Visible-Light Organophotoredox-Catalyzed Synthesis of Precursors for Horner-Type Olefinations

Article abstract of DOI:10.1002/ejoc.201800213

A metal-free photoredox-catalyzed α-heteroarylation of 2-bromophosphonoacetic esters allows the synthesis of precursors for Horner-olefinations from indoles in a single step. Numerous functional groups are tolerated in this photoinduced radical coupling u

Full text of DOI:10.1002/ejoc.201800213

A Journal of
Accepted Article
Title: Visible Light Organophotoredox-catalyzed Synthesis of Horner
Precursors
Authors: Marco Matthias Nebe, Daniel Loeper, Fabian Fürmeyer, and
Till Opatz
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800213
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European Journal of Organic Chemistry
10.1002/ejoc.201800213
Visible Light Organophotoredox-catalyzed Synthesis of
Horner Precursors
Marco M. Nebe, Daniel Loeper, Fabian Fürmeyer, and Till Opatz*
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, D-55128 Mainz,
Germany
E-mail: opatz@uni-mainz.de
URL: http://www.chemie.uni-mainz.de/OC/AK-Opatz/index.php
Abstract
A metal-free photoredox-catalyzed α-heteroarylation of 2-bromophosphonoacetic esters
allows the synthesis of precursors for the Horner- (HWE-)olefination from indoles in a single
step. Numerous functional groups are tolerated in this photoinduced radical coupling under
mild conditions and the subsequent reaction with aldehydes generates 2-(indol-2-yl)acrylates
in high yield.
Introduction
Photoredox–catalyzed reactions have become important tools in nowaday’s arsenal of
preparative methods.[1] They permit e.g. the formation of C–P and C–C-bonds under
exceptionally mild conditions and tolerate many functional groups in the respective
substrates, which makes them powerful tools for the modification and construction of
[2]
complex heterocyclic structures. While the field of photoredox-catalysis is still dominated by
metal-based systems such as luminescent complexes of Ru and Ir, cheap and readily
available organic dyes have been recognized as attractive alternatives with a favourable
ecological footprint.[3] An interesting example of such a transformation is the coupling of
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European Journal of Organic Chemistry
10.1002/ejoc.201800213
2-bromomalonates 1 with indoles and other electron-rich heterocycles, catalyzed by the
excited Ru-complex Ru(bpy)
3
Cl (2) as reported by Stephenson and co-workers (Scheme
2
1
a).[4] This reaction has inspired numerous synthetic transformations as well as mechanistic
investigations since.[5] A related enantioselective addition of 2-bromomalonates to chiral
enamines, generated in situ from a corresponding aldehyde, was reported by MacMillan and
coworkers who employed the same photocatalyst.[6] This transformation was later modified
by the groups of König and Zeitler by using eosin Y (4) as a metal–free photocatalyst
(
Scheme 1b).[7] Recently, Liu and coworkers developed a visible light catalyzed
difluoromethylenephosphonation of arenes and heteroarenes using Ir(ppy)
3
(5) as a
photocatalyst (Scheme 1c).[8] On the basis of these reports, we wondered whether
2
-bromophosphonoacetate 7 could be used in a similar fashion. Their addition to the
-position of differently substituted indoles (6) would result in the formation of synthetically
2
useful precursors for the Horner-type olefination of aldehydes in a single step (Scheme 1d).[9]
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European Journal of Organic Chemistry
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Scheme 1: a) Stephensons Ru-catalyzed photocatalytic radical functionalization of
heterocycles; b) Zeitlers photo-organocatalyzed enantioselective α-functionalisation of
aldehydes; c) Lius difluoromethylenephosphonation of (hetero)arenes d) regioselective
radical functionalisation of indoles 6, generating Horner-precursors 8
Results and Discussion
Our initial studies aimed at identifying suitable reaction conditions for the
phosphonoacetylation by investigating the model reaction depicted in Table 1. Triethyl
2
-bromophosphonoacetate (7) was used as the radical precursor, which was obtained in
8% yield by NBS-bromination of commercially available triethyl phosphonoacetate. N-Boc-
