Visible-light-promoted conversion of alkyl benzyl ether to alkyl ester or alcohol via O-α-sp3 C-H cleavage

Article abstract of DOI:10.1021/acs.orglett.5b00663

A mild and high-yielding visible-light-promoted conversion of alkyl benzyl ethers to the alkyl esters or alkyl alcohols was developed. Mechanistic studies provided evidence for a radical chain reaction involving the homolytic cleavage of O-α-sp³ C-H bonds in the substrate as one of the propagation steps. We propose that α-bromoethers are key intermediates in the transformation.

Full text of DOI:10.1021/acs.orglett.5b00663

Letter
pubs.acs.org/OrgLett
Visible-Light-Promoted Conversion of Alkyl Benzyl Ether to Alkyl
Ester or Alcohol via O‑α-sp3 C−H Cleavage
Ping Lu, Tianyuan Hou, Xiangyong Gu, and Pixu Li*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering, and Materials Science,
Soochow University, 199 RenAi Road, Suzhou, Jiangsu 215123, China
S
* Supporting Information
ABSTRACT: A mild and high-yielding visible-light-promoted
conversion of alkyl benzyl ethers to the alkyl esters or alkyl alcohols
was developed. Mechanistic studies provided evidence for a radical
chain reaction involving the homolytic cleavage of O-α-sp3 C−H
bonds in the substrate as one of the propagation steps. We propose
that α-bromoethers are key intermediates in the transformation.
he research on the cleavage of an inert sp3 C−H bond
Tadjacent to heteroatoms has attracted interest from the
organic chemistry community.1 One of the general activation
modes is the oxidation of a heteroatom to the corresponding
radical cation, which activates the adjacent C−H bond. The
reactions involving N-α-sp3 C−H bond cleavage often go
through this pathway.2 However, this strategy is hardly applicable
to the cleavage of an O-α-sp3 C−H bond presumably due to the
high electronegativity of the O-atom. As a result, the chemistry
involving the cleavage of the O-α-sp3 C−H bond remains
underdeveloped.3
would result in the formation of an oxacarbenium ion.
Consequently, new chemistry could be developed by taking
advantage of the reactive oxacarbenium ion or its related species.
The reactions involving the oxacarbenium ion via hydride
abstraction of benzyl ethers or allyl ethers using an
oxoammonium salt have been reported by Bobbitt,11 Bailey,12
Leadbeater,13 and Garcia-Mancheno14 recently. Herein, we
̃
present a visible-light-promoted direct conversion of alkyl benzyl
ether to alkyl ester or alcohol initiated by O-α-sp3 C−H bond
cleavage (Scheme 1).
Scheme 1. Photoredox Reactions of O-α-sp3 C−H Cleavage
Ir/Ru polypyridyl complex catalyzed reactions under visible
light irradiation have emerged as a powerful tool in redox
chemistry.4 Many reports of visible-light-promoted N-α-sp3 C−
H bond cleavage have appeared in the literature because amine
nitrogen could be oxidized by a number of sensitizers.5,6 In
contrast, the cleavage of the C−H bond adjacent to an O-atom
under visible light conditions is rarely reported, presumably due
to the above-mentioned difficulty in the oxidation of oxygen lone
pairs. Stephenson et al. took advantage of the relatively low
oxidation potential of the p-methoxyphenyl (PMP) group and
developed an elegant visible-light-mediated deprotection proto-
col of PMB ether.7 However, this protocol cannot be applied to
the benzyl ether because an electron-rich PMP group is required.
MacMillan et al. reported a visible-light-mediated O-α-sp3 C−H
bond cleavage using a thiyl radical to abstract benzylic hydrogen,
followed by the combination of the benzylic radical with a p-
dicyanobenzene radical anion or imine to form new C−C bond.8
Very recently, the same research group reported a photoredox
direct α-arylation of ether using a sulfate radical to activate the
C−H bond in ether.9 In continuation of our interest in the
activation of the sp3 C−H bond adjacent to the heteroatom,10 we
envisioned that a practical activation mode for ether to
participate in a visible-light-promoted photoredox reaction
would go through a hydrogen atom transfer (HAT) process.
