Visible light mediated reductions of ethers, amines and sulfides

Article abstract of DOI:10.1016/j.jphotochem.2016.05.014

Visible light-mediated photoredox catalysis enables the chemoselective reduction of activated carbon–heteroatom bonds as a function of reduction potential. The expansion of the scope of C–X bond reductions towards less activated motifs, such as ethers, amines and sulfides, is important to both organic synthesis and macromolecular degradation method development. In the present report, exploration of photoredox catalysis in alcoholic solvents mediated a decrease in the super-stoichiometric use of ⁱPr₂NEt and HCO₂H in the reduction of α-keto ethers, amines and sulfides. Additionally, in the absence of fragmentation, [Formula presented] bond formation was afforded, suggesting an intermediate ketyl radicals are present in these transformations.

Full text of DOI:10.1016/j.jphotochem.2016.05.014

Accepted Manuscript
Title: Visible Light Mediated Reductions of Ethers, Amines
and Sulfides
Author: Timothy M. Monos Gabriel Magallanes Leanne J.
Sebren Corey R.J. Stephenson
PII:
S1010-6030(16)30292-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.05.014
JPC 10233
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Accepted date:
23-4-2016
18-5-2016
Please cite this article as: Timothy M.Monos, Gabriel Magallanes, Leanne
J.Sebren, Corey R.J.Stephenson, Visible Light Mediated Reductions of Ethers,
Amines and Sulfides, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2016.05.014
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Visible Light Mediated Reductions of Ethers, Amines and Sulfides
Timothy M. Monos, Gabriel Magallanes, Leanne J. Sebren, Prof. Corey R. J. Stephenson
Department of Chemistry, University of Michigan, Ann Arbor, 48109, USA
1

Highlights
•
Method for the reduction of α–keto ethers amines and sulfides at room temperature using visible
light and an Ir(III) photocatalyst.
•
•
Reactions run in ethanol with minimal additive loadings.
Lignin model substrates containing the β–O–4 motif were reduced in fair to good yields.
2

Abstract
Visible light-mediated photoredox catalysis enables the chemoselective reduction of activated carbon–
heteroatom bonds as a function of reduction potential. The expansion of the scope of C–X bond
reductions towards less activated motifs, such as ethers, amines and sulfides, is important to both organic
synthesis and macromolecular degradation method development. In the present report, exploration of
photoredox catalysis in alcoholic solvents mediated a decrease in the super-stoichiometric use of iPr2NEt
and HCO2H in the reduction of α–keto ethers, amines and sulfides. Additionally, in the absence of
fragmentation, C–C bond formation was afforded, suggesting an intermediate ketyl radicals are present in
these transformations.
3

1. Introduction
Photoredox catalysis has emerged as a premier method for selective radical generation by single
electron transfer (SET) [1]. In this mode of catalysis, reactivity is enabled through the photophysical
excitation of a metal or organic dye photosensitizer, which can subsequently participate in bimolecular
electron transfer in the excited state [2]. Visible light mediated photocatalysis has mitigated the hazards
and environmental incompatibility of radical chemistry conducted with tin reagents and organic radical
initiators [3]. While a number of functionalities have been reduced using photoredox catalysis [1],
reductions of less labile carbon–heteroatom bonds – including ethers, amines and sulfides – are less
prevalent in the literature. These motifs comprise a large portion of organic functional groups, and as
such, represent a challenging goal to engage for applications in small molecule synthesis and
macromolecular degradation.
One such macromolecule that can benefit from the development of reduction chemistry is lignin. This
biopolymer comprises about 30-40% of plant biomass, and is largely discarded as a waste product in the
paper pulping industry due to the irregular aryl ether polymeric composition that is highly resistant to
chemical degradation [4]. Recently, methods utilizing homogenous transition metal catalysis for the
activation of the C–O bonds in lignin biomass have been described [5]. Yet, identifying scalable,
chemically efficient reactions for the selective depolymerization of lignin into valuable aromatic products
remains the central challenge of emerging technologies.
Recently, we disclosed an approach leveraging photoredox catalysis for the chemoselective reduction
of the most prominent polymeric linkage in lignin, the β–O–4 motif (Figure 1, A-B) [6]. This protocol
consisted of a stoichiometric oxidation followed by a photocatalytic reduction to selectively cleave the β–
O–4 bond in a number of model lignin substrates. This method was selective in the presence of γ–hydroxy
groups as well as unprotected phenols. Based upon the current mechanistic understanding of ketyl
chemistry, we proposed a mechanism for the photoredox reductive cleavage (Figure 1) whereby a ketone
4

substrate (1) is first reduced by an Ir(II) species generated from a reductive quenching event with a
tertiary amine. Fragmentation of 2 about the Cα–O (red) bond affords an acetophenone radical and, after
protonation, guaiacol (4). Finally, the acetophenone radical can be reduced by an H-atom transfer process
[7].
i
Despite the success of this reaction, the super-stoichiometric use of Pr2NEt and HCO2H was thought
to be a prohibiting factor in developing scale-up applications. In this report, we describe our
advancements in understanding the photochemical degradation of model lignin substrates, as well as
describe the adaptation of this reductive cleavage method for α–keto amines and sulfides (Figure 1, C).
A) First Generation Approach
Visible Light Mediated Reduction
1 mol % Ir(III)
TEMPO Oxidation
i
OH
OMe
O
OMe
3 equiv. Pr2NEt
3 equiv. HCO2H
O
OMe
1.05 equiv.
[4-AcNH-TEMPO]BF4
O
O
HO
β
4
β
4
β
+
4
RO
RO
RO
visible light, MeCN, RT
silica, DCM, RT
OH
dimeric model lignin
OH
OH
B) Proposed Mechanism
NR2
R
Ir(III)*
hν
NR2
NR2
R
O
OMe
O
O
OMe
O
OMe
Ir(III)
Ir(II)
R
O
HO
+
OH
OH
OH
3
4
1
NR2
R
anionic ketyl radical
2
C) Second Generation Reduction
OMe
1 mol % Ir(III)
2 equiv. Pr2NEt
i
O
O
OMe
O
HO
1 equiv. HCO2H
β
4
β
4
+
RO
RO
OH
visible light, EtOH, RT
OH
O
H2N
O
same conditions
same conditions
+
Y
generally
O
Y = N, S
S
+
2
Figure 1: Ketone Reduction-Fragmentation Aproach for α–heteroatom cleavage.
5

