Bifunctional copper-based photocatalyst for reductive pinacol-type couplings

Article abstract of DOI:10.1021/acscatal.9b01718

A bifunctional copper-based photocatalyst has been prepared that employs a pyrazole-pyridine ligand incorporating a sulfonamide moiety that functions as an intramolecular hydrogen-bond donor for a photochemical PCET process. In typical reductive PCET processes, the photocatalyst and H-bond donor must have an appropriate redox potential and pK_a, respectively, to promote the PCET. When working in concert in a bifunctional catalyst such as Cu(pypzs)(BINAP)BF₄, the pK_a of the H-bond donor can have an acidity that is orders of magnitude less and still efficiently promote the PCET process. A reductive pinacol-type coupling can be performed using a base-metal derived photocatalyst to afford valuable diols (24 examples, 46-99% yield), from readily available aldehydes and ketones.

Full text of DOI:10.1021/acscatal.9b01718

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Article
Bifunctional Copper-Based Photocatalyst
for Reductive Pinacol-Type Couplings
Antoine Caron, Émilie Morin, and Shawn K. Collins
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01718 • Publication Date (Web): 11 Sep 2019
Downloaded from pubs.acs.org on September 11, 2019
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Page 1 of 7
ACS Catalysis
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Bifunctional Copper-Based Photocatalyst for Reductive Pinacol-
Type Couplings.
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Antoine Caron, Émilie Morin and Shawn K. Collins*
Département de Chimie, Centre for Green Chemistry and Catalysis, Université de Montréal, CP 6128 Station
Downtown, Montréal, Québec H3C 3J7,Canada
ABSTRACT: A bifunctional copper-based photocatalyst has been prepared that employs a pyrazole-pyridine ligand
incorporating a sulfonamide moiety that functions as an intramolecular hydrogen-bond donor for a photochemical PCET
process. In typical reductive PCET processes, the photocatalyst and H-bond donor must have an appropriate redox potential
and pKa respectively to promote the PCET. When working in concert in a bifunctional catalyst such as
Cu(pypzs)(BINAP)BF4, the pKa of the H-bond donor can have an acidity that is orders of magnitude less and still efficiently
promote the PCET process. A reductive pinacol-type coupling can be performed using a base-metal derived photocatalyst
to afford valuable diols (24 examples, 46-99 % yield), from readily available aldehydes and ketones.
Photocatalysis can impact sustainable chemistry through promoting
unique reactivity and achieving novel transformations, under
relatively mild conditions.1 Consequently, much interest has been
placed on synthesis employing visible-light and inexpensive
photocatalysts at low catalyst loadings. In the last half-dozen years,
copper-based photocatalysis has attracted considerable attention. Cu-
based complexes are generally easily prepared, can promote
photochemistry through outer- and inner-sphere mechanisms, can
exhibit long excited state lifetimes and are excellent excited-state
reductants.2 As such, they are attractive photocatalysts for a variety of
ketones, generating neutral ketyl radicals. Herein we report on the
synthesis and evaluation of a bifunctional copper-based photocatalyst
in reductive pinacol-type PCET processes.
Past Work:
HA
Concerted transfer of a proton from an external acid (
)
PC
and an electron from a seperate photocatalyst (
R
)
R
N
R
N
R
PC*
R
R
PC
R
transformations,
including proton-coupled electron transfers
R
