Iron-Catalyzed Decarboxylative Olefination of Unstrained Carbon-Carbon Bonds Relying on Alkoxyl Radical Induced Cascade

Article abstract of DOI:10.1021/acs.orglett.9b02675

An iron-catalyzed decarboxylative olefination of unstrained carbon-carbon bonds via alkoxyl radical induced C-C bond cleavage is presented. This protocol features mild conditions (room temperature, redox-neutral), good substrate scope and functional group

Full text of DOI:10.1021/acs.orglett.9b02675

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron-Catalyzed Decarboxylative Olefination of Unstrained Carbon−
Carbon Bonds Relying on Alkoxyl Radical Induced Cascade
Pin Gao,† Hao Wu,† Jun-Cheng Yang, and Li−Na Guo*
Department of Chemistry, School of Science, Xi’an Key Laboratory of Sustainable Energy Material Chemistry, and MOE Key
Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an 710049, China
S
* Supporting Information
ABSTRACT: An iron-catalyzed decarboxylative olefination of unstrained carbon−carbon bonds via alkoxyl radical induced C−
C bond cleavage is presented. This protocol features mild conditions (room temperature, redox-neutral), good substrate scope
and functional group compatibility, as well as excellent stereoselectivity, thus providing a facile access to the distal alkenyl
ketones.
atalytic C−C bond activation of cyclic systems has drawn
much progress have been gained, most of the above
decarboxylative reactions suffer from some drawbacks such as
the use of precious late transition-metal catalysts or high
reaction temperature, which would limit their further large-
scale application. Therefore, development of a low-cost metal-
catalyzed and mild decarboxylative strategy is still highly
desirable. Iron, as a cheap, readily accessible, environmentally
benign metal, has already shown its unique catalytic utility in
some transformations.10 As part of our continuing interest in
C−C cleavage reactions,4i,j,9h we herein report a mild, redox-
neutral iron-catalyzed decarboxylative ketoalkylation of α,β-
unsaturated carboxylic acids through alkoxyl radical-triggered
unstrained C−C bond cleavage, which provides a facile access
to a variety of distally alkenyl ketones.
At the outset, we examined the reaction of cyclopentyl silyl
peroxide 1a with cinnamic acid 2a under the cheap iron
catalysis. To our delight, the reaction occurred and gave the
desired δ-alkenyl ketone 3a in 65% yield in the presence of
Fe(OAc)2 (10 mol %) and Bphen (10 mol %) in EtOH at
room temperature for 16 h (Table 1, entry 1). Control
experiments revealed that the ligand plays an essential role in
this transformation (entry 2). Thus, a variety of ligands were
evaluated, and Bphen was found to be superior to others (for
details, see the SI). Screening of solvents disclosed that EtOH
was the most suitable solvent (entries 3−5). Different iron
catalysts including FeCl2, ferrocene, FeSO4·7H2O, FeCl3, and
Fe powder were then tested (entries 6−10). The results
showed that ferrocene gave the best yield of 3a, while Fe
powder did not display any catalytic activity. Reducing the
loading of catalyst and ligand to 5 mol % diminished the yield
of 3a to 74%. Other catalysts such as Cu(OAc)2, Co(OAc)2·
C
the attention of chemists in recent years and has
provided an attractive strategy for the synthesis of function-
alized linear molecules.1 Over the past few years, significant
advances have been made in this field, especially in the
unstrained C−C bond cleavage.2 Among them, the alkoxyl
radical triggered C−C bond cleavage has emerged as a
powerful tool to obtain diverse functionalized ketones,
carboxylic acids, and so on.3,4 Cycloalkanols and their
derivatives have been thoroughly explored as efficient alkoxyl
radical precursors. Recently, Knowles, Zuo, Zhu, and others
have described hydrogenation, amination, and bromination of
unstrained carbon−carbon bonds via oxidative C−C bond
cleavage of cycloalkanols.5 The group of Maruoka and
Sakamoto as well as our group reported the amidation,
alkynylation, silylation, and borylation of unstrained carbon−
carbon bonds via reductive unstrained C−C bond cleavage of
cycloalkyl silyl peroxides.6 In these transformations, a carbonyl
group and a new chemical bond have been constructed
simultaneously, providing a series of functionalized long-chain
ketones. Nevertheless, to the best of our knowledge, the
decarboxylative olefination of unstrained carbon−carbon
bonds via alkoxyl radical induced C−C bond cleavage remains
unexplored. Thus, further exploring the application of alkoxyl
radical induced C−C bond cleavage is still desirable.
