Palladium-Catalyzed Reductive Coupling Reaction of Terminal Alkynes with Aryl Iodides Utilizing Hafnocene Difluoride as a Hafnium Hydride Precursor Leading to trans-Alkenes

Article abstract of DOI:10.1002/asia.201701775

Herein, we describe a reductive cross-coupling of alkynes and aryl iodides by using a novel catalytic system composed of a catalytic amount of palladium dichloride and a promoter precursor, hafnocene difluoride (Cp₂HfF₂, Cp=cyclopentadienyl anion), in the presence of a mild reducing reagent, a hydrosilane, leading to a one-pot preparation of trans-alkenes. In this process, a series of coupling reactions efficiently proceeds through the following three steps: (i) an initial formation of hafnocene hydride from hafnocene difluoride and the hydrosilane, (ii) a subsequent hydrohafnation toward alkynes, and (iii) a final transmetalation of the alkenyl hafnium species to a palladium complex. This reductive coupling could be chemoselectively applied to the preparation of trans-alkenes with various functional groups, such as an alkyl group, a halogen, an ester, a nitro group, a heterocycle, a boronic ester, and an internal alkyne.

Full text of DOI:10.1002/asia.201701775

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Accepted Article
Title: Palladium-Catalyzed Reductive Coupling Reaction of Terminal
Alkynes with Aryl Iodides Utilizing Hafnocene Difluoride as a
Hafnium Hydride Precursor Leading to trans-Alkenes
Authors: Norio Sakai, Keita Takahashi, and Yohei Ogiwara
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10.1002/asia.201701775
Chemistry - An Asian Journal
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Palladium-Catalyzed Reductive Coupling Reaction of Terminal
Alkynes with Aryl Iodides Utilizing Hafnocene Difluoride as a
Hafnium Hydride Precursor Leading to trans-Alkenes
Keita Takahashi, Yohei Ogiwara and Norio Sakai*[a]
Abstract: Herein, we describe a reductive cross-coupling of alkynes
and aryl iodides using a novel catalytic system composed of a
catalytic amount of palladium dichloride and a promoter precursor,
hafnocene difluoride (Cp2HfF2), in the presence of a mild reducing
reagent, a hydrosilane, leading to a one-pot preparation of trans-
alkenes. In this process, a series of coupling reactions efficiently
proceeds via the following three steps: (i) an initial formation of
hafnocene hydride from hafnocene difluoride and a hydrosilane, (ii) a
catalyzed coupling reaction.[3b] In this context, Negishi and co-
workers succeeded in a facile construction of alkenes via two
processes: the hydroalumination (Al-H) of alkynes with DIBAL-H
and a Pd- or Ni-catalyzed cross-coupling reaction.[4a] Moreover,
as examples that use a group 14 metal, the work of Hiyama and
Denmark involved the hydrosilylation of alkynes with
a
hydrosilane (Si-H) in the presence of a Pt catalyst and a
subsequent Hiyama cross-coupling reaction, which led to the
preparation of alkenes.[5b-e] Wnuk and co-workers demonstrated
subsequent hydrohafnation toward alkynes, and (iii)
a final
transmetalation of the alkenyl hafnium species to a palladium complex. an initial hydrogermination of alkynes with a hydrogermane (Ge-
This reductive coupling could be chemoselectively applied to the
preparation of trans-alkenes with various functional groups, such as
an alkyl group, a halogen, an ester, a nitro group, a heterocycle, a
boronic ester, and an internal alkyne.
H) in the presence of AIBN, which was followed by a Pd-catalyzed
cross-coupling with alkenylgermanes that produced alkenes.[6a]
Stille and co-workers had already found that the combination of a
hydrostannylation of alkynes with a hydrostannane (Sn-H) in the
presence of AIBN and a Stille coupling would provide alkenes.[7a]
Also, the Maleczka, Jr., group developed a synthetic approach
that combined the catalytic hydrostannylation of alkynes with an
organotin compound and a hydrosilane as a reducing agent to
prepare alkenes.[7b] Furthermore, recent examples of reductive
coupling via a transition metal hydride were demonstrated by
three groups, Lalic,[8a,8b] Nakao,[8c] and Hull,[8d] who independently
demonstrated a reductive cross-coupling that involved a catalytic
hydrocupration of alkynes with a copper hydride intermediate (Cu-
H) prepared from a Cu catalyst and a hydrosilane (Scheme 1b).
