Regioselective Chromatic Orthogonality with Light-Activated Metathesis Catalysts

Article abstract of DOI:10.1002/anie.201500740

The ability to selectively guide consecutive chemical processes towards a preferred pathway by using light of different frequencies is an appealing concept. Herein we describe the coupling of two photochemical reactions, one the photoisomerization and consequent activation of a sulfur-chelated latent olefin-metathesis catalyst at 350nm, and the other the photocleavage of a silyl protecting group at 254nm. Depending on the steric stress exerted by a photoremovable neighboring chemical substituent, we demonstrate the selective formation of either five- or six-membered-ring frameworks by light-triggered ring-closing metathesis. The orthogonality of these light-induced reactions allows the initiation of these processes independently and in interchangeable order, according to the wavelength of light used to promote them. A guiding light: By the combination of a light-activated sulfur-chelated olefin-metathesis catalyst with a photocleavable bulky silyl protecting group, two orthogonal pathways enabled the selective synthesis of five- or six-membered heterocycles according to the order in which the starting material was irradiated with light of different wavelengths. Thus, protecting-group removal and metathesis gave either a dihydropyran or a dihydrofuran.

Full text of DOI:10.1002/anie.201500740

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201500740
German Edition: DOI: 10.1002/ange.201500740
Photochemistry
Regioselective Chromatic Orthogonality with Light-Activated
Metathesis Catalysts**
Efrat Levin, Sudheendran Mavila, Or Eivgi, Eyal Tzur, and N. Gabriel Lemcoff*
Abstract: The ability to selectively guide consecutive chemical
processes towards a preferred pathway by using light of
different frequencies is an appealing concept. Herein we
describe the coupling of two photochemical reactions, one the
photoisomerization and consequent activation of a sulfur-
chelated latent olefin-metathesis catalyst at 350 nm, and the
other the photocleavage of a silyl protecting group at 254 nm.
Depending on the steric stress exerted by a photoremovable
neighboring chemical substituent, we demonstrate the selective
formation of either five- or six-membered-ring frameworks by
light-triggered ring-closing metathesis. The orthogonality of
these light-induced reactions allows the initiation of these
processes independently and in interchangeable order, accord-
ing to the wavelength of light used to promote them.
tion of ruthenium olefin-metathesis initiators has clear uses in
polymer synthesis,[9] the development of a photocatalytic
orthogonal process could expand the utility of these catalysts
for other synthetic applications. Herein, we show how the
order of light irradiation of a latent catalyst and a protected
substrate influences the selectivity for the generation of either
a five- or six-membered cycloalkene isomer as the preferred
final product.
The activation of latent precatalysts 1 and 2 (Scheme 1) by
light encouraged us to search for applications of this process
in novel light-guided synthetic pathways. Thus, 1 and 2 were
irradiated at two different wavelengths in the presence of the
benchmark ring-closing-metathesis (RCM) substrate diethyl
A
rguably one of the most impressive examples of efficient
use of energy is the way nature has exploited photons to
advance living systems. The wavelength of light determines
the exact energy delivered by a photon, and chemists, like
nature, have harnessed this energy in processes called photo-
chemical reactions.[1] A novel concept developed by Bochet
and co-workers[2] is the use of chromatically orthogonal
procedures as sequential chemical reactions that are carried
out selectively in the presence of light of different energies.[3]
These processes have been developed mostly by the use of
complex functional-group-deprotection schemes, in which the
photoinduced removal of a specific protecting group is
necessary in the presence of another protecting group that
remains intact.[4]
Scheme 1. Photoisomerization of sulfur-chelated ruthenium benzyli-
dene complexes.
diallylmalonate (DEDAM). As previously reported, photo-
isomerization of the precatalysts by irradiation at 350 nm
afforded the active trans isomers and led to concurrent ring
closing of DEDAM.[8a,10] However, when solutions of 1 and 2
in dichloromethane were irradiated with ultraviolet light at
254 nm, the cis isomer remained intact, and, as expected, no
RCM reaction occurred (see the Supporting Information).
Having shown that olefin metathesis could be induced with
UV chromatic selectivity, we decided to combine the photo-
activation of RCM with an orthogonal photodeprotection
reaction. We chose to pursue this approach by using the
tris(trimethylsilyl)silyl protecting group (supersilyl),[11,12]
a photochemically removable protecting group that may be
photocleaved at the selected wavelength, 254 nm, and is not
deprotected at 350 nm.
