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Controlling the Redox Properties of Organic Catalysts and Organic Photocatalysts – CO2 Reduction by Renewable Organo-Hydrides and Photocatalyzed Polymerization using Visible Light
The efficient chemical reduction of CO2 to fuels has been of interest to scientists for decades with growing concerns about the impact of CO2 on climate and future global energy demands motivating increasing efforts to meet this challenge. One conversion of specific interest — the reduction of CO2 to methanol (CH3OH) — is the focus of my thesis. Arguments here involve CH3OH’s utility as a practical C1 source for chemical synthesis and its attractive properties as a fuel not demanding the massive changes to the transportation fuels infrastructure required for a hydrogen economy.
My thesis focuses on understanding the role of pyridine in catalyzing the conversion of CO2 to CH3OH. In particular, I employed quantum chemical simulations as an invaluable tool to probe the redox properties of a number of pyridine-derived intermediates involved in the catalytic cycle of CO2 reduction. Accurate determination of redox properties, e.g. reduction potentials and hydricity is important to paint a detailed picture of energetics involved in the transformation of transient species during the course of CO2 reduction, and thus the role of the catalytic species is revealed. One central aspect is the determination of the driving force to effect hydride transfer. 1,2-dihydropyridine (PyH2) is a potent recyclable organo-hydride donor because it is driven by its proclivity to regain aromaticity; this mimics important aspects of the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process.
The aspect of controlling redox properties of molecules was applied to organic photocatalyst that affects photo-polymerization. In collaboration with the Stansbury’s group, we elucidated the mechanism of polymer synthesis involving methylene blue chromophore with a sacrificial sterically-hindered amine reductant and an onium salt oxidant. The combination of these components yield interesting results: light-initiated free-radical polymerization continues over extended time intervals (hours) in the dark after brief (seconds) low-intensity illumination. We proposed that these observations are due to the latent production of free radicals from energy stored in a redox potential through a 2e-/1H+ transfer process, which transforms the methylene blue chromophore to its high energy closed-shell intermediate of leuco methylene
Visible-Light-Promoted C–S Cross-Coupling via Intermolecular Charge Transfer
Disclosed is a mild, scalable, visible-light-promoted
cross-coupling
reaction between thiols and aryl halides for the construction of C–S
bonds in the absence of both transition metal and photoredox catalysts.
The scope of aryl halides and thiol partners includes over 60 examples
and therefore provides an entry point into various aryl thioether
building blocks of pharmaceutical interest. Furthermore, to demonstrate
its utility, this C–S coupling protocol was applied in drug
synthesis and late-stage modifications of active pharmaceutical ingredients.
UV–vis spectroscopy and time-dependent density functional theory
calculations suggest that visible-light-promoted intermolecular charge
transfer within the thiolate–aryl halide electron donor–acceptor
complex permits the reactivity in the absence of catalyst
Mechanism of Homogeneous Reduction of CO<sub>2</sub> by Pyridine: Proton Relay in Aqueous Solvent and Aromatic Stabilization
We employ quantum chemical calculations to investigate
the mechanism
of homogeneous CO<sub>2</sub> reduction by pyridine (Py) in the Py/p-GaP
system. We find that CO<sub>2</sub> reduction by Py commences with
PyCOOH<sup>0</sup> formation where: (a) protonated Py (PyH<sup>+</sup>) is reduced to PyH<sup>0</sup>, (b) PyH<sup>0</sup> then reduces
CO<sub>2</sub> by one electron transfer (ET) via nucleophilic attack
by its N lone pair on the C of CO<sub>2</sub>, and finally (c) proton
transfer (PT) from PyH<sup>0</sup> to CO<sub>2</sub> produces PyCOOH<sup>0</sup>. The predicted enthalpic barrier for this proton-coupled
ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH<sup>0</sup> to CO<sub>2</sub>. However, when PT is mediated by one to three
water molecules acting as a proton relay, the barrier decreases to
29.5, 20.4, and 18.5 kcal/mol, respectively. The water proton relay
reduces strain in the transition state (TS) and facilitates more complete
ET. For PT mediated by a three water molecule proton relay, adding
water molecules to explicitly solvate the core reaction system reduces
the barrier to 13.6–16.5 kcal/mol, depending on the number
and configuration of the solvating waters. This agrees with the experimentally
determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a p<i>K</i>
<sub>a</sub> for PyH<sup>0</sup> of 31 indicating that
PT preceding ET is highly unfavorable. Moreover, we demonstrate that
ET precedes PT in PyCOOH<sup>0</sup> formation, confirming PyH<sup>0</sup>’s p<i>K</i>
<sub>a</sub> as irrelevant for
predicting PT from PyH<sup>0</sup> to CO<sub>2</sub>. Furthermore,
we calculate adiabatic electron affinities in aqueous solvent for
CO<sub>2</sub>, Py, and Py·CO<sub>2</sub> of 47.4, 37.9, and
66.3 kcal/mol respectively, indicating that the anionic complex PyCOO<sup>–</sup> stabilizes the anionic radicals CO<sub>2</sub>
<sup>–</sup> and Py<sup>–</sup> to facilitate low barrier
ET. As the reduction of CO<sub>2</sub> proceeds through ET and then
PT, the pyridine ring becomes aromatic, and thus Py catalyzes CO<sub>2</sub> reduction by stabilizing the PCET TS and the PyCOOH<sup>0</sup> product through aromatic resonance stabilization. Our results suggest
that Py catalyzes the homogeneous reductions of formic acid and formaldehyde
en route to formation of CH<sub>3</sub>OH through a series of one-electron
reductions analogous to the PCET reduction of CO<sub>2</sub> examined
here, where the electrode only acts to reduce PyH<sup>+</sup> to PyH<sup>0</sup>
C–N Cross-Coupling via Photoexcitation of Nickel–Amine Complexes
C–N
cross-coupling is an important class of reactions with
far-reaching impacts across chemistry, materials science, biology,
and medicine. Transition metal complexes can elegantly orchestrate
diverse aminations but typically require demanding reaction conditions,
precious metal catalysts, or oxygen-sensitive procedures. Here, we
introduce a mild nickel-catalyzed C–N cross-coupling methodology
that operates at room temperature using an inexpensive nickel source
(NiBr<sub>2</sub>·3H<sub>2</sub>O), is oxygen tolerant, and proceeds
through direct irradiation of the nickel–amine complex. This
operationally robust process was employed for the synthesis of diverse
C–N-coupled products (40 examples) by irradiating a solution
containing an amine, an aryl halide, and a catalytic amount of NiBr<sub>2</sub>·3H<sub>2</sub>O with a commercially available 365 nm
LED at room temperature without added photoredox catalyst and the
amine substrate serving additional roles as the ligands and base.