On the other hand, as an example of the use of a group 4 early
transition metal, Negishi and co-workers achieved the synthesis
of alkenes via the hydrozirconation of alkynes with a zirconocene
chloride hydride (Zr-H) and a subsequent Pd- or Ni-catalyzed
Introduction
Reductive cross-coupling between alkynes and electrophiles is an
attractive and useful method to synthesize highly valuable
polysubstituted alkenes from readily available starting materials.[1]
One procedure of this reaction generally proceeds via a two-step
process involving an initial preparation of alkenyl metals through
hydrometalation between alkynes and a variety of metal hydrides
([M]-H) and a subsequent cross-coupling reaction of the alkenyl
metals with appropriate counterparts in the presence of transition
metals, such as a palladium and a nickel catalyst (Scheme 1).
Thus far, the in-situ generation of an alkenyl metal intermediate
via the hydrometalation of an alkyne in the first step of the process
has involved a number of previously developed metal hydrides
(Scheme 1a).[3-7] As examples using group 12 main metals, Sato
and co-workers, as well as Fu et al., reported the initial
hydrozincation of alkynes with a zinc hydride (Zn-H) derived from
zinc halide and lithium hydride, with a subsequent Pd-catalyzed
cross-coupling of the organozinc compounds and electrophiles,
affording alkenes.[2a,b] Also, Suzuki and Miyaura et al. developed
a process for the synthesis of alkenes via a combination of a
hydroboration of alkynes with catecholborane (B-H) and a Pd-
cross-coupling process (Scheme 1c).[9] Because group
4
metallocene hydrides, such as zirconocene hydride (Schwarz's
reagent) and hafnocene hydride, exhibit a high level of syn-
selectivity at
a terminal alkyne moiety in a series of
hydrometalation, these hydrides have been employed as a
suitable intermediate for the selective preparation of trans-
alkenes.[10] Also, because group 4 organometallic reagents
generally show
a
higher nucleophilicity, compared with
organoboron reagents and organosilicon reagents, these metal
hydrides preserve the advantage of functioning as a cross-
coupling partner without the use of a basic activator.[1] Despite
having such useful chemical properties, the direct handling of
these group
4 metallocene-based hydrides is extremely
troublesome due to high air-sensitivity. Also, when preparing a
metallocene hydride from the corresponding metal halide and
dialkyl metals, the use of a strong reducing reagent, such as
LiAlH4 or NaBEt3H, and a strong base, such as n-BuLi or Grignard
reagents, is required, which leads to undesired side reactions of
sensitive substrates and to a restriction of available substrates.[11]
[a]
Keita Takahashi, Dr. Y. Ogiwara, Prof. Dr. N. Sakai
Department of Pure and Applied Chemistry, Faculty of Science and
Technology, Tokyo University of Science (RIKADAI), Noda, Chiba
278-8510 (Japan)
E-mail: sakachem@rs.noda.tus.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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Thus, in order to overcome these serious problems, a skillful
process has been developed that combines the in-situ generation
of a group 4 metallocene hydride from an air-stable group 4
metallocene difluoride and a hydrosilane as a mild reducing
reagent.[12] This procedure has been applied to useful
4-methoxystilbene (3a) in a 17% yield (entry 1). After screenings
of several hydrosilanes, the yield was greatly improved when
using alkoxysilanes, particularly when the use of (EtO)3SiH
afforded 3a in a 90% yield (entries 2−7). When the reaction was
conducted with Cp2HfCl2 instead of Cp2HfF2, no target alkene was
furnished (entry 8). Moreover, when the reaction was conducted
in the absence of Cp2HfF2, the desired reductive coupling did not
occur and the starting iodoarene was recovered, which
demonstrated that Cp2HfF2 is indispensable for this reductive
coupling (entry 9). In order to compare the reactivity of other group
4 metallocene-based promoters, the same reactions were carried
out with either Cp2TiF2 or Cp2ZrF2. Consequently, Cp2TiF2 did not
work effectively, whereas Cp2ZrF2 had the same effect as Cp2HfF2
(entries 10 and 11). Thus, when each reaction was re-examined
with 1 mol % of PdCl2, Cp2HfF2 showed extremely high catalytic
activity in the present coupling, providing alkene 3a in an excellent
transformations that involve
a
reduction of esters,[12b] the
asymmetric hydrosilylation of imines,[12a,12c,12f,12g] the homo-
coupling of amides,[12h] the hydrosilylation of dienes,[12i] and a
defluorination of fluorinated alkenes or pyridines.[12j,12k,12l]
However, the series of catalysts employed in these reports are
limited to titanocene difluoride, and a reductive cross-coupling
reaction that uses hafnocene difluoride, have not be reported yet.
