Rhodium-catalysed anomalous dimerization of styrenes involving the cleavage of the ortho C-H bond

Article abstract of DOI:10.1039/b806285d

The dimerization of styrene derivatives in the presence of a rhodium catalyst proceeds to give stilbene derivatives, in which the ortho C-H bond of styrenes is cleaved and functionalized. The Royal Society of Chemistry.

Full text of DOI:10.1039/b806285d

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Rhodium-catalysed anomalous dimerization of styrenes involving
the cleavage of the ortho C–H bondw
Mamoru Tobisu,*a Isao Hyodo,b Masahiro Onoeb and Naoto Chatani*b
Received (in College Park, MD, USA) 14th April 2008, Accepted 20th August 2008
First published as an Advance Article on the web 10th October 2008
DOI: 10.1039/b806285d
The dimerization of styrene derivatives in the presence of a
rhodium catalyst proceeds to give stilbene derivatives, in which
the ortho C–H bond of styrenes is cleaved and functionalized.
Transition metal-catalysed dimerization of alkenes represents
an industrially important process for preparing higher alkenes
from simple and small alkenes.1 In addition to the practical
importance, the mechanistic aspects of these dimerization
processes have received much attention as a model system for
the polymerization and oligomerization of alkenes.2 Among
the alkenes for use in catalytic dimerization, styrene derivatives
have been extensively studied. Catalytic dimerization of styr-
enes generally produces a head-to-tail isomer,3 while there are a
few reports on catalyst systems that can afford tail-to-tail4 or
head-to-head5 isomers in a regioselective manner (Scheme 1).
Although the regiochemical outcome depends on the catalyst
system employed, a new carbon–carbon bond is formed bet-
ween the vinylic carbons in all reported systems. Herein, we
report a new mode for the catalytic dimerization of styrenes, in
Scheme 1 Catalytic dimerization of styrenes.
which the new carbon–carbon bond is formed between the
benzene ring at the 2-position and the vinylic carbon.
dimerization of 9-(4-vinylphenyl)anthracene, provided unam-
In the course of our studies on the development of new
biguous confirmation of our structure assignment (Fig. 1).z
catalytic reactions mediated by a rhodium–silyl species gene-
Intrigued by the unusual mode of dimerization, we system-
rated in situ,6 we examined the reaction of styrene (1) with
atically examined a variety of transition metals for their ability to
hexamethyldisilane in the presence of a rhodium catalyst
catalyse the dimerization of styrene. Clearly, the stoichiometric
(Scheme 2). As a result, vinylsilane 2, which was presumably
amount of hexamethyldisilane is not required for the dimeriza-
produced via
a
silyl-metallation/b-hydride elimination
tion process, and that, instead, would induce an undesired
dehydrogenative silylation. Thus, we investigated a catalytic
reaction in the absence of hexamethyldisilane (Table 1). In
contrast to the result shown in Scheme 2, the use of [RhCl(cod)]2
did not afford any dimerization products (entry 4). On the other
hand, [Rh(OH)(cod)]2 and [Rh(OMe)(cod)]2 exhibited promising
catalytic activity (entries 5 and 6), while other metal based
catalysts, including Ru3(CO)12, Ni(cod)2 and Pd(PPh3)4, were
totally inactive (entries 1–3). Comparable catalytic activity was
obtained when a mixture of [RhCl(cod)]2, H2O and Na2CO3 was
sequence,7 was obtained in 30% yield, along with byproduct
3, the molecular weight of which, by GC-MS analysis, turned
1
out to be twice that of 1. However, the H NMR spectrum of
the dimer 3 was, to our surprise, absolutely different from those
of either the expected head-to-tail, tail-to-tail, or head-to-head
dimers. Intensive assignment of the 1H NMR spectra led us to
establish the structure of 3, in which the new carbon–carbon
bond is formed between the benzene ring at the 2-position and
the vinylic carbon (Scheme 2). X-ray crystallographic analysis
of the anthracene derivative 4, which was obtained through the
a Frontier Research Base for Global Young Researchers, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-0871,
Japan. E-mail: tobisu@chem.eng.osaka-u.ac.jp;
Fax: +81 6 6879 7396; Tel: +81 6 6879 7395
b Department of Applied Chemistry, Faculty of Engineering, Osaka
University, Suita, Osaka 565-0871, Japan.
