Iron(II)-Catalyzed Heck-Type Coupling of Vinylarenes with Alkyl Iodides

Article abstract of DOI:10.1021/acs.orglett.8b04024

An iron(II)-catalyzed radical alkyl Heck-type reaction of alkyl iodides with vinylarenes under mild conditions has been reported. t-Butyl peroxybenzoate (TBPB) behaves simultaneously as a radical relay initiator, a precursor for the generation of alkyl ra

Full text of DOI:10.1021/acs.orglett.8b04024

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iron(II)-Catalyzed Heck-Type Coupling of Vinylarenes with Alkyl
Iodides
Haigen Xiong,†,‡ Yajun Li,† Bo Qian,† Rongbiao Wei,† Erik V. Van der Eycken,*,§,||
and Hongli Bao*,†,‡
†Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, State Key Laboratory of Structural Chemistry, Center for
Excellence in Molecular Synthesis, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155
Yangqiao Road West, Fuzhou, Fujian 350002, P. R. of China
‡University of Chinese Academy of Sciences, Beijing, 100049, P. R. of China
§Laboratory for Organic & Microwave-Assisted Chemistry (LOMAC), Department of Chemistry, University of Leuven (KU
Leuven), Celestijnenlaan 200F, Leuven, Belgium
||Peoples’ Friendship University of Russia (RUDN University), Miklukho-Maklaya Street 6, Moscow 117198, Russia
S
* Supporting Information
ABSTRACT: An iron(II)-catalyzed radical alkyl Heck-type reaction of alkyl
iodides with vinylarenes under mild conditions has been reported. t-Butyl
peroxybenzoate (TBPB) behaves simultaneously as a radical relay initiator, a
precursor for the generation of alkyl radical from alkyl iodides, and an oxidant to
recycle iron(II)/iron(III). Unactivated primary, secondary, and tertiary alkyl
iodides are compatible with the reaction conditions. The mechanistic studies suggest that a radical-polar-crossover pathway
might be involved in the catalytic cycle.
Scheme 1. Intermolecular Alkyl Heck-type Reaction
he Mizoroki-Heck coupling reaction has been confirmed
Tas one of the most significant methods for the
construction of carbon−carbon bond in organic synthesis
since 1970s.1 However, compared to the rapid development of
coupling of aryl or vinyl halides with alkenes,2 the coupling of
olefins and alkyl halides bearing β-hydrogen(s) has rarely been
documented.3 The reason is that the alkyl-metal species is
inclined to undergo β-hydride elimination before insertion of
alkene.4 Additionally, the rate of oxidative addition of alkyl
halide to metal-center is slower than that of aryl halide.5 To
overcome the issues mentioned above, radical strategies for
alkyl Heck (or Heck-type) reaction have been established.6
Exploratory studies on radical Heck-type reaction with
activated alkyl halides have been reported by Lei, Nishikata,
and Thomas, in the presence of nickel salt,7 ruthenium salt,8
copper salt,9 and iron salt10 as the catalyst, respectively. The
generated alkyl radicals are stabilized by the linked electron-
withdrawing groups through conjugative effect, thus limiting
the substrate scopes. Radical Heck-type reactions of unac-
tivated alkyl halides have been pioneeringly achieved by
Oshima with a ligated cobalt catalyst (Scheme 1a).11 However,
a stoichiometric amount of trimethylsilylmethylmagnesium
chloride is crucial to smoothly promote the reaction. In recent
years, palladium salts were found to be good catalysts for
radical alkyl Heck-type reaction through an open-shell
pathway.12,13 Fu14 and Gevorgyan15 independently described
elegant palladium-catalyzed Heck reactions of unactivated alkyl
halides under photoirradiation (Scheme 1a). A range of alkyl
halides, especially tertiary alkyl halides bearing β-hydrogen(s),
can be involved. However, these reactions need to proceed
under light irradiation and to use noble metal as the catalyst.
