Aerobic Oxidation of Olefins and Lignin Model Compounds Using Photogenerated Phthalimide-N-oxyl Radical

Article abstract of DOI:10.1021/acs.joc.6b01704

A metal-free protocol to generate phthalimide-N-oxyl (PINO) radicals from N-hydroxyphthalimide (NHPI) via a photoinduced proton-coupled electron transfer process is reported. Using donor-substituted aromatic ketones, such as 4,4′-bis(diphenylamino)benzophenone (DPA-BP), PINO radicals are efficiently produced and subsequently utilized to functionalize olefins to afford a new class of alkyl hydroperoxides. The DPA-BP/NHPI/O₂ photocatalytic system exhibits high efficiency toward the aerobic oxidation of β-O-4 lignin models.

Full text of DOI:10.1021/acs.joc.6b01704

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Article
Aerobic Oxidation of Olefins and Lignin Model Compounds
Using Photogenerated Phthalimide-N-oxyl (PINO) Radical
Jian Luo, and Jian Zhang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01704 • Publication Date (Web): 09 Sep 2016
Downloaded from http://pubs.acs.org on September 16, 2016
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Aerobic Oxidation of Olefins and Lignin Model
Compounds Using Photogenerated Phthalimide-N-
oxyl (PINO) Radical
Jian Luo and Jian Zhang*
Department of Chemistry, University of Nebraska–Lincoln, Lincoln, Nebraska, 68588, United
States
Email: jzhang3@unl.edu
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ABSTRACT: A metal–free protocol to generate phthalimide-N-oxyl (PINO) radicals from N-
hydroxyphthalimide (NHPI) via a photoinduced proton-coupled electron transfer (PCET) process
is reported. Using donor-substituted aromatic ketones such as 4,4'-bis(diphenylamino)-
benzophenone (DPA-BP), PINO radicals are efficiently produced and subsequently utilized to
functionalize olefins to afford a new class of alkyl hydroperoxides. The DPA-BP/NHPI/O2
photocatalytic system exhibits high efficiency toward the aerobic oxidation of β-O-4 lignin
models.
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INTRODUCTION
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One of the prominent challenges in organic synthesis is to develop efficient and inexpensive
catalytic systems for selective oxidation of organic compounds under mild and environment-
friendly conditions.1 Using ubiquitous molecular oxygen in combination with organic nitroxyl
radicals such as TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl)2 or PINO (phthalimide-N-oxyl)3
is a powerful catalytic approach toward sustainable aerobic oxidation.4 However, the generation
of short lived PINO radicals in situ from its parent hydroxylamine, N-hydroxyphthalimide
(NHPI), often requires toxic metals such as CoII, MnII, PbIV, and VIV.5 Although several metal-
free processes have been reported,6 using light as the clean and traceless reagent to
photochemically activate NHPI has not been well studied and understood.7 Herein, we report the
use of donor-substituted aromatic ketones under the irradiation of a household fluorescent lamp
to activate NHPI via a proton-coupled electron transfer (PCET) process. The resulting PINO
radicals are further utilized for the aerobic oxidation of olefins and lignin model compounds.
Aromatic ketones such as benzophenone (BP) and its derivatives have been extensively studied
for photochemical homolytic X-H (X = C, N, O) bond activation.8 The underlining mechanism
of this hydrogen abstraction reaction is highly dependent on the nature of the ketone’s excited
state (n,π* or π,π*/CT; CT = charge transfer) and the hydrogen donor’s ionization potential as
well as the X-H bond dissociation energy (BDE). It was found that the rate of hydrogen
abstraction of phenols, a popular hydrogen donor substrate due to its biological relevance,9 is
faster with ketones that exhibit the lowest π,π*/CT. Leigh et al. attributed such favorable kinetics
to a coupled electron/proton transfer that is facilitated by hydrogen-bonded exciplex formed
between phenol and ketone.10 Recently, Meyer, Wenger, and Dempsey, among others, further
provided detailed kinetic parameters of this process using photoexcited N-containing
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heterocyclic fluorophores as the hydrogen acceptor.11 In view of the comparable thermochemical
parameters of phenol (pKa = 30.0 in CH3CN and E (PhOHꢀ+/0) = 1.25 V vs. Fc+/0, Fc = ferrocene)
and NHPI (pKa = 23.5 in CH3CN and E (NHPIꢀ+/0) = 1.2 V vs. Fc+/0),12 we envisioned that the
activation of NHPI may be facilitated by the photoexcited aromatic ketones with the lowest
π,π*/CT via PCET (Figure 1a). Figure 1b lists three donor-substituted benzophenones that
exhibit lowest CT, namely, DMA-BP (4,4'-bis(dimethylamino)-benzophenone, also named as
Michler’s ketone), Cz-BP (4,4'-bis(9-carbazolyl)-benzophenone), and DPA-BP (4,4'-
bis(diphenylamino)-benzophenone). A rough thermochemical analysis was performed for Cz-BP
and provided supportive thermodynamic parameters for the hydrogen atom transfer (HAT) from
NHPI (Figure 1c, see Supporting Information S-2 for details): electron transfer in both stepwise
ET-PT (electron transfer-proton transfer) (ꢀGo = +3.7 kcal mol-1) and PT-ET (ꢀGo = +24.6 kcal
mol-1) processes would likely encounter high-energy intermediates; in contrast, the PCET step
(ꢀGo = –33.7 kcal mol-1) exhibits significantly more favorable thermochemical energetics to
generate PINO radicals.