2. Results and Discussion
2.1 Reaction design:
In order to affect the reduction
of unstrained C–O, C–N and C–S
bonds, we targeted a reduction-
fragmentation approach. Previous
reports suggested the direct
reduction of Csp3–O bonds (Ered = –
2.23 V vs. SCE [8]) is thermodynamically unfavorable using Ir(III) or Ru(II) based photocatalysts, thus a
method targeting an initial carbonyl reduction (Ered = –2.10 vs SCE [9]) seemed more feasible. This
approach has been meaningfully demonstrated by Molander, with Sm(II) reductants [10], followed by
Hasegawa [11] and Olivier [12] with photoredox catalysts (Figure 2). For both Molander and Hasegawa,
the effective reductants were very reducing and, as a result, engaged aliphatic ketones to afford
fragmentation products in high yields. More recently, Olivier’s example demonstrated phenyl ketone
substituted aziridines are privileged substrates for photoredox catalysis to induce fragmentation. Inspired
by these reports, single electron transfer catalyzed by Ir(II) reductants provided similar reactivity in the β–
O–4 lignin substrates (1) which contained an unstrained α–keto ether motif (Figure 1). In this case, the
reaction efficiency correlated well with the pKa of the fragmenting group, and this methodology was
effective for model lignin substrates in which the fragmenting group was guaiacol (4).
6

In order to address the excess of terminal
reductant additives, we targeted using
alcoholic solvents for an additional function
of terminal H-atom donors [13]. An initial
screening of alcoholic and ether solvents as
H-atom donors revealed ethanol as
competent solvent in the debromination of
diethylbromomalonate [14] Figure 3,
a
(
entries 1-4). Despite observing good
conversions of diethylbromomalonate, the
reduction of model lignin 6 failed to convert
under the same reaction conditions (Figure 3, entry 5). This could be overcome by the addition of
iPr2NEt; yet complete consumption of starting material was not observed in a 12-hour reaction period
(Figure 3, entry 6-8). Additional time did not reveal further starting material consumption. Upon the
addition of formic acid, full consumption of the starting material in 12 hours was observed. Taken
together, these results suggested the reductive quenching cycle which engaged the amine as the electron
and H-atom source was the most prominent process; however, the use of ethanol could promote the
reductive fragmentation of 6, likely by facilitating Brønsted acid activation better than in acetonitrile did
in the first generation conditions. Additionally, Brønsted acid activation by weakly acidic guaiacol did not
promote reactivity in the same manner as formic acid. Lastly, control experiments proved light and
photocatalyst were necessary for this transformation [14].
2.2 Reaction Scope
The optimized conditions (Figure 3, entry 11) proved effective in the reductive fragmentation of
model lignin substrates 8, 9, and 10 (Table 1). While the measured reduction potentials of each of these
substrates were nearly identical, –1.77 V vs. SCE [9b], full consumption of 8, 9, and 10 occurred over
7

different reaction times – 8 reduced in 24 hours, 9 in 48 and 10 in 60 hours. This suggested that the
overall fragmentation process was kinetically favored in the least alkoxy substituted aryl ketone
substrates. Isolation of the reduced lignin fragments (8a, 9a, 10a) was afforded in moderate to low yield,
suggesting the required reaction time for full starting material consumption was also allowing for
additional product reactivity and degradation.* Interestingly, substrate 11, was isolated in high yield,
implicating the β–hydroxy motif of 8, 9 and 10, to be a promoter of further reactivity. More broadly, this
protocol was effective in fragmenting benzyloxyacetaldehyde (12) and chalcone epoxide (13). The
reduction of 12 was only possible in acetonitrile solvent; we speculate this reactivity difference is due to
the formation of an acetal of 12 in the alcoholic solvents.
Table 1 Substrate Scope of Ether Reduction
While investigating a broader reduction scope, the developed reaction conditions reduced α–keto
amines in good yields (
Table 2). Substrates 17 and 18 converted quickly (6-12 hours) to the fragmented products, while substrate
16 needed a significantly longer reaction time (>24 hours). In addition to amines, α–keto sulfides were
efficiently reduced without prior oxidation to sulfoxides or sulfones. In these reductions, the sulfur
fragment was isolated as the disulfide rather than the mercaptan. One possibility for disulfide formation
* Yields isolated for substrates 8, 9, 10 were isolated in 85-90% yield when reacted under the first generation
conditions (see Fig 1 (A)).
8

occurs via fragmentation to generate a thiyl radical and acetophenone enolate, whereby two thiyl radicals
combine to form a disulfide. Additionally, similar to the ether and amine reductions, an anionic
thiophenol species could be generated which later participates in a separate oxidation event to afford the
disulfide species. Importantly, neither thiophenol nor disulfide mediated α–heteroatom fragmentations
were observed as a background reaction.
Table 2: Substrate Scope of Amine and Sulfide Reductions
Overall, sulfide substrates containing electron-withdrawing arene substituents (24-26) fragmented
efficiently, to afford moderate to good yields of the reduction products. Conversely, substrates 21-23 were
sluggish to reduce, but cleanly afforded the fragmentation products. Substrates 27 and 28 were the
slowest to reduce, and did not fully convert after reacting for more than 48 hours. Lastly, in contrast to the
equivalent ether and amine substrates, the alkyl sulfides (29, 30) cleanly fragmented without any detected
pinacol product formation. Substrate 30 was sensitive to the addition of formic acid, but cleanly
9

fragmented with only 1 equivalent of iPr2NEt. These results highlight that efficient reductive cleavage of
α–keto sulfides can be achieved through photoredox catalysis.
In addition to the observed fragmentation reactivity, selected substrates exhibited pinacol coupling
exclusively. This was first observed for substrates 14 and 15 (Table 3), whereby the products 14a and
15a were afforded in low yields. Reactivity for fragmentation or pinacol coupling is hypothesized to be
due to the stability of the intermediate ketyl species balanced with the electronic influence of both the
phenyl and oxygen substituents (Figure 4). For substrates 8-11, fragmentation products were afforded as
guaiacol is weakly basic and a good leaving group. Substrates 13-15 contain α–hydroxy or alky-ether
substituents which disfavor fragmentation from the ketyl intermediate due to the higher basicity of the
alcohol fragments. With this slow fragmentation step, the ketyl intermediate can be stabilized by the
protic solvent, allowing for a lifetime long enough to form the pinacol products. The apparent outlier of
this trend, substrate 12, fragments because the intermediate ketyl species is too unstable and forces
fragmentation before pinacol coupling can occur. Importantly, the fragmentation of 12 was only possible
in acetonitrile; fragmentation was not observed in ethanol as the diethylacetal was hypothesized to
prevent reactivity. Overall, similar ketyl C–C bond forming processes have been demonstrated by
Knowles [15] and Rueping [16], whereby the C–C bond formation is affected by an initial single electron
reduction of a phenyl ketone or aldehyde.
10

