Independence from the Sequence of Single-Electron Transfer of Photoredox Process in Redox-Neutral Asymmetric Bond-Forming Reaction

Article abstract of DOI:10.1021/acs.joc.6b00445

A catalytic cycle initiated by the oxidative quenching of the excited photosensitizer (Ir?(ppy)₃) is established for the enantioselective coupling between (N-arylamino)methanes and (N-methanesulfonyl)aldimines catalyzed by Ir-based photosensitizer and a chiral (arylamino)phosphonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate under visible light irradiation. This achievement clearly demonstrates the insensitivity of this redox-neutral asymmetric reaction to the sequence of the key redox events involved in the synergistic catalysis.

Full text of DOI:10.1021/acs.joc.6b00445

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Brief Communication
Independence from the Sequence of Single-Electron Transfer of
Photoredox Process in Redox-Neutral Asymmetric Bond-Forming Reaction
Tomohito Kizu, Daisuke Uraguchi, and Takashi Ooi
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b00445 • Publication Date (Web): 13 May 2016
Downloaded from http://pubs.acs.org on May 14, 2016
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Independence from the Sequence of Single-Electron Transfer of Photoredox Process in Redox-
Neutral Asymmetric Bond-Forming Reaction
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Tomohito Kizu1, Daisuke Uraguchi1, Takashi Ooi*2
Address: 1. Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8602, Japan.
2. Institute of Transformative Bio-Molecules (WPI-ITbM); Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University; CREST, Japan Science and Technology Agency (JST), Nagoya 464-8602, Japan.
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
*E-mail: tooi@apchem.nagoya-u.ac.jp
Notes
The authors declare no competing financial interest.
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Abstract
A catalytic cycle initiated by the oxidative quenching of the excited photosensitizer (Ir*(ppy)3) is established for the
enantioselective coupling between N-arylaminomethanes and N-methanesulfonyl (Ms) aldimines catalyzed by Ir-
based photosensitizer and a chiral arylaminophosphonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate under
visible light irradiation. This achievement clearly demonstrates the insensitivity of this redox-neutral asymmetric
reaction to the sequence of the key redox events involved in the synergistic catalysis.
The photoredox activation of organic molecules with a catalytic amount of photosensitizers under visible
light irradiation has recently been emerging as a powerful strategy for generating reactive radical species under very
mild conditions.1,2 The distinctive feature of the photoredox catalysis is that it could facilitate unique bond cleavage
and formation reactions, which are not possible within conventional methodological frameworks. The catalytic
process is generally initiated by a single-electron transfer (SET) between a photo-excited sensitizer and an electron-
donor/acceptor substrate to afford the corresponding transient redox-active intermediate and an ion-radical species.
This intermediate generated from the sensitizer subsequently experiences another SET event, regenerating the parent,
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ground-state photosensitizer. By considering the number of incoming or outgoing electrons during the overall
transformation, the photoredox process can be categorized as a net-oxidative, net-reductive, or redox-neutral system.3-
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Among these, the redox-neutral reaction, wherein both oxidants and reductants to the sensitizer are incorporated
into a desired product without any unproductive wastes, is particularly intriguing.5 Another important characteristic
of this system is its insensitivity to the sequence of redox events. Namely, the photo-catalyst can engage in SET
processes with both electron-donors and acceptors at the appropriate points in the catalytic cycle because both exist
in the same reaction vessel. Therefore, in principle, an optimal sensitizer could be identified for either oxidative or
reductive initiation of the redox-neutral reactions. However, little attention has been paid to constructing pairs of
complementary, productive catalytic systems, although such an endeavor may provide meaningful insights into the
mechanism of these redox-neutral reactions.6
Recently, we have developed an enantioselective redox-neutral α-coupling of N-arylaminomethanes with
N-sulfonyl aldimines by utilizing the synergistic catalysis of an Ir-pyridylphenyl complex and chiral ionic Brønsted
acid 1·HBArF (Fig. 1),7-10 which is initiated by the reductive quenching of the visible-light-exited IrIII-complex with
the aminomethanes.11-13 On the basis of the successful establishment of the reductive quenching system for this
redox-neutral coupling reaction, we were interested in the possibility of developing a complementary oxidative
quenching system with a view of deepening the knowledge of the photoredox process. Herein, we describe the
details of our study on closing the loop of a redox cycle initiated by SET from an exited Ir-pyridylphenyl complex to
the imine.
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Figure 1. P-Spiro Chiral Arylaminophosphonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borates 1·HBArF.
BArF = [3,5-(CF3)2C6H3]4B
Figure 2. Working Hypothesis for the Redox-Neutral Coupling Reaction via Oxidative Quenching Process.
