"ene" Reactions of Singlet Oxygen at the Air-Water Interface

Article abstract of DOI:10.1021/acs.joc.6b01030

Prenylsurfactants [(CH₃)₂C=CH(CH₂)_nSO₃^- Na⁺ (n = 4, 6, or 8)] were designed to probe the "ene" reaction mechanism of singlet oxygen at the air-water interface. Increasing the number of carbon atoms in the hydrophobic chain caused an increase in the regioselectivity for a secondary rather than tertiary surfactant hydroperoxide, arguing for an orthogonal alkene on water. The use of water, deuterium oxide, and H₂O/D₂O mixtures helped to distinguish mechanistic alternatives to homogeneous solution conditions that include dewetting of the π bond and an unsymmetrical perepoxide transition state in the hydroperoxide-forming step. The prenylsurfactants and a photoreactor technique allowed a certain degree of interfacial control of the hydroperoxidation reaction on a liquid support, where the oxidant (airborne ¹O₂) is delivered as a gas.

Full text of DOI:10.1021/acs.joc.6b01030

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Article
Ene Reactions of Singlet Oxygen at the Air-Water Interface
Belaid Malek, William Fang, Inna Abramova, Niluksha Walalawela, Ashwini A. Ghogare, and Alexander Greer
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01030 • Publication Date (Web): 06 Jul 2016
Downloaded from http://pubs.acs.org on July 13, 2016
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‘Ene’ Reactions of Singlet Oxygen at the AirꢀWater Interface
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Belaid Malek†, William Fang†, Inna Abramova†, Niluksha Walalawela†,‡, Ashwini A. Ghogare†,‡,
and Alexander Greer†,‡,*
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†Department of Chemistry, Brooklyn College of the City University of New York, Brooklyn,
New York 11210, United States
‡Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, 365
Fifth Avenue, New York, New York 10016, United States
Email: agreer@brooklyn.cuny.edu
Abstract
Prenylsurfactants [(CH3)2C=CH(CH2)nSO3 Na+ (n = 4, 6, 8)] were designed to probe the ‘ene’
—
reaction mechanism of singlet oxygen at the airꢀwater interface. Increasing the number of carbon
atoms in the hydrophobic chain caused an increase in the regioselectivity for a secondary rather
than tertiary surfactant hydroperoxide arguing for an orthogonal alkene on water. The use of
water, deuterium oxide, and H2O/D2O mixtures helped to distinguish mechanistic alternatives to
homogeneous solution conditions that include dewetting of the π bond and an unsymmetrical
perepoxide transition state in the hydroperoxideꢀforming step. The prenylsurfactants and a
photoreactor technique enabled a certain degree of interfacial control of the hydroperoxidation
reaction on a liquid support, where the oxidant (airborne 1O2) is delivered as a gas.
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Introduction
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The properties of reactive oxygen species (ROS) at interfaces in organic chemistry are of
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interest,1ꢀ6 but answers to basic questions are needed, namely: How can ROS such as O2 be
probed at interfaces? One way is to use trapping agents that reside at the interface, which is the
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topic of this paper. We report the use of prenylsurfactants 1ꢀ3 for probing O2 at the airꢀwater
interface (Figure 1), and find that an increase in chain length minimizes contact of the alkene
group with water unlike 1O2 reactions in the bulk solution phase.
Previous papers have reported the properties of ROS at interfaces, such as aerosol and
organic film oxidation by beams of hydroxyl radicals using reflection/absorption IR
spectroscopy.7 (Airꢀwater interface reactions are often monitored by viscosity or surface
potential, area and pressure.8) Micellar studies have analyzed the kinetics of O2 in the two
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phases,9,10 but did not consider the residence of O2 at the interface. Flow reactors for O2
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generation in water are coming into use,11ꢀ18 and can be applied to interfacial O2 studies.19ꢀ22
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Also, compound hydrophobicity has been exploited for selectivity in “on water” organic
reactions.23ꢀ28 But further insight is needed into the interaction of 1O2 at airꢀwater interfaces.