R
N
(PCETs). PCETs are non-classical redox processes in which a proton
R2
PC
PC
and electron are exchanged in a concerted manner, and can proceed
R2
,
PC*
R2
PC
A
R1
O
via both oxidative and reductive manifolds. In the reductive homolytic
activation of ketones, some photocatalysts (PCs) may not possess an
excited state capable of promoting the PCET process. As such, the PC
in its excited state abstracts an electron from an electron donor
(reductive quenching), such as a tertiary amine, and it is the ground
state radical anion that participates in the PCET process (Figure 1, top
left). In some instances, it has been proposed that the radical cation
of the tertiary amine can act to promote the single-electron transfer
(SET) from the PC radical anion via hydrogen bonding, or 2-
electron/3-center bonding.7 If the excited state of a photocatalyst has
a sufficiently high excited state potential, it will donate an electron
directly via the excited state to a carbonyl (oxidative quenching) that
is simultaneously activated via hydrogen-bonding, typically achieved
with an added acid in the reaction mixture (Figure 1, top right). The
PCET process generates a ketyl radical for further chemistry, and the
PC is regenerated via SET with an electron-doner, again typically a
tertiary amine. Considering the aforementioned mechanisms, it was
proposed that connecting both the PC and the acid activator within
the same catalyst structure could improve PCET processes. Through
proximity, the rate of PCET could be increased, improving yields.
Association of the ketone (or aldehyde) with the bifunctional catalyst
could result in the steric environment influencing the stereoselectivity
of the resulting chemistry of the ketyl radical. Preferably, designing a
bifunctional catalyst would take place from PCs that have already
demonstrated ease of synthesis, tunability, and application in PCET
processes. Consequently, heteroleptic copper-based complexes were
selected as ideal candidates; our group has previously evaluated
libraries of heteroleptic copper-based complexes having one diamine
and one bisphosphine ligand for photocatalysis, and reported their
activity in a PCET process involving the homolytic activation of
R1
O
H
R1
O
H
N
H
A
R
N
R
R
R
R
R
This Work: Bifunctional Catalysis for PCET
Design of a bifunctional Cu-based photocatalyst (
PC+
a
)
R2P
N
Cu
N
R2P
N
H
Design principles:
Nature of diamine and bisphosphine to control photophysical properties
Where to install H-bonding sites, how to modulate pKa
b
Concerted transfer of a proton and an electron from the
PC+
same photocatalyst (
)
R2
O
R1
PC
H
R2
R2
HEH
HEH
R1
O
H
O
R1
+
*
H
PC+
PC
H
H
Application:
Photochemical Pinacol-type coupling of aldehydes and ketones
Figure 1. Bifunctional catalysis for photocatalytic PCET
reactions.
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To investigate the possibility of using a bifunctional catalyst for
acid (dppa) as an external acid and Hantzsch ester (HEH) as a
hydrogen-atom donor, the catalyst 2 afforded the corresponding diol
1 in 90 % yield (Table 1). However, when the reaction was repeated in
the absence of dppa, no trace of the desired diol was observed. As
such, it was proposed to exchange the dq ligand for diamines
containing hydrogen bond donors. The ligands pypz and pypza were
identified as they had been previously utilized in asymmetric
photocatalysis.12 In addition to being readily available in two
synthetic steps, the pypza ligand was especially attractive, as
diversification of the NH2 group could be envisioned for further fine-
