Photoredox activation and anion binding catalysis in the dual catalytic enantioselective synthesis of β-amino esters

Article abstract of DOI:10.1039/c3sc52265b

The enantioselective oxidative C-H functionalization of tetrahydroisoquinoline derivatives is achieved through the merger of photoredox and asymmetric anion-binding catalysis. This combination of two distinct catalysis concepts introduces a potentially general approach to asymmetric transformations in oxidative photocatalysis.

Full text of DOI:10.1039/c3sc52265b

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DOI: 10.1039/C3SC52265B
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ARTICLE TYPE
Photoredox Activation and Anion Binding Catalysis in the Dual
Catalytic Enantioselective Synthesis of β-Amino Esters
a
b
a
b
Giulia Bergonzini, Corinna S. Schindler, Carl-Johan Wallentin, Eric N. Jacobsen,* and Corey R. J.
Stephenson*
a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The enantioselective oxidative C-H functionalization of tetrahydroisoquinoline derivatives is achieved
through the merger of photoredox and asymmetric anion-binding catalysis. This combination of two
distinct catalysis concepts introduces a potentially general approach to asymmetric transformations in
oxidative photocatalysis.
1
0
Introduction
Photoredox catalysis has emerged recently as a powerful tool for
1
organic synthesis. Irradiation of photoactive catalysts with
visible light can be parlayed to the practical generation of
synthetically useful high-energy organic intermediates such as
1
2
2
3
3
4
4
5
0
5
0
5
0
5
2
3,4
free radicals and radical anions and cations. Successful efforts
to induce enantioselective catalytic control in such
photocatalyzed processes have relied thus far on covalent
organocatalysis: photoredox aminocatalysis as introduced by
Fig. 1 Merger of photoredox and anion-binding catalysis in oxidative
2a,5
MacMillan, and the combination of photoredox catalysis with
5
0
enatioselective C-H functionalization of amines
6
,7
N-heterocyclic carbene catalysis as developed by Rovis. The
main text of the article should appear here.
Results and Discussion
Since many reactions initiated by visible light photocatalysis
8
involve the reductive generation of halide anions, we considered
In initial experiments, the proper choice of solvent systems
was revealed as a key challenge. In relatively polar media
(CH CN, CH Cl , DMF), with CCl as the stoichiometric oxidant,
3 2 2 4
9
whether the use of chiral anion-binding catalysts in combination
with photocatalysis would provide opportunities for new types of
1
0,11
enantioselective transformations
In one specific embodiment 55 the oxidative photocatalytic Mannich reaction of N-
of this idea, we envisioned that iminium ion equivalents
generated under mild conditions from tertiary amines by
phenyltetrahydroisoquinoline (1a) and silyl ketene acetal 2
afforded 3a cleanly in the presence of chiral thiourea catalysts,
1
2
photocatalyzed oxidation might be trapped in stereoselective
but always in racemic form.
In contrast, unreacted
nucleophilic addition reactions under the influence of a chiral H-
bond donor catalyst (Figure 1). We describe here the successful 60 oxidations in non-polar solvents such as MTBE, which are
development of a dual catalyst strategy for the enantioselective
oxidative alkylation of tetrahydroisoquinolines with silyl ketene
acetals.
tetrahydroisoquinoline 1a could be recovered from attempted
optimal for attaining high enantioselectivity in anion-binding
thiourea catalysis. The failure of the photoredox reaction under
these conditions could be ascribed simply to the lack of solubility
of the photocatalyst, even when relatively non-polar complexes
We chose to evaluate the proposed dual catalytic reaction
design in enantioselective oxidative Mannich reactions to access 65 such as Ru(bpy)
tetrahydroisoquinoline derived -amino esters such as 3a (Table
enantioselective oxidative Mannich reaction could be achieved by
performing the photocatalytic oxidation in CH CN and switching
(PF
)
were employed. Ultimately, the
3
6
2
13 14
,
1
).
In this context, we reported recently a photocatalytic
3
method for the oxidative generation of iminium ion precursors
solvents to MTBE for the thiourea-catalyzed alkylation reaction.
In this manner, full conversion of 1a to the corresponding
from simple N-aryltetrahydroisoquinolines such as 1a using
15
16 17
bromotrichloromethane as the stoichiometric oxidant. , ,
Coupling this protocol to an enantioselective thiourea-catalyzed
addition of enolate equivalents such as silyl ketene acetal 2 would
require identification of the appropriate photoredox and chiral
thiourea catalysts, as well as reaction conditions suitable for both
transformations.
70 iminium ion in CH
LEDs for 16 hours using
3
CN was observed after irradiation with blue
equiv of CCl and
2
4
,
tetrahydroisoquinoline 3a was obtained in 50% ee upon
alkylation with 2 in the presence of chiral thiourea 5a at –78 °C.
