C-H bond activation/borylation of furans and thiophenes catalyzed by a half-sandwich iron N-heterocyclic carbene complex

Article abstract of DOI:10.1002/asia.201000140

A coordinatively unsaturated iron-methyl complex having an N-heterocyclic carbene ligand, [Cp^*Fe-(L^Me)Me] (1; Cp ^*=η⁵-C₅Me₅, L ^Me=1,3,4,5-tetramethyl-imidazol-2-ylidene), is synthesized from the reaction of [Cp^*Fe(TMEDA)Cl] (TMEDA=N,N,N',N'- tetramethylethylenediamine) with methyllithium and L^Me. Complex 1 is found to activate the C-H bonds of furan, thiophene, and benzene, giving rise to aryl complexes, [Cp^*Fe(L^Me)-(aryl)] (aryl=2-furyl (2), 2-thienyl (3), phenyl (4)). The C-H bond cleavage reactions are applied to the dehydrogenative coupling of furans or thiophenes with pinacolborane (HBpin) in the presence of tert-butylethylene and a catalytic amount of 1 (10 mol% to HBpin). The borylation of the furan/thiophene or 2-substituted furans/thiophenes occurs exclusively at the 2-or 5-positions, respectively, whereas that of 3-substituted furans/thiophenes takes place mainly at the 5-position and gives a mixture of regioisomers. Treatment of 2 with 2 equiv of HBpin results in the quantitative formation of 2-boryl-furan and the borohydride complex [Cp ^*Fe(L^Me)(H₂Bpin)] (5). Heating a solution of 5 in the presence of tert-butylethylene led to the formation of an alkyl complex [Cp^*Fe-(L^Me)CH₂CH₂tBu] (6), which was found to cleave the C-H bond of furan to produce 2. On the basis of these results, a possible catalytic cycle is proposed.

Full text of DOI:10.1002/asia.201000140

DOI: 10.1002/asia.201000140
ꢀ
C H Bond Activation/Borylation of Furans and Thiophenes Catalyzed by a
Half-Sandwich Iron N-Heterocyclic Carbene Complex
Tsubasa Hatanaka, Yasuhiro Ohki,* and Kazuyuki Tatsumi*[a]
Abstract: A coordinatively unsaturated
iron-methyl complex having an N-het-
erocyclic carbene ligand, [Cp*Fe-
genative coupling of furans or thio-
phenes with pinacolborane (HBpin) in
the presence of tert-butylethylene and
a catalytic amount of 1 (10 mol% to
HBpin). The borylation of the furan/
thiophene or 2-substituted furans/thio-
phenes occurs exclusively at the 2- or
5-positions, respectively, whereas that
and gives a mixture of regioisomers.
Treatment of 2 with 2 equiv of HBpin
results in the quantitative formation of
2-boryl-furan and the borohydride
ACHTUNGTRENNUNG =
(LMe)Me] (1; Cp*=h5-C5Me5, LMe
1,3,4,5-tetramethyl-imidazol-2-ylidene),
is synthesized from the reaction of
complex [Cp*FeACTHNGUTRENUNG )ACHTGNUTREN(NNUG H2Bpin)] (5).
(LMe
Heating a solution of 5 in the presence
of tert-butylethylene led to the forma-
tion of an alkyl complex [Cp*Fe-
[Cp*Fe
G
(TMEDA=
N,N,N’,N’-tetramethylethylenediamine)
with methyllithium and LMe. Complex 1
of
3-substituted
furans/thiophenes
(LMe)CH2CH2tBu] (6), which was
ACHTUNGTRENNUNG
ꢀ
ꢀ
is found to activate the C H bonds of
takes place mainly at the 5-position
found to cleave the C H bond of furan
to produce 2. On the basis of these re-
sults, a possible catalytic cycle is pro-
posed.
furan, thiophene, and benzene, giving
rise to aryl complexes, [Cp*Fe
ACHTUNGTRENNUNG(aryl)] (aryl=2-furyl (2), 2-thienyl (3),
phenyl (4)). The C H bond cleavage
reactions are applied to the dehydro-
(LMe)-
ACHTUNGTRENNUNG
ꢀ
Keywords: borylation · C H acti-
vation · hydrocarbons · iron · N-
ꢀ
heterocyclic carbenes
Introduction
noble metals, such as Ru, Rh, Ir, Pd, and Pt. Thus the ability
to use cheaper, less toxic, abundant metals, particularly Fe,
remains an elusive challenge in this field.[5]
The development of the efficient derivatization of hydrocar-
bons is one of the most important goals for chemistry.[1] In
this regard, considerable attention has been paid to the acti-
There are two notable examples of iron-catalyzed aromat-
ꢀ
ic C H bond activations. Jones et al. have reported that a
ꢀ
vation of C H bonds carried out by transition-metal com-
zero-valent iron complex [Fe
A
G
plexes.[1,2] Although some interesting catalytic C H bond ac-
es the insertion of CNCH2tBu into a C H bond of benzene
under irradiation, resulting in the formation of aldimine
PhCH=NCH2tBu.[6] Nakamura et al. have recently achieved
the direct arylation of phenyl pyridines or aryl imines cata-
ꢀ
ꢀ
tivation reactions have been reported,[3] the first example of
ꢀ
a highly efficient and selective C H bond transformation
was the ruthenium-catalyzed hydroarylation of olefins.[4] The
ꢀ
vast majority of existing C H bond activation catalysts use
lyzed by [FeACTHUGNTRENNU(G acac)3] with chelating ligand (1,10-phenanthro-
line or 4,4’-di-tert-butyl-2,2’-bipyridine), although this reac-
tion requires stoichiometric amounts of PhMgBr,
ZnCl2·TMEDA, and 1,2-dichloro-2-methylpropane.[7] In this
ꢀ
paper, we report the catalytic C H bond activation/boryla-
tion of furans and thiophenes carried out by a half-sandwich
iron complex of the methyl-substituted N-heterocyclic car-
bene (denoted as LMe). The catalyst is a newly synthesized
[a] T. Hatanaka, Prof. Dr. Y. Ohki, Prof. Dr. K. Tatsumi
Department of Chemistry
Graduate School of Science, and Research Center for Materials Sci-
ence, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
Fax : (+81)52-789-2943
E-mail: i45100a@nucc.cc.nagoya-u.ac.jp
ohki@mbox.chem.nagoya-u.ac.jp
coordinatively unsaturated iron complex, [Cp*FeACTHNUTRGNEUNG
(LMe)Me]
(1; Cp*=h5-C5Me5, LMe =1,3,4,5-tetramethyl-imidazol-2-yli-
dene). This complex is related to complex A in Scheme 1,
which we have previously reported as capable of the stoi-
ꢀ
chiometric C H bond activation and borylation of heteroar-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000140.
enes.[8] We have also conducted a series of reactions of the
Chem. Asian J. 2010, 5, 1657 – 1666
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1657

FULL PAPERS
Single crystals of 1 were grown from a concentrated
hexane solution, and the molecular structure was deter-
mined by X-ray diffraction. There are two independent mol-
ecules in the asymmetric unit. Their structures are nearly
identical, so that the following discussion is based on one
molecule shown in Figure 1. Complex 1 is a 16-electron FeII
Scheme 1.
new iron complexes with nearly stoichiometric amount of
substrate, to gain mechanistic insights.