Carboxylic acids as one of the most abundant and
inexpensive basic chemicals are widely used in biological and
chemical synthesis. The decarboxylation of α,β-unsaturated
carboxylic acids is an important and useful process in organic
synthesis. In addition to classical transition-metal catalysis,7
radical-mediated decarboxylative coupling has been established
as a reliable protocol for the formation of C−C and C−
heteroatom bonds.8 For example, a number of radical-induced
decarboxylative alkylations of α,β-unsaturated carboxylic acids
have been presented by different research groups.9 Although
Received: July 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02675
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
or electron-withdrawing groups were engaged efficiently in this
transformation, affording the δ-alkenyl ketones 3b−3n in
moderate to good yields. A variety of functional groups
including hydroxy (3c), methoxy (3l), halogen (3d−3f, 3m,
3n), trifluoromethyl (3g, 3j), and cyano (3h) groups were
compatible with the reaction conditions. 2-Furylacrylic acid
also proves to be efficient, delivering the desired product 3o in
moderate yield. β-Methyl cinnamic acid was also suitable,
giving the product 3p in 54% yield. Besides the cinnamic acids,
other acrylic acids were also applicable for this transformation.
Dienoic acid 2q afforded the expected product 3q in 52% yield
with excellent regioselectivity. 3-Benzoylacrylic acid 2r
delivered the desired product 3r in 41% yield by using
Fe(OAc)2 as the catalyst. Remarkably, the substrate with steric
hindrance, such as estrone-derived acrylic acid 2s, reacted
smoothly to provide the desired product 3s in 40% yield.
However, 3,3-dimethylacrylic acid failed to undergo this
transformation, and only phenyl butyl ketone was isolated in
41% yield as a byproduct. When α,β-unsaturated carboxylic
acids with a phenyl group at the α-position were used as
substrates under the standard conditions, no desired
decarboxylation product was observed, and only some of the
other unidentified byproducts were detected in this case.
Finally, it is noteworthy that this decarboxylative alkenylation
reaction proceeded with excellent stereoselectivity, and only E-
isomers were obtained in almost all cases.
Table 1. Optimization of the Reaction Conditions
a
entry
variations from the standard conditions
yield (%)
1
none
65
trace
50
37
48
51
83 (74)
50
46
trace
17
6
trace
trace
0
2
without ligand
3
4
5
6
7
8
9
10
11
12
13
14
15
MeCN as the solvent
toluene as the solvent
DCM as the solvent
FeCl2 as the catalyst
ferrocene as the catalyst
FeSO4·7H2O as the catalyst
FeCl3 as the catalyst
Fe powder as the catalyst
Cu(OAc)2 as the catalyst
Co(OAc)2·4H2O as the catalyst
Pd(OAc)2 as the catalyst
Ni(OAc)2·4H2O as the catalyst
without catalyst
b
a
Reaction conditions: catalyst (10 mol %), 4,7-diphenyl-1,10-
phenanthroline (Bphen) (10 mol %), 1a (0.2 mmol, 1.0 equiv), 2a
(0.3 mmol, 1.5 equiv) in solvent (2.0 mL) at room temperature for 16
h under N2. Yields of isolated product. Ferrocene (5 mol %) and
b
Bphen (5 mol %) were used.
Then the generality and limitations of cycloalkylsilyl
peroxides 1 were evaluated (Scheme 2). An array of 1-
arylcyclopentyl silyl peroxides 1 reacted well to deliver the
corresponding δ-alkenyl ketones 4a−4j in good yields. The α
and β-naphthyl substrates 1k and 1l gave the corresponding
products 4k and 4l in moderate yields. The 1-alkylcyclopentyl
silyl peroxide 1m also afforded the desired product 4m in 79%
yield. 1,2-Disubstituted cyclopentyl silyl peroxides 1n and 1o
(derived from loxoprofen) underwent C−C bond cleavage
regioselectively, furnishing 4n and 4o in 58% and 42% yields,
respectively. Peroxides derived from 2-indanone and norcam-
phor also resulted in the expected products 4p and 4q in 43%
and 59% yields. Interestingly, when 1-phenylcyclohexyl silyl
peroxide 1r was subjected to the standard conditions, the ε-
alkenyl ketone 4r was obtained along with a small amount of
α-alkenyl ketone 4r′ as byproduct. The formation of 4r′
involves a C−C bond cleavage/1,5-HAT/decarboxylative C−
C(sp2) bond formation cascade. Luckily, it could be inhibited
by using an increased concentration (1.0 M) of reaction
system. Other cyclohexyl substrates gave the corresponding ε-
alkenyl ketones 4s and 4t in moderate yields under slightly
modified conditions. Satisfactorily, the peroxides with large-
sized rings including seven-, eight- and even 12-membered ring
also successfully produced the expected products 4u−4w in
42−93% yields. Additionally, the acyclic alkylsilyl peroxide 1x
underwent the C−C bond cleavage/decarboxylative coupling
process to give the alkene 4x in 64% yield with perfect E-
selectivity (eq 1).
