Versatile friedel-crafts-type alkylation of benzene derivatives using a molybdenum complex/ortho-chloranil catalytic system

Article abstract of DOI:10.1002/chem.200801105

A variety of molybdenum complexes catalyze Friedel-Crafts-type alkylation reactions of benzene derivatives with alkenes and alcohols in the presence of an organic oxidant, o-chloranil. The utilization of [Mo(CO)₆] and two equivalents of o-chloranil catalytically furnished the hydroarylation product of norbornene with p-xylene at 80°C, whereas [Cr(CO)₆] and [W(CO)₆] failed to catalyze the same reaction, thus indicating the importance of the molybdenum source. The best results were obtained when a molybdenum(II) complex [CpMoCl(CO)₃] (Cp = cyclopentadienyl) was used as a precatalyst. The hydroarylation reactions also took place with styrenes, cyclohexenes, and 1 -hexene as olefin substrates. The electrophilic-substitution mechanism was proposed on the basis of the ortho/para selectivities and the Markovnikov selectivities observed for the hydroarylation products. Our hypothesis was further corroborated by the fact that in the presence of the [CpMoCl(CO)₃]/o-chloranil catalytic system, secondary, benzylic, or allylic alcohols participated in the alkylation of benzenes with similar selectivities.

Full text of DOI:10.1002/chem.200801105

FULL PAPER
DOI: 10.1002/chem.200801105
Versatile Friedel–Crafts-Type Alkylation of Benzene Derivatives Using a
Molybdenum Complex/ortho-Chloranil Catalytic System
Yoshihiko Yamamoto* and Kouhei Itonaga[a]
Abstract: A variety of molybdenum
complexes catalyze Friedel–Crafts-type
alkylation reactions of benzene deriva-
tives with alkenes and alcohols in the
presence of an organic oxidant, o-chlor-
anil. The utilization of [Mo(CO)6] and
two equivalents of o-chloranil catalyti-
cally furnished the hydroarylation
product of norbornene with p-xylene at
tance of the molybdenum source. The
best results were obtained when a mo-
lybdenum(II) complex [CpMoCl(CO)3]
(Cp=cyclopentadienyl) was used as a
precatalyst. The hydroarylation reac-
tions also took place with styrenes, cy-
clohexenes, and 1-hexene as olefin sub-
strates. The electrophilic-substitution
mechanism was proposed on the basis
of the ortho/para selectivities and the
Markovnikov selectivities observed for
the hydroarylation products. Our hy-
pothesis was further corroborated by
the fact that in the presence of the
[CpMoCl(CO)3]/o-chloranil
catalytic
system, secondary, benzylic, or allylic
alcohols participated in the alkylation
of benzenes with similar selectivities.
Keywords: Brønsted acid catalysts ·
carbenium ions · chloranil · Frie-
del–Crafts alkylation · molybdenum
808C,
whereas
[Cr(CO)6]
and
[W(CO)6] failed to catalyze the same
reaction, thus indicating the impor-
Introduction
chiometric amounts of strong bases results in the formation
of salt waste. Moreover, this method is incompatible with a
wide range of reactive functional groups. To resolve these
The development of an efficient method for the alkylation
of benzenes has been of prime importance in organic syn-
thesis because of the ubiquity of substituted benzene deriva-
tives in natural products, pharmaceuticals, and functional
materials. The Friedel–Crafts alkylation is one of the most
common methods for the direct alkylation of aromatic
rings.[1] Conventional Friedel–Crafts alkylation reactions
generally require stoichiometric amounts of Lewis or
Brønsted acid promoters, which produce a large amount of
salt waste after neutralization; hence, considerable effort
has been devoted to the development of an alternative envi-
ronmentally friendly catalytic process.
À
issues, the transition-metal-catalyzed direct C H alkylation
of benzenes with olefins has been utilized since the first
truly efficient catalysts were identified by Murai et al.[3] Al-
though catalytic C H alkylation reactions enable the intro-
duction of an alkyl chain under neutral conditions, most ex-
isting methods require aromatic substrates bearing a direct-
ing group and/or specific olefins, thereby limiting the scope
of this methodology.[4,5]
The strong demand for an environmentally friendly pro-
cess that is capable of yielding alkylbenzenes with a wide
scope has led to the recent innovative development of cata-
lytic Friedel–Crafts alkylation reactions. In particular, metal
trifluoromethanesulfonates (triflates) have served as effec-
tive Lewis acid catalysts in Friedel–Crafts alkylation reac-
tions. Kobayashi and co-workers employed hafnium(IV) tri-
flate together with LiClO4 in nitromethane as the solvent
for Friedel–Crafts reactions.[6] In their method, alkylben-
zenes and the parent benzene were alkylated with benzyl
and tert-alkyl chlorides in good yields, even at room temper-
ature.[6b] The study by Kobayashi and co-workers also re-
À
Another promising method is the ortho metalation/alkyla-
tion approach, which provides access to ortho-substituted
benzene derivatives with precise regiocontrol.[2] However,
this strategy also has certain disadvantages. The use of stoi-
[a] Prof. Dr. Y. Yamamoto, K. Itonaga
Department of Applied Chemistry
Graduate School of Science and Engineering
Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152–8552 (Japan)
Fax : (+81)3-5734-3339
E-mail: omyy@apc.titech.ac.jp
vealed that scandium
ACHTUNGTRENN(UNG III) triflate acted as a catalyst in the
Friedel–Crafts acylation.[7] Yamamoto and co-workers syn-
thesized a-tocopherol by ScACTHNUTRGNE(UNG OTf)3-catalyzed cyclocoupling
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801105.
of hydroquinone and allylic alcohol precursors.[8] Fukuzawa
Chem. Eur. J. 2008, 14, 10705 – 10715
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10705

and co-workers further established the general scope of the
Sc(OTf)3-catalyzed Friedel–Crafts benzylation and allylation
nil. This finding is in striking contrast to the report by Shi-
mizu and co-workers, which stated that [Mo(CO)6] catalyzed
the hydroarylation of styrenes and cyclohexenes with ani-
sole.[12d] Hence, in this study, we focused our attention on
this unique catalytic system, which is a combination of a mo-
lybdenum complex and o-chloranil, for the alkylation of
benzenes. The obtained results, including the synthetic scope
and a plausible mechanistic scenario, will be presented
herein.
