Orthogonal photocleavage of a monochromophoric linker

Article abstract of DOI:10.1021/ja1047736

The 4-acetyl-2-nitrobenzyl moiety, substituted in both benzylic and phenacyl positions with leaving groups, has been proposed as a monochromophoric photocleavable linker. The attached groups can be disconnected selectively and orthogonally upon irradiation.

Full text of DOI:10.1021/ja1047736

Published on Web 08/04/2010
Orthogonal Photocleavage of a Monochromophoric Linker
ˇ
Laxminarayana Kammari, Toma´sˇ Solomek, Bokolombe Pitchou Ngoy, Dominik Heger, and Petr Kla´n*
Department of Chemistry, Faculty of Science, Masaryk UniVersity, Kamenice 5/A8, 625 00 Brno, Czech Republic
Received June 1, 2010; E-mail: klan@sci.muni.cz
moiety (1 or 2) substituted in both the benzylic and the phenacyl
Abstract: The 4-acetyl-2-nitrobenzyl moiety, substituted in both
benzylic and phenacyl positions with leaving groups, has been
proposed as a monochromophoric photocleavable linker. The
attached groups can be disconnected selectively and orthogonally
positions with carboxylic or alkoxy LGs (Scheme 1).
In step 1 (Scheme 1), the LG (RO-) is released exclusively from
the benzylic position by direct irradiation. This part of the ANB
chromophore is essentially a 2-nitrobenzyl moiety, a well-
established PPG, the photochemistry of which is based on a formal
upon irradiation.
intramolecular 1,5-H shift in the primary reaction step, followed
by complex transformations to give the liberated molecule.10,11 At
Linkers or anchoring groups bind organic molecules to a support
in solid-state synthesis, combinatorial chemistry, or drug delivery.1,2
They should be stable under given conditions and disconnect
selectively when required. Molecules are typically separated from
the linker in such a way that the linker moiety remains attached to
the support, which can be regenerated in a subsequent chemical
step.1 Photoinitiated (“reagentless”) removal of a chromophoric
linker is an attractive alternative, but the linker’s photochemical
properties must fulfill several criteria in order to allow rapid and
efficient release. They are generally same as those required for
photoremovable protecting groups (PPGs).3-5 PPGs are inherently
orthogonal6 to non-photochemically removable protecting groups.
Monochromatic light of different wavelengths has already been
reported to differentiate photoreleases from multiprotected sub-
strates.7
this point, “Ar” in 3 represents an unspecified chemically modified
aromatic group. The second leaving moiety (R′CO2H) in the
phenacyl position of 3 is still protected and can be separated from
the reaction mixture if required.
In step 2, R′CO2H is photochemically liberated in the presence
of a hydrogen atom donor. An analogous reaction has been
described for unsubstituted phenacyl groups which release the LGs
from the R-position by initially abstracting hydrogen from another
molecule.12,13 This step is in fact a safety-catch14 photoprocess, in
which the protected substrate is released only by the simultaneous
application of light and a chemical activator.15 In the case that R′
is a solid support, step 2 is the support regeneration process.
Scheme 2. Synthesis of the ANB Derivativesa
The aim of this work was to design a photocleavable mono-
chromophoric linker connecting two molecules (or a support)
through different functionalities, which can be disconnected
selectively and orthogonally upon irradiation. Such an approach
differs from that utilized with linkers that can be removed using a
combination of photochemical and dark processes1,2,8 or employing
a bichromophoric system.7,9 Here we report on a linker that
combines the properties of two well-known photoremovable groups
(2-nitrobenzyl and phenacyl) in a single chromophore. The work
represents a pilot study; the design and synthesis of the new linker
and some mechanistic considerations are presented here. To our
knowledge, this work is the first successful attempt to use a single
chromophore that possesses properties of a linker (or a dual
photoremoVable protecting group), from which the leaving groups
(LG) can be sequentially released.
Strategy. The structure of the proposed monochromophoric
photocleaVable linker is based on a 4-acetyl-2-nitrobenzyl (ANB)
Scheme 1. Photochemistry of a Monochromophoric Linker
a Reagents and conditions: (i) guanidinium nitrate, H2SO4, -5 °C; (ii)
AIBN, NBS, CH3CN, reflux; (iii) triethylamine (TEA), AcOEt (for 7a) or
NaI, DMF (for 7b); (iv) Br2, CHCl3, 40 °C; (v) TEA, AcOEt; (vi) p-TsOH,
benzene, reflux; (vii) aqueous NaOH, tetrabutylammonium hydrogen sulfate,
CH2Cl2; (viii) I2, acetone, 55 °C; (ix) Br2, CHCl3, 45 °C.
