Iron-catalyzed AlkylAlkyl negishi coupling of organoaluminum reagents

Article abstract of DOI:10.1246/cl.180954

The first iron-catalyzed cross-coupling reaction of alkyl halides with alkylaluminum reagents (alkylalkyl Negishi coupling) is developed using an iron/bisphosphine catalyst system. The reaction shows high functional group tolerance: various primary alkyl halides possessing a non-protected indole, carboxyl, or hydroxy group are coupled with primary alkylaluminum reagents in good yields. Potassium fluoride plays a key role to promote the reaction by generating an aluminate species, which facilitates the transmetalation between the organoaluminum and the iron catalyst.

Full text of DOI:10.1246/cl.180954

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Iron-catalyzed Alkyl–Alkyl Negishi Coupling of Organoaluminum Reagents
Ryosuke Agata, Shintaro Kawamura, Katsuhiro Isozaki, and Masaharu Nakamura*
Advance Publication on the web December 20, 2018
doi:10.1246/cl.180954
© 2018 The Chemical Society of Japan
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1
Iron-catalyzed Alkyl–Alkyl Negishi Coupling of Organoaluminum Reagents
Ryosuke Agata,1,2 Shintaro Kawamura,†1,2 Katsuhiro Isozaki,1,2 and Masaharu Nakamura*1,2
1International Research Center for Elements Science (IRCELS), Institute for Chemical Research (ICR),
Kyoto University, Uji, Kyoto 611-0011, Japan
2Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering,
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
E-mail: masaharu@scl.kyoto-u.ac.jp
cat. transition-metal
1
2
3
4
5
6
7
8
9
The first iron-catalyzed cross-coupling reaction of
Alkyl–X
+
M–Alkyl
Alkyl–Alkyl
alkyl halides with alkylaluminum reagents (alkyl–alkyl
Negishi coupling) is developed using an iron/bisphosphine
catalyst system. The reaction shows high functional group
tolerance: various primary alkyl halides possessing a non-
protected indole, carboxyl, or hydroxy group are coupled
with primary alkylaluminum reagents in good yields.
Potassium fluoride plays a key role to promote the reaction
by generating an aluminate species which facilitates the
Previous work
(a) Pd, Ni, Co, or Cu catalyst (X = I, Br, Cl, F, OTs, etc.; M = Mg, Zn, B)
(b) Fe catalyst (X = I, Br, F, OTs; M = Mg, B)
Kumada-type coupling (M = Mg) : low functional group tolerance
Suzuki-type coupling (M = B) : require i-PrMgCl to activate alkylborane
10 transmetalation between the organoaluminum and the iron
11 catalyst.
(c) This work
Ph
cat. Fe(OAc)2
cat. Bn-Nixantphos
KF
N
Alkyl–X
+
Alkyl–Alkyl
12 Keywords: Iron, Cross coupling, Organoaluminum
O
X = Cl, Br
R = Alkyl or i-Bu
R2Al–Alkyl
up to 90%
14 examples
Ph2P
PPh2
13
Cross-coupling reaction forming Csp3–Csp3 bonds is one
Bn-Nixantphos 1
14 of the key chemical transformations in synthetic organic
15 chemistry.1 In the last two decades, transition-metal-
16 catalyzed alkyl–alkyl cross-coupling reactions have been
17 extensively developed by mainly nickel and palladium
18 catalysts (Figure 1a).2–7 The first alkyl–alkyl coupling was
19 reported by Knochel and co-workers using dialkylzinc
20 reagents and alkyl iodides bearing a terminal olefin in the
21 presence of nickel catalyst.3 This achievement has led to the
22 development of a variety of alkyl–alkyl cross-coupling
23 reactions with not only alkylzinc,4 but also alkyl Grignard,5
24 and alkylboron reagents,6 applied in the synthesis of bio-
25 active natural products and pharmaceuticals.4d,6c
✔ high functional group tolerance: CN, CO2R, CO2H, OH, NHR
50
51 Scheme 1. Transition-metal-catalyzed cross coupling between alkyl
52 electrophiles and alkylmetal reagents.