ACHTUNGTRENNUNG
of substituted benzene derivatives, in which environmentally
friendly benzyl and allyl alcohols can be used as replace-
ments for the corresponding halides.[9] Since then, various
combinations of Lewis acid catalysts and alkylating agents
have been utilized for the Friedel–Crafts alkylation reac-
tions of benzene derivatives,[10] especially in methods that
use late-transition-metal catalysis.[11,12] The catalytic Friedel–
Crafts alkylation reactions have been further extended to
asymmetric versions with chiral catalysts.[13] In this context,
it should be noted that the conventional method of Brønsted
acid catalysis has also gained renewed interest as a metal-
free process.[14]
In contrast to the above catalytic methods that utilize al-
cohols, halides, and their congeners as alkylating agents, the
development of catalytic Friedel–Crafts alkylation reactions
that enable the use of simple olefins with predictable
chemo- and regioselectivities still remains a challenging sub-
ject in organic chemistry. To this end, certain intramolecular
reactions have been accomplished recently by means of car-
bophilic Lewis acid catalysts.[15] However, the development
of efficient and selective intermolecular variants is a formi-
dable task, although a wide variety of both aromatic and
olefinic substrates can be utilized. To the best of our knowl-
edge, to date, there are only a few selective Friedel–Crafts
alkylation catalysts with limited olefins. The hydroarylation
of olefins has been recognized as a side reaction of transi-
Results and Discussion
Optimization of the catalytic system: To develop an optimal
catalytic system, we first examined several Group 6 transi-
tion-metal complexes as precatalysts (Scheme 1 and
Scheme 1. Reaction of norbornene with p-xylene.
Table 1). A solution of norbornene (1 mmol) in p-xylene
(6 mL) was heated at 808C for 24 h in the presence of cata-
lytic amounts of the precatalyst (10 mol%) and o-chloranil
(20 mol%). The use of [Mo(CO)6] gave the expected adduct
1a, albeit in 20% yield (Table 1, entry 1). Although the
yield was not considerable, it indicated the significance of
the molybdenum complex. Notably, compared to [Mo(CO)6]
the related complexes [Cr(CO)6] and [W(CO)6] exhibited no
catalytic ability at all (Table 1, entries 2 and 3). In the ab-
sence of the molybdenum complex, no hydroarylation prod-
´
tion-metal-promoted olefin metathesis, and Szymanska-
Buzar and co-workers reported the efficient hydroarylation
of a strained olefin, norbornene, with benzenes at room
[16]
À
temperature by means of a W Sn bimetallic catalyst. An-
other bimetallic catalyst that comprises Pt and Ag has also
been reported to be efficient for the hydroarylation of
simple olefins such as propene, cyclohexene, and cyclopen-
tene, in addition to norbornene.[17] There have also been a
few cases that have utilized more reactive alkenes, such as
styrenes, 1,3-dienes, and methyl vinyl ketone, for the catalyt-
ic Friedel–Crafts-type alkylation of benzenes.[18]
Table 1. Influence of Group 6 transition-metal precatalysts and quino-
nes.[a]
We previously reported the unprecedented single-step as-
sembly of a palladium(IV) complex with a novel palladas-
pirocycle framework from commercially available [Pd2-
Entry
[MLn]
Quinone (mol%)
Product Yield
[%]
ACHTUNGTRENNUNG(dba)3] (dba=trans,trans-dibenzylideneacetone), o-chloranil,
1
2
3
4
5
6
7
8
[Mo(CO)6]
[Cr(CO)6]
[W(CO)6]
–
o-chloranil (20)
o-chloranil (20)
o-chloranil (20)
o-chloranil (20)
o-chloranil (20)
o-chloranil (20)
o-chloranil (20)
o-chloranil (20)
1a
nd
nd
2
1a
1a
1a,
1a
20
and norbornene.[19] Interestingly, the spiropalladacycle was
formed though the twofold oxidative cyclization of each
molecule of norbornene and o-chloranil at the palladium
center. In extending this novel metallacyclic chemistry to
other transition-metal elements, we examined the reaction
of [Mo(CO)6] with norbornene and o-chloranil in benzene
at reflux to observe whether it was possible to obtain a
threefold oxidative cyclization. To our surprise, a small
amount of exo-2-phenylnorbornane was detected instead of
the anticipated metallacycle product. The presence of this
phenyl group was attributed to the solvent; furthermore, it
was concluded that the molybdenum species formed in situ
was involved in this hydroarylation because no reaction
took place in the absence of either [Mo(CO)6] or o-chlora-
trace
87
78
50
11
A
N
R
A
9
G
p-chloranil (20)
1a
trace
10
N
3,5-di-tert-butyl-1,2-benzo- nd
quinone (20)
11
12
13
U
–
nd
1a
1a
o-chloranil (10)
o-chloranil (30)
73
84
[a] Reagents and conditions: Norbornene: 1 mmol, [MLn]: 10 mol%,
p-xylene: 6 mL, 808C, 24 h. nd=not detected.
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Catalysis of Friedel–Crafts Alkylation Reactions
FULL PAPER
Table 2. Optimization of the [CpMoCl(CO)3]/o-chloranil-catalyzed reac-
tion of norbornene with p-xylene.[a]
uct was obtained, and instead a trace amount of 2, the
hetero-Diels–Alder cycloadduct between norbornene and o-
chloranil, was detected (Table 1, entry 4).[20] To improve the
yield, several readily accessible molybdenum complexes
were tested under the same conditions. Commercially avail-
able molybdenum(II) complexes, such as [CpMoCl(CO)3]
Entry
Catalyst
loading
AHCTUNGTRENGN[UN mol%]
Volume of
p-xylene
[mL]
T [8C]
t
Yield of
1a [%]
1
2
3
4
5
6
7
8
5
5
5
5
5
10
5
5
6
6
3
1
3
3
3
3
80
80
80
80
24 h
36 h
24 h
24 h
30 min
5 min
15 min
15 min
69
76
72
69
73
81
74
trace
(Cp=cyclopentadienyl) and [{MoACHTNUTRGNEUNG(OAc)2}2], proved to be ef-
ficient precatalysts providing 1a in 87 and 78% yield, re-
spectively (Table 1, entries 5 and 6). On the other hand, sim-
ilar molybdenum(II) complexes with Cp or p-allyl ligands,
such as [{CpMo(CO)3}2] and [(h-C3H5)MoCl(AN)2(CO)2]
(AN=MeCN), were less effective (Table 1, entries 7 and 8).
The correlation between the ligands and catalytic ability is
not clear at this stage.
138 (reflux)
150 (MW)
150 (MW)
90 (MW)
[a] Norbornene: 1 mmol, [CpMoCl(CO)3]/o-chloranil: 1:2. MW=micro-
wave irradiation.
