Synthesis and degradation of photolabile dendrimers based on o-nitrobenzyl ether photolabile cores

Article abstract of DOI:10.1039/b617289j

Two dendrimer cores, 1a and 2a, that contain o-nitrobenzyl photolabile moieties, lack hydrolytically sensitive ester linkages and possess three and six sites for dendron attachment, respectively, have been alkylated to provide methylated core analogs 1b and 2b as well as second-generation benzylaryl ether dendrimer 1c and third-generation dendrimer 2c. These dendrimers undergo clean photocleavage as indicated by the evolution of isosbestic points in the UV spectra during photolysis. In addition, the nature of the photodegradation products was confirmed by observing the photolyses by both ¹H NMR and GPC. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b617289j

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and degradation of photolabile dendrimers based on
o-nitrobenzyl ether photolabile coresw
Robert M. Kevwitch and Dominic V. McGrath*
Received (in Montpellier, France) 27th November 2006, Accepted 26th March 2007
First published as an Advance Article on the web 11th April 2007
DOI: 10.1039/b617289j
Two dendrimer cores, 1a and 2a, that contain o-nitrobenzyl photolabile moieties, lack
hydrolytically sensitive ester linkages and possess three and six sites for dendron attachment,
respectively, have been alkylated to provide methylated core analogs 1b and 2b as well as second-
generation benzylaryl ether dendrimer 1c and third-generation dendrimer 2c. These dendrimers
undergo clean photocleavage as indicated by the evolution of isosbestic points in the UV spectra
during photolysis. In addition, the nature of the photodegradation products was confirmed by
1
observing the photolyses by both H NMR and GPC.
heterochiral diastereomers, detectable by NMR but not separ-
Introduction
able by flash column chromatography, a consequence of the
racemic nature of the chiral starting material. Alkylation of
core 1a afforded photolabile dendrimers 1b and 1c. Compound
1
–5
The ability to degrade dendrimers
in a controlled fashion
has the potential to impact previously established uses of
dendrimers, as well as introduce new paradigms for their roles
1
b was synthesized by the reaction of photolabile core 1a with
6
–8
9–14
in chemical systems.
Photolabile dendrimers
allow the
methyl iodide in the presence of K CO (Scheme 1). Second-
2
3
alteration of the nature of a discrete macromolecular system
by a non-invasive stimulus, which may have relevance to
generation dendrimer 1c was synthesized by treating trifunc-
tional photolabile core 1a with second-generation benzylaryl
22
1
5
biological applications. We reported the first examples of
photolabile dendrimers that contained specific, photodegrad-
ether bromide [G-2]Br under similar conditions (Chart 1).
Hexafunctional core 2a is, in a sense, a design improvement
over trifunctional core 1a. Replacing the methyl group with a
second nitrophenyl ring serves three purposes. First, it intro-
duces enhanced photocleavage due to the presence of an
1
6–21
able o-nitrobenzyl-based linkages
in the interior and
demonstrated their fragmentation upon irradiation with ultra-
9
violet light. However, these dendrimers possessed hydrolyti-
cally sensitive ester linkages at the core, and their syntheses,
although convergent, involved manipulations on the photo-
labile moieties prior to the final dendron attachment to the
core. We sought to incorporate the photolabile linkage into a
core moiety onto which we could attach separately prepared
dendrons of varying generation and exterior functionality,
thereby minimizing reaction chemistry on the photosensitive
units. Accordingly, cores 1a and 2a serve this need, as well as
meet several design criteria, including containing (a) the well-
23
additional nitro group. Second, it increases dendritic branch-
ing by doubling the number of attachment points. Third, it
eliminates the presence of stereoisomers since there are no
longer chiral centers in the structure. Two hexafunctional
photolabile dendrimers, 2b and 2c were synthesized by alkyla-
tion of core 2a. Hexafunctional core 2a was treated with
2 3
methyl iodide in the presence of K CO and 18-crown-6 to
afford first-generation dendrimer 2b (Scheme 1). Treatment of
hexafunctional core 2a with second-generation benzylaryl
22
1
7
studied o-nitrobenzyl group; (b) several sites (three and six,
ether bromide [G-2]-Br under similar conditions provided
respectively) for attachment of dendrons; and (c) no hydro-
nominally third-generation dendrimer 2c (Chart 1).
1
0
lytically sensitive ester linkages. Herein is described the
synthesis and degradation of photolabile dendrimers prepared
from these cores.
All dendrimers (1b, 1c, 2b and 2c) were thermally stable
(
acetone, 55 1C, 12 h; solid state, rt, three months) and were
1
13
readily characterized by H and C NMR, mass spectrometry
and elemental analysis.
