Bichromatic Photosynthesis of Coumarins by UV Filter-Enabled Olefin Metathesis

Article abstract of DOI:10.1002/adsc.201700316

Herein, we report on a two-step bichromatic synthesis of coumarins involving UV-A and UV-C light. The first step is a UV-A-photoinduced ruthenium-catalyzed cross-metathesis (CM) reaction of 2-nitrobenzyl-protected 2-hydroxystyrenes with acrylates, using an external solution of 1-pyrenecarboxaldehyde as a UV filter. Irradiation in the absence of the filter permanently inhibits the light-activated catalyst due to photocleavage of the photolabile protecting group (PPG) and ensuing phenolate chelation to the ruthenium. The simple removal of the external filter after CM allows further photochemical reactions with UV-C light to achieve more complex architectures, such as the coumarins presented in this work. (Figure presented.).

Full text of DOI:10.1002/adsc.201700316

Title: Bichromatic Photosynthesis of Coumarins by UV Filter Enabled
Olefin Metathesis
Authors: Or Eivgi, Revannath Sutar, Ofer Reany, and N. Gabriel
Lemcoff
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700316
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10.1002/adsc.201700316
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Bichromatic Photosynthesis of Coumarins by UV Filter
Enabled Olefin Metathesis
Or Eivgi,a Revannath L. Sutar,a,b Ofer Reanyb and N. Gabriel Lemcoffa,c*
a Department of chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel, 84105.
E-mail: Lemcoff@bgu.ac.il
b Avinoam Adam Department of Natural Sciences, The Open University of Israel, Ra’anana, Israel, 43537.
c
Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva, Israel,
84105
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. Herein, we report on a two-step bichromatic synthesis of coumarins involving UV-A and UV-C light. The
first step is a UV-A photoinduced ruthenium catalyzed cross-metathesis (CM) reaction of 2-nitrobenzyl protected 2-
hydroxystyrenes with acrylates, using an external solution of 1-pyrene carboxaldehyde as a UV filter. Irradiation in the
absence of the filter permanently inhibits the light activated catalyst due to photocleavage of the photolabile protecting
group (PPG) and ensuing phenolate chelation to the ruthenium. The simple removal of the external filter after CM
allows further photochemical reactions with UV-C light to achieve more complex architectures, such as the coumarins
presented in this work.
Keywords: Photochemistry; Catalysis; Olefin Metathesis; Photolabile Protecting groups; Ruthenium; Coumarins
Light is unambiguously the most significant and sustainable source of energy on our planet. Inspired by
nature, which harnesses light to synthesize life, chemists have studied and mastered photons to overcome
energy barriers and promote chemical transformations.[1] Indeed, the use of light has brought about a large
number of applications in organic synthesis, chemical biology and the attractive field of photocatalysis.[2] The
expansion of chemical orthogonality[3] in the field of photochemistry was pioneered by Bochet et al., who
coined the term 'chromatic orthogonality' in his seminal works on the orthogonal removal of photolabile
protecting groups (PPGs) by using two different wavelengths.[4] This ingenious and important approach has
since been used in many synthetic applications,[5] including the all-photochemical total synthesis of a natural
product.[6] In a related strategy, selective PPG cleavage has been recently accomplished through the exploitation
of differences in molar extinction coefficients between two PPGs in the presence of UV absorbing "sunscreen"
molecules. Thus, by using an external solution of phenanthrene as a molecular light filter, 2-nitrobenzylethers
could be selectively removed in the presence of photolabile tris(trimethylsilyl)silyl ("sisyl") ethers upon
irradiation with 254 nm light.[7] Naturally, the ingress of 'chromatic orthogonality' in photocatalysis, is quite
appealing. Olefin metathesis is undoubtedly one of the most important catalytic reactions established in the last
decades. Stimulated by the development of well-defined, air-stable and extremely efficient complexes, olefin
metathesis has become a handy tool for organic synthesis practitioners and for industrial applications.[8] Our
work with latent ruthenium based sulfur-chelated Hoveyda-Grubbs analogues (Figure 1) prompted us to further
study these catalysts in chromatic orthogonal sequences.[9] The active (trans-dichloro) complexes can be
generated from the more stable and dormant cis-dichloro isomers either by heating, needed to overcome the
high barrier of activation for thermal isomerization, or by irradiation, where UV light (UV-A) disrupts the
sulfur-ruthenium bond and induces photoisomerization to the active configuration.