Next, we examined the influence of quinones on catalytic
efficacy. The use of p-chloranil and 3,5-di-tert-butyl-1,2-ben-
zoquinone together with [CpMoCl(CO)3] hardly yielded 1a,
thus implying that both the o-quinone structure and the
electron-withdrawing chlorine substitution are indispensable
for efficient catalysis (Table 1, entries 9 and 10). The optimal
ratio of Mo/o-chloranil was identified as 1:2 (Table 1,
entry 5 versus entries 12 and 13), and no reaction occurred
in the absence of o-chloranil (Table 1, entry 11).
The other reaction parameters were also optimized
(Table 2). A lower catalyst loading of 5 mol% resulted in a
decrease in the yield (69%). Similar results were obtained
for longer reaction times or a decrease in the amount of the
solvent (Table 2, entries 1–4). At a lower temperature of
508C, the reaction proceeded very slowly, even for an in-
creased catalyst loading of 10 mol%, whereas an increase in
the temperature had a favorable effect on the hydroaryla-
tion. Upon heating the solution in p-xylene to reflux, 1a was
obtained in a comparably good yield of 73%, albeit with a
decreased reaction time of 30 minutes (Table 2, entry 5). En-
couraged by this result, we then applied microwave heating
to the present hydroarylation.
crowave heating conducted at 908C, but otherwise under
the same conditions, yielded only a trace amount of 1a
within 15 minutes (Table 2, entry 8).
Scope and limitations of the catalytic hydroarylation: The
results of the hydroarylation of norbornene with various
benzenes under the optimal conditions are compiled in
Table 3. Benzene and its substituted derivatives were sub-
jected to hydroarylation to furnish exo-2-arylnorbornanes
1b–g stereoselectively. Moderate-to-high yields of the isolat-
ed product were obtained by using either one of the two
heating methods. The low yields of 1b were ascribed to its
low boiling point and the low nucleophilicity of benzene
(Table 3, entries 1 and 2). Anisole afforded the correspond-
ing product 1e as mixtures of ortho and para isomers in the
ratio of o/pꢀ58:42 (Table 3, entries 7 and 8). On the other
hand, only one of the two possible isomers was formed in
the case of m-xylene, 4-methylanisole, and 4-chloroanisole
(Table 3, entries 3, 4, and 9–12). The absence of meta ad-
ducts suggests that an electrophilic-substitution mechanism
Microwave irradiation of a so-
Table 3.
Entry
[CpMoCl(CO)3]/o-chloranil-catalyzed reactions of norbornene with various benzenes.[a]
ACHTUNGTRENNUNG
lution of norbornene and the
catalyst (10 mol%) in xylene
at 1508C for 5 min in a sealed
vessel resulted in a high yield
of 81% (Table 2, entry 6).
Benzene[a]
T [8C]
t
Product
Yield [%]
1
2
benzene
benzene
80
24 h
15 min
1b
1b
31
48
150 (MW)
3
4
m-xylene
m-xylene
80
24 h
15 min
1c
1c
83
79
With
a catalyst loading of
150 (MW)
5 mol%, the reaction proceed-
ed efficiently within 15 min to
produce a yield comparable to
that obtained under conven-
tional heating conditions that
required a much longer reac-
tion time (Table 2, entry 7
versus entry 3). Although inter-
esting but controversial micro-
wave effects have been report-
ed,[21] the present acceleration
of hydroarylation can be attrib-
uted to simple thermal effects
because the reaction with mi-
5
6
mesitylene
mesitylene
80
24 h
15 min
1d
1d
75
52
150 (MW)
7
8
anisole
anisole
80
24 h
15 min
1e
1e
84[b]
76[b]
150 (MW)
9
10
4-methylanisole
4-methylanisole
80
24 h
15 min
1 f
1 f
67
76
150 (MW)
11
12
4-chloroanisole
4-chloroanisole
80
24 h
15 min
1g
1g
54
62
150 (MW)
[a] Norbornene: 1 mmol, [CpMoCl(CO)3]/o-chloranil: 1:2 (10 or 5 mol% for entries 7 and 8), p-xylene: 6 mL
(3 mL for MW). [b] The ortho/para isomer ratios were 58:42 for entries 7 and 8.
Chem. Eur. J. 2008, 14, 10705 – 10715
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10707

Y. Yamamoto and K. Itonaga
Table 4.
Entry
[CpMoCl(CO)3]/o-chloranil-catalyzed hydroarylation of styrenes.[a]
ACHTUNGTRENNUNG
is
involved
in
the
[CpMoCl(CO)3]/o-chloranil
Styrenes
T [8C]
t [h]
Product
Yields [%]
ortho/para
catalytic system (see below).
In contrast to the above
arenes, less nucleophilic chlor-
obenzene and aniline deriva-
tives, such as N,N-dimethylani-
line and acetanilide, failed to
undergo addition to norbor-
nene under the same condi-
tions.
1
2
150 (MW)
40
1
3a
3a
78
90
26:74
18:82
24
3
4
150 (MW)
150 (MW)
1
1
3b
3c
88
84
24:76
20:80
5
6
150 (MW)
40
1
3d
3d
82
80
2:98
24
0:100
7
8
150 (MW)
60
1
3e
3e
89
69
20:80
14:86
To evaluate the generality in
terms of the olefin substrate,
other alkenes were employed
for the hydroarylation. First,
the coupling of styrene and its
derivatives with anisole was
examined (Scheme 2) because
the use of [Mo(CO)6] to cata-
lyze this particular reaction
has already been reported.[12d]
Styrene was subjected to mi-
crowave-irradiation conditions
for 1 h to afford 3a in 78%
yield with an ortho/para ratio
of 26:74, which indicates that
24
9
10
11
150 (MW)
40
1
24
1
3 f
3 f
3g
85
83
78
0:100
0:100
22:78
150 (MW)
12
13
14
40
24
1
3g
3h
3h
74
89
77
11:89
20:80
14:86
150 (MW)
40
24
[a] Styrene: 1 mmol, [CpMoCl(CO)3]/o-chloranil: 1:2 (10 mol%), anisole: 6 mL (3 mL for MW). Ar=o- or
p-MeOC6H4.
lene, to furnish the corresponding products 3d–h in high
yields in favor of the para isomers (Table 4, entries 5, 7, 9,
11, and 13). It should be noted that the hydroarylation of
the styrenes with anisole also proceeded at 40–608C and the
corresponding products were obtained in comparable yields
to those obtained from microwave heating but with slightly
higher para selectivities (Table 4, entries 2, 6, 8, 10, 12, and
14). On the contrary, the reaction of styrene with p-xylene
resulted in a mixture of inseparable products that included
Scheme 2.
enes.