E-mail: chatani@chem.eng.osaka-u.ac.jp;
Fax: +81 6 6879 7396; Tel: +81 6 6879 7397
w Electronic supplementary information (ESI) available: Experimental
details and spectral data of new compounds. CCDC reference number
684849. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b806285d
Scheme 2 Unexpected formation of 3.
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Table 2 Rh-catalysed dimerization of styrenesa
Yield
(%)
Product
Entry Styrene
Fig. 1 ORTEP view of anthracene derivative 4.
Table 1 Catalyst screening for the dimerization of styrenesa
1
2
3
4
5
R = H (1)
R = CF3 (5)
R = F (7)
R = Me (9)
R = OMe (11)
3
6
8
10
12
24
63
50
44
24
6
42
GC yield
of 3 (%)
Entry
Catalyst
Additive
1b
2b
3b
4
5
6
Ru3(CO)12
Ni(cod)2
Pd(PPh3)4
[RhCl(cod)]2
[Rh(OH)(cod)]2
[Rh(OMe)(cod)]2
[RhCl(cod)]2
None
None
None
None
None
None
0
0
0
7
57
0
13
15
12
7
H2O (0.10 mmol)
+ Na2CO3 (0.20 mmol)
t-BuOH (0.10 mmol)
+ Na2CO3 (0.20 mmol)
t-BuOH (0.20 mmol)
+ Na2CO3 (0.40 mmol)
8
[RhCl(cod)]2
[RhCl(cod)]2
22
9c
24d
a
Reaction conditions: 1 (1.0 mmol), [RhCl(cod)]2 (0.10 mmol),
t-BuOH (0.20 mmol), Na2CO3 (0.40 mmol), and dioxane (0.5 mL) at
160 1C for 20 h in a sealed tube.
a
Reaction conditions: 1 (1.0 mmol), catalyst (0.05 mmol), additive,
b
and dioxane (0.5 mL) at 130 1C for 20 h in a sealed tube. 0.10 mmol
c
of the catalyst was used. At 160 1C with 0.10 mmol of the catalyst.
Isolated yield.
d
not shown in Table 2). Electron-rich styrenes, as in 9 and 11,
were also dimerized under these conditions (entries 4 and 5). We
next examined the effect of the position of the trifluoromethyl
group on the efficiency of catalytic dimerization. When the meta-
substituted isomer 13 was used, the corresponding dimer 14 was
obtained in a modest yield (entry 6). The new carbon–carbon
bond was formed at the less-hindered site in a regioselective
manner. The ortho-substituted isomer 15, wherein the steric
demand is posed, can also be successfully applied to catalytic
dimerization (entry 7). It should be noted that styrenes contain-
ing a substituent on the vinyl moiety, such as propenyl- and
isopropenylbenzene, did not afford any dimerized products.10
Although the mechanism of dimerization involving the clea-
vage of the ortho C–H bond remains elusive, two possible
pathways that account for the observed product are illustrated
in Scheme 3. The first possible path begins with the oxidative
cyclization of the p-complex 17, affording a metallacycle 18
containing a Rh(III) center (path A). A 1,2-hydride shift sub-
sequently gives a hydride–Rh(III) complex 19. The overall
process from 17 to 19 can be viewed as a formal oxidative
addition of the ortho C–H bond of styrenes.11 Insertion of
another styrene into the aryl–Rh bond in 19, followed by an
intramolecular hydrometallation, forms a metallacycle 21. The
used as the catalyst (entry 7). Thus, we subsequently examined a
variety of alcohol additives in place of H2O. Among the alcohols
examined,
t-BuOH proved to be the optimal choice (entry 8).8,9 Under the
conditions using t-BuOH, we also examined the effect of solvents
(mesitylene, ethylcyclohexane, DMF, and t-BuOH) and bases
(Cs2CO3, CsF, CsOPiv, Et3N, and DBU), although the yield of 3
did not improve. However, conducting the reaction at higher
temperature (160 1C) led to a slight increase in yield (entry 9). It
should be noted that other types of dimerization products (head-
to-tail, head-to-head and tail-to-tail) were not observed in this
rhodium catalyst system.
