Aromatic aldols and 1,5-diketones as optimized fragrance photocages

Article abstract of DOI:10.1039/c2pp05286e

Aromatic aldols and 1,5-diketones with abstractable γ-hydrogen atoms are highly photoactive cage molecules for the release of fragrance carbonyl compounds (aldehydes and Michael ketones, respectively). Aldols 3a-d are easily accessible by Mukaiyama addition and are cleaved to form the substrates with high quantum yields under solar radiation. By tuning the properties of the chromophores, a series of δ-damascone cages 5 were developed that can be used for selective and fast (5a,e) or slow (5b,d) release of fragrances under air and solar irradiation. The intermediates of the Norrish II process were observed by laser transient absorption spectroscopy.

Full text of DOI:10.1039/c2pp05286e

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PAPER
Aromatic aldols and 1,5-diketones as optimized fragrance photocages†
Axel G. Griesbeck,*a Olga Hinze,a Helmut Görner,b Ursula Huchel,c Christian Kropf,c Uta Sundermeierc and
Thomas Gerkec
Received 7th September 2011, Accepted 12th January 2012
DOI: 10.1039/c2pp05286e
Aromatic aldols and 1,5-diketones with abstractable γ-hydrogen atoms are highly photoactive cage
molecules for the release of fragrance carbonyl compounds (aldehydes and Michael ketones,
respectively). Aldols 3a–d are easily accessible by Mukaiyama addition and are cleaved to form the
substrates with high quantum yields under solar radiation. By tuning the properties of the chromophores,
a series of δ-damascone cages 5 were developed that can be used for selective and fast (5a,e) or slow
(5b,d) release of fragrances under air and solar irradiation. The intermediates of the Norrish II process
were observed by laser transient absorption spectroscopy.
investigated processes of electronically excited carbonyl
Introduction
compounds.13–15
A molecule that is part of a photocage (is “photocaged”) is
released during UV-vis irradiation. This allows the selective
delivery in the presence of bases, acids, metals and thus avoids
Results and discussion
undesired reaction conditions.1,2 The electromagnetic radiation
serves as a specific trigger for the liberation of the desired mol-
ecule. Many biologically active compounds have been the focus
of this concept for several decades. Efficient and well-perform-
ing photocages were reported for the release of numerous biomo-
lecules and other pharmaceutically relevant compounds.3–7
Another version of the photocage concept emphasizes the
property that exclusively visible light is the trigger mechanism
that releases the active molecule. This concept is advisable for
pheromones that are to be cleaved only under daylight con-
ditions.8 Fragrances for textile applications are another class of
compounds because release from textiles is undesired when
stored in the dark. Prerequisites for these compounds are thermal
stability and on/off controllable photochemical release in the
visible part of the solar spectrum. Numerous examples of photo-
cages have been described that are based either on hydrogen
atom transfer or on electron transfer initiation.1,2,9–12 The most
convincing photocage approach for the release of fragrances is
the photochemical generation of the same kind of species, i.e.
two carbonyl compounds that can act as fragrances resulting
from one single photocage. The corresponding cleavage reaction
is the Norrish II photoreaction, one of the most intensively
In order to realize this concept, the appropriate chromophore is
connected to the photocaged molecule in a way that γ-hydrogen
transfer and subsequent Norrish II cleavage become efficient. We
have tested this approach for aryl alkyl ketones on the basis of
acetophenone release. This approach is useful for the liberation
of simple terpene hydrocarbons such as β-pinene (1) as was
demonstrated for the photocage 2. This cage was prepared by the
t-butoxide-mediated metal-free coupling protocol developed by
Brown et al.16 This cage exclusively cleaves to the fragrance 1
and acetophenone when irradiated with UV light (Scheme 1).17
A quantum yield of >90% was determined by valerophenone
actinometry.18
It was the goal of our further investigation to demonstrate that
the benzoyl chromophore is useful also for the release of
fragrances from more complex organic substrates.