ACHTUNGTRENNUNG[CpMoCl(CO)3]/o-chloranil-catalyzed hydroarylation of styr-
the methoxyphenyl group was exclusively introduced at the
a position to the styrene phenyl group (Table 4, entry 1).
Similarly, p-chlorostyrene and p-methylstyrene produced 3b
and 3c in 88 and 84% yield, respectively, with similar iso-
meric ratios (Table 4, entries 3 and 4), whereas the use of p-
methoxystyrene resulted in a complex mixture as a result of
a self-reaction. The hydroarylation also proceeded efficient-
ly with 1-methyl- and 2-methylstyrene, diphenylethene, and
bicyclic analogues, such as indene and 1,2-dihydronaphtha-
the desired 5 (Scheme 3). This result is probably because p-
xylene has a rather low nucleophilicity that is comparable to
styrene. In addition, the hydroarylation was accompanied by
the dimerization of styrene, as exemplified by the reaction
conducted in chlorobenzene at 808C, which produced 1,3-di-
phenyl-1-butene in 11% yield (Scheme 3).[22]
The present catalytic hydroarylation is not limited to nor-
bornene and styrenes. We conducted experiments in which
cyclohexene reacted with anisole with both microwave irra-
diation at 1508C and conventional heating at 608C to afford
a 56:44 mixture of ortho and para cyclohexylanisole in 84
and 53% yields, respectively (Scheme 4). A reaction with
the less electron-rich p-xylene resulted in a lower yield of
the corresponding adduct (34%). The use of 1-methylcyclo-
hexene as an olefin substrate gave suggestive results. Its hy-
droarylation with anisole under microwave-irradiation con-
ditions exclusively produced (1-methylcyclohexyl)anisole in
74% yield with a high para selectivity (i.e., o/p=2:98). The
same Markovnikov-type regioselectivity in terms of the ary-
lation position was observed in the hydroarylation per-
formed under conventional heating conditions at 408C,
Scheme 3. ACHTUNGTRENNUNG[CpMoCl(CO)3]/o-chloranil-catalyzed reactions of styrene.
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Chem. Eur. J. 2008, 14, 10705 – 10715

Catalysis of Friedel–Crafts Alkylation Reactions
FULL PAPER
lation products in excellent yields.[17] The use of the Zeise
dimer under otherwise identical conditions enabled the hy-
droarylation of simple olefins: the hydroarylation of pro-
pene afforded isopropylbenzene as an exclusive product and
tolylcyclohexanes were obtained in the ratio of o/m/p=
31:6:63 from cyclohexene and toluene. These observations
À
allowed the authors to conclude that arene C H activation
by the platinum complexes might have been operative in the
former, whereas a Friedel–Crafts-type electrophilic substitu-
tion mechanism might have been involved in the latter using
the Pt/Ag system or the Zeise dimer.
Scheme 4.
hexenes.
ACHTUNGTRENNUNG[CpMoCl(CO)3]/o-chloranil-catalyzed hydroarylation of cyclo-
In our case, the ortho/para products were exclusively ob-
tained, thus indicating that the electrophilic-substitution
mechanism was involved. As an alternative possibility, the
electrophilic attack by high-oxidation-state molybdenum
species on electron-rich benzenes can give rise to aryl–mo-
lybdenum intermediates 6 with ortho/para selectivity, which
might undergo the insertion of olefins (Scheme 6). However,
albeit with a diminished yield and ortho/para selectivity.
Moreover, the hydroarylation of 1-hexene with anisole was
carried out under microwave-irradiation conditions to
obtain 3-hexylanisole and the expected 2-hexylanisole in a
46% combined yield (Scheme 5); both regioisomeric prod-
Scheme 6. Electrophilic attack by high-oxidation-state molybdenum spe-
cies on the aromatic ring.
Scheme 5.
hexene.
ACHTUNGTRENNUNG[CpMoCl(CO)3]/o-chloranil-catalyzed hydroarylation of 1-
the Markovnikov products obtained from styrenes and 1-
methylcyclohexene and the reaction with 1-hexene that
yields the 3-hexyl isomer together with the 2-hexyl product
strongly support the intermediacy of carbocationic species.
All these facts corroborate the presence of an Friedel–
Crafts-type electrophilic-substitution pathway in the
[CpMoCl(CO)3]/o-chloranil-catalyzed hydroarylation reac-
tion.
ucts contained ortho and para isomers. The formation of 3-
hexylanisole indicates that a carbocation species was pro-
duced and underwent a 1,2-hydride shift. In addition, the
ortho/para-selective substitution patterns mentioned above
were also responsible for the electrophilic aromatic substitu-
tion mechanism involving carbocation intermediates.
Mechanistic rationale of molybdenum/o-chloranil-catalyzed
hydroarylation reactions: As described in the introduction,
catalytic Friedel–Crafts-type electrophilic alkylation and
If this is the case, a question arises about the role of the
putative high-oxidation molybdenum species derived from
the molybdenum precursors and o-chloranil: Do they act as
Lewis acids or do they play some other role? In relation to
this issue, the involvement of protons has been disclosed or
suspected in certain homogeneous transition-metal catalyses.
Dyker and co-workers reported the gold-catalyzed hydroar-
ylation of a,b-unsaturated ketones and later found that the
same reaction could be catalyzed by Brønsted acids such as
HCl, para-toluenesulfonic acid (p-TsOH), and HBF4.[18a]
The groups of Hashmi and Nair also reported that the addi-
tion of electron-rich arenes to aldehydes proceeded with
both gold and Brønsted acid catalysis.[23] Similar Brønsted
acid catalyzed hydroarylation reactions of norbornene and
styrene with anilines were found independently by the
groups of Bergman and Coates.[24] In the molybdenum-cata-
lyzed hydroarylation reaction observed herein, the Brønsted
acid catalysis by tetrachlorocatechol, which was concomi-
tantly generated through the oxidation of the molybdenum
complexes with o-chloranil and subsequent protonolysis of
the putative molybdenum tetrachlorocatecholate species,
might be responsible for the observed reactions (see below).
À
transition-metal-catalyzed directed C H alkylation reactions
have been developed and employed in a complementary
manner. However, it is often difficult to clearly distinguish
which mechanism is operative when a transition-metal cata-
lyst with latent electrophilic character is used in combina-
tion with benzene substrates that possess no directing group.
