Unprecedented double C-C bond cleavage of a cyclopentadienyl ligand

Article abstract of DOI:10.1021/ja027234y

Double C-C bond cleavage of a cyclopentadienyl ligand proceeded to titanacyclopentadienes when 2 equiv of nitriles were added and the resulting two-carbon unit and three-carbon unit were converted into a benzene derivative and a pyridine derivative, respectively, in one-pot. Copyright

Full text of DOI:10.1021/ja027234y

Published on Web 07/17/2003
Unprecedented Double C-C Bond Cleavage of a Cyclopentadienyl Ligand
Zhenfeng Xi,*,† Kimihiko Sato,‡ Ye Gao,† Jianming Lu,† and Tamotsu Takahashi*,‡
Peking UniVersity-Hokkaido UniVersity Joint Lab, Department of Chemistry, Peking UniVersity, Beijing 100871,
China, and Catalysis Research Center and Graduate School of Pharmaceutical Sciences, Hokkaido UniVersity, and
CREST, Japan Science and Technology Corporation (JST), Sapporo 060-0811, Japan
Received June 9, 2002; E-mail: tamotsu@cat.hokudai.ac.jp
Carbon-carbon bond cleavage is one attractive target of organic
chemistry.1-3 However, in most cases, the carbon-carbon bond
cleavage reaction is limited to the ring opening, or elimination of
small molecules, intramolecular rearrangement reactions, or metal-
complex formation of the intermediates. From the point of view of
organic synthesis, conversion of the cleaved fragments into useful
organic molecules is more attractive and important. In this
contribution, we report an unprecedented double C-C bond
cleavage of a cyclopentadienyl ligand and transformation of the
resulting two pieces such as a two-carbon unit and a three-carbon
unit into a benzene derivative and a pyridine derivative, respectively,
in one-pot (Figure 1).4
trideutero-2,6-diphenylpyridine 3a-d3 with more than 97% deute-
rium incorporation (eq 2). Their 13C NMR spectra clearly showed
the positions of those deuteriums in 2a-d2 and 3a-d3. This indicates
that the two CH moieties in tetraethylbenzene 2a are from the Cp
ligand of 1a and the remaining three CH moieties of the Cp ligand
were trapped as diphenylpyridine 3a.
Furthermore, to obtain the direct evidence for the C-C bond-
tearing reactions of the Cp ligand into two pieces, we synthesized
13C-enriched Cp2TiCl2 (4.7 times 13C enriched in Cp ligands) and
carried out the same reaction as for deuterium experiment. The
results are shown in (eq 3) and the 13C NMR study clearly showed
that C5 and C6 carbons of 1,2,3,4-tetraethylbenzene were 13C-
enriched (4.7 times) and C3, C4, and C5 carbons of 2,6-
diphenylpyridine were also 13C-enriched (4.6 times). This result is
completely consistent with the deuterium experiments.
Figure 1. Formation of a benzene derivative and a pyridine derivative
from a cyclopentadienyl ligand via cleavage of two C-C bonds and selective
C-C formation.
Reaction of titanacyclopentadiene 1a,5 which was generated in
situ from Cp2TiBu2 and 2 equiv of 3-hexyne, with 2 equiv of PhCN
in THF at reflux for 1 h resulted in the formation of 1,2,3,4-
tetraethylbenzene, 2a, and 2,6-diphenylpyridine, 3a, in 60 and 52%
yields, respectively (eq 1). In addition to PhCN, a variety of nitriles,
As a whole, two carbon-carbon bonds of a cyclopentadienyl
ligand were cleaved and torn into two pieces, a two-carbon unit
and a three-carbon unit. The two-carbon unit was trapped by
titanacyclopentadienyl moiety to form tetraethylbenzene 2a, and
the three-carbon unit was trapped by 2 equiv of benzonitrile to
afford diphenylpyridine 3a.
On the other hand, reaction of 1a with 13C-labeled PhCN gave
the benzene derivative 2a in 59% yield in which 13C was not
incorporated. Pyridine was obtained in 52% yield, and the carbons
attached to a Ph group were 13C enriched.
In addition to the above monocyclic titanacyclopentadiene 1a,
bicyclic titanacyclopentadiene 1b, 1c, and 1d could also undergo