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Here, surfactants 1ꢀ3 are used for an interfacial O2 reaction. A feature of 1ꢀ3 is the
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prenyl (2ꢀmethylꢀ2ꢀbutene) group, which has been shown to react with O2 in organic
compounds.29ꢀ36 Our prenylsurfactants were designed so that the sulfonate anion is solvated and
removed from the 2ꢀmethylꢀ2ꢀbutene group revealing airꢀwater interface effects. Our hypothesis
1
was that prenylsurfactants that vary in chainꢀlength are adjustable to probe interfacial O2
reactivity where longꢀchain prenyls would extend into the gap between the liquid surface and the
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solid sensitizer. Our previous work showed that 2 reacted with airborne O2,37,38 but no
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information on a possible chainꢀlength dependence was available.
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Here, we report on the screening of surfactants 1ꢀ3 in an attempt to probe the role of
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chainꢀlength on interfacial O2 reactions. In particular, we (1) synthesized the prenylsurfactants
1ꢀ3 using an oxidation/Wittig/Strecker reaction sequence and computed their solubility, (2)
quantitated their reactivity with 1O2 as percent yield, (3) determined the regioselective preference
for secondary over tertiary hydroperoxides, and (4) utilized homogeneous conditions as a means
to turn off the regioselectivity.
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Figure 1. A reactor for the delivery of airborne O2 to 1ꢀ3 at the airꢀwater interface. Red light is
directed via an optical fiber to a silica plate coated with a phthalocyanine sensitizer that sits
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above the water solution. The reaction of O2 with the prenyl groups led to the formation of
secondary (a) and tertiary (b) hydroperoxides.
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Results and Discussion
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Synthesis and Solubility. The synthesis of prenylsurfactants 1ꢀ3 was carried out in 3
steps (Scheme 1). First, the reaction of bromoꢀalkanols 4 or 5 with PCC was required, which led
to aldehydes 6 and 7, respectively. Second, a Wittig reaction was carried out on 6 or 7 with iꢀ
propyl phosphonium iodide, followed by a reaction with n-butyl lithium leading to mixtures of
bromoꢀ and iodoalkenes 8ꢀ11. Third, a Strecker reaction was carried out using either 9ꢀbromoꢀ2ꢀ
methylnonꢀ2ꢀene or mixtures of 8 and 9, or 10 and 11 with sodium sulfite in refluxing DMFꢀ
water solution (1:1). Surfactant 2 was known from previous work.37 After workup, offꢀwhite
solid products of 1ꢀ3 were obtained, which bore marginal water solubility. Computed solubilities
of 1ꢀ3 were calculated using the ACD algorithm.39 The computed partition coefficients (log P) in
Table 1 show a ~2ꢀlog increase as the surfactant chain length increased from 7 to 11 carbons.
Literature reports on other olefin sulfonates have shown that as chain length decreases the water
solubility increased.40 Next, we placed surfactants 1ꢀ3 into a singlet oxygen reactor and sought
reactivity patterns based on chain length.
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Scheme 1. Synthesis of Heptene, Nonene, and Undecene Sulfonates 1−3
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6
O
PCC
7
8
H
Br
HO
Br
n
n
CH2Cl2, r.t., 2 h
9
4 (n = 1)
5 (n = 5)
yield: 72%
yield: 81%
6 (n = 1)
7 (n = 5)
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1. THF, -10 °C, 45 min
+
i-PrPPh3
I
Br
I
n
n
2. n-BuLi, then 6 or 7
THF, -78 °C, 1 h
3. r.t., 1 h
8 (n = 1)
10 (n = 5)
9 (n = 1)
11 (n = 5)
8:9 = ~1:1 yield: 36%
10:11 = ~1:1 yield: 36%
Na2SO3
DMF-water (1:1),
SO3Na
Br
n
or
8 + 9 (~1:1)
10 + 11 (~1:1)
reflux, 12 h
yield: 100%
yield: 70%
yield: 91%
1 (n = 1)
2 (n = 3)
3 (n = 5)
Singlet Oxygen Reactor. Our reactor uses liquid samples of 1, 2 or 3 irradiated from
above with red light to a silica plate coated with aluminium (III) phthalocyanine chloride
tetrasulfonic acid (AlPcS) (Figure 2). AlPcS41ꢀ46 produces 1O2 where our reactor is unique in that
the plate coated with the phthalocyanine is placed above the solution and is physically separated
from the water phase. There is an airꢀgap distance of 0.4 mm and 1.5 mm from the sensitizer
plate to the water surface at the walls of the cuvette and middle of meniscus, respectively. When
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we carry out a O2 lifetime (τꢁ) measurement by NIR luminescence, a 0.5ꢀms lifetime for
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airborne O2 is found in the air gap above the airꢀliquid interface (Figure S24, Supporting
1
Information). We also investigated the airꢀgap distance to see whether the O2 distance traveled
would relate to product yield, which it does. When the sensitizer plateꢀtoꢀwater distance was
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increased from 0.4 mm to 0.9 mm and then to 1.4 mm measured at the cuvette walls, airborne
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singlet oxygenation of 2 showed 82%, 17% and 0% hydroperoxide yields, respectively (Figure
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S15, Supporting Information), demonstrating the O2 requirement. The next step was to examine
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the hydroperoxidation efficiency in surfactants 1ꢀ3.