Moreover, a slight modification of the catalyst can alter the
mechanistic profile dramatically. For example, Tilley and co-
workers revealed that indolylpyridine-supported platinum
complexes catalyzed the hydroarylation of norbornene with
benzene at 1408C to afford exo-2-arylnorbornanes in moder-
ate yields.[5g] Notably, chlorobenzene exhibited a higher re-
activity than benzene and toluene, whereas fluorobenzene
and p-xylene failed to undergo hydroarylation. The case in
which toluene was used, the corresponding adducts were ob-
tained in the ratio of o/m/p=20:50:30. On the other hand,
the combination of a similar platinum–indolylpyridine com-
plex and silver salts catalyzed the reaction of norbornene
with benzene at lower temperatures to furnish the hydroary-
Chem. Eur. J. 2008, 14, 10705 – 10715
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10709

Y. Yamamoto and K. Itonaga
In fact, small amounts of tetrachlorocatechol were detected
in the crude reaction mixture.
We examined several Brønsted acids as catalysts for the
hydroarylation of norbornene with p-xylene (Table 5). We
form zwitterionic intermediate 8, in which subsequent
proton abstraction by the oxygen atom of the catecholate
ligand takes place to ultimately generate Brønsted acid spe-
cies 9, which behaves as a proton donor. The reaction of
norbornene with [D10]p-xylene executed by microwave heat-
ing at 1508C in the presence of 10 mol% [CpMoCl(CO)3]
and 20 mol% o-chloranil produced a mixture of [D10]1a
(m/z 210 [M+]) and [D9]1a (m/z 209 [M+]) as confirmed by
GC—mass-spectrometric analysis (see Figure S2 in the Sup-
porting Information). The formation of [D10]1a is in good
agreement with the proposed mechanism in which the ben-
zene substrates are the proton sources. Despite several at-
tempts to isolate an active species from [CpMoCl(CO)3] and
o-chloranil, we completely failed to obtain an identifiable
Table 5. Reaction of norbornene with p-xylene in the presence of molyb-
denum or Brønsted acid catalysts.[a]
Entry
Catalyst
Conditions[b]
Yield of 1a [%][c]
1
2
3
4
5
6
7
8
[Mo][d]
A
B
A
B
A
A
A
A
87 (69)
79 (74)
76 (80)
86 (74)
nd
[Mo][d]
TfOH
TfOH
TsOH
tetrachlorocatechol
nd
T
48[e]
1
A
79[e]
complex. To our surprise, the H NMR spectroscopic analy-
sis of the solution of [CpMoCl(CO)3] and o-chloranil in
C6D6 revealed the disappearance of the Cp ligand, thus indi-
cating that this ligand has no direct influence on the yield
and isomeric ratio of the products.
[a] Norbornene: 1 mmol, cat.: 10 mol%. [b] A: p-xylene: 6 mL, 808C,
24 h; B: p-xylene: 3 mL, 1508C (MW), 15 min. [c] Yields in parentheses
were obtained with 5 mol% catalyst. [d] [CpMoCl(CO)3]/o-chloranil: 1:2.
[e] Small amounts of 1,4-dimethyl-2-(4-methylbenzyl)benzene were de-
tected by 1H NMR spectroscopic analysis of the crude reaction mixtures.
[f] o-Chloranil (20 mol%) was added.
To support the above mechanistic scenario, the commer-
cially available molybdenum(V) compound [MoCl5][27] was
used as a highly electrophilic molybdenum source. It was
observed that [MoCl5] exhibited moderate catalytic activity,
even in the absence of the oxidant o-chloranil. Under con-
ventional heating conditions, a 10 mol% loading of the cata-
lyst produced 1a in 48% yield (Table 5, entry 7). In this
case, [MoCl5] was probably reduced to [MoCl4Ln] (L=
ligand) by p-xylene, thus resulting in the concomitant evolu-
tion of hydrogen chloride, which might be responsible for
the hydroarylation.[28,29] This suggestion was corroborated by
the detection of a small amount of 1,4-dimethyl-2-(4-methyl-
benzyl)benzene in the crude reaction mixture. Thus, the
combined use of [MoCl5] and o-chloranil provided a superi-
or result (Table 5, entry 8). According to these observations,
it is reasonable to consider that the optimal Mo/o-chloranil
ratio of 1:2 derived from the above experimentations is nec-
essary because the molybdenum catalyst might be reduced
by the aromatic substrates and excess o-chloranil reoxidizes
the resultant inactive lower-oxidation-state species. Alterna-
tively, the molybdenum(II) complexes might undergo a two-
fold oxidation with two equivalents of o-chloranil to pro-
duce molybdenum(VI) biscatecholate species.
observed that our catalyst compares well with trifluorome-
thanesulfonic acid (TfOH), a superacid that possesses an
acidity much greater than 100% sulfuric acid,[25] and catalyz-
es the expected hydroarylation in good yield under both the
conventional and microwave-heating conditions (Table 5,
entries 3 and 4). In stark contrast, a typical Brønsted acid,
namely, TsOH, was reported to promote the hydroarylation
of norbornene with toluene; however, an excess loading is
required for reactions utilizing a weak acid promoter.[26] In
fact, 10 mol% TsOH cannot produce 1a under the same
conditions (Table 5, entry 5). In this context, a similarly
weak Brønsted acid, tetrachlorocatechol, is also assumed to
be an inferior promoter. Indeed, it failed to catalyze the re-
action of norbornene with p-xylene (Table 5, entry 6).
Although there is no experimental data that clarifies how
the combination of molybdenum complexes and o-chloranil
catalyzes the hydroarylation of alkenes, we have proposed a
likely catalytic species (Scheme 7). First, molybdenum com-
plexes are oxidized by o-chloranil to produce the higher-oxi-
dation-state molybdenum catecholate species 7. As a result
of its electrophilic character, 7 might react with benzenes to
Having revealed the Brønsted acid character of our cata-
lytic system, we reasoned that alcohols would be viable sub-
strates for the [CpMoCl(CO)3]/o-chloranil-catalyzed cou-
pling with electron-rich benzenes.
Catalytic coupling of alcohols with electron-rich benzenes:
Recently, the catalytic Friedel–Crafts alkylation of benzenes
with alcohols as alkylating agents has gained considerable
attention as an environmentally friendly process compared
with traditional methods that use halides, thereby producing
stoichiometric amounts of salt waste.[9–12] As a result of the
poor ability of hydroxy groups as leaving groups, excess
amounts of Brønsted acids are generally required to obtain
acceptable yields of the products. Recently, several Brønsted
acid-catalyzed protocols were successfully applied to allylic,
Scheme 7. Possible mechanism for the formation of Brønsted acid species
9.