First, we investigated aromatic aldols as a group of potential
photocages. Four photocages were investigated from the
aDepartment of Chemistry, University of Cologne, Greinstr, 4,
50939 Köln, Germany
bMax-Planck-Institut für Bioanorganische Chemie, 45413 Mülheim an
der Ruhr, Germany
cHenkel AG & Co. KGaA, Henkelstr, 67, 40589 Düsseldorf, Germany.
E-mail: griesbeck@uni-koeln.de
†This paper is part of a themed issue on photoremovable protecting
groups: development and applications.
Scheme 1 Synthesis of the β-pinene photocage 2 and photochemical
release of the fragrance 1; 9-BBN = 9-borabicyclo[3.3.1]nonane.
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Table 1 Photochemistry of caged aldols
Norrish II
% Cyclization/
Conditionsa
cleavagec (%)
eliminationc
Φc
d
3a
3a
3b
3b
3c
3d
Acetonitrile
Methanol
Methanol
DPGb
Ethanol
Methanol
10
90
100
100
90
60/30
—/5
—
—
—/10
—/5
n.d.
0.77
0.78
n.d.
0.80
0.47
95
a Solar simulator irradiation, AM 1.5, concentrations: 10−3 M photocage
3a–d. b Dipropylene glycol. c By 1H NMR spectroscopy. d Quantum
yields of substrate conversion using the valerophenone actinometry
(with ΦVP = 0.7).
Scheme 2 Synthesis of the aldol photocages 3a–d by titanium and tin
enolate methods.
corresponding aldehydes or ketones and propiophenone: the iso-
butyraldehyde, n-octanal, melonal and the α-ionone photocages
3a–d. The aldols 3a–c were available under Mukaiyama con-
ditions19 in high yields and in high syn diastereoselectivities
whereas the ionone derivative 3d was available via tin enolate
coupling in moderate yields as a mixture of three diastereo-
isomers (Scheme 2).20
Scheme 4 Synthesis of the δ-damascone photocages 5a,b.
From this series, octanal, melonal and α-ionone are widely
used as fragrances. Aldols have been prepared several times as
precursors to functionalized Norrish II precursors21,22 but to the
best of our knowledge the photocleavage of aldols has yet not
been investigated. Prior to the photochemical experiments, the
thermal stability under the reaction conditions was checked and
no thermal contribution to cleavage was detected, only increas-
ing amounts of dehydration after several hours at 60 °C in
benzene solution.
The aldols 3a–d were irradiated with simulated solar radiation
(AM 1.5 solar simulator) under several solvent conditions
(benzene, methanol and dipropylene glycol (DPG)). In all cases,
the photochemical release of the caged carbonyl compound was
observed (Scheme 3).
There is a pronounced solvent dependence of the cleavage/
cyclization ratio for γ-branched substrates 3a,c that tend to form
more cyclobutanol under aprotic conditions. In contrast to that,
the substrates 3b,d preferentially result in cleavage products, i.e.
serve as efficient photocages for the corresponding fragrance
compounds (Table 1). It is remarkable that the elimination pro-
ducts were always formed as the nonconjugated ketones which
indicates a predominant photochemical pathway.
Fig. 1 UV-vis spectra of photocages 5a and 5b in CH3CN.
In order to examine if this approach works also for non-
hydroxylated α-branched photocages, we examined other substi-
tution and functional group patterns at the caged part of the mol-
ecule. The 1,5-diketones 5a and 5b were available from
δ-damascone (4) and propiophenone and 4-methoxypropiophe-
none, respectively, by Ce(III)enolate addition23 with >95% dia-
stereoselectivity (Scheme 4). The relative anti configuration of
Scheme 5 Photorelease of δ-damascone and propiophenones.
these 1,5-diketones was determined by NOESY measurements.
In the visible part of the UV-vis spectra, all compounds have
weak absorption shoulders and thus can be excited by solar
irradiation (Fig. 1).