Photocatalytic processes that enable efficient chemical
reactions to occur have been widely used and studied.[5] At
times, light promotes these catalytic processes by activation of
a dormant catalyst.[6] For example, the lower activity of cis-
dichloro ruthenium olefin-metathesis catalysts with respect to
the corresponding trans-dichloro isomers[7] has been exploited
for the activation of sulfur-chelated ruthenium benzylidenes
by a photoisomerization process.[8] Whereas the photoactiva-
[*] E. Levin, Dr. S. Mavila, O. Eivgi, Prof. N. G. Lemcoff
Department of Chemistry, Ben-Gurion University of the Negev
Beer-Sheva 84105 (Israel)
E-mail: lemcoff@bgu.ac.il
To study the orthogonality of the system, we prepared 4-
[tris(trimethylsilyl)silyloxy]hepta-1,6-diene (4) from 1,6-hep-
tadien-4-ol (3; Scheme 2a). First, the supersilyl protecting
group in 4 was fully cleaved after irradiation for 20 min at
254 nm in CH2Cl2 with methanol to regenerate 3 (Sche-
me 2b). Moreover, irradiation of 4 at 350 nm for 1 h did not
cleave the protecting group: an essential requisite for the
orthogonal system. In a parallel experiment, the olefin-
Dr. E. Tzur
Department of Chemical Engineering
Shamoon College of Engineering
Ashdod 77245 (Israel)
[**] The Israel Science Foundation is gratefully acknowledged for
financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500740.
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CH2Cl2 was prepared. Irradiation of this solution with UV
light, first at 350 nm and later at 254 nm, produced the
expected compound 6 with good overall conversion. In
contrast, the alternative path, starting with irradiation at
254 nm, provided disappointing results. Even though the
complex did not undergo isomerization at 254 nm, it strongly
absorbs light in this region, thus leading to a sunscreen
effect.[8a,13] Longer irradiation times were required for the
photoinduced deprotection owing to short-wave UV absorp-
tion by the ruthenium complex, which eventually led to the
decomposition of the precatalyst. Furthermore, undesired
cycloisomerization[14] was observed. To further investigate
potential causes for the occurrence of these side reactions, we
irradiated a solution of DEDAM and 1 at 254 nm in the
presence of a simple protected alcohol, 2-[tris(trimethylsi-
lyl)silyloxy]pentane (7; Scheme 4). Whereas irradiation for
Scheme 2. Orthogonal deprotection–RCM sequence. a) Synthesis of
the supersilyl-protected compound 4 from alcohol 3 (69% yield).
b) Irradiation of 4 at 254 nm for 20 min in the presence of methanol
(10 equiv) in CH2Cl2 (quantitative conversion). c) Ring-closing meta-
thesis of 4 provided cyclopentene 5 with 88% conversion after
irradiation for 18 h at 350 nm in CH2Cl2. DMAP=4-dimethylamino-
pyridine.
metathesis reactivity of compound 4 in the presence of
precatalyst 1 was tested. As expected, UV irradiation of
a solution of 4 and 1 in CH2Cl2 at 350 nm afforded the RCM
product 5 (88% yield; Scheme 2c).
Scheme 4. Influence of the silyl ether 7 on the irradiation of DEDAM
with precatalyst 1 at 254 nm in CH2Cl2.
Once the individual orthogonal processes were secured,
a commutative chromatic orthogonal experiment was set up
on the basis of two converging pathways (Scheme 3). Path a
involved the addition of the precatalyst 1 to a solution of 4,
followed by irradiation at 350 nm to produce cyclopentene 5.
30 min without the protected 2-pentanol did not affect the
starting materials, the cycloisomerized product
8 was
obtained when was added. Thus, cycloisomerization
7
occurred only when the silyl group was present in the reaction
vial. This result falls in line with previous reports of cyclo-
isomerization promoted by ruthenium–benzylidene-type cat-
alysts in the presence of trimethylsilyl vinyl ether[15] and
suggests that decomposition products of supersilyl photo-
cleavage affect the latent form of sulfur-chelated ruthenium
benzylidene complexes as well. Ruthenium hydride formation
in the presence of alcohols or vinyl ethers has been reported
previously;[16] however, we could not discern any cycloisome-
rization process without the addition of the silyl group and
UV irradiation at 254 nm, even after adding methanol.