Although the catalytic activity was not satisfactory at this
point, we decided to examine the effect of substituents using
these catalytic conditions (Table 2). Gratifyingly, introduction of
a trifluoromethyl group at the para position of styrene drama-
tically increased the yield of the dimer (entry 2). Similarly, the
use of 4-fluorostyrene furnished the product in an improved
yield (entry 3). In contrast, the reactions of styrenes bearing
other electron-withdrawing substituents, such as ester (18%),
nitro (0%) and cyano (0%) groups, were disappointing (data
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Notes and references
z Compound 4. C44H32, M = 560.70, triclinic, a = 11.4861(2),
b = 12.8423(2), c = 21.3357(4) A, a = 96.9557(7), b = 96.5045(7),
3
ꢀ
g = 100.6850(7)1, U = 3039.99(9) A , T = 113(2) K, space group P1,
Z = 4, m(Cu Ka) = 0.523 mmꢁ1, 57 689 reflections measured, 4723
unique (Rint = 0.0956) which were used in all calculations. The final
R1 and wR2 were 0.0898 and 0.2162 (I 42s(I)).
1 C. Janiak, Coord. Chem. Rev., 2006, 250, 66, and references therein.
2 J. Skupinska, Chem. Rev., 1991, 91, 613, and references therein.
3 For selected examples, see: (a) M. G. Barlow, M. J. Bryant, R. N.
Haszeldine and A. G. Mackie, J. Organomet. Chem., 1970, 21, 215;
(b) K. Kawamoto, A. Tatani, T. Imanaka and S. Teranishi, Bull.
Chem. Soc. Jpn., 1971, 44, 1239; (c) F. Dawans, Tetrahedron Lett.,
1971, 12, 1943; (d) T. Tsuchimoto, S. Kamiyama, R. Negoro, E.
Shirakawa and Y. Kawakami, Chem. Commun., 2003, 852; (e) C.
Sui-Seng, L. F. Groux and D. Zargarian, Organometallics, 2006,
25, 571; (f) N. Galdi, C. D. Monica, A. Spinella and L. Oliva, J.
Mol. Catal. A: Chem., 2006, 243, 106.
4 (a) G. Wu, A. L. Rheingold and R. F. Heck, Organometallics,
1987, 6, 2386; (b) G. Wu, S. J. Geib, A. L. Rheingold and R. F.
Heck, J. Org. Chem., 1988, 53, 3238.
5 (a) W. P. Kretschmer, S. I. Troyanov, A. Meetsma, B. Hessen and
J. H. Teuben, Organometallics, 1998, 17, 284; (b) T. Kondo, D.
Takagi, H. Tsujita, Y. Ura, K. Wada and T.-a. Mitsudo, Angew.
Chem., Int. Ed., 2007, 46, 5958.
6 (a) Y. Ishii, N. Chatani, S. Yorimitsu and S. Murai, Chem. Lett.,
1998, 27, 157; (b) M. Tobisu, Y. Kita and N. Chatani, J. Am.
Chem. Soc., 2006, 128, 8152; (c) M. Tobisu, Y. Ano and N.
Chatani, Chem.–Asian J., 2008, 8, 201.
7 For a recent review on catalytic dehydrogenative silylation of
alkenes, see: B. Marciniec, Coord. Chem. Rev., 2005, 249, 2374.
8 Yields with other additives are as follows: phenol (0%), 1-methyl-
cyclohexanol (16%), 2-adamantanol (15%), t-amyl alcohol (18%),
and AcOH (0%).
9 The exact role of these additives is currently unclear. One possibi-
lity is the formation of [Rh(OBu-t)(cod)] species. See: S. K.
Agarawal and R. C. Mehrotra, J. Indian Chem., 1985, 62, 805.
10 Notes: (a) In all cases shown in Table 2, the starting styrenes were
completely consumed after 20 h. Relatively poor material balance is
due to the oligomerization of styrenes, since no other significant
products were observed by GC analysis of the crude reaction mixture;
(b) Vinyl heteroaromatics did not give the corresponding dimers;
(c) Attempts at cross-coupling with other alkenes, such as trimethyl-
vinylsilanes and norbornene, have been unsuccessful thus far.