Both photocages 5a,b release the fragrance δ-damascone as a
mixture of cis- and trans-isomers when irradiated with simulated
solar light (Scheme 5, under AM 1.5 conditions). The total
yields of products that are initiated by Norrish II γ-hydrogen
transfer is >96% for both photocages (vide infra). From 5a, two
Scheme 3 Photolyses of the aldol photocages 3a–d.
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Norrish-Yang products24 were formed, the cyclobutanols 6 and
7, respectively, in a total of 30% yield with the trisubstituted
cyclobutanol 6 as the major component (>85%). No cyclobuta-
nols were however detected (GC-MS) from the methoxy cage
5b. Photocage 5b proved to be an excellent source for the light-
induced release of δ-damascone. Independently, we detected that
the cis–trans-isomerization of 4 occurs directly also under sun-
light conditions25 and thus the release of these diastereoisomers
from 5a,b does not alter the overall performance of the
fragrance.
An astonishing difference in behaviour was observed not only
for the product selectivity of photocages 5a and 5b, the quantum
yields for product formation were also extremely different under
aerated conditions. The quantum yields for cleavage are compar-
able for 5a under nonpolar and polar protic conditions whereas a
30 times reduction in quantum yield was determined for 5b
under air (Table 2). When the photochemical decomposition of
5a under air was finished after 1 h (10–3 M in methanol), com-
plete conversion of 5b needed 48 h under identical conditions.
above transient properties are very similar upon 248 nm photoly-
sis of 5b (not shown). The lifetime of the transient of 5a upon
248 nm photolysis is much shorter, τo = 40 ns in argon-saturated
cyclohexane, 60 ns in acetonitrile and 120 ns in trifluoroethanol.
The absorption spectrum has a maximum at λmax < 360 nm and
k
ox = 2–3 × 109 M−1 s−1. The transient of 5a might be attributed
to a 1,4-biradical formed by intramolecular hydrogen transfer.
This is consistent with the properties of monoketones, with λmax
< 360 nm and a lifetime in the 10–100 ns range, whereas that of
the excited triplet carbonyl state as precursor is much
shorter.14,26 This may be in contrast to the diketones, where both
lifetimes are rather similar. One literature example refers to 1,5-
diaryl-1,5-diketones.27 The transient of 5b could be attributed to
both the excited triplet carbonyl state or the subsequently formed
1,4-biradical. The lifetime increase in polar solvents might be
attributed to an increasing π,π* character of the triplet ketone.
For comparison, the triplet–triplet absorption spectrum of
p-methoxyacetophenone has a maximum at 360 nm, taking that
of acetophenone at <300 nm.28 The lifetime of the triplet
ketones at ambient temperature is 5 ns for parent valerophenone
and 0.8 μs for p-methoxyvalerophenone.29 The important point
is that for 5b in air-saturated methanol Φc is much smaller than
under argon (Table 2) since the lifetime is tenfold shorter
(Table 3). Oxygen is able to quench both the excited triplet car-
bonyl state precursor and the 1,4-biradical.27 Thus, the assign-
ment of the observed transient of 5b is presented as being
tentative. In general, the T–T and biradical spectra are not very
specific (frequently without observable maximum) and a signifi-
cant red-shift upon introduction of a p-methoxy group is also
expected for the biradical.14,26,27
A long-lived transient with an absorption maximum at λmax
=
400 nm appears upon 308 nm photolysis of 5b in solution
(Fig. 2). The decay follows first-order kinetics with a lifetime τo
of 0.2 μs in argon-saturated cyclohexane, 1.5 μs in acetonitrile
and 4 μs in trifluoroethanol. The decay is quenched by oxygen
with a rate constant of kox = 2–3 × 109 M−1 s−1. The transient
lifetime under air is thus markedly shorter, τo = 0.1 μs in cyclo-
hexane and 0.2 μs in acetonitrile or methanol. In 2-propanol, the
lifetime is not shorter than in acetonitrile, i.e. H-atom abstraction
from 2-propanol does virtually not take place. Moreover, the
The optimal design for a photocage that selectively releases
the desired fragrance by a Norrish II cleavage mechanism with
controlled rate and without further side products includes:
absorption in the visible range (Fig. 1), low but sufficient triplet
energy to direct hydrogen transfer selectively to the energetically
preferred γ-position (Scheme 6), slower hydrogen transfer that
results in moderate quantum yields especially under air where
most triplets are quenched prior to H transfer (Table 2). The two
latter properties do also reflect the increasing ππ* character of
the excited triplet state in the methoxy-substituted cage 5b.30
The slow but selective release (see Tables 2 and 4) under air is
an important feature for the desired applications.
