Enantioselective Lewis Acid Catalyzed ortho Photocycloaddition of Olefins to Phenanthrene-9-carboxaldehydes

Article abstract of DOI:10.1002/anie.201808919

Visible-light irradiation (λ=457 nm) enabled the enantioselective ortho photocycloaddition of olefins to phenanthrene-9-carboxaldehydes (15 examples, 46–93 % yield, 82–98 % ee). A chiral oxazaborolidine Lewis acid (20 mol %) was employed as the catalyst. It operates by coordination to the aldehyde inducing a bathochromic absorption shift beyond the nπ* absorption of the uncomplexed aldehyde. At long wavelengths the Lewis acid complex is exclusively excited; within the complex, one enantiotopic face of the aromatic aldehyde is efficiently shielded. Lewis acid coordination also alters the type selectivity and the simple diastereoselectivity of the photocycloaddition.

Full text of DOI:10.1002/anie.201808919

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Accepted Article
Title: Enantioselective Lewis Acid Catalyzed ortho Photocycloaddition
of Olefins to Phenanthrene-9-carboxaldehydes
Authors: Simone Stegbauer, Christian Jandl, and Thorsten Bach
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808919
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10.1002/anie.201808919
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
Photochemistry
Enantioselective Lewis Acid Catalyzed ortho Photocycloaddition of Olefins
to Phenanthrene-9-carboxaldehydes**
Simone Stegbauer, Christian Jandl, and Thorsten Bach*
Dedicated to all past and present members of the TUM family
Abstract. Visible light irradiation (
λ
= 457 nm) enabled the
reasoned that chromophores with a ππ* absorption at long
wavelength might allow for a selective excitation of the Lewis acid
complex without interference of a racemic non-catalytic reaction.
We have now shown that phenanthrene-9-carboxaldehydes fulfill
the above-mentioned requirements and we report in this
communication on their enantioselective catalytic photocyclo-
addition to olefins.
enantioselective ortho photocycloaddition of olefins to
phenanthrene-9-carboxaldehydes (15 examples, 46-93% yield, 82-
98% ee). A chiral oxazaborolidine Lewis acid (20 mol%) was
employed as the catalyst. It operates by coordination to the alde-
hyde inducing a bathochromic absorption shift beyond the nπ*
absorption of the uncomplexed aldehyde. At long wavelength the
Lewis acid complex is exclusively excited and within the complex
one enantiotopic face of the aromatic aldehyde is efficiently
shielded. Lewis acid coordination also alters the type selectivity
and the simple diastereoselectivity of the photocycloaddition.
O
9
10
I
n recent years there has been a constantly increasing interest in
the enantioselective catalysis of photochemical reactions. Several
approaches have been reported to face the challenge of selectivity
control in the excited state.[1] Among others, it has been shown that
acids can be employed to activate an α,β-unsaturated carbonyl
compound by coordination to the carbonyl group and by altering its
absorption properties.[2] Important contributions include the use of
chiral Brønsted acids,[3] the use of a chiral Sc-based Lewis acid in
combination with a triplet sensitizer,[4,5] and the use of chelating
Rh-based Lewis acids.[6] Chiral AlBr3-activated oxazaborolidines[7]
have been introduced by our group to promote enantioselective
photocatalysis and have been established as very successful
catalysts.[8] They operate on α,β-unsaturated carbonyl compounds
by lowering the energy of the allowed (ε ≥ 10000 M−1 cm−1) ππ*
transition and therefore shift this UV/Vis absorption band, which
typically occurs around 250-290 nm, by ca. 40-50 nm to longer
wavelength (bathochromic shift ∆λ). Since α,β-unsaturated
carbonyl compounds exhibit a weak (ε ≤ 100 M−1 cm−1) nπ*
transition which occurs also bathochromic relative to the ππ*
transition, there is in solution a competition between an excitation
of the respective Lewis acid complex and the uncomplexed
substrate. As a consequence undesired racemic background reac-
tions occur and the catalyst loading for an effective enantio-
selective photochemical reaction catalyzed by a chiral oxazaboro-
lidine has been high (50 mol%) so far.[8] A possible improvement
would be expected if it was possible to shift the above-mentioned
ππ* transition bathochromically beyond the nπ* transition of the
respective substrate. Since an identical energy difference ∆E
corresponds to a higher difference of ∆λ at longer wavelength, we
1a
Figure 1. UV/Vis spectrum of compound 1a (c = 0.25 m
in the presence of variable equivalents (eq.) of EtAlCl2.
