1-[3-(Diethylamino)phenyl]ethyl (DEAPE): A Photolabile Protecting Group for Hydroxyl and Carboxyl Groups

Article abstract of DOI:10.1021/acs.joc.8b01137

Herein we demonstrate that the photolabile protecting group (PPG), the 1-[3-(diethylamino)-phenyl]ethyl (DEAPE) group, has dramatically different photochemical properties from the parent 3-(diethylamino)benzyl (DEABn) PPG. The new PPG, derived from DEABn by adding a methyl group to the benzylic carbon, has similar reactivity as DEABn in releasing alcohol in MeCN/water; however, it is more efficient than DEABn in releasing carboxylic acid. In particular, it can release carboxylic acid efficiently in aprotic solvents and the PPG itself converts to 3-diethylaminostyrene. Photochemical removal of DEAPE can also be conveniently carried out with sunlight. The results in this work suggest that there probably exist multiple reaction pathways in cleaving the benzylic C-O bond and they can be affected by the benzylic substitution and the reaction conditions.

Full text of DOI:10.1021/acs.joc.8b01137

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Article
1-[3-(Diethylamino)phenyl]ethyl (DEAPE): A Photolabile
Protecting Group for Hydroxyl and Carboxyl Groups
Xiong Ding, and Pengfei Wang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01137 • Publication Date (Web): 23 Aug 2018
Downloaded from http://pubs.acs.org on August 23, 2018
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1-[3-(Diethylamino)phenyl]ethyl (DEAPE): A
Photolabile Protecting Group for Hydroxyl and
Carboxyl Groups
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Xiong Ding and Pengfei Wang*
Department of Chemistry, University of Alabama at Birmingham, 901 14th Street South,
Birmingham, Alabama 35294, United States
wangp@uab.edu.
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
required according to the journal that you are submitting your paper to)
ABSTRACT
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Herein we demonstrate that the photolabile protecting group (PPG), 1-[3-(diethylamino)-
phenyl]ethyl (DEAPE) group, has dramatically different photochemical properties from the
parent 3-(diethylamino)benzyl (DEABn) PPG. The new PPG, derived from DEABn by adding a
methyl group to the benzylic carbon, has similar reactivity as DEABn in releasing alcohol in
MeCN/water; however, it is more efficient than DEABn in releasing carboxylic acid. In
particular, it can release carboxylic acid efficiently in aprotic solvents and the PPG itself
converts to 3-diethylaminostyrene. Photochemical removal of DEAPE can also be conveniently
carried out with sunlight. The results in this work suggest that there probably exist multiple
reaction pathways in cleaving the benzylic C-O bond and they can be affected by the benzylic
substitution and the reaction conditions.
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Introduction
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Photolabile protecting groups (PPGs) have demonstrated increasingly important role in a
broad range of research areas. Some of their creative applications in synthesis,1 traceless release
of biologically important molecules,2-5 polymers,6-7 and surface patterning8-9 were reviewed
recently. Despite the significant advance in the field, there is still demanding for new PPGs to
complement the existing arsenal.10-11 We have recently developed a series of structurally simple
PPGs for carbonyl, hydroxyl, diol, amino, and carboxyl groups.11-12 For example, the trityl-type
hydroxyl PPG, e.g., ,-diphenyl-3-(diethylamino)benzyl PPG (as in 1)13-14 and the benzyl-type
hydroxyl PPG, e.g., 3-diethylaminobenzyl (DEABn) group (as in 2),15 are both effective and
structurally simple PPGs for releasing hydroxyl groups upon UV irradiation (Scheme 1). These
two PPGs have similar chemical and photochemical efficiency in releasing substrates, even
though the presumed tritylium intermediate (I) of 1 would be much stable than the benzylic
cation intermediate (II) of 2. We also noticed that the DEABn PPG released alcohol more
efficiently than released carboxylic acid, albeit that carboxylate is a better leaving group. Herein
we report that a slight structural variation of the DEABn group can result in a new PPG that not
only has improved efficiency in releasing carboxylic acid in protic solvent but also can release
the substrate effectively in aprotic solvent, which should further expand application scope of the
PPG. Results from this work also provide new insights into the reaction mechanism.
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Scheme 1. Trityl-Type and Benzyl-Type PPGs
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Results and Discussion
The new PPG, 1-[3-(diethylamino)phenyl]ethyl (DEAPE) PPG as shown in 3 (Scheme 2), is
derived from DEABn by adding a methyl group to the benzylic carbon. We inferred that UV
irradiation of ether 3 would lead to the ion pair of benzylic cation 5 and alkoxide/carboxylate 6,
and subsequent proton abstraction by 6 from 5 would provide alcohol/carboxylic acid ROH and
styrene product 4. Since benzylic cation 5 can be the proton source for released alkoxide
/carboxylate 6, we reasoned that the reaction could probably take place in aprotic solvent.
