Iron-Catalyzed C(sp3)-C(sp3) bond formation through C(sp3)-H functionalization: a cross-coupling reaction of alcohols with alkenes

Article abstract of DOI:10.1002/anie.200903960

Synthetic convenience: A wide range of substrates proved to be well tolerated by the novel title transformation. This protocol provides an economical and convenient strategy for the efficient access to structurally diverse secondary alcohols (see scheme).

Full text of DOI:10.1002/anie.200903960

Angewandte
Chemie
DOI: 10.1002/anie.200903960
Iron Catalysis
3
3
3
À
À
Iron-Catalyzed C(sp ) C(sp ) Bond Formation through C(sp ) H
Functionalization: A Cross-Coupling Reaction of Alcohols with
Alkenes**
Shu-Yu Zhang, Yong-Qiang Tu,* Chun-An Fan, Fu-Min Zhang, and Lei Shi
Iron as one of the most abundant metals on earth and has
been increasingly explored in modern catalysis to discover its
unique and novel reactivity towards carbon–carbon and
carbon–heteroatom bond formation, which have been typi-
cally achieved by rare and expensive transition metal catalysts
(e.g. Ru, Rh, and Pd). Owing to its inexpensive and environ-
mentally friendly characteristics, considerable effort has been
directed towards iron catalysis and as a result a series of novel
iron-catalyzed organic transformations have been developed.
2
À
À
These mainly involve C(sp ) C(sp), C(sp) C(sp), and C-
3
[1,2]
À
(sp ) N bond formations.
To our knowledge, however,
3
3
À
iron-catalyzed C(sp ) C(sp ) bond-forming reactions through
direct C H functionalization remain rare.
The formation of C C bonds through direct C H
functionalization undoubtedly belongs to an important and
challenging area of organic chemistry.[4] In particular, the
[3]
À
À
À
Scheme 1. Carbon–carbon cross-coupling reactions of alcohols with
olefins through copromotion with late transition metals and Lewis
acids.
À
direct C C cross-coupling of alcohols with olefins through
À
direct functionalization of a C H bond at the a position is one
À
of the most attractive reactions as it constructs a new C C
solvents were firstly examined in the presence of a catalytic
amount of FeCl3 (Table 1, entries 1–5), and gratifyingly it was
found that DCE (1,2-dichloroethane) gave 83% yield of 1c
(Table 1, entry 5). These control experiments indicated that
DCE was essential to this coupling reaction; DCE was even
effective as an additive in a reaction medium that could not
bond, directly introduces an active hydroxy functional group,
and is atom economic involving few steps.[5] Recently, our
research group has demonstrated that of a late-transition-
metal catalyst (Rh, Ru, or Pd) and a Lewis acid can be used to
copromote this kind of cross-coupling reaction (Scheme 1).[6]
However, the chemical disadvantages of these protocols
center around using such expensive and toxic transition
metals as well as excess amounts of Lewis acids. An
alternative catalytic system with more sustainable perspec-
tives is therefore highly appealing. Herein, we present our
preliminary experimental results of a novel FeCl3-catalyzed
C(sp ) C(sp ) bond-forming cross-coupling reaction of alco-
hols with alkenes.
To explore our iron catalysis system, 3-phenylpropanol
(1a) and 1,1-diphenylethylene (1b) were selected as model
substrates for screening iron catalysts and solvents for the
general experimental procedure. As shown in Table 1, the
Table 1: Optimization of cross-coupling reaction conditions.[a]
3
3
À
Entry
[Fe]
Solvent
3a [%][b]
1
2
FeCl3
FeCl3
toluene
THF
trace
–
3
FeCl3
DMF
–
4
5
6
7
8
9
10
11
12
FeCl3
FeCl3
FeCl3
FeCl2
Fe(acac)3
FeCl3·6H2O
Fe(ClO4)3·9H2O
FeCl3 (0.02 equiv)
FeCl3 (0.10 equiv)
nBuBr
DCE
trace
83
63
22
–
72
36
42
64
nBuBr/DCE[c]
DCE
[*] S.-Y. Zhang, Prof. Dr. Y.-Q. Tu, Prof. Dr. C.-A. Fan, Dr. F.-M. Zhang,
Dr. L. Shi
DCE
DCE
DCE
DCE
State Key Laboratory of Applied Organic Chemistry and
Department of Chemistry
Lanzhou University, Lanzhou 730000 (China)
Fax: (+86)931-891-5557
DCE
E-mail: tuyq@lzu.edu.cn
[a] General reaction conditions: a mixture of alcohol 1a (0.2 mmol),
alkene 1b (0.3 mmol), and solvent (2 mL) in the presence of iron catalyst
(0.03 mmol) was stirred at 658C under argon for 8–12 hours. [b] Yield of
isolated product. [c] 1.0 equivalent of DCE was added to 2 mL of nBuBr.
acac=acetylacetonate, DMF=N,N-dimethylformamide, THF=tetrahy-
drofuran.