the same reactions and afforded 2b, 2c, and 2d in 50, 72, and 70%
yields, respectively, along with the formation of 3a in 56, 47, and
45% yields, respectively. Results are shown in Table 1To under-
stand whether this reaction proceeds via an intramolecular pattern
or an intermolecular way, a mixture of 1e-d10 (R ) Pr) and 1f (R
) Me) in a 1:1 molar ratio was treated with PrCN. As shown in eq
4, only the corresponding intramolecular coupling products, 2e-
d2 and 2f were obtained. This clearly indicates that the benzene
such as p-F-C6H4CN, p-MeC6H4CN, and PrCN were found to be
effective for the above novel reaction, resulting in the formation
of 3b, 3c, and 3d, respectively, as well as the formation of 2a.
Treatment of the reaction mixtures with DCl or I2 instead of
hydrolysis also afforded the same products 2 and the corresponding
pyridine derivatives 3 (eq 1), which indicates that free benzene
derivatives 2 and free pyridine derivatives 3 were formed in this
reaction. Monitoring of the reaction of 1a with 2 equiv of PhCN at
40 °C revealed that 2a was formed first and then 3a appeared with
a time-lag.
(C5D5)2TiCl2 was prepared from C5D5Li and TiCl4 and used for
this reaction instead of Cp2TiCl2. Surprisingly, the reaction of 1a
with PhCN afforded dideuterated tetraethylbenzene 2a-d2 and 3,4,5-
† Peking University-Hokkaido University Joint Lab, Department of Chemistry,
Peking University.
‡ Catalysis Research Center and Graduate School of Pharmaceutical Sciences,
Hokkaido University, and CREST, Japan Science and Technology Corporation
(JST).
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J. AM. CHEM. SOC. 2003, 125, 9568-9569
10.1021/ja027234y CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Formation of Benzene Derivatives and Pyridine
Derivatives from Cyclopentadienyltitanacyclopentadienes
In addition, we synthesized a tetraethyltitanacyclopentadiene with
one methylcyclopentadienyl ligand and a pentadeuterated Cp ligand.
When it was treated with PhCN, only 2a and 4-methyl-2,5-diphenyl-
pyridine were obtained as benzene derivatives and pyridine deriva-
tives, respectively, from the methylcyclopentadienyl ligand. No for-
mation of 3a from the methylcyclopentadienyl ligand was observed.
This clearly shows that the pyridine is formed from the rest of the
C3 unit of cyclopentadienyl ligand after formation of the benzene
derivative.
Recently Rosenthal and co-workers reported an interesting ring
opening of the Cp ligand of bicyclic bis-η5-cyclopentadienyltitana-
cyclopentadienes such as 1b and an intramolecular C-C coupling
to form a stable η4,η3-dihydroindenyltitanium complex bearing a
Ti-allyl moiety.2k,m Although we must await for further investiga-
tions to elucidate the reaction mechanism, one possible reaction
path is via indene derivative formation by coupling of a Cp ligand
with titanacyclopentadiene moiety.
Acknowledgment. We thank Mr. Y. Kuzuba for his assistance
in experiments. A part of this work was supported by National
Science Fund for Distinguished Young Scholars (29825105), the
Major State Basic Research Development Program (G2000077502-
D), and National Natural Science Foundation of China.
Supporting Information Available: Experimental details and
spectra data for all new compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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After the reaction of 1a with 2 equiv of PhCN, 0.1 equiv of
unreacted PhCN was recovered in the reaction mixture. Hydrolysis
and distillation of the reaction mixture gave 1.21 equiv of ammonia
which was detected by indophenol method. Since 3a was formed
in 52% yield, more than 90% of N was detected. The origin of
ammonia may be titanium nitride or titanium imide complexes.
However, we did not observe the clean formation of some specified
species.
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