Table 2 Absorption maxima and quantum yields of Norrish II
cleavage of 1,5-diketones 5a,b in cyclohexane and methanola
Solvent
λmax (nm)
Φc/argon
Φc/air
5a
5a
5b
5b
Cyclohexane
Methanol
Cyclohexane
Methanol
242
245
266
273
0.4
0.6
0.6
0.5
0.3
0.6
0.2
0.02
a In gas-saturated solution, λirr = 254 nm; substrate consumption was
followed by UV absorption spectra, conversion of 5a and 5b follows an
exponential curve and Φc was determined by the valerophenone
actinometry.
When further increasing this effect by donor substitution like
in the 3,4-dioxolane 5c, we did no longer observe any Norrish II
photochemistry (Table 4). The analogous 3,4-dimethoxy deriva-
tive 5d showed comparable reactivity as 5b, however, cyclobuta-
nol formation was again observed and the 4-methyl derivative 5e
behaved like the unsubstituted cage 5a. Overall, we have
Table 3 Lifetimes (μs) of the transients from 5b in solutiona
Solvent
τ0/argon
τ/air
Cyclohexane
Acetonitrile
Methanol
0.17
1.5
2
0.10
0.2
0.2
Trifluoroethanol
4
0.25
Fig. 2 Transient absorption spectra of 5b in argon-saturated (a) cyclo-
hexane and (b) trifluoroethanol at 20 ns (○), 1 μs (4) and 10 μs (□)
after the 308 nm pulse; insets: kinetics at 400 nm as indicated.
a Upon photolysis at 308 nm, λobs = 400 nm.
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Perkin-Elmer FT-IR-S 1600 FT spectrometer. For the GCMS-
analysis of the crude product mixtures as well as for quantum
yield determination of the solar simulation experiments, a GC
(Agilent 6890)–MS (Agilent 5975) system was used with a 30 m
(250 μm diameter) HP-5MS column (Macherey-Nagel, 5% phe-
nylmethyl siloxane). Hydrogen was used as the mobile phase
and the 50–300 M method was used for all measurements:
column preheated to 50 °C, temp. program 25 °C min−1 to
300 °C.
For the laser flash measurements, an excimer laser (Lambda
Physik, EMG 200, pulse width of 20 ns and energy <50 mJ) was
used for excitation at 308 nm, another (EMG 201 MSC) for
248 nm. The absorption signals were measured with a digitizer
(Tektronix 7912AD) and an Archimedes 440 computer for data
handling.
Scheme 6 Norrish type II hydrogen transfer from 5a–5e and sub-
sequent processes.
Table 4 Photolyses of the δ-damascone cages 5a–5e
Synthesis of caged aldols 3a–d: general procedure for 3b and 3d
Full conv. (h)a
Cleavage
Cyclization
To a stirred solution of 1.34 g (10 mmol) of propiophenone in
20 ml of dichloromethane were successively added 12 ml
(12 mmol) of TiCl4 (1 M in CH2Cl2) and 1.85 g (14 mmol) of
tributylamine at −78 °C under argon atmosphere (method A).