Herein, we report the full details of a novel Pd-catalyzed reductive
three-component coupling reaction of alkynes, aryl iodides, and a
hydrosilane in the presence of hafnocene difluoride, leading to a
selective one-pot preparation of trans-alkenes. In particular, we
describe how a metal hydride precursor, an air-stable hafnocene
difluoride, functions as the central and key promoter of this cross-
coupling series (Scheme 1d).
Table 1. Optimization of Reaction Conditions.
PdCl2 (5 mol %)
Cp2MX2 (1 equiv)
a) a main group metal hydride
[Pd, Ni, etc.]
hydrosilane (2.5 equiv)
R3
R2
H
H
[M]
PMP
R3
+
[M]
H
X
Ph
PMP
2a
0.1 mmol
I
Ph
R1
R2
toluene 1 mL
80 °C, 8 h
1a
3a
cross-coupling
hydrometalation
R1
R2
R1
2 equiv
PMP = 4-MeOC6H4
[M] = Zn, B, Al
Si, Ge, Sn
Entry
Hydrosilane
Cp2MX2
Conv. of 2a
GC yield [%]
[%][a]
b) a transition metal and a hydrosilane
[Pd]
R3
R2
H
[Cu]
R2
H
R3
[Cu]–X , Si–H
1
2
Et3SiH
Cp2HfF2
Cp2HfF2
Cp2HfF2
Cp2HfF2
Cp2HfF2
Cp2HfF2
Cp2HfF2
Cp2HfCl2
–
95
15
73
99
99
98
96
21
1
17
X
R1
R2
R1
R1
PhMe2SiH
(Me2SiH)2O[b]
PMHS[c]
0
c) group 4 transition metal hydride
3
6
[Pd, Ni]
R2
R2
H
[Zr]
H
[Zr]–H
R1
X
4
11
R1
R1
5
(EtO)2MeSiH
(nPnO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
85
regioselective
6
45
d) this work
[Pd]
7
90
R2
H
[Hf]
H
R2
[Hf]–F , Si–H
X
R1
8
0
R1
air-stable reagents
R1
regioselective
hafnocene difluoride and a hydrosilane
9
0
10
11
12[d]
13[d]
14[d]
15[d]
Cp2TiF2
Cp2ZrF2
Cp2ZrF2
Cp2HfF2
60
91
55
96
99
99
1
Scheme 1. Reductive cross-coupling reaction between alkynes and
electrophiles using a variety of metal hydrides.
91
3
94 (89)[e]
82
Results and Discussion
[f]
Cp2HfF2
Optimization of the reductive cross-coupling of phenylacetylene
(1a) with 4-iodoanisole (2a) as a model reaction was initially
[g]
Cp2HfF2
12
performed with various parameters including group
4
metallocenes and hydrosilanes (Table 1). When the reaction of
phenylacetylene (1a: 2 equiv) with 4-iodoanisole (2a) was
performed in the presence of 5 mol % of PdCl2, 1 equiv of Cp2HfF2
and 2.5 equiv of Et3SiH as a reducing agent in toluene at 80 °C,
the desired three-component coupling proceeded to obtain trans-
[a] The conversion was determined by GC analysis. [b] 1.25 equiv. [c]
Polymethylhydrosiloxane (Si–H: 2.5 equiv). [d] PdCl2 (1 mol %), toluene (2
mL). [e] Isolated yield. [f] Cp2HfF2 (50 mol %). [g] Cp2HfF2 (10 mol %).
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yield (entries 12 and 13). On the other hand, when the reaction
was conducted with less than a stoichiometric amount of Cp2HfF2
(50 mol % or 10 mol %), alkene 3a was obtained in 82% and 12%
yields, respectively (entries 14 and 15). Therefore, the results
strongly imply that regeneration of a hafnocene hydride with an
excess amount of the hydrosilane is rather difficult. Also, when
decreasing in equivalent of the alkyne, the yield of alkene 3a was
drastically reduced. [13]
With the optimal conditions in hand, the scope of aryl iodides
was investigated in the reductive three-component coupling
reaction using phenylacetylene (1a) (Table 2). Iodobenzene and
aryl iodides bearing a methyl group, regardless of the location of
a substituted group, efficiently functioned as a coupling partner to
give the desired alkene products 3b−3e in good yields. Also, a
substrate with a bulky substituent, a t-butyl group, produced
alkene 3f in an 88% yield. In contrast, when using substrates
including either a dimethylamino or a methylthio group, a
decreased yield was observed for each of the products 3g and 3h.