At the outset, we designed a new catalytic cycle starting with SET from an excited photosensitizer to N-
sulfonyl imine 3 (Fig. 2). This oxidative quenching system requires an appropriate photosensitizer with an
extremely high exited state oxidation potential (step a); this is necessitated by the relatively high reduction potential
of N-Ms benzaldimine (3a) (Ered = –1.45 V vs SCE). Tris(2-(2-pyridyl)phenyl)iridium [Ir(ppy)3 (2a)] was found to
have a sufficiently high reducing ability in its excited state (Eox of Ir*(ppy)3 = –1.73 V).14 However, an initial trial
in the reaction of 3a with Ph2NMe (4a) under the influence of 1·HBArF in toluene at ambient temperature showed
that 2a was not suitable for our purposes (Table 1, entry 1). Careful re-investigation of the designed redox cycle
revealed a mismatch of redox potential at the amine oxidation stage (step b), where the reduction potential of
IrIV(ppy)3 (Ered = +0.77 V) is slightly lower than that of Ph2NMe (4a) (Eox = +0.86 V).15 Since the reduction
+
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potential of these Ir-complexes is known to be increased by introducing electron-withdrawing groups to the phenyl
moiety of the ppy ligand, the iridium complex bearing 5-fluoro-2-(2-pyridyl)phenyl (Fppy) groups was used as a
sensitizer.16 In the presence of Ir(Fppy)3 (2b) (Ered = +0.97 V), the coupling took place as expected, albeit with low
conversion (36% yield determined by 1H NMR with mesitylene as an internal standard), to give the diamine product
5a in 24% isolated yield with 93% ee (entry 2). This unsatisfactory conversion is presumably because the back
electron transfer from the anion radical of the imine to the transient IrIV complex (dashed arrow in Fig. 2) or the
cation-radical of the amine competes with the desired downstream steps. With this in mind, we attempted the
reaction with deuterated aminomethane 4a-d3 to assess any possible rate difference stemming from the kinetic isotope
effect (KIE) for assigning the rate-determining step.17 After 24 h of irradiation, 1H NMR analysis of a crude aliquot
indicated that the corresponding 5a-d2 was formed in only 13% yield (KIE = 2.8:1) (eq 1). Furthermore, subjecting
an equimolar mixture of 4a and 4a-d3 to the standard conditions resulted in the formation of 5a and 5a-d2 in a ratio
of 3.4:1 (eq 2). These experiments suggest that the deprotonation of the cation-radical of the amine 4a is the rate-
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determining step. In other words, if we could accelerate the generation of the aminomethyl radical, the efficiency
of the entire catalytic cycle could be improved. This hypothesis prompted us to employ Ph2NCH2SiMe3 (4b)13,18
instead of 4a, expecting rapid generation of the carbon-radical via spontaneous release of a trimethylsilylium ion
from the initially generated cation-radical of 4b. Indeed, the conversion of the starting substrates was markedly
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improved by using 4b as the amine component under otherwise similar conditions, but the chemical yield of the
product 5a was decreased with concomitant production of various byproducts (entry 3). After considering the
oxidation potential of 4b (Eox = +0.60 V), we applied 2a to the reaction with 4b, which delivered a considerable
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increase in the isolated yield of 5a, probably due to the attenuated oxidation ability of the transient IrIV(ppy)3 (entry
+
4). Then, we pursued further optimization of the reaction parameters (entries 5-8). In regard to the stoichiometry
of the coupling partners, using 4b in slight excess enhanced the chemical yield, although the addition of two
equivalents of 3 was beneficial in our previous study.7 This observation corroborates the importance of rapid
generation of the aminomethyl radical for overall reaction efficiency (entry 5). It should be noted that the desired
process was consistently accompanied by the formation of a pinacol-type coupling product 6 from the anion-radical
of 3a, inevitably reducing the yield of 5a. Evaluation of the solvent effect in terms of reactivity and selectivity
revealed that ethereal solvents were effective and cyclopentyl methyl ether (CPME) in particular was found to be
optimal (entries 6 and 7).19 Finally, the loading of sensitizer 2a, which was not completely soluble in CPME under
the reaction conditions, was reduced with a slight change in the ratio to 1·HBArF to ensure maximum efficiency of
excitation under visible light irradiation (entry 8).
Table 1. Optimization of the Reaction Conditionsa
1
·HBArF (X mol%)
Ms
H
Ms
Ph
N
N
HN
Ph
2
Ir(L)3 (Y mol%)
NPh2
+
Ph
Ph
visible light
rt, 24 h
G
3a
4
5a
yield
(%)c
0
ee
(%)d
–
entry
L (2)b
G (4)
3a:4
X,Y
solvent
1
2
3
ppy (2a)
H (4a)
H (4a)
2:1
2:1
2:1
4, 1
4, 1
4, 1
toluene
toluene
toluene
Fppy (2b)
Fppy (2b)
24
93
Me3Si (4b)
15
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ppy (2a)
ppy (2a)
ppy (2a)
ppy (2a)
ppy (2a)
Me3Si (4b)
Me3Si (4b)
Me3Si (4b)
Me3Si (4b)
Me3Si (4b)
2:1
4, 1
4, 1
toluene
toluene
Et2O
45
71
79
79
80
86
91
92
93
93
1:1.5
1:1.5
1:1.5
1:1.5
4, 1
4, 1
CPMEe
CPMEe
7
3, 0.5
8
a The reactions were performed with 0.10 mmol of a yield determining reagent (3a or 4b) with
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1·HBArF and 2 at ambient temperature for 24 h under argon atmosphere with visible light
b
irradiation (15
pyridyl)phenyl.
W
c
white LED).
ppy
=
2-(2-pyridyl)phenyl, Fppy
= 5-fluoro-2-(2-
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d
Isolated yield.
Enantiomeric excesses were determined by chiral
stationary phase HPLC. e CPME = cyclopentyl methyl ether
With the optimized reaction conditions in hand, the substrate generality of the oxidative quenching system
was investigated.
