Intermolecular formyloxyarylation of alkenes by photoredox meerwein reaction

Article abstract of DOI:10.1021/acscatal.5b00314

The intermolecular formyloxylation-arylation of stilbenes occurs in the presence of diazonium salts, a photocatalyst, visible light, and DMF. The photo-Meerwein addition products are obtained in good yields up to 76%. We propose the formation of an iminiu

Full text of DOI:10.1021/acscatal.5b00314

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Article
Intermolecular Formyloxy-Arylation of
Alkenes by Photoredox Meerwein Reaction
Chang-Jiang Yao, Qiu Sun, Namrata Rastogi, and Burkhard König
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00314 • Publication Date (Web): 01 Apr 2015
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Intermolecular Formyloxy-Arylation of Alkenes by Photoredox
Meerwein Reaction
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Chang-Jiang Yao,ab Qiu Sun,a Namrata Rastogi,a Burkhard König*a
a. Department of Chemistry and Pharmacy, Universität Regensburg Universitätsstrasse 31, 93040 Regensburg (Germany)
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b. Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, People’s Republic of China
ABSTRACT: The intermolecular formyloxylation-arylation of stilbenes occurs in the presence of diazonium salts, a photocata-
lysts, visible light and DMF. The photo-Meerwein addition products are obtained in good yields up to 76 %. We propose the
formation of an iminium ion intermediate, which is hydrolyzed to the product.
KEYWORDS: Formyloxylation-arylation, Meerwein reaction, photocatalysis, diazonium salts, N, N-dimethylformamide
1. INTRODUCTION
The arylation of alkenes is an important transformation
in organic synthesis. First reports of the reaction using
aryl radicals generated from diazonium salts by the
Meerwein protocol date back more than 100 years.1 How-
ever, the classic reaction conditions of the Meerwein
arylation require metal salts, such as copper oxides, and
aqueous media, leading to side reactions and only moder-
ate product yields.2 Therefore many improved protocols
for arylations with diazonium salts have been reported
over the years.3 The recently introduced Photo-Meerwein
arylation uses visible light induced electron transfer from
a photoredox catalyst, e.g. Eosin Y or Ru(bipy)3Cl2, to
reduce the diazonium salt to aryl radicals providing a
clean conversion under mild conditions.4 Most of the
reported applications of the Photo-Meerwein arylation
apply an arylation-elimination sequence regenerating the
alkene double bond. The equally important Meerwein
arylation addition reaction, in which a nucleophile adds
to the carbenium-ion intermediate obtained from aryl
radical addition and oxidation of the benzyl radical, is less
explored in its photocatalytic variant. We recently report-
ed the intermolecular amino arylation of alkenes by pho-
toredox Meerwein addition using nitriles as nucleophile
and subsequent Ritter reaction,5 but there are still few
reports about photoredox oxy arylations employing dia-
ryliodonium.6 Here we describe the related formyloxy
arylation of alkenes by Photo-Meerwein addition in the
presence of dimethylformamide (Scheme 1). The three
component reaction allows the facile functionalization of
styrene derivatives.
Scheme 1. Types of photo Meerwein arylation reac-
tions: (a) photo Meerwein arylation-elimination, (b)
photo Meerwein arylation-addition with nitriles and
with dimethylformamide
(a) Photo Meerwein arylation-elimination
N2BF4
R1
R1
+ HBF4 + N2
R
R
photocatalyst
visible light
(b) Photo Meerwein arylation-addition
N2BF4
R1
R1
O
R1
R2CN, H2O
HN
R2
R
R
R
R
photocatalyst
visible light
Ritter type
amination
-
+ BF4 + N2
This work
O
N2BF4
H
N
Ar
Ar
O
Ar
R
R
O
H
photocatalyst
visible light
H2O
-
+ BF4 + N2
2. RESULTS AND DISCUSSION
Diazonium salts are labile towards base. The selection
of a non-basic nucleophile to trap the carbenium-ion
intermediate arising from aryl radical addition to the
alkene and oxidation by back electron transfer to the
photocatalysts (see below for the mechanistic proposal) is
therefore essential. N,N-Dimethylformamide (DMF) is
widely used as a dipolar aprotic solvent. DMF can act as
electrophile7 in reactions with organometallic reagents
and as a nucleophile or ligand in coordination chemistry.8
In visible light photoredox catalysis DMF was recently
employed to generate the Vilsmeier reagent.9
Irradiating 4-chlorophenyl diazonium tetrafluoroborate
(1a) and styrene (2a) in the presence of eosin Y (2 mol %)
in 0.5 mL of DMF with 530 nm green light gave the alkene
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Page 2 of 5
formyloxylation arylation product 3aa in 41 % yield (Table Furthermore, we examined the influence of the water
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4
5
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1, entry 10). The molecular structure of the compound was
ambiguously confirmed by X-ray single crystal analysis
(Figure 1). No significant product formation is observed
in the absence of the photocatalyst or the light source.
