Alcohol Etherification via Alkoxy Radicals Generated by Visible-Light Photoredox Catalysis

Article abstract of DOI:10.1021/acs.orglett.0c03058

A mechanistically divergent method is described that, employing a commercially available hypervalent iodine(III) reagent, generates alkoxy radicals from 1°, 2°, and 3° alcohols and allows their use in the functionalization of C(sp3)-H and C(sp2)-H bonds. This visible-light photoredox catalysis produces alkyl ethers via 1,5/6-hydrogen atom transfer or aryl ethers via 1,5-addition. This mild methodology provides a practical strategy for the synthesis of acetals, orthoesters, tetrahydrofurans, and chromanes.

Full text of DOI:10.1021/acs.orglett.0c03058

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Letter
Alcohol Etherification via Alkoxy Radicals Generated by Visible-
Light Photoredox Catalysis
́
Alexandra R. Rivero, Peter Fodran, Alica Ondrejkova, and Carl-Johan Wallentin*
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ABSTRACT: A mechanistically divergent method is described
that, employing a commercially available hypervalent iodine(III)
reagent, generates alkoxy radicals from 1°, 2°, and 3° alcohols and
allows their use in the functionalization of C(sp3)−H and C(sp2)−
H bonds. This visible-light photoredox catalysis produces alkyl
ethers via 1,5/6-hydrogen atom transfer or aryl ethers via 1,5-
addition. This mild methodology provides a practical strategy for
the synthesis of acetals, orthoesters, tetrahydrofurans, and
chromanes.
Scheme 1. Synthesis of Ethers and Alkoxy Radical
Generation
thers constitute a common framework in many bioactive
E
natural products, pharmaceuticals, and cosmetics.1 More-
over, ethers are broadly applied in industrial contexts as
solvents, refrigerants, and more recently, lubricants and fuels.2
During the past decades, transition-metal catalysis became a
heavily utilized tool for the synthesis of ethers, overcoming the
functional group incompatibility observed for more classical
methods.3 However, these cross-coupling reactions are limited
to aryl halides and are typically conducted under high
temperatures to achieve effective reductive elimination for
palladium- and nickel-based systems or effective oxidative
addition for copper-based processes (Scheme 1a).4 Photoredox
catalysis has recently emerged as a powerful alternative that
allows for direct C−O coupling using milder conditions.5 A
recent application of this methodology is the room-temper-
ature etherification developed by MacMillan and co-workers
that allows for the formation of C−O bonds by employing dual
photoredox and nickel catalysis. Nevertheless, this novel
approach is limited to aryl bromides.6 We reasoned that
alkoxy radicals generated from alcohols under photoredox
catalysis,7 could be applied to avoid the use of halides as
substrates in the construction of ethers. Photoredox catalysis
has been employed to generate alkoxy radicals directly from
alcohols via reductive quenching7e or by oxidative quenching
in combination with stoichiometric oxidants7b or redox-active
auxiliaries (Scheme 1b). Most commonly used alkoxy radical
precursors from alcohols, such as nitrite esters,8 sulfenyl
ethers,9 peroxides,10 hypohalites,11 N-alkoxypyridine-2-thio-
nes,12 and N-alkoxyphthalimides,13 are sometimes challenging
to synthesize, unstable, or toxic. On the other hand,
hypervalent iodine(III) reagents (HIR),14a which have been
used for the direct generation of oxygen-centered radicals from
alcohols under mild photoredox conditions via oxidative
quenching (Scheme 1b),14b−d are commercially available,
easy to handle, and nontoxic.
Inspired by the recent applications of HIR in photoredox
catalysis, we decided to further explore their potential to
develop a synthetically useful C−O bond-forming method-
ology. We hypothesized that direct attachment of HIR to the
hydroxyl group might, upon oxidative quenching, promote the
Received: September 13, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03058
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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well-known 1,5-hydrogen atom transfer (1,5-HAT) for C-
(sp3)−H remote funtionalization15 and also the underexplored
1,5-addition to unsaturated functionalities for C(sp2)−H
functionalization. This strategy would provide access to a
diverse set of oxacyclic frameworks in a redox-neutral fashion
(Scheme 1c).
