Transition-metal-catalyzed benzylation of arenes and heteroarenes

Article abstract of DOI:10.1002/anie.200460666

Friedel-Crafts-type benzylation of various aromatic and heteroaromatic compounds with benzylic acetates, alcohols, and carbonates was promoted under mild reaction conditions by a series of Rh, Ir, Pd, and Pt catalysts (see scheme; R¹ = H, Me; R² = H, Cl, MeO; R³ = H, Ac, CO₂Me). The desired products were obtained in high yields and with high selectivities.

Full text of DOI:10.1002/anie.200460666

Communications
Homogeneous Catalysis
Initially we were attracted by the reaction of benzyl alcohol
and its derivatives with arenes leading to diarylmethane-type
compounds. The diarylmethane motif is found in a number of
biologically active compounds such as piritrexim,[11a] trimetho-
prim,[11a] avrainvilleol,[11b] papaverine,[11c] beclobrate,[11d] and
anastrozole[11e] and in a precursor in the synthesis of letro-
zole.[11e] Diarylmethane derivatives are used to prepare elec-
troactive and photoactive oligomers and polymers based on
fluorenes.[12] Friedel–Crafts reactions of benzylic alcohols and
halides have been studied with traditional Brønsted and Lewis
acids,[13] more recently with lanthanide and actinide triflates,[14]
and also with heterogeneous catalysts.[15] Surprisingly few
transition-metal catalysts based on palladium and ruthenium
have been described to date for this type of reaction.[16] While
palladium complexes have been used only on a stoichiometric
basis for this reaction,[17] ruthenium complexes required high
temperatures (2008C) and gave the corresponding diaryl-
methanes generally in low to moderate yield (22–77%).[18]
In our search for new and improved catalysts we studied
the benzylation of o-xylene with 1-phenylethyl acetate as a
model reaction (Scheme 1). A variety of Brønsted acids,
transition-metal salts, and organometallic complexes were
tested under mild conditions (808C). Selected results are
given in Table 1. In the presence of catalytic amounts
Transition-Metal-Catalyzed Benzylation of
Arenes and Heteroarenes**
Kristin Mertins, Irina Iovel, Jette Kischel,
Alexander Zapf, and Matthias Beller*
Dedicated to Professor Rꢀdiger Selke
on the occasion of his 70th birthday
The majority of known fine chemicals, pharmaceuticals,
agrochemicals, dyes, etc. contain aromatic and/or hetero-
aromatic substructures. To vary and improve the properties of
these compounds functionalization reactions of the aromatic
core are of fundamental importance to organic chemistry. In
the past and still today most aromatic products are prepared on
the industrial level by well-known classic transformations such
as Friedel–Crafts alkylations, Friedel–Crafts acylations, nitra-
tions, and halogenations.[1] Although these methods work
reliably with a variety of substrates, they often have major
drawbacks such as drastic reaction conditions (high temper-
ature, strong acidic conditions), low regioselectivity, and large
amounts of (salt) by-products. In addition it is common that the
À
construction of specific C C bonds on the aromatic core
requires several steps including introduction of activating
groups and protection and deprotection steps. Therefore, the
À
development of new direct C C coupling reactions of arenes
Scheme 1. Benzylation of o-xylene with 1-phenylethyl acetate.
Table 1: Screening of catalysts for the model reaction.[a]
has become an important topic in organometallic chemistry
and catalysis. Recent elegant examples of such catalytic
functionalization reactions include the addition of olefins to
acetophenones (Murai reaction)[2] and aromatic imines,[3] novel
additions of olefins to electron-rich arenes[4] and heterocycles,[5]
the addition of arenes to alkynes,[6] the carbonylation of
heterocycles,[7] the intramolecular cyclization of arylalkynes,[8]
and the C-arylation of heteroarenes.[9]
Entry
Catalyst
T [8C]
Conv. [%][b]
Yield [%][c]
1
2
3
4
5
6
7
8
HCl
p-TSA
H2SO4
HOAc
80
80
80
80
80
80
80
80
50
RT
80
80
50
RT
80
80
80
50
RT
80
80
50
RT
80
80
5
52
9
0
2
0
0
6
61
0
0
79
48
0
99
99
75
16
0
8
[W(CO)6]
18
100
0
In the past we have mainly focused on transformations of
[MesW(CO)3]
MnCl2·4H2O
Co(OAc)2·4H2O
RhCl3·nH2O
RhCl3·nH2O
[Rh(cod)Cl]2
IrCl3·nH2O
IrCl3·nH2O
IrCl3·nH2O
IrBr3·4H2O
[{Ir(cod)Cl}2]
H2[PdCl4]·6H2O
H2[PdCl4]·6H2O
H2[PdCl4]·6H2O
[Pd(PPh3)4]
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
H2[PtCl6]· 6H2O
PtCl2
À
aryl chlorides using the C Cl bond as an activating and
directing functionality.[10] Because of the obvious advantages
0
À
9[d]
10[d]
11
12[d]
13[d]
14[d]
15
16
17
18
19
20
21
22
23
24
25
100
83
0
100
100
100
19
0
of direct C H transformations over palladium-catalyzed
reactions of aryl halides, we initiated a program to search
for new catalysts for C C coupling reactions of arenes.
Herein, we report our first results and show that various
transition-metal catalysts are capable of benzylating activated
and nonactivated arenes and heteroarenes with high yield and
selectivity under mild conditions.
À
100
75
0
99
65
0
[*] Dipl.-Chem. K. Mertins, Dr. I. Iovel, J. Kischel, Dr. A. Zapf,
Prof. Dr. M. Beller
0
0
Leibniz-Institut fꢀr Organische Katalyse
Universitꢁt Rostock e.V. (IfOK)
Buchbinderstrasse 5–6, 18055 Rostock (Germany)
Fax: (+49)381-466-9324
100
100
100
8
99
99
99
7
E-mail: matthias.beller@ifok.uni-rostock.de
[Pt(cod)Cl2]
0
0
[**] This work was supported by the State of Mecklenburg-Western
Pomerania and the Bundesministerium fꢀr Bildung und Forschung
(“Nachhaltige Aromatenchemie”). We thank Prof. Dr. M. Michalik,
Dr. W. Baumann, Dr. C. Fischer, H. Baudisch, and S. Buchholz (all
IfOK) for excellent assistence with the analyses.
[a] Reaction conditions: 10 mL o-xylene, 0.5 mmol 1-phenylethyl acetate,
10 mol% catalyst, 20 h. Mes=mesitylene, cod=cycloocta-1,5-diene.
[b] GC conversion based on 1-phenylethyl acetate. [c] GC yield.
[d] 8 mol% catalyst.
238
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460666
Angew. Chem. Int. Ed. 2005, 44, 238 –242