content, the type and amount of the photocatalyst, and
the number of equivalents of styrene on the reaction
yield. Product 3aa was obtained in 76 % yield (Table 1,
entry 3) when the diazonium salt 1a (0.2 mmol) was re-
acted with 4 equiv. of styrene (2a) in the presence of 2
mol% of [Ru(bpy)3]Cl2(H2O)6 and 1 equiv. of water in 0.5
mL of DMF at room temperature.
Table 1. Optimizing Reaction Conditions.a
O
Photocatalyst
(x mol%)
Cl
N2BF4
O
H
9
+
Ph
visible light, DMF/H2O
25 oC, N2
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Cl
1a
2a
3aa
Entry
1
Conditions
Yield % b
Ru(bpy)3Cl2(H2O)6 (0.7 mol% )
Ru(bpy)3Cl2(H2O)6 (1.3 mol% )
Ru(bpy)3Cl2(H2O)6 (2.0 mol% )
Ru(bpy)3Cl2(H2O)6 (2.7 mol% )
2a (2.0 equiv.)
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Figure 1. Molecular structure of compound 3aa in the
solid state
2
71
3
76
Next, we explored the scope of the diazonium salts and
the results are summarized in Table 2. Aryl diazonium
4
71
salts bearing electron-withdrawing, -neutral, and
-
5
64
66
75
donating substituents react smoothly affording the corre-
sponding products in good yields. Several functional
groups including ester, nitro, halide, ether, and alkyl
groups are tolerated in the photoreaction. Heteroaryl
diazonium salt 1j gave product 3ja only in 35% yield along
with the photo-Meerwein arylation-elimination product
in 15% yield.
6
2a (3.0 equiv.)
7
2a (6.0 equiv.)
8
1.0 h
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Table 2. Scope of the aryl diazonium salts.a
9
2.0 h
60
41
Ph
Ph
Ph
O
10
11
Eosin Y (2.0)
O
H
O
H
O
H
Cl
O2N
F3C
Me
MeO
Br
F
O
O
Sulforhodamine B (2.0 mol%)
Rose Bengal (2.0 mol%)
Rhodamine B acid 2.0 mol%)
1a (0.8 M)
44
43
3ba, 69%
3ca, 69%
3aa, 76%
Ph
Ph
Ph
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20
21
22
O
H
O
H
O
H
O
O
O
47
3da, 66%
3ea, 72%
3fa, 70%
3ia, 74%
Ph
Ph
Ph
48
70
O
H
O
H
O
H
EtO2C
O
O
O
1a (0.6 M)
3ga, 68%
3ha, 74%
MeO2C
S
1a (0.2 M)
56
Ph
Ph
O
Me
O
H
Br
No H2O
64
71
O
O
3x, 52%
3ja, 35%
H2O (2.0 equiv.)
H2O (5.0 equiv.)
H2O (10.0 equiv.)
No photocatalyst
No light
aThe reaction was carried out with 1 (0.2 mmol), styrene 2
(0.8 mmol), [Ru(bpy)3]Cl2(H2O)6 (0.004 mmol), and 1.0
equiv of water in 0.5 mL of DMF. Yields of isolated prod-
ucts are given.
65
58
Different aryl alkenes were used in the reaction and the
results are summarized in Table 3. Styrenes with electron-
withdrawing, -neutral, and -donating substituents at the
para position yield the corresponding products in moder-
ate to good yields under standard reaction conditions.