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Figure 1. (a) Scheme of PCET from NHPI to an aromatic ketone. (b) Structures of DMA-BP,
Cz-BP, and DPA-BP. (c) Thermochemical analysis of excited state PCET for Cz-BP and NHPI.
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RESULTS AND DISCUSSION
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We first chose a stoichiometric addition reaction to test the feasibility and efficiency of
photoinduced generation of PINO radicals.13 It is known that in the presence of molecular
oxygen PINO can be effectively trapped by olefins to form the 1,2-dioxygenated product13 that
can be potentially transformed into a variety of derivatives.14 We examined the reaction of
styrene (1a) with NHPI in MeCN in the presence of a ketone photocatalyst (2 mol %) and
oxygen (1 atm) under the irradiation of a 26W white compact fluorescent lamp (CFL) for 4 h. To
our delight, DPA-BP gave rise to the dioxygenated product hydroperoxide 2a with an excellent
yield (92%, Table 1, entry 1). Blue LED (λmax = 465 nm) was also an effective light source for
DPA-BP due to its considerable absorbance above 400 nm (Figure S1) albeit a longer reaction
time (12 h) was required (Table 1, entry 2). Control experiments confirmed the essential role of
light irradiation, oxygen, and the photocatalyst (entries 3-5). Cz-BP gave the essentially same
yield as that of DPA-BP (90%, entry 6). Interestingly DMA-BP gave the lowest yield (12%,
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entry 7). H NMR spectroscopy revealed a significant decomposition of DMA-BP (Supporting
Information, Figure S9), possibly due to its reactive N-methyl C-H bond in the presence of
PINO.15
Since hydrogen bonding (H-bond) between NHPI and the photo-excited ketone is essential for
the proposed PCET process, solvents with strong hydrogen bonding ability (either as acceptors
or donors) are expected to negatively affect the abstraction of hydrogen atom and subsequent
reactions. Indeed, strong H-bond acceptors such as DMF and DMSO significantly compete with
the ketone to form H-bond with NHPI and completely quenched the reaction (Supporting
Information, Table S4, entry 2-3).16 H-bond donors such as methanol also completely (in pure
MeOH) or partially (in a solvent mixture of MeCN and MeOH, v:v=10:1) quenched the reaction
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Table 1. Initial studies for the photo-induced dioxygenation of styrene.a
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Entry
Photocatalyst
DPA-BP
DPA-BP
DPA-BP
DPA-BP
None
Yield (%)b
92
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89c
No reactiond
Tracee
Trace
90
Cz-BP
DMA-BP
12f
aReaction conditions: photocatalyst (2.0 mol %), styrene (52 mg,
0.5 mmol), NHPI (98 mg, 0.6 mmol), O2 (1 atm), 5 mL MeCN,
26 W CFL, room temperature for 4h. Isolated yield. A blue
LED was used as the light source and 12h reaction time.
dWithout light. Without O2. Decomposition of DMA-BP was
observed.
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(Table S4, entry 4-5), despite methanol’s weaker Stern-Volmer quenching constant (20 M-1) for
*DPA-BP than that of NHPI (330 M-1) (Figures S6-8).17 Lastly, additives acting as strong H-
bond acceptors such as fluoride (as TBAF, tetra-n-butylammonium fluoride) also inhibited the
reaction via the competitive binding with NHPI (Table S4, entry 6).18 On the other hand, the
reaction proceeded smoothly in MeCN and acetone (Table S4, entry 7), which are weaker H-
bond acceptor compared with DMF and DMSO, and do not disrupt the H-bond interaction of the
photo-excited ketone with NHPI.
On the basis of the thermochemical calculation and control experiments described above, a
possible reaction pathway is outlined (Scheme 1, inset). First, the photoexcited ketone (*Cat.)
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Scheme 1. Photochemical aerobic oxidation of olefins.
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abstracts a hydrogen atom from NHPI via PCET, forming the ketyl (Cat.-H) and PINO radicals.
