Base-Directed Photoredox Activation of C-H Bonds by PCET

Article abstract of DOI:10.1021/acs.joc.0c00333

Photoredox catalysis using proton-coupled electron transfer (PCET) has emerged as a powerful method for bond transformations. We previously employed traditional chemical oxidants to achieve multiple-site concerted proton-electron transfer (MS-CPET) activation of a C-H bond in a proof-of-concept fluorenyl-benzoate substrate. As described here, photoredox oxidation of the fluorenyl-benzoate follows the same rate constant vs driving force trend determined for thermal MS-CPET. Analogous photoredox catalysis enables C-H activation and H/D exchange in a number of additional substrates with favorably positioned bases. Mechanistic studies support our hypothesis that MS-CPET is a viable pathway for bond activation for substrates in which the C-H bond is weak, while stepwise carboxylate oxidation and hydrogen atom transfer likely predominate for stronger C-H bonds.

Full text of DOI:10.1021/acs.joc.0c00333

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Article
Base-directed photoredox activation of C–H bonds by PCET
Maraia E. Ener, Julia Whitney Darcy, Fabian S Menges, and James M. Mayer
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c00333 • Publication Date (Web): 04 May 2020
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Base-directed photoredox activation of C–H bonds by PCET
Maraia E. Ener, Julia W. Darcy, Fabian S. Menges, and James M. Mayer*
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Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, james.mayer@yale.edu
Supporting Information Placeholder
ABSTRACT: Photoredox catalysis using proton-coupled
electron transfer (PCET) has emerged as a powerful method
for bond transformations. We previously employed traditional
chemical oxidants to achieve multiple-site concerted proton-
electron transfer (MS-CPET) activation of a C–H bond in a
proof-of-concept fluorenyl-benzoate substrate. As described
here, photoredox oxidation of the fluorenyl-benzoate follows
the same rate constant vs driving force trend determined for
thermal MS-CPET. Analogous photoredox catalysis enables C–
H activation and H/D exchange in a number of additional
substrates with favorably positioned bases. Mechanistic
studies support our hypothesis that MS-CPET is a viable
Recently, C–H bond activation by MS-CPET was reported
using traditional chemical (“thermal”) oxidants and a proof-of
concept substrate (1) containing favorably positioned
pathway for bond activation for substrates in which the C–H
bond is weak, while stepwise carboxylate oxidation and
hydrogen atom transfer likely predominates for stronger C–H
bonds.
a
carboxylate base (Figure 1A).5 For 1, initial intermolecular ET
is concerted with intramolecular proton transfer (PT) to the
carboxylate base. Conversion of radical intermediate 1• to the
observed lactone product (1-lac) involves additional loss of
1e–/1H+. The substrate scope of such reactions is restricted by
incompatibility of carboxylate bases with many thermal
oxidants such as ferrocenium cations.5a The direct reaction of
an oxidant [electron poor] with a base [electron rich] is a
known limitation of MS-CPET reactivity.6
Introduction
Selective activation and functionalization of C–H bonds
under mild conditions remains
a primary challenge in
synthetic chemistry.1 Photoredox catalysis has emerged as a
method for performing difficult bond transformations by
harnessing the redox power of photocatalyst electronic
excited states.2 Recent photoredox strategies have selectively
activated strong X–H bonds by multiple-site concerted proton-
electron transfer (MS-CPET).3 In this mechanism, electron
transfer (ET) with a photocatalyst occurs simultaneously with
proton transfer to a separate, basic site. For polar X–H bonds,
hydrogen bonding of the base assists the concerted reaction
(Scheme 1A). In contrast, nonpolar C–H bonds have typically
been activated by stepwise heteroatom oxidation and then
hydrogen atom transfer (HAT; Scheme 1B).3a,4
We elected to explore photocatalysis as a path to mitigate
incompatibilities and enable further investigation of the
fundamental aspects governing MS-CPET reactivity at C–H
bonds. During the preparation of this manuscript, the Knowles
and Alexanian groups reported a termolecular MS-CPET
system in which the photocatalyst and phosphate base form a
noncovalent adduct that is capable of activating sp3 C–H
bonds.7 We report here a photoredox system that oxidizes 1
and related substrates, achieves non-oxidative base-directed
C–H activation on additional substrates, and investigates the
competition between MS-CPET (Scheme 1C) and stepwise
ET/HAT (Scheme 1B) mechanisms.