Angewandte
Chemie
Table 2: Benzylation of different substrates.[a]
Entry
Substrate
Main product
Catalyst
T [8C]
t [h]
Conv.[b] [%]
100
Yield[c] [%]
62
Regiosel. (o/m/p)
1[d]
H2[PtCl6]·6H2O
140
20
–
2[d]
H2[PtCl6]·6H2O
RT
24
76
57
87
3[d]
4[d]
H2[PtCl6]·6H2O
IrCl3·nH2O
50
50
24
24
100
100
87
79
83
84
5[d]
6[d]
7[e]
H2[PtCl6]·6H2O
IrCl3·nH2O
50
50
80
24
24
24
100
100
100
99
99
92
17:0:83
17:0:83
–
H2[PtCl6]·6H2O
8[f]
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
120
120
120
23
24
24
100
100
100
96
62
85
<1:99
<1:99
–
9[f]
10[f]
11[f]
12[f]
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
120
120
20
20
100
100
84
96
82:18[g]
77
13[f,h]
IrCl3·nH2O
80
120
80
20
24
20
100
100
100
92
94
91
63:5:32[i]
14[f]
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
–
15[d,h]
77
16[j]
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
50
80
20
20
100
100
99
67
<1:99
<1:99
17[d,h]
[a] Reaction conditions: 0.5 mmol 1-phenylethyl acetate, 10 mol% catalyst, 20–24 h. [b] GC conversion based on 1-phenylethyl acetate. [c] GC yield.
[d] In 10 mL substrate. [e] 10 mmol substrate in 8 mL benzene. [f] 10 mmol substrate in 8 mL dioxane. [g] 2-/3-. [h] 8 mol% catalyst. [i] 2-/4-/5-.
[j] 10 mmol substrate in 8 mL CH2Cl2.
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239