Synthesis of the ANB Derivatives. Compounds 1a,b and 2 in
Scheme 2 represent doubly substituted monochromophoric photo-
cleavable ANB linkers. Compounds 7a and 9-11 are model
compounds used for the quantum yield measurements and other
mechanistic investigations. The synthesis (see the Supporting
Information) of 1 and 2 started with 4-methylacetophenone (4),
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C O M M U N I C A T I O N S
which was nitrated to give 4-methyl-3-nitroacetophenone (5) using
nitroguanidine.16 In the next several steps, both the benzylic and
phenacyl positions were substituted with either carboxylate or
alkoxy LGs. In the case of 1a,b, the synthetic procedures included
sequential bromination and nucleophilic displacement steps via the
intermediates 6 and 7a,b to give the target products with reasonable
overall chemical yields (∼25%). The preparation of 2 was carried
out using similar steps; only the carbonyl group in 8 was protected
as an acetal in the nucleophilic substitution reactions and subse-
quently released to give the synthetic precursor 11. The overall
chemical yield over six synthetic steps was ∼7%. Compound 9
was prepared from 8 using a procedure similar to that described
above.
Since it already presented an impediment, this solvent was not
utilized for this particular study.
The side product, photochemically formed from the ANB moiety,
has not been identified yet. However, the individual photorelease
steps were studied on the model compounds 7a and 9-11.
Mechanism of Step 1. The 4-acetyl-2-nitrobenzyl derivatives
7a (ester) and 11 (ether) and their acetal derivatives 9 and 10 were
prepared to evaluate the acetyl group’s influence on the photo-
chemical behavior of the ANB moiety. Exhaustive irradiation of
7a and 11 gave the corresponding acid and alcohol, respectively,
in ∼90% chemical yield (GC). The disappearance quantum yields
(Φ) for the model compounds are listed in Table 2. Photodegra-
dation of the esters was an order of magnitude less efficient than
that of the ethers. The quantum yield was not determined for 7a
and 9 in methanol because the acyloxy substituent undergoes slow
solvolysis in the dark, as in 1a,b. Photodegradation of 2-nitrobenzyl
esters has already been reported to be rather inefficient (Φ is usually
below 0.1) compared to that of 2-nitrobenzyl ethers (Φ can reach
0.5 or more).11,17-19 The quantum yields obtained in this work
and listed in Table 2 are thus in agreement with this general
tendency.
Photolysis of the ANB Derivatives. To demonstrate that the
ANB moiety is capable of orthogonal photochemical release of two
different LGs, compounds 1a,b and 2 were treated in the following
manner:
For step 1 (Scheme 1), a degassed solution of the ANB linker
(5 × 10-3 M) in acetonitrile was irradiated with a medium-pressure
mercury lamp (450 W) through a Pyrex filter (>290 nm). When
the starting material was consumed (>95%), the chemical yields
(88-94%, Table 1) of the released acid (R1CO2H) or alcohol
(R3OH, Scheme 2) were determined using gas chromatography
(GC). The acids were completely liberated from the esters 1a,b in
∼4 h, while the alcohol was released from the ether 2 in ∼30 min
under the given conditions. Only negligible amounts (<3%) of LG
were released from the phenacyl position after prolonged irradiation.
Table 2. Step 1: Quantum Yields of Disappearance (Φ)
compound
Φ in CH3CNa
Φ in CH3OHa
7a (ketone)
9 (acetal)
0.051 ( 0.001
0.048 ( 0.001
n.d.b
n.d.b
11 (ketone)
10 (acetal)
0.88 ( 0.01
0.77 ( 0.01
0.61 ( 0.02
0.86 ( 0.02
Table 1. Chemical Yields of Leaving Groups Photoreleasea
a Degassed solutions (5 × 10-3 M) were irradiated at 313 ( 5 nm
benzaldehyde in methanol (Φ ) 0.41) as an actinometer.20 The irradiated
solutions were analyzed using GC. Each sample was measured at least three
times; the standard deviation of the mean is indicated. b n.d. ) not
determined. Esters 7a and 9 in methanol were unstable in dark.
yield (%) of [LG] releaseb
(optical bench).