After 30 min, 1.53 g (12 mmol) of octanal was added to the
mixture and stirred at −78 °C for 2 h. The reaction mixture was
quenched with water and extracted twice with ether. The organic
phase was washed with water and subsequently with brine and
dried over Na2SO4. The obtained crude oil was purified by SiO2-
column chromatography (cyclohexane : ethyl acetate = 9 : 1) to
give (2RS,3SR)-3-hydroxy-2-methyl-1-phenyl-decan-1-one (3b;
2.1 g, 80%).
5a
5b
5c
5d
5e
1.5
48
—
48
1.5
+
+
−
+
+
+
−
−
+
+
a 10−3 M in methanol, >98% conversion, solar simulation, GC-MS
detection of product formation.
developed superior photochemical cage compounds for volatile
aldehydes and the important fragrance δ-damascone.
The photocage 3d was prepared following a corresponding tin
enolate route (method B): To a stirred solution of 1.34 g
(10 mmol) of propiophenone in 20 ml of dichloromethane were
successively added 4.50 g (11 mmol) of tin(II)triflate, 1.35 g
(12 mmol) of N-ethylpiperidine, and after 15 min 2.50 g
(13 mmol) of α-ionone in 20 ml of dichloromethane of at 0 °C
under argon atmosphere. Standard workup resulted in 970 mg
(30%) of 3d.
Conclusion
In summary, the selective and controlled release of several
families of fragrances was achieved by Norrish II cleavage of
acetophenone and propiophenone derivatives: terpene alkenes,
aliphatic aldehydes and ketones (α-ionone) or α,β-unsaturated
ketones (δ-damascone).
Experimental
Spectroscopic data for the aldol photocages 3a–d
Materials and methods
1
3a: yield: 90%; H-NMR (300 MHz, CDCl3): 7.93–7.99 (2H,
All reactions were carried out under nitrogen atmosphere in
oven-dried and/or flame-dried glassware with magnetic stirring.
Unless otherwise mentioned, all reagents were purchased from
commercial sources and were used without further purification.
All solvents employed in the reactions were distilled over appro-
priate drying agents prior to use.
Reactions were monitored by thin-layer chromatography on
silica-gel precoated sheets. Chromatograms were visualized by
fluorescence quenching (λ = 254 nm) or by staining using anisal-
dehyde stains. Flash column chromatography was performed
using silica gel 60 (particle size 0.040–0.063 mm) purchased
from Acros. Chemical yields refer to pure isolated substances.
1H and 13C NMR spectra were recorded on Bruker DPX-300,
DRX-500, or AV-600 spectrometers. Mass spectra and accurate
mass determinations were obtained with a Finnigan MAT 900S
by electrospray ionization. Infrared spectra were recorded with a
m), 7.57–7.63 (1H, m), 7.46–7.52 (2H, m), 3.68 (1H, dd, J =
2.7, 6.9 Hz), 3.64 (syn; 1H, dd, J = 2.7, 8.1 Hz), 1.78 (1H, dqq,
J = 8.1, 6.6, 6.9 Hz), 1.25 (3H, d, J = 6.9 Hz), 1.04 (3H, d, J =
6.6 Hz), 0.96 (3H, d, J = 6.9 Hz); 13C-NMR (75 MHz, CDCl3):
205.9 (Cq), 135.8 (Cq), 133.4 (CH), 128.8 (CH), 128.4 (CH),
76.6 (CH), 41.8 (CH), 30.7 (CH), 19.1 (CH3), 19.0 (CH3), 10.7
(CH3); IR [cm−1]: 3503, 2963, 1678, 1451, 1215, 972, 710;
HRMS (EI 70 eV, C13H18O2): m/z 163.08 ([M]+, –C3H7), calcd.:
163.0759, found:163.007, error: <5 ppm.