Our group previously described a radical decarboxylative
alkyl Heck-type reaction catalyzed by triflic acid (Scheme 1b)
and iron catalysts.16,17 We found that t-butyl peroxybenzoate
(TBPB) is a good initiator for the iron-catalyzed alkyl Heck-
type reactions through a radical relay process.18,19 Considering
that iron is an inexpensive and environmentally benign
catalyst,20 we reported herein a general iron-catalyzed radical
alkyl Heck-type reactions with both unactivated and activated
alkyl iodides (Scheme 1c).10
Received: December 17, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b04024
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Initially, 4-tert-butylstyrene (1a) and alkyl iodide (2a) were
used as the model substrates to explore the alkyl Heck-type
reaction (Table 1). 5 mol % of metal catalysts, such as
Scheme 2. Scope of Alkyl Iodides
a
Table 1. Optimization of the Reaction Conditions
b
entry
catalyst (mol %)
solvent
THF
THF
THF
THF
THF
THF
THF
CH3CN
DME
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
Pd(OAc)2 (5)
NiCl2 (5)
CuI (5)
50
50
50
50
50
50
50
50
50
50
25
70
trace
trace
trace
trace
25
66
70
9
Fe(OAc)2 (5)
Fe(OTs)3 (5)
Fe(OTf)3 (5)
Fe(OTf)2 (5)
Fe(OTf)2 (5)
Fe(OTf)2 (5)
Fe(OTf)2 (5)
Fe(OTf)2 (5)
Fe(OTf)2 (5)
9
65
c
10
11
12
1,4-dioxane
1,4-dioxane
1,4-dioxane
73 (72)
trace
73
a
Reaction conditions: alkene 1a (1 mmol, 1 equiv), alkyl iodide 2a (3
a
b
Reaction conditions: alkene (1 mmol, 1 equiv), alkyl iodide 2 (3
mmol, 3 equiv), TBPB (3 mmol, 3 equiv), solvent (4 mL), 3 h. Yield
of product was determined by crude. H NMR. Isolated yield in
parentheses.
1
c
mmol, 3 equiv), Fe(OTf)2 (5 mol %), TBPB (3 mmol, 3 equiv), 1,4-
b
dioxane (4 mL), 50 °C, 3 h. Isolated yields. Alkene (1 mmol, 1
equiv), alkyl iodide 2 (2 mmol, 2 equiv), TBPB (2 mmol, 2 equiv),
DME (4 mL), 2 h.
Pd(OAc)2, NiCl2, CuI, and Fe(OAc)2, were employed to run
the reaction. Unfortunately, only a trace amount of alkylated
product was detected in these cases when the reaction was
performed at 50 °C in THF for 3 h (Entries 1−4). To our
delight, 25% yield of the coupling product (3aa) was obtained
by using Fe(OTs)3 as the catalyst (Entry 5). Inspired by this
result, other iron catalysts were tested. While catalyst Fe(OTf)3
afforded product 3aa in 66% yield, Fe(OTf)2 offered a better
yield, as high as 70% (Entries 6 and 7). Then, the solvents
were screened, and up to 72% isolated yield of 3aa could be
produced in 1,4-dioxane (Entry 10 vs Entries 8 and 9). When
the reaction was performed at a lower temperature (25 °C),
only a trace amount of product was detected (Entry 11). A
higher temperature could not increase the yield of product
(Entry 12).
a
Scheme 3. Scope of Aryl Olefins
With the optimal reaction conditions in hand, the scope of
alkyl iodides was investigated (Scheme 2). Both of the acyclic
and cyclic secondary alkyl iodides smoothly couple with 4-tert-
butylstyrene (1a) to deliver the corresponding products (3ab−
3ae) in 51−71% yields. Tertiary alkyl iodide is transformed
into product (3af) in 61% yield. A variety of primary alkyl
coupling products (3ag−3aj), possessing long-chain alkyl,
phenyl, chloride, and trifluoromethyl groups, are obtained in
39−50% yields. Perfluoroalkyl iodides are also compatible with
this Heck-type reaction, affording the desired products (3ak−
3am) in moderate to good yields (66−76%). In addition, 1-
chloro-2-iodo-tetrafluoroethane is used as a coupling partner,
providing the product (3an) in 86% yield. 2-Iodo-heptafluor-
opropane is applicable to the reaction, giving 81% yield of
product 3ao. Difluoroacetylated alkene 3ap could be attained
through this transformation in 58% yield. However, 3-
iodooxetane or tert-butyl 3-iodoazetidine-1-carboxylate fails
to afford the corresponding product.
a
Reaction conditions: alkene 1 (1 mmol, 1 equiv), alkyl iodide 2a (3
mmol, 3 equiv), Fe(OTf)2 (5 mol %), TBPB (3 mmol, 3 equiv), 1,4-
dioxane (4 mL), 50 °C, 3 h. Isolated yields.
is employed to react with alkyl iodide 2a, leading to the
generation of product 3ba in 61% yield. The aryl alkenes
bearing electron-withdrawing (halides) and electron-donating
groups (3ca−3ma) are tolerated in the reaction with moderate
to good efficiency (56−80%). 2-Vinyl naphthalene and 6-vinyl
tetrahydronaphthalene afford the alkyl-Heck products (3na
and 3ma) in 56% and 61% yields, respectively. Particularly, the
estrone derivative containing a CC double bond is readily
converted into the corresponding product (3pa, 63% yield).
