Enantioselective rhodium/ruthenium photoredox catalysis: en route to chiral 1,2-aminoalcohols

Article abstract of DOI:10.1039/c6cc04397f

A rhodium-based chiral Lewis acid catalyst combined with [Ru(bpy)₃](PF₆)₂ as a photoredox sensitizer allows for the visible-light-activated redox coupling of α-silylamines with 2-acyl imidazoles to afford, after desilylation, 1,2-amino-alcohols in yields of 69-88% and with high enantioselectivity (54-99% ee). The reaction is proposed to proceed via an electron exchange between the α-silylamine (electron donor) and the rhodium-chelated 2-acyl imidazole (electron acceptor), followed by a stereocontrolled radical-radical reaction. Substrate scope and control experiments reveal that the trimethylsilyl group plays a crucial role in this reductive umpolung of the carbonyl group.

Full text of DOI:10.1039/c6cc04397f

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Enantioselective rhodium/ruthenium photoredox catalysis en
route to chiral 1,2-aminoalcohols
Jiajia Ma,a Klaus Harmsa and Eric Meggers*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcomm
A rhodium-based chiral Lewis acid catalyst combined with
[Ru(bpy)3](PF6)2 as photoredox sensitizer allows for the
visible-light-activated redox coupling of α-silylamines with 2-
acyl imidazoles to afford, after desilylation, 1,2-amino-
alcohols in yields of 69-88% and with high enantioselectivity
(54-99% ee). The reaction is proposed to proceed via an
electron exchange between the α-silylamine (electron donor)
and the rhodium-chelated 2-acyl imidazole (electron
acceptor), followed by a stereocontrolled radical−radical
reaction. Substrate scope and control experiments reveal
that the trimethylsilyl group plays a crucial role in this
reductive umpolung of the carbonyl group.
Organosilicon compounds are of tremedeous utility in
organic synthesis.1 For example, silyl groups introduced in the
α-position of amines serve as convenient precursors for the
mild generation of α-aminoalkyl radicals (Figure 1).2 The silyl
group in α-silylamines not only serves as a redox handle to
facilitate a single electron oxidation (SET, single electron
transfer),3 but also results in a subsequent rapid cleavage of
the C-Si bond under release of α-aminoalkyl radicals, which
then may either add to a variety of electron-deficient alkenes
or become involved in iminium chemistry after further
oxidation. Here we report a less common reactivity of α-
silylamines, namely a visible-light-induced redox-coupling with
ketones,4 and demonstrate how to perform this reaction under
conditions of enantioselective catalysis.5
Fig. 1 α-Aminoalkyl radical generation and follow-up chemistry with α-
silylamines serving as convenient radical precursors.
electron-acceptor. In order to expand the substrate scope, we
turned our attention to dual catalysis, namely using a chiral
Lewis acid in combination with a separate photosensitizer.7,8
Encouragingly, the chiral-at-metal rhodium-based Lewis acid
Λ-
RhO9-11 together with the established photosensitizer
[Ru(bpy)3](PF6)2 (Rubpy) catalyzed the photoactivated reaction
of the benzoyl imidazole 1a with the α-silylamine 2a to provide
under C-C bond formation the silylether product 3a’ in 68%
and with 41% ee (Table 1, entry 1).12 Removing the
trimethylsilyl (TMS) group with TBAF ahead of the workup
increased the yield of the corresponding alcohol 3a to 73%
(entry 2). The enantioselectivity of the aminoalcohol coupling
product was then stepwise improved by increasing the steric
bulk of the substituent at the imidazole nitrogen (R1, entries 3-
We recently reported the photoredox coupling of tertiary
amines with 2-trifluoroacetyl imidazoles under stereoselective
formation of 1,2-aminoalcohols, catalyzed by a chiral iridium
complex as dual chiral Lewis acid / photoredox catalyst.6
However, this method is limited to ketones containing α-CF3
groups, apparently to render the carbonyl compound a better
a.Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, 35043 Marburg,
Germany. Email: meggers@chemie.uni-marburg.de
5), with the best result obtained for
substituent, reaching 93% ee. Importantly, replacing the
rhodium-catalyst with the analogous iridium congener
IrO13-
a 2-phenylphenyl
b.College of Chemistry and Chemical Engineering, Xiamen University, Xiamen
361005, People’s Republic of China.
Λ
-
15
Electronic Supplementary Information (ESI) available: Experimental details and
analytical data. See DOI: 10.1039/x0xx00000x
abolished product formation, both in the presence or
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absence of an additional photosensitizer (entries 6 and 7).