Results and Discussion
Figure 1. Molecular structure of 1 with thermal ellipsoids at the 50%
probability level. One of two molecules found in the asymmetric unit is
Synthesis and Structure of [Cp*Fe
The amide ligand, N(SiMe3)2, acts as a base, and we have
previously demonstrated that Fe{N(SiMe3)2}2 reacts readily
(LMe)Me] (1)
ACHTUNGTRENNUNG
ꢀ
shown. Selected bond distances (ꢁ) and angles (8) for 1: Fe(1) C(1)=
AHCTUNGTRENNUNG
ꢀ
ꢀ
ꢀ
1.978(5), Fe(1) C(2)=1.935(3), C(2) N(1)=1.363(5), C(2) N(2)=
1.368(4), C(1)-Fe(1)-C(2)=90.1(2), N(1)-C(2)-N(2)=103.3(2).
AHCTUNGTRENNUNG
with thiols and serves as a convenient precursor for the syn-
thesis of iron bis-thiolate complexes and iron thiolate/sulfide
clusters.[9] We have also reported that the reaction of
complex coordinated by Cp*, LMe, and CH3 ligands. The
plane consisting of the iron atom, methyl carbon, and car-
bene carbon of LMe, is nearly perpendicular to the Cp*
[Cp*Fe{NACHTUNGTRENNUNG
(SiMe3)2}][10] with the mesityl-substituted imidazo-
lium salt (HLMes)(Cl) (LMes =1,3-dimesityl-imidazol-2-yli-
ꢀ
dene) followed by treatment with methyllithium gives rise
plane, with an inter-plane angle of 94.53(11)8. The Fe C-
to [Cp*Fe
[Cp*Fe{N
(LMes)Me].[8] However, an analogous reaction of
(LMe) bond length of 1.935(3) ꢁ and the Fe CH3 bond of
ꢀ
N
1.978(5) ꢁ are comparable to those for the analogous com-
(LMes)Me] (Fe C(LMes
plex [Cp*FeCATHUNGTERNNUGN ACHTUNGTRENNGNU ) 1.931(2) ꢁ; Fe CH3
ꢀ
ꢀ
2.009(2) ꢁ). On the other hand, they are slightly shorter
than those for electronically saturated iron complexes,
synthesis of [Cp*Fe
of [Cp*Fe(TMEDA)Cl] (TMEDA=N,N,N’,N’-tetramethyl-
ACHTUNGTRENNUNG
(LMe)Me] (1). Alternatively, the reaction
[12]
(LMe
)
and 2.034(4)–
ꢀ
E
1.949(10)–2.007(5) ꢁ for Fe C
ethylenediamine)[11] with methyllithium at ꢀ788C followed
by addition of LMe at 08C led to the formation of a dark
orange solution, from which 1 was isolated as orange crys-
tals in 54% yield (Scheme 2). Complex 1 is sensitive to air/
moisture and, thus, needs to be handled under inert condi-
tions. However 1 is thermally stable in solution and only
gradually decomposes over several days in boiling
[D18]octane. The paramagnetic nature of 1 was indicated by
2.130(6) ꢁ for Fe CH3.
[13]
ꢀ
ꢀ
C H Bond Activation of Furan, Thiophene, and Benzene
ꢀ
Complex 1 was found to activate the C H bonds of furan,
thiophene, and benzene, providing the 2-furyl, 2-thienyl, and
phenyl complexes 2–4, respectively (Scheme 3). In the case
of the reactions with furan and thiophene, complexes 2 and
3 were produced stoichiometrically at room temperature (2)
or 608C (3) according to NMR, whereas the isolated crystal-
line yields were 74% (2) and 53% (3). However, the reac-
tion with benzene required 7 days at 808C and gave the
1
the analysis of the H NMR spectrum in which the methyl
signals for Cp* and LMe were observed at d=41.8 (Cp*), 8.4
(LMe) and ꢀ47.5 ppm (LMe). We were not able to find the
ꢀ
signal assignable to Fe CH3 in the region from ꢀ500 to
+500 ppm.
Scheme 2.
Scheme 3.
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1657 – 1666

ꢀ
C H Bond Activation of Furans and Thiophenes
ꢀ
ꢀ
phenyl complex 4 in only 40% yield by NMR (36% isolated
crystalline yield). Interestingly, the 2-furyl or 2-thienyl com-
plexes 2 or 3 were selectively prepared from benzene solu-
and 4 are close to the Fe C
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
(LMe) (1.935(3) ꢁ) bond lengths in 1, respectively.
The reaction of 1 with 100 equiv of furan was found to
ꢀ
tions. This is indicative of much slower C H bond activation
obey pseudo-first-order kinetics with respect to the concen-
tration of 1. The kinetic data for the conversion of 1 into 2
were obtained by monitoring the decay of [1] by using
1H NMR spectroscopy in the range of 10 to 508C, and an
Eyring plot gave the activation parameters, DH° =
9.3(2) kcalmolꢀ1 and DS° =ꢀ44.2(8) eu. The kinetic isotope
effect (KIE) for this reaction was estimated to be kH/kD =
3.8(2) at 258C, for which the rate constants kH and kD were
of benzene than those of furan and thiophene. In the reac-
tions illustrated in Scheme 3, the methyl ligand abstracts a
hydrogen atom from the arenes, and is liberated as methane,
1
which was detected in the H NMR spectra of the reaction
mixtures. The 1H NMR spectra of complexes 2–4 are similar,
exhibiting broad signals in the ranges of d=50.1–47.0 (Cp*),
8.6–7.6 (LMe), and ꢀ38.3–ꢀ42.4 ppm (LMe). Interestingly, the
NMR data demonstrates that complexes 2 and 3 were
determined from the reactions of
1 with furan and
ꢀ
formed as single isomers, hence the regio-selective C H
[D4]furan, respectively. The KIE value and the large nega-
tive activation entropy of the reaction suggest that an inter-
bond activation of furan and thiophene occurred at the 2-
position. The X-ray diffraction established the molecular
structures of 3 and 4, as shown in Figure 2 (selected bond
distances and angles are listed in Table 1). A low quality
ꢀ
molecular C H bond cleaving process is involved in the
rate-determining step.
Catalytic Borylation of Furans and Thiophenes
ꢀ
Having achieved the successful C H bond activation of
arenes by 1, we were motivated to try catalytic formation of
aryl-boranes, which are useful building blocks for Suzuki–
Miyaura coupling reactions (Scheme 4).[14]
In the presence of tert-butylethylene (TBE), a catalytic
amount of 1 (10 mol% with respect to pinacolborane
(HBpin)) was found to carry out the dehydrogenative cou-
pling between furan/thiophene and HBpin at 60–708C
(Scheme 4). For this reaction, it was necessary to keep the
concentration of HBpin low in the reaction mixture. Thus
1 equiv of HBpin was divided into 10 portions, and each was
added at intervals of 1 hour (see the Experimental Section).