The latter then quickly adds to styrene 1a to give the benzyl radical 3a, which further binds with
O2 and transforms into a peroxyl radical 4a that abstracts a hydrogen atom from another NHPI to
afford the 1,2-dioxygenation product 2a. This nearly thermoneutral reaction (BDE of ROO–H
88.5 kcal mol–1 and O–H in NHPI: 88 kcal mol–1)19 does proceed at an appreciable rate (e.g. 7.2
× 103 M–1 s–1).19b The ketone photocatalyst Cat. is regenerated from the ketyl radical Cat.-H by
coupling with O2, and the thus generated peroxyl radical undergoes a similar hydrgon abstraction
from NHPI and release the PINO radical and H2O2.20
A series of substituted styrenes were used to investigate the scope of this transformation under
the optimized conditions (Scheme 1). Excellent to good yields of the dioxygenated product were
obtained. No significant detrimental effect was observed for electron-donating substituents such
as methoxyl (2b, 84% yield) or tert-butyl (2c, 86% yield). This is in contrast with the previous
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observation of product decomposition when p-methoxy-styrene was used as the substrate.13b
Interestingly, the reaction rate decreases as the electron-withdrawing capability of the substituent
groups increases, possibly due to the slower addition of the electrophilic PINO radical to the C-C
double bond. For example, p-bromo-styrene (2d, 86% yield) and 2,3,4,5,6-pentafluoro-styrene
(2e, 77% yield) require a longer reaction time of 12 h and 24 h, respectively. p-Methoxyl-(E)-β-
methylstyrene 1f was a compatible substrate and provided a mixture of diastereoisomers 2f (dr =
74:26, determined by 1H NMR). (E)-1,2-Diphenylethene 1g gave a high yield (94%, dr = 89:11,
determined by 1H NMR) of the major diastereoisomer 2g with a slightly longer reaction time (6
h), which can be attributed to the steric effect induced by the two bulky phenyl groups. However,
when non-aromatic alkenes, such as cyclohexene, was used as substrate in the same reaction, no
expected product was obtained possibly due to the unstable radical intermediate.
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Scheme 2. Reactivity of DPA-BP/NHPI/O2 photocatalytic system towards lignin β-O-4 models.
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Encouraged by this result, we next tested the efficiency of the DPA-BP/NHPI/O2 system
toward the aerobic oxidation of secondary benzylic alcohols. This fundamental reaction has
recently attracted much attention in the depolymerization of lignin, a biopolymer that can be
potentially used to provide value-added monomers for biorefineries.21 It is known that the
oxidation of benzylic position of the most dominant β-O-4 linkage structure in lignin models
facilitates the subsequent C-O bond cleavage (~14 kcal mol–1 decrease of BDE22), to allow for
the access to high-value aromatic products, including a recently reported photoredox approach.23
To date, most methods for the oxidation of lignin and its related model compounds24 employ
transition-metal-based oxidants at elevated temperatures, affording products with low selectivity
and poor yield.25 Only a few metal-free approaches such as laccase enzymes26 and
TEMPO/HNO3/HCl27 have been employed for aerobic alcohol oxidation in lignin models.
We first tested DPA-BP as the photocatalyst (1.5 mol %) and NHPI as the co-catalyst (15
mol %) for the aerobic oxidation of p-methoxyl-α-methylbenzyl alcohol 5 (Table 2). After
irradiation of the reaction mixture with a 26 W CFL under 1 atm O2 for 24 h in acetone, the
ketone product 6 was obtained in an excellent yield (97%, Table 2, entry 1). Solvent screening
revealed CH3CN is another effective solvent (93%, Supporting Information, Table S5). Similar
to dioxygenation of styrene, polar solvents such as strong H-bond acceptor (DMF and DMSO)
and H-bond donator (methanol) strongly quenched the reaction (Table S5). Light irradiation,
photocatalyst, NHPI, and molecular oxygen are all essential to this reaction (Table S6). In the
presence of less NHPI (5 mol% and 10 mol%), the reaction rate decreased (Table S6, entries 7-
8); however, as the ratio of NHPI increased to 20 mol%, lower selectivity was observed (Table
S6, entry 9). Our catalytic system also compares favorably to the metal-based
Co(AcO)2/NHPI/O2 catalytic system3a under the same reaction condition (48% yield, Table 2,
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entry 3). Cz-BP gave a lower yield (42%, entry 4) and DMA-BP was ineffective (entry 5). The
synergistic catalytic activity of photocatalyst and PINO is essential for this reaction. For
instance, moderately oxidative photocatalyst such as [Ru(bpy)3]2+, which does not have H-bond
interaction with NHPI, exhibited a diminished activity (6% yield, entry 6). In the absence of
NHPI, despite their higher reduction potentials (EM+/M > +1.80 V) than that of the benzylic
alcohol 5 (E+/0 > +1.67 V) (Supporting Information, Table S1 and Figure S13), [Ru(bpz)3]2+ (bpz
= 2,2'-bipyrazine) and Acr-Mes+ (9-mesityl-10-methylacridinium) resulted in a poor yield of the
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Table 2. Initial studies for the photo-induced oxidation of benzylic alcohol.a
Entry
Photocatalyst
DPA-BP
Conversion (%)
Yield (%)b
1
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4
5
6
7
8
100
89
50
43
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97
84c
48d
42
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DPA-BP
None
Cz-BP
DMA-BP
[Ru(bpy)3]2+
[Ru(bpz)3]2+
Acr+-Mes
6
6
15e
41e
15e
39e
aReaction condition: photocatalyst (1.5 mol %), 5 (50 µL, 0.35
mmol), NHPI (7.5 mg, 15 mol %), O2 (1 atm), 5 mL acetone, 26 W
CFL, room temperature for 24h. bIsolated yield. cA blue LED as the
d
light source was used and 48h reaction time. 2 mol % Co(AcO)2,
NHPI 15 mol %, benzoic acid 10 mol %, dark. eNo NHPI.