Scheme 2. Design of a New Benzyl-Type PPG
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We thus compared three benzyl-type PPGs as in 7-9 (Table 1). The un-substituted DEABn
PPG in 7 serves as a control, and compound 8 has the DEAPE PPG and compound 9 has the 2-
[3-(diethylamino)phenyl]-2-propyl (DEAPP) PPG. Even though the benzylic carbon of the
DEAPP PPG is a tertiary carbon center, compound 9 is thermally stable. It remained intact in
MeCN/water (4:1) at 50 oC for 2 hours at pH = 7 or 2.
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Table 1. Comparison of Bn-Type PPGs in Releasing Alcohol in MeCN/watera
rxn.
time
(min)
conv.
(%)
released
substrate
(%)
PPG-OH
(amide)
(%)
Styrene
(%)
caged
substrate
Entry
1
2
3
4
5
6
5
8
5
8
5
8
76
94
75
96
84
96
98
98
>99
99
96
58 (28)
53 (34)
79
61
80
-
-
3
2
12
12
7
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8
8
9
9
99
74
aA 5.0 mM solution in CD3CN/D2O (4:1) in 5 mm NMR tubes was irradiated with a 450 W medium
pressure mercury lamp equipped with a Pyrex filter sleeve ( > 300 nm) without deaeration. The yields
were determined by 1H NMR analysis and calculated based on the corresponding conversion.
We first compared the photoreactions of 7-9 in MeCN in the presence of protic solvent water
(Scheme 3). These PPGs showed similar chemical efficiency in MeCN/water (4:1) (Table 1,
entries 1-6). For compound 7, irradiation for 8 min led to release of the alcohol (10) in 98% yield
(based on 94% conversion) (Table 1, entry 2). The PPG mainly converted to the corresponding
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PPG-OH (i.e., 3-diethylaminobenzyl alcohol (11)) in 53% yield and N-(3-diethylamino)benzyl
acetamide (12) in 34% yield. The latter was derived from trapping of the benzylic cation with
acetonitrile followed by hydrolysis of the resulting nitrilium ion.15 Compounds 8 and 9 also
released the alcohol efficiently; however, their PPGs mainly converted to the respective PPG-
OH, e.g., 13 (61%) from 8 (Table 1, entry 4) and 14 (74%) from 9 (Table 1, entry 6) after 8 min
of irradiation. The PPG-OHs were unstable under the irradiation conditions. In the case of 8 the
yield of 13 decreased from 79% after 5 min of irradiation to 61% after 8 min. We did not observe
the respective amide in the reaction mixture of 8 and 9. The expected styrene products from the
corresponding PPG of 8 and 9 were only produced in a minor amount, e.g., 2~3% of 4 from 8
and 12% of 15 from 9 (Table 1, entries 3-6).
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Scheme 3. Photoreaction of 7-9 in MeCN/water
We then examined photoreactions of 7 and 8 in MeCN or DMF without a protic co-solvent.
Contrary to what we expected, conversion of 7 and 8 was low in both solvents. For example,
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conversion of 8 was only 7% in MeCN and 9% in DMF after 10 min of irradiation. These results
suggest that protic solvent such as water is crucial to the photoreactions; it stabilizes the charge-
separation transition state and species developed along the reaction pathway of benzylic C-O
bond heterolysis through effective solvation. Formation of the PPG-OH and the respective
styrene from 8 and 9 in the presence of water indicates possible involvement of competing SN1
and E1 pathways. Less styrene formation from 8 than from 9 can probably be attributed to an
additional SN2 pathway being available for 8 with its leaving group on the secondary carbon.
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Contrary to their similar reactivity in releasing alcohol, DEABn and DEAPE displayed
different behaviors in releasing carboxylic acid. We showed previously that DEABn was less
efficient in releasing alcohol than releasing acid in MeCN/water, albeit carboxylate is a much
better leaving group than alkoxide.15 With DEABn, ester 16 showed different reactivity in
CD3CN/D2O (4:1) (313 nm (MeCN/H2O) = 2,200 M-1 cm-1) and in CD3CN (308 nm (MeCN) = 2,300 M-1


cm-1). In CD3CN/D2O (4:1), 15 min of irradiation released the acid in 82% yield (based on 70%
conversion) (Table 2, entry 1), which was less efficient than that of the ether counterpart under
the same reaction conditions (Table 1, entries 1 and 2). These results are consistent with our
earlier observations.15 In CD3CN without protic co-solvent, photoreaction of 16 released the acid
in 62% yield (based on only 33% conversion) under the same conditions (Table 2, entry 2), much
less efficient and clean than the reaction in CD3CN/D2O (4:1).