Experiments to optimize the conditions for the borylation of
furan and thiophene are outlined in Table 2. The reaction
using 1:1 ratio of HBpin and furan was found to give a 4:1
mixture of 2-boryl-furan and 2,5-di-boryl-furan (entry 1),
whereas the selective formation of 2-boryl-furan was ach-
ieved with a 1:6 ratio of HBpin and furan (entry 2). In con-
trast, a 1:2 ratio of HBpin and thiophene was found to sup-
press the formation of 2,5-di-boryl-thiophene (entries 5–7).
Whereas the yield of 2-boryl-thiophene was much lower
than that of 2-boryl-furan, addition of excess tert-butylethy-
lene was found to improve the yield (entries 6 and 7). The
use of catecholborane (HBcat) instead of HBpin for boryla-
tion of furan and thiophene resulted in the formation of 2-
boryl-furan in 6.2% yield and 2-boryl-thiophene in 3.9%
yield under the same conditions for entries 2 and 6, respec-
tively. Among the solvents employed, i.e., benzene, THF,
fluorobenzene, good yields were attained if benzene or THF
was used (entries 2, 3, 6, and 7), and thus the following cata-
lytic reactions were carried out in benzene to facilitate the
monitoring of the reactions by using NMR spectroscopy.
The catalytic borylation of furan derivatives and thio-
phene derivatives are summarized in Table 3. The product
Figure 2. Molecular structures of 3 and 4 with thermal ellipsoids at the
50% probability level.
Table 1. Selected bond distances (ꢁ) and angles (8) for complexes 3 and
4.[a]
3
4
ꢀ
Fe C(1)
1.935(4)
1.973(4)
1.360(3)
–
92.46(17)
103.3(3)
1.9403(18)
1.9613(17)
1.363(2)
1.359(2)
92.64(7)
ꢀ
Fe C(2)
ꢀ
C(1) N(1)
C(1) N(2)
C(1)-Fe-C(2)
N(1)-C(1)-N(2)
ꢀ
103.50(15)
Dihedral Angle
C(1)-Fe-C(2)/aryl
0
85.19(5)
[a] Labeling scheme as shown. [Fe]=Cp*Fe.
structure of 2 was also identified by X-ray analysis (see the
Supporting Information). Complex 3 has crystallographic Cs
symmetry, with the iron, the carbene carbon, and the 2-
thienyl group on the mirror plane. In contrast, the phenyl
group of 4 is close to perpendicular to the plane consisting
1
yield was determined by the H NMR analysis of the reac-
of
C
G
tion mixture using C6Me6 as an internal standard. All of the
products were isolated by Kugelrohr distillation or column
chromatography, and their isolated yields are given in paren-
85.19(5)8. The Fe CAHCTUNGTRENNUNG
Fe CACHTUNGTRENNUNG
ꢀ
Chem. Asian J. 2010, 5, 1657 – 1666
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1659

T. Hatanaka, Y. Ohki, and K. Tatsumi
FULL PAPERS
ꢀ
HBpin, entries 1–3). Regio-selective C H bond cleavage
provided only 5-borylated products from the reactions of 2-
methyl- and 2-trimethylsilyl-furan. The borylation of 3-
methylfuran took place mainly at the 5-position, and the
product was a 92:8 mixture of 2-boryl-4-methylfuran and 2-
ꢀ
boryl-3-methylfuran. These examples demonstrate that C H
bond activation is favored for the less hindered position
neighboring the oxygen atom. The borylation of 3-phenyl-
furan (entry 4) gave a similar result to that of 3-methylfuran,
while the yield was moderate (51%). Catalytic reactions
using either 2-methoxyfuran or 2-cyanofuran gave no bory-
lated product, and the stoichiometric reactions of 1 with
these substrates did not provide characterizable iron species.
While the borylation of 3-methylthiophene and 2-phenyl-
thiophene (entries 7 and 8)
Scheme 4.
theses. The borylation of methylfurans and 2-trimethylsilyl-
furan gave good yields of boryl-furans (73–94% based on
yielded only 32% and 52%
borylated products, analogous
Table 2. Optimization of the catalytic borylation reactions of furan or thiophene.[a]
Entry Substrate HBpin/arene/
TBE[b]
Solvent Product
T [8C] t [h] Yield by NMR
reactions with 2-methylthio-
phene and 2-trimethylsilylthio-
phene (entries 5 and 6) gave 5-
boryl-thiophenes in good yields
(88% and 90%).
[%][c]
1
1:1.1:2
benzene
60
11
41/10
2
3
4
5
6
7
1:6:2
1:6:2
1:6:2
1:2:2
1:2:10
1:2:10
benzene
THF
C6H5F
benzene
benzene
THF
60
60
60
70
70
70
11
11
11
11
11
11
75[d]
74
54
13
24
Mechanistic Investigation
23
To gain mechanistic insights
into the catalytic borylation of
furan, stepwise reactions using
iron complexes, HBpin, tert-bu-
tylethylene, and furan, were
carried out as follows.
[a] The reactions were carried out with 0.58 mmol of HBpin, in the presence of a catalytic amount of 1
(10 mol% to HBpin, and 1.7–9.1 mol% for the substrates). 10ꢂ0.058 mmol of HBpin was added at 1 h inter-
vals. [b] HBpin=pinacolborane (4,4,5,5-tetramethyl-1,3,2-dioxaborolane), TBE=tert-butylethylene (3,3-di-
methyl-1-butene). [c] The yield based on HBpin was determined by 1H NMR using C6Me6 as an internal stan-
dard. [d] The product was isolated in 66% yield by Kugelrohr distillation.
Reactions of Complexes 2–4
with Pinacolborane
Table 3. Borylation of furan derivatives and thiophene derivatives catalyzed by 1.[a]
Entry
Substrate
HBpin/arene/TBE
Product
T [8C]
t [h]
Yield by NMR [%][b]
94 (82)
Treatment of the 2-furyl com-
plex 2 with an excess of HBpin
in C6D6 led to the formation of
2-boryl-furan and the borohy-
1
1:6:2
70
11
2
3
1:2:2
1:6:2
70
70
11
11
73 (72)
dride complex [Cp*Fe
AHCTUNRTGEG(NUNN H2Bpin)] (5) (Scheme 5). Im-
ACHTUNGTRENNUNG
(LMe)-
76 (70)[c]
portantly, this reaction ap-
peared not to be affected by
the presence of tert-butylethy-
lene and/or furan, indicating
that this step accounts for the
4
1:2:2
70
11
51 (46)[c]
5
6
1:3:10
1:3:10
70
70
11
11
88 (67)
90 (84)
formation
of
boryl-furans
during the catalytic reactions.
In fact, complexes 2 and 5 were
observed in the H NMR spec-
7
8
1:3:10
70
70
11
11
32 (30)[c]
52 (50)
1
tra of the catalytic reaction
mixture. In the reaction of 2
with HBpin, 2-boryl-furan and
5 were generated quantitatively
according to analysis of the
NMR spectra, whereas the rele-
vant reactions of the 2-thienyl
complex 3 or the phenyl com-
1:1.5:10
[a] The reactions were carried out with 0.58 mmol of HBpin, in the presence of a catalytic amount of 1
(10 mol% to HBpin, and 1.7–9.1 mol% for the substrates). 10ꢂ0.058 mmol of HBpin was added at 1 h inter-
vals. [b] The yield based on HBpin was determined by the 1H NMR using C6Me6 as an internal standard. The
yield of isolated product is given in parentheses. [c] The product was obtained as a mixture of the 5-borylated
heteroarene (major) and the 2-borylated isomer (minor) in the ratios of 92:8 (entry 3), 90:10 (entry 4), or 96:4
1
(entry 7), respectively, based on the H NMR analysis.