We postulated that HAT from alkyl benzyl ether to electrophilic
species such as a trichloromethyl radical would afford a benzyl
radical. Facile single-electron oxidation of the benzyl radical
Our investigation of new visible-light-mediated reactions of
ether used benzyl ether as the model substrate since the
homolytic fission of benzylic C−H bond affords a stable benzylic
radical. The oxidation of the benzylic radical of ether by visible-
light-excited sensitizer is likely to occur due to the formation of a
stable phenyl oxonium ion. We initiated our study by examining
the photochemical oxidation of (3-(benzyloxy)propyl)benzene
(1a) in the presence of CBrCl3. However, a messy reaction was
observed after irradiating the reaction mixture under a 14 W
compact fluorescent light for 12 h (Table 1, entry 1). To our
surprise, addition of EtOAc to the reaction mixture upon
disappearance of 1a afforded 3-phenylpropyl acetate (2a) in 99%
yield (Table 1, entry 2). Ethyl acetate was then used as a reagent
in the reaction (Table 1, entries 3−7). It turned out that 20 equiv
of ethyl acetate were necessary to afford 2a in good yield.
Received: March 5, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00663
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Table 2. Substrate Scope Screening
EtOAc
(equiv)
GC
entry
catalyst
solvent
CH2Cl2
yield
1
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(ppy)3
−
−
1
messy
b
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DMSO
DMF
99%
3
21%
30%
60%
77%
99%
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
7%
4
2
5
5
6
10
20
20
20
20
20
20
20
20
20
20
20
20
7
8
9
FIrpic
10
11
12
13
14
15
16
17
18
Ru(bpy)3Cl2·6H2O
Ru(ppy)3(PF6)2
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
Ir(dtb-bpy)(ppy)2PF6
−
MeCN
HCCl3
20%
5%
CH2ClCH2Cl
CH2Cl2
CH2Cl2
trace
trace
Ir(dtb-bpy)(ppy)2PF6, no
light
a
Reactions were performed with 1a (0.5 mmol), CBrCl3 (1.0 mmol),
EtOAc, and catalyst (1 mol %) in solvent (2 mL), irradiated under 14
b
W CFL for 12 h at rt. EtOAc was added after the starting material
had been consumed.
Ru(bpy)3Cl2·6H2O, Ru(ppy)3(PF6)2, Ir(ppy)3, and Bis[2-(4,6-
difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)
(FIrpic) were found to be ineffective (Table 1, entries 8−11). A
survey of solvents demonstrated that CH2Cl2 was the optimal
solvent for this reaction (Table 1, entries 12−16). In the absence
of light or catalyst, only a trace amount of product was obtained,
indicating the reaction is a visible-light-promoted reaction (Table
1, entries 17−18).
a
Reaction conditions: substrate 1 (0.5 mmol), Ir(dtb-bpy)(ppy)2PF6
(1 mol %), CBrCl3 (1.0 mmol), and EtOAc (10 mmol) in CH2Cl2 (2
b
mL), irradiated under 14 W CFL for 12 h at rt. Isolated yield.
c
Reacted for 24 h.
a
Table 3. Transesterification with Different Esters
Typically, the conversion of benzyl ether to the corresponding
acetate requires two separate steps, hydrogenolysis of benzyl
ether to afford alcohol and a subsequent esterification reaction.
The direct conversion from alkyl benzyl ether to alkyl ester using
an alkyl acetate as the reagent has never been reported in the
literature. Therefore, substrate scope investigation and mecha-
nistic study are warranted. First, we explored the scope of
different ethers (Table 2). Excellent yields were generally
obtained for different alkyl benzyl ethers (Table 2, entries 1−
7). The benzylic C−H adjacent to the O-atom is selectively
cleaved over the normal benzylic C−H bond, including the
benzylic C−H bond of the p-methoxyphenyl group. Alkyl benzyl
ether bearing a bromo group at the alkyl chain was found to
afford the corresponding product in high yield too (Table 2,
entry 8, 95%). Substituents, such as methyl and halides, at the
benzene ring were all tolerated and afforded the corresponding
products in good yields (Table 2, entries 9−12). However,
secondary benzyl ether with a methyl or phenyl group at the
benzylic position gave lower yields (Table 2, entries 13 and 14),
probably due to the steric effect.