We began our investigations with benziodoxolone 2, derived
from 4-phenylbutanol. First, we focused on the 1,5-HAT
process as a model reaction. As shown in Table 1 (summary of
etherification of 4-phenyl butanol. Unfortunately, several
studies showed that compound 2 cannot be formed using
optimized conditions (see Supporting Information S12−14).
With the optimal conditions in hand, we next investigated
the scope of the alkoxy benziodoxolone derivatives. All
benziodoxolone derivatives were prepared using a modified
method developed by Baker et al.,16 employing commercially
available 1-acetoxy-1,2-benziodoxol-3(1H)-one and dibromo-
methane as the solvent. These conditions allowed us to reduce
the required stoichiometry of the alcohol to only 1.5 equiv as
compared to the solvolytic conditions reported by Baker16 (see
Supporting Information S4). As shown in Scheme 2, double-
Table 1. Summary of the Optimization Conditions for the
Photoredox-Catalyzed Etherification of Phenylbutoxy
Benziodoxolone 2
a
Scheme 2. Photocatalytic Etherification via 1,5-HAT
ab
,
entry Ir(ppy)3 (mol %)
base
solvent
yield of 3 (%)
1
2
3
4
5
6
7
8
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
2.0
10.0
5.0
DMF
DCE
THF
PhCF3
0
5
5
28
40
42
46
50
35
32
44
44
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
K2HPO4
Amberlite
pyridine
lutidine
DMAP
pyridine
pyridine
pyridine
pyridine
pyridine
9
10
11
12
13
14
15
c
52
0
0
no catalyst
5.0
d
a
Reaction conditions: 2 (0.1 mmol), Ir complex (2.0−10.0 mol %),
base (1.0 equiv), and solvent (2 mL) under light irradiation using blue
b
LED at rt for 16h under N2. Yield of compound 3 determined by 1H
c
NMR using ethylene carbonate as an internal standard. Reaction
time 3 h. Reaction performed in the absence of light. IB =
d
a
benziodoxolone.
Reaction conditions: 2 (0.5 mmol), fac-Ir(ppy)3 (5.0 mol %),
pyridine (1.0 equiv), and CH3CN (10 mL) under light irradiation
using blue LED at rt for 3 h under N2. Yields are those of isolated
products.
key observations), we started screening for the optimal solvent
in which benziodoxolone 2 would be converted into 2-
phenyltetrahydrofuran 3 in the presence of photocatalyst fac-
Ir(ppy)3. Among all the solvents investigated, acetonitrile
showed the best result, giving the desired tetrahydrofuran 3 in
40% yield (NMR) after irradiation with blue LEDs for 16 h
(entry 5). Subsequently, the etherification was optimized by
varying reaction parameters and exploring various organic and
transition-metal-based catalysts. None of these variations
showed a notable increase in the reaction yield (see Supporting
Information S11). However, the addition of base was
somewhat beneficial, with pyridine providing the highest
yield (entries 6−10). Any variation of the photocatalyst
loading resulted in a decrease in the reaction yield (entries 11
and 12). Optimal conditions were established using fac-
Ir(ppy)3 (5.0 mol %) as the photocatalyst, pyridine (1.0 equiv)
as the base, and acetonitrile (0.05 M) as solvent under blue
LEDs irradiation for 3 h (52% yield, entry 13). Control
experiments revealed that both photocatalyst and light were
essential for the etherification reaction given that no product
was detected when the reaction was performed in their absence
(entries 14 and 15). Finally, we explored the direct
activated (benzylic and α-heteroatom) C−H bonds produced
the acetals 3a, 3b, and 3c in 75%, 92%, and 91% yield,
respectively. These three examples illustrate that this δ-
C(sp3)−H etherification is not affected by the electronic
nature and tolerates some steric bulk of the substituents on the
arene. The use of only α-heteroatom-activated C−H bonds
allowed the formation of noncyclic acetal 3d with slightly
decreased yield (52%). This result prompted us to question
whether this methodology applies to the synthesis of
nonsymmetric orthoesters, which are versatile synthetic
intermediates known for their typically problematic syn-
thesis.17 We were pleased to obtain orthoester 3e in 46%
yield. The substrate used for the optimization of the reaction
conditions, as well as the p-fluoro derivative, gave the
corresponding tetrahydrofurans 3f and 3g in 48% (43%
when scaled up to 1.0 mmol) and 51% yields, respectively.