7
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European Journal of Organic Chemistry
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tryptamine 6a was chosen as the model hetarene for this coupling reaction since the blocked
-position of the indole moiety would prevent potential regioselectivity issues during the
screening process. The 1-position of the indole was left unprotected. Tryptamine 6a also
3
[4]
performed well as substrate in the reaction described by Stephenson and co-workers. Upon
employing the original reaction conditions which use Ru(bpy) Cl as a photocatalyst and
-methoxy-N,N-diphenylaniline (3) as a reductive quencher, the reaction proceeded smoothly
3
2
4
providing the desired product in 74% yield after 20 h when a 462 nm blue LED was used as
the light source (entry 1). Comparable results were obtained by utilizing a method described
by the Weaver group, employing Ir(ppy) and dicyclohexylisobutylamine as a reductive
3
quencher (entry 2).[10] While these methods both showed good applicability to the desired
coupling reaction, they required the stoichiometric addition of the employed amines, which
are not commercially available or rather costly at best. Moreover, the use of expensive and
potentially toxic late transition metal catalysts is not ideal with respect to sustainability and
cost. Zeitler and coworkers have demonstrated that xanthene dyes such as rose bengal or
eosin Y offer comparable redox potentials and reaction kinetics, so we wondered whether
they might permit the reductive radical generation from 2-bromophosphonoacetate esters
and the closure of a redox-neutral catalytic cycle without any additional quenchers added.[7]
Both rose bengal and eosin Y were tested in combination with 2,6-lutitine as an inexpensive,
non-redox-active base. Upon irradiating a solution containing 2.5 mol% of the respective
xanthenes dye with a 520 nm green LED for 94 h, the desired product was obtained in 19%
and 43% yield, respectively (entries 3 and 4). Interestingly, irradiation with blue light under
otherwise identical conditions increased the yield to 32% for rose bengal (entry 5) and even
to 82% for eosin Y (entry 6) in a shorter reaction time. This observation was somewhat
unexpected when considering the absorption spectra of these dyes, both showing a
maximum absorption for green light.[11] Screening of various solvents revealed DCM to be the
most suitable for this reaction, providing the product in 83% yield after 20 h (entry 7). The
yield dropped to 71% when the catalyst loading was lowered to 1 mol% (entry 8). Due to
initial problems with the reproducibility as well as the long reaction times, the reaction setup
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European Journal of Organic Chemistry
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was modified and white LED stripes were used for irradiation instead of a single high power
blue LED. This resulted in a more rapid conversion and good reproducibility of the individual
runs.[12] In this way we were able to obtain compound 8a in 78% isolated yield after 14 hours
of irradiation. Unfortunately, two equivalents of the brominated phosphonoacetate 7 were
required and the yield dropped significantly to <15%, when only 1.25 equiv. were added
(
(
entry 10). As expected, no reaction could be observed in the absence of light or the catalyst
entries 11 and 12).
Table 1. Screening and optimization of reaction conditions for the photocatalytic addition of
bromophosphonoacetate 7a)
Entry cat. (mol%)
Ru(bpy) Cl
Ir(ppy) (1)
rose bengal (2.5)
eosin Y (Na ) (2.5)
h
additive (equiv.)
solvent
DMF
t (h)
20
20
94
94
72
48
20
20
14
22
48
Yield (%)b)
74%
1
2
3
4
5
6
7
8
9
1
1
1
3
2
(1)
blue LED
(p-CH
3
OC
6
H
4
2
)NPh (2)
i
3
blue LED
K
2
CO
3
(2), Cy
2
NBu (0.5)
MeCN
MeCN
MeCN
MeCN
MeCN
DCM
78%
green LED
green LED
blue LED
2,6-lutidine (2)
2,6-lutidine (2)
2,6-lutidine (2)
2,6-lutidine (2)
2,6-lutidine (2)
2,6-lutidine (2)
2,6-lutidine (2)
2,6-lutidine (2)
2,6-lutidine (2)
2,6-lutidine (2)
19%
2
43%
rose bengal (2.5)
eosin Y (2.5)
eosin Y (2.5)
eosin Y (1)
eosin Y (2.5)
eosin Y (2.5)
eosin Y (2.5)
-
32%
blue LED
82%
blue LED
83%
blue LED
DCM
71%
white LED-stripes
white LED-stripes
-
DCM
78%c)
<15%
N.R.
