Iron-Catalyzed Radical Cleavage/C?C Bond Formation of Acetal-Derived Alkylsilyl Peroxides

Article abstract of DOI:10.1002/asia.201901695

A novel radical-based approach for the iron-catalyzed selective cleavage of acetal-derived alkylsilyl peroxides, followed by the formation of a carbon–carbon bond is reported. The reaction proceeds under mild reaction conditions and exhibits a broad substrate scope with respect to the acetal moiety and the carbon electrophile. Mechanistic studies suggest that the present reaction proceeds through a free-radical process involving carbon radicals generated by the homolytic cleavage of a carbon–carbon bond within the acetal moiety. A synthetic application of this method to sugar-derived alkylsilyl peroxides is also described.

Full text of DOI:10.1002/asia.201901695

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Iron-Catalyzed Radical Cleavage/C–C Bond Formation of Acetal-
Derived Alkylsilyl Peroxides
Authors: Yoko Shiozaki, Shunya Sakurai, Ryu Sakamoto, Akira
Matsumoto, and Keiji Maruoka
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Iron-Catalyzed Radical Cleavage/C–C Bond Formation of Acetal-
Derived Alkylsilyl Peroxides
Yoko Shiozaki,[a] Shunya Sakurai,[a] Ryu Sakamoto,[a] Akira Matsumoto,[b] and Keiji Maruoka*[a],[b],[c]
Abstract: A novel radical-based approach for the iron-catalyzed
selective cleavage of acetal-derived alkylsilyl peroxides, followed by the
formation of a carbon–carbon bond is reported. The reaction proceeds
under mild reaction conditions and exhibits a broad substrate scope
with respect to the acetal moiety and the carbon electrophile.
Mechanistic studies suggest that the present reaction proceeds through
a free-radical process involving carbon radicals generated by the
homolytic cleavage of a carbon–carbon bond within the acetal moiety.
A synthetic application of this method to sugar-derived alkylsilyl
peroxides is also described.
2a).[5] A similar cleavage reaction has been achieved using a series
of anomeric nitrate esters and N-phthalimido glycosides of
carbohydrates under reductive conditions (Scheme 2b).[6] Very
recently, the light-driven isomerization of carbohydrate derivatives
has been accomplished via proton-coupled electron-transfer
activation of the anomeric O–H bond in the presence of an Ir
photocatalyst (Scheme 2c).[7] However, examples of the trapping of
in-situ-generated carbon radicals via C–C bond cleavage of acetal
derivatives with carbon electrophiles have not been reported yet.
Our group has been developing an alternative method for the
generation of alkoxy radicals from bench-stable alkylsilyl peroxides,
which readily undergo β-scission in the presence of metal catalysts
to afford the corresponding alkyl radicals.[8] Therefore, we were
interested in the possibility of generating carbon radicals via the
iron-catalyzed selective C–C bond cleavage of acetal-derived
alkylsilyl peroxides and the subsequent C–C bond formation with
carbon electrophiles (Scheme 2d). Here, we wish to report our
initial results on this topic.
Acetals are synthetically useful intermediates in organic synthesis,
and their basic structures are ubiquitous in carbohydrate
chemistry.[1] Accordingly, the development of versatile approaches
for the skeletal transformation of these structures is of great
importance. The hitherto reported cleavage patterns of semicyclic
acetals[2] can be divided into three categories: 1) Ionic cleavage of
the endocyclic C1–O bond (Scheme 1a);[3] 2) ionic cleavage of the
exocyclic C1–O bond (Scheme 1b);[4] and 3) homolytic cleavage of
the C1–C2 bond (Scheme 1c). Several examples of the homolytic
cleavage of the C1–C2 bond of semicyclic acetal derivatives have
recently been reported (Scheme 2).[5–7]
Scheme 1. Hitherto reported cleavage patterns of semicyclic acetals.
The initial generation of alkoxy radicals from carbohydrate
derivatives using hypervalent diacetoxyiodobenzene and iodine,
followed by the cleavage of a C–C bond of the tetrahydrofuran ring
to furnish linear iodo or acetoxy formates has recently been
realized under photo-irradiation or heating conditions (Scheme
Scheme 2. Homolytic cleavage of the C–C bond of semicyclic acetal derivatives.
[a]
Y. Shiozaki, S. Sakurai, Dr. R. Sakamoto, Prof. Dr. K. Maruoka
Department of Chemistry, Graduate School of Science, Kyoto
University, Sakyo, Kyoto 606-8502, Japan
E-mail: maruoka.keiji.4w@kyoto-u.ac.jp
Dr. A. Matsumoto, Prof. Dr. K. Maruoka
Graduate School of Pharmaceutical Sciences, Kyoto University,
Sakyo, Kyoto 606-8501, Japan
Prof. Dr. K. Maruoka
School of Chemical Engineering and Light Industry, Guangdong
University of Technology, Guangzhou 510006, China
2-Triethylsilylperoxytetrahydro-2H-pyrane (1a) can be readily
prepared from 3,4-dihydro-2H-pyran in two steps (Scheme 3).[9]
The reaction of 1a and 1,1-diphenylethene (2a, 2.0 equiv) in the
presence of FeCl3 (10 mol%) was carried out in DMSO at 80 °C
for 1 h to furnish 6,6-diphenylhex-5-en-1-yl formate (3aa) as the
ring-opened product in 77% yield (Table 1, entry 1).[10] Increasing
the catalyst loading from 10 mol% to 20 and 40 mol% under
otherwise identical conditions afforded 3aa in 81% and 70% yield,
respectively (entries 2 and 3). The reaction did not proceed in the
absence of FeCl3. The influence of the reaction temperature was
[b]
[c]
Supporting information for this article is given via a link at the end of
the document.