Enantioselective oxidative Mannich reactions of 1a and silyl
75 ketene acetal 2 were evaluated with a variety of thiourea catalysts
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Table 1 Thiourea catalyst optimization
Table 2 Counterion effects
a
b
Determined by HPLC on commercial chiral columns. Isolated yield
c
1
after purification by chromatography on silica gel. Determined by
H
4
0
NMR spectroscopic analysis of the crude reaction mixture relative to 2,5-
dimethylfuran as the internal standard. Range of enantiomeric excess
d
a
o
b
c
entry
thiourea
T (°C)
[1a] (M) yield[%]
ee[%]
achieved over >10 runs.
catalyst
1
2
3
4
5
6
7
8
9
-
4
–78
–78
–78
–78
–78
–78
–60
–60
–60
–60
–60
0.1
0.1
0.1
0.1
0.1
0
N/A
10
50
20
80
29
93
44
87
75
68
63
57
63
37
68
63
63
72
5a
5b
5c
5d
5c
epi-5c
5e
5f
0.1
0.05
0.05
0.05
0.05
0.05
d
1
1
0
1
85
–97
ent-5g
a
b
Reaction temperature for the alkylation step in MTBE. Yields
1
determined by H NMR spectroscopic analysis of the crude reaction
mixture relative to 2,5-dimethylfuran as the internal standard.
5
0
5
0
5
0
5
c
d
Determined by HPLC on commercial chiral columns. Pyrrolodine
configuration is (R), tert-leucine configuration is (S).
(
Table
1).
Only
catalysts
bearing
both
3,5-
1
1
2
2
3
3
bis(trifluoromethyl)aniline and tertiary amide components were
found to promote useful reaction rates at –78 ˚C. Constrained
arylpyrrolidine-derived thiourea catalysts of the general structure
5
1
8
,
which have proven effective in
a
wide range of
1
9,20
enantioselective transformations,
induced significantly higher
ee’s than catalysts bearing a less constrained tertiary amide
fragment (e.g. 4). In that context, the relative configuration of the
arylpyrrolidine unit relative to the tert-leucine component had a
significant impact on reaction enantioselectivity (entries 7-8),
pointing to the importance of the precise disposition of the aryl
group relative to the thiourea anion-binding site. No
straightforward correlation between the expanse of the -aryl
substituent on the pyrrolidine and reaction enantioselectivity was
observed (Table 1, entries 3-6). In contrast, substitution on the
ortho-position of the arylpyrrolidine resulted in significant
improvements in enantioinduction, although the electronic
properties of the substituent had little impact (entries 9-11).
Further reaction optimization using catalyst 5c revealed a
significant improvement in enantioselectivity when the reaction
was conducted at –60 °C with an initial concentration of 0.05M
(entries 5 and 7). After extensive evaluation of different
arylpyrrolidine-derived analogs of 5, thiourea 5g was identified
as an optimal catalyst for the model transformation of 1a to 3a.
During the course of the optimization studies outlined above,
we observed that the identities of both the photocatalyst
counterion and the halogen atom source had significant effects on
Fig. 3 Substrate scope
4
5
5
6
5
0
5
0
enantioselectivity in the thiourea-catalyzed silyl ketene acetal
addition reaction (Table 2). Consistently lower enantioselectivity
was observed when using BrCCl as the stoichiometric oxidant
3
compared with CCl , a result that is attributable to more favorable
interactions of the chloride-bound thiourea catalyst in the
4
2
1
enantioselectivity-determining transition structure.
When
was used as the photocatalyst in place of
together with CCl as the stoichiometric oxidant,
Ru(bpy)
Ru(bpy)
3
(PF
6 2
)
3
Cl
2
4
highly variable enantioselectivities were obtained (e.g. 61-97% ee
with catalyst 5g). This unpredictable behavior associated with the
use of the Ru(bpy) (PF ) photocatalyst appears to be tied to the
3
6 2
heterogeneous nature of the alkylation reaction mixture. Although
only 2 mol % of PF6 is present in the reaction medium and the
–
iminium ion intermediate is predominantly associated with
–
–
–
chloride (Cl /PF
6 6
= ca. 51:1), the PF may impart enhanced
solubility properties to the iminium ion, and thereby enable the
2
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racemic, background reaction to a greater extent.
Acknowledgements
With optimized conditions in hand we next explored the
substrate scope of the reaction (Figure 2). The time required for
full conversion of the N-aryltetrahydroisoquinoline derivatives 1
in the oxidation reaction was highly dependent on the electronic
nature of the substrate, with electron-rich derivatives being more
readily oxidized. To ensure complete conversion to the iminium
ion, the amine was treated under the photochemical oxidation
conditions for 16 h in all cases. The position of substituents on
the N-aryl group influenced the enantioselectivity of the oxidative
alkylation addition more profoundly than did their electronic
properties. Thus, ortho-substituted substrates generally provided
higher ee compared to para-substituted analogs (3l vs 3b; 3j vs
Financial support for this research from the NIH-NIGMS (PO1
GM69721 and R01-GM43214) to ENJ and R01-GM096129 to
CRJS), the Alfred P. Sloan Foundation, Amgen, Boehringer
Ingelheim, Eli Lilly, and Novartis is gratefully acknowledged.