As is evident from the representative results summarized in Table 2, good to high
enantioselectivities were generally observed, although the chemical yields were case-sensitive. With respect to the
aromatic imine component 3, the electronic and steric properties of the substituted phenyl groups affected the reaction
profile subtly (entries 1-7); however, the conversion of 3-thiophenylaldimine 3i was low, and the formation of many
unidentified aromatic byproducts was detected in the case of 2-naphthylaldimine 3j (entries 8 and 9). The problem
associated with the incorporation of a naphthyl unit seemed to be inherent, as became apparent when a 1-naphthyl
group was appended to the amine coupling partner (entry 10). With other arylaminomethanes 4, variation in the
aromatic substituents was possible, and more dilute condition was essential for avoiding precipitation of the less
soluble amines or their coupling products, to ensure light irradiation efficiency (entries 11 and 12). Interestingly,
electron-deficient amine 4f underwent very sluggish reaction, probably because of its higher oxidation potential; thus
2b was employed as the requisite photosensitizer (entry 13). The scope of the reaction was extended from
diarylaminomethanes to alkyl arylaminomethanes, which also selectively reacted at the methyl carbon. This
allowed access to a range of diamines 5 with an alkyl chain in moderate yields and with excellent enantioselectivities
(entries 14-16).
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Table 2. Substrate Scopea
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yield
(%)b
71
ee
(%)c
97
91
87
96
91
78
93
96
93
91
87
81
89
90
97
91
entry
Ar' (3)
R, R' (4)
prod
1
2
4-MeC6H4 (3b)
4-FC6H4 (3c)
4-ClC6H4 (3d)
4-MeSC6H4 (3e)
3-MeC6H4 (3f)
3-MeOC6H4 (3g)
2-MeC6H4 (3h)
3-Thiophenyl (3i)
2-Naphthyl (3j)
Ph (3a)
Ph, Ph (4b)
Ph, Ph (4b)
5b
5c
5d
5e
5f
70
3
Ph, Ph (4b)
64
4
Ph, Ph (4b)
50
5
Ph, Ph (4b)
67
6
Ph, Ph (4b)
72
5g
5h
5i
7
Ph, Ph (4b)
59
8d
9
Ph, Ph (4b)
36
Ph, Ph (4b)
40
5j
10
11d
12d
13d,e
14
15d
16
1-Naphthyl, Ph (4c)
3-MeC6H4, Ph (4d)
4-MeC6H4, 4-ClC6H4 (4e)
4-BrC6H4, 4-BrC6H4 (4f)
iPr, Ph (4g)
28
5k
5l
Ph (3a)
78
Ph (3a)
83
5m
5n
5o
5p
5q
Ph (3a)
54
Ph (3a)
66
4-MeC6H4 (3b)
4-MeC6H4 (3b)
iPr, Ph (4g)
86
nHex, Ph (4h)
62
a Unless otherwise noted, reactions were performed with 0.10 mmol of 3 and 0.15 mmol of 4 with
1·HBArF (3.0 mol%) and 2a (0.5 mol%) in 0.5 mL of CPME under argon atmosphere with visible
light irradiation (15 W white LED). b Isolated yield. c Enantiomeric excesses were determined
by chiral stationary phase HPLC. d 1.0 mL of toluene was used as solvent. e Ir(Fppy)3 (2b) was
used instead of Ir(ppy)3 (2a).
In conclusion, we have demonstrated the feasibility of establishing a reverse catalytic cycle that initiates
with the oxidative quenching of the excited photosensitizer for the asymmetric coupling reaction between N-
arylaminomethanes and N-Ms arylaldimines under the synergistic catalysis of Ir(ppy)3 and chiral
arylaminophosphonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate 1·HBArF. This study sheds light on the
insensitivity of redox-neutral transformations to the sequence of redox events, revealing new aspects of these
photoredox reactions. Further efforts are being made to achieve a precise and deeper understanding of the reaction
mechanism.
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Experimental Section
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General Informations: Infrared (IR) spectra were recorded on film as absorption wavenumbers (cm-1). 1H NMR
spectra were recorded at ambient temperature on a 400 MHz FT-NMR spectrometer. Chemical shifts are reported
in ppm from tetramethylsilane (0.00 ppm) resonance as the internal standard (CDCl3). Data are reported as follows:
chemical shift, integration, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, m = multiplet,
br = broad), and coupling constants (Hz). 13C NMR spectra were recorded at ambient temperature on a 151 MHz
FT-NMR spectrometer with complete proton decoupling. Chemical shifts are reported in ppm from the solvent
resonance as the internal standard (CDCl3; 77.16 ppm). The high resolution mass spectra (HRMS) were conducted
on FT-ESI mass analyzer. Analytical thin layer chromatography (TLC) was performed on Merck precoated TLC
plates (silica gel 60 GF254, 0.25 mm). Flash column chromatography was performed on silica gel 60 (spherical,
40-50 µm; Kanto Chemical Co., Inc.), silica gel 60 (Merck 1.09385.9929, 230–400 mesh). Enantiomeric excesses
were determined by HPLC analysis using chiral columns (ϕ 4.6 mm x 250 mm, DAICEL CHIRALPAK AD-3 (AD3)
and CHIRALCEL OD-3 (OD3)) with hexane (H), and 2-propanol (IPA) as eluent. Toluene, diethylether (Et2O),
and tetrahydrofuran (THF) were supplied from Kanto Chemical Co., Inc. as “Dehydrated” and further purified by
passing through neutral alumina under nitrogen atmosphere. Cyclopentyl methyl ether (CPME) was kindly supplied
by Zeon Corporation. Other simple chemicals were purchased and used as such.