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examined at a loading of 20 mol% of FeCl3, but neither raising nor
lowering the temperature increased the yield of 3aa (entries 4–7).
However, the choice of solvent was found to be crucial (entries 2
and 8–17). Although MeCN and DCE are not suitable solvents for
this transformation (entries 8 and 10), the addition of DMSO (1.0
equiv) to these solvents afforded 3aa in higher yields (entry 9 vs.
11). This additive effect was not observed when dioxane was used
as the solvent (entries 16 vs. 17). Various Fe salts such as FeCl2,
FeCl3, Fe(acac)3, Fe(III)-EDTA, K3[Fe(CN)6], Fe(NO3)3·9H2O, and
Fe(ClO4)3·6H2O were tested as potential catalysts, but none rivaled
the performance of FeCl3 (cf. Supporting Information).
With the optimized conditions in hand, we subsequently carried
out the reaction using alkylsilyl peroxides with semicyclic and
acyclic acetal moieties, as well as several carbon electrophiles
(Table 2). The reaction of 1a with 2a could be conducted on a gram
scale, although the product 3aa was obtained in slightly lower yield
(entry 1). The reaction of other carbon electrophiles, such as
acrylamide 2b, isocyanide 2c with 1a successfully afforded the
oxindole derivative 3ab[10a,10c] and phenanthridine derivative
3ac,[10b] respectively (entries 2 and 3). In addition, the reaction with
pharmaceutically important quinoxaline derivative 2d was also
possible (entry 4). With respect to the acetal moiety of the alkylsilyl
peroxide substrate, the five-membered semicyclic acetal derivative
1b was also applicable to the reaction with 2a to give the
corresponding formate 3ba in moderate yield (entry 4). Additionally,
alkylsilyl peroxide 1c derived from an acyclic acetal, smoothly
furnished alkylated 3ca in good yield (entry 5).
Scheme 3. Synthesis of 1a.
Table 2. Radical cleavage/C–C bond formation of alkylsilyl peroxides (1).a
Table 1. Optimization of the radical cleavage/C–C bond formation of 1a.a
Entry
1
FeCl3 (mol%)
Temp (°C)
80
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
MeCN
MeCN
DCE
Yield (%)b
77
10
20
40
20
20
20
20
20
20
20
20
20
20
20
20
20
20
2
80
81 (72)c
70
3
80
4
r.t.
30
5
40
77
6
60
72
7
100
80
70
8
39
9d
10
11d
12
13d
14
15d
16
17d
80
47
80
24
80
DCE
49
80
Benzene
Benzene
DMF
12
80
11
80
45
80
DMF
50
80
Dioxane
Dioxane
50
aReaction conditions: 1 (0.10 mmol), 2 (0.20 mmol), and FeCl3 (20 mol%) in
DMSO (1.0 mL) at 80 °C for 1 h under an Ar atmosphere. bIsolated yield. c1.2 g of
1a was used. dReaction conditions: 1 (0.20 mmol), 2 (0.10 mmol), and FeCl3 (20
mol%) in DMSO (1.0 mL) at 80 °C for 1 h under an Ar atmosphere.
80
12
aReaction conditions: 1a (0.10 mmol), 2a (0.20 mmol), and FeCl3 (10~40 mol%)
in the indicated solvent (0.10 M) for 1 h under an Ar atmosphere. bThe yield was
determined by 1H-NMR spectroscopy using 1,1,2,2-tetrachloroethane as the
internal standard. cIsolated yield. dDMSO (0.10 mmol) was added.
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Having established the generality of this method, we
investigated the substrate scope for alkylsilyl peroxides derived
from carbohydrates, which are a representative class of naturally
abundant acetal compounds. The application of this method to
carbohydrate derivatives could potentially allow the efficient
construction of building blocks with multiple chiral centers and
functional groups.[11] Indeed, the reaction of alkylsilyl peroxide 4a,
which is derived from 2-deoxy-D-ribose with 2a afforded the
corresponding formate 5aa in high yield without affecting its acetal
moiety (Table 3, entry 1). The pyranoside derivatives 4b and 4c,
which bear methyl- and acetyl-protected hydroxy groups,
respectively, were also tolerated, and furnished the desired
products that contain three stereogenic centers (entries 2 and 3). In
addition to pyranoside derivatives, our method could also be
successfully applied to furanoside derivative 4d (entry 4). The
reaction of the second furanoside derivative 4e confirmed that the
benzoyl protecting group was suitable for this transformation (entry
5). It is also noteworthy that linear aldose derivative 4f, which has
both a silyl-protected 1,3-diol and a free hydroxy group, afforded
the corresponding product 5fa after fragmentation at the C–C bond
of the polyol main chain (entry 6).
good results in the radical cleavage reaction of 1a (Scheme 4a).