53
Herein, we report a new iron-catalyzed alkyl–alkyl
54 cross-coupling reaction of non-activated alkyl halides with
55 alkylaluminum reagents by using catalytic amounts of
56 Fe(OAc)2 and Bn-Nixantphos 115 (Figure 1c). The addition
57 of potassium fluoride12c was found to be the key to attain the
58 facile coupling reaction. This reaction system shows high
59 functional group compatibility, which has not been achieved
60 by previous iron catalysts.
26
Recently, increasing attention has been focused on
61
We chose the coupling reaction between (6-
27 iron as a sustainable catalyst in organic synthesis due to its
28 non-toxic, environmentally benign nature, and high-
29 abundance in the Earth’s crust.8,9 Considerable efforts have
30 thus been devoted to developing iron-catalyzed cross-
31 coupling reactions forming Csp2–Csp3, Csp2–Csp2, Csp–Csp2,
32 and Csp–Csp3 bonds to date.10–12 However, there are only
33 limited reports for Csp3–Csp3 bond-forming reactions. Chai,
34 Cárdenas, and Kambe reported iron-catalyzed Kumada-type
35 alkyl–alkyl coupling between alkyl Grignard reagents and
36 alkyl halides (Figure 1b).13 Our group previously reported
37 the Suzuki-type alkyl–alkyl coupling by using
38 alkylboronates as nucleophiles.14 There is still room for
39 improvement in functional group tolerance and substrate
40 scope on electrophiles and/or nucleophiles, because these
41 reactions require the use of Grignard reagents and highly
42 reactive organolithium to activate the organoboron
43 nucleophiles. In contrast, we previously reported the
44 synthetic utility of organoaluminum reagents in iron-
45 catalyzed Negishi-type coupling reactions, which enable the
46 compatibility with a range of functional groups under mild
47 activation condition.12 Therefore, we envisioned the
48 applicability of organoaluminum reagents to iron-catalyzed
49 alkyl–alkyl coupling to overcome the above limitations.
62 bromohexyl)benzene (2a) and trioctylaluminum as a model
63 reaction. After the screening of catalyst precursors, ligands,
64 additives, and reaction conditions, a wide bite angle
65 bisphosphin 1 was found to give the desired coupling
66 product 3a in 83% yield (entry 1). Comparable catalytic
67 activities (64 and 72%) were observed by using its
68 congeners, TBS-Nixantphos and Xantphos (entries 2 and
69 3).16 It should be noted that these reaction conditions
70 suppressed the undesired b-hydrogen elimination
71 completely and olefin 4a and 1-octene were not observed at
72 all (see SI, Table S1). On the other hand, a structurally
73 flexible bisphosphine ligand, DPEphos, resulted in low
74 selectivity, likely due to the dissociation of one phosphine
75 from the iron center, which enables competitive side
76 reactions such as the b-hydrogen elimination (entry 4). A
77 narrow bite angle bisphosphine, SciOPP, did not give the
78 desired product 3a, despite its effectiveness in the iron-
79 catalyzed cross-coupling reaction of alkyl halides (entry
80 5).10b When i-Pr-Pybox4c and N-heterocyclic carbene (NHC)
81 precursor IPr·HCl4b were used, almost no desired product 3a
82 was obtained (entries 6 and 7). Other mono- and
83 bisphosphine, diamine,10a and NHC precursors13b were

2
1
2
Table 1.
Effect of reaction parameters in iron-catalyzed cross
28 product was obtained (entries 12–14). Notably, the addition
29 of fluoride anion is critical for attaining the rapid conversion
30 of the alkyl bromide, which can be accounted for facilitating
31 the transmetalation step by the formation of a highly
32 reactive aluminate species.12c,17 These results indicate that
33 the Fe(OAc)2/1 catalyst with KF is crucial to achieve high
34 catalytic activity and suppress undesired side reactions.