[**] This work was supported by the NSFC (grant nos. 20621091,
20672048, and 20732002) and the “111” Program of the Chinese
Education Ministry.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903960.
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promote the present cross-coupling reaction (Table 1,
(Table 2, entries 7 and 8). Generally, our coupling reaction
was carried out within 8 hours, but a prolonged reaction time
(16–24 h) was essential for those reactions that involved the
in situ formation of alkenes b from tertiary alcohols (Table 2,
entry 13–16). For entries 12, 15, and 16 of Table 2, a substrate-
based diastereoselectivity (> 99:1 to 1:1) was observed, thus
highlighting the fact that the level of diastereocontrol was
dependent to the relative bulkiness of the R2 and R3 groups in
b.
To elucidate a possible reaction pathway, some additional
experiments were carefully performed. For example,
PhCH2CH2CHO instead of the starting alcohol 1a was
treated with the alkene 1b under the standard reaction
conditions, however, no hydroacylation reaction was
observed. Clearly this result indicated that the known
“oxidation/hydroacylation/reduction” or “transfer-hydroge-
native coupling” process was not involved in our current
cross-coupling reaction of alcohols a and alkenes b.[9,10]
To further clarify the hydrogen transfer from alcohols a to
alkenes b, several deuterium-labeling experiments were
conducted. As shown in Scheme 2, 1,1-dideuterated ethanol
10a and 1b were subjected to the cross-coupling reaction, and
only the 1,3-dideuterated product 17c was obtained, in which
entry 6). Subsequently, our evaluation turned to the iron
salts. FeCl2 and Fe(acac)3 proved to be less effective (Table 1,
entries 7–10), and decreasing the loading of FeCl3 led to lower
yields of 1c (Table 1, entries 11 and 12).
Also, in combination with FeCl3 in DCE, we tested several
ligands such as TMEDA (N,N,N’,N’-tetramethylethylenedi-
amine), NEt3, and DACH (trans-1,2-diaminocyclohexane),
which are commonly used for iron catalysis, and some
additives such as lithium or copper salts,[7,8] but no significant
improvement in yield was observed. As a result, FeCl3
(0.15 equiv)/DCE was selected as the general catalysis
system for our coupling reaction (Table 1, entry 5).
After the above optimization, the generality and substrate
scope of this cross-coupling reaction were investigated. As
indicated in Table 2, a series of primary alcohols 1a–9a were
treated with alkenes 1b–5b or tertiary alcohols 6b–9b as their
precursors (which are more environmentally friendly and
easily available than alkenes), and smoothly gave the desired
secondary alcohols 1c–16c in moderate to good yields.
Notably, the coupling reaction using b-aryl alcohol 7a or 8a
did not proceed to completion, even after 24 h, and the
reaction only afforded 51% and 48% yield, respectively
Table 2: Cross-coupling reaction of primary alcohols with various alkenes or tertiary alcohols.[a]
Entry
Substrate
alcohol
Product
Yield Entry
[%][b]
Substrate
Product
Yield
[%][b]
alkene
2b
alcohol
alkene
1
2
1a
2a
2c
3c
83
81
9
9a
1a
1b
3b
10c
11c
53
82
1b
1b
10
3
3a
4c
72
11
1a
4b
12c
88
62
4
5
4a
5a
1b
1b
5c
6c
78
70
12
13
1a
1a
5b
6b
13c
1c
(>99:1)[d]
82
6
6a
1b
7c
77
14
5a
7b
14c
78
51
58
(1:1)
7
8
7a
8a
1b
1b
8c
9c
15
16
1a
5a
8b
9b
15c
16c
(90)[c]
48
93
(1:1)
(92)[c]
[a] Reaction conditions: alcohols a (0.2 mmol) were treated with alkenes b (0.3 mmol) in the presence of FeCl3 (0.03 mmol) and DCE (2 mL) at 658C for 8–
24 hours. [b] Yield of isolated product. [c] Yield based on the recovered starting material. [d] Only one diastereomer was isolated, and its relative
stereochemistry is temporarily assigned as syn according to our previous report.[6c]
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obtained as well as 1b (H/D = 85:15), which had partial
deuteration of the terminal hxdrogen atom.[11] Notably, no
C1- or C3-deuterated products were observed. One additional
deuterium-labeling experiment [Eq. (2), Scheme 4] clearly
excluded the possibity that the hydrogen atom at C2 of the
deuterium-free cross-coupling product 18c directly
exchanged with the deuterium of 11a in this coupling
system. This fact demonstrated a preferential occurrence of
the hydrogen exchange between the active hydrogen atom of
alcohols and the terminal hydrogen atom of alkenes under the
standard reaction condition.[12,13] Moreover D2O, as an alter-
native active source of hydrogen, was subjected to the current
reaction system [Eq. (3), Scheme 4], and the deuterium of
D2O could also be partially installed at the C2 position of the
product. These above-mentioned results reveal that the active
hydroxy hydrogen atom could undergo a fast iron-catalyzed
regioselective hydrogen exchange between alkenes and
alcohols under the current iron catalysis,[12] thus delivering
the final C2-deuterated products.
