Second, electrochemical oxidation of GOase by mediators was studied
using cyclic voltammetry. The electron-transfer rates between GOase
and various mediators at different pH values were determined, showing
a dependence on both the redox potential of the mediator and the pH.
This observation suggests that the oxidation of GOase by mediators
at pH 7–9 likely occurs via a concerted proton-coupled electron-transfer
(PCET) mechanism under anaerobic conditions. Finally, this electrochemical
GOase activation method was successfully applied to the development
of a bioelectrocatalytic GOase-mediated aerobic oxidation of benzyl
alcohol derivatives, cinnamyl alcohol, and aliphatic polyols, including
the desymmetrizing oxidation of 2-ethynylglycerol, a key step in the
biocatalytic cascade used to prepare the promising HIV therapeutic
islatravir.
Adenosine
triphosphate (ATP) provides the driving force necessary
for critical biological functions in all living organisms. In synthetic
biocatalytic reactions, this cofactor is recycled in situ using high-energy stoichiometric reagents, an approach that generates
waste and poses challenges with enzyme stability. On the other hand,
an electrochemical recycling system would use electrons as a convenient
source of energy. We report a method that uses electricity to turn
over enzymes for ATP generation in a simplified cellular respiration
mimic. The method is simple, robust, and scalable, as well as broadly
applicable to complex enzymatic processes including a four-enzyme
biocatalytic cascade in the synthesis of the antiviral molnupiravir.
We have investigated the effect of axial ligands on the ability of cobaloximes to catalyze the generation of transferable hydrogen atoms from hydrogen gas and have learned that the active catalyst contains one and only one axial ligand. We have, for example, shown that Co(dmgBF2)2 coordinates only one Ph3P and that the addition of additional Ph3P (beyond 1 equiv) to solvated Co(dmgBF2)2 does not affect its catalytic turnover for H• transfer from H2.
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