tuning. As such, the catalysts Cu(pypz)(BINAP)BF4(3) and
Cu(pypza)(BINAP)BF4 (4) were prepared in hopes that the new
ligands would participate in hydrogen bonding. Each catalyst was
capable of promoting the PCET reaction of acetophenone in the
presence of dppa (78 and 95 % of 1 with catalysts 3 and 4 respectively).
However, only low yields (14-25 %) of the desired diol 1 were observed
in the absence of dppa. As such, the pypza ligand was further modified
through sulfonylation with p-fluorosulfonyl chloride to afford the
pypzs ligand, in hopes that the sulfonamide moiety would help
photocatalytic PCET reactions, the pinacol-type reductive coupling of
ketones was selected for study.1 The photochemical pinacol-type
coupling has been previously reported by Rueping and co-workers
using an Ir-based catalyst system,7 while some organic dyes have also
been show to effective photocatalysts.11a The pinacol process provides
access to valuable diol products and alternative synthetic routes call
for excess amounts of reducing metals and trialkylsilane-based
protection strategies to prevent inhibition with the metal-based
reagents.11
1
2
3
4
5
6
7
8
9
Table
1.
Pinacol-Type
Coupling
Employing
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18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
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54
55
56
57
58
59
60
Bifunctional Cu-Based Photocatalysts.
catalyst (2 mol %)
additive (5 mol %)
HO
Me
Ph
Ph
Me
OH
HEH (2 equiv)
O
Ph
Me
THF [50 mM]
1
augment
H-bonding.
Gratifyingly,
Cu(pypzs)(BINAP)BF4(5)
Blue LEDs, 18 h
promoted the reductive pinacol coupling of acetophenone both with
dppa (86 % 1) and without dppa (76 % 1). No PCET-type reactivity was
observed for Cu(pypzs)(BINAP)BF4 in the absence of light. Attempts
at replacing HEH with an alternative H-atom dodnor like Et3SiH
provided no trace of diol 1. Also, the PCET pinacol-type processes
employed blue LEDs for irradiation. The reductive pinacol coupling of
acetophenone was also attempted with Cu(pypzs)(BINAP)BF4 under
purple LEDs, but identical yields of 1 (76 %) were obtained. No
background pinacol coupling of acetophenone was observed under
purple LEDs in the absence of catalyst. Interestingly, attempting to
promote the PCET process to form 1 using Cu(dq)(BINAP)BF4 with
added pypzs to replace dppa did not afford any product. In comparing
Ph2
P
Ph2
NH
N
N
P
N
N
Cu
Cu
P
P
Ph2
Ph2
BF4
BF4
3
Cu(pypz)(BINAP)BF4
2
Cu(dq)(BINAP)BF4
O
S
p-FC6H4
NH
NH
N
H2N
O
Ph2
P
Ph2
NH
P
N
Cu
Cu
N
P
1
N
P
the relative acidities, the pKa of dppa ( 3.72, DMSO) is orders of
Ph2
Ph2
1
magnitude lower than either
a
pyrazole N-H
(
14.2)
or a
BF4
Cu(pypzs)(BINAP)BF4
BF4
1
sulfonamide (PhSO2NH2 16.1, DMSO) moiety present in the pypzs
ligand. As such, the success of the bifunctional catalyst
Cu(pypzs)(BINAP)BF4 is likely a result of the proximity of the
hydrogen-bond donor and the photoactive complex.
5
4
Cu(pypza)(BINAP)BF4
Catalyst
Additive
Yield 1
Next, the scope of the pinacol-type couplings were explored using
both ketones and aldehydes (Table 2). Both phenylmethylketone and
(4-phenyl)phenylmethylketone underwent coupling to afford the
corresponding diols 1 and 6 in 76 and 99 % yields. A variety of
halogen-substituted methylketones all were homocoupled in good to
excellent yields. Substrates substituted with 3-bromo-, 4-bromo-, 3-
bromo-4-fluoro- and 2-chloro- all afforded the corresponding diols in
good to excellent yields (7010, 74096 %). A variety of other
functional groups were compatible. Pinacol-type coupling with a
phenylmethylketone having a 3-pyridyl substituent afforded the