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ketone product (< 40% yield, Table 2, entry 7-8). Moreover, the formation of H2O2 as the other
reduction product of O2 was confirmed by the standard iodide test (Figure S15).
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Next, we sought to apply the DPA-BP/NHPI/O2 photocatalytic system to a series of β-O-4
lignin models 7, including several with an additional hydroxymethyl fragment that features in
lignin (7e, 7i, and 7k), to determine its efficiency and selectivity (Scheme 2). Overall, the
benzylic carbonyl compounds 8 were obtained in excellent isolated yields (81-99%). Except for
substrate 7a, which required 60 h to complete the conversion (98% yield), most lignin models
with electron-donating substituents proceeded in a faster reaction rate (24-36 h). This oxidation
protocol is also effective toward several β-O-4-linked diols (7e, 7i, and 7k) (> 81% yield). In
particular, the yield for 8i represents a two-fold increase comparing to that of a previously
reported TEMPO/HNO3/HCl catalyst system.27 Considerably more challenging lignin models (7j
and 7k), which contain free phenols and tend to undergo decomposition25b or inhibit catalysts,28
exhibited good reactivity with excellent yields of corresponding ketone 8j (92%) and 8k (81%),
respectively. Such high chemoselectivity is reasonable because the stronger O-H bond of phenols
(BDE: 87.7 kcal mol–1)12 cannot sufficiently compete with the Cα–H in benzylic alcohol (BDE is
75~85 kcal mol–1)3d to react with PINO radical via hydrogen abstraction. Moreover, compared
with NHPI (pKa = 23.5 in CH3CN), phenol (pKa = 30.0 in CH3CN) is a weaker H-bond donor so
that the interaction between NHPI and DPA-BP will not be significantly weakened in the
presence of the phenolic substrate (4 eq.) under the reaction condition.
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Scheme 3 illustrates the proposed mechanism for benzylic alcohol oxidation. A photoinduced
PCET between *Cat. and NHPI generates the reactive PINO radical, which abstracts a hydrogen
atom from the benzylic alcohol to afford the α-hydroxybenzylic radical 9 and regenerate NHPI.
This is a thermodynamically favorable step on the basis of the BDE of Cα–H (75~85 kcal mol–1)
in benzylic alcohol3d and O–H (88 kcal mol–1) in NHPI.19b,29 Radical 9 is intercepted by O2 to
generate the peroxy radical 10, which abstracts a hydrogen atom from NHPI and release the
ketone and H2O2 as the products. The regeneration of photocatalyst Cat. from ketyl Cat.-H
follows a similar fashion as the transformation from ketyl 9 to ketone 6.
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Scheme 3. Proposed reaction mechanism for the oxidation of benzylic alcohols.
•–
It is known that superoxide (O2 ), commonly formed by single electron reduction by excited
photocatalysts, can undergo a hydrogen abstraction reaction.30 However, it usually exhibits lower
reactivities compared to peroxyl radical.31 We further ruled out the role of this mechanism in the
generation of PINO radicals in our catalytic system: when a structural analogue of 5 that contains
the active C-H bond, p-(1-ethoxyethyl)-anisole, was subjected to the DPA-BP/NHPI catalyst
system in the absence of O2, the ketone product 6 was successfully generated, presumably due to
the decomposition of the α-benzyl radical following the hydrogen abstraction by PINO
(Supporting Information, Figure S14).
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In summary, we have demonstrated a new example of utilizing proton-coupled electron
transfer in chemical photocatalysis. The hydrogen bond between DPA-BP and NHPI is used to
facilitate the formation of synthetically useful PINO radicals via the photoinduced unidirectional
PCET pathway where the electron and proton are simultaneously transferred in a single
elementary step.11b-d This strategy serves as a useful addition to the photoinduced bidirectional
PCET recently employed by Knowles et al. to generate ketyl and amidyl radicals.32 Both PCET
mechanisms allow for a rapid reaction rate due to the decreased activation barriers. The obtained
PINO radicals can be used to access a new class of alkyl hydroperoxides. Furthermore, the DPA-
BP/NHPI/O2 photocatalytic system exhibits high efficiency and selectivity for the aerobic
oxidation of benzylic alcohols including β-O-4 lignin models. Our metal-free catalytic system
holds a great potential utility for the future development of green aerobic oxidation methods.