Table 3: Pinacol Products
3. Conclusions
The continued development of mild and selective carbon–heteroatom bond reductions is critical for both
synthesis and biomass conversion processes. Using a Ir(III) photocatalyst, we have demonstrated the
reduction of a variety of unstrained α–keto ethers, amines and sulfides using decreased amounts of
terminal reductants. This demonstrates progress towards the minimization of reagent stoichiometry, one
of the implicit goals of catalytic biomass valorization. Lastly, a radical pathway for C–C bond formation
by ketyl dimerization was demonstrated, and appears to be highly influenced by substrate electronics and
reaction conditions, both of which we are currently investigating.
4. Experimental
4.1 General Information:
All other chemicals were used as received. Reactions were monitored by TLC and visualized with
a dual short wave/long wave UV lamp. Column flash chromatography was performed using 230-400
mesh silica gel or via automated column chromatography. Preparative TLC purifications were run on
silica plates of 1000 µm thickness. NMR spectra were recorded on Varian MR400, Varian Inova 500,
11

Varian Vnmrs 500, or Varian Vnmrs 700 spectrometers. Chemical shifts for 1HNMR were reported as
13
, parts per million, relative to the signal of CHCl3 at 7.27 ppm. Chemical shifts for CNMR were
reported as , parts per million, relative to the center line signal of the CDCl3 triplet at 77.16 ppm.
Chemical shifts for 19FNMR were reported as , parts per million, relative to the signal of a
trifluorotoluene internal standard at –63.72 ppm. The abbreviations s, br. s, d, dd, br. d, ddd, t, q, br.
q, qi, m, and br. m stand for the resonance multiplicity singlet, broad singlet, doublet, doublet of
doublets, broad doublet, doublet of doublet of doublets, triplet, quartet, broad quartet, quintet,
multiplet and broad multiplet, respectively. IR spectra were recorded on a Perkin-Elmer Spectrum BX
FT-IR spectrometer fitted with an ATR accessory. Mass Spectra were recorded at the Mass
Spectrometry Facility at the Department of Chemistry of the University of Michigan in Ann Arbor,
MI on an Agilent Q-TOF HPCL-MS with ESI high resolution mass spectrometer. Gas
Chromatography yields were run on a Shimadzu GC-MS QP2010 SE with an Rx1 5sil MS column.
Yields were calculated based on linear calibration curve of 4-methoxyacetophenone. Electrochemical
data was collected on a CHI600E potentiostat with the accompanying CH Instruments software. LED
lights and the requisite power box and cables were purchased from Creative Lighting Solutions
(http://www.creativelightings.com) with the following item codes: CL-FRS5050-12WP-12V (4.4W
blue LED light strip), CL-FRS5050WPDD-5M- 12V-BL (72 W LED strip), CL-PS94670-25W (25
W power supply), CL-PS16020-150W (150 W power supply), CL-PC6FT-PCW (power cord), CL-
TERMBL-5P (terminal block).
Unless stated otherwise, all reactions were run on a 1.0 mmol scale in a 2 or 4 dram vial equipped
with stir bar and cap. One 4w LED strip was wrapped in a circle around the reaction with the reaction
about 3-4 inches from the light source. At this distance the temperature of the reactions did not
exceed 35 ˚C.
Preparation of Starting Materials:
4.1.1 Phenylethanone Amines: 2-bromoacetophenone (2.0 g, 10.05 mmol) and ethanol (50 mL, 0.2
M in starting material) were added to a dry round botom flask equipped with a magnetic stir
bar. Aniline (1.83 mL, 20.10 mmol, 2 eq.) was added drop-wise, as a white precipitant
formed immediately upon addition. This was allowed to stir for 3 hours, upon which the
solution was filtered, concentrated in vacuo, and recrystallized using ethanol.
1
4.1.1.1 1-phenyl-2-(phenylamino)ethan-1-one (16) - H NMR (400 MHz, CDCl3) 8.03 (d, J = 7.3
Hz, 2H), 7.65 (t, J = 7.4 Hz, 1H), 7.53 (t, J = 7.7 Hz, 2H), 7.31 – 7.22 (m, 3H), 6.81 (t, J = 7.6
Hz, 3H), 4.67 (s, 2H). 13C NMR (176 MHz, CDCl3) 194.98, 146.95, 134.89, 133.86, 129.37,
128.88, 127.75, 117.94, 113.13, 50.41. IR (neat) 3410.9, 1686.3, 1601.9, 1508.4, 1444.8,
1356.9, 1321.6, 1261.8, 1219.9, 1178.8, 1147.7, 984,864.4, 743.6, 684.2, 663.5. HRMS (m/z)
– 212.1070 [M+H]+. Rf (7:3 Hexanes:EtOAc) = 0.64
1
4.1.1.2 4-methy-N-(2-oxo-2-phenylethyl)-N-phenylbenzenesulfonamide (17) - H NMR (700 MHz,
CDCl3) 7.95 (dd, J = 8.2, 1.1 Hz, 2H), 7.60 (d, J = 7.4 Hz, 1H), 7.59 – 7.55 (m, 2H), 7.47 (t,
J = 7.8 Hz, 2H), 7.32 – 7.23 (m, 5H (2+3), overlap with CHCl3), 7.18 – 7.14 (m, 2H), 5.05 (s,
13
2H), 2.44 (s, 3H). C NMR (176 MHz, CDCl3) 193.87, 143.86, 139.74, 135.61, 135.06,
133.78, 129.53, 129.22, 128.86, 128.85, 128.39, 128.16, 77.34, , 76.98, 57.78, 21.75. IR
(neat): 3061.9, 2917.1, 1707.2, 1595.3, 1491.8, 1447.4, 1335.1, 1364.6, 1305.8, 1291.6,
1212.6, 1184.4, 1103.8, 1089.9, 1029.7, 1011.6, 1000.4, 980.1, 881.8, 935.3, 912.4, 809.2,
768.2, 755.0, 768.2, 729.9, 667.0, 692.9 HRMS (m/z) = 366.1158 [M+H].
12