M in CH2Cl2)
The UV/Vis spectrum of aldehyde[9] 1a (Figure 1) revealed an
absorption at 316 nm (ε = 13780 M−1 cm−1) which is assigned to a
ππ* transition (1Lb band).[10] The shoulder at 361 nm (ε = 1940 M−1
cm−1) has been previously assigned to the phenanthrene 1La band[10]
but it seems to partially overlap with the nπ* band of the aldehyde
carbonyl group. Indeed, photochemical reactions of 1a were
observed (vide infra) up to an excitation wavelength of 419 nm.
Upon successive addition of the Lewis acid EtAlCl2, a strong
absorption became visible with a maximum at λ = 387 nm. The
absorption reached saturation upon addition of 15 equivalents of
EtAlCl2 indicating complete complexation of 1a. The extinction
coefficient was determined as ε = 16180 M−1 cm−1 which supports
an assignment of this band to a ππ* transition. The band tails into
the long-wavelength region of the spectrum and there is a
significant absorption beyond λ = 420 nm. Photochemical
experiments with achiral Lewis acids suggested a catalytic
photocycloaddition reaction to be possible at λ ≥ 420 nm.
Experiments with chiral Lewis acids commenced with the
previously reported[8a-c] catalyst 3a. Addition of 2,3-dimethyl-2-
butene to substrate 1a occurred at −78 °C in CH2Cl2 (Table 1)
exclusively at the C9/C10 double bond and resulted in product 2a
which is formally the product of an ortho photocycloaddition.[11,12]
The highest enantioselectivity was achieved at λ = 457 nm (entries
1-3) and the Lewis acid was subsequently optimized by varying the
aryl substituent (Ar) at the boron atom (for the optimization studies,
see the Supporting Information). Another known[13] Lewis acid 3b
with a single substituent in ortho position of the phenyl ring gave a
significant increase in ee (entry 4). Eventually, the 2,6-
[∗]
M. Sc. S. Stegbauer, Dr. C. Jandl, Prof. Dr. T. Bach
Department Chemie and Catalysis Research Center (CRC)
Technische Universität München
Lichtenbergstr. 4, D-85747 Garching
Fax: +49 (0)89 289 13315
E-mail: thorsten.bach@ch.tum.de
Homepage: http://www.oc1.ch.tum.de/home_en/
Supporting information and the ORCID identification
number(s) for the author(s) of this article can be found
under: http://dx.doi.org./10.1002/....
1
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10.1002/anie.201808919
Angewandte Chemie International Edition
Table 2: Lewis acid Catalyzed, Enantioselective Photocycloaddition
Reactions of Phenanthrene-9-carboxaldehydes (1) and 2,3-
Dimethyl-2-butene
dimethylphenyl derivative 3c turned out to be the ideal Lewis acid
(entry 5) and it fulfilled our expectations with regard to catalytic
turnover (entries 6, 7). A loading of 10 mol% was sufficient to
provide a high yield (81%) and a high enantioselectivity (94% ee).
Table 1: Photocycloaddition Reaction of Phenanthrene-9-carbox-
aldehyde (1a) and 2,3-Dimethyl-2-butene in the Presence of Chiral
Lewis acids 3
yield[b]
[%]
ee[c]
[%]
entry[a] substrate
X
Y
product
1
2
1a
1b
1c
1d
1e
1f
H
2-Me
2-Cl
3-F
H
H
2a
2b
2c
2d
2e
2f
79
75
89
78
66
85
92
79
89
93
85
94
84
88
93
92
86
92
82
90
92
96
Lewis loading
acid[b] [mol%]
t[a]
[h]
yield[c]
[%]
entry[a] λ[a] [nm]
ee[d] [%]
3
H
4
H
1
2
3
4
5
6
7
424
435
457
457
457
457
457
3a
3a
3a
3b
3c
3c
3c
50
50
50
50
50
25
10
14
9
53
43
49
54
62
79
81
56
70
79
84
97
94
94
5
3-Me
H
H
6
5-Me
6-CF3
6-F
11
11
11
19
26
7
1g
1h
1i
H
2g
2h
2i
8
H
9
2-Me
3-Me
3-F
6-CF3
6-CF3
6-CF3
10
11
1j
2j
1k
2k
[a] The reaction was performed at −78 °C with a substrate concentration of c
= 20 mM in CH2Cl2 at the indicated wavelength ( ) and for the indicated period
of time (t). The olefin was used in excess (30 eq.).
[a] The reaction was performed at −78 °C with a substrate concentration of c
λ
[b]
3a: Ar = 2,4,6-
= 20 mM in CH2Cl2 at the indicated wavelength (λ) and for the indicated period
of time (t). The olefin was used in excess (30 eq.). [b] Yield of isolated product.