corresponding diol 11 in 58 % yield. Diol 12 was isolated in 46 % having
pendant methyl esters substituents. Other substituted aryl ketones
that were not compatible under previously reported conditions using
Ir-based catalysts (Ir(dfCF3ppy)2(bpy)PF6 (1 mol %), NBu3 (3 equiv),
DMF, Blue LEDs, 18 h), were viable coupling partners when using
bifunctional catalyst 5. The diol 13 formed via pinacol coupling of 4-
SMe substituted methylketone was isolated in 80% yield, was only
isolated in 15 % yield under Ir-catalysis. In addition, diol 14, adorned
with pinacolborate esters that could be used as handles for further
functionalization of the diols, was formed in 75 % yield using Cu-
catalyst 5, but was isolated in less than 10% yield when using the Ir-
based conditions. Diarylketones also underwent pinacol-type
coupling in good yields, the p-tolyl substituted diol 15 was obtained in
58 % yield, while the diol 16 adorned with thiophene and p-tolyl
groups was formed in 62 % yield.
(%)a
1
2
Cu(dq)(BINAP)BF4
Cu(dq)(BINAP)BF4
dppa
90
0
-
3
Cu(pypz)(BINAP)BF4
Cu(pypz)(BINAP)BF4
Cu(pypza)(BINAP)BF4
Cu(pypza)(BINAP)BF4
Cu(pypzs)(BINAP)BF4
Cu(pypzs)(BINAP)BF4
Cu(pypzs)(BINAP)BF4
Cu(dq)(BINAP)BF4
Cu(pypzs)(BINAP)BF4
Cu(pypzs)(BINAP)BF4
-
dppa
78
25
95
14
86
76
0
4
-
5
dppa
6
-
7
dppa
8
-
9b
10
11c
12d
13e
-
ppyzs
0
-
-
-
0
76
0
a Isolated yields. 1:1 ratio of diastereomers observed in all cases. b
Using Et3SiH instead of HEH. No light. d Using purple LEDs (394
c
nm). e Using purple LEDs (394 nm) with no catalyst. BINAP: (rac)-
2,2′-bis(diphenylphosphino)-1,1′-binaphthalene. pypz: 3-(2-
pyridyl)-pyrazole. pypza 5-amino-3-(2-pyridyl)-pyrazole. pypzs: 5-
(4-fluorosulfonyl)amino-3-(2-pyridyl)-pyrazole.
A number of aldehydes also underwent pinacol-type coupling with
the bifunctional copper-based catalyst 5 (Table 2). Diols formed from
the homocoupling of benzaldehyde (17, 99 %), 2-chlorobenzaldehyde
(18, 93 %), 4-bromobenzaldehyde (19, 99 %), 3-bromobenzaldehyde
(20, 99 %), 4-methoxybenzaldehyde (21, 66 %), 2-
hydroxybenzaldehyde (22, 74 %) and 4-ethylbenzaldehyde (23, 85 %)
The catalyst Cu(dq)(BINAP)BF4 2 had been identified as a promising
candidate for PCET processes.9 Indeed, in the reaction of
acetophenone with Cu(dq)(BINAP)BF4 using diphenyl phosphoric
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ACS Catalysis
could all be isolated. Polycyclic derivatives were also compatible as 1-
naphthyl aldehyde afforded the corresponding diol 24 in 57% yield.
1
2
3
4
Table 2. Scope of the Reductive Pinacol-Type Coupling Employing a Bifunctional Cu-Based Catalysta
5
6
7
O
p-FC6H4
S
NH
O
Ph2
P
NH
N
Cu
8
9
N
P
Ph2
BF4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(2 mol %)
HEH (2 equiv)
HO
R1
R2
R1
OH
O
R1 R2
R2
THF [50 mM]
Blue LEDs, 18 h
a
Ketones
Ph
Br
Br
Br
F
Cl
HO
Me
HO
Me
HO
Me
HO
Me
HO
Me
HO
Me
Me
OH
Me
OH
Me
OH
Me
OH
Cl
Me
OH
Me
OH
Ph
Br
Br
F
Br
1
6
7
8
9
10
, 89 %
, 76 %
, 99 %
, 96 %
, 90 %
, 74 %
N
Me
CO2Me
Bpin
Me
S
SMe
HO
Me
HO
HO
Me
HO
Me
HO
HO
Me
Me
OH
Me
Me
Me
OH
Me
OH
Me
OH
OH
S
OH
Me
MeO2C
Bpin
Me
MeS
N
b
c
11
, 58 %
13
14
15
16
, 62 %
, 80 %
, 75 %
, 58 %
12
, 46 %
Ir(dfCF3ppy)2(bpy)PF6 (1 mol %), NBu3 (3 equiv), DMF,
13
Blue LEDs , 18 h, 15 % of , < 10 % of
14
b
Aldehydes
Br
Br
OMe
Cl
HO
H
HO
H
HO
H
HO
H
HO
H
HO
H
H
OH
Cl
H
OH
H
OH
HO
H
OH
H
OH
H
OH
OH
Br
Br
20
MeO
17
18
19
21
22
, 74 %
, 99 %
, 93 %
, 99 %
, 99 %
, 66 %
1:1.5 cis:trans
Me
N
Me
HO
H
N
S
S
HO
H
HO
H
HO
H
HO
H
HO
H
H
OH
H
OH
H
OH
H
OH
H
H
OH
OH
S
S
N
Me
N
Me
23
, 85 %
24
25
27
, 57 %
, 82 %
, 54 %
1:0.8 cis:trans b
26
, 68 %
28
, 82 %