Table 2. Comparison of Bn-Type PPGs in Releasing Carboxylic Acidsa
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solvent
rxn Time
(min)
conv. (%) released
substrate (amide)
(%) (%)
41 (24)
PPG-OH Styrene 4
caged
substrate
Entry
(%)
1
2
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4
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6
7
8
9
CD3CN/D2O 4:1
CD3CN
CD3CN/D2O 4:1
CD3CN/D2O 4:1
CD3CN
CD3CN
CH3CN
CD3CN (N2)
CD3CN
CD3CN
15
15
10
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20
15
15
15
70
33
85
95
83
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28
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61
53
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95(91b)
96
40 (40b)
75
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98
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10
11
18
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CD3CN/D2O 4:1
84
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aA 5.0 mM solution in CD3CN/D2O (4:1) in 5 mm NMR tubes was irradiated with a 450 W medium
pressure mercury lamp equipped with a Pyrex filter sleeve ( > 300 nm) without deaeration. The yields
were determined by 1H NMR analysis and calculated based on the corresponding conversion. bIsolated
yield.
However, DEAPE showed much improved efficiency in releasing acid compared with
DEABn. For instance, ester 17 in CD3CN/D2O (4:1) (314 nm (MeCN/H2O) = 2,400 M-1 cm-1,

) released the acid in 96% yield (based on 85% conversion) after only 10 min of
irradiation, and in 94% yield (based on 95% conversion) after 15 min irradiation. The PPG
converted to the PPG-OH and the expected styrene, i.e., compound 4 (Table 2, entries 3 and 4).
Contrary to DEABn, photoreaction of 17 (308 nm (MeCN) = 2,500 M-1 cm-1, was equally

efficient in MeCN in the absence of water. For example, release of the acid was in 92% yield
(based on 83% conversion) after 10 min of irradiation and in 95% yield (based on 88%
conversion) after 15 min of irradiation in MeCN (Table 2, entries 5-8). PPG product 4 was in
61% yield at 10 min and decreased to 53% yield at 15 min; however, it was in 75% yield at 20
min if the reaction was under nitrogen atmosphere (Table 2, entry 8). It appeared that the
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presence of oxygen would not affect release of the carboxylic acid but accelerate decomposition
of styrene 4 under irradiation. In a larger scale run of the reaction in CH3CN, the reaction
outcome (Table 2, entry 7) was close to the small scale run in CD3CN (Table 2, entry 6). The
released acid was isolated in 91% yield (based on 89% conversion), in agreement with the
calculated yields based on 1H NMR analysis.
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DEAPE showed much better efficiency in releasing acid than releasing alcohol in MeCN,
different from its reactivity shown in MeCN/water solution. For example, in MeCN, conversion
of ether 8 after 10 min irradiation was only 7% (versus 83% conversion of 17). These
experimental observations suggest that the ester group perhaps played a role in facilitating the
benzylic C-O bond scission. We first postulated a possible pathway that involved a photo-
induced single electron transfer process (PET) upon irradiation of 17 (i.e., pathway A, Scheme
4).16-20 The resulting ionic diradical intermediate 17’ would undergo intramolecular proton
abstraction to provide the radical cation of 4 and the corresponding radical anion of the released
acid. Subsequent SET within the pair would result in styrene 4 and carboxylic acid 20.
Scheme 4. Proposed Reaction Mechanism of Photo Deprotection
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To test this hypothesis, we examined photoreaction of 18 that possesses a CD3 group
instead of a CH3 group as in 17. After irradiation for 6 min under the same reaction conditions,
conversion of 17 and 18 were 57% and 56%, respectively, and after irradiation for 15 min,
conversion of 17 and 18 were 88% and 82%, respectively (Table 2, entry 6 vs. 9). The small
hydrogen isotope effect (even for a non-linear hydrogen transfer), suggests that fragmentation of
17’ was probably not starting with proton abstraction from the benzylic methyl group by the
radical anion of the ester moiety or via a concerted process, thus pathway A might not operate.
The photoreaction of 19 with a tert-butyl group did not provide further mechanistic insights.
Irradiation of 19 in CD3CN resulted in a complex mixture containing multiple products derived
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from the carboxylic acid part with the desired acid in only 18% yield (based on 90% conversion)
(Table 2, entry 10). However, in CD3CN/D2O (4:1), the carboxylic acid was released in 84%
yield (based on 94% conversion) (Table 2, entry 11) while the PPG portion converted to a
complex mixture.