Further research with other solvents revealed that
reduced irradiation times were possible for the photoinduced
deprotection when 1,1,2,2-tetrachloroethane (TCE) was
used.[17] This finding permitted the photocleavage of the
supersilyl group in the presence of the ruthenium complex
just by changing the solvent. The adjusted procedure involved
mixing diene 4 with methanol (100 equiv) and catalyst
1 (11 mol%) in TCE. First, irradiation at 254 nm provided
the alcohol in 30 min; next, irradiation at 350 nm for 21 h
(CH2Cl2 was added at this stage to enhance the photo-
isomerization of precatalyst 1) gave the RCM product in over
90% yield (see the Supporting Information for full exper-
imental details). Cleavage of the protecting group in the
presence of the catalyst in this way without triggering
metathesis (or other unwanted side reactions) thus provided
the foundation for a chromatic orthogonal catalytic process.
The examples described above are commutative; that is,
the order of irradiation does not influence the final result.
Scheme 3. Commutative chromatically orthogonal sequences.
Methanol (20 equiv) was then added, and irradiation at
254 nm afforded compound 6. Path b required the irradiation
of a solution of 4 at 254 nm for 20 min, which led to removal of
the silyl protecting group; then 1 was added to the mixture,
and irradiation at 350 nm afforded RCM product 6 (see the
Supporting Information for full experimental details).
The photochemical reactions presented in Scheme 3,
although one-pot processes, require the external addition of
reagents after the first step. A more challenging endeavor
would be to perform the reactions just by changing the order
of light irradiation without any additional interference
throughout the entire process, that is, in a closed system.
Thus, a solution of diene 4, precatalyst 1, and methanol in
2
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Table 1: RCM of triene 10 or 11 with benzylidene ruthenium precata-
Inspired by the documented property of a supersilyl func-
tional group to influence the stereochemical outcome of
reactions,[18] and also by the pioneering studies on triene
substrates by Schmidt and Nave,[19] we set out to develop
a noncommutative olefin-metathesis catalytic cycle in which
the order of irradiation with light of different wavelengths
could be used to guide the selectivity of a reaction.
For this purpose, compound 11 containing three carbon–
carbon double bonds and a supersilyl ether group was
prepared. Diol 9 was selectively allylated to afford triene
10, which was protected with a tris(trimethylsilyl)silyl group
to provide substrate 11 (Scheme 5).
lysts.[a]
Catalyst
Conversion [%][b]
10!12
10!13
11!14
11!15
Grubbs I
Hoveyda–Grubbs I
Grubbs II
Hoveyda–Grubbs II
catalyst 1
catalyst 2
42
51
18
36
49
82
99
99
87
1
1
13
10
3[d]
12[d]
26
74
90
18[c]
12[c]
82[c]
88[c]
54[d]
87[d]
[a] Reactions were carried out at room temperature in CH2Cl2 with
methanol (10 equiv) and a catalyst loading of 6 mol% unless otherwise
1
indicated. [b] Conversion was measured by H NMR spectroscopy and
GC–MS. [c] The reaction was carried out with a catalyst loading of
10 mol% and irradiation at 350 nm for 18 h. [d] The reaction was carried
out with a catalyst loading of 15 mol% and irradiation at 350 nm for
60 h.
Scheme 5. Synthesis of the triene supersilyl compound 11. a) Allyl
bromide, NaH, THF (58%); b) (Me3Si)3SiCl, DMAP, CH2Cl2 (49%).
The regioselectivity of the ring-closing metathesis reac-
tion was studied both in the presence and absence of the
supersilyl protecting group with different ruthenium catalysts.
Compounds 10 and 11 may independently produce two
different RCM products,[20] namely, five- and six-membered
heterocycles (Scheme 6), depending on which double bonds
participate in the reaction (the formation of a cyclobutene
would be disfavored owing to its high steric strain).
Scheme 7. Chromatically orthogonal system.
catalyst 1 in CD2Cl2, and the solution was irradiated at 350 nm
for 68 h. The total conversion observed was approximately
80%, with the five-membered-ring compound 14 obtained as
the major product (14: 62%, 15: 15%). Next, methanol was
added, and irradiation at 254 nm for 2.4 h promoted the
desired photocleavage to give the dihydrofuran product 12
with 6:1 selectivity over 13 (10: 20%, 12: 69%, 13: 11%;
Table 2, entry 1). The orthogonal pathway began with the
irradiation of 11 at 254 nm for 40 min to afford alcohol 10.
Catalyst 1 was then added, and irradiation at 350 nm for 2 h
delivered RCM products 13 and 12 in a 9:1 ratio with full
conversion (Table 2, entry 4).
Following the successful development of this noncommu-
tative sequence, closed-system experiments were carried out
in which all the reactive components were within the reaction
vessel from the start. Once again, upon irradiation first at
350 nm and then at 254 nm, good conversion was observed,
and 12 was synthesized selectively (Table 2, entries 2 and 3).