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Figure 2. Singlet oxygen reactor where airborne O2 forms by redꢀlight irradiation of a silica
plate with aluminum (III) phthalocyanine chloride tetrasulfonic acid, AlPcS, deposited on its
bottom side. Singlet oxygen migrates through air to surfactants 1ꢀ3 at the liquid surface
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underneath. The surfactant traps airborne O2 at the airꢀwater interface, a location that contains
no sensitizer.
Table 1 shows the amount of hydroperoxides formed based on chain length. The yield of
hydroperoxides was found to be greater in 3 (85%) compared to 2 (82%) and 1 (76%) after 1 h
(entries 1, 2 and 4). The increase is nearly linearly related to the length of the prenylsurfactant.
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Other conditions, namely using deuterated solvents to increase the O2 lifetime, were then
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investigated. The hydroperoxide yields for the reaction of 2 with O2 at the airꢀwater interface
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increased by ~3ꢀfold in D2O compared to H2O (the yield of 2a and 2b was 82% in D2O and
~25% in H2O) (Table 1, entries 2 and 6) and there were increases in yield when the ratio of
D2O:H2O changed from 1:3 to 3:1 (entries 3ꢀ5). Given enough time the use of D2O or H2O will
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reach the same final yields, even though deuterated solvents reach higher O2 product yields
more rapidly compared to protio solvents. That is, a 20ꢀfold enhanced product yield would have
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been expected if the hydroperoxidation of 2 occurred in bulk solution phase, where the O2
lifetime is longer in D2O (τꢁ = 69 ꢂs) compared to H2O (τꢁ = 3.5 ꢂs).47ꢀ49 Thus, the results are
consistent with partial solvation of 2 and point to a dependence between chain length and
regioselectivity, as will be seen next.
Regioselective Ene 1O2 Reactions. Table 1 shows the relative amount of hydroperoxides
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a and b formed from the reaction of airborne O2 with 1ꢀ3. Notice the regioselective preference
for hydroperoxide a over b increased from a ratio of 2.5:1 to 3.2:1 for 1 compared to 3 at the airꢀ
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water interface (Table 1, entries 1, 2 and 7). H NMR spectra show higher amounts of the
secondary hydroperoxide formed as the surfactant chain length increased (Figure 3). However,
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the regioselectivity is lost for the reaction of airborne O2 with solvated surfactant 2 in aqueous
acetonitrile (Table 1, entries 8 and 9), which is similar to the lack of regioselectivity in the
homogeneous singlet oxygen reaction with 2ꢀmethylꢀ2ꢀpentene36 in acetonitrile (Table 1, entry
10). The data reported in Table 1 (entries 1ꢀ9) used a concentration of 1 mM for surfactants 1ꢀ3
to examine the yield and regioselectivity, where the reactions were carried to high percent
conversion of hydroperoxides a and b, since we previously found micelle formation for 2 above
10 mM and the loss of regioselectivity at 25 mM concentration.38 Previous work38 also indicated
the importance of added Ca2+ ions in prenylsurfactant 2 photoperoxidation where salting out led
to an increase in its suspending power and a loss of regioselectivity. The regioselectivity can also
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be reversed, reports of trisubstituted alkenes with bulky allylic substituents form tertiary over
secondary hydroperoxides,50,51 which is a reversal of the regioselectivity that we observe with
formation of secondary over tertiary hydroperoxide in 1ꢀ3.
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Our observed regioselectivity was not due to hydroperoxide instability. We examined the
hydroperoxides a and b and found them to be reasonably stable; for example, complete
decomposition of 2a required 1 h at 100 °C, and 2b required 2 h at 185 °C, where the
decomposition products were not scrutinized. Thus, alternative mechanisms such as the Schenck
rearrangement (radical ꢁOOH migration) that would afford alternative hydroperoxides can be
ruled out.52ꢀ54 As will be seen in the next section, the regioselectivity of hydroperoxide a over b
is attributed to the prenyl groups of 1ꢀ3 extended away from the water surface.