Scheme 1. Photoredox PCET mechanisms for X–H and C–H
bond activation. B represents a generic base, X is N or O.
This work involves case C.
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To probe the identity of the active oxidant during these
reactions, Stern-Volmer quenching experiments were
employed. The excited state *[IrIII
reacts rapidly with
1
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4
5
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7
8
]
Co(acac)3 with kq  108 M-1s-1 (Figure 1B, top red arrow).10
This forms [IrIV], which can oxidize 1. The direct reaction of
*[IrIII] with 1 is slower, (kq  106 M-1s-1, see below). Nearly
equimolar Co(acac)3 and substrate 1 are employed in the
reaction mixture, and [IrIV] persists in solution longer than
*[IrIII], so we expect [IrIV] to be the predominant oxidant
under oxidative photoredox conditions (Figure 1B, red
pathway).
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C–H bond activation was also probed under net-redox
neutral photoredox conditions, in the absence of Co(acac)3
terminal oxidant. No formation of 1-lac is observed under
these conditions. However, 1 can be transiently oxidized to 1•
by *[IrIII] (Figure 1B, blue pathway). Formation of reduced
photosensitizer ([IrII]) is observed upon quenching with 1, by
comparison of transient spectra to that of electrochemically-
reduced photosensitizer, and to [IrII] photochemically
produced in the presence of an established reductive
quencher, tritolylamine.10 This confirms electron transfer
Figure 1. A) Oxidation of substrate 1. B) Photoredox cycle
showing the flash-quench pathway for oxidation of 1 under
oxidative conditions (red), and transient C–H activation of 1
by *[IrIII] under redox neutral conditions (blue). Q represents
the Co(acac)3 quencher for oxidative photoredox. C) Ir
photocatalysts in this work.
from
constant for luminescence quenching of *[IrIIItBu] by 1 of
approximately kq 4(±2)
106 M-1s-1 (comparing
1 . Stern-Volmer analysis indicates a rate
to *IrIII
=
x
Results and Discussion
luminescence lifetimes at varying concentration of 1). There is
significant batch-to-batch variability in the rate constant vs
concentration dependence12 (also see SI), possibly due to
small impurities (e.g., water hydrogen-bonding to the
carboxylate in 1) that could affect the observed reactivity.
The 2-electron oxidation of 1 to 1-lac proceeds under
oxidative photoredox conditions (Figures 1A and 1B). A 410
nm LED was used to irradiate a 3-5 mM solution of 1
(generated in situ by deprotonation of the carboxylic acid with
0.8 eq tetrabutylammonium acetate, TBAOAc), with 10 mol%
iridium photocatalyst [IrIIIH] or [IrIIItBu],8 and ~1.2 eq of
Co(acac)3 terminal oxidant, in deoxygenated acetonitrile. After
2-4 hours of irradiation, up to 90% formation of the 1-lac final
product was observed (1H NMR), with respect to equivalents
of base and terminal oxidant.9 Importantly, omitting light,
photocatalyst, terminal oxidant, or added base results in no or
only trace 1-lac.10
Additional evidence for transient C–H activation under net
redox-neutral conditions was obtained from deuterium
incorporation studies (Scheme 2). The fluorenyl C–H of 1 in
solution does not exchange with methanol-OD within 24
hours, in the absence of photocatalyst and light.5a However,
irradiating a solution of 1-h (denoting isotopic substitution at
the fluorenyl carbon) and *[IrIII] with ~2% v/v d4-methanol in
d3-MeCN led to 80% conversion to 1-d (1H NMR and HR-ESI-
MS). Enrichment is envisioned to occur because the carboxylic
acid proton of intermediate 1• exchanges with ~0.5 M CD3OD
to give 1•-CO2D competitively with its reduction to 1-d by the
quenching product [IrII] (≤ 150 µM) under these net redox
neutral conditions. Therefore, deuterium incorporation can be
used as a marker for transient C–H activation by *[IrIII].