3b: yield: 80%; b.p = 175–180 °C; 1H NMR (300 MHz,
CDCl3): 7.93–7.97 (2H, m), 7.53–7.62 (1H, m), 7.46–7.52 (2H,
m), 4.03 (1H, m), 3.47 (1H, dq, J = 2.8, 7.2 Hz), 3.07 (s, 1H,
O–H), 1.25–1.58 (13H, m), 0.88 (3H, t, J = 6.9 Hz); 13C NMR
(75 MHz, CDCl3): 205.9 (Cq), 135.9 (Cq), 133.4 (CH), 128.8
(CH), 128.5 (CH), 71.3 (CH), 44.5 (CH), 34.3 (CH2), 31.8
590 | Photochem. Photobiol. Sci., 2012, 11, 587–592
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(CH2), 29.6 (CH2), 29.3 (CH2), 26.1 (CH2), 22.6 (CH2), 14.1
(CH3), 11.0 (CH3); IR [cm−1]: 3469, 2921, 1672, 1446,
1213,966, 705; HRMS: (EI 70 eV, C17H26O2): m/z 244.18
([M]+, –H2O), calcd.: 244.1827, found: 244.183, error: <5 ppm.
13C-NMR (75 MHz, CDCl3) (δ, ppm): 213.7 (Cq), 202.6 (Cq),
163.4 (Cq), 131.9 (CH), 130.5 (CH), 129.8 (CH), 124.0 (CH),
113.8 (CH), 62.5 (CH), 55.5 (Cq), 50.1 (CH2), 44.3 (CH), 41.7
(CH2), 33.1 (Cq), 31.4 (CH), 30.7 (CH), 29.8 (CH3), 20.6 (CH3),
19.8 (CH3), 18.8 (CH3), 12.7 (CH3).
3c: yield: 75%; 1H-NMR (300 MHz, CDCl3): 7.94 (d, J = 7.2
Hz, 2H), 7.64 (t, J = 7.2 Hz, 1H), 7.54 (t, J = 7.8 Hz, 2H), 5.08
(t, J = 7.2 Hz, 1H), 3.73–3.64 (m, 2H), 3.08 (s, 1H), 2.14–2.03
(m, 1H), 1.97–1.73 (m, 4H), 1.64 (s, 3H), 1.58 (s, 3H), 1.24 (d,
J = 6.9 Hz, 3H), 0.95 (d, J = 6.6 Hz, 3H); 13C-NMR (75 MHz,
CDCl3): 205.7 (Cq), 135.8 (Cq), 133.4 (CH), 131.1 (Cq), 128.8
(CH), 128.4 (CH), 124.7 (CH), 75.3 (CH), 41.7 (CH), 35.3
(CH), 32.5 (CH2), 25.7 (CH3), 25.3 (CH2), 17.7 (CH3), 15.7
(CH3), 10.7 (CH3); IR [cm−1]: 3442, 3148, 2970, 1676, 1486,
1375, 1215, 1118, 968, 709; HRMS: (EI 70 eV, C17H26O2): m/z
256.18 ([M]+, –H2O), calcd.: 274.192, found: 274.191, error:
<5 ppm.
5a: yield: 65% (M = 326.47 g mol−1); GC/MS (50–300 M),
m/z: 326, 293, 203, 161, 123, 105, 69, 41; tR (50–300 M) =
10.507 min; IR [film, cm−1]: 2960, 1705, 1678, 478, 472;
1H-NMR (600 MHz, CDCl3): 7.94 (d, J = 7.2 Hz, 2 H), 7.58 (t,
J = 7.2 Hz, 1 H), 7.48 (t, J = 7.7 Hz, 2 H), 7.55–7.52 (m, 1H),
5.43 (d, J = 10.2 Hz, 1 H), 3.56 (quin, J = 7.6 Hz, 1 H),
2.57–2.53 (m, 1H), 2.48–2.41 (m, 2H), 2.48 (d, J = 2.4 Hz, 1
H), 2.44 (d, J = 9.0 Hz, 1H), 2.18 (d, J = 10.0 Hz, 1 H), 1.97 (d,
J = 18.0 Hz, 1 H), 1.69 (dd, J = 5.2 Hz, 18.0 Hz, 1 H), 1.58 (s,
1 H), 1.17 (d, J = 6.8 Hz, 3 H), 1.07 (d, J = 6.7 Hz, 3 H), 0.97
(s, 3 H), 0.94 (s, 3 H), 0.78 (d, J = 6.9 Hz, 3 H); 13C-NMR
(150 MHz, CDCl3): 213.2(Cq), 204.1 (Cq), 136.9 (Cq), 132.9
(CH), 131.9 (CH), 128.7 (CH), 128.3 (CH), 124.0 (CH), 62.6
(CH), 50.1 (CH2), 44.7 (CH), 41.7 (CH2), 33.1 (Cq), 31.8 (CH),
30.7 (CH), 29.8 (CH3), 20.7 (CH3), 19.8 (CH3), 18.8 (CH3),
12.6 (CH3); HRMS: (EI 70 eV, C22H30O2): m/z 308.21 ([M]+,
–H2O), calcd.: 308.21, found: 308.214, error: <5 ppm.