Table 1. Effects of Increasing the Chain Length of Surfactant on Computed Solubility,
Percent Yield and Ratio of Hydroperoxides Formed by Ene 1O2 Reactions a
surfactant b
singlet oxygen
computed
% yield
product
entry compound
chain
solvation
state
log P a
(a + b) c
ratio a:b d
interfacial
solvent
D2O
length
1.40 ±
0.43
not
1
2
7
air−liquid
air−liquid
76 ± 3
82 ± 2
2.5:1
2.8:1
1
2
solvated
2.39 ±
0.43
not
9
D2O
solvated
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not
D2O:H2O
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7
8
3
4
9
9
air−liquid
air−liquid
air−liquid
air−liquid
air−liquid
air−liquid
air−liquid
ꢀ
64
58
2.8:1
2
2
solvated
(3:1, v:v)
9
not
D2O:H2O
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2.8:1
2.8:1
2.8:1
3.2:1
1.3:1
1:1
solvated
(1:1, v:v)
not
D2O:H2O
5
9
39
2
solvated
(1:3, v:v)
not
6
9
H2O
D2O
~25 ± 10
85 ± 2
46 ± 5
100 ± 1
ꢀ
2
solvated
3.46 ±
0.43
not
7
11
9
3
solvated
CH3CN:
8
2 e
solvated
solvated
solvated
H2O (9:1)
CD3CN:
9
2 e
9
D2O (9:1)
2ꢀmethylꢀ
10
ꢀ
CH3CN
1.4:1
2ꢀpentene f
a
b
Log P values were computed with an ACD program. Samples of 1 mM surfactant in 0.6 mL
c
were irradiated by a 669ꢀnm laser via an optical fiber above the sensitizer solid for 1 h. Trace
products such as ketones or epoxides from alternative mechanisms (i.e. nonꢀ1O2 reactions) were
not observed. d Errors are expressed as mean ± standard deviation. e Ref. 38. f Ref. 36.
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Figure 3. H NMR spectra of secondary (a) and tertiary (b) hydroperoxides from the reaction of
1O2 with 1ꢀ3 at the airꢀwater interface.
Mechanistic Aspects. Our chainꢀlength study has enabled quantitative measurements of
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‘ene’ reactions of O2 at the airꢀwater interface. Two types of correlations were found as the
chain length increased in surfactants 1ꢀ3. First, the hydroperoxide percent yield increased nearly
linearly from 76% to 85%, in which spatial separation is likely important based on the number of
−CH2− groups, where the distance between surfactant head and the methyl termini range from
10.9 to 16.0 Å (Figure 4). Second, the regioselectivity also correlated linearly; Figure 4 (red line)
shows the plot of the data from entries 1, 2 and 4 in Table 1, in which each additional −CH2−
group in the chain accounted for a ~3% increase in a over b. Further support for an interfacial
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mechanism is the loss of regioselectivity when 1ꢀ3 were dissolved in homogeneous solution,
similar to that seen for 2ꢀmethylꢀ2ꢀpentene dissolved in organic solvents.36
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Mechanistically, the longer chain minimizes contact between the prenyl group and water,
for preferential allylic hydrogen abstraction of the methyl groups by airborne 1O2. The methylene
allylic hydrogens are less accessible, making the methyl hydrogen abstraction favorable. This
interpretation, along with the D2O result (Table 1, entries 2 and 3) where sparse product yield
increase compared to H2O, point to partial solvation of an unsymmetrical perepoxide transition
state55 in the productꢀdetermining step (Figure 5). Figure 5 shows the proposed perepoxide a‡
that would be expected for the reason of methyl desolvation to favor hydroperoxide a over b. In
addition to perepoxides a‡ and b‡, acyclic zwitterionic species may also form as partially
solvated intermediates. The 7 to 11 carbon length series that we have chosen also gives an
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approximate guide to mixed substrate and solvent quenching of O2. For O2 quenching, longꢀ
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chain prenylsurfactants react with airborne O2 with greater chemical quenching efficiency,
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while shortꢀchain prenylsurfactants suffer from greater solvent physical quenching of O2 due to
the close proximity to the liquid surface. The solubility of 1O2 should be greater in the longꢀchain
than the shortꢀchain surfactant so that prenyl chemical quenching (1O2 + RH → ROOH) and
appearance of hydroperoxide is facilitated for 3, while 1O2 is wasted through physical quenching
3
for 1 (1O2 + H2O → O2 + H2O). This notion is similar to facial selectivity from favored
1
chemical over physical quenching of O2 in oxazolidinoneꢀfunctionalized enecarbamates in the
literature.56,57
Comments are also in order concerning apparatus design, surfactant packing and
1
branching. We have utilized the delivery of O2 topꢀdown from a sensitizer plate to reach prenyl
sites at the airꢀwater interface. We have not conducted bottomꢀup studies where a solvated
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sensitizer produces O2 in solution, or aerosol studies with sunlight and solvated aromatics.58,59
As discussed above, prenyls in long chain surfactants extend further into the gap between the
liquid surface and solid sensitizer. But, our experiments did not assess the effects of
prenylsurfactants with shorter chains than 1, or with longer chains than 3. Thus, a wider
mechanistic picture is not yet available for possible regiospecificity (saturation in Figure 4) in the
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hydrogen abstraction step of O2 with prenylsurfactant. One would expect the regioselectivity
and yield to become nonlinear when the carbon length is less than 7 and greater than 11.