A variety of factors influence the efficacy of this photoredox
transformation (see Supporting Information [SI] for
additional discussion). Notably, using TBAOAc for in situ
deprotonation enhances long-term photocatalyst stability,
compared to tetrabutylammonium hydroxide (TBAOH) or
DBU (1,8-diazabicyclo[5.4.0]undec-7-ene). We presume that
the stoichiometric amount of acetic acid formed during in situ
Scheme 2. Deuterium incorporation under net-redox
deprotonation reduces the nucleophilicity of the
1
neutral photoredox conditions is
activation.
a marker for C–H
carboxylate, attenuating degradation of the photocatalysts as
a result of nucleophilic attack on the ligands. However, both Ir
catalysts degrade slowly under these basic conditions over
many hours, as evidenced by red-shifting of the UV-visible
absorption and emission spectra (see SI).11
Photo-oxidant stability is also enhanced when using < 1
equivalent of base, and adding protic solvents (e.g., MeOH).
However, additions > 5% v/v of MeOH significantly slow or
halt the conversion of 1 to 1-lac, consistent with studies
employing thermal oxidants.5a No additional products are
formed under these more protic conditions. The selection of a
terminal oxidant is somewhat limiting, as many weak oxidants
react directly with 1 (e.g., N-bromo succinimide, methyl
viologen), and other reagents common to photoredox (e.g.,
aryl diazonium salts) form reactive radicals that could
interfere with the desired mechanistic analysis. Co(acac)3
meets the experimental limitations for our purposes,2a but its
competitive absorbance throughout the visible region renders
it less than optimal.
Table 1. Deuterium incorporationa and lactone formationb
for various substrates under photoredox conditions.
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E1/2 of the ferricenium and triarylaminium oxidants used.5a,5c
The approximate rate constant for photoredox C–H bond
cleavage in 1 by *[IrIIItBu] fits this same correlation (Figure 2),
supporting an MS-CPET mechanism.18
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Figure 2. Plot of rate constant vs. oxidant strength for
oxidation of 1. Red circles and trendline are thermal
oxidations;5a cyan star is oxidation by *[IrIIItBu] (this work).
Star size encompasses uncertainty in kMS-CPET and E(oxidant).
a
D incorporation under net redox-neutral conditions (~3 mM
substrate acid, 0.8 eq TBAOAc, 10 mol% [IrIII], 2% d4-MeOD (v/v)
in MeCN, irradiated overnight, 410 nm.) %D determined by HR-
ESI-MS. “2D” indicates double deuteration. %D not determined
for 4 due to low solubility in MeCN. b Oxidative photoredox of 1-6
(~6 mM substrate acid, 0.9 eq TBAOAc, 10 mol% [IrIII], 1.2 eq
Co(acac)3), MeCN, irradiated overnight, 410 nm) gave
corresponding lactone product by NMR and/or HR-ESI-MS,
(“+ox”). No lactone product observed for 7. Substrates 8-14 not
screened for formation of lactones.
To explore whether [IrIV] and/or *[IrIII] can directly oxidize
some carboxylates to carboxyl radicals, we estimated the
reduction potentials from cyclic voltammetry and optical
estimates of the E0-0 energies (Table 2, Supporting
Information Section 4). These values show that the ground
state IrIV species are capable of oxidizing carboxylates by
outer-sphere electron transfer, and very likely the excited
states as well. This is consistent with Glorius’ report that
This H/D exchange methodology has allowed us to explore
photoredox activation of C–H bonds in other substrates with a
positioned base (Table 1), including many that were
unreactive with thermal oxidants (5-10). Deuterium
incorporation was observed for substrates 1-10 (NMR and/or
HR-ESI-MS). In addition, irradiating 1-6 under oxidative
photoredox conditions – with Co(acac)3 – showed some
formation of the oxidized lactone product (1H NMR and/or
HR-ESI-MS, denoted “+ox” in Table 1).