coupling of 2a with trioctyltaluminuma
Fe(OAc)2 (5 mol %)
1 (10 mol %)
Ph
(C8H17)3Al (1.5 equiv)
Ph
Ph
KF (1.5 equiv)
4a
5a
+
Ph
THF, 40 ºC, 8 h
Br
C8H17
2a
3a
35
Table 2 illustrates the scope and limitations of the
H
36 present reaction under optimal conditions. A variety of
Yield (%)b
37 Table 2.
Substrate scope of alkyl halides and alkylaluminum
Entry Changes from the optimal conditions
3a 4a 5a RSMc
38 reagentsa
none
83
64
72
0
0
0
16
7
0
12
0
1
2
TBS-Nixantphos instead of 1
Xantphos instead of 1
DPEPhos instead of 1
SciOPP instead of 1
Yield (%)b
77
Entry
1
Alkyl–X
( )4
Product
15
3
20 23 37
0
4
( )4
C8H17
( )4
C2H5
( )4
CH3
0
0
4
29 50
0
5
2b
2b
2b
3ba
3bb
3bc
i-Pr-Pybox instead of 1
IPr·HCl instead of 1
1
9
84
7
6
Br
21 35
7
FeCl2 instead of Fe(OAc)2
Fe(OPiv)2 instead of Fe(OAc)2
Fe(OTf)2 instead of Fe(OAc)2
17 17 33
26
0
8
2
3
65
12
70
70
0
0
0
16
16
8
9
4
10
11d
12
13
14
FeCl2 and KOAc (30 mol %) instead of Fe(OAc)2 76
0
without 1
<1 23 36
37
92
95
without Fe(OAc)2
without KF
0
0
0
5
5
CN
CN
Br
<1
3
4
5
6
7
8
73 (48)
4
5
2c
2c
3ca
3cb
aReactions were carried out on 0.2–0.8 mmol scale. bDetermined by
quantitative GC analysis using undecane as an internal standard.
C8H17
CN
c
d
87 (30)
62 (54)
Recovery of starting material 2a. FeCl2 (5 mol %) was treated with
C2H5
KOAc (30 mol %) in MeOH/THF for 1 h and then the solvent was
O
O
removed in vacuo.
R
2d
2e
OEt
3d
3e
6
OEt
N
C8H17
Br
O
O
O
O
O
Ph2P
PPh2
Ph2P
PPh2
Ph2P
PPh2
7c
OH
37
OH
R = Bn: Bn-Nixantphos 1
R = TBS: TBS-Nixantphos
Xantphos
DPEphos
C8H17
Br
t-Bu
t-Bu
Cl
Br
C8H17
8c
O
N
O
N
90 (44)
(60)
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
N
NH
NH
OH
N
N
P
P
2f
3f
OH
t-Bu
9c
i-Pr-Pybox
SciOPP
IPr·HCl
2g_Br
2g_Cl
3g
9
C8H17
Br
10
OH
OH
11 ineffective in this reaction, where significant amounts of by-
12 products, 4a, 5a, and 1-octene were formed (see SI, Table
13 S1). The present coupling reaction thus requires the use of
14 wide bite angle bisphosphine ligands possessing rigid
15 framework to achieve high selectivity of the coupling
16 product.
The counter anion of the iron salts showed remarkable
18 influence on the product yield and selectivity. As shown in
19 entry 8, FeCl2 showed lower reactivity and selectivity than
20 Fe(OAc)2. On the other hand, the iron salts having
10c
3g
(38)
C8H17
Cl
Cl
Cl
62 (49)
11
12
2h
2i
3h
3i
C8H17
C8H17
C8H17
Br
Br
0
17
3ja
3jb
37
0
13
2j
Br
C8H17
10e
21 oxoanions, Fe(OPiv)2 and Fe(OTf)2
resulted in much
39
40 aReactions were carried out on a 1 mmol scale using Fe(OAc)2 (5
41 mol %), 1 (10 mol %), and alkylaluminum reagents (1.5 equiv) in THF
42 at 40 ºC for 8–44 h, see the SI for details of the reaction conditions for
43 each entry. bDetermined by quantitative GC analysis using undecane as
44 an internal standard. Isolated yields are given in parentheses.
45 cTrioctylaluminum (2.5 equiv) was used.
