Visible light photoredox organocatalysis: A fully transition metal-free direct asymmetric α-alkylation of aldehydes

Article abstract of DOI:10.1039/c2gc35118h

Rose Bengal, a dye sensitizer, was found to be active as a visible light photoredox catalyst for the direct enantioselective α-alkylation of aldehydes in environmentally benign and simple conditions. Starting from a SOMO converse mechanism, the catalytic

Full text of DOI:10.1039/c2gc35118h

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Visible light photoredox organocatalysis: a fully transition metal-free direct
asymmetric α-alkylation of aldehydes†
Kassim Fidaly, Claire Ceballos, Annie Falguières, Maité Sylla-Iyarreta Veitia, Alain Guy and
Clotilde Ferroud*
Received 25th January 2012, Accepted 5th March 2012
DOI: 10.1039/c2gc35118h
method for the enantioselective α-alkylation, α-arylation and
α-trifluoroalkylation of aldehydes via the formation of an
enamine.
However the potential toxicity of the ruthenium or the iridium
salts as well as their future limited availability is the major weak-
ness of these transition metal-based methods for the manufactur-
ing of fine chemicals and pharmaceuticals.
Rose Bengal, a dye sensitizer, was found to be active as a
visible light photoredox catalyst for the direct enantioselec-
tive α-alkylation of aldehydes in environmentally benign and
simple conditions. Starting from a SOMO converse mechan-
ism, the catalytic activity of the imidazolidinone organocata-
lyst was improved using an electrophilic cocatalyst. The
resulting methodology and optimal conditions led to efficient
and transition-metal-free direct photoredox organocatalysis.
Herein we present a versatile transition-metal-free process,
using a fully organic visible light photoredox catalyst.
The development of new and green activation modes in selective
synthesis is a fundamental objective for organic chemists. Asym-
metric organocatalysis is an important area that has been inten-
sively studied over the past few years,1 one that is complementary
to transition-metal-based catalysis.2 This kind of process has
become very attractive since environmentally friendly and metal-
free transformations are very much in demand.
In 2007, MacMillan introduced the concept of organo- singly
occupied molecular orbital (SOMO) catalysis.3 This one electron
mode of activation has enabled the development of several
useful transformations, such as C–C4 and C–Heteroatom5 bond
formation. In the organo-SOMO catalysis, the central need of an
oxidant in a stoichiometric amount (CAN) prompted MacMillan
to develop a photoredox catalytic system via a SOMO converse
mechanism.6
Previously in our laboratory, the photo-oxidation of tertiary
amines9 mediated by an organosensitizer has been widely devel-
oped to make a C–C bond on the α position of the nitrogen atom
via the formation of highly reactive iminium ions (Scheme 1).
Among the various organic sensitizers, xanthene dyes have
given rise to active interest in the last few years due to their high
absorption in the visible domain. For instance, several useful
transformations induced by the photo-oxidation of activated ter-
tiary amines such as tetrahydroisoquinoline derivatives were per-
formed by C.-H. Tan et al.10 and B. König & D. P. Hari11 using
organic dyes like Rose Bengal or Eosin Y.
In 2011, K. Zeitler’s group first published an alternative12 to
the use of the ruthenium photocatalyst for the enantioselective
Light can be considered as an ideal reaction condition for
environmentally friendly, green chemical synthesis as it is abun-
dant, non toxic and generates no waste.
Recently, the application of visible light photoredox catalysis
has emerged as a growing field in organic synthesis7 in particular
for applications using combinations of metal and organocata-
lysts.2 The applications in SOMO catalysis were performed
using the well-known organometallic polypyridyl complexes
such as [Ru(bpy)3]2+ or fac-Ir(ppy)3 as photoredox catalysts.7d,8
In this SOMO converse mechanism termed photoredox organo-
catalysis, the cooperative combination of a photocatalytic
process with an organocatalytic cycle offers an efficient catalytic
Laboratoire de Transformations Chimiques et Pharmaceutiques – ERL
3193, Cnrs, ESPCI Paris Tech, Cnam. 2 rue Conté, 75003 Paris,
France. E-mail: clotilde.ferroud@cnam.fr; Tel: (+33)-1-4027-2402;
Fax: (+33)-1-4271-0534
†Electronic supplementary information (ESI) available: General pro-
cedures, experimental data, photosensitizers properties, spectroscopic
data. See DOI: 10.1039/c2gc35118h
Scheme 1 Previous laboratory work showing the competitive pathways
in the photo-oxidation of tertiary amines.