Communications
(10 mol%) of various Brønsted acids (HCl, H2SO4, HOAc, p-
Interestingly, even nonactivated substrates such as benzene
(62%) and more challenging starting materials such as 4-
chloroanisole (99%)[20] and 2-methoxybenzaldehyde (67%)
gave the corresponding products selectively. It is important to
note that the last example constitutes one of the very rare
cases of a selective Friedel–Crafts reaction in the presence of
an aldehyde group!
toluenesulfonic acid) the benzylated product 1-(3,4-dimeth-
ylphenyl)-1-phenylethane (1) was obtained at best in low
yields (ꢀ 2%; Table 1, entries 1–4); however, to our surprise a
variety of Rh, Ir, Pd, and Pt salts proved to be suitable
catalysts for the test reaction. In addition, also [(mesitylene)-
W(CO)3] gave a significant yield (61%) of the desired
product (Table 1, entry 6). At 808C excellent results (99%
yield of 1) were obtained in the presence of hydrated IrCl3,
H2[PdCl4], and H2[PtCl6] (Table 1, entries 12, 17, and 21,
respectively).[19] Good to quantitative yields of 1 (up to 99%)
were observed even at room temperature using IrCl3 and
H2[PtCl6] (Table 1, entries 14 and 23, respectively). Although
for convenience most of the screening were conducted for
20 h, it is noteworthy that the reaction with H2[PtCl6] at 808C
is complete within 1 h. In some cases a significant difference
between the conversion and yield of 1 is observed (e.g.,
Table 1, entries 2, 6, 10). Here, side reactions of 1-phenylethyl
acetate such as elimination, oligomerization, and polymeri-
zation of the resulting styrene take place.
Finally, we set out to study the scope and limitation of the
benzylating agent in more detail (Scheme 2). As shown in
Scheme 2. Benzylation of arenes with different benzylation reagents.
cat.=H2[PdCl6]·6H2O, IrCl3·nH2O.
With reliable catalysts for the benzylation of o-xylene in
hand, we were interested in the benzylation of other arenes
and heteroarenes (Table 2) and in the use of different
benzylating agents (Table 3). Although a number of the
active transition-metal salts listed in Table 1 also catalyze
these reactions, only the results with the most active and
selective catalysts (IrCl3 and H2[PtCl6]) are listed in Tables 2
and 3. As expected, a variety of electron-rich arenes such as
toluene (87%), anisole (> 99%), 1,4-dimethoxybenzene
(92%), and 2-naphthol (96%) gave the corresponding
benzylated products in good to excellent yield. In addition,
heteroarenes, for example, 2,5-dimethylfuran (85%), benzo-
furan (91%), and 2-methylthiophene (96%) react very well.
Table 3 apart from 1-phenylethyl acetate also benzyl acetate
and 4-chlorobenzyl acetate react with o-xylene in the
presence of H2[PtCl6] at 808C (Table 3, entries 1, 4). In
addition, benzyl methyl carbonate (66%) and benzyl alcohol
(48%) also gave the desired product in moderate yields
(Table 3, entries 2, 3). More interestingly, the reaction of
benzyl acetate and chlorobenzene proceeded in good yield
(57%). It is important to note that this latter reaction is one of
the few successful Friedel–Crafts benzylations that work well
also with deactivated arenes.
With regard to the mechanism, the presented reactions
clearly operate by means of electrophilic aromatic substitu-
Table 3: Reactions with different benzylation reagents.[a]
Entry
Product
R’
Catalyst
T [8C]
Conv.[b] [%]
Yield[c] [%]
Regiosel. (o/m/p)
1
2
3
Ac
CO2Me
H
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
80
80
80
100
100
100
45
66
48
<1:99
<1:99
<1:99
4
Ac
H2[PtCl6]·6H2O
80
100
65
28:0:72
5
6
CO2Me
Ac
H2[PtCl6]·6H2O
H2[PtCl6]·6H2O
80
100
100
42
79
1:99
–
140
7
8
Ac
Ac
H2[PtCl6]·6H2O
IrCl3·nH2O
80
80
100
100
80
88
40:7:53
40:7:53
9
Ac
H2[PtCl6]·6H2O
80
100
80
–
10
11
Ac
Ac
H2[PtCl6]·6H2O
IrCl3·nH2O
80
80
93
80
93
76
35:0:65
38:0:62
12
Ac
H2[PtCl6]·6H2O
100
100
57
42:1:57
[a] Reaction conditions: 10 mL substrate, 0.5 mmol benzylating agent, 10 mol% catalyst, 20–24 h. [b] GC conversion based on benzylating agent.
[c] GC yield.
240
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Angew. Chem. Int. Ed. 2005, 44, 238 –242