This result indicates that iron (II) species as well as iron (III)
species would be involved in the catalytic cycle. When the reaction
of 1a with 2a was conducted in the presence of the radical inhibitor
2,6-di-tert-butyl-p-cresol (BHT), the yield of 3aa decreased
significantly (Scheme 4b). Furthermore, a similar reaction was
conducted in the presence of the radical scavenger 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO), which inhibited the
formation of the C–C bond with 2a. Instead, TEMPO adduct 6 was
isolated in 64% yield (Scheme 4c). These observations provide
evidence for the in-situ generation of an alkyl radical, which is
triggered by the homolytic C–C bond cleavage of alkylsilyl peroxide
1a.
Table 3. Substrate scope for sugar-derived alkylsilyl peroxides 4.a
Scheme 4. Control experiments.
Based on these results, a plausible reaction mechanism is
proposed in Scheme 5. First, the iron catalyst cleaves the O–O
bond of 1a via a single-electron transfer (SET) process,[10,12] that
yields oxy-radical 7 and a silanoxide. Oxy-radical 7 then undergoes
a β-scission to furnish alkyl radical 8, which subsequently reacts
with 2a to afford intermediate carbon radical 9. This radical is
oxidized by the iron catalyst to give the corresponding carbocation
10. Finally, 10 reacts with the silanoxide as a base to furnish 3aa.
aReaction conditions: 4 (0.10 mmol), 2a (0.20 mmol), and FeCl3 (20 mol%) in
DMSO (1.0 mL) at 80 °C for 1 h under an Ar atmosphere. bIsolated yield.
To gain insight into the reaction mechanism, we performed
several control experiments (Scheme 4). The use of FeCl2 provided
Scheme 5. Proposed reaction mechanism.
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[4]
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In conclusion, we have developed a novel method for the
formation of C–C bonds using carbon radicals that are generated
in-situ via the iron-catalyzed selective C–C bond cleavage of
acetal-derived alkylsilyl peroxides. The synthetic utility of this
approach has been clearly demonstrated using several types of
carbohydrate derivatives. Further investigations into our
methodology using alkylsilyl peroxides for the synthesis of various
functionalized molecules are currently in progress in our laboratory.
[5]
Experimental Section
A solution of 1,1-diphenylethene (2a, 35 μL, 0.20 mmol, 2.0 equiv) and
FeCl3 (3.2 mg, 0.020 mmol, 20 mol%) in DMSO (1.0 mL) was treated with
alkylsilyl peroxide 1a (23 mg, 0.10 mmol). After being stirred for 1 h at 80 °C
under an Ar atmosphere, the reaction mixture was cooled to room
temperature, quenched with brine and extracted five times with
hexane/EtOAc (v/v = 1/1). The combined organic layers were dried over
Na2SO4 and concentrated under reduced pressure. The residue was
analyzed by 1H-NMR spectroscopy in order to determine the NMR yield of
3aa using 1,1,2,2-tetrachloroethane (11 μL, 0.10 mmol) as the internal
standard (81% NMR yield based on 1a). The crude product was purified by
flash column chromatography on silica gel using hexane/EtOAc (v/v = 10/1)
as the eluent to afford 3aa as a colorless oil (20 mg, 72%).
[6]
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Acknowledgements
[9]
This work was supported by JSPS KAKENHI grants JP26220803
and JP17H06450 (Hybrid Catalysis). S.S. thanks for a Grant-in-Aid
for JSPS Fellowship for Young Scientists.
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2081–2085.
Keywords: iron-catalysis • C–C bond cleavage • C–C bond
formation • radical reaction • alkylsilyl peroxides • sugar derivatives
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Entry for the Table of Contents (Please choose one layout)
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Yoko Shiozaki, Shunya Sakurai, Ryu
Sakamoto, Akira Matsumoto, and Keiji
Maruoka*
Page No. – Page No.
Iron-Catalyzed Radical Cleavage/C–C
Bond Formation of Acetal-Derived
Alkylsilyl Peroxides
A novel radical approach for the iron-catalyzed selective cleavage of acetal-derived
alkylsilyl peroxides followed by a carbon–carbon bond formation is reported. The
reaction proceeds under mild reaction conditions and exhibits a broad substrate
scope with respect to the acetal moiety and the carbon electrophiles. Mechanistic
studies suggest that the present reaction proceeds via a free radical process
including carbon radicals that are generated by the homolytic cleavage of carbon–
carbon bond within the acetal moiety. The synthetic utility of this method based on
sugar-derived alkylsilyl peroxides is also described.
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