0d)
1
DCM
DCM
2
white LED-stripes
DCM
48
N.R.
a)
1
b)
Reaction conditions: 6a (1 equiv.), 7 (2 equiv.), solvent (60 µmol/mL), cat., additive, 25 °C, h; determined via
c)
d)
H-NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as an internal standard; isolated yield; 1.25 equiv.
7
; N.R.: no reaction
Under the optimized conditions, the scope of this reaction was investigated (Scheme 2).
N-acylated or N-sulfonylated tryptamines reacted readily, providing the corresponding
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European Journal of Organic Chemistry
10.1002/ejoc.201800213
products 8a–d in high yields. The reaction also afforded N-benzylamine 8e, as well as ester
8f in only slightly reduced yield. The protected tryptophan derivative 8g could be obtained in
68% yield, making this reaction potentially attractive for the late stage functionalization of
peptides. In general, 3-unsubstituted indoles only provided the respective 2-substituted
products, highlighting the complete regioselectivity of this reaction. Potentially sensitive alkyl
as well as aryl bromides (8h,i) were well tolerated in this reaction, as were donor-substituted
indoles (8j), although the yield was slightly reduced in the latter case. For the reaction
involving indole-3-carbaldehyde, the corresponding phosphonoacetate 8k could only be
isolated in 15% yield, which can presumably be explained by radical side reactions like at the
aldehyde function. While unsubstituted indole reacted readily providing the desired product 8l
in 76% yield, no conversion of the starting material could be observed for N-functionalized
indoles like N,N’-Di-Boc-tryptamine 6a’, N-benzylindole (6l’) or N-Cbz-indole (6l’’). This is
most likely due to steric hindrance imposed by the protecting group, preventing an addition of
the bulky phosphonoacetate moiety. Interestingly however, in the case of N-methylindole,
27% of the expected 2-substituted indole 8m alongside with 28% of the 3-substituted product
8m’ could be obtained. When 2-methylindole was employed in the reaction, no addition
occurred and only the previously reported oxidative dimerization of the starting material to
the oxidized dimer 9 took place when the reaction was not performed under an inert
atmosphere.[13] Attempts to employ other heterocycles than indoles in this reaction were
unsuccessful and no conversion could be observed for substrates like pyrrole, benzofuran,
benzoxazole, benzimidazole or benzothiazole.
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European Journal of Organic Chemistry
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Scheme 2: Photocatalytic phosphonoacetylation of indoles.
A plausible reaction mechanism is displayed in Scheme 3. We propose that by photon
absorption and intersystem crossing (ISC) of the photocatalyst eosin Y (EY), an excited
triplet state EY* is formed, which can transfer an electron to bromophosphonoacetate 7

+
generating the eosine Y radical cation EY , bromide, and the radical 10. The latter can then
add to the indole aromatic system, providing the benzylic radical 11, which can thereafter
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European Journal of Organic Chemistry
10.1002/ejoc.201800213
rearomatize to the obtained product 8 through single electron transfer (SET) to EY+ and
subsequent deprotonation. An alternative reductive catalytic cycle as discussed by König and
Zeitler for the addition of bromomalonate to enamines seems less likely for the present
reaction.[7] This mechanism, originally proposed by MacMillan and co-workers, relies on an
initial oxidation of a catalytic amount of enamine, generating the catalytic species necessary
to start the actual catalytic cycle.[6] Such a reaction course seems improbable for our case,
since a corresponding initial oxidation of indole would require an oxidation potential
exceeding that of the excited eosin Y.[14] The same applies for lutidine, which we believe to
act as a mere acid scavenger for the HBr liberated during the course of the reaction and
promoting the rearomatization step. When lutidine was replaced by the reductive quencher
4-methoxy-N,N-diphenylaniline (3), a similar yield and reaction progress was observed,
supporting the hypothesis of an oxidative reaction cycle. It can however not be ruled out that
a catalytic amount of bromophosphonoacetate 7 is deprotonated to the corresponding anion
which could act as a reductive quencher to initiate a reductive catalytic cycle.