22 higher yields than the chloride, albeit displaying lower
23 selectivity than Fe(OAc)2 (entries 9 and 10, see also SI,
24 Table S2). As shown in entry 11, pretreatment of FeCl2 with
25 potassium acetate dramatically improved the yield and
26 selectivity compared with FeCl2 alone. In the absence of
27 bisphosphine ligand 1, Fe(OAc)2, or KF, almost no desired

3
1
2
3
4
5
6
7
8
9
alkylaluminum reagents were cross-coupled with 1-
bromodecane (2b). The reactions of trioctylaluminum or
triethylaluminum with 2b gave the corresponding coupling
products 3ba and 3bb in 77% and 65% yields, respectively
(entries 1 and 2). In contrast, trimethylalminum afforded
only 12% yield of product 3bc, where decene was obtained
as the major by-product (entry 3).18
49 product yield and selectivity, this consecutive
50 transformation displays the synthetic potential of the
51 present coupling reaction.23
52
Based on the above-described results and previous
53 mechanistic studies on iron-catalyzed cross-coupling
54 reactions,11d–f a plausible reaction mechanism featuring an
55 FeI/FeII/FeIII manifold is presented in Figure 1. The catalytic
56 cycle starts from the bisphosphine-iron(I) intermediate A
57 which forms by the reduction of Fe(OAc)2 in the presence of
58 ligand 1.11b,24 The reaction of A with the alkyl halide 2
59 forms the alkyl radical intermediate and the iron(II)
We then studied the scope of alkyl halides and to our
delight the present reaction showed high functional group
10 tolerance. Alkyl bromides bearing a reactive functional
11 group, such as cyano or ester group, reacted with
12 alkylaluminum reagents to give the corresponding coupling
13 products 3ca-d in good yields (62–87%, entries 4–6).
14 Importantly, alkyl halides possessing a non-protected indole,
15 carboxyl, and hydroxy group19 also participated in the
16 reactions to afford the corresponding products 3e-g in 37–
17 90% yield, despite the catalyst poisoning effect of these
18 functional groups in iron-catalyzed coupling reactions of
19 alkyl halides (entries 7–9).10b An even more challenging
20 primary alkyl chloride bearing a hydroxy group 2g_Cl can
21 also be cross-coupled with trioctylaluminum reagent to give
22 the product 3g in 38% yield (entry 10). We consider that
23 organoaluminum reagent deprotonate the acid proton and
24 the resulting aluminum salts of the substrate undergo facile
25 cross-coupling reaction.12b,20 The reaction of 1-bromo-6-
26 chlorohexane (2h) showed high chemoselectivity and
27 proceeded exclusively at the bromide site to afford 3h in
28 62% yield (entry 11). The reaction of secondary alkyl
29 bromide, bromocycloheptane (2i) with trioctylaluminum
30 reagent did not produce the desired coupling product,
31 whereas undesired by-products, cycloheptane, cycloheptene,
32 and 1,1’-bi(cycloheptane) were obtained in 14%, 46%, and
33 12% yields, respectively (entry 12). The reaction of
34 (bromomethyl)cyclopropane (2j)21 gave the corresponding
35 ring-opening/coupling product 3ja in 37% yield without the
36 formation of 3jb indicating the intermediacy of alkyl
37 radicals (entry 13).11,13b
60 intermediate B. Transmetalation of
B
with the
61 organoaluminate species generates the organoiron(II)
62 intermediate C, which can undergo rapid recombination
63 with the alkyl radical to produce the organoiron(III)
64 intermediate D.11d–f The catalytic cycle then completes by
65 reductive elimination to give the Csp3–Csp3 coupling product
66 3, regenerating the iron(I) intermediate A. We consider that
67 the wide bite angle ligand promotes the reductive
68 elimination and the acetate ligand occupying the vacant
69 coordination sites25 suppresses the undesired b-hydrogen
70 elimination.
71
X
K[(Alkyl)3AlF]
O
O
Alkyl–X
P
FeII
2
KX
(Alkyl)2AlF
P
B
Alkyl
O
O
P
P
P
Alkyl
FeI
O
FeII
O
P
A
C
P
Alkyl
FeIII
Alkyl
O
P
P
P
O
Alkyl–Alkyl
Bn-Nixanthos
3
D
(X = Br, Cl)
72
73
Figure 1.
A possible reaction mechanism.