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10.1002/adsc.201700316
Advanced Synthesis & Catalysis
Figure 1. Hoveyda-Grubbs type sulfur-chelated photoswitchable olefin metathesis catalysts.
Thus, by taking advantage of the wavelength selective photoactivation of these complexes, a regioselective
chromatic orthogonal ring closing metathesis (RCM) protocol that could produce either dihydropyran or
dihydrofuran compounds with excellent regio-selectivity was achieved.[10] The chromatic orthogonality
between photocleavage of the sisyl ethers[7b] (UV-C) and photoactivation of the catalyst (UV-A) was also used
in a novel strategy where sisyl groups were directly attached to the NHC ligand of the catalyst (catalyst B,
figure 1), to make catalysts that can be activated with one wavelength and permanently deactivated with
another.[11] Notwithstanding the high efficacy of olefin metathesis, there are still some challenging reactions
where routine olefin metathesis falls short.[12] For example, in his influential report on cross-metathesis (CM)
reactions, Grubbs states “While electronic and steric parameters of olefins account as key contributing factors
in ways olefins are classified, other factors are often implied in determining selectivity, including chelating
ability of certain functional groups to metal catalysts.”[12a] A classic example for this CM inadequacy is the
reaction of 2-hydroxystyrene derivatives, which is considered impractical because the substrates form stable
inactive chelates with the ruthenium catalysts.[13] Protection of the phenol functionality with a suitable
protecting group may circumvent the chelation inhibition; however, this route involves the known cost of
additional protection and deprotection steps. To simplify the deprotection step, a PPG may be used to
accomplish a functional group tolerant cleavage which does not require additional physical intervention.[14] In
order to combine this process with photoinduced olefin metathesis, the activation of the catalyst and the
removal of the PPG must be chromatically orthogonal to one another. Unfortunately, the activation wavelength
for S-chelated olefin metathesis complexes also cleaves most PPGs commonly available. Thus, to achieve
photoinduced olefin metathesis reactions in cases where a PPG is needed, we contemplated introducing UV
absorbing molecules. Herein, we report an expansion of the "sunscreen" methodology to the UV-A region and
combination of this technique with photoinduced catalytic CM reactions. Thus, photoinduced CM of 2-
nitrobenzyl protected 2-hydroxystyrenes with acrylate esters in the presence of an external solution of 1-pyrene
carboxaldehyde may be rendered feasible due to the large difference between the molar absorption coefficients
of the protected styrene and the catalyst. This methodology was applied to the photochemical generation of
various coumarin derivatives in a two-step photochemical protocol, in which irradiation with 380 nm light in
the presence of an external 1-pyrene carboxaldehyde solution is followed by irradiation with 254 nm light.
2-nitrobenzyl PPG was selected for the protection of the phenol moiety due to its low molar absorptivity in the
380 nm region and, as previously mentioned, because it is one of the most commonly used PPGs in
synthesis.[14-15] Styrenes bearing large ortho-substitution are sterically demanding CM substrates;[12a] thus, the
most active sulfur-chelated ruthenium complex to date, catalyst A (Figure 1), was selected for the task.[9a] We
deemed 1-pyrene carboxaldehyde as a reasonable molecular filter for this reaction due to its high molar
extinction coefficient between 320 nm and 420 nm, its low toxicity relative to other substituted pyrenes and its
commercial availability (Figure 2).
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10.1002/adsc.201700316
Advanced Synthesis & Catalysis
Figure 2. Molar absorption coefficients of 3a, catalyst A and 1-pyrene carboxaldehyde in CH2Cl2.
Thus, the initial challenge was to determine whether the S-chelated catalyst could be activated by light without
photocleavage of the PPG. In the absence of an external 1-pyrene carboxaldehyde solution, when 1a was mixed
with methyl acrylate (2a) in the presence of catalyst A and irradiated at 380 nm, no CM was observed.