It seemed that the formation of a Lewis acid/Lewis base complex
between the hafnocene hydride and these heteroatoms inhibited
both the initial hydrohafnation and a subsequent transmetalation.
Also, the coupling with a hydroxy-substituted iodoarene did not
produce the expected alkene, and the starting iodoarene was
recovered. Interestingly, when using an iodoarene with an
acetoxy group, the hydroxy-substituted coupling product 3i was
obtained in a 44% yield. During a series of reductive couplings,
the deprotection of an acetyl group occurred. Moreover, an
iodoarene with a triethylsiloxy group provided the corresponding
alkene 3j in a high yield. In this case, the silyl group adequately
protected the hydroxy group, which remained even after a
common column purification. Iodoarenes with a fluorine, a
chlorine, and a bromine substituent were also converted to the
corresponding alkenes 3k−3m in moderate to good yields. Among
the reducible functional groups, an ester substituent was very
tolerant to a hafnium hydride and produced alkene 3n in a 76%
yield.[14] In contrast, functional groups, such as ketones or amides,
were not applied to the reductive coupling system involving a
hafnocene hydride. Thus, a ketone moiety was initially protected
as an acetal group, followed by a two-step conversion composed
of reductive coupling and subsequent deprotection to give the
desired ketone product 3o in a 45% yield. An aryl iodide with a
cyano group gave the coupling adduct 3q’ with a formyl group that
was over-reduced with a hafnocene hydride. When using an
iodoarene with a nitro group, the desired alkene 3r was obtained
in a satisfactory yield. However, by-product 3r’ bearing an over-
reduced amino group was simultaneously obtained in a 22% yield.
In these two cases, no formation of the corresponding deiodinated
Table 2. Substrate Scope of Aryl Iodides.[a,b]
PdCl2 (1 mol %)
Cp2HfF2 (1 equiv)
(EtO)3SiH (2.5 equiv)
Ar
Ph
Ar
I
+
Ph
toluene 2 mL
80 °C, 13 h
1a
2 equiv
2
3
0.5 mmol
tBu
NMe2
R
Ph
Ph
Ph
Ph
R = 4-H 3b: 94%
4-Me 3c: 71%
3-Me 3d: 88%
2-Me 3e: 81%
3f: 88%
3g: 45%
3j: 89%
OH
OSiEt3
SMe
Ph
3i: 0%[c] (44%)[d]
Ph
3h: 43%
O
O
X
OEt
Me
Ph
X = F 3k: 53%
Ph
Ph
Ph
3o: 0%[e] (45%)[f]
3n: 76%
Cl 3l: 71%
Br 3m: 90%
O
CN
+
H
O
Ph
NEt2
3q: trace
3r': 22%
3q': 65%
Ph
Ph
3p: 0%
NO2
NH2
+
Ph
Ph
3r: 70%
3s: 87%
products was observed. Also,
a substrate possessing a
N
S
Ph
Ph
Ph
N
Ph
naphthalene ring gave the corresponding alkene 3s in a good
yield. In heterocycles, 3-iodopyridine afforded alkene 3t in a
relatively good yield. In contrast, 2-iodopyridine showed low
reactivity in this coupling, and the starting iodoarene was
recovered. When 2-iodothiophene was treated with our optimal
conditions, the corresponding alkene 3v was obtained in a
practical yield. In the case of 1,4-diiodobenzene, double cross-
coupling proceeded and produced 1,4-divinylbenzene derivative
3w in a 40% yield with the detection of a trace amount of a mono-
coupling product, trans-4-iodostilbene. Remarkably, under our
base-free conditions, an aryl iodide containing a boronic ester
could be chemoselectively converted to alkene 3x with the boron
moiety in an excellent yield.
3t: 64%
3u: 21%[g]
3v: 60%
Ph
B(pin)
Ph
3w: 40%
3x: 81%
[a] Isolated yield. [b] When the desired coupling proceeded, the formation of
an alkene dimer was slightly observed. [c] 4-Iodophenol was used as a
starting material. [d] 4-Iodophenyl acetate was used as a starting material. [e]
4'-Iodoacetophenone was used as a starting material. [f] 2-(4-Iodophenyl)-2-
methyl-1,3-dioxolane was used as a starting material. [g] 12% (100 ˚C, 24 h).