11 A similar mechanism is proposed in the oxidative addition of the
ortho C–H bond of acetophenones. See: (a) F. Kakiuchi, S. Sekine,
Y. Tanaka, A. Kamatani, M. Sonoda, N. Chatani and S. Murai,
Bull. Chem. Soc. Jpn., 1995, 68, 62; (b) T. Matsubara, N. Koga, D.
G. Musaev and K. Morokuma, J. Am. Chem. Soc., 1998, 120,
12692; (c) T. Matsubara, N. Koga, D. G. Musaev and K.
Morokuma, Organometallics, 2000, 19, 2318.
Scheme 3 Mechanistic possibilities.
elimination of the benzylic b-hydrogen in 21 then affords a
hydride–Rh(III) complex 22, which finally leads to a dimer 3 by
reductive elimination. An alternative mechanism is mediated by
a hydride–Rh complex (formal valence of Rh could be I or III),
which could be generated in situ by the reaction of the rhodium
precursor with added alcohol or residual water (path B). Hydro-
metallation of 1 provides an alkyl–rhodium species 23, which
then leads to an aryl–Rh complex 24 via a 1,4-transposition of
the rhodium center.12,13 The reaction of 24 with another styrene
via a Mizoroki–Heck-like mechanism (hydrometallation/b-hy-
dride elimination) furnishes the final product 3 while regenerat-
ing the hydride–rhodium species. The required regiochemistry
for the initial hydrometallation (1 - 23) is unfavorable com-
pared with the formation of 26. Thus, path B would be viable
only when the reaction of benzyl–rhodium complex 26 with
another styrene is relatively slow.14
12 For a review, see: S. Ma and Z. Gu, Angew. Chem., Int. Ed., 2005,
44, 7512.
In summary, we have described an unusual rhodium-
catalysed dimerization of styrenes, which involves the cleavage
of an ortho C–H bond. Although a substantial number of
reports have appeared involving the catalytic ortho C–H bond
functionalization of benzenes bearing polar coordinating groups
(i.e., pyridine, ketone, etc.),15 the reaction described herein
represents the first catalytic ortho C–H functionalization of
styrenes.16
13 For examples of a 1,4 shift of an alkyl–metal species, see: (a) J. C.
Calabrese, M. C. Colton, T. Herskovitz, U. Klabunde, G. W.
Parshall, D. L. Thorn and T. H. Tulip, Ann. N. Y. Acad. Sci., 1983,
415, 302; (b) K. Oguma, M. Miura, T. Satoh and M. Nomura, J.
Am. Chem. Soc., 2000, 122, 10464; (c) L. Wang, Y. Pan, X. Jiang
and H. Hu, Tetrahedron Lett., 2000, 41, 725; (d) T. Matsuda, M.
Shigeno and M. Murakami, J. Am. Chem. Soc., 2007, 129, 12086.
14 Preliminary labeling studies indicated that the majority of the
cleaved ortho hydrogen atom was incorporated into the methyl
group of the product (see supporting information for details).
These results agree with both of our proposed mechanisms.
15 For recent reviews, see: (a) F. Kakiuchi and N. Chatani, Adv.
Synth. Catal., 2003, 345, 1077; (b) A. R. Dick and M. S. Sanford,
Tetrahedron, 2006, 62, 2439; (c) K. Godula and D. Sames, Science,
2006, 312, 67; (d) Topics in Organometallic Chemistry, ed. N.
Chatani, Springer-Verlag, Berlin, 2007, vol. 24.
This work has been carried out under the Program of Promotion
of Environmental Improvement to Enhance Young Researchers’
Independence, the Special Coordination Funds for Promoting
Science and Technology, Ministry of Education, Culture,
Sports, Science and Technology, Japan. We also thank the Instru-
mental Analysis Center, Faculty of Engineering, Osaka University,
for their assistance with the MS, HRMS, and elemental
analyses.
16 Carbon–carbon triple bond directed ortho C–H bond functionali-
zation has been reported recently: N. Chernyak and V. Gevorgyan,
J. Am. Chem. Soc., 2008, 130, 5636.
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