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Chem. Asian J. 2010, 5, 1657 – 1666

ꢀ
C H Bond Activation of Furans and Thiophenes
Scheme 5.
Scheme 6. A possible pathway for the formation of 5 and borylfuran
([Fe]=Cp*Fe, HBpin=pinacolborane.
plex 4 with HBpin in C6D6 provided 72% of 2-boryl-thio-
phene or 68% of borylbenzene, with concomitant formation
of 5 in 86% (from 3) or 78% (from 4) yield.
[Cp*FeACHTUNGTRNEUNG
(LMe)H], and the hydride intermediate would react
with another equivalent of HBpin to give 5.
The borohydride complex 5 is a diamagnetic 18e complex
Formation of the Furyl Complex 2 from the Borohydride
Complex 5
1
of FeII, and its H NMR exhibited signals assignable to Cp*
and Fe-H-B at d=1.96 (15H) and ꢀ16.4 ppm (2H, w1/2
=
175 Hz), respectively. In the 11B NMR spectrum, a reso-
nance at d=36.3 ppm appeared, which is characteristic for a
borohydride group bound to a metal.[8,15] The crystal struc-
ture of 5 revealed that the H2Bpin ligand coordinates to
Treatment of a C6D6 solution of 5 with an excess of tert-bu-
tylethylene at 708C resulted in the formation of the borylal-
kane (tBuCH2CH2Bpin) and a new alkyl complex [Cp*Fe-
AHCTUNGTRENNUNG
(LMe)CH2CH2tBu] (6) (Scheme 7). A new set of paramag-
1
ꢀ
iron through Fe-H-B interactions (Figure 3). The Fe B
netic signals appeared in the H NMR at d=42.5 (Cp*), 7.6
bond length of 2.0158(14) ꢁ is in the range of known Fe-H-
(LMe), and ꢀ43.5 ppm (LMe), with chemical shifts similar to
B complexes (1.855(2)–2.304(21) ꢁ).[8,16] The Fe C
bond length of 5 (1.9357(13) ꢁ) is comparable to those for
1, 3, and 4 (1.935(4)–1.9403(18) ꢁ).
(carbene)
those for the Fe CH3 complex 1. Despite efforts to use vari-
ꢀ
ꢀ
ous conditions, isolation of 6 has been unsuccessful. Howev-
er, we were able to convert 6 quantitatively into a stable
CO adduct [Cp*FeACTHNUTRGNEUNG
(LMe)(CO)CH2CH2tBu] (7), which was
isolated in 92% yield based on 5. The CO adduct 7 was
characterized spectroscopically and by X-ray crystallograph-
Figure 3. Molecular structure of 5 with thermal ellipsoids at the 50%
ꢀ
probability level. Selected bond distances (ꢁ) and angles (8): Fe B=
ꢀ
ꢀ
ꢀ
2.0158(14), Fe C(1)=1.9357(13), Fe H(1)=1.499(17), Fe H(2)=
ꢀ
ꢀ
ꢀ
1.500(15),
B H(1)=1.382(15),
B H(2)=1.330(18),
C(1) N(1)=
ꢀ
1.3718(17), C(1) N(2)=1.3700(18), B-Fe-C(1)=94.66(5), O(1)-B-O(2)=
108.73(10), B-Fe-H(1)=43.3(5), B-Fe-H(2)=41.3(6), Fe-B-H(1)=
48.0(7), Fe-B-H(2)=48.1(6), N(1)-C(1)-N(2)=102.32(11).
A possible pathway for the reaction of 2 with HBpin is
shown in Scheme 6. We assume that this reaction proceeds
with retention of the FeII oxidation state, because oxidative
addition reaction is uncommon for FeII organo-iron com-
plexes. As illustrated in the bracket in Scheme 6, we propose
that the coordination of HBpin to iron would occur through
an Fe-H-B interaction, resulting in close proximity between
the boron atom and the furyl group. Sigma-bond metathesis
between the Fe-(C4H3O) group and HBpin could occur to
give 2-boryl-furan and a putative iron-hydride intermediate
Scheme 7.
Chem. Asian J. 2010, 5, 1657 – 1666
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1661

T. Hatanaka, Y. Ohki, and K. Tatsumi
FULL PAPERS
1
ic analysis. H NMR spectroscopy signals, characteristic for
multiple species. These results and the reactions of 1 with
iron-bound methylene protons, registered at d=1.00 and
0.86 ppm, and the alpha-methylene carbon signal was ob-
served at d=4.7 ppm in the 13C{1H} NMR spectrum. The
CO stretching frequency of n˜ =1855 cmꢀ1 in the IR spectrum
is lower than those of the analogous complexes [Cp*Fe-
arenes (Scheme 3) indicate that the iron-alkyl group is cru-
ꢀ
cial for C H bond activation.
A Possible Pathway for Catalytic Borylation
On the basis of the above results, we propose a possible cat-
alytic cycle in Scheme 8. One step in this proposed catalytic
cycle is the borylation of an iron-furyl group in 2 with
A
(n˜ =1889 cmꢀ1),
(LMes)(CO)I] (n˜ =
[CpFe-
A
ACHTUNGTRENNUNG
1938 cmꢀ1).[8,13n,17] Although the iron center is chiral, com-
plex 7 crystallizes in a centric space group (Pbca) in a 50:50
enantiomeric mixture. Three independent molecules are
found in the asymmetric unit, and one of these is shown in
ꢀ
Figure 4. The Fe CH2 bond lengths range from 2.078(2) to
ꢀ
2.081(2) ꢁ, and are comparable to the Fe C
A
lengths in [Cp*Fe(CO)2R] (2.057(3)–2.146(10) ꢁ, R=C3H7,
C3H6Br, CH2SiMe2OH, (CH2)4OSiMe2NPh2, h1-C5H5).[18]
Figure 4. Molecular structure of 7 with thermal ellipsoids at the 50%
probability level. One of three molecules found in an asymmetric unit is
ꢀ
shown. Selected bond distances (ꢁ) and angles (8): Fe(1) C(1)=
ꢀ
ꢀ
ꢀ
Scheme 8. A possible catalytic cycle ([Fe]=Cp*Fe, HBpin=pinacol-
borane).
1.710(2), C(1) O(1)=1.170(3), Fe(1) C(2)=2.081(2), C(2) C(3)=
ꢀ
ꢀ
ꢀ
1.525(3), Fe(1) C(8)=1.953(2), C(8) N(1)=1.376(2), C(8) N(2)=
1.368(2), C(1)-Fe(1)-C(2)=86.16(10), C(2)-Fe(1)-C(8)=95.91(9), C(8)-
Fe(1)-C(1)=94.11(8),
Fe(1)-C(1)-O(1)=179.1(2),
N(1)-C(8)-N(2)=
102.13(18).