1
Moreover, catalyst decomposition was clearly indicated by the appearance of H-NMR signals in the 9.6-10.6
ppm region, concomitant with a diminishing benzylidene signal (16.9 ppm).[16] Interestingly, even a minimal
amount of phenol deprotection, was sufficient to permanently inhibit the ruthenium catalyst and thwart CM
(only 5 mol% catalyst is used, and only a fraction of this catalyst becomes active during irradiation). Then, the
reaction was tested again, with varying concentrations of external 1-pyrene carboxaldehyde solution. Increasing
the filter solution concentration, led to improved reaction conversions until a concentration of 0.1M was
reached (Figure 3). Adding more 1-pyrene carboxaldehyde (over 0.1M) did not offer any further advantage.
Notably, the catalyst decomposition signals observed in the absence of the external 1-pyrene carboxaldehyde
were not observed when the UV filter protocol was followed, i.e. when the external 1-pyrene carboxaldehyde
solution is used the precatalyst is stable. Additional optimization of the reaction conditions (e.g. acrylate
concentration, reaction times) allowed improved reaction yields with minimal homo-dimerization of the
styrene.
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10.1002/adsc.201700316
Advanced Synthesis & Catalysis
Figure 3. Light-induced CM of 1a and 2a with varying concentrations of external 1-pyrene carboxaldehyde solution.
Reaction conditions: A solution of 0.15M 1a, 5 equiv of 2a and 5% mol of catalyst A in 0.5 mL of CD2Cl2 was prepared in
a 5mm NMR tube under Ar. The tube was placed inside a 10 mL pyrex test tube filled with 5 mL of a CH2Cl2 1-pyrene
carboxaldehyde solution of the desired concentration and irradiated in a Rayonet photoreactor for 70h. Conversion was
monitored using 1H NMR with 1,3,5-trioxane as internal standard.
To probe the reaction scope, the UV filter assisted CM of various 2-nitrobenzyl protected 2-hydroxystyrenes
and different acrylate esters was studied (Table 1). CM reaction of the α-substituted styrene (1c), was not
successful, possibly due to increased steric hindrance and the known difficulty of producing tri-substituted
double bonds in olefin CM reactions.[12a, 12b] All other photoinduced CM reactions proceeded with good yields,
in the case of acrylates, only the E isomer was obtained. Additionally, as a further example for this method,
light induced CM with cis-1,4-diacetoxy-2-butene (entry 8, 3h) was accomplished with good isolated yields.
Table 1. UV filter assisted light induced CM reactions of 2-nitrobenzyl protected 2-hydroxystyrenes.a)
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10.1002/adsc.201700316
Advanced Synthesis & Catalysis
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10.1002/adsc.201700316
Advanced Synthesis & Catalysis
a)
Reaction conditions: 0.15M of styrene (1), 10 equiv. of 2, 5%
mol of catalyst A in CH2Cl2. 0.1M 1-pyrene carboxaldehyde
solution in CH2Cl2 as the external filter solution. Irradiation in a
b)
Rayonet photoreactor with 380 nm lamps for 90h
Isolated
yields after column chromatography. c) 5 equiv of 2c were used.
Having shown the feasibility of this process, the methodology was applied in a two-step all photochemical
synthesis of coumarin derivatives. Coumarins are highly important structures, common in plants and
microorganisms,[17] and have numerous applications in the life sciences, medicine[18] and in chemistry,
especially in the field of fluorescent dyes and visible light PPGs.[5e, 14, 19] Due to their significance, a vast
number of syntheses of coumarin based structures have been reported over the years.[20] Among them, very
efficient RCM based syntheses by Grubbs and Schmidt.[21] Recently, photochemical syntheses of coumarins
have been reported by Feng and Metternich;[22] while, Pirrung in 2003 reported the photochemical cyclization
of 2-nitrobenzyl protected coumarate esters to coumarins.[23] However, the photochemical synthesis of
coumarin derivatives via a CM reaction, has not been reported. Thus, we envisioned a protocol involving two
photochemical steps, where the UV filtered CM reaction at 380 nm would be followed by removal of the
external tube containing the 1-pyrene carboxaldehyde solution and irradiation with 254 nm light, bringing
about a chain of three photochemical reactions: photodeprotection of the PPG, E/Z double bond isomerization
and subsequent cyclization to form the coumarin. Indeed, this tandem irradiation process enforced by the
external filter, led to the efficient synthesis of several coumarin derivatives 4a-4g, as summarized in Table 2.