Finally, we examined the reactivity of secondary and tertiary
alkoxy radicals, which are considered challenging due to rapid
β-fragmentation based side reactions.18a To our delight, both
B
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secondary and tertiary alkoxy benziodoxolones yielded the
desired tetrahydrofurans 3h−3k in good to excellent yields
(56−97%). The different yields obtained for the various
substrates investigated are effected by byproducts formations
coupled to the characteristics of the benziodoxolone substrat
(see Supporting Information S15).
In general, alkoxy radicals react via 1,5-HAT or β-
fragmentation due to the formation of less energetic C-
centered radicals acting as a thermodynamic driving force.18
However, when neither of these two paths is feasible, the
resulting RO• has a propensity to add to unsaturated systems,
as recently demonstrated by Dagousset and co-workers.19
Consequently, we decided to explore the possibility of applying
our methodology to the synthesis of chromanes, which to the
best of our knowledge has never been achieved using alkoxy
radicals generated under photoredox catalysis.20
When initially exploring the conditions identified as optimal
for suitable 1,5-HAT substrates, with substrates deemed
suitable to undergo 1,5-addition, we observed that no base
additive was needed to achieve full conversion. Subjecting 3-
phenyl-1-propanol to these conditions afforded chromane 3l in
a 55% yield after only 1 h. The scope of this novel
photocatalyzed intramolecular aryl etherification was next
investigated, and the results are summarized in Scheme 3.
(3-(3,4,5-trimethoxyphenyl)propanol) produced chromane
3m as a single isomer in 56% yield. Alcohol 1n (3-(4-
methoxyphenyl)propanol) gave rise to chromane 3n as an
inseparable mixture of two regioisomers in a 3:1 ratio in 44%
yield. The regioselectivity increased to 5:1 when the phenyl
ring was substituted with ethyl carboxylate in the para-position
(alcohol 1o, methyl 4-(3-hydroxypropyl)benzoate), producing
chromane 3o as an inseparable mixture of regioisomers in 37%
yield. Synthetically useful halogens were also compatible with
this transformation. The o-bromo and p-chloro substituent in
the phenyl ring of the alcohols yielded the corresponding
chromanes 3p and 3q in 52% and 54% yield, respectively, and
in both cases as inseparable mixtures of two regioisomers in a
1:1 ratio. When naphthyl-substituted alcohol was employed
(alcohol 1t, 2-methyl-4-(naphthalen-1-yl)butan-2-ol), the two
regioisomers formed, chromanes 3t and 3t′, were isolated
separately in 49% and 11% yields, respectively. This method
outperforms the existing methods by requiring lower temper-
atures, avoiding the use of a stoichiometric oxidant21a and
allowing electron-rich substrates to undergo etherification.21
The proposed mechanism for both etherification processes is
depicted in Scheme 4. For the formation of tetrahydrofurans
Scheme 4. Proposed Mechanisms Consistent with Gathered
Data
a
Scheme 3. Photocatalytic Etherification via 1,5-Addition
we propose that the reaction is initiated by single-electron
reduction of benziodoxolone A (Ered = −1.07 V vs standard
calomel electrode (SCE) in CH3CN) by the photoexcited state
of IrIII* (E1/2
*
= −1.73 V vs SCE in CH3CN),22
III/IV
producing oxidized catalyst IrIV, 2-iodobenzoate, and the
oxygen-centered radical B. Subsequently, B undergoes 1,5-
HAT, forming the translocated carbon-centered benzylstabi-
lized radical C. The Ir(ppy)3 catalyst is regenerated from IrIV
by SET oxidation of carbon radical C to carbocation D. The
oxidation potential of an alkyl-substituted benzylic radical such
as C is Eox = +0.35 V vs SCE in CH3CN23 and the oxidation
potential of oxidatively quenched Ir(ppy)3 is Eox = +0.77 V vs
SCE in CH3CN,22 making the turnover event of the catalyst
exergonic. Finally, carbocation D undergoes an intramolecular
ring closure to form tetrahydrofuran E. It is worth mentioning
that alkyl-substituted alcohols, such as decanol and 3-
cyclohexylpropanol, with lower radical and carbocation
a
Reaction conditions: 2 (0.5 mmol), fac-Ir(ppy)3 (5.0 mol %) and
CH3CN (10 mL) under light irradiation using blue LED at rt for 1 h
under N2. Yields refer to those of isolated products.