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Preparation of Trimethylsilylmethylamines:
Procedure for the Synthesis of Ph2NCH2SiMe3 (4b):13 Synthesis of the title compound was performed by following
the literature procedure with slight modification. To a solution of diphenylamine (1.69 g, 10.0 mmol) in THF (20.0
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mL) was added n-butyl lithium (2.6 M in nhexane, 4.60 mL, 12.0 mmol) dropwise at 0 °C. After being stirred for
20 min at 0 °C, (iodomethyl)trimethylsilane (1.60 mL, 11.5 mmol) was added. The reaction mixture was allowed
to warm to room temperature and stirred for overnight. The reaction was then quenched by the addition of saturated
aqueous solution of NH4Cl and the aqueous phase was extracted with Et2O twice. The combined organic phases
were dried over Na2SO4, filtered, and concentrated. The residue was purified by column chromatography on silica
gel (H/EA = 1:0–150:1 as eluent). The resulting liquid was further purified by distillation to give 4b (0.891 g, 3.49
mmol, 35%) as colorless liquid. N-phenyl-N-((trimethylsilyl)methyl)aniline (4b): 1H NMR (400 MHz, CDCl3) δ
7.24 (4H, t, J = 7.7 Hz), 6.98 (4H, d, J = 7.7 Hz), 6.91 (2H, t, J = 7.7 Hz), 3.30 (2H, s), –0.06 (3H, s). Other physical
data were identical in all respects to those previously reported.13
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1
N-Phenyl-N-((trimethylsilyl)methyl)naphthalen-1-amine (4c): 0.329 g, 39% as colorless liquid. H NMR (400
MHz, CDCl3) δ 7.89 (1H, d, J = 8.0 Hz), 7.80 (1H, d, J = 8.0 Hz), 7.75 (1H, d, J = 8.0 Hz), 7.51 (1H, t, J = 8.0 Hz),
7.46 (1H, t, J = 8.0 Hz), 7.40 (1H, d, J = 8.0 Hz), 7.38 (1H, t, J = 8.0 Hz), 7.11 (2H, t, J = 8.0 Hz), 6.66 (1H, t, J =
8.0 Hz), 6.57 (2H, d, J = 8.0 Hz), 3.38 (2H, s), –0.17 (9H, s); 13C NMR (151 MHz, CDCl3) δ 151.4, 144.7, 135.4,
131.9, 128.9, 128.6, 127.1, 127.0, 126.3, 126.2, 124.5, 116.8, 113.6, 44.4, –1.2; IR (film) 3057, 2951, 1595, 1574,
1497, 1395, 1354, 1294, 1248, 1207 cm−1; HRMS (ESI) Calcd for C20H24NSi+ ([M+H]+) 306.1673. Found 306.1671.
3-Methyl-N-phenyl-N-((trimethylsilyl)methyl)aniline (4d): 0.329 g, 39% as colorless liquid. 1H NMR (400 MHz,
CDCl3) δ 7.23 (2H, t, J = 7.3 Hz), 7.13 (1H, t, J = 7.7 Hz), 6.96 (2H, d, J = 7.3 Hz), 6.90 (1H, t, J = 7.3 Hz), 6.81
(1H, s), 6.79 (1H, d, J = 7.7 Hz), 6.75 (1H, d, J = 7.7 Hz), 3.28 (2H, s), 2.28 (3H, s), –0.05 (9H, s); 13C NMR (151
MHz, CDCl3) δ 149.9, 149.7, 139.0, 129.2, 129.1, 122.2(2), 122.2(0), 121.0, 120.8, 118.7, 43.9, 21.8, ‒1.1; IR (film)
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2953, 1593, 1493, 1429, 1354, 1248, 1206, 1169, 854 cm−1; HRMS (ESI) Calcd for C17H24NSi+ ([M+H]+) 270.1673.
Found 270.1671.
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4-Chloro-N-(p-tolyl)-N-((trimethylsilyl)methyl)aniline (4e): 0.543 g, 52% as colorless liquid. 1H NMR (400 MHz,
CDCl3) δ 7.11 (2H, d, J = 8.0 Hz), 7.10 (2H, d, J = 9.2 Hz), 6.97 (2H, d, J = 8.0 Hz), 6.72 (2H, d, J = 9.2 Hz), 3.22
(2H, s), 2.32 (3H, s), –0.06 (9H, s); 13C NMR (151 MHz, CDCl3) δ 149.0, 146.5, 133.2, 130.2, 128.8, 124.6, 123.4,
118.7, 44.1, 21.0, ‒1.2; IR (film) 2953, 1591, 1510, 1489, 1431, 1356, 1248, 1190, 1096, 841, 812 cm−1; HRMS (ESI)
Calcd for C17H23N35ClSi+ ([M+H]+) 304.1283. Found 304.1280.