Results and discussion
Degradation of photolabile dendrimers
Photodegradation of dendrimers 1b, 1c, 2b and 2c occurred
readily upon irradiation with UV light. Photolyses were
carried out at the lmax of each system, 340–345 nm, with a
bathochromic shift in the lmax of the system during photolysis.
The degradation of each compound was monitored by UV-Vis
Synthesis of photolabile dendrimers
Photolabile cores 1a and 2a (Scheme 1) were both prepared
from readily available piperonal in four and five steps, respec-
1
0
tively. Core 1a exists as a 1 : 3 mixture of homochiral :
1
spectroscopy and H NMR spectroscopy to assist in elucida-
Department of Chemistry, University of Arizona, Tucson, AZ 85721,
USA. E-mail: mcgrath@u.arizona.edu; Fax: 520-621-8407;
Tel: 520-626-4690
w This paper was published as part of the special issue on Dendrimers
and Dendritic Polymers: Design, Properties and Applications.
tion of the photolytic pathway. In the case of the larger
compounds 1c and 2c, photodegradation was also monitored
by gel permeation chromatography (GPC). The photodegra-
dation of all dendrimers was consistent with generation of the
1
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Dendrimers 2b and 2c were similarly irradiated separately in
chloroform and the degradations were followed by UV-Vis
spectroscopy (Fig. 1(c) and (d)). Isosbestic points were again
evident for both 2b (257, 268 and 354 nm) and 2c (258, 269 and
3
52 nm). The photodegradation of dendrimers 2b and 2c
should result in release of nitronitroso benzophenones 2d
and 2e, respectively, and the corresponding core 3 (Scheme
2
, bottom). The photolysis of both compounds in THF (12
mM) again exhibited similar spectral evolution with isosbestic
points in the UV-Vis (not shown).
The logical existence of two intermediates in the stepwise
degradation of 1b, 1c, 2b and 2c (i.e., after one and two
equivalents of their respective photoproducts 1d, 1e, 2d, or
2
e have been cleaved from the core) and the appearance of
isosbestic points, implies that these systems may be considered
‘pseudo-two-component’’ or a depolymerization process (i.e.,
P - nM) where polymer subunits and monomer units are
‘
n
9
,10,24
distinct chromophores.
1
H NMR
Supporting evidence for the identity of the photocleavage
1
products was obtained by following the photolyses by
NMR in CDCl (Fig. 2). Photolysis of 1b was accompanied
by the decrease in intensity of the methoxy singlets
d 3.92–3.96) corresponding to the six diastereomerically
H
3
(
9
related methoxy groups of 1b and the evolution of two
new methoxy peaks (d 4.08 and 3.88) corresponding to 1d.z
The correlation between disappearance of 1b and appearance
of 1d can be seen readily in terms of percentage conversion as
measured by NMR integration (Fig. 3).
1
In comparing the H NMR of the photolysis of 1b with that
Scheme 1 Synthesis of photolabile dendrimers from cores 1a and 1b.
of second-generation 1c (Fig. 2(b)), the replacement of the
methyl meta to the nitro group in 1b with a benzylaryl ether
dendron in 1c is readily apparent. The three (rather than six)
diastereomerically related methoxy groups of 1c appear as
three singlets in a 1 : 2 : 1 ratio, consistent with a statistical
mixture of 1 : 3 homochiral/heterochiral diastereomers, at
respective nitroso ketones and the 1,3,5-tris(hydroxymethyl)-
benzene core (3) (Scheme 2).
UV-Visible spectrophotometry
3
.87–3.89 ppm. Photodegradation was accompanied by the
Compound 1b was photolyzed at 345 nm in chloroform
solution (12 mM) and monitored by UV-Vis spectroscopy
decrease in intensity of these peaks and the evolution of the
methoxy peak corresponding to photoproduct 1e at 4.03 ppm,
with a good correlation according to integration data (Fig. 3).
The apparent doublet at 3.90 ppm and singlet at 3.93 ppm are
attributed to partially degraded dendrimer with one and two
(
Fig. 1(a)). Photodegradation was carried out at ambient
temperature and there was no noticeable temperature change
during the reaction. The evolution of isosbestic points at 280
and 322 nm suggested a clean transformation into photopro-
duct 1d according to Scheme 2. The photolysis of 1b in THF
9
equivalents of 1e removed from the core, respectively. These
peaks were not present after complete photodegradation.
1
The photocleavage of 2b was followed by H NMR (Fig.
(
12 mM) exhibited a similar spectral evolution (not shown).