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Based on the experimental observations, we proposed other possible pathways for the
photoreaction of 17 (Scheme 4). Thus in pathway B, direct benzylic C-O cleavage can take place
in the S1 state of 17, leading to benzylic cation 21 and carboxylate 22. In the presence of a protic
solvent, e.g., in MeCN/water solution, the competing SN1 and E1 mechanisms would lead to
solvolysis product 13 and elimination product 4, respectively, from benzylic cation 21, along
with released carboxylic acid 20 (Table 2, entries 3 and 4). The similar pathway is probably also
involved in releasing alkoxide in 7-9. However, for a poor leaving group like alkoxide, solvation
of the leaving group in the transition state by protic solvent molecules would be crucial to bond
cleavage. Alternatively, a SN2-like mechanism (i.e., pathway C) would release acid 20 and
produce solvolysis product 13 without producing styrene 4. Pathway C should be more important
for releasing poor leaving group alkoxide than releasing carboxylate. In fact, in the reaction of
ether 8, there were much more PPG-OH 13 than styrene 4 (e.g., 79% vs. 3%, Table 1, entry 3)
while in the reaction of ester 17, there were less solvolysis product 13 than styrene 4 (e.g., 28%
vs. 47%, Table 2, entry 3). Another reason for the higher production of elimination than
solvolysis could be the involvement of pathway D in the photolysis of 17 (Scheme 4). Thus,
intermediate 17’ can undergo homolysis of the benzylic C-O bond to release carboxylate 22 and
produce cationic diradical intermediate 23.16-20 This open-shell diradical intermediate might
convert to closed-shell benzylic cation 21, since it was estimated that the triplet of intermediate
23 was ~11.2 kcal/mol higher of energy than ground state benzylic cation 21 based on density
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functional theory (DFT) computation (B3LYP/6-31G(d,p)).21 Intermediate 21 would lead to
styrene 4 as in pathway B. Pathways B-D would all contribute to the overall efficiency of
photochemical release of carboxylic acid 20 in the presence of protic solvent but only pathway D
would operate in aprotic solvent. We inferred that pathway D might be available for DEABn as
well. However, DEABn released carboxylate in lower efficiency than that of DEAPE (Table 2,
entries 2 and 6), and this difference in reactivity can be attributed to the fact that the secondary
benzylic radical intermediate of DEAPE (i.e., 23) is more stable than the primary benzylic
radical of the DEABn counterpart. For the same reason, the competing non-productive back
electron transfer (BET) process probably did not affect 17 (i.e., from 17’ to 17 in pathway D) as
much as affect 16. We attributed the BET process of DEABn for its lower efficiency in releasing
carboxylate than releasing alkoxide.15
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DEAPE released carboxylic acid not only in aprotic solvent MeCN but also in some other
aprotic solvents such as DMSO, DMF, or acetone. For example, irradiation of 17 in DMF-d7 led
to higher conversion and better yield of the styrene product from the PPG than in CD3CN (Table
3, entries 1-4). Again, styrene was not stable under the irradiation conditions, and its yield
decreased with increased time of UV exposure. Under nitrogen atmosphere in DMF, the yield of
the released acid improved slightly but styrene 4 appeared to be more stable (Table 3, entry 5).
Similar results were obtained in DMSO (Table 3, entries 6 and 7) and acetone (Table 3, entries 8
and 9), but in acetone, styrene 4 was much less stable, and the signals of the PPG in the aromatic
region of the 1H NMR became very complicated. Photochemical reactions of 17 to release the
acid also occurred in EtOAc, hexanes, benzene, and THF but the results were not better than
those shown in Table 2 and 3.
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Table 3. Irradiation of 17 in different aprotic solvents a
5
6
7
8
9
solvent
rxn
Time (%)
(min)
conv. released
Styrene
4 (%)
Entry
substrate
(%)
92
1
2
3
4
5
6
7
8
9
CD3CN
CD3CN
DMF
DMF
DMF
DMSO
DMSO
Acetone
Acetone
10
15
10
15
30b
10
15
10
15
83
88
61
53
76
66
74
70
39
13
9
10
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aA 5.0 mM solution in deuterated solvent in a 5 mm NMR tube was irradiated with a 450 W medium
pressure mercury lamp equipped with a Pyrex filter sleeve (>300 nm) without deaeration. The yields
were determined by 1H NMR analysis and calculated based on the corresponding conversion. bUnder N2
The new PPGs can also release alcohol and acid with sunlight. For example, irradiation of 9
in CD3CN/D2O (4:1) for 30 min under sunlight (with a local UV index of 6) released the alcohol
in 98% yield (based on 83% conversion). The PPG converted to the corresponding PPG-OH
(84%) and styrene (8%) (Table 4, entry 1). The photoreaction completed in 60 min with a nearly
quantitative yield of the released alcohol (Table 4, entry 2). Reaction of 17 in CD3CN under
sunlight (with a local UV index of 5) also released the acid in excellent yields (Table 4, entries 3-
6). The yield of the PPG product, i.e., styrene 4, diminished with increased time of sunlight
exposure, which is consistent with the observations made with the reactions carried out with the
UV lamp.