•
*[IrIIItBu] oxidizes benzoate to PhCO2 , which then activate a
variety of aliphatic C–H bonds by intermolecular HAT
(Scheme 1B).4a Our transient absorption experiments show
the formation of reduced photosensitizer ([IrII]) in the
presence of benzoate, further supporting this possibility. This
stepwise mechanism may be expected to outcompete a
concerted one (MS-CPET) for substrates with stronger C–H
bonds, in which MS-CPET is expected to be slower
(conceptually illustrated in Figure 3).
The substrates in Table 1 span a range of C–H bond
strengths from ~74 kcal/mol (1)5a to ~87 kcal/mol (ortho-
toluate, 7),13 and include an example with an internal
sulfonamide base (10). Importantly, no evidence of deuterium
incorporation is seen for para-carboxylate substrates 11 and
12, indicating the importance of base position for reactivity.
There is also no observed deuteration for 14, which has a
weaker pyridine base.14,15 Time course NMR studies with
substrates 1, phenyl o-toluate (5) and o-toluate (7) indicate
that the majority of deuteration occurs within the first 20-30
minutes.10 This is a rapid, simple, and regioselective method
for isotope incorporation that is complementary to a recently
reported photoredox strategy for amine-directed deuterium
and tritium incorporation.16
Table 2. Measured and estimated reduction potentials vs.
ferrocene+/0 (Fc+/0) in MeCN.a
Redox Couple
E1/2
References
E1/2(IrIVH/IrIII
)
H
estimated here
1.38 V
a
E1/2(IrIVtBu/IrIII
)
)
estimated here
1.35 V
~1.05 V
~0.95 V
~0.9 V
tBu
a
E1/2(*IrIIIH/IrII
)
H
estimated here
b
E1/2(*IrIItBu/IrII
estimated here
tBu
Preliminary studies explored functionalization of the
carbon radical intermediates generated under redox neutral
conditions. Methylvinylketone (MVK) and TEMPO have been
employed by others as traps to achieve photoredox-mediated
C–C and C–O bond formations.3a,4b,17 Acetonitrile solutions of
substrate 7 (toluate) with 10 mol% [IrIIItBu] and 2 eq of the
trap were irradiated at 410 nm for 16 hrs. HR-ESI-MS spectra
b
•
–
3a,4a,19
E1/2(RCO2 /RCO2
)
a
Quasi-reversible waves by cyclic voltammetry; see Supporting
b
Information, Section 4.1.
See discussion of the optical E0-0
energies in Supporting Information, Section 4.2.
We have probed the mechanistic possibilities of stepwise
ET/HAT vs. MS-CPET (Scheme 1B vs. C) by comparing
*[IrIIItBu] luminescence quenching by different substrates.
Benzoate (which can only undergo direct ET) quenches the
*[IrIIItBu] luminescence with an observed rate constant that is
very sensitive to conditions. For benzoate formed in situ from
benzoic acid and TBAOAc, kq = 4105 M-1s-1. In contrast, kq for
of the resulting mixtures showed anions corresponding to
TEMPO–CH2C6H4CO2– and CH3C(O)(CH2)3C6H4CO2
.
– 10
The H/D exchange and trapping results show that
photoredox C–H activation occurs for a variety of benzylic
substrates with internal carboxylates. Previously reported
rate constants for thermal oxidations of 1 correlate with the
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TBA-benzoate in dry acetonitrile (no acetic acid as potential
bonds, and deeper explorations into the fundamental aspects
of MS-CPET processes at carbon.
1
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4
5
6
7
8
hydrogen bond partner) is approximately two orders of
magnitude faster20 (4107 M-1s-1 at low concentrations, in
reasonable agreement with other reports).4a Assuming that all
luminescence quenching by benzoate/HOAc is due to ET, the
rate constant of kq = 4105 M-1s-1 gives an approximate bound
at which ET/HAT becomes competitive with MS-CPET under
these acetic acid-tuned conditions (Figure 3). For substrate 7
(toluate), which has one of the strongest C–H bonds in Table
1, the kq = 4105 M-1s-1, suggests that ET/HAT is a likely
mechanism for C–H activation.