Density functional theory calculations and kinetic isotope effect
experiments were performed to elucidate the observed C–N cross-coupling
reactivity
Predicting Hydride Donor Strength via Quantum Chemical Calculations of Hydride Transfer Activation Free Energy
We
propose a method to approximate the kinetic properties of hydride
donor species by relating the nucleophilicity (<i>N</i>)
of a hydride to the activation free energy <i>Δ<i>G</i></i><sup>⧧</sup> of its corresponding hydride
transfer reaction. <i>N</i> is a kinetic parameter related
to the hydride transfer rate constant that quantifies a nucleophilic
hydridic species’ tendency to donate. Our method estimates <i>N</i> using quantum chemical calculations to compute <i>Δ<i>G</i></i><sup>⧧</sup> for hydride
transfers from hydride donors to CO<sub>2</sub> in solution. A linear
correlation for each class of hydrides is then established between
experimentally determined <i>N</i> values and the computationally
predicted <i>Δ<i>G</i></i><sup>⧧</sup>; this relationship can then be used to predict nucleophilicity for
different hydride donors within each class. This approach is employed
to determine <i>N</i> for four different classes of hydride
donors: two organic (carbon-based and benzimidazole-based) and two
inorganic (boron and silicon) hydride classes. We argue that silicon
and boron hydrides are driven by the formation of the more stable
Si–O or B–O bond. In contrast, the carbon-based hydrides
considered herein are driven by the stability acquired upon rearomatization,
a feature making these species of particular interest, because they
both exhibit catalytic behavior and can be recycled
Organocatalyzed Birch Reduction Driven by Visible Light
The
Birch reduction is a powerful synthetic methodology that uses solvated
electrons to convert inert arenes to 1,4-cyclohexadienes—valuable intermediates
for building molecular complexity. This reaction historically requires
dangerous alkali metals and cryogenic liquid ammonia as the solvent, severely
limiting application potential and scalability. Here, we introduce
benzo[ghi]perylene imides as new organic photoredox catalysts for Birch
reductions performed at ambient temperature and driven by visible light. Using
low catalyst loadings (<1 mole percent), benzene and other functionalized
arenes can be selectively transformed to 1,4-cyclohexadienes in good yields.
Mechanistic studies support that this unprecedented visible light induced
reactivity is enabled by the ability of the organic photoredox catalyst to
harness the energy from two visible light photons to affect a single, high
energy chemical transformation, likely proceeding through a solvated electron.</p
Roles of the Lewis Acid and Base in the Chemical Reduction of CO<sub>2</sub> Catalyzed by Frustrated Lewis Pairs
We employ quantum
chemical calculations to discover how frustrated Lewis pairs (FLP)
catalyze the reduction of CO<sub>2</sub> by ammonia borane (AB); specifically,
we examine how the Lewis acid (LA) and Lewis base (LB) of an FLP activate
CO<sub>2</sub> for reduction. We find that the LA (trichloroaluminum,
AlCl<sub>3</sub>) alone catalyzes hydride transfer (HT) to CO<sub>2</sub> while the LB (trimesitylenephosphine, PMes<sub>3</sub>) actually
hinders HT; inclusion of the LB increases the HT barrier by ∼8
kcal/mol relative to the reaction catalyzed by LAs only. The LB hinders
HT by donating its lone pair to the LUMO of CO<sub>2</sub>, increasing
the electron density on the C atom and thus lowering its hydride affinity.
Although the LB hinders HT, it nonetheless plays a crucial role by
stabilizing the active FLP·CO<sub>2</sub> complex relative to
the LA dimer, free CO<sub>2</sub>, and free LB. This greatly increases
the concentration of the reactive complex in the form FLP·CO<sub>2</sub> and thus increases the rate of reaction. We expect that the
principles we describe will aid in understanding other catalytic CO<sub>2</sub> reductions