However, the previously developed second-generation chiral-
at-metal rhodium-based Lewis acid
Λ- , in which the
RhS 16
benzoxazole ligands are replaced with benzothiazoles,
provided in combination with [Ru(bpy)3]2+ the best results
with 76% yield and 95% ee for the reaction 1d
+ 2a → 3d
(entry 8).17 Control experiments confirm that this reaction
requires both visible light and the ruthenium photosensitizer
(entries 9 and 10). Furthermore, the TMS group appears
crucial for this reaction as a substrate (2b) with the more bulky
bis(cyclopropyl)methylsilyl group affording only 7% yield of the
aminoalcohol (entry 11), whereas a related amine (2c) devoid
of any silyl group suppressed product formation completely
(entry 12).
Table 1 Initial experimentsa
Fig. 2 Substrate scope with respect to α-silylamines.
was well tolerated, providing the C-C-coupling product 3g in
78% yield and with a slightly reduced 93% ee. A reaction with
N-(trimethylsilymethyl)-1,2,3,4-tetrahydroquinoline provided
the 1,2-amino alcohol 3h with 70% yield and 95% ee. N,N-
Diaryl-N-(trimethylsilylmethyl)amines also gave satisfactory
results (products 3i and 3j).
entry cat
sensb
h
νc cpd
1
cpd
2
t (h) yield (%)d ee (%)e
1
Λ-
Λ-
Λ-
Λ-
Λ-
Λ-
Λ-
Λ-
Λ-
Λ-
Λ-
Λ-
RhO Rubpy yes 1a
RhO Rubpy yes 1a
RhO Rubpy yes 1b
RhO Rubpy yes 1c
RhO Rubpy yes 1d
IrO Rubpy yes 1d
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
14 68 (3a’
14 73 (3a
14 74 (3b
)
41
41
82
88
93
0
2
)
3
)
4
14 73 (3c
14 72 (3d
24 50 (3d
24 43 (3d
)
5
)
)
)
)
6
7
IrO none
yes 1d
3
8
RhS Rubpy yes 1d
RhS Rubpy none 1d
5
76 (3d
95
n.d.
n.d.
79
n.d.
9
20 n.r.
20 n.r.
20 7 (3d
20 n.r.
10
11
12
RhS none
yes 1d
RhS Rubpy yes 1d
)
RhS Rubpy yes 1d
aReaction conditions: To a mixture of 1a
(1.0 mol% or none), and catalyst (4 mol% or none) in MeCN/DMAC (4:1)
(1ml) was added the amine 2a (0.15 mmol). The reaction mixture was
-d (0.1 mmol), [Ru(bpy)3](PF6)2
-c
degassed and stirred at r.t. under nitrogen and, except for entry 9,
irradiated with a 23 W CFL. Afterwards, the solvent was evaporated to
dryness and the residue was redissolved in THF, then treated with TBAF
(0.5 mmol). The TBAF addition was omitted for entry 1. The product
was isolated by flash chromatography.
bRubpy = [Ru(bpy)3](PF6)2.
c23 W compact fluorescent lamp (CFL) at a distance of approx. 5 cm
from Schlenk tube.
dIsolated yields. n.r. = no reaction.
eDetermined by HPLC on chiral stationary phase. n.d. = not determined.
Having discovered this reaction, we next investigated the
substrate scope with respect to α-trimethylsilyalkylamines (Fig.
2). Accordingly, both para- and meta-methylated N-phenyl
moieties within the α-silylamines afforded the individual
products (3e and 3f) in very good yields (80-81%) and with
excellent enantioselectivities (97-98% ee). An N-benzyl group
Fig. 3 Substrate scope with respect to 2-acyl imidazoles.
2 | J. Name., 2012, 00, 1-3
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demonstrated by trapping with an electron deficient alkene.
These experiments support the mechanistic picture that the
trimethylsilyl group exerts an important role as an additional
Lewis acid22,23 to activate the rhodium-coordinated 2-acyl
imidazole and facilitates a reduction by lowering the ligand-
centered LUMO, whereas the related di(cyclopropyl)-
methlysilyl group is too bulky to interact with the rhodium-
coordinated substrate.
Next, the substrate scope with respect to 2-acyl imidazole was
evaluated. Fig. 3 reveals that both aromatic as well as aliphatic
substituents at the acyl group are tolerated (substituent R in Fig. 3),
providing the respective 1,2-amino alcohols 3k-s in 70-88% yield
and with enantioselectivities of 54-99% ee. With respect to
substituents in the aromatic moiety, methyl groups (products 3k
and 3l) and
a phenyl group (product 3q) gave excellent
enantioselectivities of 96-98% ee. An acetoxy substituent also
afforded the product 3p with high yield and high enantioselectivity.