Experimental
Materials
Substrates 1 (including both isotopic variants 1-h and 1-d),
2, S15 and 6 were synthesized as described previously.5a,21
Substrate
3 was synthesized as described below. The
remaining chemicals were obtained from commercial sources
and used without further purification unless otherwise noted.
Co(acac)3, and substrates 10 and S20 were obtained from Alfa
Aesar; substrate S16 was obtained from Alfa chemicals;
substrates 4, 5, 8, and 14 were obtained from Santa Cruz
Biotechnology; substrate 13 was obtained from Acros
Organics; substrate 7 was obtained from Tokyo Chemical
Industry; tetrabutylammonium acetate (TBAOAc) was
obtained from Sigma Aldrich and dried in a vacuum oven;
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benzoic
tetrabutylammonium hydroxide (TBAOH, 1M solution in
methanol), tetrabutylammonium benzoate (TBAOBz), [IrIII
acid,
1,3,5-trimethoxybenzene
(TMB),
]
H
and [IrIIItBu], and substrates 9, 12, S17, S18, S19, S21, S22,
S23 were obtained from Sigma Aldrich. Phthalic anhydride
was obtained from Sigma Aldrich and recrystallized from
chloroform before use. Substrate 11 was obtained from Trans
World Chemicals. Anaerobic acetonitrile (Burdick Jackson low
water) was sparged with argon, and was plumbed directly
into the glovebox and used without additional
purification/drying. Deuterated solvents were obtained from
Cambridge Isotope Lab, deoxygenated by freeze-pump-thaw,
and stored in an N2-filled glovebox.
Figure 3. Conceptual transition between concerted and stepwise
C–H activation mechanisms with a single oxidant. Boxed numbers
represent measured kq for substrates 1 and 7. The blue line
(ET/HAT), representing measured kq for benzoate, is independent
of C–H bond strength. The red line (MS-CPET) represents the k vs.
driving force relationship for thermal oxidations of substrate
1.5a,5b Inset: schematic of MS-CPET (red) and ET/HAT (blue)
mechanisms.
Synthesis
2-(bis(4-methoxyphenyl)methyl)benzoic
acid
(3).
Phenolphthalein (1.0 g, 3.1 mmol, 1 eq), K2CO3 (1.3 g, 9.3
mmol, 3 eq), and acetone (20 mL) were stirred at reflux for 10
minutes to ensure deprotonation. Then, iodomethane (3.1 mL,
6.2 mmol, 2 eq) was added. The reaction was quenched after
22 hrs with 20 mL 2 M NaOH. The aqueous layer was re-
acidified with 1 M HCl until a white precipitate formed. The
aqueous layer was extracted with Et2O (3  40 mL), dried over
Na2SO4, and concentrated. The crude product was purified on
a silica column using pure EtOAc to yield the triply methylated
methyl 2-(bis(4-methoxyphenyl)methyl)benzoate. The methyl
ester (3.1 mmol) was deprotected by refluxing in KOHaq (1.7 g
in 50 mL H2O) for 5 hours. Then the reaction mixture was
cooled to room temperature and acidified with ~40 mL 1 M
HCl until a white precipitate formed. The aqueous layer was
extracted with Et2O (2  40 mL) and dried over Na2SO4. The
resulting white solid was recrystallized from MeOH; yield 110
The substrates in Table 1 span the range of bond strengths
between that of 1 and 7, and are expected to span the
transition between mechanisms. Trityl substrates 2, 3, and 4
have similar C–H bond strengths to 1 and probably follow an
MS-CPET mechanism, while phenyl toluate (5) may undergo
predominantly ET/HAT, or
a
combination of both
mechanisms. Limited solubility of these substrates (< 20 mM)
has precluded Stern-Volmer analysis. Similarly, the available
data do not allow drawing mechanistic conclusions for the
reduction of the fluorenyl radical, the last part of the isotope
exchange pathway (Scheme 2, Supporting Information 6.2).