Scheme 2. Deuterium-labeled experiment using 10a.
the deuterium at the C3 position of 17c originated entirely
from the a deuterium atom that was adjacent to the hydroxy
group of 10a. This reaction outcome is consistent with what
we have observed previously[6] in this type of cross-coupling of
alcohols and alkenens catalyzed by non-iron catalysts, and
therefore additionally demonstrating that the pathway of our
iron-catalyzed cross-coupling reaction was quite different
from the “oxidation/hydroacylation/reduction” or “transfer-
hydrogenative coupling” processes. Moreover, one crossover
experiment was investigated using a 1:1 mixture of 1,1-
dideuterated 10a and undeuterated 1a (Scheme 3), and
Together with the experiments shown in Schemes 2–4, the
À
ionic C H bond dissociation at the a position of alcohols
could be excluded unambiguously in our iron-catalyzed cross-
À
coupling reaction, and a possible homolysis of the C H bond
at the a position in the alcohols catalyzed by iron was
supported. In light of this mechanistic implication, the radical
scavenger PhSH was subjected to the current cross-coupling
reaction, and the positive inhibiting effect in this control
experiment further indicates the presence of the free radical
species resulting from homolysis.
According to the above experimental results and our
Scheme 3. Deuterium-labeled crossover experiment.
previous reports,[6] a catalytic mechanism for this iron-
catalyzed cross-coupling reaction is proposed in Scheme 5.
3
À
Firstly, iron-initiated activation for cleavage of the C(sp ) H
afforded a statistical distribution of deuterium labels in the
products: the isotopic distribution patterns were analyzed by
1H NMR spectroscopy. This result implies that the hydrogen-
transfer process from alcohol to alkene might proceed
discretely in an intermolecular fashion.
In addition to the above preliminary investigation using
10a (Scheme 2), 11a was also examined to probe the transfer
of the active hydrogen atom [Eq. (1), Scheme 4]. Surprisingly,
the C2-deuterated compound 18c (H/D = 87:13) was
bond adjacent to oxygen[14] of alcohols a is involved in the
formation of A. Then a radical pair B forms followed by
simultaneous free-radical addition and disassociation to
afford [Fe]IV H and a free-radical species C. Subsequently,
À
the metal hydride ([Fe]IV H) undergoes an outer-sphere-type
À
hydrogen transfer to give the coupling product c, thus
regenerating the iron-catalyst [Fe]III for the next catalytic
cycle.
Scheme 4. Deuterium-labeled experiment probing the active hydrogen
atom.[12] Reaction conditions: a) 1. standard reaction conditions,
2. aqueous work-up.
Scheme 5. Proposed catalytic mechanism.
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Communications
Zhang, Angew. Chem. 2009, 121, 3875 – 3878; Angew. Chem. Int.
Ed. 2009, 48, 3817 – 3820.
[3] Up to now, only three such transformations have been reported
In summary, we have developed a novel iron-catalyzed
C(sp3)-C(sp3) bond-forming reaction between alcohols and
3
À
olefins through direct C(sp ) H functionalization, and vari-
in the presence of a stoichiometric amount of peroxides, wherein
ous substrates proved to be well tolerated. This protocol
provides an economical and convenient strategy for efficient
access to structurally diverse secondary alcohols. An exten-
sive investigation of this reaction is underway in our
laboratory.
À
oxidative activation of the C H bond at the benzylic, alkanic, or
alpha position adjacent to a heteroatom was proposed, see: a) Z.
Li, L. Cao, C.-J. Li, Angew. Chem. 2007, 119, 6625 – 6627; Angew.
Chem. Int. Ed. 2007, 46, 6505 – 6507; b) Z. Li, R. Yu, H. Li,
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2008, 47, 7497 – 7500; c) Y. Zhang, C.-J. Li, Eur. J. Org. Chem.
2007, 4654 – 4657.