The internal alkenes trans-beta-methylstyrene and ben-
zalacetone provide the addition products in 57% or 47%
yield, resp., but with only low diastereoselectivity (d.r. 2:1).
The X-ray crystal structure analysis of compound 3al (see
supporting information) shows the configuration of its
0
Trace
aAll reactions were carried out on a scale of 0.2 mmol of
the diazonium salt at 25 C. See the ESI for the detailed
o
b
c
structures of the photocatalysts. Isolated yields given.
dNo light.
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ACS Catalysis
um ion yields the formyloxylation product 3f. Alternative-
major diastereoisomer. Alpha-methylstyrene yields the
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2
3
4
5
6
7
8
corresponding addition product in only 10%, with elimi-
nation product dominating. The scope of alkenes is lim-
ited to alkenyl arenes. Double bonds with rigid cyclic
structure, such as indene, or alkenes bearing no aryl sub-
stituent, such as p-benzoquinone and methyl acrylate, do
not convert to the corresponding product in significant
yields.
ly, benzyl radical 5 reacts first with DMF giving 6b, which
is oxidized by Ru(III) yielding iminium ion 7. The addi-
tion of a radical intermediate to DMF has been proposed
previously.9,10 The addition of TEMPO to the reaction
mixture allows to trap the aryl radical addition intermedi-
ate, which supports this being an intermediate during the
photoreaction. The use of DMF-d7 gave the product with
deuterium label at the formyl position (see supporting
information for data), while in reactions with added D2O
instead of water no deuterium incorporation was ob-
served.
9
Table 3. Scope of alkenes.
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O
Cl
O
H
Ru(bpy)3Cl2 6H2O
(2.0 mol%)
R3
R3
R1
N2BF4
+
R2
Scheme 2. Proposed mechanism of the photo Meer-
wein addition reaction with DMF.
R2
3
R1
440 nm,
DMF/H2O
25 oC, N2
Cl
Cl
R3
1a
2
R1
Cl
R2
byproduct
Me
O
H
O
H
O
O
H
Cl
Cl
Cl
O
3aa, 76%
O
3ab, 61%
3ac, 74%
F
Cl
F
O
H
O
H
O
H
Cl
Cl
Cl
O
O
O
3ad, 66%
3ae, 73%
3af, 69%
NO2
S
O
H
O
H
Cl
Cl
3. CONCLUSION
O
H
Cl
O
O
O
3ag, 60%
3ah, 48%
3ai, trace
The intermolecular formyloxylation-arylation of stilbenes is
possible by photoredox Meerwein addition reaction in the
presence of diazonium salts and DMF. The addition products
are obtained in good yields up to 76 %. We propose the for-
mation of an iminium ion intermediate, which is hydrolyzed
to the target product.
Cl
O
O
H
O
MeO
H
O
H
O
Cl
O
O
Br
3aj, 57% (dr, 2:1)
3al, 47% (dr, 4:1)
3ak, 10%
4. EXPERIMENTAL SECTION
aThe reaction was carried out with 1a (0.2 mmol), styrene
2 (0.8 mmol), [Ru(bpy)3]Cl2(H2O)6 (0.02 eqiv), and 1
equiv of water in 0.5 mL of DMF. Yields of isolated prod-
ucts. In the case of compound 3ak the elimination prod-
uct is the major product.
General information
Unless otherwise noted, commercial reagents and starting
materials were used either as p.a. grade or treated accord-
ing to literature known procedures. Proton NMR spectra
were recorded on a Bruker Advance 300 MHz spectrome-
ter in CDCl3 solution with internal solvent signal peak at
If N,N-dimethylacetamide is used instead of DMF the
corresponding product 3X is obtained in 52%. A plausible
mechanism of the photoreaction is proposed in Scheme 2
based on previous work, the trapping of radical interme-
diates and experiments using labelled compounds. The
aryl radical 4 is produced by a single-electron transfer
from the excited state of the photocatalyst [Ru(bpy)3]2+*
to the diazonium salt 1f. Addition of the aryl radical to
alkene 2 generates the corresponding benzyl radical in-
termediate 5, which is either oxidized by Ru(III) to give an
carbenium-ion intermediate 6a and reacts with the car-
bonyl oxygen of N,N-dimethylformylamide to generate
the iminium ion intermediate 7. Hydrolysis of the imini-
13
7.26 ppm. C NMR were recorded at 75 MHz spectrome-
ter in CDCl3 solution and referenced to the internal sol-
vent signal at 77.00 ppm. Proton NMR data are reported
as follows: chemical shift (ppm), multiplicity (s = singlet,
d = doublet, t = triplet, q = quartet, dd = doublet of dou-
blets, m = multiplet), and coupling constants (Hz). All
reactions were monitored by thin-layer chromatography
using Merck silica gel plates 60 F254; visualization was
accomplished with short wave UV light (254 nm).