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General Information. All solvents and reagents were purchased from commercial sources
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and, unless otherwise noted, used without further purification. H and 13C NMR spectra were
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measured in CDCl3 at 300 or 400 MHz and 100 MHz, respectively. 19F NMR spectra were
measured in CDCl3 at 376 MHz. The chemical shift references were as follows: (1H), 7.26 ppm
(CDCl3); (13C), 77.0 ppm (CDCl3). High resolution mass spectrometry was conducted on a Q-
TOF mass spectrometer equipped with an ESI or EI mode. Thin-layer chromatography (TLC)
was performed on 0.25 mm hard-layer silica G plates containing a fluorescent indicator.
Developed TLC plates were visualized with a hand-held UV lamp. All the compounds were
purified by flash column chromatography with silica gel with the product purity higher than 95%
1
(calculated from H-NMR spectra). UV-vis, fluorescence excitation and emission spectra were
measured in CH3CN. Cyclic voltammetry (CV) was performed using an Epsilon electrochemical
workstation: glassy carbon electrode as the working electrode, Pt wire as the counter electrode,
Ag/AgCl (KCl, 3 M) electrode as the reference electrode, and ferrocenium-ferrocene (Fc+/Fc) as
the internal standard. Scan rate: 100 mV s–1 (in the range –2.2 to +1.8 V). Bu4NPF6 (0.1 M in
MeCN) was used as the supporting electrolyte.
Photocatalysts and Starting Materials Preparation. The photocatalysts Cz-BP,33 DPA-BP,34
35
and [Ru(bpz)3](PF6)2 were synthesized using reported procedures. The lignin model
compounds 7 were prepared via a literature procedure.23
General Procedures. Dioxygenation of Styrenes. The mixture of styrene (0.5 mmol), NHPI
(98 mg, 0.6 mmol) and DPA-BP (5.0 mg, 10µmol, 2 mol%) in 5 mL MeCN was stirred under O2
atmosphere (1 atm) with irradiation of the light source (26 W CFL, distance app. 2 cm) at room
temperature. The reaction was monitored via TLC (hexanes : ethyl acetate = 5 : 1 ~ 2 : 1). Upon
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consumption of starting material, removal of the solvent under vacuum, the residue was purified
by column chromatography on silica gel using hexane/ethyl acetate (10 : 1 ~ 4 : 1) as the eluent
to yield the products.
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2-(2-Hydroperoxy-2-phenylethoxy)isoindoline-1,3-dione (2a). Prepared from styrene (57 µL,
1
52 mg, 0.5 mmol) and NHPI (98 mg, 0.6 mmol). White solid, 137 mg (92 %); mp 78-80 ºC. H
NMR (CDCl3, 400 MHz): 7.86–7.95 (2H, m), 7.74–7.86 (2H, m), 7.33–7.48 (5H, m), 5.38–5.51
(1H, m), 4.45~4.62 (2H, m). 13C NMR (CDCl3, 100 MHz): 164.8, 135.7, 134.8, 128.9, 128.8,
128.7, 127.1, 124.9, 85.5. HRMS (ESI-MS) m/z: [M+Na]+ Calcd for C16H13NO5Na+ 322.0691;
Found 322.0680.
2-(2-Hydroperoxy-2-(4-methoxyphenyl)ethoxy)isoindoline-1,3-dione (2b). Prepared from p-
methoxy-styrene (67 mg, 0.5 mmol) and NHPI (98 mg, 0.6 mmol). White solid, 144 mg (84 %);
1
mp 87-89 ºC. H NMR (CDCl3, 400 MHz): 9.42 (1H, brs), 7.84–7.92 (2H, m), 7.77–7.84 (2H,
m), 7.37 (2H, d, J = 8.7 Hz), 6.93 (2H, d, J = 8.7 Hz), 5.34–5.42 (1H, m), 4.51~4.56 (m, 2H),
3.85 (3H, s). 13C NMR (CDCl3, 100 MHz): 163.8, 134.8, 128.7, 128.6, 127.7, 123.8, 114.2, 84.9,
78.9, 55.3. HRMS (ESI-MS) m/z: [M+Na]+ Calcd for C17H15NO6Na+ 352.0797; Found 352.0788.
2-(2-(4-(tert-Butyl)phenyl)-2-hydroperoxyethoxy)isoindoline-1,3-dione (2c). Prepared from p-
(tert-butyl)-styrene (80 mg, 0.5 mmol) and NHPI (98 mg, 0.6 mmol). White solid, 152 mg (86
%); mp 86-88 ºC. 1H NMR (CDCl3, 300 MHz): 7.85–7.92 (2H, m), 7.77–7.85 (2H, m), 7.43 (2H,
d, J = 8.4 Hz), 7.36 (2H, d, J = 8.4 Hz), 5.44 (1H, dd, J1 = 7.2 Hz, J2 = 4.2 Hz), 4.49–4.59 (2H,
13
m), 1.32 (9H, s). C NMR (CDCl3, 100 MHz): 163.8, 134.8, 132.8, 128.8, 126.9, 125.7, 123.8,
85.3, 78.9, 34.5, 31.3. HRMS (ESI-MS) m/z: [M+Na]+ Calcd for C20H21NO5Na+ 378.1317;
Found 378.1310.