[c] The ee was determined by chiral HPLC analysis.
trifluorophenyl; 3b: Ar = 2-(trifluoromethyl)phenyl; 3c: Ar = 2,6-dimethylphenyl.
[c] Yield of isolated product.
[d]
The enantiomeric excess (ee) was determined
by chiral HPLC analysis.
For substrate 1c, the respective complex 1c·3c is depicted in
Figure 2 and the enantioface differentiation can be explained by an
attack of the olefin from the face that is not shielded by the 3,5-
dimethylphenyl ring (in gray) of the oxazaborolidine and that is
relative to carbon atom C10 on the Si face. Despite the fact that
complex 1c·3c provides a rationale for the stereochemical outcome
of the reaction, it cannot explain at this point the subtle differences
which are caused by the choice of aryl (Ar) groups at the boron
atom and by the substitution pattern at the phenanthrene substrate.
The substrate scope of the photocycloaddition was explored with
some functionalized phenanthrene-9-carboxaldehydes 1. Their
synthesis was best achieved by linkage of the two arene rings via a
Suzuki cross-coupling and by subsequent introduction of the
C9/C10 bond in an aldol condensation (see Supporting Infor-
mation). In order to keep the reaction times low and in order to
allow for complete conversion of all substrates a catalyst loading of
20 mol% was consistently applied (Table 2). Under these
conditions the respective products were obtained in yields of 66-
93% and with enantioselectivities of 82-96% ee. From the
preliminary data set, it appears as if substitution at C3 and C6 is
beneficial for the enantioselectivity while substituents at positions
C2 and C5 (entries 2, 3, 6) lead to a slightly reduced enantio-
selectivity. Fluoro, chloro, methyl, and trifluoromethyl substitution
was tolerated but the study on the functional group tolerance was
limited by the availability of the starting materials. In general,
compounds with Lewis basic sites (alkoxy, carbonyl) are not
recommended due to competitive binding to the Lewis acid.
The absolute configuration of the cyclobutane products was
proven by anomalous X-ray diffraction (Figure 2). Chloro
derivative 2c gave suitable crystals and the configuration at the two
newly formed stereogenic centers could be unambiguously
assigned. Irrespective of the mechanism of the photocycloaddition,
the attack of the olefin at carbon atom C10 occurred from the Si
face. It has been previously suggested[7,14] that aldehydes bind to
cationic oxazaborolidine Lewis acids by coordination of the
carbonyl group to the boron atom and by a second non-classical
hydrogen bond[15] of the hydrogen atom to the oxazaborolidine
oxygen atom.
Figure 2. Absolute configuration of product 2c as determined by
anomalous X-ray diffraction and model for the association of Lewis
acid 3c to phenanthrene-9-carboxaldehyde 1c.
In a second set of preliminary experiments, we studied the
reaction of two phenanthrene-9-carboxaldehydes 1a and 1d with
cyclopentene (Scheme 1). This olefin represents a frequently used
cyclic reaction partner in [2+2] photocycloaddition chemistry[16]
and offers a first indication for the generality of the reaction. To
our delight, the enantioselectivity of the Lewis acid catalyzed
process remained very high. Apart from the major diastereoisomers
2
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10.1002/anie.201808919
Angewandte Chemie International Edition
4a and 5a in which the carbocyclic rings are positioned on opposite
sides of the cyclobutane core (exo) the minor endo products were
also detected (d.r. = diastereomeric ratio).
Figure 3. Structure of photocycloaddition products rac-8, rac-4b,
rac-5b, rac-6b, and rac-6c.
Previous work on the ortho photocycloaddition of methyl
phenanthrene-9-carboxylate had produced evidence that its reaction
(λ = 350 nm) with 2,3-dimethyl-2-butene proceeds via a triplet
intermediate.[18] Contrary to our results, it was found that any
attempted photocycloaddition in the presence of a Lewis acid was
sluggish but it was postulated that the Lewis acid mediated process
occurred via a singlet-state mechanism. Indeed, a change of
mechanism could explain the reversal of simple diastereoselectivity
in the reaction of phenanthrene-9-carboxaldehydes with cyclo-
pentene and cyclohexene. Preliminary fluorescence studies on the
complex of 1a with EtAlCl2 revealed a strong fluorescence at an
Scheme 1. Lewis acid catalyzed, enantioselective photocyclo-
addition reactions of phenanthrene-9-carboxaldehydes 1a and 1d to
cyclopentene.
Similarly, cyclohexene produced the respective exo product 6a as
the major diastereoisomer which was formed in 90% ee from
aldehyde 1a (Scheme 2). Apart from the endo diastereoisomer,
minimal amounts of a third diastereoisomer were found (vide infra).
