Visible-light-promoted stereoselective alkylation by combining heterogeneous photocatalysis with organocatalysis

Article abstract of DOI:10.1002/anie.201108721

Dream team: Heterogeneous inorganic semiconductors and chiral organocatalysts team up for the stereoselective photocatalytic formation of carbon-carbon bonds. However, the connection between the organic and inorganic catalysts should not be too tight: Covalent immobilization inactivates the system. Copyright

Full text of DOI:10.1002/anie.201108721

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Angewandte
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DOI: 10.1002/anie.201108721
Stereoselective Photocatalysis
Visible-Light-Promoted Stereoselective Alkylation by Combining
Heterogeneous Photocatalysis with Organocatalysis**
Maria Cherevatskaya, Matthias Neumann, Stefan Fꢀldner, Christoph Harlander,
Susanne Kꢀmmel, Stephan Dankesreiter, Arno Pfitzner, Kirsten Zeitler, and Burkhard Kçnig*
Dedicated to Dr. Wolf-Dieter Haack on the occasion of his 80th birthday
The application of sensitizers to utilize visible light for
chemical reactions is an established method.[1] Several
recent publications[2] have impressively demonstrated the
versatile use of visible light for various transformations, such
as the conversion of alcohols to alkyl halides,[3] and [2+2],[4]
[3+2],[5] and [4+2][6] cycloadditions as well as carbon–
carbon[7] and carbon–heteroatom bond formations.[8] The
combination of organocatalysis with visible-light photoredox
catalysis using ruthenium or iridium complexes[9] or organic
dyes[9d] as photocatalysts allows for an expansion to enantio-
selective reactions.[10] Although inorganic semiconductors,
such as titanium dioxide, have been widely used for the
photocatalytic degradation of organic waste,[11] the number of
examples in which they photocatalyze bond formation in
organic synthesis is still limited.[12] Kisch and co-workers[13]
Table 1: Enantioselective alkylations using MacMillan’s chiral secondary
amine and inorganic semiconductors as photocatalysts.
Entry
Photocat.[a]
l [nm][b]
t [h]
T [8C]
Yield 9
ee [%][d]
[%][c]
1
2
3
4
5
6
7
8
1
440
440
440
440
530
530
440
440
440
440
455
455
20
20
3
20
20
20
55
60
76
40
55
65
69
40
84
49
41
69
71
72
74
83
72
81
71
84
72
83
71
80
1[e]
1[f]
1
20
20
20
20
20
20
20
3
À10
2
2
3
3
4
4
20
À10
20
À
explored CdS-mediated bond formations, and oxidative C C
coupling reactions with titanium dioxide[14] are known.
However, bond formations on heterogeneous photocatalysts
typically proceed without control of the stereochemistry and
mixtures of isomers are obtained.[15,16] We demonstrate herein
that the combination of stereoselective organocatalysis with
visible-light heterogeneous photoredox catalysis promotes
the stereoselective formation of carbon–carbon bonds in good
selectivity and yield. The approach combines the advantages
of heterogeneous catalysis (robust, simple, and easy-to-
separate catalyst material) with the stereoselectivity achieved
in homogeneous organocatalysis.[17,18]
À10
9
20
10
11
12
À10
20
À10
4[f]
4[f]
10
[a] 64 mg of photocatalyst per 1 mmol of 6 in 2.5 mL of degassed
CH3CN. [b] High-power LED (440, 455, or 530 nm Æ10 nm, 3 W,
LUXEON as indicated). [c] Yield of isolated product. [d] Determined by
HPLC on a chiral stationary phase or by NMR spectroscopy using a chiral
diol.[27] [e] Photocatalyst reused. [f] Irradiation in a microreactor in
1.5 mL of CH3CN.
The enantioselective a-alkylation of aldehydes developed
by MacMillan et al.[9a] was selected as a test reaction to apply
inorganic heterogeneous photocatalysts (Table 1). Five semi-
conductors were used: commercially available white TiO2
(1),[19] the same material surface-modified covalently with
a Phos-Texas Red dye increasing the absorption of visible
light (Phos-Texas-Red-TiO2, 2), yellow PbBiO2Br, which
absorbs blue light, and PbBiO2Br as bulk material (3) and
in nanocrystalline form (4). TiO2 (1) with an average particle
size of 21 nm is a stable and inexpensive semiconductor with
a band gap of 3.2 eV, but the unmodified powder absorbs only
weakly up to 405 nm as a result of to defects and surface
deposits.[20] Its absorption range can be extended into the
visible range by structure modification[21] or surface modifi-
cation with dyes.[22,23] The Texas Red derived dye 10[24]
(Scheme 1) was covalently anchored on TiO2 yielding 2,
which absorbs at 560 nm (see the Supporting Information for
the synthesis of 10 and the characterization of 2). PbBiO2Br 3
and 4 were prepared by different synthetic routes leading to
different particle sizes of the semiconductors: PbBiO2Br bulk
material 3 with a band gap of 2.47 eV was prepared by high-
temperature solid-phase synthesis,[25] while the nanocrystal-
line material 4 was obtained from synthesis in aqueous
solution leading to an average calculated particle size of (28 Æ
6) nm and an optical band gap of 2.56 eV. Yellow CdS (5) has
a band gap of 2.4 eV and was prepared as previously
reported.[26]
[*] M. Sc. M. Cherevatskaya, M. Sc. M. Neumann, Dr. S. Fꢀldner,
M. Sc. C. Harlander, Dipl.-Chem. S. Kꢀmmel, M. Sc. S. Dankesreiter,
Prof. Dr. A. Pfitzner, Priv.-Doz. Dr. K. Zeitler, Prof. Dr. B. Kçnig
Fakultꢁt fꢀr Chemie und Pharmazie, Universitꢁt Regensburg
Universitꢁtsstrasse 31, 93040 Regensburg (Germany)
E-mail: burkhard.koenig@chemie.uni-regensburg.de
[**] Financial support from the Deutsche Forschungsgemeinschaft
(GRK 1626, chemical photocatalysis) is acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108721.
4062
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Angew. Chem. Int. Ed. 2012, 51, 4062 –4066