4.1.2 Phenylethanone Sulfides: Prepare a solution of thiol and base ethanol solution, 0.2 M with
respect to the thiol. Allow for equilibration and cooling for 30 minutes. Separately, prepare a
solution of 2-bromoacetophenone in ethanol, and add slowly to the reaction. Allow
appropriate time for reaction. Quench the reaction with an equimolar amount of acid, and
then precipitate the product by adding a large excess of water. Crude crystals can be
recrystallized in MeOH-H2O.
1
4.1.2.1 1-phenyl-2-(phenylthio)ethan-1-one (21) - H NMR (700 MHz, CDCl3) 7.96
(dd, J = 8.4, 1.2 Hz, 2H), 7.96 (dd, J = 8.4, 1.2 Hz, 2H), 7.60 (ddt, J = 8.6, 7.2,
1.2 Hz, 1H), 7.48 (dd, J = 8.1, 7.4 Hz, 2H), 7.42 –7.38 (m, 2H), 7.32 – 7.28 (m,
2H), 7.26 – 7.21 (m, 1H), 4.29 (s, 2H). 13C NMR (176 MHz, CDCl3) 133.47,
130.52, 129.05, 128.67, 128.67, 127.11, 41.2. HRMS (m/z) = 229.0682 [M+H]+.
IR (neat) 3074.1, 1668.8, 1596, 1578.4, 1479.2, 1444.9, 1416.1, 1273.2, 1185.8,
1133.9, 1072.5, 1011.2, 941.7, 898, 804.3, 740.5, 722, 66.6, 648.1, 614.1 Rf
(EtOAc/Hexanes (1:5)) = 0.50
1
4.1.2.2 1-(4-methoxyphenyl)-2-(phenylthio)ethan-1-one (22) - H NMR (500 MHz, CDCl3) 7.98 –
7.91 (m, 2H), 7.43 – 7.37 (m, 2H), 7.32 – 7.27 (m, 2H), 7.25 – 7.20 (m, 1H), 6.97 – 6.92 (m,
2H), 4.25 (s, 2H), 3.89 (s, 3H). 13C NMR (176 MHz, CDCl3) 192.68, 163.76, 135.03,
131.02, 130.34, 129.00, 128.37, 126.94, 113.83, 55.49, 40.95. IR (neat) 1658.2, 1601.4,
1572, 1508.6, 1480.4, 1420.3, 1436.7, 1395.9, 1310.5, 1262.6, 1199.9, 1173.8, 1023.7, 991.6,
817.8, 734.3, 690, 633.1. HRMS (m/z) 259.0682 [M+H]+ Rf (EtOAc/Hexanes 1:5) 0.55.
4.1.2.3 2-((4-methoxyphenyl)thio)-1-phenylethan-1-one (23)- 1H NMR (500 MHz, CDCl3) 7.93 (d,
J = 7.8 Hz, 2H), 7.57 (d, J = 7.3 Hz, 1H), 7.47 (t, J = 7.6 Hz, 2H), 7.37 (d, J = 8.6 Hz, 2H),
6.83 (d, J = 8.6 Hz, 2H), 4.14 (s, 2H), 3.80 (s, 3H). 13C NMR (176 MHz, CDCl3) 194.29 (s),
159.73 (s), 135.45 (s), 134.65 (s), 133.29 (s), 132.64 (s), 130.48 (s), 128.94 (s), 128.65 (d, J =
18.3 Hz), 126.30 (s), 124.52 (s), 114.82 (s), 114.63 (d, J = 18.7 Hz), 77.18 (s), 76.99 (s),
76.81 (s), 55.29 (s), 42.79 (s). IR (neat) 2937.2, 2835.3, 1674.0, 1591.1, 1492.3, 1461.7,
1447.4, 1406.0, 1274.1, 1242.8, 1196.2, 1172.8, 1133.5, 1104.0, 1075.9, 1027.9, 1014.0,
1075.9, 1027.9, 1014.0, 825.8, 798.3, 748.4, 723.8, 686.9. HRMS (m/z): 259.0788 [M+H]+.
4.1.2.4 2-(phenylthio)-1-(4-(trifluoromethyl)phenyl)ethan-1-one (24) - 1H NMR (700 MHz, CDCl3)
8.04 (d, J = 8.1 Hz, 15H), 7.74 (d, J = 8.1 Hz, 15H), 7.41 – 7.36 (m, 14H), 7.30 (ddd, J = 6.6,
2.4, 0.8 Hz, 14H), 7.28 (t, J = 1.3 Hz, 3H), 7.27 – 7.25 (m, 4H), 4.26 (s, 16H). 13C NMR (176
MHz, CDCl3) 193.18 (s), 138.21(s), 135.08(s), 134.80 (q, J = 32.8 Hz), 134.70 (s), 134.52
(s), 134.10 (s), 131.18 (s), 129.35 (s), 129.21 (s), 125.98 (s), 125.88 (q, J = 3.7 Hz), 123.65
19
(q, J = 272.7 Hz), 121.33 (s), 41.42 (s). F NMR (471 MHz, CDCl3) ppm = –64.17 (s, 3 F).
IR (neat) 2896.7, 1679.3, 1580.0, 1511.7, 1482.4, 1438.6, 1409.9, 1393.0, 1326.1, 1311.2,
1311.2, 1285.7, 1195.7, 1162.8, 1111.1, 1065.1, 1026.9, 1015.1, 992.4, 964.0, 900.1, 854.4,
839.0, 825.5, 739.9, 701.1, 689.8. HRMS (m/z) = 297.055 [M+H]+. Rf (9:1 Hexanes:EtOAc)
= 0.36 (stains light green in vanillin)
1
4.1.2.5 1-(naphthalen-2-yl)-2-(phenylthio)ethan-1-one (25) - H NMR (700 MHz, CDCl3) 8.44 (s,
1H), 8.02 (dd, J = 8.6, 1.6 Hz, 1H), 7.96 – 7.86 (m, 3H), 7.63 (t, J = 7.4 Hz, 1H), 7.57 (t, J =
7.5 Hz, 1H), 7.43 (d, J = 7.5 Hz, 2H), 7.30 (t, J = 7.5 Hz, 2H), 7.25 (t, J = 7.3 Hz, 1H), 4.41
13
(s, 2H). C NMR (176 MHz, CDCl3) 194.20, 135.86, 134.95, 132.85, 132.56, 130.87,
130.73, 129.79, 129.25, 128.88, 128.73, 127.94, 127.35, 127.02, 124.36, 41.53. IR (neat)
13