Φ was determined using a solution of 2-nitro-
linker
step 1c
step 2c
1a
1b
2
88 [PhCH2CO2H]
94 [PhCO2H]
90 [CH3(CH2)14CO2H]
97 [PhCH2CO2H]
93 (91d) [PhCH2OH]
90 (89d) [CH3(CH2)14CO2H]
a Degassed solutions (5 × 10-3 M) in acetonitrile were irradiated at λ
> 290 nm to >95% conversion. All data are the average of at least three
measurements. b The optimized acid (for 1a,b; Scheme 2) or alcohol (for
2) releases in the presence of 2-propanol (IPA; at least 2 mol equiv) as
Intramolecular photoreduction of the nitro group and subsequent
release of a leaving moiety from the benzylic position of the
2-nitrobenzyl chromophore has been systematically studied for the
past 30 years.3-5,10,11,21-24 A light-induced intramolecular 1,5-
hydrogen shift in the primary chemical step to form aci-nitro
intermediates, followed by formation of benzoxazolidines and
subsequent ring opening to release the LG and give a 2-nitrosobenz-
aldehyde derivative, is a well-established mechanism. It has been
reported that the triplet state (3n,π*) of 4-nitroacetophenone is
formed rapidly upon irradiation25 and that the reduction in the
presence of H-atom donors occurs essentially on the nitro group
instead of the acetyl group.26,27 Therefore, we anticipated that the
excited-state nature of the ANB group must be favorable for an
intramolecular reduction of the NO2 group and subsequent LG
release, and that the intermediate 3 (Scheme 1) is a 2-nitrosobenz-
aldehyde derivative as shown in Scheme 3 (in red). Indeed,
irradiation of 11 at 313 nm produced 12, and its structure was
established by its in situ reaction with benzylamine28 to give the
3-(N-benzylamino)anthranil 13 in ∼60% isolated chemical yield
(Scheme 3). In the case of 7a, the longer irradiation time caused
extensive secondary decomposition of the reaction intermediate;
the complex mixture of side products was not identified.
a
H-atom donor (GC). c See Scheme 1. d In acetonitrile/methanol
mixture (50:50, v/v).
For step 2, 2-propanol (IPA, a hydrogen atom donor) was added
to the solution. The reaction mixture was then purged with argon
and irradiated with the same light source as before. The chemical
yields of acid (R2CO2 H ) R4CO2H, Scheme 2) release were
90-97% (GC). More than 2 mol equiv of IPA was needed to
accomplish complete photorelease of the acid (Table 1). Higher
amounts of the reagent did not improve the chemical yields but
cut the irradiation time. For example, complete photorelease of
phenylacetic acid from the intermediate 3, formed from 1a, in the
presence of 1000 and 2 mol equiv of IPA was achieved in 11 and
50 h, respectively. The order of steps 1 and 2 cannot be reversed;
both LGs are released from both 1 and 2 in the presence of a H-atom
donor upon irradiation.
In the case of 2, the initial concentration was kept equal to or
below 5 × 10-3 M, because the released benzyl alcohol in step 1
acted as a hydrogen donor. In such a case, palmitic acid was
partially released already in the first step; thus, a safety-catch
obstacle was disabled. This observation was subsequently confirmed
by a series of irradiation experiments using variable benzyl alcohol
concentrations. The acyloxy substituent in the benzylic position of
1a,b was found to undergo thermal hydrolysis in methanol to
produce the corresponding methyl ester (∼10% conversion in 24 h).
Mechanism of Step 2. The 2-nitrosobenzaldehyde 3 (or 12)
formed in step 1 (Scheme 3) possesses three functional groups
which could potentially be photoactive. In the presence of hydrogen
atom donors, aromatic ketones and aldehydes are known to be
photoreduced to the corresponding alcohols or pinacols,29-31 and
aromatic nitroso compounds can give products of two- and four-
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Scheme 3. A “Step 1” Photoproduct and Its Trapping
chromophore preparation, the necessity to use UV radiation (<350
nm), and lower quantum efficiencies of the release in some cases.
Several approaches for improving the monochromophoric linker
properties and the mechanism of step 2 are currently under
investigation in our laboratory.
Acknowledgment. This paper is dedicated to Prof. Richard S.
Givens on the occasion of his 70th birthday. Support for this work
was provided by the Grant Agency of the Czech Republic (203/
09/0748), the Ministry of Education, Youth and Sports of the Czech
Republic (MSM0021622413), and the European Union (CETO-
COEN; CZ.1.05/2.1.00/01.0001). We thank J. Wirz for valuable
comments and J. Frantisek (Ratiochem, Ltd., Brno) for mass
spectrometric data. T.S. acknowledges the Scholarship for talented
Ph.D. students of the Brno City.
electron reduction (hydroxylamines and anilines, respectively).31-33
Photorelease of a LG attached to the R-position of the phenacyl
chromophore is known to be initiated by hydrogen abstraction by
the triplet excited carbonyl group (Scheme 4a).13,34 Therefore, we
attempted to identify the intermediates responsible for LG release
after 3 (Scheme 3) is photolyzed in the presence of 2-propanol.
Density functional theory quantum chemical calculations showed
Supporting Information Available: Experimental procedures,
analytical data for all new compounds, and quantum chemical calcula-
tion results. This material is available free of charge via the Internet at
http://pubs.acs.org.
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