Couplings of various alkynes with 4-iodoanisole (2a) were then
examined under our coupling conditions, and the results are
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summarized in Table 3. For example, a series of couplings with
phenylacetylene derivatives bearing an electron-donating group
efficiently proceeded to give the corresponding alkenes 4a−c in
good yields. However, when employing a phenylacetylene with an
o-methyl-substituted group, the expected alkene was not
obtained, and, instead, a 2-methylstyrene that was simply
reduced with a hafnocene hydride was selectively obtained. An
aromatic alkyne containing a chlorine group that shows a
moderate electron-withdrawing effect also produced the coupling
product 4e in a 71% yield. In contrast, aromatic alkynes with a
relatively strong electron-withdrawing group, such as a nitro and
an ester group, managed to undertake the desired reductive
coupling to produce the corresponding products 4f and 4g in
rather low yields, probably due to the fact that an electron-
withdrawing group would repress the hydrometalation of an
alkyne.[15] In a terminal alkyne with an ester moiety, hydride of the
hafnium hydride initially added to an electron-deficient β-carbon
of the alkyne, followed by a subsequent transmetalation at the α-
position to give 1,1’-disubstituted alkene 4g. Unfortunately, an
aliphatic alkyne was not applicable to our coupling protocol due
to the low level of stability of the alkenyl hafnium intermediate.
Also, an internal alkyne, such as diphenylacetylene, did not
produce the desired alkene. Thus, we turned the negative result
into an advantage, and attempted the following coupling. Namely,
when a terminal alkyne containing an internal alkyne moiety was
employed, the desired coupling selectively proceeded at the
terminal alkyne moiety to provide alkene 4j in a good yield.
As control experiments, the reaction of Cp2HfF2 with 2 equiv of
phenylacetylene (1a) and 2.5 equiv of (EtO)3SiH was initially
performed at 80 °C for 15 min, followed by hydrolysis to form
styrene (5) in a 60% GC yield (eq 1 in Scheme 2). Also, as a
sequential procedure, when in-situ generated alkenyl hafnium
species was treated with 1 equiv of 4-iodoanisole (2a) and a PdCl2
catalyst at 80 °C for 8 h, the desired alkene 3a was obtained in a
91% GC yield (eq 2 in Scheme 2). These results show that the
key intermediate in the series of reductive cross-coupling is an
alkenyl hafnium intermediate.
Cp2HfF2 (0.5 mmol)
(EtO)3SiH (2.5 equiv)
X
H2O
HfCp2
Ph
(1)
Ph
Ph
toluene 2 mL
80 °C, 15 min
1a
5
X = H or halogen
2 equiv
60% GC yield
Cp2HfF2 (0.5 mmol) PdCl2 (1 mol %)
2a
(EtO)3SiH (2.5 equiv)
(1 equiv)
PMP
Ph
(2)
Ph
toluene 2 mL
80 °C, 15 min
80 °C, 8 h
1a
3a
2 equiv
91% GC yield
Scheme 2. Mechanistic Studies.
On the basis of the results obtained by the control experiments,
a plausible mechanistic pathway of the present reductive coupling
is shown in Scheme 3. Initially, the reaction of hafnocene
difluoride with (EtO)3SiH easily produced hafnocene hydride A.[16]
Then, hydrometalation between an alkyne and the hafnocene
hydride A occurred to form alkenyl hafnium species B. In the
process, as well as the results with a zirconium hydride [10,15], the
syn-addition to a terminal alkyne of hafnocene hydride A
stereoselectively proceeded, and, simultaneously, the hafnium
side of A was regioselectively added at the terminal position of an
alkyne in order to avoid steric hindrance. Then, the
transmetalation of an alkenyl moiety on alkenyl hafnium species
B to aryl palladium species C, which was generated via the
oxidative addition of an aryl iodide to a Pd(0) catalyst, easily
occurred to produce the corresponding alkenyl aryl palladium
species D. On the basis of the observed formation of the alkene
dimer, it was assumed that a palladium(0) species would be
generated via the reductive elimination of a dialkenyl palladium(II)
complex. Finally, a reductive elimination of species D occurred,
which led to the preparation of the desired trans-alkene with the
regeneration of a Pd(0) catalyst and the formation of a hafnocene
Table 3. Substrate Scope of Alkynes.[a,b]
PdCl2 (1 mol %)
Cp2HfF2 (1 equiv)
(EtO)3SiH (2.5 equiv)
R1
PMP
R1
2 equiv
R2
+
PMP
I
R2
4
toluene 2 mL
80 °C, 13 h
1
2a
0.5 mmol
PMP = 4-MeOC6H4
PMP
PMP
PMP
Me
MeO
Cl
4a: 72%
4-Me 4b: 91%
4e: 71%
3-Me 4c:
2-Me 4d:
86%
0%[c]
PMP
PMP
CO2Et
PMP
Me
dihalide.