3073.2, 3048.4, 2952.3, 2898.7, 2898.7, 1676.0, 1622.7, 1581.6, 1505.9, 1478.8, 1466.1,
1436.0, 1389.7, 1355.9. 1293.9, 1271.1, 1244.5, 1164.5, 1124.3, 1093.9, 1070.5, 1024.1,
1024.1, 984.5, 972, 944.7, 925.8, 890.3, 864.5, 810.1, 767.9, 748.6, 727.7, 685.7. HRMS
(m/z): 301.0658 [M+Na+]. Rf (9:1 Hexanes:EtOAc) = 0.32
1
4.1.2.6 2-((3-chlorophenyl)thio)-1-phenylethan-1-one (26) - H NMR (700 MHz, CDCl3) 7.96 (dd,
J = 8.4, 1.2 Hz, 1H), 7.61 (ddd, J = 2.4, 1.8, 1.2 Hz, 1H), 7.52 – 7.47 (m, 1H), 7.38 (t, J = 1.6
Hz, 1H), 7.27 (m, 1 H) overlap with chloroform, 7.22 (t, J = 7.5 Hz, 1H), 7.21 – 7.19 (m, 1H),
4.32 (s, 1H). 13C NMR (126 MHz, CDCl3) 193.72, 137.10, 135.41, 133.82, 130.21, 129.80,
128.92, 128.80, 128.16, 127.24, 41.02. IR (neat) 3052.3, 2943.0, 2913.8, 1686.2, 1593.2,
1573.4, 1561.6, 1561.6, 1464.3, 1445.8, 1404.1, 1382.3, 1382.3, 1322.5, 1308.1, 1288.9,
1196.9, 1180.2, 1116.2, 1086.1, 1086.1, 1077.5, 1026.0, 999.3, 980.0, 884.0, 884, 871.3,
775.7, 765.0, 751.5, 687.0, 680.2. HRMS (m/z): 263.0292 [M+H+]. Rf (9:1 Hexanes:EtOAc)
= 0.33
1
4.1.2.7 ((2-fluorophenyl)thio)-1-phenylethan-1-one (27) - H NMR (700 MHz, CDCl3) 7.96 (dd, J =
8.4, 1.2 Hz, 1H), 7.61 (tt, J = 7.41, 1.21 Hz, 1H), 7.51 – 7.48 (m, 1H), 7.38 (t, J = 1.6 Hz,
1H), 7.27(dt, J = 7.48,1.57 Hz, 1H overlap with CDCl3), 7.22 (t, J = 7.5 Hz, 1H), 7.21 – 7.19
13
(m, 1H) , 4.32 (s, 2H). C NMR (126 MHz, CDCl3) 193.98 (s) 162.17 (d, J = 246.5 Hz), ,
135.52 (s), 134.05 (s), 134.04 (s), 133.65 (s), 129.96 (d, J = 8.0 Hz), 128.80 (d, J = 9.5 Hz),
19
124.74 (d, J = 3.8 Hz), 121.33 (d, J = 17.6 Hz), 116.02 (d, J = 22.5 Hz), 40.51 (s). F NMR
(471 MHz, CDCl3) -109.06 (ddd, J = 9.4, 7.5, 5.5 Hz). IR (neat) 3070.3, 2947.6, 2911.4,
1679.9, 1593.5, 1578.8, 1565.6, 1465.8, 1445.9, 1397.5, 1317.6, 1284.4, 1261.9, 1220.1,
1191.4, 1180.6, 1160.0, 1123.1, 1070.2, 1031.3, 999.4, 983.7, 927.8, 888.2, 852.9, 826.9,
806.3, 806.3, 743.3, 687.4, 678.4. HRMS (m/z) = 269.0407 [M+ Na+]. Rf (9:1
Hexanes:EtOAc) = 0.40
1
4.1.2.8 2-methyl-1-phenyl-2-(phenylthio)propan-1-one (28) - H NMR (700 MHz, CDCl3) 8.25 (d,
J = 8.4 Hz, 2H), 7.54 (td, J = 7.7, 1.0 Hz, 1H), 7.46 (t, J = 7.7 Hz, 2H), 7.38 – 7.32 (m, 3H),
7.32 – 7.24 (m, 2H, overlap with CHCl3), 1.57 (s, 6H). 13C NMR (176 MHz, CDCl3) 201.20
(s), 137.90 (s), 137.16 (s), 132.49 (s), 131.78 (s), 130.48 (s), 130.16 (s), 129.59 (s), 128.84
(s), 55.67 (s), 27.59 (s). IR (neat) : 3060.3, 2967.4, 2928.0, 1665.8, 1595.6, 1574.2, 1473.5,
1460.9, 1438.7, 1383.5, 1364.5, 1304.0, 1259.7, 1157.3, 1118.8, 1088.3, 1068.0, 1024.9,
1002.0, 975.5, 881.6, 792.4, 750.0, 732.1, 702.2. HRMS (m/z) = 257.0994 [M+H+] Rf (9.5:0.5
Hexanes:EtOAc) = 0.42
4.1.2.9 2-(phenylthio)-2,3-dihydro-1H-inden-1-one (29) - 1H NMR (700 MHz, CDCl3) ppm 3.13
(dd, J=17.54, 3.75 Hz, 8 H) 3.63 (dd, J=17.71, 7.66 Hz, 9 H) 4.08 (dd, J=7.83, 3.75 Hz, 8 H)
7.21 - 7.30 (m, 29 H) 7.34 - 7.42 (m, 15 H) 7.44 - 7.52 (m, 15 H) 7.54 - 7.67 (m, 9 H) 7.78 (d,
J=7.66 Hz, 7 H). 13C NMR (176 MHz, CDCl3) ppm 34.79, 50.34, 124.58, 126.29, 127.12,
127.46, 127.67, 127.84, 128.95, 132.30, 133.28, 135.26, 135.43, 152.06, 202.23. IR (neat)
3058.7, 2906.7, 1763.5, 1721.7, 1602.1, 1580.3, 1602.1, 1580.3, 1481.7, 1470.7, 1437.9,
1419.7, 1325.4, 1299.4, 1273.8, 1205.8, 1185.6, 1173.6, 1146.0, 1087.8, 1020.8, 1008.1,
957.1, 892.4, 849.7, 792.9, 780.4, 740.6, 729.8, 711.0, 689.5. HRMS (m/z) = 241.0682
[M+H+] Rf (9:1 Hexanes:EtOAc)= 0.46
1
4.1.2.102-(benzylthio)-1-phenylethan-1-one (30) - H NMR (500 MHz, CDCl3) ppm 3.67 (s, 2 H)
3.76 (s, 2 H) 7.17 - 7.28 (m, 2 H) 7.32 (t, J=7.46 Hz, 2 H) 7.34 - 7.38 (m, 2 H) 7.43 - 7.49 (m,
13
2 H) 7.52 - 7.60 (m, 1 H) 7.93 (d, J=8.07 Hz, 2 H) C NMR (176 MHz, CDCl3): 194.40,
137.26, 135.36, 133.32, 129.26, 128.68, 128.64, 128.52, 127.22, 36.06, 35.82. IR (neat):
14