HBpin, and formation of an iron-hydride intermediate
[Cp*FeACHTUNGTRNEUNG
(LMe)H]. This is followed by the insertion of tert-bu-
ꢀ
The alkyl complex 6 was probably formed by means of
dissociation of HBpin from 5 to generate the iron-hydride
tylethylene into the Fe H bond of the iron-hydride. The re-
sulting Fe-alkyl complex 6 abstracts a proton from the 2-po-
sition of furan to regenerate complex 2, which restarts the
cycle. Although we have not been able to observe [Cp*Fe-
intermediate [Cp*Fe
tert-butylethylene into the Fe H bond. The liberated HBpin
would react with to provide the alkylborane
(tBuCH2CH2Bpin) and iron-hydride [Cp*Fe
(LMe)H]. The
(LMe)H], followed by the insertion of
ACHTUNGTRENNUNG
ꢀ
6
(LMe)H], generation of this intermediate is in agreement
ACHTUNGTRENNUNG
with the formation of 5 from 2+HBpin and the formation
of 6 from 5+tert-butylethylene. The Fe-alkyl complex 6 pro-
motes the C H bond activation of furan, as shown in
Scheme 7, whereas the borohydride complex 5 does not acti-
N
iron-hydride species is again transformed into 6 in the pres-
ence of excess tert-butylethylene. As one can expect, treat-
ment of 6 with excess HBpin resulted in the formation of 5
and alkylborane (tBuCH2CH2Bpin).
ꢀ
ꢀ
vate the C H bond of furan, but gradually decomposes
upon heating with excess furan.
Complex 6, generated from 5 and tert-butylethylene, was
ꢀ
found to promote the C H bond cleavage of furan at room
Borohydride complex 5 is more stable than [Cp*Fe-
temperature to give rise to complex 2 and 2,2-dimethylbu-
tane (Scheme 7). The sequential formation of 2 from 5, by
way of 6, suggests that tert-butylethylene serves not only as
a hydrogen scavenger, but also as the source for the iron-
(LMe)H], and the iron-hydride intermediate opts for com-
ACHTUNGTRENNUNG
plexation with HBpin to form 5 rather than the reaction
with TBE to generate 6. Thus, to facilitate the formation of
6 from 5, which is essential for the catalytic cycle, the con-
centration of HBpin needs to be low, as was described earli-
er in this paper. A low concentration of HBpin is also im-
portant to reduce the reaction between 6 and HBpin that
wastes both tert-butylethylene and HBpin by hydroboryla-
tion.
ꢀ
alkyl group in 6, which is responsible for the C H bond acti-
vation of furan. In contrast to the facile formation of 2 from
the alkyl complex 6, treatment of a C6D6 solution of the bor-
ohydride complex 5 with 100 equiv of furan or thiophene at
608C led to gradual decomposition of the iron complex into
1662
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1657 – 1666

ꢀ
C H Bond Activation of Furans and Thiophenes
Cp*), 8.4 (6H, LMe), ꢀ47.5 ppm (6H, LMe); elemental analysis (%) calcd
Conclusions
for C18H30N2Fe: C 65.46, H 9.16, N 8.48; found: C 65.83, H 8.97, N 8.35.
Synthesis of [Cp*FeACHTNUGTRENNUG )ACHTNUGTERN(NUNG 2-C4H3O)] (2): A toluene (7 mL) solution of 1
(LMe
A coordinatively unsaturated Cp*Fe complex 1 was found
to catalyze the dehydrogenative coupling of furans/thio-
phenes with pinacolborane (HBpin) in the presence of tert-
butylethylene. Although Ir[19], Rh[19b,19i,20], and Re[21] cata-
lysts have been reported, this is the first example of an iron-
(56 mg, 0.17 mmol) and furan (49 mL, 0.67 mmol) was stirred at room
temperature for 1 day. The solvent was removed under reduced pressure,
and the residue was extracted with hexane/Et2O (12:3 mL) and centri-
fuged. The solution was concentrated to ꢁ1 mL, and cooled at ꢀ308C to
give [Cp*FeACHTNUTRGENNUG )ACHTUNGTRNEN(UNG 2-C4H3O)] (2) as brownish-green crystals (48 mg,
(LMe
0.13 mmol, 74%). 1H NMR (C6D6): d=47.0 (15H, Cp*), 38.0 (1H,
C4H3O), 7.6 (6H, LMe), ꢀ38.3 (6H, LMe), ꢀ69.4 (1H, C4H3O), ꢀ93.0 ppm
(1H, C4H3O); elemental analysis (%) calcd for C21H30N2OFe: C 65.97,
H 7.91, N 7.33; found: C 65.65, H 8.02, N 7.27.
ꢀ
catalyzed borylation of aromatic C H bonds. The borylation
of furans and thiophenes occurred regio-selectively at the
less hindered 2- or 5-positions. The N-heterocyclic carbene
ligand remained intact during the course of the catalytic
processes. An advantage of N-heterocyclic carbene ligands
is their strong s-donating ability, which leads to strong bond-
ing between various metals.[22]
Synthesis of [Cp*Fe
ogous to that for 2. The reaction of 1 (66 mg, 0.20 mmol) with thiophene
(32 mL, 0.40 mmol) at 608C for 3 days gave [Cp*Fe (2-C4H3S)] (3) as
(LMe
(LMe
ACHTUNGTRENNNUG )ACHTUNGTREG(NUNN 2-C4H3S)] (3): The synthetic procedure is anal-
A
)ACHTUNGTRENNUNG
1
brownish-green crystals (42 mg, 0.11 mmol, 53% yield). H NMR (C6D6):
d=47.0 (15H, Cp*), 20.3 (1H, C4H3S), 8.6 (6H, LMe), ꢀ21.5 (1H,
C4H3S), ꢀ38.8 (6H, LMe), ꢀ66.1 ppm (1H, C4H3S); elemental analysis
(%) calcd for C21H30N2SFe: C 63.31, H 7.59, N 7.03, S 8.05; found:
C 63.20, H 7.544, N 7.07, S 7.57.
A series of reactions, including complexes 1 and 6 with
furan, complex 2 with HBpin, and complex 5 with tert-butyl-
ethylene under heating, have provided insight into the cata-
Synthesis of [Cp*FeACTHNUTRGNEUNG
(LMe)Ph] (4): Method A, the reaction of 1 with ben-
ꢀ
lytic pathway: a) The iron complexes responsible for C H
zene: A benzene (10 mL, 0.11 mol) solution of 1 (65 mg, 0.20 mmol) was
stirred at 808C for 7 days. The solvent was removed under reduced pres-
sure, and the residue was extracted with Et2O (7 mL) and centrifuged.