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4-Bromo-N-(4-bromophenyl)-N-((trimethylsilyl)methyl)aniline (4f): 0.782 g, 66% as yellow liquid. H NMR
(400 MHz, CDCl3) δ 7.33 (4H, d, J = 9.0 Hz), 6.84 (4H, d, J = 9.0 Hz), 3.24 (2H, s), –0.03 (9H, s); 13C NMR (151
MHz, CDCl3) δ 148.3, 132.2, 122.8, 113.8, 44.1, ‒1.1; IR (film) 2953, 1578, 1483, 1433, 1356, 1248, 1188, 1072,
1009, 841, 810 cm−1; HRMS (ESI) Calcd for C16H20N79Br2Si+ ([M+H]+) 411.9726. Found 411.9728.
Procedure for the Synthesis of (N-Alkyl-N-Phenylaminomethyl)trimethylsilane: This protocol is a modified
literature procedure.13
To a solution of N-n-hexylaniline (1.25 g, 7.04 mmol) in THF (14.0 mL) and
n
hexamethylphosphoric triamide (2.80 mL) was added n-butyl lithium (2.6 M in hexane, 3.25 mL, 8.45 mmol)
dropwise at 0 °C. After being stirred for 30 min at 0 °C, (iodomethyl)trimethylsilane (1.20 mL, 8.61 mmol) was
added. The reaction mixture was allowed to warm to room temperature and stirred for 1 d. The reaction was then
quenched by the addition of saturated aqueous solution of NH4Cl and the aqueous phase was extracted with Et2O
twice. The combined organic phases were dried over Na2SO4, filtered, and concentrated. The residue was purified
by column chromatography on silica gel (H/EA = 160:1–120:1 as eluent) to give 4h (0.99 g, 3.76 mmol, 53%) as
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colorless liquid. N-n-Hexyl-N-phenylaminomethyltrimethylsilane (4h): 1H NMR (400 MHz, CDCl3) δ 7.17 (2H,
dd, J = 7.1, 8.8 Hz), 6.60 (2H, d, J = 8.8 Hz), 6.57 (1H, t, J = 7.1 Hz), 3.26–3.21 (1H, m), 2.81 (2H, s), 1.60–1.51
(2H, m), 1.38–1.22 (6H, m), 0.89 (3H, t, J = 7.0 Hz), 0.07 (9H, s); 13C NMR (151 MHz, CDCl3) δ 149.4, 129.1, 114.7,
112.0, 52.8, 41.9, 31.9, 27.0, 25.9, 22.9, 14.2, ‒0.9; IR (film) 2953, 2928, 1599, 1504, 1456, 1364, 1248, 1225, 1119,
989, 841 cm−1; HRMS (ESI) Calcd for C16H30NSi+ ([M+H]+) 264.2142. Found 264.2135.
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Representative Procedure for the Asymmetric Coupling between N-Ms Imine
3
and
Trimethylsilylmethylamines 4: To a dried test tube were weighted N-sulfonyl imine 3a (18.8 mg, 0.10 mmol),
1·HBArF (5.74 mg, 0.003 mmol), and Ir(ppy)3 (2a, 0.33 mg, 0.0005 mol) under Ar. Then, CPME (0.50 mL) was
introduced at room temperature. After addition of Ph2NCH2SiMe3 (0.15 mmol, 39.0 µL), evacuation of the tube
followed by refill with Ar was conducted three times. The test tube was placed in a water bath and illuminated with
15 W white LED (approximate distance was 2 cm from the light source) for 24 h. After the specified time has
elapsed, reaction mixture was concentrated.
The residue was subjected to the purification by column
chromatography on silica gel (H/CH2Cl2/EA = 6:2:1, then H/EA = 4:1 to 2:1 as eluent) to afford 5a (29.2 mg, 0.080
mmol, 80%). The enantiomeric excess of 5a was determined to be 93% by HPLC analysis. (R)-N-(2-
(diphenylamino)-1-phenylethyl)methanesulfonamide (5a): 1H NMR (400 MHz, CDCl3) δ 7.38–7.29 (5H, m), 7.26
(4H, t, J = 7.8 Hz), 6.99 (2H, t, J = 7.8 Hz), 6.92 (4H, d, J = 7.8 Hz), 5.19 (1H, d, J = 5.8 Hz), 4.77 (1H, dt, J = 8.7,
5.8 Hz), 4.04 (1H, dd, J = 14.6, 8.7 Hz), 3.89 (1H, dd, J = 14.6, 5.8 Hz), 2.55 (3H, s). HPLC OD3, H/IPA = 10:1,
flow rate = 1.0 mL/min, λ = 254 nm, 13.9 min (minor), 28.3 min (major). The other physical data were identical in
all respects to those previously reported.7
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(R)-N-(2-(diphenylamino)-1-(p-tolyl)ethyl)methanesulfonamide (5b): 27.7 mg, 71%, 97% ee. 1H NMR (400 MHz,
CDCl3) δ 7.27 (4H, td, J = 8.7, 7.3 Hz), 7.21 (2H, d, J = 8.2 Hz), 7.16 (2H, d, J = 8.2 Hz), 7.00 (2H, td, J = 7.3, 0.9