However, more strongly electron withdrawing or electron accepting
substituents led to inferior results. For example, while a chloro
substituent still provided the product 3m with high yield (88%) and
satisfactory enantioselectivity (93% ee), a methyl ester or methoxy
group in para-position afforded the products 3n and 3o only with
54% ee and 86% ee, respectively.
tBu
S
Re face blocked
N
(2-Ph)Ph
N
+ N
R3
Rh
O
Me3Si
N
R1
tBu
A proposed mechanism is illustrated in Fig. 4. Accordingly,
visible light activated [Ru(bpy)3]2+ oxidizes in a well-established
mechanism the silylmethylamine to the corresponding radical
cation, which subsequently undergoes a rapid desilylation to
provide an α-aminomethyl radical.2,3,18 To take into account
the observed silyl effect, we propose that the released
trimethylsilyl group is captured by the rhodium-coordinated 2-
S
N
R2
I
III
intermediate
intermediate
Fig. 5 Crystal structure of a derivative of intermediate I (CCDC no. 1480695)
and the proposed structure of intermediate III with the indicated
stereocontrolled radical-radical recombination.
In conclusion, we have discovered a visible-light-driven
enantioselective radical-radical cross-coupling promoted by
the cooperativity between a rhodium-based chiral Lewis acid
and a ruthenium photosensitizer and in which a trimethylsilyl
group appears to play a crucial role as an in situ released Lewis
acid. In this way synthesized nonracemic 1,2-aminoalcohols
represent a reductive umpolung of the carbonyl reactivity
triggered by a photoinduced electron transfer.24,25
acyl imidazole (intermediate I) to afford the very electron
deficient silylated intermediate II. This is followed by an
electron transfer from the the reduced sensitizer to regenerate
the photosensitizer in the ground state and provide a rhodium
coordinated, silylated ketyl (intermediate III), which then
undergoes a radical-radical recombination19,20 with the α-
aminomethyl radical to form the rhodium-coordinated C-C
coupling product (intermediate IV). Replacement of the
product (3) by a new substrate (1) will then initiate a new
catalytic cycle. Thus, in this mechanism, Rubpy serves as a
light-activated electron shuttle between the α-silylamine
(electron donor) and the 2-acyl imidazole (electron acceptor),
followed by a radical-radical coupling, whose stereochemistry
is controlled by the chiral rhodium Lewis acid. Fig. 5 shows a
crystal structure of a derivative of intermediate
I and the
structure of the proposed intermediate III which can
rationalize the observed S-configuration of the products.
Fig. 6 Mechanistic experiments.
We thank the German Research Foundation (DFG) for
financial support of this research (ME 1805/13-1).
Notes and references
‡Representative reaction: A 10 mL Schlenk tube was charged with Λ-RhS (5.20
mg, 0.0060 mmol), [Ru(bpy)3](PF6)2 (1.30 mg, 0.0015 mmol) and 1m (39.3 mg,
0.15 mmol). A solution of 2j (103 mg, 0.45 mmol) in MeCN/DMAC (4:1, 1 mL) was
added. After degassing via three freeze-pump-thaw cycles, the Schlenk tube was
sealed and positioned at a distance of 5 cm from a 23 W compact fluorescent
lamp. The reaction was stirred at r.t. for 15 h under nitrogen. Afterwards, the
mixture was concentrated under reduced pressure, redissolved in THF (2 mL),
and TBAF (1.0 mL, 1.0 M in THF, 1.0 mmol) was added. The mixture was stirred at
r.t. for 0.5 h and quenched with saturated aqueous solution of NH4Cl. After
Fig. 4 Proposed mechanism for the observed silyl effect in the visible-light
activated Rh/Ru dual catalysis. SET = single electron transfer.
extraction with EtOAc (3
× 10 mL), the combined organic layers were
Control experiments shown in Fig. 6 demonstrate that the
silyl group, although crucial for observing the C-C-coupling
product with an 2-acyl imidazole, is actually not required for
the production of intermediate α-aminomethyl radicals,21 as
concentrated and subjected to a flash chromatography on silica gel (EtOAc/n-
hexane = 1:7 to 1:4) to afford the product 3s as a white solid (53.0 mg, 0.13 mmol,
86% yield). Enantiomeric excess of 99% was determined by HPLC (Chiralpak OD-H
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Marsch, G. Hilt and E. Meggers, Nature, 2014, 515, 100–103.
4 | J. Name., 2012, 00, 1-3
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