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2-(2-(4-Bromophenyl)-2-hydroperoxyethoxy)isoindoline-1,3-dione (2d). Prepared from p-
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bromo-styrene (91 mg, 0.5 mmol) and NHPI (98 mg, 0.6 mmol). White solid, 162 mg (86 %);
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mp 93-95 ºC. H NMR (CDCl3, 300 MHz): 7.85–7.92 (2H, m), 7.78–7.85 (2H, m), 7.55 (2H, J
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=8.4Hz, d), 7.33 (2H, J = 8.4 Hz, d), 5.40 (1H, J1 = 7.8 Hz, J2 = 3.6Hz, dd), 4.44–4.56 (2H, m).
13C NMR (CDCl3, 100 MHz): 163.8, 134.9, 134.8, 131.9, 128.8, 128.7, 123.9, 123.0, 84.8, 78.5.
HRMS (ESI-MS) m/z: [M+Na]+ Calcd for C16H12BrNO5Na+ 399.9797; Found 399.9786.
2-(2-Hydroperoxy-2-(perfluorophenyl)ethoxy)isoindoline-1,3-dione (2e). Prepared from
2,3,4,5,6-pentafluoro-styrene (97 mg, 0.5 mmol) and NHPI (98 mg, 0.6 mmol). White solid, 149
mg (77 %); mp 93-95 ºC. 1H NMR (CDCl3, 300 MHz): 9.70 (1H, brs), 7.87–7.92 (2H, m), 7.80–
7.87 (2H, m), 5.86 (1H, J1 = 9.0 Hz, J2 = 3.0 Hz, dd), 4.74–4.84 (1H, m), 4.50–4.59 (1H, m). 19F
13
NMR (CDCl3, 282 MHz): 140.35 (2F, m), 151.75 (1F, m), 160.87 (2F, m). C NMR (CDCl3,
+
100 MHz): 136.8, 135.0, 128.6, 124.0, 77.8. HRMS (EI-MS) m/z: [M]+ Calcd for C16H8NF5O5
389.0323; Found 389.0315.
2-((1-Hydroperoxy-1-(4-methoxyphenyl)propan-2-yl)oxy)isoindoline-1,3-dione (2f). Prepared
from p-methoxyl-(E)-β-methylstyrene (74 mg, 0.5 mmol) and NHPI (98 mg, 0.6 mmol). White
solid, 140 mg (81 %); mp 103 ºC (decomposition). 1H NMR (CDCl3, 300 MHz): 10.03 (1H, brs),
7.86–7.94 (2H, m), 7.77–7.86 (2H, m), 7.42 (1.4H, J = 8.0 Hz, d), 7.29 (0.6H, J = 8.0 Hz, d),
6.94 (2H, J = 8.0 Hz, d), 5.08 (1H, s), 4.78–4.95 (0.74H, m), 4.60–4.67 (0.26H, m), 3.83 (3H, s),
13
1.31 (3H, J = 8.0 Hz, d). C NMR (CDCl3, 100 MHz): 164.70, 159.95, 134.85, 130.00, 128.77,
123.84, 133.89, 87.89, 83.98, 55.28, 14.66. HRMS (ESI-MS) m/z: [M+Na]+ Calcd for
C18H17NO6Na+ 366.0954; Found 366.0951.
2-(2-Hydroperoxy-1,2-diphenylethoxy)isoindoline-1,3-dione (2g). Prepared from (E)-1,2-
diphenylethene (90 mg, 0.5 mmol) and NHPI (98 mg, 0.6 mmol). White solid, 176 mg (94 %);
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mp 136-137 ºC. H NMR (CDCl3, 300 MHz): 10.24 (1H, brs), 7.67–7.81 (4H, m), 7.12–7.33
(10H, m), 5.94 (0.89 H, J = 4.2 Hz, d), 5.61 (0.11H, J = 4.2 Hz, d), 5.48 (0.11H, J = 4.2 Hz, d),
5.26 (0.9H, J = 4.2 Hz, d). 13C NMR (CDCl3, 100 MHz): 164.1, 134.7, 133.9, 133.5, 128.9,
128.8, 128.6, 128.5, 128.0, 127.8, 123.7, 89.1, 87.9. HRMS (ESI-MS) m/z: [M+Na]+ Calcd for
C22H17NO5Na+ 398.1004; Found 398.1016.