10710
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Chem. Eur. J. 2008, 14, 10705 – 10715

Catalysis of Friedel–Crafts Alkylation Reactions
FULL PAPER
benzylic, propargylic alcohols, and 1-hydroxy-d-ribofura-
nose.[14] We then examined the catalytic efficacy of our mo-
lybdenum-based Brønsted acid catalyst toward the Friedel–
Crafts alkylation of benzene derivatives with alcohols as the
alkylating agents.
At the outset, exo-norborneol 10 was allowed to react
with p-xylene and anisole (Table 6). With a catalyst loading
of 10 mol%, 10 was heated at 808C in p-xylene for 24 h to
deliver the expected 1a in a yield slightly lower than that
from norbornene (Table 6, entry 1 versus Table 1, entry 5).
Under the microwave-irradiation conditions (1508C, 15 min)
1a was also afforded, albeit in a diminished yield of 60%
(Table 6, entry 2). The decrease in yield was ascribed to the
pre-equilibrium between 10 and a norbornyl cation inter-
mediate with water. Because p-xylene is an inefficient nucle-
ophile, the regeneration of 10 might have competed with the
attack by p-xylene on the cationic intermediate. In contrast,
the reaction with a stronger nucleophile, namely, anisole,
gave 1e in excellent yields, irrespective of the heating condi-
tions. The ortho/para selectivities were identical to those ob-
served for the reactions of norbornene (Table 3, entries 7
and 8). Benzyl alcohol 11a also reacted with p-xylene and
anisole to furnish benzylated products 12a and 12b in good-
to-excellent yields (Table 6, entries 5–8). On the other hand,
the reaction of 1-phenylethanol
(11b) with p-xylene resulted in
the formation of 5 but only in a
Table 6.
Entry
ACHTUNGTRENNUNG
[CpMoCl(CO)3]/o-chloranil-catalyzed coupling of alcohols.[a]
Alcohol
Conditions
Product
Yield [%] ortho/
low yield (Table 6, entry 9),
which was partly because of the
concomitant formation of 1,3-
diphenyl-1-butene, thus indicat-
ing of the formation of styrene
by the dehydration of 11b. The
more efficient nucleophile ani-
sole, however, uneventfully fur-
nished 3a in excellent yield
(Table 6, entries 10 and 11).
The inefficiency of p-xylene
as a nucleophile was observed
again in the reaction with cin-
namyl alcohol 13a. Although
13a was consumed within 7 h at
808C, the reaction led to a com-
plex mixture. In contrast, the
reaction with anisole completed
uneventfully, even at 408C
within 6 h, and produced 14a in
68% yield with the selectivity
para
p-xylene (6 mL), [Mo] (10 mol%),
808C, 24 h
1
2
10
1a
1a
78
60
–
10 p-xylene (3 mL), [Mo] (10 mol%),
1508C (MW), 15 min
10 anisole (6 mL), [Mo] (5 mol%), 808C,
24 h
10 anisole (3 mL), [Mo] (5 mol%), 1508C
(MW), 15 min
–
55:45
56:44
3
4
1e
1e
88
91
p-xylene (6 mL), [Mo] (10 mol%),
808C, 24 h
5
6
11a
12a 76
12a 70
–
–
11a p-xylene (3 mL), [Mo] (10 mol%),
1508C (MW), 1 h
anisole (6 mL), [Mo] (10 mol%), 808C,
24 h
7
8
11a
12b 91
12b 95
47:53
49:51
11a anisole (3 mL), [Mo] (10 mol%),
1508C (MW), 1 h
p-xylene (6 mL), [Mo] (10 mol%),
808C, 24 h
9
11b
5
23[b]
88
–
10
11b anisole (6 mL), [Mo] (10 mol%), 408C,
24 h
11b anisole (3 mL), [Mo] (10 mol%),
1508C (MW), 1 h
3a
21:79
11
12
3a
99
25:75
14:86
of
ortho/para=12:88
anisole (3 mL), [Mo] (5 mol%), 608C,
2 h
(Scheme 8). Furthermore, the
decreased catalyst loading of
13a’
14a 80
14b 93
5 mol% gave
a comparable
anisole (3 mL), [Mo] (5 mol%), 608C,
4 h
13
14
15
13b
4:96
32:68[c]
17:83[d]
yield and selectivity, albeit with
a longer reaction time. Carrying
out the reaction in a higher
concentration (anisole=3 mL)
led to a diminished reaction
time, and a slightly better yield
was obtained at 608C.
anisole (3 mL), [Mo] (5 mol%), 608C,
6 h
13d
14d 58
14e 58
anisole (3 mL), [Mo] (5 mol%), 608C,
6 h
13e
Under these modified condi-
tions, isomeric allylic alcohol
13a’ was converted within 2 h
into the same product 14a but
with a higher yield of 80%
(Table 6, entry 12). This result
is probably because the hy-
droxy group in 13a’ occupies
the allylic and benzylic posi-
tions, and, hence, it has a much
anisole (3 mL), [Mo] (5 mol%), 408C,
5 h
16
15a
16a 88[e]
–
[a] Alcohol: 1 mmol, [CpMoCl(CO)3]/o-chloranil: 1:2. [b] Inseparable mixture with 1,3-diphenyl-1-butene
(19%). [c] Regioisomer 14d’ was formed (14d/14d’=9:1). [d] Olefinic isomer 14e’ was formed (14e/14e’=
8:1). [e] An unidentified by-product was also formed in a trace amount.
Chem. Eur. J. 2008, 14, 10705 – 10715
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10711

Y. Yamamoto and K. Itonaga
mer ratio of 80:20. In contrast, the use of TfOH (5 mol%)
as a catalyst led to extensive oligomerization of the furan
ring, and a decrease in both the yield (46%) and regioselec-
tivity (70:30) were observed. Encouraged by these results,
we then optimized the molybdenum-catalyzed reaction. A
better yield of 65% was obtained at the expense of regiose-
lectivity, when isomeric alcohol 13a’ was employed in place
of 13a under otherwise identical conditions. With the hope
for further improvement, we examined precatalyst
[{CpMo(CO)3}2], which has no electron-withdrawing chlor-
ine ligand and is hence expected to give rise to milder acid
species. In fact, the oligomerization of the aromatic sub-
strate seemed to be well suppressed. As a consequence, the
reaction reached completion within 3 h, and the highest
yield (68%) and regioselectivity (17a/17b=80:20) were
achieved. In a similar manner, the use of benzhydrol as an
alcohol component produced 18 in 85% yield, whereas the
reaction of more labile propargyl alcohol 15a was carried
out at a lower temperature to give 19, albeit in 37% yield.