Second-generation dendrimer 1c was photolyzed in chloro-
2(c)) in CDCl3 and indicated the disappearance of both
methoxy peaks (d 3.96 and 3.76) of 2b and the appearance
of four methoxy peaks (d 4.16, 3.95, 3.94 and 3.85) consistent
with the structure of the unsymmetrical benzophenone 2d. The
form (12 mM, 343 nm) and monitored by UV-Vis spectroscopy
Fig. 1(b)). The absorbance spectrum of 1c contained the same
(
bands as that of 1b with the addition of the benzylaryl ether
dendron band at ca. 285 nm. The evolution of this band was
minor, as would be expected, while the chromophore at 343
nm underwent the bathochromic shift characteristic of o-
nitrobenzyl degradation. Isosbestic points at 256 and 327 nm
was evidence of clean photodegradation into photoproducts
1
appearance of several other peaks throughout the H NMR
during the photolysis is attributed to photolytic intermediates
during the reaction, as these peaks disappear upon complete
photolysis.
1
e according to Scheme 2. The photolysis of 1c in THF
1
z The identity of 1d was verified by comparison to the H NMR
(
(
12 mM) exhibited a similar spectral evolution in the UV-Vis
not shown).
spectrum, provided by Dr C. P. Holmes of Affymax Research
Institute, Palo Alto, CA, of an authentic sample.
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Chart 1 Structural formulas of second-generation dendrimer 1c and third-generation dendrimer 2c.
Photocleavage of 2c was similarly monitored by following Gel permeation chromatography
1
3
the photolysis by H NMR in CDCl (Fig. 2(d)) and a
Evidence for the existence of the photolytic intermediates
observed in the NMR experiments was obtained by monitor-
ing the photolyses by GPC. A solution of dendrimer 1c in
comparison with the data from the photocleavage of 2b reflects
the replacement of two methoxy groups with a benzyl
aryl ether dendron. The single methoxy peak corresponding
to dendrimer 2c at 3.64 ppm decreased in intensity with
concomitant evolution of two chemical shift nonequivalent
methoxy peaks corresponding to photoproduct 2e at 4.09 and
CH
column (Fig. 4). Initially, the only species present is dendrimer
c. Upon photolysis, it was evident that the high molecular
2 2
Cl was photolyzed in intervals and run through a GPC
1
3
.87 ppm. Photolytic intermediates are observed in this case as
well, largely between 3.8–3.9 ppm, that subside upon complete
photodegradation.
3
Fig. 1 UV-Vis spectra of the photolysis of 12 mM solutions (CHCl )
of (a) 1b, (b) 1c, (c) 2b and (d) 2c at t = 0, 2, 4, 6, 8, 10, 15, 25, 35,
Scheme 2 Proposed degradation products from the photolysis of
dendrimers 1b, 1c, 2b and 2c.
45 min.
1
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1
Fig. 2 H NMR spectra of the photolysis of 5 mM solutions (CDCl
3
) of (a) 1b, (b) 1c, (c) 2b and (d) 2c at t = 0, 10, 20, 30, 40, 50, 60, 90, 120
(
1c and 2c only), 150 (1c and 2c only), 180 min (proceeding upwards).
Scheme 3 Schematic representation of the sequential photodegrada-
tion of a three-arm dendrimer (A) into a two-arm dendrimer (B), a
one-arm dendrimer (C) and full degradation (D).
2 2
Photolytic degradation 2c in CH Cl was also monitored by
GPC. Conversion into smaller molecular weight components
is also evident (Fig. 4). Dendrimer 2c (28.03 min) degraded by
loss of one dendron to give two-arm dendrimer B (28.03 min)
as well as dendritic fragment 2e (30.57 min) (Scheme 2).
Dendrimer 2c and partially intact dendrimer B appear to
coelute in the GPC because complete photolysis results in
the disappearance of the peak at 28.03 min and no other peaks
in the GPC can be attributed to B. Subsequently, B degraded
by further loss of dendron to give one-arm dendrimer C (28.84
min) and release of another equivalent of fragment 2e. Finally,
photolysis was complete when dendrimer C underwent further
photolysis to release the final equivalent of fragment 2e as well
as core 3. Core 3 was not seen in the GPC.
Fig. 3 Conversion of 1b to 1d and 1c to 1e as monitored by
1
integration of the appropriate peaks in the H NMR (CDCl
3
) spectra
(
1b: 3.97–3.91 ppm; 1d: sum of 4.09–4.06 and 3.90–3.87 ppm; 1c:
3.90–3.86 ppm; 1e: 4.05–4.02 ppm; TMS peak used for integration
reference).
weight dendrimer degrades into smaller molecular weight
components (Scheme 3). Photodegradable dendrimer 1c
(
29.18 min) degraded by loss of one dendron to give a two-
arm dendrimer corresponding to B in Scheme 3 (30.32 min) as
well as dendritic fragment 1e (32.72 min). Subsequently, B
degraded by further loss of dendron to give a one-arm
dendrimer corresponding to C (33.70 min) and release of
another equivalent of fragment 1e. Finally, photolysis was
complete when dendrimer C released the final equivalent of
fragment 1e as well as core 3. Core 3 was not seen in the GPC
trace.