Procedure for the preparation of aryl diazonium
tetrafluoroborates11
The appropriate aniline (10 mmol) was dissolved in a
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Kirschstein, J. Org. Chem. 2007, 72, 9609-9616. g) M. R.
mixture of 3.4 mL of hydrofluoroboric acid (50%) and 4
mL of distilled water. The reaction mixture was cooled to
0°C using an ice-water bath, and then sodium nitrite
(NaNO2) solution (0.69 g in 1.5 mL) was added drop wise.
The resulting reaction mixture was stirred for 40 min at
0-5°C and the obtained precipitate was collected by filtra-
tion, dried and re-dissolved in a minimum amount of
acetone. Diethyl ether was added until precipitation of
diazonium tetrafluoroborate, which is filtered, washed
several times with small portions of diethyl ether and
dried under vacuum.
1
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General procedure for the reaction of arenediazoni-
um tetrafluoroborates with alkenes:
A 5 mL snap vial equipped with magnetic stirring bar was
charged with the photocatalyst [Ru(bpy)2]Cl2 (H2O)6 (0.004
mmol, 0.02 equiv.), arenediazonium tetrafluoroborate (0.2
mmol, 1 equiv.), alkene (0.8 mmol, 4.0 equiv.), water (0.2
mmol, 1 equiv), and DMF (0.5 mL). The reaction mixture was
degassed by three “pump-freeze-thaw” cycles via a syringe
needle. The vial was irradiated through the vial’s plane bot-
tom side using 450 nm blue LEDs with cooling device main-
taining a temperature around 25 0C. After 1.5 h of irradiation,
10 mL of water were added to the reaction mixture, which
was then extracted with dichloromethane (3 x 30 mL). The
combined organic phases was dried over Na2SO4 and concen-
trated in vacuum. The residue was purified by flash column
chromatography using petrol ether/ethyl acetate (30:1) as
eluent.
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ASSOCIATED CONTENT
Supporting Information. NMR and MS spectra of all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
8
a) S. Kobayashi, M. Sugiura, C. Ogawa, Adv. Synth. Catal.
2004, 346, 1023-1034. b) A. Jutand, Chem. Rev. 2008, 108,
2300-2347; c) S. Yamaguchi, H. Shinokubo, A. Osuka, Inorg.
Chem. 2009, 48, 795-797. d) T. Cottineau, M. Richard-Plouet,
J.-Y. Mevellec, L. Brohan, J. Phys. Chem. C 2011, 115, 12269-
12274; e) M. C. Das, H. Xu, Z. Wang, G. Srinivas, W. Zhou, Y.-
F. Yue, V. N. Nesterov, G. Qian, B. Chen, Chem. Commun.
2011, 47, 11715-11717. f) C. Femoni, M. C. Iapalucci, G. Longoni,
S. Zacchini, Dalton Trans. 2011, 40, 8685-8694. g) H. Arora, J.
Cano, F. Lloret, R. Mukherjee, Dalton Trans. 2011, 40, 10055-
10062. h) E. A. Mikhalyova, S. V. Kolotilov, M. Zeller, L. K.
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Dalton Trans. 2011, 40, 10989-10996. i) Y. Nakao, H. Idei, K. S.
* E-Mail: Burkhard.Koenig@ur.de (B. König)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the GRK 1626 (Chemical Photocatalysis) of Ger-
man Science Foundation (DFG) for financial support and the
Lindau fellowship of Sino-German Center for C.J.Y.
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22 examples, yield up to 76%
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ACS Paragon Plus Environment