Hz), 6.94 (4H, dd, J = 8.7, 0.9 Hz), 5.08 (1H, d, J = 5.5 Hz), 4.73 (1H, dt, J = 9.2, 5.5 Hz), 4.01 (1H, dd, J = 15.1,
9.2 Hz), 3.87 (1H, dd, J = 15.1, 5.5 Hz), 2.54 (3H, s), 2.34 (3H, s). HPLC OD3, H/IPA = 10:1, flow rate = 1.0
mL/min, λ = 210 nm, 12.8 min (minor), 16.1 min (major). The other physical data were identical in all respects to
those previously reported.7
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(R)-N-(2-(diphenylamino)-1-(4-fluorophenyl)ethyl)methanesulfonamide (5c): 25.9 mg, 70%, 91% ee. 1H NMR
(400 MHz, CDCl3) δ 7.30 (2H, dd, J = 8.7, 5.5 Hz), 7.27 (4H, td, J = 7.3, 0.9 Hz), 7.04 (2H, t, J = 8.7 Hz), 7.00 (2H,
t, J = 7.3 Hz), 6.92 (4H, dd, J = 7.3, 0.9 Hz), 5.26 (1H, d, J = 5.7 Hz), 4.76 (1H, dt, J = 9.2, 5.7 Hz), 4.01 (1H, dd, J
= 15.1, 9.2 Hz), 3.87 (1H, dd, J = 15.1, 5.7 Hz), 2.56 (3H, s). HPLC AD3, H/IPA = 10:1, flow rate = 1.0 mL/min,
λ = 210 nm, 11.3 min (minor), 14.4 min (major). The other physical data were identical in all respects to those
previously reported.7
(R)-N-(1-(4-chlorophenyl)-2-(diphenylamino)ethyl)methanesulfonamide (5d): 26.0 mg, 64%, 87% ee. 1H NMR
(400 MHz, CDCl3) δ 7.34 (2H, d, J = 8.7 Hz), 7.31–7.24 (6H, m), 7.02 (2H, tt, J = 7.3, 0.9 Hz), 6.93 (4H, d, J = 7.3
Hz), 5.16 (1H, d, J = 5.6 Hz), 4.75 (1H, dt, J = 9.2, 5.6 Hz), 3.98 (1H, dd, J = 15.1, 9.2 Hz), 3.88 (1H, dd, J = 15.1,
5.6 Hz), 2.58 (3H, s). HPLC OD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 16.8 min (minor), 30.6 min
(major). The other physical data were identical in all respects to those previously reported.7
(R)-N-(2-(diphenylamino)-1-(4-(methylthio)phenyl)ethyl)methanesulfonamide (5e): 19.9 mg, 50%, 96% ee. 1H
NMR (400 MHz, CDCl3) δ 7.30–7.19 (8H, m), 7.00 (2H, t, J = 7.8 Hz), 6.93 (4H, d, J = 7.8 Hz), 5.13 (1H, d, J = 5.5
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Hz), 4.73 (1H, dt, J = 9.2, 5.5 Hz), 4.00 (1H, dd, J = 15.1, 9.2 Hz), 3.87 (1H, dd, J = 15.1, 5.5 Hz), 2.55 (3H, s), 2.47
(3H, s). HPLC OD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 29.0 min (minor), 42.4 min (major). The
other physical data were identical in all respects to those previously reported.7
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(R)-N-(2-(diphenylamino)-1-(m-tolyl)ethyl)methanesulfonamide (5f): 24.8 mg, 67%, 91% ee. 1H NMR (400 MHz,
CDCl3) δ 7.30–7.21 (5H, m), 7.15–7.09 (3H, m), 7.00 (2H, t, J = 7.8 Hz), 6.94 (4H, d, J = 7.8 Hz), 5.05 (1H, d, J =
5.7 Hz), 4.73 (1H, dt, J = 9.2, 5.7 Hz), 4.01 (1H, dd, J = 15.1, 9.2 Hz), 3.89 (1H, dd, J = 15.1, 5.7 Hz), 2.56 (3H, s),
2.33 (3H, s). HPLC AD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 9.0 min (minor), 9.5 min (major).
The other physical data were identical in all respects to those previously reported.7
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(R)-N-(2-(diphenylamino)-1-(3-methoxyphenyl)ethyl)methanesulfonamide (5g): 27.7 mg, 72%, 78% ee. H
NMR (400 MHz, CDCl3) δ 7.31–7.24 (5H, m), 7.00 (2H, t, J = 7.3 Hz), 6.97–6.91 (5H, m), 6.87–6.82 (2H, m), 5.04
(1H, d, J = 5.6 Hz), 4.74 (1H, dt, J = 9.2, 5.6 Hz), 4.02 (1H, dd, J = 15.1, 9.2 Hz), 3.90 (1H, dd, J = 15.1, 5.6 Hz),
3.78 (3H, s), 2.59 (3H, s). HPLC AD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 14.5 min (minor), 15.6
min (major). The other physical data were identical in all respects to those previously reported.7
(R)-N-(2-(diphenylamino)-1-(o-tolyl)ethyl)methanesulfonamide (5h): 21.7 mg, 59%, 93% ee. 1H NMR (400 MHz,
CDCl3) δ 7.38 (1H, dd, J = 6.9, 2.3 Hz), 7.28 (4H, t, J = 7.3 Hz), 7.22 (1H, dd, J = 7.3, 1.8 Hz), 7.20 (1H, td, J = 7.3,
1.8 Hz), 7.15 (1H, d, J = 7.3 Hz), 7.01 (2H, t, J = 7.3 Hz), 6.95 (4H, d, J = 7.3 Hz), 5.06 (1H, dt, J = 9.2, 5.0 Hz),
5.02 (1H, d, J = 5.0 Hz), 4.01 (1H, dd, J = 14.6, 9.2 Hz), 3.82 (1H, dd, J = 14.6, 5.0 Hz), 2.54 (3H, s), 2.26 (3H, s).