Scheme 8. ACHTUNGTRENNUNG[CpMoCl(CO)3]/o-chloranil-catalyzed coupling of cinnamyl al-
cohol with anisole.
better leaving ability. Similarly, secondary alcohol 13b also
provided an excellent yield (93%) of 14b (Table 6,
entry 13), whereas tertiary analogue 13c produced a com-
plex product mixture. Crotyl and prenyl alcohols 13d,e
were then subjected to the optimal reaction conditions. As a
result, 13d produced an inseparable mixture of the expected
product 14d and its regioisomer 14d’ in 58% yield with a
ratio of 14d/14d’=9:1 (Table 6, entry 14), whereas 13e fur-
nished an inseparable mixture of 14e and its olefinic isomer
14e’ in 58% yield with a ratio of 14e/14e’=8:1 (Table 6,
entry 15). In contrast, no arylation product was formed from
allyl or methallyl alcohols, thus indicating the necessity of
an internal olefin moiety for the present catalytic reaction.
When 1,3-diphenyl-2-propyn-1-ol (15a) was subjected to a
catalyzed arylation at 408C, the corresponding para-propar-
gylated anisole derivative 16a was formed as an exclusive
regioisomer in a high yield (Table 6, entry 16). At a higher
temperature, the yield was decreased as a result of unidenti-
fiable side reactions.
To demonstrate the applicability of the molybdenum cat-
alysis to acid-labile substrates, we then examined the reac-
tion of 13a or 13a’ with 2-methylfuran, which has been re-
ported to undergo acid-catalyzed polymerization.[30] First,
the reaction with cinnamyl alcohol 13a was attempted with
the [CpMoCl(CO)3]/o-chloranil catalyst system (Scheme 9).
Although the oligomerization of 2-methylfuran was ob-
served, the desired adducts 17a and 17b were formed as an
inseparable mixture in 57% combined yield with a regioiso-
Catalytic [2:1] condensation of benzaldehydes with anisole:
The catalytic twofold Friedel–Crafts-type addition of elec-
tron-rich benzenes to aryl aldehydes and their congeners
provides straightforward access to triphenylmethane deriva-
tives, which have widespread applications.[23,31,32] We finally
examined the catalytic condensation of benzaldehydes with
anisole (Scheme 10 and Table 7). In the presence of
10 mol% molybdenum catalyst, benzaldehyde (1 mmol) was
heated in anisole (3 mL) at 808C for 16 hours, thus resulting
in the formation of triarylmethane 20a in 58% yield as an
inseparable mixture of regioisomers (Table 7, entry 1). The
1H NMR spectroscopic analysis revealed that a para,para
adduct was formed predominantly, and a minor product was
tentatively assigned as a ortho/para isomer (para,para/ortho,-
para=87:13). The yield was improved to 87% when the cat-
alyst loading was increased to 20 mol% (Table 7, entry 2).
Scheme 10. ACHTUNTRGEN[UNG CpMoCl(CO)3]/o-chloranil-catalyzed condensation of ben-
zaldehydes with anisole.
Table 7. Catalytic condensation of benzaldehydes with anisole.
Entry
R
Catalyst
loading [mol%] ACHTUNTRGNEUNG[8C]
T
t [h] Product Yield para,para/
[%] ortho,para
1
2
3
4
5
6
7
H
H
H
Cl 20
Cl 20
Me 20
Me 20
10
20
20
80
80
16 20a
58
87
68
86
72
81
63
87:13
87:13
75:25
86:14
75:25
86:14
75:25
8
1
4
1
20a
20a
20b
20b
150 (MW)
80
150 (MW)
80
24 20c
20c
150 (MW)
1
Scheme 9. Catalyzed reactions of 2-methylfuran.
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Chem. Eur. J. 2008, 14, 10705 – 10715

Catalysis of Friedel–Crafts Alkylation Reactions
FULL PAPER
spectra were recorded on a Varian Gemini 2000 NMR spectrometer in
solution with CDCl3 at 258C. The 1H NMR chemical shifts d are reported
in ppm relative to the singlet at d=7.26 ppm for CHCl3. Splitting pat-
terns are designated as follows: s=singlet, d=doublet, t=triplet, q=
quartet, quint=quintet, sext=sextet, sept=septet, m=multiplet. Cou-
pling constants are reported in Hz. The 13C NMR spectra were fully de-
coupled and are reported in ppm relative to the triplet at d=77.0 ppm
for CDCl3. The EI mass-spectrometric measurements were performed on
a Shimadzu GCMS-QP2010 plus mass spectrometer. Elemental analyses
were performed on a Perkin Elmer 2400II CHNS/O elemental analyzer.
The microwave-irradiation experiments were carried out on a single-
mode microwave reactor (CEM Discover LabMate), closed reaction ves-
sels were used, and the temperature was monitored by an online IR de-
tector. All the transition-metal complexes were purchased and used as re-
In the same manner, adducts 20b and 20c were also formed
in high yield starting from p-chlorobenzaldehyde and p-tolyl-
aldehyde (Table 7, entries 4 and 6). Carrying out these reac-
tions under microwave-irradiation conditions shortened the
reaction time to 1 hour, although both the yield and regiose-
lectivity were lowered (Table 7, entries 3, 5, and 7).
Conclusion
Herein, we have succeeded in identifying a new Friedel–
Crafts alkylation catalyst system, which consists of a molyb-
denum(II) complex and an organic oxidant, [CpMoCl(CO)3]
and o-chloranil, respectively. A strained alkene, norbornene,
underwent hydroarylation with alkylated benzenes and ani-
sole derivatives at 808C to produce exo-2-arylnorbornanes
in moderate-to-good yields. The use of microwave irradia-
tion at 1508C shortened the reaction time to 15 min, thus re-
sulting in the formation of hydroarylation products in com-
parable yields. The same catalyst system also promoted the
hydroarylation of styrenes and cyclic and linear alkenes at
lower temperatures. On the basis of the ortho/para and Mar-
kovnikov selectivities observed for the products, we assumed
that the oxidation of the molybdenum complex by o-chlor-
anil generates an intermediary molybdenum catecholate
species, which then undergoes electrophilic reaction with
aromatic substrates before ultimately producing an elusive
Brønsted acid catalytic species. In accordance with this as-
sumption, exo-norborneol, benzylic, allylic, and propargylic
alcohols underwent arylation by using the [CpMoCl(CO)3]/
o-chloranil catalytic method, and alkylated aromatic prod-
ucts with ortho/para and Markovnikov selectivities were
produced.