1670.2, 1595.7, 1449.8, 1394.7, 1292.3, 1197.4, 998.8, 751.1, 685.3, 638.4, 552.6, 536.0,
526.3, 497.6, 480.1, 447.3, 417.9, 412.5. HRMS (m/z) = 265.0658 [M+Na+].
1
4.1.2.111-([1,1'-biphenyl]-4-yl)-2-(phenethylthio)ethan-1-one (31)- H NMR (700 MHz, CDCl3)
1
ppm H NMR (700 MHz, CDCl3) 8.06 (d, J = 8.4 Hz, 2H), 7.70 (d, J = 8.4 Hz, 2H)), 7.64
(dd, J = 8.2, 1.1 Hz, 2H), 7.49 (t, J = 7.7 Hz, 2H), 7.42 (t, J = 7.4 Hz, 1H), 7.30 (dd, J = 9.5,
5.7 Hz, 2H), 7.22 (dd, J = 7.2, 5.3 Hz, 3H), 3.84 (s, 2H), 2.93 (dd, J = 8.9, 6.4 Hz, 2H), 2.89 –
2.83 (m, 2H). 13C NMR (176 MHz, CDCl3) ppm 13C NMR (176 MHz, cdcl3) 194.16 (s),
146.22 (s), 140.22 (s), 139.95 (s), 133.96 (s), 129.55 (s), 129.12 (s), 128.73 (s), 128.63 (s),
128.45 (s), 127.48 (s), 127.43 (s), 126.57 (s), 37.20 (s), 35.72 (s), 33.79 (s). IR (neat): 2914.2,
2139.9, 2183.1, 2172.7, 2066.2, 1977.0, 2017.31971.9, 1955.0, 1660.3, 1599.9, 1560.5,
1485.1, 1452.8, 1419.8, 1310.9, 1288.9, 1270.1, 1204.5, 1139.5, 1159.9, 1072.8, 1034.3,
1002.6, 927.7 855.8, 845.8, 714.5, 698.5, 988.2, 647.5, 633.9, 609.3, 615.0, 620.1, 604.0
HRMS (m/z) = 333.1308 [M+H+]. Rf (9:1 Hexanes:EtOAc): 0.55 (brown in anisaldehdye
stain).
4.2 General Reaction Procedure:
Phenyl ketone (0.50 mmol – 1.0 mmol) was added to a round bottom or 4 dram vial with iPr2NEt
(2 equiv.), HCO2H (1 equiv.) and photocatalyst [Ir(ppy)2(dtbbpy)PF6] (1 mol %). These reactants were
diluted in EtOH (5 mL, 0.20 M in starting material), and irradiated by 1x4W Blue LED strip until
reaction completion (6-96 hours). At this point the ethanol was removed in vacuo, and the resulting
oil was diluted in water and extracted with ethyl acetate. The organic portion was washed with 4 N
HCl(aq), saturated sodium bicarbonate solution, brine and finally dried with Na2SO4, after which it was
concentrated to an oil. If the starting material contained acetophenone as the phenacyl fragment, 1 eq.
of PhTMS was added to the oil and the mixture was diluted in CDCl3. This was analyzed via1H NMR
to obtain an accurate acetophenone yield. If the starting material yields an acetophenone derivative
heavier than acetophenone, then the PhTMS standardization step was omitted. After which the crude
reaction was purified by SiO2 chromatography to afford the fragmentation products.
4.2.1 2,3-diphenylbutane-1,2,3,4-tetraol (14c) - 1H NMR (500 MHz, CDCl3) (1.4:1 racemic:meso)
7.38 – 7.28 (m, 3H), 7.26 – 7.16 (m, 3H), 6.99 (bs 1H), 6.98 (bs, 1H), 4.22 (d, J = 11.7 Hz,
13
1H), 3.98 (dd, J = 28.3, 11.8 Hz, 2 H), 3.52 (d, J = 11.8 Hz, 1H). C NMR (126 MHz,
CDCl3) 140.53, 139.45, 128.03, 127.82, 127.71, 127.62, 1227.23, 127.10, 80.25, 79.55,
66.47, 66.32. IR (neat): 3379.4, 2946.3, 2246.0, 1956.1, 1601.2, 1492.5, 1446.3, 1384.2,
1184.8, 1119.3, 1066.0, 1051.7, 953.3, 906.8, 841.5, 761.8, 729.1, 702.4, 645.0, 620.8, 611.2.
HRMS (m/z) = 292.2268
1
4.2.2 1,4-bis(benzyloxy)-2,3-diphenylbutane-2,3-diol (15c) - H NMR (500 MHz, CDCl3) 7.35 –
7.28 (m, 3H), 7.26 – 7.18 (m, 5H), 7.17 – 7.12 (m, 2H), 4.42 (q, J = 11.9 Hz, 2H), 4.05 (d, J =
13
9.7 Hz, 1H), 3.89 (s, 1H), 3.77 (t, J = 12.6 Hz, 1H). C NMR (176 MHz, CDCl3) 141.27,
137.77, 128.48, 127.97, 127.88, 127.38, 127.18, 127.03, 79.30, 74.71, 73.77. IR(neat) =
3548.0, 1495.1, 1446.3, 1403.8, 1359.9, 1301.6, 1252.2, 1208.2, 1127.6, 1101.4, 1066.4,
15