Contrary to what we expected, electron-rich and electron-poor
aromatic arenes produced the corresponding chromanes with
similar yields (3m−3q, 56−54%). Remarkably, secondary and
tertiary alcohols were also viable substrates affording the
corresponding compounds 3r−3u in up to quantitative yield.
Unfortunately, the process takes place with poor regioselec-
tivity: only alkoxy benziodoxolone derived from alcohol 1m
C
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stabilization capacity, were not suitable reaction partners in this
transformation.
Gothenburg, Sweden; orcid.org/0000-0003-1983-9378;
Email: carl.wallentin@chem.gu.se
Regarding the formation of chromanes, the first step is the
same as previously (the single electron reduction of the
benziodoxolone). Next, the oxygen-centered radical B′ can
either add to the ipso position of the aryl ring leading to the
formation of carbon radical C′ or to the ortho position forming
carbon radical C′′. These cyclohexanedienyl radicals, C′ and
C′′, can be oxidized (Eox= ca. 0.1 V vs SCE)24 to the
corresponding carbocations D′ and D′′, while reducing the IrIV
back to IrIII. Finally, rearomatization produces the final
chromane E′ in both cases. For carbocation D′ this
rearomatization process is preceded by a rearrangement. The
mixture of regioisomers obtained (Scheme 3) is most probably
due to a competition between 1,5- and 1,6-addition as well as
between carbon and oxygen migration in the rearrangement
step of the spirocyclic intermediate. Given the regioselectivity
ratios observed for the various aryl rings investigated, it is not
possible to derive a qualitative rational for mechanistic pathway
preferences. The regioisomer distribution observed in the
formation of chromanes 3n−3q and 3t−3t′ does not suggest
that there is a clear trend coupled to stereoelectronic effects;
however, carbon migration is more likely to occur than oxygen
migration for the electron-deficient derivatives 3o−3p.25a
Accordingly, the regioisomeric ratio obtained for these
compounds most probably reflect the 1,5- vs 1,6-addition
preferences for these compounds. Chromane 3m was formed
as a single regioisomer, which suggests that the reaction
selectively progresses via the 1,6-addition pathway. However,
similarly stabilized cationic spirocyclic compounds have been
suggested to rearrange via oxygen migration involving a
stepwise mechanism initiated with a C−O bond cleavage
followed by a 1,4-addition.25b Taken together, the regioiso-
meric ratios observed for most substrates probably stems from
competition between 1,5- and 1,6-addition. For very electron-
rich systems, the picture might be more complicated.
Authors
Alexandra R. Rivero − Department of Chemistry and Molecular
Biology, University of Gothenburg, 412 96 Gothenburg, Sweden
Peter Fodran − Department of Chemistry and Molecular
Biology, University of Gothenburg, 412 96 Gothenburg, Sweden
Alica Ondrejkova − Department of Chemistry and Molecular
Biology, University of Gothenburg, 412 96 Gothenburg,
́
Sweden; orcid.org/0000-0001-7806-5064
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c03058
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by The Swedish Research
Council (2018-04871), The Carl Tryggers foundation, and
Marie Skłodowska-Curie actions (grant to P. F. grant number
799943 SUPER) funded by the European Union under the
Horizon 2020 Program (H2020-MSCA-IF-2017).
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03058.
Experimental procedures and characterization data of
products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
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Carl-Johan Wallentin − Department of Chemistry and
Molecular Biology, University of Gothenburg, 412 96
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