Both methyl and tert-butyl acrylates were used as CM partners, with the bulkier acrylates giving slightly higher
yields in shorter times. The photochemical processes were viable with both electron withdrawing and electron
donating groups on the styrenes, showing the versatility of the process. Brominated and chlorinated compounds
were tolerated (entries 2, 3 and 6, Table 2) and did not undergo photodehalogenation[24] even under the
energetic UV-C light irradiation. It is worth noting that the availability of a halogen atom on the coumarin
widens the scope for further functionalization, e.g. as a Suzuki-Miyaura cross-coupling partner.
Table 2. UV filter assisted, two-step photochemical synthesis of coumarins.a)
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10.1002/adsc.201700316
Advanced Synthesis & Catalysis
a) Reaction conditions: 0.15M styrene (1), 10 equiv of 2 and 5% mol of catalyst
A in CH2Cl2. 0.1M 1-pyrene carboxaldehyde solution in CH2Cl2 as the
external filter solution. Irradiation in a Rayonet photoreactor with 380 nm
lamps for 80-100h and with 254 nm lamps for 10-30h. b) Isolated overall yields
after column chromatography.
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10.1002/adsc.201700316
Advanced Synthesis & Catalysis
In conclusion, we have carried out a catalytic light induced CM reaction between 2-nitrobenzyl protected 2-
hydroxystyrenes and acrylate esters in the presence of an external 1-pyrene carboxaldehyde solution. The
differences in the molar absorptivity between the ruthenium complex and the 2-nitrobenzyl chromophore were
exploited to selectively activate the ruthenium catalyst without removal of the 2-nitrobenzyl PPG, thus
permitting the progress of the CM reaction. In the absence of the external filter, this scheme resulted in catalyst
inhibition due to photodeprotection of the PPG and consequent formation of an unresponsive chelate with the
metal center. This is the first time that the “sunscreen” methodology is applied to allow photoactivation of a
catalyst, opening a new avenue for catalytic light activated sequences. This procedure was successfully applied
to a two-step all photochemical synthesis of various coumarin derivatives. Thus, a series of important
molecules may now be synthesized in two simple photochemical events by irradiating the reaction components
with 380 nm in the presence of an external UV filter solution, followed by irradiation at 254 nm, without
needing any additional external intervention other than the treatment by light of different wavelengths in the
right sequence.
Experimental Section
General procedure for UV filter assisted CM:
0.196 mmol of protected styrene (1), 10 equivalents of 2 and 0.05 equivalents of catalyst A, were dissolved in 1.3 mL of
CH2Cl2 in a 4 mL vial. The vial was purged with argon, capped and submerged inside of a 20 mL vial charged with 14 mL
of 0.1M solution of 1-pyrene carboxaldehyde in CH2Cl2. The external vial was capped, and the top was covered with
aluminium foil exposing only the bottom half (photographs included in the supporting information). The reaction was
irradiated using a Rayonet photoreactor equipped with 16 lamps of 380nm (RPR-3800) for approximately 90 hours (or
1
until no further conversion was recorded). Reaction progress was monitored using H-NMR or GC-MS, the external vial
was removed and volatiles were evaporated. The residue was purified directly using flash chromatography.
General procedure for bichromatic photochemical synthesis of coumarins:
The general UV filter assisted CM procedure was followed. Without the purification step, the internal vials residue was re-
dissolved in CH2Cl2 (5 mL) and transferred to a quartz tube (15 mL, 10 mm diameter). Silica gel could be added until
solution became clear. The tube was purged with argon, and capped, and the reaction was irradiated using a Rayonet
photoreactor equipped with 16 lamps of 254 nm (RPR-2537) for 10-30 h. Upon completion, the quartz tube content was
filtered, solvent was evaporated under vacuum, and the crude product was purified using flash chromatography.
Acknowledgements
The United States – Israel Binational Science Foundation (Grant No. 2014116) is gratefully acknowledged for partial funding of this
research. The Open University is acknowledged for partial financial support.
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Advanced Synthesis & Catalysis
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COMMUNICATION
Bichromatic Photosynthesis of Coumarins by UV
Filter Enabled Olefin Metathesis
Adv. Synth. Catal. Year, Volume, Page – Page
Or Eivgi, Revannath L. Sutar, Ofer Reany and N.
Gabriel Lemcoff*
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