1048.8, 1034.2, 914.6, 818.6, 738.5, 703.1, 616.3, 600.2, 582.1, 558.8, 545.1, 536.3, 528.8,
512.9, 505.8, 501.9, 487.1, 480.3, 46739, 452.7, 445.7. HRMS (m/z) = 477.2036 [M+Na+]
1
4.2.3 N,N'-(2,3-dihydroxy-2,3-diphenylbutane-1,4-diyl)bis(N-phenylacetamide) (20c) - H NMR
(700 MHz, (CD3)2SO) [meso:racemic (1:2)] 7.22 (t, J = 7.1 Hz, 2H), 7.19 (t, J = 7.0 Hz,
2H), 7.17 – 7.13 (m, 2H), 6.88 (t, J = 7.1 Hz, 1H), 6.60 (s, 1H), 6.43 (s, 2H), 4.95 (d, J = 14.5
Hz, 1H), 4.39 – 4.30 (m, 1H), 3.95 (d, J = 14.6 Hz, 2H), 3.82 (d, J = 14.6 Hz, 2H), 1.59 (s,
3H), 1.46 (s, 6H). 13C NMR (126 MHz, (CD3)2SO) 185.51, 185.34, 174.61, 174.36, 174.31,
144.71, 142.07, 141.67, 129.26, 127.90, 127.75, 127.62, 127.37, 127.00, 126.55, 126.21,
87.38, 82.15, 81.14, 58.31, 58.10, 22.98, 22.75. IR (neat): 3224.1 (broad OH), 3054.6,
1624.9, 1591.9, 1494.4, 1443.5, 1397.8, 1354.1, 1298.4, 1241.3, 1176.1, 1116.6,1068.4,
1019.7, 784.0, 741.2, 884.0, 843.8, 765.9, 746.8, 726.1, 695.8, 635.5, 623.9, 608.7, 558.9,
551.9, 526.6, 502.4, 491.2, 484.4, 477.9, 467.4, 449.3, 436.4, 421.3, 411.3. HRMS (m/z) =
509.2435 [M+H+].
1
4.2.4 1-(4-(benzyloxy)-3,5-dimethoxyphenyl)-3-hydroxypropan-1-one (10a): H NMR (500 MHz,
CDCl3): δ 7.47 (d, J = 7.2 Hz, 2H), 7.36-7.29 (m, 3H), 7.21 (s, 2H), 5.12 (s, 2H), 4.04 (t,
J=5.3 Hz, 2H), 3.89 (s, 6H), 3.21 (t, J=5.3 Hz, 2H). 13C NMR (126 MHz, CDCl3) = 199.45,
153.62, 141.98, 137.40, 132.21, 128.57, 128.37, 128.22, 105.75, 75.18, 58.38, 56.47. IR
(neat) 2929.3, 1675.1, 1585.9, 1455.0, 1413.9, 1320.5, 1157.6, 1126.7, 700.9 HRMS (m/z) =
317.1381 [M+1]+ Rf: 0.21 (7:3 EtOAc:Hexanes)
5. Acknowledgements
Financial support from the National Science Foundation (CHE-1440118), the Camille and Henry
Dreyfus Foundation, and the University of Michigan is gratefully acknowledged. We thank Dr.
John D. Nguyen, Dr. Bryan Matsuura, Dr. Märkus Kärkäs, Dr. Verner Lofstrand, Dr. Daryl
Staveness, Joel Beatty and Mitchell Keylor for their helpful discussions.
6. References
[1] For photoredox catalysis reviews: (a) J.M.R. Narayanam, C.R.J. Stephenson, Visible light photoredox
catalysis: applications in organic synthesis, Chem. Soc. Rev. 40 (2010) 102–113. doi:10.1039/B913880N.(b) J.W.
Tucker, C.R.J. Stephenson, Shining Light on Photoredox Catalysis: Theory and Synthetic Applications, J. Org.
Chem. 77 (2012) 1617–1622. doi:10.1021/jo202538x. (c) C.K. Prier, D.A. Rankic, D.W.C. MacMillan, Visible
Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis, Chem. Rev. 113
(2013) 5322–5363. doi:10.1021/cr300503r. (d) Y. Xi, H. Yi, A. Lei, Synthetic applications of photoredox catalysis
with visible light, Org. Biomol. Chem. 11 (2013) 2387–2403. doi:10.1039/C3OB40137E. (e) D.P. Hari, B. König,
Synthetic applications of eosin Y in photoredox catalysis, Chem. Commun. 50 (2014) 6688–6699.
doi:10.1039/C4CC00751D. (f) R.A. Angnes, Z. Li, C.R.D. Correia, G.B. Hammond, Recent synthetic additions to
the visible light photoredox catalysis toolbox, Org. Biomol. Chem. 13 (2015) 9152–9167.
doi:10.1039/C5OB01349F. (g) D.A. Nicewicz, T.M. Nguyen, Recent Applications of Organic Dyes as Photoredox
Catalysts in Organic Synthesis, ACS Catal. 4 (2014) 355–360. doi:10.1021/cs400956a.
16