Furthermore, a caveat of 1ꢀ3 is that they are branched, which for surfactants is a property known
to cause looser packing compared to unbranched isomers.60 However, 1ꢀ3 are branched to the
same extent relative to each other, whereas the chain length is not. Our reasoning for using a
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prenyl group, i.e. branched trisubstituted alkene was due to their higher reactivity with O2
compared to that of a disubstituted alkene31,61 for easier monitoring of the reaction in the former.
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Figure 4. Regioselectivity effects on surfactants 1ꢀ3 treated with airborne 1O2. A saturation limit
is approached in increasing the number of carbon atoms in the chain. Compounds that dissolve in
common organic solvents generally show no regioselectivity, such as 2ꢀmethylꢀ2ꢀpentene in the
blueꢀcolored box (Ref. 36). The upper axis shows the molecular lengths of the surfactants.
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Figure 5. Proposed mechanisms for unsymmetrical and synchronous attack of 1O2 on the π bond
considering desolvation of the methyl groups thus favoring hydroperoxide a over b.
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Conclusion
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A physicalꢀorganic chemistry approach has been developed to probe O2 at the airꢀwater
interface with the use of prenylsurfactant probes 1ꢀ3 and a photoreactor technique. Upon
screening 1ꢀ3, we find that they are effective but to different extents as interfacial traps of
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airborne O2 (Figure 6). One implication of singlet oxygen at the airꢀwater interface is its
potential for broader use for selective oxidations in organic chemistry. Future studies could be
undertaken to (i) examine a hydrophobic group that causes color formation upon reaction with
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airborne O2 (where workup and NMR analysis and precision are not limiting factors); (ii)
compare product selectivity by theoretical methods for varying interfacial mechanisms; (iii)
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examine O2 regioselectivity based on surface tension, e.g. a perfluorinated surfactant with a
lower surface tension than regular surfactants; (iv) examine kH/kD kinetic isotope effects on H/D
substituted prenyl groups for mechanistic insight and (v) expand on reactor equipment design
and user accessibility to deliver ROS to airꢀliquid interfaces where the oxidant is delivered as a
gas.
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Figure 6. Mechanistic summary of ‘ene’ reactions of airborne 1O2 with surfactants 1ꢀ3 at the airꢀ
water interface in subꢀmicellar concentrations.
Experimental Section
Reagents, Instrumentation and Computations. Diethyl ether, DMF, CH2Cl2, CHCl3,
ethanol, acetone, THF, pentane, hexanes, ethylacetate, D2O (99.8 %), DMSOꢀd6 (99.5%), CDCl3,
pyridinium chlorochromate (PCC), sodium sulfite (Na2SO3), sodium sulfate (Na2SO4), nꢀ
butyllithium, iꢀpropyl triphenylphosphonium iodide, benzoic acid, aluminum (III)
phthalocyanine chloride tetrasulfonic acid (AlPcS), 5ꢀbromopentanꢀ1ꢀol, 9ꢀbromononanꢀ1ꢀol, 9ꢀ
bromoꢀ2ꢀmethylnonꢀ2ꢀene, celite, silica, and porous Vycor glass were obtained commercially
and were used as received. Water was purified using a deionization system. NMR data were
1
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recorded on a spectrometer operating at 400 MHz for H NMR and 100.6 MHz for C NMR.