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Aerobic oxidation of benzyl alcohol 5. The mixture of 5 (50 µL, 0.3 mmol), NHPI (7.3 mg, 45
µmol, 15 mol%) and DPA-BP (2.3 mg, 4.5 µmol, 1.5 mol%) in 5 mL acetone was stirred under
O2 atmosphere (1 atm) with the irradiation of light source (26 W CFL, distance app. 2 cm) at
1
room temperature. The reaction was monitored via H-NMR or TLC (hexanes : ethyl acetate =
20 : 1). Upon consumption of starting material (24 h), removal of the solvent under vacuum, the
residue was purified by column chromatography on silica gel using hexane/ethyl acetate (20:1)
as the eluent to yield the product 6.36 1H NMR (CDCl3, 300 MHz): 7.32 (2H, J = 12.0 Hz, d).
6.97 (2H, J = 12.0 Hz, d), 3.90 (3H, s), 2.59 (3H, s). HRMS (ESI-MS) m/z: [M+H]+ Calcd for
+
C9H11O2 151.0759; Found 151.0742.
Aerobic oxidation of lignin model compounds 7. The mixture of lignin model compound 7a~l
(0.2 mmol), NHPI (8.0 mg, 50 µmol, 25 mol%) and DPA-BP (2.6 mg, 5.0 µmol, 2.5 mol%) in 5
mL acetone was stirred under O2 atmosphere (1 atm) with irradiation of the light source (26 W
CFL, distance app. 2 cm) at room temperature. The reaction was monitored via TLC (hexanes :
ethyl acetate = 10 : 1 ~ 4 : 1). Upon consumption of starting material, removal of the solvent
under vacuum, the residue was purified by column chromatography on silica gel using
hexane/ethyl acetate (50 : 1 ~ 10 : 1) as the eluent to yield the product 8a~l.
2-(2-Methoxyphenoxy)-1-phenylethan-1-one (8a).37 Prepared from 7a (48.8 mg, 0.2 mmol).
White solid, 47.4 mg (98 %). 1H NMR (CDCl3, 400 MHz): 8.06 (2H, J = 9.0 Hz, d), 7.63 (1H, J
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= 6.0 Hz, t), 7.52 (2H, J = 6.0 Hz, t), 6.93~7.00 (2H, m), 6.88 (2H, J = 3.0 Hz, d), 5.37 (2H, s),
3.91 (3H, s). HRMS (ESI-MS) m/z: [M+Na]+ Calcd for C15H14O3Na+ 265.0841; Found
265.0853.
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1-(4-Methoxyphenyl)-2-phenoxyethan-1-one (8b).37 Prepared from 7b (48.8 mg, 0.2 mmol).
White solid, 48.2 mg (99 %). 1H NMR (CDCl3, 400 MHz): 8.05 (2H, J = 12.0 Hz, d), 7.25~7.35
(2H, m), 6.90~7.07 (5H, m), 5.24 (2H, s), 3.91 (3H, s). HRMS (ESI-MS) m/z: [M+Na]+ Calcd
for C15H14O3Na+ 265.0841; Found 265.0854.
1-(4-Methoxyphenyl)-2-(p-tolyloxy)ethan-1-one (8c).23 Prepared from 7c (51.6 mg, 0.2 mmol).
1
White solid, 50.7 mg (99 %). H NMR (CDCl3, 400 MHz): 8.02 (2H, J = 8.0Hz, d), 7.16 (1H, J
= 8.0 Hz, t), 6.98 (2H, J = 8.0 Hz, d), 6.66–6.87 (3H, m), 5.19 (2H, s), 3.89 (3H, s), 2.32 (3H, s).
+
HRMS (ESI-MS) m/z: [M+H]+ Calcd for C16H17O3 257.1178; Found 257.1183.
2-(2-Methoxyphenoxy)-1-(4-methoxyphenyl)ethan-1-one (8d).23 Prepared from 7d (54.8 mg,
1
0.2 mmol). White solid, 53.5 mg (98 %). H NMR (CDCl3, 400 MHz): 8.06 (2H, J = 9.3Hz, d),
6.92–7.01 (4H, m), 6.88 (2H, d, J = 3.9Hz), 5.31 (2H, s), 3.91 (3H, s), 3.90 (3H, s). HRMS (ESI-
MS) m/z: [M+Na]+ Calcd for C16H16O4Na+ 295.0946; Found 295.0948.
3-Hydroxy-2-(2-methoxyphenoxy)-1-(4-methoxyphenyl)propan-1-one (8e).23 Prepared from 7e
1
(60.8 mg, 0.2 mmol). White solid, 58.2 mg (98 %). H NMR (CDCl3, 400 MHz): 8.11 (2H, J =
9.0Hz, d), 6.81–7.05 (6H, m), 5.40 (1H, J = 6.9 Hz, t), 4.02–4.15 (2H, m), 3.90 (3H, s), 3.88
(3H, s), 3.14 (1H, J = 6.9, t). HRMS (ESI-MS) m/z: [M+Na]+ Calcd for C17H18O5Na+ 325.1052;
Found 325.1047.