a
Reaction conditions: 1a or 1g (0.5 mmol), Ir(dtb-bpy)(ppy)2PF6 (1
mol %), CBrCl3 (1.0 mmol), and ester (10 mmol) in CH2Cl2 (2 mL),
irradiated under 14 W CFL for 12 h at rt. Isolated yield. Isolated
yield of 3a.
b
c
However, ethyl benzoate did not undergo transesterification
reaction. Only 3-phenyl-1-propanol was isolated in 64% yield
(Table 3, entry 5).
We then began the study to understand the reaction
mechanism. First, the initiation of the reaction was investigated
using a fluorescence quenching experiment. The results (see
Next, we turned our attention to the reaction with different
esters, as this would lead to different ester products. Various alkyl
carboxylic acid esters participated in the reactions forming the
corresponding esters in good yields (Table 3, entries 1−4).
B
DOI: 10.1021/acs.orglett.5b00663
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Organic Letters
Letter
Supporting Information) showed that CBrCl3 is much more
efficient in quenching the visible-light-excited Ir(dtb-bpy)-
(ppy)2PF6, indicating that an oxidative quenching of the excited
catalyst by CBrCl3 afforded the Br− anion and ·CCl3 radical. The
resulting ·CCl3 radical should be responsible for the abstraction
of the H-atom from (3-(benzyloxy)propyl)benzene to give the
corresponding benzylic radical. Indeed, chloroform was formed
as a stoichiometric product by in situ NMR monitoring. This
result is consistent with the report by Stephenson et al.7 Next, it is
important to answer what happened to the benzylic radical. In
situ 1H NMR spectra of the reactions of 1a and 1a-d2 (the two
benzylic hydrogens in 1a were replaced by deuteriums)
performed in CD2Cl2 in the absence of EtOAc showed that a
distinctive proton signal at 6.98 ppm increased as the reaction of
1a proceeded. No such signal was observed in the reaction of 1a-
d2 (Figure 1, eqs 1 and 2). The chemical shift of the benzylic
Scheme 2. Investigations of the Role of Ester
oxonium ion generated from benzyl ether. When substrate 1a
and 0.5 equiv of dodecan-1-ol were subjected to the standard
conditions, both 2a and dodecyl acetate were observed in the
product mixture (Scheme 2, eq 3). This result suggested alcohol
might be one of the key intermediates in the reaction pathway.
On the basis of the above-mentioned results, a reaction
mechanism was proposed in Scheme 3. The reaction is initiated
Scheme 3. Proposed Mechanism
Figure 1. NMR study.
proton adjacent to the O-atom in an oxonium ion should be
further downfield than 6.98 ppm. Therefore, this proton signal
was attributed to α-bromoether. In the reaction with EtOAc
(Figure 1, eq 3), the 6.98 ppm signal could not be seen because
the α-bromoether intermediate was converted to the product 2a.
The α-bromoether could be formed via two possible pathways:
(1) the benzylic radical is oxidized to an oxonium ion, followed
by nucleophilic addition of the Br− anion formed in the oxidative
quenching step; or (2) the benzylic radical abstracts a bromine
atom from CBrCl3 to form the α-bromoether, to generate a new
·CCl3 radical. The former pathway would be a radical-ionic
crossover pathway which relies on continuous irradiation. In
contrast, the latter pathway is a radical chain propagation
pathway. Therefore, a light on/off switch experiment was
performed (see Supporting Information). The reaction of 1a
did not stop completely in the absence of light. Instead, the
product 2a kept forming after turning off the light. So the radical
chain propagation mechanism probably is operative in the
reaction. It is one of the few Ru/Ir complex-catalyzed visible-
light-promoted radical chain reactions.
It was important to find out the role of ester and how the acyl
group was transferred. In the reaction of dodecyl acetate,
dodecanol was observed as one of the products (Scheme 2, eq 1).