Scheme 3: Proposed Mechanism for the photocatalytic addition of bromphosphonoacteate 7
to indoles.
Next, the Horner-type olefination of the aryl-phosphonoacetates 8 was investigated. Again,
the N-Boc-tryptamine derived compound 8a was chosen as a model compound. Optimization
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European Journal of Organic Chemistry
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studies showed that the best results for this transformation could be obtained by the use of
DBU in dichloromethane (Scheme 4a).[15] In this fashion, aldehydes like benzaldehyde or
cinnamaldehyde could be transformed to the corresponding E-olefins 14a and 14b
1
1
exclusively in 63% and 80% yield. The E-configuration was confirmed via H- H-NOE-
spectroscopy as well as (in the case of 14a) by X-ray crystallography (scheme 4b). With
aliphatic aldehydes like valeraldehyde and isobutyraldehyde, not only the E-olefins 14c and
14d, but also the respective Z-isomers 14c’ and 14d’ could be obtained, which was again
1
1
determined by H- H-NOE-spectroscopy. The isomeric products derived from valeraldehyde
could be separated by flash column chromatography, yielding the E-isomer 14c as major
product in 79% yield. In the case of isobutyraldehyde, a 1.5:1 mixture of diastereomers was
obtained in 40% yield which could only be separated by preparative HPLC. A possible
explanation for the reduced yield and selectivity for the latter example could be the increased
steric demand of the isopropyl group, which may impair the formation of the intermediate
phosphoxetane. This effect is likely to be more prominent for the phosphoxetane
diastereomer yielding the E-olefin in which the bulky indole moiety is syn- to the isopropyl
group. Not surprisingly, ketones such as acetone or cyclohexanone are no suitable coupling
partners since there is significant bulk and charge stabilization in the deprotonated
phosphonates 8. Using paraformaldehyde as the carbonyl compound led to a complex
mixture of products (as judged by TLC), from which the desired olefin could not be isolated.
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European Journal of Organic Chemistry
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Scheme 4: a) Horner-type olefination of indolyl-phosphonoacetates 8; b) X-Ray crystal
structure of compound 14a, solid state at 193K (thermal ellipsoids, 30% probability).
As discussed above, all of the Horner-type olefinations performed showed the expected
E-selectivity, albeit to a varying extent. It appeared tempting to develop a modification of this
protocol predominantly furnishing the isomeric Z-olefins which would provide control over the
double bond geometry. Indolyl-diphenoxyphosphonoacetate 16 was thus chosen as an
intermediate which could be reacted in an Ando variant of the Horner reaction (Scheme 5).[16]
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European Journal of Organic Chemistry
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The preparation of the required diphenoxy bromophosphonoacetate 15 turned out to be not
as straightforward as in the case of alkyl derivative 7. A chemoselectivity in favour of the
monobrominated product was rather challenging to achieve, as was its purification, which
was extensively investigated by Brückner and co-workers.[17] NBS-bromination of ethyl
diphenylphosphonoacetate, as described for the triethyl derivative, provided only 47% of
bromophosphonoacetate 15. Execution of the photocatalytic coupling to N-Boc-tryptamine 6a
under the previously established standard conditions provided the corresponding aryl
phosphonoacetate 16 in 34% yield. When this compound was subjected to the Horner
olefination with benzaldehyde, the E-olefin 14a was exclusively obtained in 61% yield,
similarly to our earlier findings in the alkyl series. We speculated that the Z-isomer potentially
formed during the reaction might be photochemically isomerized to the E-olefin which is not
an uncommon process for cinnamoyl-derivatives.[18] We therefore decided to repeat the
reaction using valeraldehyde, providing the aliphatic -unsaturated esters 14c/14c’, which
are less prone to isomerisation due to the hypsochromic shift of the absorption maximum of
the product olefin. Here, an E/Z-ratio of 4.3:1 and 59% isolated yield of the diastereomeric
mixture were observed. This is only a slight improvement towards the formation of the Z-
isomer when compared to the 7.2:1 ratio obtained for the triethyl derivative while the yield
was considerably lower.