HRMS data were collected on a timeꢀofꢀflight mass spectrometer. Calculations for octanolꢀtoꢀ
water partitioning were carried out with the ACD program.39
Synthesis of Sodium 6ꢀMethylheptꢀ5ꢀEneꢀ1ꢀSulfonate (1). Yield 90 mg (16%), purity
95%. Step 1: Synthesis of 5-bromo-1-pentanal (6). 5ꢀBromoꢀ1ꢀpentanol 4 (0.55 g, 3.3 mmol)
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reacted with PCC (1.02 g, 4.90 mmol) in 50 mL CH2Cl2 for 2 h at 26 °C. Fifty milliliters of Et2O
was added and the solution was filtered with celite to remove unreacted PCC. The residue was
concentrated under vacuum yielding 390 mg (70%) of 5 as darkꢀcolored oil. Step 2: Synthesis of
a mixture of 7-bromo-2-methylhept-2-ene (8) and 7-iodo-2-methylhept-2-ene (9). iꢀ
Propylphosphonium iodide (1.28 g, 3.34 mmol) in 9 mL THF reacted with nꢀbutyllithium (1.1
mL, 2.6 mmol) for 1.75 h at 0 °C. The solution was chilled to ꢀ78 °C then 5ꢀbromopentanal 6
(0.35 g, 2.1 mmol) was added with stirring for 10 min. The reaction was brought up to room
temperature and stirred for an additional 1 h, after which the residue was extracted with pentane.
It was then filtered and washed with water, and dried over Na2SO4. Solvent was removed by
vacuum yielding 85 mg (23%) of a mixture of 8 and 9. 1H NMR (CDCl3, 400 MHz): δ 5.12 (t, J
= 7.2 Hz, 1H), 3.41 (t, J = 6.8 Hz, 1H), 3.19 (t, J = 6.8 Hz, 1H), 2.00 (m, 2H), 1.84 (m, 2H), 1.64
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(s, 3H), 1.60(s, 3H), 1.43 (m, 2H). C NMR (CDCl3, 100.6 MHz): δ 132.4, 124.2, 34.3, 33.5,
32.7, 31.0, 28.7, 27.4, 27.2, 26.0, 18.0, 7.5. Step 3: Synthesis of sodium 6-methylhept-5-ene-1-
sulfonate (1). A Strecker reaction of 8 and 9 (ratio ~1:1) (85 mg, 0.4 mmol) and Na2SO3 (0.10 g,
0.80 mmol) was carried out under N2 atmosphere in refluxing DMFꢀwater (1:1) (4 mL) for 12 h.
The reaction was cooled to room temperature, upon which deionized H2O (2 mL) and then
ethanol (2 mL) were added. The solution was filtered and partitioned with CH2Cl2 (4 × 4 mL)
and the CH2Cl2 fractions were removed. A white solid 1 was obtained after evaporating off the
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water; mp 227ꢀ229 °C; H NMR (D2O, 400 MHz): δ 5.25 (t, J = 8 Hz, 1H), 2.92 (t, J = 8 Hz,
1H), 2.05 (m, 2H), 1.74 (m, 5H), 1.64 (s, 3H), 1.60 (s, 3H), 1.47 (m, 2H). 13C NMR (D2O, 100.6
MHz): δ 133.7, 124.3, 50.9, 27.9, 26.8, 24.8, 23.6, 16.9. IR (neat) ν 2934, 2916, 2851, 1464,
1354, 1177, 1127, 1044 cmꢀ1. HRMS (TOF) m/z found [C8H15SO3]ꢀ, 191.0752; calcd
[C8H15SO3]ꢀ, 191.0742.