HPLC OD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 11.9 min (minor), 19.8 min (major). The other
physical data were identical in all respects to those previously reported.7
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(R)-N-(2-(diphenylamino)-1-(thiophen-3-yl)ethyl)methanesulfonamide (5i): 13.7 mg, 36%, 96% ee. H NMR
(400 MHz, CDCl3) δ 7.34 (1H, dd, J = 5.0, 3.2 Hz), 7.30–7.22 (5H, m), 7.07 (1H, dd, J = 5.0, 0.9 Hz), 7.00 (2H, td,
J = 7.8, 0.9 Hz), 6.93 (4H, dd, J = 7.8, 0.9 Hz), 4.95–4.88 (2H, m), 4.10 (1H, dd, J = 14.6, 8.2 Hz), 3.95 (1H, dd, J =
14.6, 5.5 Hz), 2.60 (3H, s). HPLC OD3, H/IPA = 4:1, flow rate = 1.0 mL/min, λ = 210 nm, 19.5 min (major), 24.3
min (minor). The other physical data were identical in all respects to those previously reported.7
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(R)-N-(2-(diphenylamino)-1-(naphthalen-2-yl)ethyl)methanesulfonamide (5j): 16.6 mg, 40%, 93% ee. 1H NMR
(400 MHz, CDCl3) δ 7.85 (1H, d, J = 8.5 Hz), 7.84–7.79 (2H, m), 7.78 (1H, brs), 7.53–7.48 (2H, m), 7.46 (1H, dd, J
= 8.5, 1.8 Hz), 7.27 (4H, t, J = 7.3 Hz), 7.00 (2H, t, J = 7.3 Hz), 6.97 (4H, d, J = 7.3 Hz), 5.21 (1H, d, J = 5.4 Hz),
4.95 (1H, dt, J = 9.2, 5.4 Hz), 4.11 (1H, dd, J = 15.1, 9.2 Hz), 3.98 (1H, dd, J = 15.1, 5.4 Hz), 2.53 (3H, s). HPLC
OD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 19.5 min (minor), 24.3 min (major). The other physical
data were identical in all respects to those previously reported.7
(R)-N-(2-(naphthalen-1-yl(phenyl)amino)-1-phenylethyl)methanesulfonamide (5k): 10.9 mg, 26%, 91% ee. 1H
NMR (400 MHz, CDCl3) δ 7.89 (1H, d, J = 8.4 Hz), 7.81 (1H, d, J = 8.4 Hz), 7.60 (1H, d, J = 8.4 Hz), 7.46 (2H, t, J
= 8.4 Hz), 7.39–7.27 (6H, m), 7.23–7.12 (3H, m), 6.78 (1H, t, J = 8.4 Hz), 6.65 (2H, d, J = 8.4 Hz), 5.01–4.87 (2H,
m), 4.19 (1H, dd, J = 15.2, 8.8 Hz), 3.97 (1H, dd, J = 15.2, 5.6 Hz), 2.49 (3H, s). HPLC AD3, H/IPA = 10:1, flow
rate = 1.0 mL/min, λ = 223 nm, 11.4 min (major), 13.0 min (minor). The other physical data were identical in all
respects to those previously reported.7
(R)-N-(1-phenyl-2-(phenyl(m-tolyl)amino)ethyl)methanesulfonamide (5l): 30.5 mg, 78%, 87% ee. 1H NMR (400
MHz, CDCl3) δ 7.40–7.29 (5H, m), 7.26 (2H, t, J = 8.0 Hz), 7.17 (1H, t, J = 8.0 Hz), 6.98 (1H, tt, J = 8.0, 1.2 Hz),
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6.92 (2H, dd, J = 8.0, 1.2 Hz), 6.84 (1H, d, J = 8.0 Hz), 6.75 (1H, d, J = 8.0 Hz), 6.73 (1H, s), 5.06 (1H, d, J = 5.6
Hz), 4.76 (1H, dt, J = 8.8, 5.6 Hz), 4.01 (1H, dd, J = 15.2, 8.8 Hz), 3.87 (1H, dd, J = 15.2, 5.6 Hz), 2.55 (3H, s), 2.28
(3H, s). HPLC AD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 9.1 min (minor), 9.6 min (major). The
other physical data were identical in all respects to those previously reported.7
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(R)-N-(2-((4-chlorophenyl)(p-tolyl)amino)-1-phenylethyl)methanesulfonamide (5m): 32.4 mg, 83%, 81% ee. 1H
NMR (400 MHz, CDCl3) δ 7.40–7.27 (5H, m), 7.15 (2H, d, J = 8.8 Hz), 7.10 (2H, d, J = 8.8 Hz), 6.84 (2H, d, J =
8.8 Hz), 6.73 (2H, d, J = 8.8 Hz), 5.14 (1H, d, J = 6.0 Hz), 4.73 (1H, dt, J = 8.8, 6.0 Hz), 4.00 (1H, dd, J = 15.2, 8.8
Hz), 3.80 (1H, dd, J = 15.2, 6.0 Hz), 2.57 (3H, s), 2.31 (3H, s). HPLC OD3, H/IPA = 10:1, flow rate = 1.0 mL/min,
λ = 210 nm, 13.2 min (minor), 28.7 min (major). The other physical data were identical in all respects to those
previously reported.7