The solution was concentrated to ca. 1 mL, and cooled at ꢀ308C to give
[Cp*FeLMePh] (4) as reddish-orange crystals (28 mg, 0.071 mmol, 36%
bond activation are the Fe-alkyl complexes 1 and 6. b) The
reactions of the Fe-aryl complexes with HBpin provide
boryl-arenes and the hydride intermediate [Cp*FeACTHNUTRGNEUNG
(LMe)H],
which is trapped by HBpin to form the borohydride com-
plex 5. c) The additive tert-butylethylene serves not only as
a hydrogen scavenger, but also as the source for the alkyl
group in 6, which is generated by means of insertion of tert-
ꢀ
1
yield). H NMR (C6D6): d=50.1 (15H, Ph), 15.0 (1H, Ph), 7.9 (6H, LMe),
2.8 (2H, Ph), ꢀ9.4 (2H, Ph), ꢀ42.4 ppm (6H, LMe); elemental analysis
(%) calcd for C34H40N2Fe: C 70.41, H 8.22, N 7.14; found: C 70.81,
H 8.45, N 7.21 ppm. Method B, the reaction of [Cp*Fe
ACHTUNGTREN(NGNU TMEDA)Cl] with
butylethylene into the Fe H bond of [Cp*FeACTHNUTRGNEUNG
(LMe)H].
phenyllithium: Analogous to the synthesis of 1, the reaction of [Cp*Fe-
AHCTUNGTERG(NNNU TMEDA)Cl] (475 mg, 1.39 mmol) with a Bu2O solution of phenyllithium
(1.9m, 0.73 mL, 1.39 mmol) and LMe (172 mg, 1.38 mmol) in toluene
(50 mL) yielded 4 (183 mg, 0.47 mmol, 34%) as reddish-orange crystals.
Experimental Section
Kinetic Study of the Reaction of 1 with Furan
General Procedures
Furan (900 mL, 12.4 mmol) was added to a C6D6 (4.0 mL) solution of 1
(41 mg, 0.12 mmol) and C6Me6 (20 mg, 0.12 mmol, internal standard).
Portions of this solution were charged into seven NMR tubes, and the
tubes were capped with a J-Young valve. The Cp* signal of 1 (d 41.8 at
All manipulation was carried out under a nitrogen or argon atmosphere
by using standard Schlenk techniques and/or in a glove box. THF, tolu-
ene, Et2O, benzene, and hexane were purified by the method of
Grubbs,[23] by which the solvents were passed over columns of activated
alumina and a supported copper catalyst supplied by Hansen & Co. Ltd.
Fluorobenzene was dried over calcium hydride, and distilled prior to use.
Deuterated benzene (C6D6) was vacuum-transferred from sodium prior
to use. 1H, 11B{1H}, and 13C{1H} NMR spectra were acquired by using a
JEOL ECA-600. The 1H NMR signals were referenced to the residual
proton peak of the deuterated solvent. The 11B chemical shift was mea-
sured relative to an external reference of (Me3N)BH3 in C6D6 (d=
ꢀ9.1 ppm). The 13C chemical shifts were measured relative to the carbon
signals for the deuterated solvents. Infrared spectra were recorded by
using a JASCO A3 spectrometer. Elemental analyses were performed by
using a LECO-CHNS-932 elemental analyzer in which the air-sensitive
crystalline samples were sealed in silver capsules under nitrogen. X-ray
diffraction data were collected by using a Rigaku AFC8 or a Rigaku FR-
1
258C) was monitored by H NMR spectroscopy at 10–508C and was inte-
grated at intervals. Rate constants k were determined to be 8.93(13)ꢂ
10ꢀ5 (108C), 1.20(15)ꢂ10ꢀ4 (158C), 1.50(4)ꢂ10ꢀ4 (208C), 2.27(8)ꢂ10ꢀ4
(258C), 2.93(5)ꢂ10ꢀ4 (308C), 4.84(8)ꢂ10ꢀ4 (408C), and 7.60(12)ꢂ10ꢀ4
(508C). An Eyring plot (see the Supporting Information) yielded the ac-
tivation parameters, DH° =9.3(2) kcalmolꢀ1 and DS° =ꢀ44.2(8) eu.
Kinetic Isotope Effect (KIE) for the Reaction of 1 with Furan
Deuterated furan (178 mL, 2.45 mmol) was added to an NMR tube con-
taining a C6D6 (0.80 mL) solution of 1 (8.1 mg, 0.025 mmol) and C6Me6
(4.0 mg, 0.025 mmol, internal standard). The Cp* signal of 1 was moni-
tored periodically by 1H NMR at 258C, and the rate constant was ob-
tained as kD =5.97(7)ꢂ10ꢀ5 (258C). kH/kD was found to be 3.8(2).
Typical Procedure for the Catalytic Dehydrogenative Coupling between
Furans/Thiophenes and Pinacolborane
E equipped with a CCD area detector using graphite-monochromated
[11]
MoKa radiation. [Cp*Fe
(TMEDA)Cl]
and LMe (1,3,4,5-tetramethyl-
imidazol-2-ylidene)[24] were prepared according to literature procedures.
Synthesis of [Cp*Fe
(LMe)Me] (1): An Et2O solution of methyllithium
(0.96m, 1.15 mL, 1.10 mmol) was added to a THF (40 mL) solution of
[Cp*Fe
A benzene (8 mL) solution containing 1 (19 mg, 0.058 mmol), furans or
thiophenes, tert-butylethylene, and C6Me6 (19 mg, 0.12 mmol, as an inter-
nal standard), was charged into a Schlenk tube with a J-Young valve. The
reaction mixture was stirred for 1 hour at 60 or 708C before addition of
the first portion (500 mL) of a benzene (5.0 mL) solution containing pina-
colborane (83.5 mL, 0.58 mmol). The remaining nine portions (9ꢂ500 mL)
of pinacolborane solution were added successively at 1 hour intervals.
The reaction mixture was further stirred for 1 h at 60 or 708C after com-
pletion of the addition of pinacolborane, and thus the total reaction time
was 11 h. The volatiles were removed under reduced pressure, and the
residue was dissolved in C6D6. The product yield was determined by
1H NMR spectroscopy, using C6Me6 as an internal standard. The product
ACHTUNGTRENNUNG
(TMEDA)Cl] (380 mg, 1.11 mmol) at ꢀ788C. The reaction mix-
ture was allowed to warm gradually to 08C with stirring, producing a
dark-bluish-green solution. A THF (8 mL) solution of LMe (138 mg,
1.11 mmol) was added to the reaction mixture. After warming to room
temperature, the reaction mixture was stirred for several hours. The sol-
vent was removed under reduced pressure, and the residue was extracted
with hexane (40 mL) and centrifuged. The solution was concentrated to
ꢁ15 mL, and cooled at ꢀ308C to give [Cp*Fe
(LMe)Me] (1) as orange
ACHTUNGTRENNUNG
1
crystals (199 mg, 0.60 mmol, 54% yield). H NMR (C6D6): d=41.8 (15H,
Chem. Asian J. 2010, 5, 1657 – 1666
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1663

T. Hatanaka, Y. Ohki, and K. Tatsumi
FULL PAPERS
was isolated by Kugelrohr distillation or by column chromatography on
silica gel eluted with CH2Cl2. Known organic products were identified by
comparison of the NMR signals with those for authentic samples,[19f–h,j,25]
and unreported compounds were characterized based by using NMR
spectroscopy, EI-MS, and elemental analysis. Detailed procedures and
characterization of compounds are found in the Supporting Information.
evaporated under reduced pressure to remove unreacted tert-butylethy-
lene. The residue was again dissolved in C6D6 (0.6 mL), and the solution
was charged into an NMR tube. Addition of pinacolborane (6.5 mL,
0.045 mmol) gave 5 and tBuCH2CH2Bpin in >99% and 81% yields, re-
spectively, based on 6.