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Chemie
the semiconductor surface may lead to its rapid oxidative
photodecomposition. The non-immobilized catalyst, mostly
present in solution as enamine, will only very rarely encounter
the surface as the free amine and is thereby protected from
oxidative decomposition.
Our attempts to use CdS (5) for this transformation were
not successful. A comparison of the relevant potentials of the
widely employed photocatalyst [Ru(bpy)3]Cl2 and the inves-
tigated semiconductors explains the observation. [Ru(bpy)3]+
is proposed as the electron donor with a potential of À1.33 V
(SCE). The conduction band potential of TiO2 at À2.0 V
(SCE) in acetonitrile is sufficient for this step, while the
corresponding reported potential for CdS in acetonitrile of
À1.05 V may be too low (Figure 1).[28,29] On the other hand,
the reductive potential for the quenching of excited [Ru-
Scheme 1. Compounds for covalent surface immobilization on TiO2:
Phos-Texas-Red (10) and the chiral organocatalyst 11.
The a-alkylation of aldehyde 7 in the presence of
20 mol% of the secondary amine 8 as a chiral catalyst and
unmodified TiO2 afforded product 9 in moderate yield and
good enantioselectivity after extended irradiation time
(Table 1, entry 1), as only a small fraction of the visible light
at 440 nm was absorbed. TiO2 can be reused giving similar
results (Table 1, entry 2). With higher light intensity in
a microreactor set up (Table 1, entry 3) the reaction time
can be reduced to 3 h. Lowering the reaction temperature to
À108C increased the stereoselectivity of the reaction to
83% ee, but slowed down the reaction significantly (Table 1,
entry 4). When surface-modified TiO2 2 was used, the
reaction could be run with green light (530 nm; Table 1,
entries 5 and 6) yielding 65% product in 81% ee at À108C.
PbBiO2Br (3) absorbs in the visible range and catalyzed the
reaction with blue light (Table 1, entries 7 and 8). However,
its surface area, only 0.17 m2 gÀ1, is low compared to that of
TiO2 (50 m2 gÀ1). This explains the still rather long reaction
time. Nanocrystalline PbBiO2Br (4) has a larger surface area
(10.8 m2 gÀ1) and at room temperature and 440 nm irradiation
the product was isolated with a yield of 84% and 72% ee after
20 h (Table 1, entry 9). Again, the stereoselectivity increased
to 83% ee at À108C, but with lower conversion (Table 1,
entry 10). The reaction times could be reduced to 3 and 10 h
(Table 1, entries 11 and 12, respectively), with yields of 69%
and ee values of 80%. The reuse of 4 is possible, but black
organic surface deposits led to significantly slower conver-
sions.
Figure 1. Band gaps (in eV) and redox potentials (in V vs. SCE) of
common inorganic semiconductors in comparison with redox poten-
tials of molecular photocatalysts and redox potentials of some photo-
catalytic key steps. *Estimated change of the flat band potential of
PbBiO2Br in acetonitrile. Values given for Ru relate to [Ru(bpy)3]2+
;
values for Ir are related to fac-[Ir(ppy)3]. bpy = 2,2’-bipyridyl; ppy =
phenylpyridyl.
The mechanism of the alkylation reaction presumably
follows the proposed pathway for photoredox catalysis (see
the Supporting Information for a scheme). Electron transfer
from the conduction band of the semiconductor to the
halogenated carbonyl compound results in the loss of
a bromide anion and generates the a-carbonyl radical,
which adds to the enamine obtained by condensation of the
chiral catalyst with octanal. The a-amino radical is then
oxidized by a hole of the valence band yielding the iminium
ion that releases catalyst and product.
In an attempt to create a completely heterogeneous
catalyst system we prepared the chiral amine phosphonate
ester 11 (Scheme 1; see the Supporting Information for the
synthesis) and immobilized it on TiO2. However, the catalyst
system is inactive and no product formation could be
observed under conditions identical to those used before.
The close proximity of the secondary amine organocatalyst to
(bpy)3]2+* leading to the oxidation of the a-amino radical
intermediate is estimated to be + 0.84 V (SCE), which is
matched by the hole potentials (all vs. SCE in acetonitrile) of
TiO2 (+ 1.0 V)[29,30] and CdS (+ 1.35 V).
The combination of heterogeneous inorganic and homo-
geneous organic catalysts is applicable to other substrates,
such as bromoacetophenone (12a). For the conversion of the
more difficult to reduce dinitro benzylbromide (12b) iridium
complexes are required in the case of homogeneous photo-
catalysis.[9c] However, the estimated conduction band poten-