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α-alkylation of aldehydes. In this case, using a green LEDs
irradiation system, the authors have shown the ability of Eosin Y
to perform this reaction with attractive results, however low
temperatures and somewhat longer reaction times were needed.
In this work, we intend to develop a Rose Bengal photoredox
catalysis taking particular advantage of room temperature mild
conditions with limited irradiation times.
As enamine compounds are known to be oxidized easier than
the corresponding amines,13 our preliminary studies began with
the screening of various photosensitizers as photoredox catalysts
in asymmetric organocatalytic C–C bond formation according to
the methodology developed by MacMillan et al. Between a large
choice set, we selected sensitizers in line with their prior use in
the photo-oxidation of tertiary amines. Some organic dyes were
particularly attractive as they exhibited a remarkable resemblance
with the redox properties of the metal complexes previously
used by MacMillan (Fig. 1).6d,8d,14
To test the ability of these dyes to act as photoredox catalysts,
we selected the direct and enantioselective α-alkylation of alde-
hydes as a representative transformation.‡ Experimentally, the
alkylation protocol was first performed in DMF with the combi-
nation of the imidazolidinone 1 and a photosensitizer S as the
catalytic system for the α-alkylation of hydrocinnamaldehyde 2a
by diethyl bromomalonate 3a used as a SOMO electrophilic pre-
cursor (Scheme 2).
domain and to act both as a strong oxidant in the excited state S*
and as an efficient reductant in its semi-reduced form S˙−
(Fig. 2).
As outlined in Fig. 3, the mechanistic picture of this whole
organocatalytic cycle could involve two closely interwoven cata-
lytic cycles. We speculated that a more stable triplet state in a
polar solvent such as DMF will promote the mechanism, avoid-
ing any thermodynamically favourable back electron transfer. In
fact, in this “light part” of the mechanism, the high-energy inter-
mediate S* would efficiently strip one electron from a sacrificial
enamine so that the semi-reduced form S˙− with a high
reduction potential could initiate the formation of the electron-
deficient radical from the halide.
Besides, the “dark part” would resume the organocatalytic
cycle. This part began with the condensation of the catalyst 1
with the aldehyde 2 providing the enamine 5. In the presence of
a radical, the somophilic enamine could play the role of a trap-
ping radical species. The generation of an electron-rich α-amino
radical 6 that is more readily oxidizable than the corresponding
enamine should enforce the formation of the iminium ion 7 by
reducing the halide 3 as a propagation step directly or via a
second photocatalytic cycle as described. The enamine for-
mation, the liberation of the catalyst from the iminium intermedi-
ate, and the interconnection of the two cycles with the radical
trapping are the major steps of this mechanism.
As
a
central design consideration, the photoactivation
According to the general procedure described in Scheme 2, a
number of organosensitizers studied were effective for this
reveals the ability of the photosensitizer to absorb in the visible
Fig. 1 Absorption and redox properties of the sensitizers used as photoredox catalysts (λmax: in CH3CN; Potential values: V/SCE).6d,8d,14
Scheme 2 General procedure for the α-alkylation of hydrocinnamalde-
hyde 2a by diethyl bromomalonate 3a.
Fig. 2 Photoredox pathways.
1294 | Green Chem., 2012, 14, 1293–1297
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Fig. 3 Proposed mechanism for the α-alkylation of aldehydes.
transformation, albeit with different yields and enantioselectiv-
ities (Table 1, entries 1–7). The reaction can be conducted using
different light sources.15 Nevertheless, light as well as the
photocatalyst were proved to be essential for this transformation
(entries 14–15).
Even if Eosin B seems to be the best sensitizer with regard to
reduction potential (Fig. 1) for the halide reduction, Rose Bengal
gives the best compromise between the yield and the ee (Table 1,
entries 7 and 10). These results show that the reduction potential
of sensitizers is not the only parameter to be considered. We
speculated here that the triplet quantum yield plays a significant
role in the photoactivation steps.16 Under a 150 W high pressure
Hg lamp light or even an economic eco-compatible 24 W
6500 K fluorescent bulb light, the reaction was achieved in only
2 h at room temperature and did not require any heating or
cooling system.