With the more active precatalyst 2, a high conversion of 88%
was observed with high selectivity of around 30:1. For the
alternative pathway (Table 2, entries 6 and 7), substrate 11
Scheme 6. Possible RCM products derived from 10 and 11.
In line with the findings of Schmidt,[19] when substrate 10
was used, the first-generation ruthenium olefin-metathesis
catalysts produced five- and six-membered rings as approx-
imately 1:1 mixtures (Table 1). However, the second-gener-
ation catalysts provided compound 13 with a six-membered
ring as the major product. Notably, the observed regioselec-
tivity with 2 for the formation of a dihydropyran structure is
surprisingly high.[19a]
When the bulky substrate 11 was used, a reversal of
selectivity was expected with a preference for the five-
membered-ring product. Indeed, all ruthenium catalysts
tested showed good selectivity for the formation of 14.
Notably, the use of the supersilyl protecting group also
enhanced the selectivity for the five-membered-ring product
with the second-generation catalysts, in contrast with previous
results by Schmidt and Nave when using bulky esters.[19b]
Finally, the orthogonal sequence was examined with
triene 11 (Scheme 7). Thus, triene 11 was dissolved with
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Table 2: Chromatic synthesis of dihydrofuran 12 and dihydropyran 13.[a]
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Entry Catalyst MeOD System
[equiv]
Irradiation order
Conv. [%][b]
(12/13)
1[c]
2[c]
3[c]
4[d]
5[d]
6[e]
7[e]
1
1
2
1
2
1
2
20
5
10
5
5
100
80
sequential 350 nm!254 nm
80 (6:1)
46 (10:1)
88 (28:1)
closed
closed
350 nm!254 nm
350 nm!254 nm
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sequential 254 nm!350 nm 100 (1:9)
sequential 254 nm!350 nm 100 (1:5)
closed
closed
254 nm!350 nm
254 nm!350 nm
54 (1:6)
32 (1:4)
[a] Reactions were carried out at room temperature in CD2Cl2 with
a catalyst loading of 10 mol% unless otherwise indicated. [b] Conversion
was measured by 1H NMR spectroscopy. [c] Irradiation time: 50–68 h at
350 nm, 40–160 min at 254 nm. [d] Irradiation time: 40 min at 254 nm,
2–16 h at 350 nm. [e] The reaction was carried out in [D2]TCE with
a catalyst loading of 11 mol%; CD2Cl2 (0.4 mL) was also added, and the
mixture was irradiated at 350 nm overnight.
was mixed with methanol and a catalyst in [D2]TCE, and
irradiation at 254 nm provided alcohol 10 after 30 min. The
addition of CD2Cl2 and irradiation at 350 nm overnight gave
the desired metathesis product 13 with 6:1 selectivity over the
five-membered-ring product 12 when precatalyst 1 was used.
Notably, in this pathway, the use of the less active precatalyst
1 gave better results, probably as a result of lessened
decomposition during the short-wave irradiation period.
In conclusion, we designed, developed, and demonstrated
a proof of concept for a novel catalytic chromatically
orthogonal system based on a light-sensitive protecting
group and a light-triggered latent olefin-metathesis catalyst.
Different synthetic pathways were followed by taking advant-
age of the chromatic selectivity of the latent sulfur-chelated
olefin-metathesis precatalyst. Photoisomerization of a cis-
dichloro ruthenium benzylidene complex and photocleavage
of a silyl ether protecting group at different wavelengths
enabled the selective production of either dihydrofuran or
dihydropyran scaffolds, depending on the order in which the
processes were activated. Further development of this con-
cept may lead to the use of chemical systems in which light of
different wavelengths can be used to produce desired com-
pounds for smart applications or in environments in which
actual physical intervention is precluded.
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Keywords: chromatic orthogonality · cyclization · heterocycles ·
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[D2]TCE. For experimental details, see the Supporting Informa-
tion.
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Received: January 26, 2015
Revised: February 26, 2015
Published online: && &&, &&&&
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Communications
Photochemistry
A guiding light: By the combination of
a light-activated sulfur-chelated olefin-
metathesis catalyst with a photocleavable
bulky silyl protecting group, two orthog-
onal pathways enabled the selective syn-
thesis of five- or six-membered hetero-
cycles according to the order in which the
starting material was irradiated with light
of different wavelengths. Thus, protect-
ing-group removal and metathesis gave
either a dihydropyran or a dihydrofuran.
E. Levin, S. Mavila, O. Eivgi, E. Tzur,
N. G. Lemcoff*
&&&& — &&&&
Regioselective Chromatic Orthogonality
with Light-Activated Metathesis Catalysts
6
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