GB is grateful to the Spanish Ministry of Education and Science
5
5
5
(
MEC) for a FPU predoctoral fellowship (AP2009-0950).
Postdoctoral support from the Alexander von Humboldt
Foundation (Feodor Lynen fellowship to CSS) and the Swedish
Research Council (to CJW) is gratefully acknowledged.
6
0
1
1
2
2
3
3
4
0
5
0
5
0
5
0
Notes and references
a
3
d). In contrast, reaction enantioselectivities were very sensitive
Department of Chemistry, University of Michigan, 930 N University
Ave, Ann Arbor, USA. Fax: +1734-647-4865; Tel: +1-734-763-8283; E-
mail: crjsteph@umich.edu
to the electronic nature substituents on the tetrahydroisoquinoline
ring, with electron-rich systems generally affording lower ee’s
and/or yields (e.g. 3e, 3h, 3i).
6
7
5
0
b
Department of Chemistry, Harvard University, Cambridge, USA. Fax:
+
1-617496-3688; Tel: +1-617-496-1880; E-mail:
In order to better establish the synthetic utility of this chiral
tetrahydroisoquinoline synthetic methodology, we sought to
develop a protocol for N-dearylation of the products to provide
Jacobsen@chemistry.harvard.edu
† Electronic Supplementary Information (ESI) available: Detailed
experimental procedures and spectroscopic data of all new compounds.
See DOI: 10.1039/b000000x/
2
2
access to the more useful secondary amine derivatives.
Electron-rich anilines have favorable redox properties that render
‡
Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
2
3
them prone to N-aryl bond cleavage under oxidative conditions.
Accordingly, compound 3l, which was generated with nearly
perfect enantioselection in the oxidative Mannich reaction, was
chosen as a model substrate for dearylation studies. Application
of standard oxidative protocols for PMP or PMB removal (CAN,
1
2
For recent reviews, see (a) J. M. R. Narayanam, C. R. J. Stephenson,
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MacMillan, Chem. Rev. 2013, 113, 5322.
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Science 2008, 322, 77. (b) J. M. R. Narayanam, J. W. Tucker, C. R. J.
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PhI(TFA) , or DDQ) led to rapid consumption of 3l but low
2
yields of the desired secondary amine due to product
decomposition via over-oxidation. We anticipated that the use of
an oxidant more closely matched to the oxidation potential of the
substrate would have a better chance of avoiding decomposition
of the relatively sensitive secondary amine product. Evaluation of
3
+
Fe(III)-based oxidants led to the identification of [Fe(bpy) ] ,
3
2
011, 50, 951. (f) J. D. Nguyen, E. M. D’Amato, J. M. R.
2
4
formed in situ upon treatment of [Fe(bpy) ](PF ) with CAN, as
3
6 2
Narayanam, C. R. J. Stephenson, Nature Chem. 2012, 4, 854.
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a highly effective oxidant for this transformation, leading to clean
conversion to the desired amine 6. Comparison of the optical
rotation of 6 with the reported value enabled the assignment of its
absolute configuration. Acylation of 6 with acetyl chloride
provided 7 (95% ee, 87% yield starting from 3l) with no
significant compromise of enantiomeric purity.
3
4
5
(a) D. A. Nagib, M. E. Scott, D. W. C. MacMillan, J. Am. Chem. Soc.
2
009, 131, 10875. (b) H.-W. Shih, M. N. Vander Wal, R. L. Grange,
(
1)
D. W. C. MacMillan, J. Am. Chem. Soc. 2010, 132, 13600.
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photochemical transformations, as described by Bach et al. For
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Nature 2005, 436, 1139. (b) C. Müller, A. Bauer, T. Bach, Angew.
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6
7
Conclusions
In summary, we have developed an approach to the oxidative,
enantioselective C-H functionalization of tetrahydroisoquinoline
derivatives. A mild method for the selective dearylation of
products bearing N-o-anisyl groups was developed, thereby
enabling further derivatization of the tetrahydroisoquinoline
scaffold. Further applications of photoredox /chiral anion binding
dual catalysis are anticipated and are the subject of current study.
4
5
8
For selected examples, see refs 2, 5 and (a) J. D. Nguyen, J. W.
Tucker, M. D. Konieczynska, C. R. J. Stephenson, J. Am. Chem. Soc.
2
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5
0
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Sci. 2012, 3, 633.
Visible light photoredox catalysis has been successfully combined
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1
1
0
1
5
2
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6
950.
1
1
2
3
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1
4
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1
1
8
9
See the Supporting Information for catalysts evaluated in addition to
the catalysts presented in Table 1.
S. E. Reisman, A. G. Doyle, E. N. Jacobsen, J. Am. Chem. Soc. 2008,
1
30, 7198. (b) R. R. Knowles, S. Lin, E. N. Jacobsen, J. Am. Chem.
Soc. 2010, 132, 5030. (c) J. A. Birrell, J.-N. Desrosiers, E. N.
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