Our catalyst system not only compares well with TfOH in
terms of the protonation ability toward norbornene, but also
has a better applicability toward acid-labile substrates, such
as 2-methylfuran. In addition, the present catalyst system
has a significant practical advantage: both of the catalyst
precursors [CpMoCl(CO)3] and o-chloranil are stable, easy-
to-handle solids and, unless mixing in an aromatic solvent,
they do not exhibit acidity. Therefore, directly employing a
harmful strong acid reagent, such as TfOH, which is a hy-
groscopic and corrosive liquid, can be avoided.
ceived, except for [(h-C3H5)MoClACHTNUGRTNEUNG(MeCN)2(CO)2], which was prepared
according to a previous report.[33] All the products except 1 f,g were
known products.
General procedure for [CpMoCl(CO)3]/o-chloranil-catalyzed arylations:
hydroarylation of norbornene with p-xylene
1) Conventional
heating
method:
[CpMoCl(CO)3]
(28.00 mg,
0.100 mmol) and o-chloranil (49.23 mg, 0.200 mmol) were mixed with p-
xylene (3 mL) in a 20-mL round-bottom flask at room temperature in an
argon atmosphere. The solution was immediately darkened and slight gas
evolution was observed. After the mixture had been stirred for 30 min at
room temperature, a solution of norbornene (93.99 mg, 1.00 mmol) in p-
xylene (3 mL) was added by syringe. The reaction mixture was heated on
an oil bath at 808C for 24 h. After cooling to room temperature, the solu-
tion was diluted with diethyl ether (5 mL) and filtered through a pad of
alumina. The insoluble materials were further washed with diethyl ether
(40 mL), and the filtrate was concentrated in vacuo. The resulting residue
was purified by flash column chromatography on silica gel eluting with
hexane to give 1a as a colorless oil (173.4 mg, 87%).
2) Microwave heating method: [CpMoCl(CO)3] (27.36 mg, 0.098 mmol)
and o-chloranil (47.95 mg, 0.195 mmol) were mixed with p-xylene (1 mL)
in a 6-mL reaction tube at room temperature in an argon atmosphere.
The solution immediately darkened and slight gas evolution was ob-
served. After the mixture had been stirred for 30 min at room tempera-
ture, a solution of norbornene (91.80 mg, 0.98 mmol) in p-xylene (2 mL)
was added by syringe. The reaction tube was sealed with a teflon cap and
heated in a microwave reactor at 1508C for 15 min. The same purifica-
tion procedures as above gave 1a as a colorless oil (157.2 mg, 81%).
Analytical data for 1 f: colorless oil; 1H NMR (300 MHz, CDCl3, 258C):
d=1.19 (m, 1H), 1.24–1.65 (m, 6H), 1.78 (ddd, J=12.0, 9.0, 2.4 Hz, 1H),
2.29 (s, 3H), 2.28–2.35 (m, 2H), 2.95 (dd, J=9.0, 6.0 Hz, 1H), 3.84 (s,
3H), 6.73 (d, J=8.1 Hz, 1H), 6.94 (dd, J=8.1, 2.0 Hz, 1H), 7.01 ppm (d,
J=2.0 Hz, 1H); 13C NMR (75 MHz, CDCl3, 258C): d=20.7, 29.0, 30.4,
36.2, 36.8, 38.6, 40.3, 41.1, 55.4, 110.2, 126.4, 126.7, 129.2, 135.8,
155.4 ppm; MS (EI): m/z (%): 216 (55) [M+], 149 (14) [M+H+ÀC5H8],
135 (100) [M+H+ÀC5H8ÀCH2]; elemental analysis calcd (%) for
C15H20O: C 83.28, H 9.32; found: C 83.06, H 9.54.
Analytical data for 1g: colorless oil; 1H NMR (300 MHz, CDCl3, 258C):
d=1.19 (brd, J=9.9 Hz, 1H), 1.24–1.66 (m, 6H), 1.78 (ddd, J=12.8, 9.0,
2.1 Hz, 1H), 2.32 (s, 2H), 2.92 (dd, J=8.4, 5.7 Hz, 1H), 3.80 (s, 3H), 6.73
(d, J=8.7 Hz, 1H), 7.09 (dd, J=8.4, 2.4 Hz, 1H), 7.14 ppm (d, J=2.4 Hz,
1H); 13C NMR (75 MHz, CDCl3, 258C): d=28.9, 30.2, 36.2, 36.8, 38.6,
40.4, 40.9, 55.5, 111.3, 125.3, 125.8, 126.1, 138.0, 156.0 ppm; MS (EI): m/z
(%): 236 (93) [M+], 201 (31) [M+ÀCl], 168 (45) [M+ÀC5H8], 155 (100)
Although we were unable to obtain the catalytically
active Brønsted acid species stemming from the molybde-
num precatalysts and o-chloranil, further studies elucidating
the reaction mechanism and expanding the synthetic utility
of our new Friedel–Crafts alkylation catalysis will be report-
ed in due course.
[MH+ÀC5H8ÀCH2]; elemental analysis calcd (%) for C14H17ClO:
C
71.03, H 7.24; found: C 70.81, H 7.24.
Experimental Section
Acknowledgements
General: Column chromatography was performed on silica gel (Cica
silica gel 60N or Fuji Silysia FL100D) eluting with hexane or a mixed sol-
vent system (hexane/ethyl acetate). Filtration through alumina was car-
This research was partially supported by the Ministry of Education, Sci-
ence, Sports and Culture, Japan; Grant-in-Aid for Scientific Research
(B) (grant 20350045); Scientific Research on Priority Area “Chemistry
ried out on Merck standadized aluminum oxide 90. The H and 13C NMR
1
Chem. Eur. J. 2008, 14, 10705 – 10715
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10713

Y. Yamamoto and K. Itonaga
on Concerto Catalysis” (grant 20037018); and the Global COE Program
(Tokyo Institute of Technology). We thank Prof. K. Tomooka (Kyushu
University) for use of the NMR spectrometer and Profs H. Suzuki and T.
Takao (Tokyo Institute of Technology) for EI mass spectrometric meas-
urements and elemental analysis. We also thank Prof Y. Yamaguchi (Yo-
kohama National University) for helpful suggestions on the preparation
of molybdenum complexes.
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Received: June 7, 2008
Revised: August 20, 2008
Published online: October 27, 2008
Chem. Eur. J. 2008, 14, 10705 – 10715
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10715