4H2O, and Pd(OAc)2 resulted in an inferior yield of 3a
(entries 11−14). Finally, no desired product 3a could be
observed without iron catalyst (entry 15).
With the optimized conditions in hand, the scope of α,β-
unsaturated carboxylic acids was tested with peroxide 1a
(Scheme 1). Cinnamic acids bearing either electron-donating
Scheme 1. Evaluation of Various α,β-Unsaturated
a
Carboxylic Acids
a
Reaction conditions: ferrocene (10 mol %), Bphen (10 mol %), 1a
(0.2 mmol, 1.0 equiv), 2 (0.3 mmol, 1.5 equiv) in EtOH (2.0 mL) at
room temperature for 16 h under N2. Isolated yields. The E/Z ratio
was determined by 1H NMR analysis, and the data are given in
b
To shed light on this transformation, some control
experiments were conducted (Scheme 3). The use of
cyclopentyl hydroperoxide 6 instead of silyl peroxide 1a
parentheses. The reaction was performed in MeCN at 120 °C for 12
c
h with Fe(OAc)2 (5 mol %) as the catalyst. Phenyl butyl ketone was
isolated in 41% yield.
B
DOI: 10.1021/acs.orglett.9b02675
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
On the basis of the above results, a reasonable catalytic
mechanism is depicted in Scheme 4.6e,9h First, single-electron
Scheme 2. Evaluation of Cycloalkylsilyl Peroxides
Scheme 4. Plausible Reaction Mechanism
reduction of cyclopentyl silyl peroxide 1a by the Fen complex,
which was generated from Ferrocene and Bphen, gave the
alkoxy radicals A. Subsequently, β-fragmentation of A would
generate the corresponding alkyl radical B, which attacks the
CC bond of cinnamic acid 2a regioselectively to afford
benzyl radical C. Deprotonation of C with trimethyl silanolate
occurs to give radical D, followed by oxidation by Fen+1 to
deliver the diradical intermediate E. Finally, decarboxylation of
E gave the desired product 3a and released CO2. However, the
loss of carbon dioxide through a carbonium ion intermediate
could not be excluded at present.11
a
The reaction run at 1.0 M concentration.
Scheme 3. Control Experiments
In conclusion, we have developed an efficient iron-catalyzed
decarboxylative olefination of unstrained carbon−carbon
bonds via an alkoxyl radical-triggered cascade. A variety of
cycloalkyl silyl peroxides and α,β-unsaturated carboxylic acids
underwent this C(sp3)−C(sp3) bond cleavage/C(sp3)−C(sp3)
bond formation process with excellent functional group
tolerance and stereoselectivity. This mild and redox-neutral
protocol provided a useful synthetic route to the remotely
alkenyl ketones.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02675.
Experimental details, spectral data, copies of 1H and 13
NMR spectra (PDF)
C
delivered the desired product 3a in only 30% yield.
Cyclopentanol 5 could not give the target product 3a under
the standard conditions. Furthermore, it also failed to afford 3a
under different oxidative conditions. When 1.5 equiv of
TEMPO was added to the reaction of 1a and 2a, the reaction
was suppressed completely and the alkyl-TEMPO adduct 7
was isolated in 82% yield. These results suggest that an alkyl
radical intermediate is probably involved in this trans-
formation.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: guoln81@xjtu.edu.cn.
ORCID
Li−Na Guo: 0000-0002-9789-6952
C
DOI: 10.1021/acs.orglett.9b02675
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Copper Promoted Fragmentation Reactions of α-Alkoxy Hydro-
Author Contributions
†P.G. and H.W. contributed equally to this work.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Science Foundation of China (No.
21602168), the Fundamental Research Funds of the Central
Universities (xjj2017027), and Key Laboratory Construction
Program of Xi’an Municipal Bureau of Science and Technology
(201805056ZD7CG40). We also thank the Instrument
Analysis Center of Xi’an Jiaotong University for the samples
analysis.
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DOI: 10.1021/acs.orglett.9b02675
Org. Lett. XXXX, XXX, XXX−XXX