tials of TiO2 and PbBiO2Br in acetonitrile (Figure 1) should
be still sufficient and we indeed could observe the clean
conversion to the expected products in good yield and high
stereoselectivity (Scheme 2).
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Table 2: Photocatalytic Mannich reaction of N-aryltetrahydroisoquino-
lines 14 with ketones 15 and l-proline on CdS (5)[a]
Scheme 2. Alkylation of 7 with bromoacetophenone (12a) in CH3CN or
with 2,4-dinitrobenzylbromide (12b) in DMSO, using chiral amine 8,
PbBiO2Br (4), and blue light.
Entry R1
1a
Ketone
Product
t [h] Yield [%][b]
24
24
24
24
86[c]
90[d]
100[e]
87
1b
1c
H
1d
Recently, several cross-dehydrogenative couplings[31] of
tetrahydroisoquinolines by homogeneous photocatalysis
using Ir- or Ru-based complexes[5b,c,7d,8b,d,15,31] or organic
dyes[33] such as Eosin Y[8a] have been reported. Here the
photocatalytic key step is the reductive quenching of the
excited chromophore leading to an amine radical cation,
which is subsequently converted into an electrophilic iminium
species. If one considers the use of inorganic semiconductors
for this reaction, the potential of the photogenerated holes in
the valence band is of importance. Based on the band gap and
its redox potential (see Figure 1) CdS should be a suitable
heterogeneous visible-light photocatalyst for oxidations to
generate the desired amine radical cation. The combination of
proline as the organocatalyst with CdS, as the inorganic
photocatalyst, indeed promotes the clean conversion of N-
aryltetrahydroisoquinolines 14 in a photooxidative Mannich
type reaction[32b,33a] with ketones 15 upon irradiation with blue
light of 460 nm. The products 16a–d arising from the reaction
with acyclic or cyclic ketones can be obtained in good yields of
76–89% (Table 2).[34] While the reaction can also be per-
formed successfully in CH3CN with a significantly reduced
amount of ketone (see Table 2, entry 1a–c), the reaction is
most conveniently run in neat ketone if inexpensive (liquid)
ketones are employed.
The flat band potentials of some common inorganic (and
organic) semiconductors are summarized in Figure 1.[35]
Importantly, at different pH values and in different organic
solvents, these values shift significantly and the currently
available data for organic solvents are limited. However, by
comparing the semiconductor flat band potentials with the
potentials required for catalytic key steps from known
photoredox catalysts (e.g. Ru and Ir complexes, xanthene
dyes, etc.), suitable combinations of (inorganic) semiconduc-
tors with organocatalysts can be predicted.
2
3
4
OMe
18
24
15
89
79
76
H
H
[a] Unless otherwise noted all experiments were performed with amine
(1 equiv) and l-proline (0.2 equiv) in a 5 mgmLÀ1 mixture of CdS in neat
ketone (camine =0.25 molLÀ1). Reactions were run in Schlenk tubes with
an attached oxygen balloon and irradiated with high-power LEDs
(460 nm) for the time indicated. [b] Given yields correspond to isolated
product. [c] Reaction performed in CH3CN with 2 equiv of acetone; the
conversion was determined by GC analysis. [d] Reaction performed in
CH3CN with 5 equiv of acetone; the conversion was determined by GC
analysis. [e] Reaction performed in CH3CN with 10 equiv of acetone; the
conversion was determined by GC analysis.
The good availability of inorganic semiconductors with
different band gaps and redox potentials, their simple removal
from the reaction mixture, and recycling make them the
perfect partners for chiral organocatalysts in stereoselective
photocatalysis.
We have demonstrated that the appropriate combination
of heterogeneous semiconductor photocatalysts with chiral
organocatalysts can used to promote different types of
stereoselective bond formation by visible-light photocatalysis.
The yields and stereoselectivities are comparable to those of
previously reported homogeneous reactions using transition-
metal complexes or organic dyes. Electrons are exchanged in
the course of the reaction between the chiral reaction
intermediates in solution and the semiconductor surface, if
the redox potentials of substrates and band gaps match. The
covalent immobilization of the organocatalyst on the semi-
conductor surface leads to its oxidative decomposition and
must be avoided.
Received: December 11, 2011
Revised: January 17, 2012
Published online: March 13, 2012
Keywords: alkylation · organocatalysis · photocatalysis ·
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semiconductors · visible light
´
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