aYields following chromatography. The cis:trans ratios are 1:1 unless indicated otherwise. b Isolated as a single isomer. c Using 4 mol % of
catalyst .
Heterocyclic derivatives also functioned well under the reaction
conditions: diols derived from pyridine (25), quinolone (26),
thiophene (27) and benzothiophene (28) were isolated in 54-82 %
yield.
When considering a possible mechanism for the pinacol-type
coupling employing the copper-based bifunctional catalyst, the failure
of using an alternative H-atom donor such as Et3SiH was suprising.
When investigating lifetime emission quenching studies, it was shown
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Page 4 of 7
that it was not acetophenone, but HEH that quenched the excited
enough to accept an electron from HEH in the ground state (3 (pypz)
-0.18 V; 4 (pypza) -0.43 V; 5 (pypzs) -0.18 V). If a suitable electron
donor is necessary in the PCET process, then it should be possible to
use Et3SiH as an H-atom donor if another electron donor, such as a
tertiary amine, is added. Indeed, when the pinacol-type coupling was
performed using acetophenone, catalyst 5 and both Et3SiH and
iPr2NEt, the PCET process was once again productive (37 % of diol 1,
Figure 2b). In the proposed mechanism, intermediate A could then
engage in hydrogen bonding with the sulfonamide proton, pyrazole
hydrogen or combination thereof resulting in activation of the ketone
state of the catalyst. As such, upon excitation of copper-based catalyst
5, electron transfer with HEH is proposed (Figure 2a). Evano and co-
workers have similarly proposed such SET transfers to Cu-based
catalysts in the presence of suitable electron donors.1 Note that
1
2
3
4
5
6
7
8
HEH can be easily oxidized (E1/2(HEH/HEH+.)= 0.404 V) and if there are
17
traces of oxygen present, HEH is also rapidly converted to HEHO
.
In addition, all of the new catalysts (PC+) possess excited state
potentials (E1/2(PC */PC)) capable of accepting an electron from HEH; (3
+
(pypz) 2.12 V; 4 (pypza) 1.87 V; 5 (pypzs) 1.86 V). Note that although
all the new catalysts can theoretically accept an electron from HEH in
the excited state, their potentials in the resulting ground state (E1/2(PC/
PC+)) will not be sufficient to promote electron transfer to acetphenone
1
or aldehyde for proton-coupled electron transfer. A H NMR study
9
(Figure 2c) of the catalyst Cu(pypzs)(BINAP)BF4 in the presence of
acetophenone in THF-d8 indicated that both the pyrazole N-H and
sulfonamide N-H signals (Figure 2c, Ha and Hb respectively)1
underwent desheilding ( Ha = 0.073-0.193 ppm,  Hb = 0.086-
0.201 ppm), suggesting hydrogen bonding is occurring.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(E -2.48 V vs. Fc).1 Indeed, a sufficiently strong proton-donor is
½=
required to undergo activation through hydrogen bonding. Note that
none of the photocatalysts possess a potential (E1/2(PC+/PC)) high
Me
HN
Me
CO2Et
H
a
Proposed Mechanism
b
Effects of Electron and H-Atom Donors
O
p-FC6H4
CO2Et
Me
S
NH
CO2Et
H
O
Ph2
P
HEH
NH
N
N
Cu
HN
Me
HEH+
+1
P
BF4
O
Ph2
p-FC6H4
S
CO2Et
NH
BF4
O
Ph2
P
NH
N
N
HO
Me
Ph
Ph
Me
OH
(2 mol %)
O
EtO2C
EtO2C
Me
N
Cu
P
Ph
Me
R2
THF [50 mM]
Ph2
1
Blue LEDs, 18 h
5
R1
O
Me
R2
HEH[O]
76 %
with HEH (2 equiv)
37 %
with Et3SiH (2 equiv)
and iPr2NEt (2 equiv)
O
p-FC6H4
b
H
0 %
with Et3SiH (2 equiv)
R1
Ph2
P
S
O
Me
HN
Me
CO2Et
N
O
a
H
N
N
H
Cu
R2
N
P
d
Photophysical Data
(All Potentials vs. SCE)
Ph2
O
R2
CO Et
2
R1
O
R2
A
R1
R1
HEH+
R2
OH
Ph2
+1
Ph2
P
O
NH
N
N
p-FC6H4
H
P
N
C
S
O
O
H
R1
Ph2
P
R2
Cu
Cu
N
N
P
P
N
N
N
Ph2
Ph2
R2
O
R1
HO
R1
R2
1H NMR Data
Cu
R1
OH
BF4
BF4
P
BF4
2
3
Ph2