Conclusion
The results presented above demonstrate that photoredox
can be used as a mild, versatile method for base-directed C–H
activation, deuterium incorporation, and functionalization.
Photoredox conditions can replace thermal oxidants to
accomplish MS-CPET at the fluorenyl C–H bond in 1, opening
new avenues to probe fundamental aspects of this concerted
mechanism. For many substrates, two mechanistic pathways
are possible under the highly oxidizing conditions generated
during photoredox. MS-CPET is likely in competition with the
two-step pathway of rate-limiting ET followed by rapid HAT,
which has been reported in other studies.4a The stepwise
ET/HAT path is favored by a more easily oxidized carboxylate
and by stronger C–H bonds, since it is independent of the C–H
bond strength. MS-CPET may be favored when the C–H bond
is weak, and the proton acceptor (typically carboxylate here)
is basic and difficult to oxidize. We hope these insights will
facilitate further development of strategies for activating C–H
1
mg, 10.1%. H NMR (400 MHz, CD3CN) δ (ppm): 7.85 (d, 1H),
7.45 (t, 1H), 7.31 (t, 1H), 7.05 (d, 1H), 6.95 (d, 4H), 6.84 (d,
4H), 6.49 (s, 1H), 3.75 (s, 6H). 13C{1H} NMR (100 MHz, CD3CN)
δ (ppm): 169.2, 159.1 146.4, 137.3, 132.6, 131.6, 131.5, 131.4,
131.2, 127.2, 114.5, 55.8, 51.2. HR-MS (negative ion mode, for
the carboxylate anion): m/z 347.1319 (theoretical: 347.1284).
Photoredox Reactions
Photoredox reaction mixtures were prepared in a nitrogen-
filled glovebox using deoxygenated, deuterated solvent, and
placed in sealed J-Young NMR tubes. Blue/violet light
irradiation was applied between 45 minutes and 16 hours
(depending on the experiment), using
a 410 nm LED
apparatus assembled in house. Lamp components were
purchased from Mouser Electronics: 410 nm LED (e.g., LZ1-
10UB00-01U8), heat sink, focusing lens, and associated lamp
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components. The distance between lamp and J-Young tube
constants were determined from the slope of the best fit line
for rate vs. concentration of substrate plots. For Transient
Absorption experiments, excitation was performed at 430 nm
with a Spectra-Physics Nd/YAG laser (Quanta-Ray) and OPO
(basiScan). Probe light was supplied by an Edinburgh
Instruments Xe900 xenon lamp directed at a 90˚ angle to the
excitation beam. Multiple-wavelength transient absorption
changes were monitored with an Andor iStar300 series CCD
camera. Single-wavelength kinetics (luminescence and
transient absorption) were monitored at 530 nm using an
LP920-K UV-Vis detector. Samples were excited along the long
path length and probed at 90 degrees (through the short path
length dimension). Data were analyzed, fit, and plotted using
MATLAB software. References 19 and 20 are cited in the SI.
1
2
3
4
5
6
7
8
was typically 15-20 cm. No effort was made to cool the
samples during irradiation, and no filters were used. More
details for these and other experiments are given in the
Supporting Information.
For net oxidative photoredox reactions, samples typically
included 5 mM substrate, 0.8-0.9 eq TBAOAc as in situ base, 6
mM Co(acac)3 as terminal oxidant, and 75-150 μM Ir
photocatalyst in d3-acetonitrile. For net neutral photoredox
(H/D exchange) conditions, samples typically included 3 mM
substrate, ~0.8 eq TBAOAc, and ~150 μM Ir photocatalyst in
d3-acetonitrile with 2% (v/v) d4-methanol. Deuterium
incorporation was first assessed by 1H NMR spectroscopy,
then the sample was diluted to 1 mM in acetonitrile, and a
High Resolution Mass Spectrum (HR-MS) was collected. 1H
NMR spectra were collected in the Yale Chemical and
Biophysical Instrumentation Center on either a 400 MHz or
500 MHz Agilent NMR spectrometer. HR-MS was performed
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ASSOCIATED CONTENT
Supporting Information
Additional experimental procedures and analytical methods.