X
F
X
H
Ar1
O2N
(EtO)3SiH
[Hf]
H
[Hf]
[Hf]
4f: 15%
4g: 18%[d]
4h: 0%
–
(EtO)3SiF
F
Ar1
A
B
PMP
Ph
PMP
[Hf] = Cp2Hf
X = H or halogen
Ph
PdCl2
4i: 0%[e]
4j: 76%
Ph
2 x B
Ar1
2[Hf]XCl
Pd
+
Ar1
[a] Isolated yield. [b] When the desired coupling proceeded, the formation of
an alkene dimer was slightly observed. [c] 2-Methylstyrene was obtained in a
36% yield. [d] 24 h. [e] 0% (100 ˚C, 24 h).
Ar2
Pd(0)
B
Ar1
Pd
Ar2
I
Ar2
Ar2
I
–
[Hf]XI
–
Pd(0)
C
Ar1
D
Scheme 3. Proposed reaction mechanism for reductive cross-coupling via a
hafnocene hydride.
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J = 7.5 Hz, 2 H, ArH), 7.44 (d, J = 8.5 Hz, 2 H, ArH), 7.33 (t, J = 7.5 Hz, 2
H, ArH), 7.22 (t, J = 7.5 Hz, 1 H, ArH), 7.06 (d, J = 16.5 Hz, 1 H, CH), 6.97
(d, J = 16.5 Hz, 1 H, CH), 6.89 (d, J = 8.5 Hz, 2 H, ArH), 3.81 (s, 3 H, CH3);
13C NMR (125 MHz, CDCl3) δ = 159.3, 137.6, 130.1, 128.6, 128.2, 127.7,
127.2, 126.6, 126.2, 114.1, 55.3; LRMS (EI) m/z (%) 210 (M+, 100), 195
(15), 165 (23), 152 (13), 105 (10), 89 (11).
Conclusions
In conclusion, we have described how a key intermediate,
hafnocene hydride derived form an air-stable hafnocene difluoride
and a mild reducing agent, a hydrosilane, effectively promoted a
palladium-catalyzed reductive cross-coupling between alkynes
and aryl iodides, which led to the preparation of various trans-
alkenes. The principal findings include a generated hafnium
hydride regioselectively added to terminal alkynes to afford an
alkenyl hafnium intermediate, which easily undertook
transmetalation to produce a corresponding palladium(II) complex.
In addition, this protocol introduces the use of alkene derivatives
containing various functional groups such as an ester, a nitro, a
boronic ester group and a heterocycle.
Acknowledgements
The authors deeply thank Shin-Etsu Chemical Co., Ltd., for the
gift of hydrosilanes
Keywords: hafnium • palladium • cross-coupling • hydrosilane •
alkyne
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Experimental Section
General information
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Cp2ZrF2,[18] and Cp2TiF2
were prepared using a similar procedure
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Table of Contents
FULL PAPER
[Hf]–F
Si–H
[Pd] (1 mol %)
Ar
Keita Takahashi, Yohei Ogiwara and
Norio Sakai*
H
H
F
F
I
[Hf]
Hf
R1
Ar
R1
alkenyl hafnium
R1
up to 94%
30 examples
[Hf]–F
Page No. – Page No.
hafnium hydride as a key intermediate
low Pd-catalyst loading
high selectivity of E-alkene
Palladium-Catalyzed
Coupling Reaction
Reductive
of Terminal
Alkynes with Aryl Iodides Utilizing
Hafnocene Difluoride as a Hafnium
Hydride Precursor Leading to trans-
Alkenes
Hafnium-mediated reductive cross-coupling between alkynes and aryl iodide:
One-pot preparation of trans-alkenes from alkynes and aryl iodides in the presence
of an air-stable hafnocene difluoride, a mild reducing agent, a hydrosilane, and a
palladium catalyst. Hafnocene difluoride functions as a metal hydride precursor and
behaves as the central and key promoter of a series of reductive cross-coupling.
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