The alkyl Heck-type cross coupling reaction can also be
extended to various styrene derivatives (Scheme 3). Styrene 1b
B
DOI: 10.1021/acs.orglett.8b04024
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Organic Letters
Letter
1,1-Disubstitued alkene is also compatible to the reaction,
giving the alkylated alkene (3qa) in 67% yield. Notably, 4-
vinylbenzonitrile could afford the desired product (3ra) in 58%
yield, while methyl 4-vinylbenzoate and 1-nitro-4-vinylbenzene
are incompatible with the optimized reaction conditions.
Moreover, but-3-en-1-ylbenzene as an aliphatic alkene, and
benzyl acrylate or pent-1-en-3-one as a conjugated alkene, are
also not tolerated the reaction conditions and fail to afford the
desired products.
Scheme 5. Possible Mechanism
To insight into the mechanism of this alkyl Heck-type cross
coupling reaction, the preliminary experiments were performed
(Scheme 4). The control experiments proceeded separately in
Scheme 4. Preliminary Mechanistic Studies
In summary, a radical relay process for iron(II)-catalyzed
Heck-type alkylation of aryl alkenes with alkyl iodides has been
established. Activated/unactivated primary, secondary, and
tertiary alkyl iodides are compatible in this reaction to afford
various alkyl alkenes in moderate to good yields. TBPB is
employed as an internal oxidant to oxidize iron(II) species,
meanwhile, as an initiator to promote the generation of alkyl
radical from alkyl iodides. The mechanistic studies suggest that
a radical-polar-crossover pathway might be involved in the
catalytic cycle.
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.8b04024.
Experimental procedures, characterization of new
compounds, synthetic applications, mechanistic studies,
NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
the absence of TBPB or Fe(OTf)2 under standard reaction
conditions, and alkylated product (3aa) is not detected by GC-
MS in both cases (Scheme 4a,b). These results indicated that
both the iron catalyst and the TBPB are crucial for this alkyl
Heck-type transformation. Moreover, when 3 equiv of 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) is added as a radical
inhibitor, the product (3aa) cannot be observed as well
(Scheme 4c). Moreover, substrate 6-iodohex-1-ene (2q) could
only afford a cyclized Heck-type product (3aq) (Scheme 4d).
These results imply that radical species might be involved in
the reaction. With methanol as an additive, the selective
formation of product 4aa suggested that a benzylic cation
might be involved (Scheme 4e).
Therefore, based on the mechanistic experiments, a possible
mechanism for the alkyl Heck-type reaction is proposed
(Scheme 5). Fe(II) is oxidized by TBPB, generating Fe(III)
and a tert-butoxy radical (I), followed by the production of a
methyl radical (II) and acetone. The alkyl radical (III) would
arise via atom transfer process between methyl radical (II) and
alkyl iodide. The addition of the alkyl radical (III) to aryl
alkene leads to the formation of a benzyl radical (IV) which
could be further oxidized by the Fe(III) species, providing a
benzyl cation and the regenerated Fe(II) species. As a
consequence of deprotonation of the benzyl cation, the
Heck-type product is acquired.
*E-mail: hlbao@fjirsm.ac.cn.
*E-mail: erik.vandereycken@chem.kuleuven.be.
ORCID
Yajun Li: 0000-0001-6690-2662
Erik V. Van der Eycken: 0000-0001-5172-7208
Hongli Bao: 0000-0003-1030-5089
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
H.B. thanks the National Key R&D Program of China
(2017YFA0700103), the NSFC (Grant Nos 21502191,
21672213, and 21871258), the Strategic Priority Research
Program of the Chinese Academy of Sciences (Grant No.
XDB20000000), and the Haixi Institute of CAS (Grant No.
CXZX-2017-P01) for financial support. E.V.dE. acknowledges
the support of RUDN University Program 5-100.
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