Table 4. Photoreaction with sunlighta
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solvent
rxn
Time (%)
(min)
30
60
30
60
120
210
conv. released
PPG-
substrate OH
(%) (%)
98 84
Styrene
(%)
caged
substrate
Entr
y
1
2
3
4
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6
CD3CN/D2O 4:1
CD3CN/D2O 4:1
CD3CN
CD3CN
CD3CN
83
100
30
8
9
9
17
17
17
17
>99
95
ND
ND
66
67
49
39
-
-
-
-
54
91
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CD3CN
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aA 5.0 mM solution in deuterated solvent in a 5 mm NMR tube was irradiated with sunlight without
deaeration. The yields were determined by 1H NMR analysis and calculated based on the corresponding
conversion.
Conclusion
In conclusion, we demonstrated that with slight structural variation from DEABn, the
resulting DEAPE PPG showed dramatically different photochemical properties from the parent
DEABn PPG. It is not only more efficient than DEABn in releasing carboxylic acid but also can
release carboxylic acid efficiently in aprotic solvent such as MeCN, DMF, DMSO, and acetone.
In the photoreaction of releasing carboxylic acid, the DEAPE PPG mainly converted to 3-
diethylamino styrene. Comparison of the reaction results of DEAPE in releasing alcohol and acid
under different conditions provided mechanistic insights, revealing that there could be multiple
reaction pathways involved in photochemical cleavage of the benzylic C-O bond. In releasing
alcohol from DEABn (as in 7) and DEAPE (as in 8), we postulated that the reaction was mainly
taking place via a UV-triggered SN2 pathway with the protic solvent serving as the nucleophile.
There also existed a SN1 pathway for DEAPP (as in 9), as evidenced by the presence of a small
amount of styrene in the reaction products. In releasing carboxylic acid from DEAPE, in addition
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to the SN2 and SN1 pathways in protic solvent, there could be a PET-promoted pathway leading
to homolysis of the benzylic C-O bond. The resulting catonic diradical intermediate would
convert to the ground-state benzylic cation, which justified the high productivity of styrene from
the PPG in the process or releasing acid and the high efficiency of the photoreaction in aprotic
solvent where the SN2 and SN1 pathways are absent.
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General. Organic solutions were concentrated by rotary evaporation at ca. 12 Torr. Flash
column chromatography was performed employing 230-400 mesh silica gel. Thin-layer
chromatography was performed using glass plates pre-coated to a depth of 0.25 mm with 230-
400 mesh silica gel impregnated with a fluorescent indicator (254 nm). IR data are presented as
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frequency of absorption (cm-1). H NMR or 13C NMR spectra were recorded on 400 and 700
MHz NMR spectrometers; chemical shifts are expressed in parts per million (δ scale) downfield
from tetramethylsilane and are referenced to residual protium in the NMR solvent (CHCl3: δ
7.26). Data are presented as follows: chemical shift, multiplicity (s = singlet, d = doublet, t =
triplet, q = quartet, quint = quintet, m = multiplet and/or multiple resonances), coupling constant
in Hertz (Hz), integration. HRMS was conducted with ESI ionization method and with TOF
mass analyzer.
Materials. Anhydrous solvents diethyl ether (Et2O), tetrahydrofuran (THF),
dimethylformamide (DMF), and dichloromethane (DCM) were used without distillation.
Solvents for workup and column chromatography such as petroleum ether (PE), hexanes (Hex),
and ethyl acetate (EA), and other chemicals were obtained from commercial vendors and used
without further purification.