74
In conclusion, the first example of iron-catalyzed
38
We
further
conducted
a
sequential
39 hydroalumination22/cross-coupling reaction. Addition of
40 DIBAL-H to olefin 6 and the subsequent iron-catalyzed
41 cross-coupling reaction with 2b proceeded in a one-pot
42 manner to give the desired coupling product 3ka in 36%
43 yield along with the formation of minor by-product 3kb
44 (Scheme 2). Although there is still room for improving the
75 alkyl–alkyl Negishi coupling of organoaluminum has been
76 achieved employing an Fe(OAc)2/1 catalyst system.
77 Stoichiometric potassium fluoride was identified as the
78 critical additive that facilitates the coupling reactions by the
79 formation of highly nucleophilic aluminate species. This
80 new coupling method features the simple and mild
81 activation of organoaluminum nucleophiles, resulting in
82 high functional group tolerance, and thus demonstrates
83 synthetic advantages especially in high product selectivity
84 over previously reported iron-catalyzed alkyl–alkyl coupling
85 reactions.
45
HAli-Bu2
(1.5 equiv)
Al
hexane, 60 ºC, 38 h
then evaq.
6
(1.7 equiv)
7
86
Fe(OAc)2 (5 mol %)
87
This work was supported in part by the Advanced Low
1 (10 mol %)
KF (1.5 equiv)
2b
88 Carbon Technology Research and Development Program
89 (ALCA JPMJAL1504) from the Japan Science and
90 Technology Agency (JST), and JSPS Core-to-Core Program
91 "Elements Function for Transformative Catalysis and
92 Materials". R.A. thanks ‘Research Fellowships of Japan
93 Society for the Promotion of Science for Young Scientists’
94 (JSPS 11488). We are grateful to Integrated Research
C10H21
C10H21
3kb, 8% (GC yield)
+
THF, 40 ºC, 40 h
3ka, 36% (GC yield)
46
47 Scheme 2. Cross coupling of alkyl halides with terminal alkenes via
48 hydroalmination with diisobutylaluminum hydride.

4
68 11
69
For a recent mechanistic study on iron-catalyzed cross-couplings
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1
2
3
4
5
6
7
8
9
Consortium on Chemical Sciences (IRCCS). FT-ICR-MS
and 800 MHz NMR analyses were supported by the JURC
at ICR, Kyoto University. The authors thank Profs. Hideo
Nagashima and Yusuke Sunada for providing Fe(OPiv)2.
We are grateful to Tosoh Finechem Corporation and Nissan
Chemical Industries Corporation for their financial support.
70
71
72
73
74
75
76
Supporting Information is available electronically on J-
STAGE.
77
78
79
10 References and Notes
80
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†
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110
111
112
0.7%, and 24% yields, respectively. Unfavorable low-valent or
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7
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For selected papers on oxidative Tsuji–Trost allylic alkylation,
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114
115
116
coupling reaction of 1-chlorohexanol (ref. 12b), however the
reaction between 1-chlorohexanol and trioctylaluminum did not
proceed without KF.
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117 20 The reaction of 1-chlorohexanol with octylmagnesium chloride
118
did not provide the desired coupling product.
119 21 V. W. Bowry, J. Lusztyk, K. U. Ingold, J. Am. Chem. Soc. 1991,
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120
122
123 23 Other by-products decene, decane, icosane, and starting material
9
124
125
126
127
were observed in 18%, 8%, 13%, and 13% yields, respectively.
We assume that the b-hydrogen elimination from isobutyliron
intermediates affords iron hydride intermediates which cause the
undesired side reactions.
54 10
55
56
57
128 24 The reduction of iron(II) acetate by organoaluminum reagent in
58
129
130
131
132
133
THF may give dimeric iron(I) acetate species, which can
generate the monomeric iron(I) acetate A in an equilibrium
manner. For the crystal structure of origomeric iron(II) acetate,
see: B. Weber, R. Betz, W. Bauer, S. Schlamp, Z. Anorg. Allg.
Chem. 2011, 102–107.
59
60
61
62
63
134 25 a) M. Wakioka, Y. Nakamura, Q. Wang, F. Ozawa,
135
136
64
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