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European Journal of Organic Chemistry
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Scheme 5: Photoaddition and Horner-type olefination starting from phosphonoacetate 15.
Since both the photocatalytic phosphonoacetylation as well as the subsequent Horner-type
olefination ran smoothly in dichloromethane, we tried to develop a one-pot protocol for this
transformation. Addition of DBU and benzaldehyde to a solution of 8a directly after complete
conversion in the preceding photoreaction however did not result in the formation of the
desired olefin. Only a complex mixture could be detected via LC-MS after several days of
reaction instead.
Conclusion
In summary, a photoredox-catalyzed synthesis of precursors of the Horner-type olefination
from triethyl 2-bromophosphonoacetate and indoles has been developed. Eosin Y serves as
an inexpensive catalyst and no external quenchers are required. Standard white light LEDs
serve as an inexpensive energy source for this redox-neutral C–C-coupling reaction.
Olefination reaction with aromatic as well as aliphatic aldehydes gave rise to 2-(indol-2-
yl)acrylates in high yield and selectivity. This versatile and facile two-step synthesis hence
could find application in natural product total synthesis as well as for the late stage
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European Journal of Organic Chemistry
10.1002/ejoc.201800213
functionalization of indole structures such as tryptophane-moieties in peptides. The synthetic
potential of a combination of a Horner olefination followed by photoredox chemistry was
recently demonstrated by Reiser and co-workers for the synthesis of indolines and
indenones.[19] An amalgamation of this methodology with the present sequence may permit
even more elaborate photocatalytic sequences. While the origin of the higher reaction
efficiency under irradiation with light of a wavelength shorter than the absorption maximum of
the organic dye remains to be investigated, this effect has also been seen by us in other
processes such as oxidative cyanations of amines. The excitation of a reduced or oxidized
form of the dye could possibly account for this observation, as it was reported by König and
co-workers for rhodamine 6G, which possesses a similar absorption spectrum as eosin Y.[20]
Experimental Section
General procedure
A 10 mL Schlenk-tube was charged with the respective indole 6 (0.25 mmol, 1 equiv.), which
was dissolved in dry and degassed CH Cl (4.5 mL) under an atmosphere of nitrogen or
argon. Ethyl bromo(diethoxyphosphoryl)acetate 7 (150 mg, 0.50 mmol, 2 equiv.), 2,6-lutidine
60 µL, 0.50 mmol, 2 equiv.) and Na -eosin Y (4.3 mg, 6.22 µmol, 2.5 mol%) were added and
A
for the photoredox-catalytic phosphonoacetylation:
2
2
(
2
the solution was stirred under irradiation with white LED-stripes, until full consumption of the
starting material 6 could be observed via TLC or LC-MS (12–24 h). The solution was filtered
over a short plug of silica gel, the products were eluted with EtOAc and purified by column
chromatography. If not stated otherwise, automated flash column chromatography (C18,
0
%100% MeCN in H
General procedure
Phosphonoacetate 8a (0.23 mmol, 1.00 equiv.) was dissolved in dry CH
2
O) was employed.
B
for
the
Horner-type
olefination:
2
Cl
2
(2.5 mL) and
cooled to 0 °C. The respective aldehyde (1.15 mmol, 5 equiv) and DBU (1.15 mmol, 5 equiv.)
were added and the solution was stirred at 40 °C until full consumption of the starting
material was indicated by TLC (If full conversion was not achieved, another portion of base
and aldehyde were added as indicated for the respective compounds). The solution was
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European Journal of Organic Chemistry
10.1002/ejoc.201800213
filtered over a short plug of silica, the product was eluted with EtOAc and purified by column
chromatography.
Acknowledgement
The authors thank Dr. J. C. Liermann (Mainz) for NMR spectroscopy, Dr. C. J. Kampf (Mainz)
for mass spectrometry and Dr. D. Schollmeyer (Mainz) for X-ray crystallography. This work
was supported by the LESSING initiative of the Johannes Gutenberg-University.
Keywords
Photochemistry; Olefination; Heterocycles; C-C coupling; Radical reactions
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