3d: yield: 30%; 1H-NMR (300 MHz, CDCl3): 7.92 (d, J = 7.2
Hz, 2H), 7.58 (t, J = 7.5 Hz, 1H), 7.47 (d, J = 7.7 Hz, 2H), 5.47
(m, 2H), 5.28 (s, 1H), 4.22 (d, 1H), 3.62 (quin, J = 7.1 Hz, 1 H),
1.9 (m, 2H), 1.58 (m, 1 H), 1.33 (m, 9H), 1.1 (m, 1H), 0.9 (m, 2
H), 076–0.58 (m 5 H); 13C-NMR (75 MHz, CDCl3): 206.8 (Cq),
138.4 (Cq), 137.9 (CH), 133.9 (CH), 133.4 (Cq), 129.1 (CH),
128.5 (CH), 128.3 (CH), 120.6 (CH), 73.8 (Cq), 53.6 (CH), 46.4
(CH), 31.8 (CH2), 31.3 (CH3), 27.2 (CH2), 26.8 (CH3), 25.9
(CH3), 22.8 (CH3), 22.5 (CH3), 12.6 (CH3); IR [cm−1]: 3473,
2962, 1673, 1448, 1222, 970, 708; HRMS: (EI 70 eV,
C22H30O2): m/z 308.21 ([M]+, –H2O), calcd.: 308.21, found:
308.214, error: <5 ppm.
Photolyses of photocaged compounds 3a–d, 5a–5e
(a) Quantum yield determination: 5 ml Pyrex vessels of 10−3
M
solutions of the photocage compound in spectrograde solvents
were irradiated in a multilamp photoreactor (Luzchem® LZC-4)
in a carousel with 4 samples (2 photocage samples and 2 actino-
meter samples) with phosphor-coated low-pressure mercury
lamps with a maximum emission at 300 nm. The solutions were
purged with argon or with synthetic air, respectively, before
irradiation. Samples were analyzed after constant irradiation
times by a Agilent 6890 GCMS system until 20% conversion
was reached for the fastest process. Alternatively, samples were
irradiated at 308 nm in cuvettes under argon or with synthetic air
by an excimer laser (Lambda Physik, EMG 200, pulse width of
20 ns and energy <50 mJ) and analyzed by HPLC.
(b) Preparative photolyses: 50 ml of 0.1 M solutions of the
photocage in spectrograde solvents were irradiated in Pyrex
vessels in Rayonet® photoreactors with phosphor-coated low-
pressure mercury lamps with a maximum emission at 300 and
350 nm, respectively.