abs = 472 nm

abs = 373 nm
B
c

em = 520 nm

em = 539 nm
(32)
 = 4.00 ns
 = 7.42 ns
E1/2 (PC+/PC+2)= 0.51 V
E1/2 (PC+*/PC+2) = -1.87 V
E1/2 (PC+/PC+2)= 1.07 V
E1/2 (PC+/PC)= -0.18 V
E1/2 (PC+*/PC+2) = -1.23 V
E1/2 (PC+*/PC) = 2.12 V
(16)
(8)
O
S
p-FC6H4
BF4
H2N
BF4
NH
NH
Ph2
NH
O
Ph2
P
(4)
(2)
P
P
N
N
N
N
Cu
Cu
P
Ph2
Ph2
4
5
(1)

abs = 381 nm

abs = 374 nm

em = 540 nm

em = 620 nm
Ha
Hb
 = 7.46 ns
 = 8.95 ns
E1/2 (PC+/PC+2)= 0.81 V
E1/2 (PC+/PC)= -0.43 V
E1/2 (PC+*/PC+2) = -1.48 V
E1/2 (PC+/PC+2)= 1.11 V
E1/2 (PC+/PC)= -0.04 V
E1/2 (PC+*/PC+2) = -0.89 V
E1/2 (PC+*/PC) = 1.96 V
E1/2 (PC+*/PC) = 1.87 V
Chemical Shift ( )