[2] (a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Zelewsky, Ru(II) polypyridine
complexes: photophysics, photochemistry, eletrochemistry, and chemiluminescence, Coord. Chem. Rev. 84 (1988)
85–277. doi:10.1016/0010-8545(88)80032-8. (b) V. Balzani, S. Campagna A. Meijere, K.N. Houk, H. Kessler, S.V.
Ley, J.-M. Lehn, et al., Photochemistry and Photophysics of Coordination Compounds II, Springer-Verlag Berlin
Heidelberg, 2007.
[3] W.P. Neumann, Tri-n-butyltin Hydride as Reagent in Organic Synthesis, Synthesis. (1987) 665–683.
doi:10.1055/s-1987-28044.
[4] J. Zakzeski, P.C.A. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, The Catalytic Valorization of Lignin for the
Production of Renewable Chemicals, Chem. Rev. 110 (2010) 3552–3599. doi:10.1021/cr900354u.
[5] (a) M. D. Kärkäs , B. S. Matsuura, T. M. Monos, G. Magallanes, C.R.J. Stephenson, Transition-metal
catalyzed valorization of lignin: the key to a sustainable carbon neutral future, Org. Biomol. Chem. (2016) doi:
0.1039/C5OB02212F (b) P.J. Duess, J. Barta, J.G. de Vries, Homogeneous catalysis for the conversion of biomass
and biomass-derived platform chemicals, Catal. Sci. Technol.;4 (2014), 1174-1196.
[6] J.D. Nguyen, B.S. Matsuura, C.R.J. Stephenson, A Photochemical Strategy for Lignin Degradation at Room
Temperature, J. Am. Chem. Soc. 136 (2014) 1218–1221. doi:10.1021/ja4113462.
[7] J.W. Beatty, C.R.J. Stephenson, Amine Functionalization via Oxidative Photoredox Catalysis: Methodology
Development and Complex Molecule Synthesis, Acc. Chem. Res. 48 (2015) 1474–1484.
doi:10.1021/acs.accounts.5b00068.
[8] The original measurement was –2.27 to –2.34 V. Experimental conditions: epoxide [0.005 M] in DMF, [0.1] M
Bu4NClO4, reference electrode 0.1 M Ag/AgI/0.1M I–, hanging mercury drop working electrode. AgI‰Ag + I– = –
0.152 vs. SHE. K. Boujlel, J. Simonet, Cathodic Cleavage of Carbon-Oxygen Bonds (V). Direct and Indirect
Electrochemical Reduction of Epoxides, Electrochimica Acta., 1979, 24, 481–487. doi:10.1016/0013-
4686(79)85020-3.
[9] The original measurement reported was –2.10 V vs. SCE. Experimental conditions: 0.1 M tetraethylammonium
perchlorate supporting electrolyte in acetonitrile at 20 °C. Ag/AgCl (–0.042 V vs SCE) reference electrode, mercury
hanging drop working electrode. (a) F. M. Martens, J.W. Verhoeven, R.A. Gase, U.K. Pandit, Th. H. deB, oer, On
the Question of One-Electron Transfer in the Mechanism of Reduction by NADH-Models, Tetrahedron, 1978, 34, 4,
443-446. (b) Fukuzumi, K. Ishikawa, K. Hironaka, T. Tanaka, Acid catalysis in thermal and photoinduced electron-
transfer reactions, J. Chem. Soc. Perkin Trans. 2. (1987) 751–760. doi:10.1039/P29870000751. (b) see supporting
information for independently measured reduction potentials of substrates.
[10] G.A. Molander, G. Hahn, Lanthanides in organic synthesis. 2. Reduction of α–heterosubstituted ketones, J. Org.
Chem. 51 (1986) 1135–1138. doi:10.1021/jo00357a040. (b) Ered describes the reduction half- potential measured in
THF, and recalculated from Ered= –1.41 V vs. Fe+/Fe in THF: (c)R.J. Enemærke, K. Daasbjerg, T. Skrydstrup, Is
Samarium diiodide an inner- or outer-sphere electron donating agent? Chem. Commun. 1999, 343-344. (d) N.G.
Connelly, W.E. Hahn, Chemical Redox Agents for Organometallic Chemistry, Chem. Rev. 1996, 887-910.
[11]E. Hasegawa, S. Takizawa, T. Seida, A. Yamaguchi, N. Yamaguchi, N. Chiba, et al., Photoinduced electron-
transfer systems consisting of electron-donating pyrenes or anthracenes and benzimidazolines for reductive
transformation of carbonyl compounds, Tetrahedron. 62 (2006) 6581–6588. doi:10.1016/j.tet.2006.03.061. (b) Ered
describes the reduction half-potential based on a direct oxidative quench between photoexcited BDMAP* and a
ketone substrate in acetonitrile.
[12] M.-H. Larraufie, R. Pellet, L. Fensterbank, J.-P. Goddard, E. Lacôte, M. Malacria, et al., Visible-Light-Induced
Photoreductive Generation of Radicals from Epoxides and Aziridines, Angew. Chem. Int. Ed. 50 (2011) 4463–4466.
doi:10.1002/anie.201007571. (b) Ered describes the reduction half-potential from Ru(II) to Ru(III) in acetonitrile.
17

[13] (a) Amatore, C.; Badoz-Lambling, J.; Bonnel-Huyghes, C.; Pinson, J.; Savaént, J.M.; Thiébault, A.; J. Am.
Chem. Soc., 1982, 104, 1979-1986 (b) Andrieux, C.P.; Badoz-Lambling, J.; Combellas, C.; Lacombe, D.; Savaént,
J.M.; Thiébault, A.; Zann, D.; J. Am. Chem. Soc., 1987, 109, 1518-1525 (c) Bordwell, F. G.; Zhang, X.; Alnajjar,
M. S. J. Am. Chem. Soc. 1992, 114 (20), 7623–7629. (d) Alfassi, Z. B.; Golden, D. M. J. Phys. Chem. 1972, 76
(23), 3314–3319. (e) S. Mizuta, S. Verhoog, K.M. Engle, T. Khotavivattana, M. O’Duill, K. Wheelhouse, et al.,
Catalytic Hydrotrifluoromethylation of Unactivated Alkenes, J. Am. Chem. Soc. 135 (2013) 2505–2508. (f) W.J.
Boyle, J.F. Bunnett, Relative reactivities of methanol and methoxide ion as hydrogen atom donors to the p-
nitrophenyl radical, J. Am. Chem. Soc. 96 (1974) 1418–1422.
[14] See supporting information for details.
[15] (a) L.J. Rono, H.G. Yayla, D.Y. Wang, M.F. Armstrong, R.R. Knowles, Enantioselective Photoredo,, x
Catalysis Enabled by Proton-Coupled Electron Transfer: Development of an Asymmetric Aza-Pinacol Cyclization,
J. Am. Chem. Soc. 135 (2013) 17735–17738. doi:10.1021/ja4100595. (b) K.T. Tarantino, P. Liu, R.R. Knowles,
Catalytic Ketyl-Olefin Cyclizations Enabled by Proton-Coupled Electron Transfer, J. Am. Chem. Soc. 135 (2013)
10022–10025. doi:10.1021/ja404342j.
[16] M. Nakajima, E. Fava, S. Loescher, Z. Jiang, M. Rueping, Photoredox-Catalyzed Reductive Coupling of
Aldehydes, Ketones, and Imines with Visible Light, Angew. Chem. Int. Ed. 54 (2015) 8828–8832.
doi:10.1002/anie.201501556.
18