Table 1 Investigation on the light source and the photosensitizera
Entry
Light source
S
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
Hg 150 W
Hg 150 W
Hg 150 W
Hg 150 W
Hg 150 W
Hg 150 W
Hg 150 W
Fluo 24 W
Fluo 24 W
Fluo 24 W
LED 530 nm
LED 530 nm
LED 558 nm
Dark
Anthr
Napht
DCA
DCN
EB
EY
RB
EB
EY
RB
EY
3
3
6
6
4
4
2
4
4
2
3
3
16
8
72
6
0
46
87
86
quant
78
81
quant
86
66
66
trace
trace
88
81
nd
70
82
83
83
83
82
82
75
76
84
nd
nd
Next we carried out a solvent screening, with Rose Bengal
as photoredox catalyst, to ascertain the solvent scope in this reac-
tion. The mild polarity governs the two equilibriums of the pro-
posed mechanism. It is also a major concern for the solvent
separated ion pairs. So as we expected, it was found (Table 2)
that the more polar DMSO and DMF were the most suitable sol-
vents both in yield and enantioselectivity. Switching the solvent
to less polar MeCN, THF, or DME dropped the yield and
increased the reaction time.
9
10
11
12
13
14
15
RB
RB
RB
—
Fluo 24 W
8
a All the experiments were conducted with 0.5 mmol of 3a in a 0.5 M
anhydrous DMF solution. b NMR yield was determined using a
calibrated internal standard. c The ee was determined by chiral HPLC
analysis.
As generally expected in organocatalysis,17 the concentration
also displayed
a
significant effect on both yield and
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Table 2 Solvent screening for the α-alkylation of aldehydesa
Table 4 Effect of a catalytic amount of cocatalysta
Entry
Solvent
Time (h)
Yieldb (%)
eec (%)
Catalyst 1
(mol%)
Cocatalyst
(mol%)
Time
(h)
Yieldb
(%)
eec
(%)
Entry
1
2
3
4
5
anh. DME
anh. THF
anh. MeCN
anh. DMF
anh. DMSO
6
6
6
2
2
34
45
48
quant
quant
70
63
74
82
87
1
2
3
4
5
6
7
8
20
15
10
10
10
10
15
15
15
15
15
15
15
15
15
15
15
15
—
—
—
2
4
6
6
4
8
2
2
2
2
2
3
3
2
quant
68
47
46
55
42
82
53
70
80
90
83
54
91
93
93
95
92
82
80
81
73
77
78
82
84
80
74
76
82
nd
83
73
85
81
82
LiCl
5
LiCl
LiCl
LiCl
LiBr
LiPF6
LiBF4 10
LiOTf 10
LiClO4 10
MgCl2 10
LiCl
LiCl
LiCl
LiCl
LiCl
10
20
10
10
10
a All the experiments were conducted as in entry 10 Table 1 with Rose
Bengal and a 24 W fluorescent bulb. b NMR yield was determined using
a calibrated internal standard. c The ee was determined by chiral HPLC
analysis.
9
10
11
12
13
14d
15d
16e
17f
18g
enantioselectivity. Between 0.1 and 1.0 M, the best results were
obtained for 0.5 M (Table 3, entry 3).
We also examined the influence of cheaper Lewis acid salts as
cocatalysts in order to reduce the amount of catalyst 1 in this
reaction.
10
10 16
10
10
10
2
2
2
a All the experiments are conducted as in entry 10 Table 1 with Rose
Bengal and a 24 W fluorescent bulb in DMF. b NMR yield was
determined using a calibrated internal standard. c The ee was determined
by chiral HPLC analysis. d Reaction performed in DMSO. e Scale-up in
DMSO: 1 mmol of started 3a instead of 0.5 mmol. f Scale-up in DMSO:
5 mmol of started 3a. g Reaction performed in DMSO using sunlight as
the light source.