General procedure for δ-damascone photocages
1-(4-Methoxyphenyl)-2,3-dimethyl-5-(2,6,6-trimethylcyclohex-
3-enyl)pentane-1,5-dione (5b): The lithium enolate from 4-meth-
oxypropiophenone was produced from the reaction of 12 mmol
of LDA from 1.7 ml (12 mmol) of diisopropylamine and 9.1 ml
(14.5 mmol) of a 1.6 M solution of n-BuLi in n-hexane) in
30 ml of THF at −78 °C with a solution of 1.97 g (12 mmol) of
4-methoxypropiophenone in 30 ml of THF, which was added by
a syringe. The reaction mixture was stirring for 1 h. This solution
was subsequently treated with a solution of 0.52 g (1.4 mmol) of
cerium(III) chloride (pre-dried under vacuum conditions for 2 h)
in 30 ml of THF and stirred for 30 min at −78 °C. To this sol-
ution, a solution of 2.3 g (12 mmol) of δ-damascone was added
dropwise over 10 min and stirred for 3 h. After warming to r.t.,
40 ml of a saturated aqueous NH4Cl solution was added and
extracted 2× with 50 ml of diethyl ether. The combined organic
phases were washed with a water, brine and dried over MgSO4.
After filtration and solvent evaporation, the crude material was
washed twice with 20 ml of cold pentane resulting in 3.25 g of a
colorless solid (76%), m.p. = 80 °C.
(c) Irradiation with a solar light simulator. 2 ml 10−2 M sol-
utions of the photocages were irradiated by a Solar Light®
XPS200 equipped with a 150 W Xe arc lamp and a AM1.5 filter
setup as well with a IR filter using polystyrene 4 × 4 multiwell
plates with 4 ml probe volume. The solutions were irradiated
under air in all cases and samples were analyzed after constant
irradiation times by a Agilent GCMS system (vide supra).
Cyclobutanol 6: M = 326.47 g mol−1; GC/MS (50–300 M),
m/z: 326, 309, 203, 143, 123, 105, 77, 55; tR(50–300 M) =
10.85 min; IR [cm−1]: 3441, 2950, 1697, 1365, 701, 483;
1H-NMR (300 MHz, CDCl3): 7.40–7.36 (m, 4H), 7.28 (m, 1H),
5.56 (m, 1H), 5.49 (d, J = 6 Hz, 1H), 2.67 (m, 2H), 2.55–2.46
(m, 2H), 3.36–2.31 (quin, J = 6 Hz, 1H), 2.27 (d, J = 12 Hz, 1
H), 2.10–1.97 (m, 2H), 1.73–1,70 (dd, J = 18 Hz, 1 H),
GC/MS (50–300 M), m/z: 55, 69, 77, 94, 107, 123, 135, 164,
1
233, 356; H-NMR (300 MHz, CDCl3) (δ, ppm): 7.93 (d, J =
8.7 Hz, 2H), 6.93 (d, J = 6.93 Hz, 2 H), 5.46 (m, 2H), 3.87 (s,
3H), 3.52 (quin, J = 6.6 Hz, 1H), 2.74–2.15 (m, 5H), 1.93 (d, J
= 17.7 Hz, 1H), 1.65 (d, J = 17.4 Hz, 1H), 1.13–1.10 (m, 3H),
1.09–1.02 (m, 3H), 0.95–0.84 (m, 8H), 0.76 (d, J = 6.9 Hz, 1H);
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1.23–1.21 (m, 3 H), 0.98 (t, 6 H), 0.90 (t, 5H); 13C-NMR
(75 MHz, CDCl3): 214.2 (Cq), 146.9 (Cq), 132.1 (CH), 130.3
(CH), 128.5 (CH), 127.3 (CH), 124.9 (CH), 124.3(CH), 77.3
(Cq), 62.7 (CH), 53.7 (CH2), 46.7 (CH), 41.8 (CH2), 40.2 (CH2),
33.2 (CH3), 32.4 (CH), 31.7 (Cq), 30.0 (CH3), 20.8 (CH3), 20.1
(CH3), 12.7 (CH3); HRMS: (EI 70 eV, C22H30O2): m/z 326.22
[M]+, calcd.: 326.2245, found: 326.224, error: <5 ppm.
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Acknowledgements
This work was generously supported by Henkel AG & Co.
KGaA. O.H. was awarded the 2011 Henkel Laundry & Home
Care Research Award for her Ph.D. Thesis.
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592 | Photochem. Photobiol. Sci., 2012, 11, 587–592
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