Figure 2. a. Proposed reaction mechanism. b. Effects of electron and hydrogen atom donors. c. Selected 1H NMR data (THF-d4) of
catalyst 5 (in red) in the presence of increasing numbers of equivalents of acetophenone (indicated in paratheses). d. Comparison
of photophysical properties of four copper-based photocatalysts.
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Furthermore, the PCET pinacol process afforded much lower yields in
an acidity that is orders of magnitude less and still efficiently promote
the PCET process. The modularity and ease of synthesis of the copper
catalysts is noteworthy, and further modification of the steric
environment of the diamine, alteration of the hydrogen-bond donors
and substitution of the bisphosphine could all lead to new catalysts
capable of further influencing the stereoselectivities of PCET
processes. Given that H-bonding ligands are being used increasingly
in asymmetric transition metal catalysis, there could be used for
future tuning and development of asymmetric processes.
protic or polar solvents which could interfere with hydrogen-bonding
(acetophenone→1, in EtOH 11 %; in DMF 30 %).2 Following the
PCET event, the resulting ketyl radical can react with another
equivalent of the ketone or aldehyde to afford the pinacol-type
intermediate C. The desired diols are formed via H-atom transfer
(HAT) from HEH+ (or HEH). The radical cation of HEH also serves to
regenerate the copper-based catalyst via proton transfer. The nature
of the pypzs ligand present in the bifunctional catalyst 5 impacts the
photophysical properties, and results in differences with regards to
other copper-based catalysts such as Cu(dq)(BINAP)BF4, used in a
previous PCET process (Figure 2d). For example the extended -
surface of the dq ligands results in excellent absorbance for
Cu(dq)(BINAP)BF4 at 472 nm. In contrast, Cu(pypzs)(BINAP)BF4, as
well as the two other bifunctional catalysts Cu(pypz)(BINAP)BF4 and
Cu(pypza)(BINAP)BF4 exhibit maximum absorbances in a narrow
range at 373-381 nm, and only weakly absorb within the blue range. In
fact, it is surprising that Cu(pypzs)(BINAP)BF4 is so efficient when
irradiated with blue LEDs given its low absorption. All copper
catalysts have similar emission spectra with max between 520-540 nm,
with the exception of Cu(pypzs)(BINAP)BF4 which has a weak
emission at 620 nm. Interestingly, all catalysts having a N-H bond
donor have excited-state lifetimes longer than Cu(dq)(BINAP)BF4 (
= 4.00 ns), with the pypzs ligand augmenting the excited-state
lifetime of its respective catalyst the furthest ( = 8.95 ns) in
comparison to the other catalysts evaluated (Cu(pypz)(BINAP)BF4 =
7.42 ns; Cu(pypza)(BINAP)BF4 = 7.46 ns). The data supports that
bifunctional catalyst 5 acts as a photocatalyst for reductive PCET
reactions through its sufficiently strong hydrogen-bond donor.
In summary, a bifunctional copper-based photocatalyst has been
prepared that employs a designed pyrazole-pyridine based ligand
adorned with sulfonamide moiety that functions as an intramolecular
hydrogen-bond donor for a photochemical PCET process. The
catalysis is the first application of base-metal photocatalysts toward
pinacol-type couplings. The diols prepared herein are formed from a
variety of aldehydes and ketones, and in several cases the bifunctional
catalyst was capable of promoting pinacol-coupling of substrates that
were low yielding when using previously Ir-based catalyst systems. In
reductive PCET processes, the PC and H-bond donor must have an
appropriate redox potential and pKa respectively to promote the
PCET. Importantly, when working in concert in a bifunctional catalyst
such as Cu(pypzs)(BINAP)BF4, the pKa of the H-bond donor can have
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AUTHOR INFORMATION
Corresponding Author
shawn.collins@umontreal.ca
ACKNOWLEDGMENT
The authors acknowledge the Natural Sciences and
Engineering Research Council of Canada (NSERC, Discovery
1043344), Université de Montréal and the Fonds de recherche
Nature et technologie via the Centre in Green Chemistry and
Catalysis (FRQNT-2020-RS4-265155-CCVC) for generous
funding. The authors thank Prof. D. Rochefort and Mr. Simon
Généreux for help obtained CV data, Mr. D. Chartrand and
Ms. C. Minozzi for help obtaining excited state lifetime data.
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TOC GRAPHIC
O
p-FC6H4
S
bifunctional catalyst (2 mol %)
HEH (2 equiv)
Ar
NH
NH
O
O
Ph2
P
HO
R
N
N
R
OH
R
Cu
Ar
P
THF [50 mM]
Ph2
Ar
Blue LEDs, 18 h
BF4
Bifunctional catalyst enables pinacol-type coupling via photochemical PCET
(1 ) The pyrazole N-H signal is observed further downfield in the 1H NMR spectra for both catalyst 4 ( = 11.2 ppm, (CD3)2CO) and catalyst
5 ( = 12.1 ppm, (CD3)2CO).
(2 ) For additional detail, see Supporting Information.
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