Synthesis of 1-[3-(diethylamino)phenyl]ethyl 3-phenylpropyl ether (8): To the solution of
3-(diethylamino)benzaldehyde (1.80 g, 10.0 mmol) in Et2O (6.0 mL) was added CH3MgBr (3.0
M in Et2O, 3.0 mL, 18.0 mmol) at room temperature and stirred for 2 h. The reaction was then
quenched with 1.0 N HCl at 0 oC, extracted with EA, concentrated, and purified by flash column
chromatograph (Hex/EA 30:1) to provide 1-[3-(diethylamino)phenyl]ethanol (1.78 g, 92%) as a
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colorless oil. Rf = 0.1 (Hex/EA 30:1). To the PPG-OH (96.5 mg, 0.5 mmol) in DMF (3.0 mL)
was added NaH (60%, 60.0 mg, 1.5 mmol) at room temperature, followed by 3-phenylpropyl
tosylate (218.0 mg, 0.75 mmol). The reaction completed in 24 h and was then quenched with ice
water, extracted with EA. The organic layer was washed with water, dried over Na2SO4,
concentrated, and purified with flash column chromatography (Hex/EA 100:1) to provide 8
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(139.0 mg, 89%) as a colorless oil. Rf = 0.10 (Hex/EA 100:1); H NMR (700 MHz, CDCl3) δ
7.27-7.23 (m, 2 H), 7.18-7.14 (m, 4 H), 6.66 (s, 1 H), 6.60-6.56 (m, 2 H), 4.31 (q, J = 6.5 Hz, 1
H), 3.42-3.30 (m, 6 H), 2.74-2.62 (m, 2 H), 1.90 (quint, J = 7.4 Hz, 2 H), 1.44 (d, J = 6.5 Hz, 3
13
H), 1.16 (t, J = 7.1 Hz, 6 H); C NMR (176 MHz, CDCl3) δ 148.0, 145.2, 142.3, 129.2, 128.5,
128.3, 125.7, 113.5, 110.8, 109.2, 78.5, 67.9, 44.4, 32.5, 31.6, 24.3, 12.6; IR (neat) 3027, 2970,
2929, 2864, 1600, 1495, 1448; HRMS (ESI-TOF) m/e: calc for C21H30NO [M+H]+ 312.2327,
found 312.2334.
Synthesis of 2-[3-(diethylamino)phenyl]-2-propyl 3-phenylpropyl ether (9): Compound 9
was obtained in a way similar to that described for 8 and the reaction of 2-[3-(diethylamino)-
phenyl]-2-propanol with 3-phenylpropyl tosylate provided 9 in 76% yield. Rf = 0.2 Hex/EA
50:1); 1H NMR (700 MHz, CDCl3) δ 7.26-7.24 (m, 2 H), 7.18-7.15 (m, 4 H), 6.81 (s, 1 H), 6.64
(d, J = 7.6 Hz, 1 H), 6.59 (d, J = 8.2 Hz, 1 H), 3.36 (q, J = 7.0 Hz, 4 H), 3.24 (t, J = 6.6 Hz, 2
H), 2.66 (t, J = 7.9 Hz, 2H), 1.87 (quint, J = 7.9 Hz, 2 H), 1.53 (s, 6 H), 1.16 (t, J = 7.0 Hz, 6 H);
13C NMR (176 MHz, CDCl3) δ 147.8, 147.6, 142.4, 128.9, 128.4, 128.2, 125.6, 113.2, 110.5,
109.4, 62.1, 44.4, 32.6, 32.2, 28.6, 12.7; IR (neat) 3026, 2972, 2929, 2867, 1598, 1576, 1493;
HRMS (ESI-TOF) m/e: calc for C22H32NO [M+H]+ 326.2484, found 326.2483.
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Synthesis of 1-[3-(diethylamino)phenyl]ethyl octanoate (17): To the solution of 1-[3-
(diethylamino)phenyl]ethanol (309.0 mg, 1.60 mmol) in DCM (4.0 mL) at room temperature was
added octanoyl chloride (274.0 mg, 1.68 mmol) and DMAP (2.0 mg). The reaction completed
after stirring overnight and was quenched with ice water. The DCM layer was washed with
water, brine, dried over anhydrous Na2SO4, and concentrated. The residue was purified with
flash column chromatography ((PE/EA) to provide 17 (488.0 mg, 95%) as a colorless oil. Rf =
0.2 (PE/EA 50:1); 1H NMR (700 MHz, CDCl3) δ 7.17 (t, J = 8.1 Hz, 1 H), 6.64-6.59 (m, 3 H),
5.83 (q, J = 6.6 Hz, 1 H), 3.35 (q, J = 7.1 Hz, 4 H), 2.31 (m, 2 H), 1.63 (quint, J = 7.3 Hz, 2 H),
1.52 (d, J = 6.6 Hz, 3 H), 1.29-1.26 (m, 8 H), 1.16 (t, J = 7.1 Hz, 6 H), 0.87 (t, J = 7.1 Hz, 3 H);
13C NMR (176 MHz, CDCl3) δ 173.2, 147.9, 142.8, 129.4, 112.9, 111.2, 109.5, 72.6, 44.4, 34.7,
31.7, 29.1, 28.9, 25.0, 22.6, 22.3, 14.1, 12.6; IR (neat) 2963, 2927, 2858, 1732, 1603, 1581,
1499; HRMS (ESI-TOF) m/e: calc for C20H34NO2 [M+H]+ 320.2590, found 320.2594.