(R)-N-(2-(bis(4-bromophenyl)amino)-1-phenylethyl)methanesulfonamide (5n): 28.6 mg, 54%, 89% ee. 1H NMR
(400 MHz, CDCl3) δ 7.39–7.30 (7H, m), 7.26–7.22 (2H, m), 6.71 (4H, d, J = 9.2 Hz), 5.20 (1H, d, J = 7.6 Hz), 4.73
(1H, q, J = 7.6 Hz), 4.08 (1H, dd, J = 15.2, 7.6 Hz), 3.79 (1H, dd, J = 15.2, 7.6 Hz), 2.61 (3H, s). HPLC OD3,
H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 21.2 min (minor), 40.9 min (major). The other physical data
were identical in all respects to those previously reported.7
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(R)-N-(2-(isopropyl(phenyl)amino)-1-phenylethyl)methanesulfonamide (5o): 21.9 mg, 66%, 90% ee. H NMR
(400 MHz, CDCl3) δ 7.39 (4H, d, J = 8.4 Hz), 7.33 (3H, t, J = 8.4 Hz), 7.08 (2H, d, J = 8.4 Hz), 7.00 (1H, t, J = 8.4
Hz), 5.27 (1H, br s), 4.47 (1H, ddd, J = 10.8, 4.8, 1.6 Hz), 3.76 (1H, sept, J = 6.8 Hz), 3.39 (1H, dd, J = 14.0, 4.8 Hz),
3.05 (1H, dd, J = 14.0, 10.8 Hz), 2.42 (3H, s), 1.23 (3H, d, J = 6.8 Hz), 0.96 (3H, d, J = 6.8 Hz). HPLC AD3, H/IPA
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= 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 8.9 min (minor), 10.7 min (major). The other physical data were
identical in all respects to those previously reported.7
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(R)-N-(2-(isopropyl(phenyl)amino)-1-(p-tolyl)ethyl)methanesulfonamide (5p): 29.0 mg, 86%, 97% ee. 1H NMR
(400 MHz, CDCl3) δ 7.32 (2H, t, J = 7.6 Hz), 7.28 (2H, d, J = 7.6 Hz), 7.19 (2H, d, J = 7.6 Hz), 7.06 (2H, d, J = 7.6
Hz), 6.99 (1H, t, J = 7.6 Hz), 5.27 (1H, br s), 4.44 (1H, ddd, J = 10.8, 4.8, 2.0 Hz), 3.76 (1H, sept, J = 6.8 Hz), 3.36
(1H, dd, J = 14.0, 4.8 Hz), 3.04 (1H, dd, J = 14.0, 10.8 Hz), 2.42 (3H, s), 2.35 (3H, s), 1.23 (3H, d, J = 6.8 Hz), 0.95
(3H, d, J = 6.8 Hz). HPLC AD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 10.9 min (minor), 13.1 min
(major). The other physical data were identical in all respects to those previously reported.7
(R)-N-(2-(hexyl(phenyl)amino)-1-(p-tolyl)ethyl)methanesulfonamide (5q): 24.5 mg, 62%, 91% ee. 1H NMR (400
MHz, CDCl3) δ 7.30–7.23 (4H, m), 7.19 (2H, d, J = 8.0 Hz), 6.82 (2H, d, J = 8.0 Hz), 6.79 (1H, t, J = 8.0 Hz), 5.02
(1H, d, J = 5.6 Hz), 4.69 (1H, dt, J = 8.8, 5.6 Hz), 3.53 (1H, dd, J = 14.8, 8.8 Hz), 3.43 (1H, dd, J = 14.8, 5.6 Hz),
3.15 (2H, t, J = 7.8 Hz), 2.55 (3H, s), 2.36 (3H, s), 1.53–1.33 (2H, m), 1.32–1.18 (6H, m), 0.87 (3H, t, J = 7.2 Hz).
HPLC AD3, H/IPA = 10:1, flow rate = 1.0 mL/min, λ = 210 nm, 7.3 min (minor), 8.6 min (major). The other
physical data were identical in all respects to those previously reported.7
Acknowledgement
Financial support was provided by CREST-JST, a Grant-in-Aid for Scientific Research on Innovative Areas
“Advanced Molecular Transformations by Organocatalysts” from MEXT, IGER Program in Nagoya University,
Grants of JSPS for Scientific Research, and the Asahi Glass Foundation. T.K. acknowledges JSPS for financial
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
10.1021/acs.joc.xxxxxxx.
Electro- and photochemical experiments and copies of spectra (PDF)
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We had assumed that the three-dimensional structure of the transition state in the carbon-carbon bond-forming
step would be identical in both the reductive and oxidative quenching systems. Thus, the origin of the
difference in the optimal reaction conditions remains open for further mechanistic investigation and discussion.
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