2-(4-Methoxyphenyl)-2-oxoethyl acetate (8f).23 Prepared from 7f (40.2 mg, 0.2 mmol). White
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solid, 38.6 mg (96 %). H NMR (CDCl3, 400 MHz): 7.91 (2H, J = 8.0Hz, d), 6.97 (2H, J =
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8.0Hz, d), 5.30 (2H, s), 3.88 (3H, s), 2.23 (3H, s). HRMS (ESI-MS) m/z: [M+H]+ Calcd for
3
4
5
6
+
C11H13O4 209.0814; Found 209.0825.
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2-(2,6-Dimethoxyphenoxy)-1-(4-methoxyphenyl)ethan-1-one (8g).23 Prepared from 7g (60.8
mg, 0.2 mmol). White solid, 58.0 mg (96 %). 1H NMR (CDCl3, 400 MHz): 8.11 (2H, J = 8.0 Hz,
d), 7.03 (1H, J = 8.0 Hz, t), 6.98 (2H, J = 8.0 Hz, d), 6.61 (2H, J = 8.0 Hz, d), 5.16 (2H, s), 3.90
(3H, s), 3.83 (6H, s). HRMS (ESI-MS) m/z: [M+Na]+ Calcd for C17H18O5Na+ 325.1052; Found
325.1061.
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1-(3,4-Dimethoxyphenyl)-2-(2-methoxyphenoxy)ethan-1-one (8h).38 Prepared from 7h (60.8
1
mg, 0.2 mmol). White solid, 57.2 mg (94 %). H NMR (CDCl3, 400 MHz): 7.73 (1H, J1 = 9.0
Hz, J2 = 3.0Hz, dd), 7.63 (1H, J = 3.0 Hz, d), 6.90–7.00 (3H, m), 6.88 (2H, J = 3.0 Hz, d), 5.32
(2H, s), 3.98 (3H, s), 3.96 (3H, s), 3.91 (3H, s). HRMS (ESI-MS) m/z: [M+Na]+ Calcd for
C17H18O5Na+ 325.1052; Found 325.1051.
1-(3,4-Dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propan-1-one (8i).23 Prepared from
1
7i (66.8 mg, 0.2 mmol). White solid, 59.8 mg (90 %). H NMR (CDCl3, 400 MHz): 7.80 (1H, J1
= 9.0 Hz, J2= 3.0 Hz, dd), 7.64 (1H, J = 3.0 Hz, d), 6.98–7.10 (1H, m), 6.77–6.98 (4H, m), 5.42
(1H, J = 3.0 Hz, t), 4.09 (2H, J = 6.0 Hz, t), 3.97 (3H, s), 3.94 (3H, s), 3.89 (3H, s), 3.01–3.19
+
(1H, m). HRMS (ESI-MS) m/z: [M+H]+ Calcd for C18H21O6 333.1338; Found 333.1346.
1-(4-Hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)ethan-1-one (8j).38 Prepared from 7j
(58.0 mg, 0.2 mmol). White solid, 53.0 mg (92 %). 1H NMR (CDCl3, 400 MHz): 7.57–7.65 (2H,
m), 6.90–7.00 (3H, m), 6.87 (2H, J = 3.0 Hz, d), 6.39 (1H, brs), 5.30 (2H, s), 3.96 (3H, s), 3.90
+
(3H, s). HRMS (ESI-MS) m/z: [M+H]+ Calcd for C16H17O5 289.1076; Found 289.1081.
3-Hydroxy-1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propan-1-one
(8k).38
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Prepared from 7k (64.0 mg, 0.2 mmol). White solid, 51.5 mg (81 %). H NMR (CDCl3, 400
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MHz): 7.56–7.80 (2H, m), 6.81–7.05 (5H, m), 5.44 (1H, J = 6.3Hz, t), 4.05–4.12 (2H, m), 4.08
5
6
+
(3H, s), 3.89 (3H, s), 3.02 (1H, brs). HRMS (ESI-MS) m/z: [M+H]+ Calcd for C17H19O6
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8
9
319.1182; Found 319.1191.
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2-(2-Methoxyphenoxy)-1-(3,4,5-trimethoxyphenyl)ethan-1-one (8l).39 Prepared from 7l (66.8
1
mg, 0.2 mmol). White solid, 64.4 mg (97 %). H NMR (CDCl3, 400 MHz): 7.36 (2H, s), 6.89–
7.05 (2H, m), 6.89 (2H, J = 6.0Hz, d), 5.29 (2H, s), 3.95 (3H, s), 3.94 (6H, s), 3.91 (3H, s).
+
HRMS (ESI-MS) m/z: [M+Na]+ Calcd for C18H20NaO6 355.1158; Found 355.1167.
ASSOCIATED CONTENT
Supporting Information. Physical measurements, spectroscopic characterizations, CV
diagrams, and NMR spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
jzhang3@unl.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The authors are grateful for the support from the University of Nebraska-Lincoln. J. Z. is also
grateful for the Donors of the American Chemical Society Petroleum Research Fund (53678-
DNI10) and National Science Foundation (DMR-1554918) for partial support of this research.
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