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Synthesis of 1-[3-(diethylamino)phenyl]-2,2,2-trideuterioethyl octanoate (18): To the
mixture of Mg (41.0 mg, 1.7 mmol) in 2.0 mL of Et2O was added CD3I (218.0 mg, 1.50 mmol)
under N2. The reaction mixture was initiated at warm temperature and then stirred at room
temperature for 2.5 h. The solution of 3-(diethylamino) benzaldehyde (177.0 mg, 1.0 mmol) in
1.0 mL of Et2O was then added and stirred at room temperature for1.5 h, and the reaction was
quenched with ice water and neutralized with 1N HCl. The organic layer was separated, dried
over Na2SO4, and concentrated. The crude product mixture was purified with flash column
chromatography (Hex/EA 5:1, Rf = 0.3) to provide the corresponding PPG-OH (S1) (141 mg,
72%) as a colorless oil. 1H NMR (700 MHz, CDCl3) δ 7.19 (t, J = 7.9 Hz, 1 H), 6.70 (s, 1 H),
6.64 (dd, J = 7.5, 0.5 Hz, 1 H), 6.60 (dd, J = 8.3, 2.6 Hz, 1 H), 4.82 (s, 1 H), 3.36 (q, J = 7.1, 4
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H), 1.16 (t, J = 7.1 Hz, 6 H); 13C NMR (176 MHz, CDCl3) δ 148.1, 147.1, 129.5, 112.3, 111.0,
108.6, 70.9, 44.3, 12.6; IR (neat) 3369, 2969, 2929, 2873, 1600, 1495; HRMS (ESI-TOF) m/e:
calc for C12H17D3NO [M+H]+ 197.1733, found 197.1730.
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The PPG-OH (49.0 mg, 0.25 mmol), octanoyl chloride (61.0 mg, 0.37 mmol), K2CO3
(104.0 mg, 0.75 mmol), and DMAP (1 mg) in 2.0 mL of DCM were stirred at room temperature
for 4 h. The reaction mixture was filtered to remove the solid and the filtrate was concentrated.
The crude product mixture was purified with flash column chromatography (Hex/EA 20:1, Rf =
0.3) to provide ester 18 (75.0 mg, 93%) as a colorless oil; 1H NMR (700 MHz, CDCl3) δ 7.17 (t,
J = 8.01 Hz, 1 H), 6.63-6.62 (m, 2 H), 6.60 (d, J = 7.9 Hz, 1 H), 5.82 (s, 1 H), 3.35 (q, J = 7.1
Hz, 4 H), 2.31 (m, 2 H), 1.63 (quint, J = 7.1 Hz, 2 H), 1.29-1.25 (m, 8 H), 1.16 (t, J = 7.1 Hz, 6
H), 0.87 (t, J = 7.0 Hz, 3 H); 13C NMR (176 MHz, CDCl3) δ 173.2, 147.9, 142.8, 129.4, 112.9,
111.2, 109.5, 72.5, 44.4, 34.7, 31.7, 29.1, 28.9, 25.0, 22.6, 14.1, 12.6; IR (neat) 2961, 2927,
2858, 1732, 1603, 1581, 1499; HRMS (ESI-TOF) m/e: calc for C20H34NO2 [M+H]+ 320.2590,
found 320.2594.
Synthesis of 1-[3-(diethylamino)phenyl]-2,2-dimethylpropyl octanoate (19): To the
solution of 3-(diethylamino) benzaldehyde (354.0 mg, 2.0 mmol) in 5.0 mL of THF was added t-
BuMgCl (1.7 M in THF, 1.5 mL, 2.6 mmol) at 0 oC. The reaction mixture was stirred at 50 oC
for 3 h and quenched with 1.0 N HCl at 0 oC. The aqueous layer was extracted with EA, and the
organic layer was concentrated, and purified with flash column chromatography (Hex/EA 30:1,
Rf = 0.3) to provide the corresponding PPG-OH (S2) (306.0 mg, 65%) as a colorless oil; 1H
NMR (700 MHz, CDCl3) δ 7.14 (t, J = 7.9 Hz, 1 H), 6.63 (s, 1 H), 6.60-6.57 (m, 2 H), 44.33 (s,
1 H), 3.35 (m, 4 H), 1.15 (t, J = 7.1 Hz, 6 H), 0.95 (s, 9 H); 13C NMR (176 MHz, CDCl3) δ
147.3, 143.4, 128.4, 115.1, 111.3, 111.0, 83.0, 44.4, 35.6, 26.2, 12.7; IR (neat) 3451, 2968,
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2902, 2869, 1600, 1578, 1496; HRMS (ESI-TOF) m/e: calc for C15H26NO [M+H]+ 236.2014,
found 236.2015.