Angewandte
Chemie
tion. Thus, different regioisomers can be formed; however, in
a number of cases (Table 2, entries 1, 7–10, 14, 16, 17) the
benzylation proceeds regioselectively to produce only one
product. By comparing the results in the presence of simple
Brønsted acids and our transition-metal catalysts we propose
that the active benzylation reagent is not a protonated benzyl
acetate. In addition, we exclude the existence of a “free”
benzylic carbocation because labeled Ph13CH2OH does not
lead to a scrambling of the 13C atom by tropylium ion
formation. However, reaction of enantiomerically pure 1-
phenylethyl acetate with o-xylene results in a racemic mixture
of 1. Basically, the transition-metal center should stabilize the
carbocation intermediate and prevent unproductive forma-
tion of styrene and subsequent polymerization. Based on the
reactivity of the applied catalysts we rule out both a
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À
transition-metal-based C H activation pathway as well as a
simple oxidative addition to the benzyl acetate.
In summary, we have developed a new general benzyl-
ation reaction of aromatics. Arenes and heteroarenes can be
easily benzylated in the presence of several transition-metal
compounds, of which IrCl3 and H2[PtCl6] are the most active
catalysts. Often the reactions proceed under mild conditions
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Experimental Section
All reactions were carried out in air without any special precautions.
1H and 13C NMR spectra were recorded on a Bruker ARX 400
spectrometer. Chemical shifts (d) are given in ppm and the residual
solvent served as the internal standard. Gas chromatography was
performed on a Hewlett Packard HP 6890 chromatograph with a HP5
column. Mass spectra were recorded on an AMD 402/3 mass
spectrometer. Chemicals and solvents were purchased from Fluka
and Aldrich and used as received.
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General procedure for benzylation of o-xylene: In a pressure
tube, 1-phenylethyl acetate (0.50 mmol) and catalyst (10 mol%,
0.050 mmol) were dissolved in o-xylene (10 mL). Hexadecane
(50 mL) was added as internal GC standard. The reaction mixture
was stirred 20 h before aliquots were removed and analyzed by GC to
determine the yield and conversion. For isolation of the products the
reaction was quenched with water. The aqueous phase was extracted
with dichloromethane. The combined organic layers were dried over
MgSO4, and the solvents were removed under reduced pressure. The
product was purified by column chromatography on silica gel (70–
230 mesh, n-heptane/ethyl acetate (50:1)). 1H NMR (400 MHz,
CDCl3, 258C): d = 1.5 (d, J(H,H) = 7.2 Hz, 3H; CH3), 2.1 (s, 6H;
2ArCH3), 4.0 (q, J(H,H) = 7.2 Hz, 1H; CH), 6.9–7.2 ppm (m, 8H; CH
arom.); 13C NMR (101 MHz, CDCl3, 258C): d = 18.3, 18.8, 20.9, 43.2,
123.8, 124.8, 126.5, 127.3, 127.9, 128.6, 133.1, 135.4, 142.8, 145.6 ppm;
MS (70 eV): m/z [%]: 210 (40) [M+], 195 (100) [C15H15+], 91 (10)
[C7H7+].
Received: May 14, 2004
Revised: July 20, 2004
Keywords: arenes · benzylation · Friedel–Crafts reaction ·
.
iridium · platinum
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Communications
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[20] The use of 5 mol% (1 mol%) H2[PtCl6] at 808C afforded 97%
(42%) yield of 4-chloro-1-methoxy-2-(1-phenylethyl)benzene.
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