Synthesis of [Cp*FeLMe(CO)(CH2CH2tBu)] (7).
A toluene solution
Synthesis of [Cp*Fe
A
)
E
(10 mL) of (82 mg, 0.19 mmol) and tert-butylethylene (1.2 mL,
5
borane: Pinacolborane (109 mL, 75 mmol) was added to a THF (10 mL)
solution of 1 (83 mg, 0.25 mmol) at room temperature. After stirring for
1 hour, the solvent was removed under reduced pressure. The residue
was extracted with hexane (15 mL) and centrifuged. The solution was
9.3 mmol) was stirred for 4 h at 708C. After cooling to room temperature,
the reaction mixture was exposed to 1 atm of CO. The color of solution
became orange. After stirring for 2 h, the volatile materials were re-
moved under reduced pressure. Column chromatography on neutral alu-
mina, eluted with hexane under an argon atmosphere, provided an
orange eluent of [Cp*FeLMe(CO)(CH2CH2tBu)] (7). Complex 7 was iso-
lated as an orange powder (73 mg, 0.17 mmol, 92% yield). Orange crys-
tals of 7 were obtained from a hexane solution stored at ꢀ208C (25 mg,
concentrated to ꢁ5 mL, and cooled at 08C to give [Cp*Fe
(LMe
ACHTUNGTRENNUGN )ACHTUNGTREG(NNUN H2Bpin)]
(5) as purple crystals (90 mg, 0.20 mmol, 81% yield). 1H NMR (C6D6):
d=3.64 (s, 6H, LMe), 1.96 (s, 15H, Cp*), 1.43 (s, 6H, LMe), 1.33 (s, 6H,
Bpin), 1.04 (s, 6H, Bpin), ꢀ16.4 ppm (br, w1/2 =175 Hz, 2H, Fe-H-B);
11B{1H} NMR (C6D6): d=36.3 ppm (w1/2 =190 Hz). 13C{1H} NMR (C6D6):
d=203.6 (CN2C2Me2), 125.1 (CN2C2Me2), 82.9 (C5Me5), 81.0, 80.2 (Bpin),
36.3 (CN2(Me)2C2Me2), 25.5, 25.3 (Bpin), 12.3 (C5Me5), 9.7 ppm
(CN2C2Me2); elemental analysis (%) calcd for C23H41BN2O2Fe: C 62.18,
H 9.30, N 6.31; found: C 62.30, H 8.82, N 6.31.
1
31% yield). H NMR (C6D6): d=3.55 (s, 3H, LMe), 3.39 (s, 3H, LMe), 1.62
(s, 15H, Cp*), 1.54 (dt, JHH =7.7, 4.1 Hz, 1H, tBu-CH2-), 1.49 (s, 3H,
LMe), 1.33 (s, 3H, LMe), 1.08 (dt, JHH =7.7, 4.1 Hz, 1H, tBu-CH2-), 1.00
(ddd, JHH =13.1, 7.7, 4.1 Hz, 1H, Fe-CH2-), 0.86 ppm (ddd, JHH =13.1, 7.7,
4.1 Hz, 1H, Fe-CH2-); 13C{1H} NMR (C6D6): d=228.3 (CO), 199.7
(CN2C2), 125.9 (CN2C2), 125.4 (CN2C2), 89.9 (C5Me5), 52.1 (CMe3), 35.9
(CN2(Me)2C2(Me)2), 34.9 (CN2(Me)2C2(Me)2), 32.2 (tBu-CH2), 30.6
(CMe3), 10.5 (C5Me5), 9.8 (CN2(Me)2C2(Me)2), 9.5 (CN2(Me)2C2(Me)2),
4.7 ppm (Fe-CH2); IR (KBr): n˜ =1855 cmꢀ1 (s, CO); elemental analysis
(%) calcd for C24H40N2OFe: C 67.28, H 9.41, N 6.54; found: C 66.85, H
9.29, N 6.32.
Method B, from 2 and pinacolborane in an NMR tube: The reaction of 2
(5.2 mg, 0.014 mmol) with pinacolborane (8.0 mL, 0.030 mmol) in C6D6
(0.6 mL, with 4 mg of C6Me6 (0.025 mmol) as an internal standard) yield-
ed 2-borylfuran and 5 in >99% and >99%, respectively.
Method C, from 3 and pinacolborane in an NMR tube: The reaction of 3
(10 mg, 0.025 mmol) with pinacolborane (8.0 mL, 0.055 mmol) in C6D6
(0.6 mL, with 4 mg of C6Me6 (0.025 mmol) as an internal standard) gave
2-borylthiophene and 5 in 72% and 86%, respectively. Method D, from 4
Degradation of 5 in the Presence of Furan or Thiophene
and pinacolborane in an NMR tube: The reaction of
4 (12 mg,
Furan (164 mL, 2.26 mmol) was added to a C6D6 (1.6 mL) solution of 5
(10 mg, 0.023 mmol) and hexamethylbenzene (2.5 mg, 0.015 mmol, inter-
nal standard), and a portion of this solution was charged into an NMR
tube. A C6D6 (1.6 mL) solution containing 5 (10 mg), thiophene (180 mL,
2.25 mmol), and C6Me6 (2.5 mg) was prepared in a similar manner, and
was also introduced into an NMR tube. The tubes were kept at 608C in
the NMR probe. The reaction mixtures gradually exhibited unassigned
signals in the 1H NMR, indicating degradation of the iron complex. The
half-lives (t1/2) of 5 were 22 min (with furan) and 139 min (with thio-
phene).
0.031 mmol) with pinacollborane (9.5 mL, 0.065 mmol) in C6D6 (0.6 mL,
with 4 mg of C6Me6 (0.025 mmol) as an internal standard) yielded boryl-
benzene and 5 in 68% and 78% yields, respectively.
NMR Observation of Iron Complexes in the Catalytic Reaction Mixture
As described in the procedure for catalytic reactions, a C6D6 (8 mL) solu-
tion containing 1 (19 mg, 0.058 mmol), furan (251 mL, 3.45 mmol), tert-bu-
tylethylene (148 mL, 1.15 mmol), and C6Me6 (19 mg, 0.12 mmol, internal
standard), was charged into a Schlenk tube, and the solution was stirred
for 1 hour at 608C. After addition of 500 mL of a benzene (5.0 mL) solu-
tion containing pinacolborane (83.5 mL, 0.58 mmol), the solution was
again stirred at 608C. Portions of the reaction mixture were periodically
taken out and were subjected to NMR analysis. Complexes 2 and 5 were
observed in the 1H NMR. Their ratios after addition of pinacolborane
were as follows: 2:5=1.8:1 (3 min), 2.4:1 (10 min), 5.6:1 (25 min), 12.3:1
(50 min).
X-ray Crystal Structure Determination
Crystal data and refinement parameters for 1, 3--5, and 7 are summarized
in Table 4. Preliminary crystallographic study on 2 is given in the Sup-
porting Information. Single crystals were coated with oil (Immersion Oil,
type B: Code 1248, Cargille Laboratories, Inc.) and mounted on loops.