This material is available free of charge via the Internet at
http://pubs.acs.org.
with
a
Thermo Scientific Orbitrap Velos Pro Mass
Spectrometer. HR-MS data were recorded in anion mode with
a resolution (FWHM) of at least 30,000 at 400 m/z and
averaged over at least 100 spectra. The spray needle was held
at 2.5kV with an injection rate of 2.5µL/min, source
temperature 40°C, capillary temperature 225°C and sheath
gas flow rate of 8 a.u. Instrument parameters were held
constant for all MS experiments. Isotope patterns were
modeled by hand with natural abundance isotopic
distribution. Linear combinations of isotope patterns for
substrates containing 1, 2, or 3 deuterium that best matched
AUTHOR INFORMATION
Corresponding Author
* james.mayer@yale.edu
Notes
The authors declare no competing financial interests.
the experimental data were used to determine
incorporation.
% D
ACKNOWLEDGMENT
Electrochemistry
We acknowledge NIH 2R01GM50422 for funding this work.
FSM thanks the Air Force Office of Scientific Research (AFOSR)
under grants FA9550-17-1-0267 (DURIP) and FA9550-18-1-
0213. We thank Prof. Mark A. Johnson for helpful discussions,
Brian Koronkiewicz for assistance with transient absorption
experiments, and Scott Coste for help with a final experiment
and final manuscript preparation.
Cyclic voltammograms were collected to determine the
ground state potentials of the two photocatalysts, [IrH] and
[IrtBu]. The experiments were performed in a nitrogen-filled
glovebox, in acetonitrile with 0.1M TBAPF6 electrolyte. The
three electrode setup consisted of a glassy carbon working
electrode, platinum counter electrode, and pseudo reference
electrode with a silver wire encased in a fritted compartment
containing 100 mM TBAPF6. After collection of the
voltammograms for the photocatalysts, ferrocene was added
as an internal standard and voltammograms were corrected
to the Fc+/0 potential. This addition of ferrocene caused the
IrIV/III couple to become less reversible.
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9
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8. The two photocatalysts have very similar photophysical
properties, also see SI. [IrH] is a slightly more potent oxidant
(~100 mV), and [IrtBu] is slightly more robust in the presence
of carboxylates. Oxidative photoredox studies were performed
using [IrH] for historical reasons. Stern Volmer quenching
experiments intentionally used [IrtBu] due to its slightly
enhanced stability. The two catalysts were used
interchangeably in the redox-neutral deuterium exchange
studies, and appeared to be equally effective.
9. The oxidation of 1 to 1-lac is a 2e–, 2H+ reaction, but only one
equivalent of base and terminal oxidant were used. This was
due to the dark color of Co(acac)3 oxidant (which absorbed
much of the incident light) and instability of the Ir
18. The reduction reaction that re-forms the C-H or C-D bond
could similarly be MS-CPET, or could proceed through a
stepwise mechanism. See SI for discussion.; the data available
do not allow a confident assignment. See SI Section 6.2 for
discussion.
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Tetrabutylammonium Salts of Aliphatic Carboxylic Acids in
Acetonitrile. J. Electrochem. Soc. 2002, 149, D46-D50.
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concentrations. This is an indication of side reactions.
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for 1, 5-and 1, 6-Hydrogen Atom Transfer Reactions of Mono-,
Di-, and Tri-Aryl-Substituted Donors, Models for Hydrogen
Atom Transfers in Polyunsaturated Fatty Acid Radicals. J.
Phys. Chem. A 2008, 112, 1891-1896.
photocatalyst in the presence of excess TBAOAc base.
10. See SI.
11. (a) These shifts in photocatalyst absorption and emission spec-
trum only occur after irradiation. In the absence of irradiation,
the addition of carboxylate-containing substrates to MeCN
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