Conclusions
In conclusion, we have presented the synthesis and demon-
strated the photodegradation of photolabile dendrimers based
on cores that contain o-nitrobenzyl photolabile moieties, lack
hydrolytically sensitive ester linkages and possess three and six
sites for dendron attachment, respectively. A combination of
UV-Vis, NMR and GPC studies established the clean char-
acter of the photocleavage, the identity of the photolysis
products and the sequential nature of the photodegradation.
Experimental
General
NMR and mass spectra (MS) were obtained using commer-
cially available instrumentation. Tetrahydrofuran (THF) was
2
distilled under N from potassium-benzophenone ketyl. Acet-
˚
one, DMF, DMSO and methanol were dried over crushed 3 A
molecular sieves. Potassium carbonate (granular, J. T. Baker)
was dried at 100 1C at reduced pressure and stored in a
vacuum oven. Compounds 1a, 1b, 2a and 2b were synthesized
Fig. 4 GPC in CH
2
Cl
5, 20, 25, 30, 35, 170 min) and (right) 2c to 2e (t = 0, 5, 10, 15, 20, 25,
0, 35, 40, 90, 140 min).
2
of conversion of (left) 1c to 1e (t = 0, 5, 10,
1
3
1
0
as previously. Compound [G-2]Br was prepared according to
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2
the literature. All other needed reagents were purchased
5
The resulting orange oil was poured over water (30 mL) and
from commercial suppliers and used as received. Flash chro-
2
matography was performed by the method of Still et al.
extracted with CH
2
Cl
2
(3 ꢁ 10 mL). The organic layers were
6
combined, dried (MgSO ), filtered, concentrated and the
4
using silica gel (32–63 mm, Scientific Adsorbants, Inc., Atlanta
GA). Thin-layer chromatography (TLC) was performed on
precoated plates (Silica Gel HLO, F-254, Scientific Adsor-
bants, Inc.).
residue was purified by column chromatography (SiO 3 : 6 :
2
1 gradient to 5 : 4 : 1 CH Cl –hexanes–Et O) to yield 2c (178
2
2
2
mg, 79%) as a bright yellow solid: UV-Vis: l
339 nm, e 3.28
max
4
ꢂ1
ꢂ1
1
ꢁ 10 L cm mol
3
; H NMR (CDCl ) d 7.65 (s, 6 H);
7
.37–7.21 (m, 123 H); 7.00 (s, 3 H); 6.86 (s, 6 H); 6.64–6.63 (d,
Photolysis procedure
J = 2.0 Hz, 12 H); 6.62–6.61 (d, J = 2.0 Hz, 24 H); 6.52–6.51
(t, J = 2.0 Hz, 12 H); 6.50–6.49 (t, J = 2.0 Hz, 6 H); 4.98 (s, 12
H); 4.94 (s, 48 H); 4.87 (s, 24 H); 4.52 (s, 6 H); 3.64 (s, 18 H);
UV spectra were recorded in a quartz cuvette in distilled
chloroform or spectral grade THF at concentrations that
afforded absorbances of less than 5. Irradiation for UV-Vis
experiments was carried out in a quartz cuvette using a Photon
Technology International Xenon arc lamp at the appropriate
1
3
C NMR (CDCl ) d 160.4, 153.9, 147.6, 141.4, 139.3, 138.1,
3
137.0, 130.5, 128.8, 128.2, 127.8, 110.9, 110.5, 106.9, 106.6,
102.1, 101.8, 71.4, 70.4, 70.3, 70.2, 56.6; MS (MALDI in
l
max (B340–345 nm) and an irradiation (inlet and outlet) slit
dithranol) m/z 5536 (M ꢂ CH
3
OH, C347
5538.019). Anal. Calc. for C348H N O63: C, 74.98; H, 5.42;
300 6
H N O62 requires
296 6
1
width of 10 nm. Photolyses monitored by H NMR were
irradiated in a Kontes Glass Company 5 mm ꢁ 8 NMR tube
in a Rayonet with mercury lamps (350 nm irradiation).
Photolyses monitored by GPC were carried out in glass
scintillation vials with a mercury pen lamp (365 nm irradia-
tion). Throughout the course of photolyses no noticeable
temperature change was observed.
N, 1.51; Found: C, 74.75; H, 5.69; N, 1.88%.
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ꢂ1
ꢂ1
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(
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,3,5-Tris((bis(4-(3,5-bis(3,5-bis(benzyloxy)benzyloxy)benzyl
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336 | New J. Chem., 2007, 31, 1332–1336
This journal is ꢀc the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007