As reported in Table 4, lowering of the catalyst led to a
remarkably slow conversion with very poor yields (Table 4,
entries 2–3). An additional 10 mol% of anhydrous LiCl allowed
the use of only 15 mol% of catalyst 1 thus recovering both yield
and enantioselectivity (Table 4, entries 4–13). Furthermore,
switching the DMF by DMSO provided the best result for this
study (Table 4, entry 14). It could be noticed that leaving the
reaction over 16 h decreased the selectivity (Table 4, entry 15).
Moreover, we proved that this reaction can be efficiently scaled-
up (Table 4, entries 16–17). Interestingly, irradiation under
diffuse sunlight18 provided the same results as a fluorescent light
bulb (Table 4, entry 18), notably without any loss in
enantioselectivity.
After establishing the optimal reaction conditions, several
aldehydes were subjected to the direct enantioselective α-alky-
lation by diethyl bromomalonate 3a (Scheme 3). Various ali-
phatic aldehydes were found to be suitable substrates for this
reaction.
In conclusion, Rose Bengal was successfully applied as an
organic photocatalyst to the asymmetric photoredox α-alkylation
of aldehydes. The reaction proceeded smoothly under visible
light irradiation at room temperature and with short reaction
times providing good to excellent yields and enantioselectivities.
The conditions described in this report are environmentally
benign and operationally simple. Rose Bengal associated with a
cocatalytic system (catalyst 1 and LiCl) gave superior or
Table 3 Concentration effecta
Entry
Concentration (M)
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
0.10
0.25
0.50
0.75
1.0
3
3
2
0.75
0.5
66
75
quant
97
85
77
80
82
78
79
Scheme 3 Scope of the α-alkylation by fully organic photoredox orga-
nocatalysis. aReaction performed in DMF with 20 mol% catalyst
without LiCl.
a All the experiments were conducted as in entry 10 Table 1 with Rose
Bengal and a 24 W fluorescent bulb in DMF. b NMR yield was
determined using a calibrated internal standard. c The ee was determined
by chiral HPLC analysis.
equivalent results in termsafter of enantioselectivity, yield, cata-
lyst loading and particularly reaction times, when compared to
other photoredox systems previously described in the literature.
1296 | Green Chem., 2012, 14, 1293–1297
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The preliminary chemistry described herein underlines the sus-
ceptibility of the α-alkylation reaction to subtle changes in the
reaction conditions. Mechanistic studies, the design of improved
photocatalysts and their application to other types of reactions
are currently investigated in our laboratory and will be reported
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Acknowledgements
We specially thank UPMC, Cnam and CNRS. MENRT is grate-
fully acknowledged for the graduate fellowship awarded to
K. Fidaly.
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Notes and references
‡Typical procedure for enantioselective α-alkylation of aldehydes (syn-
thesis of 4a): an overnight oven dried Pyrex glass vial was equipped
with a septum and a dried magnetic stir bar. Rose Bengal (0.0025 mmol,
0.005 equiv, 2.5 mg), the imidazolidinone 1 (0.075 mmol, 0.15 equiv,
24.0 mg), anhydrous LiCl (prior oven dried, 0.050 mmol, 0.10 equiv,
2.1 mg) and diethyl bromomalonate 3a (0.50 mmol, 1.0 equiv, 84 μL)
were added successively. After purging the container with argon for
1 min, anhydrous DMSO (0.5 M, 1 mL) was added followed by hydro-
cinnamaldehyde 2a (1.0 mmol, 2.0 equiv, 132 μL) and 2,6-lutidine
(1.0 mmol, 2.0 equiv, 115 μL). Then the stirred solution was degassed
for 10 min under argon bubbling and the mixture was placed 2 cm from
the 24 W 6500 K 1425 lm fluorescent light source for irradiation until
the complete conversion of the bromide after 2 h (monitored by TLC
and/or GC/MS analysis). Then 10 mL of water was added and the result-
ing solution was extracted with EtOAc (3 × 10 mL). The combined
organic layers were dried over anhydrous MgSO4 and concentrated in
vacuo. The crude product was purified by silica gel chromatography
using cyclohexane–EtOAc (95 : 5) to afford the desired alkylation
product 4a (89% yield, 83% ee) as a colorless oil.
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16 See the ESI† for further information on the triplet quantum yield.
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various sunlight expositions, and same results (yield and ee) were
observed.
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