À
[4] a) Handbook of C H Transformations (Ed.: G. Dyker), Wiley-
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Kitamura, Y. Fujiwara, Acc. Chem. Res. 2001, 34, 633 – 639;
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Ritleng, C. Sirlin, M. Pfeffer, Chem. Rev. 2002, 102, 1731 – 1789;
h) F. Kakiuchi, S. Murai, Acc. Chem. Res. 2002, 35, 826 – 834;
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R. E. J. Beckwith, Chem. Rev. 2003, 103, 2861 – 2903; k) K.
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Typical procedure: DCE (2 mL), alcohol a (0.2 mmol), and FeCl3
(0.03 mmol) were sequentially added to a flame-dried 10 mL flask.
The mixture was stirred for 20 min before alkene b or the related
tertiary alcohol (0.3 mmol) was introduced. The resulting reaction
mixture and stirred at 658C for a further 6–18 hours until the reaction
was completed (as evident by TLC). The reaction mixture was cooled
to room temperature and diluted with ethyl acetate (3 mL) and H2O
(2 mL). The organic layer was separated, and the aqueous phase was
extracted with ethyl acetate (3 ꢀ 3 mL). The combined organic
extracts were washed with H2O (10 mL), dried over anhydrous
Na2SO4, filtered, and concentrated in vacuo. The crude product was
purified by flash column chromatography to afford the desired
product c.
[5] a) B. M. Trost, Science 1991, 254, 1471 – 1477; b) R. A. Sheldon,
Pure Appl. Chem. 2000, 72, 1233 – 1246; c) B. M. Trost, Acc.
Chem. Res. 2002, 35, 695 – 705.
Received: July 19, 2009
Revised: September 12, 2009
Published online: October 13, 2009
[6] a) L. Shi, Y.-Q. Tu, M. Wang, F.-M. Zhang, C.-A. Fan, Y.-M.
Zhao, W.-J. Xia, J. Am. Chem. Soc. 2005, 127, 10836 – 10837;
b) Y.-J. Jiang, Y.-Q. Tu, E. Zhang, S.-Y. Zhang, K. Cao, L. Shi,
Adv. Synth. Catal. 2008, 350, 552 – 556; c) S.-Y. Zhang, Y.-Q. Tu,
C.-A. Fan, Y.-J. Jiang, L. Shi, K. Cao, E. Zhang, Chem. Eur. J.
2008, 14, 10201 – 10205.
[7] For reports on FeCl3-catalyzed coupling reactions affected by
trace quantities of other metals in FeCl3, see: a) S. L. Buchwald,
C. Bolm, Angew. Chem. 2009, 121, 5694 – 5695; Angew. Chem.
Int. Ed. 2009, 48, 5586 – 5587; b) P.-F. Larsson, A. Correa, M.
Carril, P.-O. Norrby, C. Bolm, Angew. Chem. 2009, 121, 5801 –
5803; Angew. Chem. Int. Ed. 2009, 48, 5691 – 5693.
[8] Some additives such as LiCl, KCl, CuI, CuCl2, CuBr2, Cu(OAc)2,
CuO, Cu2O, NiCl2·6H2O, and CoCl2·6H2O were tested by
cooperation with FeCl3 (98%, Alfa Aesar), but no significant
improvement of the yield was observed. It should be noted that
the above non-iron additives alone, in the absence of FeCl3, did
not promote this cross-coupling reaction. In addition, the high-
purity FeCl3 (99.99%, Aldrich) was also used in this reaction and
gave 82% yield, which was analogous to that obtained using
FeCl3 (98%, Alfa Aesar) in Table 1, entry 5.
[9] For selected reviews on hydroacylation, see: a) B. Bosnich, Acc.
Chem. Res. 1998, 31, 667 – 674; b) M. Tanaka, K. Sakai, H.
Suemune, Curr. Org. Chem. 2003, 7, 353 – 367; c) G. C. Fu in
Modern Rhodium-Catalyzed Organic Reactions (Ed.: P. A.
Evans), Wiley-VCH, New York, 2005, pp. 79 – 91; d) Y. J. Park,
J.-W. Park, C.-H. Jun, Acc. Chem. Res. 2008, 41, 222 – 234, and
references therein.
À
À
Keywords: alcohols · C C coupling · C H activation ·
cross-coupling · iron
.
[1] For recent reviews, see: a) Iron Catalysis in Organic Synthesis
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À
[10] For recent reviews on transition-metal-catalyzed C C cross-
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[11] For details, see the Supporting Information.
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[12] The proposed equilibrium for iron-catalyzed H/D exchange
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3
À
[14] For other C(sp ) H bond functionalization adjacent to an
À
oxygen atom and with subsequent C C bond-formation reac-
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[13] Although this kind of H/D exchange processes haS been
mentioned in some transition-metals-catalyzed transformations
(such as Pd, Rh, Ru, and Ir), they have not yet been reported in
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