Diffraction data were collected at ꢀ1008C under a cold nitrogen stream
by using a Rigaku AFC8 equipped with a Saturn70 CCD detector or on
a Rigaku FR-E equipped with a Saturn70 CCD detector using graphite-
monochromated MoKa radiation (l=0.710690 ꢁ). Six preliminary data
frames were measured at 0.58 increments of w, to assess the crystal quali-
ty and preliminary unit cell parameters. The intensity images were also
measured at 0.58 intervals of w. The frame data were integrated using the
CrystalClear program package, and the data sets were corrected for ab-
sorption using a REQAB program. The calculations were performed
with the CrystalStructure program package. All structures were solved by
direct methods, and refined by full-matrix least squares. Anisotropic re-
finement was applied to all non-hydrogen atoms except for disordered
groups in 1, 3, and 7 (refined isotropically), and all hydrogen atoms were
put at calculated positions. In the asymmetric unit of 1, there are two
crystallographically independent molecules. The iron-bound methyl
group of one of these molecules is disordered over two positions, with
50:50 occupancy. The Cp* ligand in 3 is disordered over two equally oc-
cupied positions. The thienyl group in 3 is disordered over two positions,
with occupancy factors of 60:40. In an asymmetric unit of 7, there are
three crystallographically independent molecules. The Cp* ligand of one
of these molecules is disordered over two positions, with occupancy fac-
tors of 50:50. The tBu group in a different molecule is disordered over
two positions, with occupancy factors of 50:50. The atomic coordinates
for complexes for complexes 1, 3–5, and 7 have been deposited with the
Formation of [Cp*FeACHTUNGTRENNUNG
(LMe)CH2CH2tBu] (6) from 5 and tert-butylethylene:
A C6D6 solution (0.60 mL) containing 5 (10 mg, 0.023 mmol) and tert-bu-
tylethylene (29 mL, 0.23 mmol) was charged into an NMR tube, and a
glass capillary containing a C6D6 solution of ferrocene (0.25m) was insert-
ed as an internal standard. The NMR tube was heated at 708C for 3 h.
1
Analysis of the H NMR spectra revealed one set of paramagnetic signals
assignable to [Cp*FeACHTUNGTRENNUNG
(LMe)CH2CH2tBu] (6) at d=42.5 (15H, Cp*), 14.0
(9H, tBu), 7.6 (6H, LMe), 1.79 (2H, CH2CH2tBu), and ꢀ43.5 ppm (6H,
LMe), although we were not able to observe the signal for Fe CH2 group.
ꢀ
The signals for alkylborane (tBuCH2CH2Bpin) were observed at d=1.07
(12H, Bpin), 1.02 (2H, CH2), 0.86 (9H, tBu), 0.83 ppm (2H, CH2). The
yields of 6 and tBuCH2CH2Bpin in these NMR experiments were deter-
mined, as 90% and 97%, respectively.
Reaction of 6 with furan: Furan (6.5 mL, 0.089 mmol) was added to the
NMR tube used for the formation of 6. The tube was stood for 2 days at
room temperature, and the furyl complex 2 and 2,2-dimethylbutane were
formed in 90% and 99% yields, respectively, based on 6. The 1H NMR
signals for 2,2-dimethylbutane were observed at d=1.17 (q, 2H, CH2),
0.84 (s, 9H, tBu), 0.80 ppm (t, 3H, CH3).
Reaction of 6 with pinacolborane: Complex 6 was generated from 5
(10 mg, 0.023 mmol) and tert-butylethylene (29 mL, 0.23 mmol) in C6D6
(0.60 mL, with 4 mg of C6Me6 (0.025 mmol) as an internal standard), as
described above. This solution was transferred to a Schlenk tube and was
1664
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Chem. Asian J. 2010, 5, 1657 – 1666

ꢀ
C H Bond Activation of Furans and Thiophenes
Table 4. Crystal data for [Cp*Fe
(LMe)(CO)(CH2CH2tBu)] (7).
(LMe)Me] (1), [Cp*Fe(LMe (LMe)Ph] (4), [Cp*Fe(LMe
ACHTUNGTRENNUNG AHCNUTTGREUNNNG )ACHTUTGNRENGU(N 2-C4H3S)] (3), [Cp*FeAHTCUNGTRENNUGN HCATUNGTRENNNUG )CATHUNGTREN(NUNG H2Bpin)] (5), and [Cp*Fe-
ACHTUNGTRENNUNG
1
3
4
5
7
formula
C18H30N2Fe
330.29
C21H30N2SFe
398.39
C23H32N2Fe
392.37
C23H41BN2O2Fe
444.25
C24H40N2OFe
428.44
Mw [gmolꢀ1
]
crystal system
space group
crystal color
crystal size [mm]
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
b [8]
triclinic
P1 (#2)
orthorhombic
Pnma (#62)
yellowish green
0.1ꢂ0.05ꢂ0.05
19.803(3)
monoclinic
P21/n (#14)
yellowish brown
0.1ꢂ0.1ꢂ0.07
10.517(2)
monoclinic
P21/n (#14)
reddish purple
0.1ꢂ0.1ꢂ0.03
11.7529(18)
10.8939(15)
18.858(3)
orthorhombic
Pbca (#61)
orange
0.2ꢂ0.2ꢂ0.03
14.8602(16)
20.808(2)
¯
orange
0.2ꢂ0.08ꢂ0.02
10.618(3)
12.191(3)
14.535(4)
90.417(6)
91.075(5)
101.695(5)
1842.0(9)
4
1.191
8.15
54.8
22215
8317
435
0.098
0.181
11.6494(17)
9.3090(12)
14.033(3)
15.043(3)
46.856(5)
107.427(3)
96.122(2)
g [8]
V [ꢁ3]
2147.5(5)
4
1.232
8.04
2118.2(7)
4
2400.6(6)
4
14488(3)
24
1.178
6.39
Z
1calc [gcmꢀ3
]
1.230
7.20
55.0
16794
4802
267
1.229
6.48
55.0
18612
5359
309
m
N
]
54.9
55.0
no. of measured rflns
no. of unique rflns
no. of variables
R1[a]
16673
2529
126
0.077
0.237
1.19
128198
16510
750
0.065
0.163
1.07
0.046
0.080
1.04
2
0.036
0.088
1.01
2
[b]
wR2
GOF[c]
1.01
2
[a] I>2s(I), R1=SjjFo jꢀjFc jj/SjFo j. [b] Refined with all data, wR2=[{Sw
Np denote the numbers of reflection data and parameters.
(Fo -Fc2)2}/Sw
N
G
N
Cambridge Crystallographic Data Centre, Cambridge CB2 1EK, UK , re-
spectively). CCDC 766277 (1), CCDC 766282 (2), CCDC 766278 (3),
CCDC 766279 (4), CCDC 766280 (5), and CCDC 766281 (7) contain the
supplementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
at www.ccdc.cam.ac.uk/data_request/cif
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Acknowledgements
This research was financially supported by Grant-in-Aids for Scientific
Research (No. 18GS0207, 18064009, and 20613004) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan. We thank
Prof. Roger E. Cramer at the University of Hawaii for fruitful discussion
and for careful reading of the manuscript.
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Received: February 20, 2010
Published online: June 10, 2010
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Chem. Asian J. 2010, 5, 1657 – 1666