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Synthesis of Sodium 10ꢀMethylundecꢀ9ꢀEneꢀ1ꢀSulfonate (3). Yield 123 mg (27%),
purity 95%. Step 1: Synthesis of 9-bromo-1-nonanal (7). 9ꢀBromononanꢀ1ꢀol 5 (0.57 g, 2.55
mmol) reacted with PCC (790 mg, 3.8 mmol) in 40 mL CH2Cl2 for 2 h at 26 °C. Fifty milliliters
of Et2O was added and the solution was filtered with celite to remove unreacted PCC. The
solution was washed two additional times with Et2O. The residue was then concentrated under
vacuum yielding 467 mg (81%) of 7. Step 2: Synthesis of a mixture of 11-bromo-2-methylundec-
2-ene (10) and 11-iodo-2-methylundec-2-ene (11). iꢀPropylphosphonium iodide (1.01 g, 2.63
mmol) in 7.7 mL THF reacted with nꢀbutyllithium (0.97 mL, 2.30 mmol) for 1 h at 0 °C. The
solution was chilled to ꢀ78 °C then 9ꢀbromononanal 7 (0.47 g, 2.11 mmol) was added with
stirring for 30 min. The reaction was brought up to room temperature and stirred for an
additional 1 h, after which the residue was extracted with pentane. It was then filtered and
washed with water, and dried over Na2SO4. Solvent was removed by vacuum yielding 168 mg
(36%) of a mixture of 10 and 11. 1H NMR (CDCl3, 400 MHz): δ 5.12 (t, J = 8 Hz, 1H), 3.43 (t, J
= 8 Hz, 1H), 3.21 (t, J = 7.0 Hz, 1H), 1.97 (m, 2H), 1.86 (m, 2H), 1.71 (m, 3H), 1.62 (m, 3H),
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1.42 (m, 2H), 1.31 (m, 8H). C NMR (CDCl3, 100.6 MHz): δ 131.2, 124.8, 34.1, 33.6, 32.8,
30.5, 29.8, 29.4, 29.2, 28.8, 28.5, 28.2, 28.0, 25.7, 17.7. 7.3. Step 3: Synthesis of sodium 10-
methylundec-9-ene-1-sulfonate (3). A Strecker reaction of 10 and 11 (ratio ~1:1) (130 mg, 0.49
mmol) and Na2SO3 (0.12 g, 0.98 mmol) was carried out under N2 atmosphere in refluxing DMFꢀ
water (1:1) (8 mL) for 12 h. The reaction was cooled to room temperature, upon which deionized
H2O (4 mL) and then ethanol (4 mL) were added. The solution was filtered and partitioned with
CH2Cl2 (4 × 8 mL) and the CH2Cl2 fractions were removed. A white solid 3 was obtained after
evaporating off the water; mp 229ꢀ232 °C; 1H NMR (D2O, 400 MHz): δ 5.26 (t, J = 8 Hz, 1H),
2.91 (t, J = 8 Hz, 2H), 2.00 (m, 2H), 1.75 (m, 2H), 1.70 (s, 3H), 1.63 (s, 3H), 1.42 (m, 2H), 1.31
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(m, 8H). C NMR (D2O, 100.6 MHz): δ 133.1, 125.4, 51.1, 34.5, 29.0, 28.3, 28.2, 27.6, 27.2,
24.8, 23.9, 16.9. IR (neat) ν 2961, 2934, 1464, 1354, 1177, 1127, 1044 cmꢀ1. HRMS (TOF) m/z
found [C12H23SO3]ꢀ, 247.1376; calcd [C12H23SO3]ꢀ, 247.1368.
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Singlet Oxygen Reactor. The reactor consisted of a sensitizing glass plate made by
depositing AlPcS (4 × 10ꢀ8 mol) onto the bottom side of a 0.50 g porous silica square (shape 2.25
cm2 × 1.0 mm). The sensitizing glass plate was placed on top of a quartz cuvette (sized: 1.0 × 1.0
× 0.7 cm3). The sensitizer plate was placed 3.0 cm below the illumination source of (i) 669ꢀnm
light (irradiance 383 mW cmꢀ2 for 1 h; total dose = ~1400 J/cm2) from a diode laser with a spot
size ~1.0 cm2, where the cuvette receiving the 669 nm light increased in temperature by 3 °C
over 1 h; or (ii) 630ꢀnm light from Nd:YAG pumping an optical parametric oscillator (OPO)
producing 5ꢀns pulses at ~0.2 mJ/pulse, where singlet oxygen was directly detected by its NIR
luminescence to a photomultiplier tube (PMT) by way of a 1270ꢀnm bandpass filter (FWHM =
15 nm). τ∆ was calculated by curveꢀfitting with a leastꢀsquares procedure. The output of the red
light from the lasers yielded incident photons in a Gaussian distribution upon the sensitizer plate.