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The PPG-OH (47.0 mg, 0.20 mmol), octanoyl chloride (49.0 mg, 0.30mmol), K2CO3
(83.0 mg, 0.60 mmol) in 1.0 mL of DCM were stirred at room temperature overnight. The
reaction mixture was filtered to remove the solid and the filtrate was concentrated. The crude
product mixture was purified with flash column chromatography (Hex/EA 20:1, Rf = 0.4) to
provide ester 18 (68.0 mg, 94%) as a colorless oil;1H NMR (700 MHz, CDCl3) δ 7.11 (t, J = 8.1
Hz, 1 H), 6.58-6.55 (m, 3 H), 5.44 (s, 1 H), 3.37-2.28 (m, 4 H), 2.38-2.31 (m, 2 H), 1.64 (quint, J
= 7.3 Hz, 2 H), 1.29-1.26 (m, 8 H), 1.15 (t, J = 7.0 Hz, 6 H), 0.94 (s, 9 H), 0.87 (t, J = 7.0 Hz, 3
H); 13C NMR (176 MHz, CDCl3) δ 172.9, 147.1, 139.5, 128.3, 115.3, 111.7, 111.1, 83.0, 44.4,
35.2, 34.8, 31.7, 29.2, 29.0, 26.3, 25.1, 22.6, 14.1, 12.7; IR (neat) 2960, 2928, 2866, 1735, 1602,
1579, 1499; HRMS (ESI-TOF) m/e: calc for C23H40NO2 [M+H]+ 362.3059, found 362.3044.
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Photoreaction of 1-[3-(diethylamino)phenyl]ethyl octanoate (17): The solution of 17
(80.0 mg, 0.25mmol) in 50mL of CH3CN was allotted to 5 mm NMR tubes (1.5 mL/per tube).
The samples were irradiated with a 450 W medium pressure mercury lamp in a Hanovia
photoreactor. With the 450 W medium pressure mercury lamp, the 313 nm UV emission is
measured to be ~1 mW/cm2 (at a distance of ~4 cm from the UV lamp) with a UVX radiometer.
The radiometer with an attenuator is placed behind a quartz cuvette (1.0 cm) containing a
solution of 2.0 mM K2CrO4 in a 5% K2CO3 aqueous solution. After 15 min, the reaction
solutions were combined and washed with 1.0 N HCl (25 mL) and extracted with ether (30 mL x
2). The organic layers were combined, dried over anhydrous Na2SO4, and concentrated.
Conversion of the reaction was measured to be 89% based on 1H NMR analysis of the crude
product mixture. The crude mixture was purified with flash column chromatography (Hex/EA=
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10:1, Rf = 0.1) to provide the acid (29.0 mg, 81%). This yield was 91% based on the 89% of
conversion. The aqueous was concentrated via evaporation and the residue was purified with
flash column chromatography (Hex/EA 30:1, Rf = 0.5) to provide 3-diethylamino styrene 4 (19.0
mg, 36%) in its HCl salt form. 1H NMR (700 MHz, CD3CN) δ 7.12 (t, J = 7.9 Hz, 1 H), 6.73 (s,
1 H), 6.72-6.67 (m, 2 H), 6.61 (dd, J = 8.3, 2.3 Hz, 1 H), 5.73 (dd, J = 17.6, 0.9 Hz, 1 H), 5.17
(d, J = 10.9, 0.9 Hz, 1 H), 3.37 (q, J = 7.0 Hz, 4 H), 1.12 (t, J = 7.0 Hz, 6 H); 13C NMR (176
MHz, CD3CN) δ 149.2, 139.5, 139.0, 130.4, 114.1, 113.6, 112.7, 110.9, 45.0, 12.9; IR (neat)
3084, 2967, 2928, 2872, 1731, 1595, 1572,1495; HRMS (ESI-TOF) m/e: calc for C12H18N
[M+H]+ 176.1439, found 176.1434.
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Photoreaction with sunlight: The 5 mm NMR tubes containing 9 (5.0 mM) in
CD3CN/D2O (4:1) were placed outdoor in sunlight (with a UV index of 5 and temperature of 21
oC during the reaction time).
Quantum yield determination: A 5.0 mM solution of the sample and a 5.0 mM solution of
DEABn-protected 3-phenyl-1-propanol (with known quantum yield of 0.26)15 in NMR tube were
placed behind a standard 1 cm quartz UV cuvette contain the filter solution. Filtered light
centered at 312 nm was obtained by passing light from the 450 W medium pressure mercury
lamp through a solution of 2.0 mM K2CrO4 in a 5% K2CO3 aqueous solution.22-23 The yields of
photo reactions were determined by 1H NMR analysis.
Supporting Information Available: 1H and 13C NMR spectra of new compounds, and computation
parameters. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENT. We thank the NSF (CHE 1401063) for financial support.
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