In the reaction using S-ethyl thioacetate, (phenylmethylene)bis-
(ethylsulfane) (8) was formed along with 57% of benzaldehyde
and 97% of the product 2a (Scheme 2, eq 2). In contrast, O-ethyl
thioacetate was unreactive in otherwise identical reaction
conditions. The results showed that the sp3 O-atom of ester 2a
was from the ether 1a, indicating that the reaction might go
through a tetrahedral intermediate by nucleophilic addition to
ester instead of a nucleophilic addition of caboxylate to an
via the oxidative quenching of Ir(III)* by BrCCl3 to afford a
·CCl3 radical, which abstracts a H-atom from benzyl ether 6 to
form chloroform and benzylic radical A. In the radical chain
propagation step, the resulting benzylic radical abstracts a Br-
atom from BrCCl3 to give the corresponding α-bromoether B
and regenerates the ·CCl3 radical. Next, a nucleophilic attack of
residual alcohol in ethyl acetate to either the α-bromoether B or
oxonium bromide C affords the mixed acetal D. Proton transfer
and a second nucleophilic attack of alcohol afford benzaldehyde
diethyl acetal (E) and alcohol F. Alcohol F then undergoes
transesterification with excess ethyl acetate to afford the final
product 2. Alternatively, the conversion of B to 2 could be
triggered by a trace amount of water in solvent or EtOAc in a
similar manner.
Given the reaction pathway works as proposed, it could be
anticipated that debenzylation would occur in the absence of
ethyl acetate. So we investigated the reaction using alcohol to
replace ester. The reaction was completely shut down with the
addition of MeOH as a reagent in the reaction. Therefore,
MeOH was added after the reaction mixture was irradiated under
visible light for 12 h. To our delight, alcohol 3a was obtained in
85% isolated yield. A small amount of alkyl bromide was
observed as the byproduct. This reaction is an alternative, mild
method for benzyl ether deprotection when hydrogenolysis is
not compatible with other functionalities in a complex molecule.
A series of benzyl ethers were screened, and the results were
summarized in Table 4. Benzyl ethers of primary and secondary
alcohols afforded the corresponding products with reasonably
good isolated yields.
C
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Organic Letters
Letter
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a
Reaction conditions: substrate 1 (0.5 mmol), Ir(dtb-bpy)(ppy)2PF6
(1 mol %), and CBrCl3 (1.0 mmol) in CH2Cl2 (2 mL), irradiated
under 14 W CFL at rt. MeOH (10 mL) was added after 12 h.
b
Isolated yield; GC yield in parentheses.
In conclusion, a visible-light-mediated O-α-sp3 C−H cleavage
strategy was developed. Oxidative quenching of photoexcited
Ir(dtb-bpy)(ppy)2PF6 with BrCCl3 generates the electron-
deficient ·CCl3 radical, which abstracts the benzylic hydrogen
of alkyl benzyl ether and triggers radical chain propagation of the
benzylic radical with BrCCl3 to form a reactive α-bromoether
intermediate. The transesterification of the α-bromoether with
excess ester affords alkyl ester. This mild and high-yielding
reaction is a convenient alternative route to the traditional
transformations of alkyl benzyl ether to the corresponding alkyl
ester via a two-step hydrogenolysis and esterification procedure.
In addition, by replacing the ester reactant with MeOH, a visible-
light-promoted debenzylation reaction was developed. Mecha-
nistic study showed that the reaction is an iridium-initiated
photocatalytic radical chain propagation reaction.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterization data, and 1H and 13
C
NMR spectra for all products. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Soc. 2014, 136, 16986.
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X.; Gu, X.; Li, Y.; Li, P. ACS Catal. 2014, 4, 1897.
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: lipixu@suda.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support is provided by NSFC (21102097) and the
Scientific Research Foundation for the Returned Overseas
Chinese Scholars, the State Education Ministry.
(13) Kelly, C. B.; Ovian, J. M.; Cywar, R. M.; Gosselin, T. R.; Wiles, R.
J.; Leadbeater, N. E. Org. Biomol. Chem. 2015, 13, 4255.
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DOI: 10.1021/acs.orglett.5b00663
Org. Lett. XXXX, XXX, XXX−XXX