Our experiments for 1ꢀ3 were carried out at 1 mM, about oneꢀtenth or oneꢀthird of their
respective CMCs, where the surfactants were gradually added and spread over the liquid. Our
previous results showed that the CMC of 2 is 9.7 mM at 26 °C.22 A digital ruler with a precision
of 0.01 mm was used to measure the distance between the water surface and sensitizer plate in
the cuvette. The evaporation of water in the cuvette did not cause a measurable change over the
course of a 1 h photolysis experiment. Over the course of the photolysis experiment, AlPcS
molecules did not become disconnected and did not relocated into the water solution.
Photobleaching of the sensitizer plate occurred over ~3 h irradiation times, after which the glass
was cleaned by Soxhlet extraction and then reloaded with AlPcS. We have found with control
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experiments that redꢀlight irradiation of a piece of porous Vycor bearing no AlPcS coating did
not yield 2a and 2b. In the dark, there was no evidence of formation of hydroperoxides a and b.
Generation of Sodium 5ꢀHydroperoxyꢀ6ꢀMethylheptꢀ6ꢀEneꢀ1ꢀSulfonate (1a) and
Sodium (E)ꢀ6ꢀHydroperoxyꢀ6ꢀMethylheptꢀ4ꢀEneꢀ1ꢀSulfonate (1b). Compound 1 (1 mM) was
placed in the singlet oxygen reactor with 0.6ꢀmL D2O for 1 h, after which the D2O was
evaporated by N2 gas at room temperature. The ratio of 1a and 1b was determined by 1H NMR
analysis of the 11.2 and 10.8 ppm proton signals, respectively, in which benzoic acid was the
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internal standard. H NMR (DMSOꢀd6, 400 MHz): δ 11.28 (s, 1H), 10.82 (s, 1H), 5.54 (m, 2H),
5.50 (m, 2H), 4.87 (s, 2H), 4.84 (s, 2H) 4.12 (t, J = 8 Hz, 1H) (peaks in the aliphatic region
overlap and thus are not reported).
Generation of Sodium 7ꢀHydroperoxyꢀ8ꢀMethylnonꢀ8ꢀEneꢀ1ꢀSulfonate (2a) and
Sodium (E)ꢀ8ꢀHydroperoxyꢀ8ꢀMethylnonꢀ6ꢀEneꢀ1ꢀSulfonate (2b). Compound 2 (1 mM) was
placed in the singlet oxygen reactor with 0.6ꢀmL D2O or H2O for 1 h, after which the water was
evaporated by N2 gas at room temperature. The ratio of 2a and 2b was determined by 1H NMR
analysis of the 11.2 and 10.8 ppm proton signals, respectively, in which benzoic acid was the
1
internal standard. H NMR (DMSOꢀd6, 400 MHz): δ 11.28 (s, 1H), 10.80 (s, 1H), 5.54 (m, 2H),
5.50 (m, 2H), 4.87 (s, 2H), 4.12 (t, J = 8 Hz, 1H) 37 (peaks in the aliphatic region overlap and
thus are not reported).
Generation of Sodium 9ꢀHydroperoxyꢀ10ꢀMethylundecꢀ10ꢀEneꢀ1ꢀSulfonate (3a) and
Sodium (E)ꢀ10ꢀHydroperoxyꢀ10ꢀMethylundecꢀ8ꢀEneꢀ1ꢀSulfonate (3b). Compound 3 (1 mM)
was placed in the singlet oxygen reactor with 0.60ꢀmL D2O for 1 h, after which the D2O was
evaporated by N2 gas at room temperature. The ratio of 3a and 3b was determined by 1H NMR
analysis of the 11.2 and 10.8 ppm proton signals, respectively, in which benzoic acid was the
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internal standard. H NMR (DMSOꢀd6, 400 MHz): δ 11.28 (s, 1H), 10.82 (s, 1H), 5.54 (m, 2H),
5.50 (m, 2H), 4.87 (s, 2H), 4.84 (s, 2H) 4.11 (t, J = 8 Hz, 1H) (peaks in the aliphatic region
overlap and thus are not reported).
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Acknowledgments
We acknowledge support from the National Science Foundation (CHEꢀ1464975). We also thank
Milton Rosen and Leda Lee for comments.
Supplementary Data
HRMS, IR and NMR spectra for surfactants 1 and 3, NMR spectra for mixtures of 8ꢀ11, and a
decay trace of airborne singlet oxygen within the reactor are shown. This material is available
free of charge via the Internet at http://pubs.acs.org.
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