The reaction of aldehyde 1a with isopropylidenecyclohexane gave
mainly a single regioisomer 7 (r.r. = regioisomeric ratio) which was
obtained in 84% ee.
emission wavelength of λem = 498 nm (excitation wavelength λexc
=
400 nm) which was quenched upon addition of 2,3-dimethyl-2-
butene. Since fluorescence quenching by energy transfer to an
olefin is thermodynamically not feasible,[19] it is likely that the
excited singlet state of complex 1a·AlEtCl2 reacts with the olefin.
In conclusion, it was found that phenanthrene-9-carboxaldehydes
undergo an extensive bathochromic shift upon coordination to a
Lewis acid. The strong band (λmax = 387 nm, ε = 16180 M−1 cm−1)
which is induced upon Lewis acid coordination stretches beyond all
absorption bands of uncomplexed phenanthrene-9-carboxaldehydes
and thus enables a selective excitation of the Lewis acid complex at
long wavelength. In the presence of catalytic amounts of chiral
oxazaborolidine Lewis acid 3c, an enantioselective ortho
photocycloaddition was possible and a model for the complexation
is suggested based on literature precedence. The result shows for
the first time that the aldehyde binding motif is suitable to achieve
high enantioface differentiation in photochemical reactions.
Mechanistically, there is circumstantial evidence that the catalytic
reaction proceeds via an excited singlet state and that this fact
influences the selectivity pattern of the photocycloaddition. Further
work is required to explore the subtle influence of the substituents
at the oxazaborolidine and to understand the change of the
selectivity pattern by the Lewis acid.
Scheme 2. Lewis acid catalyzed, enantioselective photocyclo-
addition of phenanthrene-9-carboxaldehyde (1a) with cyclohexene
and isopropylidenecyclohexane to products 6a and 7.
As previously noted for [2+2] photocycloaddition reactions,[8d]
the chiral Lewis acid governs not only the enantioselectivity but
also has a significant impact on other selectivity parameters. In the
current study it was found that the formation of other constitutional
isomers was completely suppressed in the presence of the Lewis
acid. Irradiation of aldehyde 1a and 2,3-dimethyl-2-butene at λ =
366 nm, 398 nm, and 405 nm led consistently to the formation of
oxetane rac-8 (Figure 3) in a ratio of ca. 1/5 relative to rac-2a. At
lower wavelength (λ = 254 nm, 350 nm, and 366 nm) de-
carbonylation reactions were observed. No Paternò-Büchi
reaction[17] products nor any other side products from carbonyl
photochemistry were observed in the presence of the chiral Lewis
acid.
Acknowledgement
Financial support by the European Research Council under the
European Union’s Horizon 2020 research and innovation
programme (grant agreement No 665951 − ELICOS) and the
Fonds der Chemischen Industrie (Ph. D. fellowship to S. S.) is
gratefully acknowledged. We thank O. Ackermann and J.
Kudermann for their help with the HPLC and GLC analyses, and
Dr. T. S. Chung for the fluorescence measurements.
Even more remarkable was the simple diastereoselectivity of the
reaction which was almost completely inverted as compared to the
racemic reaction. Direct excitation of 1a and 1d in the presence of
cyclopentene at λ = 366 nm produced predominantly the endo
diastereoisomers rac-4b (28%, d.r. = rac-4a/rac-4b = 12/88) and
rac-5b (40%, d.r. = 9/91). In the case of cyclohexene, direct
irradiation at λ = 366 nm led in 31% yield to the endo
diastereoisomer rac-6b and the trans-configured product rac-6c in
almost identical amounts. A small fraction of the exo isomer rac-6a
was also observed and the diastereomeric ratio (d.r. = rac-6a/rac-
6b/rac-6c) was 21/35/44.
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Keywords: arenes · chromophores · cycloaddition ·
enantioselectivity · Lewis acids · photochemistry
3
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10.1002/anie.201808919
Angewandte Chemie International Edition
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4
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10.1002/anie.201808919
Angewandte Chemie International Edition
Entry of the Table of Contents
Photochemistry
S. Stegbauer, C. Jandl, T. Bach*
__________ Page – Page
Enantioselective Lewis Acid-Catalyzed
ortho Photocycloaddition of Olefins to
Phenanthrene-9-carboxaldehydes
Chromophore activation at its best: Lewis acid catalysis enables an enantio-
selective photocycloaddition of olefins to the C9/C10 double bond of phenanthrene-
9-carboxaldehydes in which up to four stereogenic centers are created. The
chromophore is significantly modified by Lewis acid coordination.
5
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