1-Chloro-4,5,6,7-tetraalkyldihydroindene formation by reaction of bis(cyclopentadienyl)titanacyclopentadienes with titanium chloride

Article abstract of DOI:10.1021/ja074180p

Reaction of titanacyclopentadienes 1 with TiCl₄ afforded 1-chloro-4,5,6,7-tetrasubstituted dihydroindene derivatives 8 in high yields. Copyright

Full text of DOI:10.1021/ja074180p

Published on Web 08/31/2007
1-Chloro-4,5,6,7-tetraalkyldihydroindene Formation by Reaction of
Bis(cyclopentadienyl)titanacyclopentadienes with Titanium Chloride
Tamotsu Takahashi,*, Zhiyi Song, Kimihiko Sato, Yuichi Kuzuba, Kiyohiko Nakajima, and
†
†
†
†
‡
†
Ken-ichiro Kanno
Catalysis Research Center, Hokkaido UniVersity, and SORST, Japan Science and Technology Agency (JST), Kita-ku,
Sapporo 001-0021, Japan, and Department of Chemistry, Aichi UniVersity of Education, Igaya, Kariya 448-8542,
Aichi, Japan
Received June 22, 2007; E-mail: tamotsu@cat.hokudai.ac.jp
Recently, we have reported the formation of unusual indene
derivatives 2 with alkyl group migration via carbon-carbon bond
cleavage from bis(cyclopentadienyl)titanacyclopentadienes 1, which
were prepared from alkyl-substituted alkynes (eq 1).1
Column separation of the reaction products with silica gel in air
after the reaction of 1a with 4 equiv of TiCl at room temperature
for 1 h gave indene 5a, dihydroindenol 6a, and indenol 7a in 39,
4
22, and 6% yield, respectively.
It was found that before treatment with silica gel in air 1-chloro-
4,5,6,7-tetraethyldihydroindene 8a was formed in high yield.
As shown in Table 1, the yield of 8a was dependent on the
4 4
amount of TiCl . When the amount of TiCl increased to 8 equiv,
We also found this transformation proceeded via Rosenthal-type
reaction of complex 3 with an alkyl group at a bridgehead carbon.
the yield was remarkably improved to 99%. With CuCl , a small
2
2
amount of 8a (11% yield) was obtained. However, other metal
We could rationalize that the alkyl group of the bridgehead of 3
transferred to give 2 by oxidation of the titanium metal center.
However, the formation 3 from 1 is still mysterious since the
formation of complex 3 involves the carbon-carbon bond cleavage
of the Cp ligand.3
chlorides, such as ZnCl , ZrCl , and TaCl , did not give the
2
4
5
compound 8a at all. Use of TiBr instead of TiCl afforded 5a in
4
4
less than 10% yield.
In order to verify the structure of 8a, Diels-Alder reaction of
8a with TCNE was carried out to obtain compound 9. The structure
of 9 was determined by X-ray analysis. The structure is shown in
Figure 1.
It clearly shows that four Et groups are in the six-membered
ring moiety but not at the bridgehead carbon of 8a. It also reveals
that the Cl atom attached to the five-membered ring with exo-
configuration.
-5
It is interesting to know if complex 4 can be formed without the
carbon-carbon bond cleavage of the Cp ligand by either intramo-
lecular Diels-Alder-type reaction of one olefin moiety of one Cp
ligand with the metallacyclopentadienyl moiety in 1 or stepwise
C-C bond formation of one Cp ligand with metallacyclopentadienyl
6
moiety in 1 (eq 2). First we focused on the possibility for the
formation of 4 from 1 prepared from alkyl-substituted alkynes.
The results for the reaction of 1b-e with 8 equiv of TiCl are
4
summarized in Table 2. The reactions of monocyclic titanacyclo-
4
pentadienes 1b and 1c with TiCl produced the corresponding 8b
and 8c in high yields (entries 1 and 2). The reactions with bicyclic
titanacyclopentadiene 1d and tricyclic titanacyclopentadiene 1e also
gave the corresponding indenes 8d and 8e in high yields, respec-
tively.
The products 8a-e were converted into indene derivatives 5a-e
after reflux in toluene for 2 h in 50-65% isolated yields.
It is important to know if the five-membered ring of 8 came
from the Cp ligand of 1 with or without C-C bond cleavage of
Rosenthal’s pioneering work revealed that a dihydroindene
titanium species similar to 4 was obtained when 4 had pyridyl
substituents instead of alkyl substituents. In this case, the pyridyl
group stabilized complex 4 by coordination. However, unfortu-
nately, there is no report for the formation of 2 with the pyridyl
substituents.
This prompted us to investigate evidence for the formation of
dihydroindene titanium complexes 4 with usual alkyl substituents
which can provide complex 2.
7
13
the Cp ligand. Therefore, we used C-enriched titanacyclopenta-
1
3
3a
diene 1a- C of which a preparative method was reported previously.
13
13
The product was converted to 5a- C, and the C content of the
five-membered ring carbons of 5a was checked.
As shown in eq 4, comparison of C NMR spectra of 5a and
13
13
13
13
5
a- C revealed that the C-enriched carbons of Cp in 1a- C were
In this paper, we would like to report the formation of 1-chloro-
13
located at five-membered ring carbons in 5a- C. Furthermore, a
deuterium-labeled experiment was also carried out as shown in
eq 5. The reaction of titanacyclopentadiene 1a-d10 deuterated on
4
,5,6,7-tetraalkyldihydroindenes by the reaction of titanacyclopen-
8
4
tadienes 1 with TiCl , where the five-membered ring moiety of
the chlorodihydroindene came from a Cp ligand without the C-C
bond cleavage.
9
4 4
the Cp ligands with TiCl produced indene 5a-d deuterated at 1-,
1
-, 2-, and 3-positions. This result is consistent with that of the
C-labeled experiment. The existence of two deuterium atoms on
1
3
†
Hokkaido University and JST.
Aichi University of Education.
‡
the C-1 carbon indicates the migration of one deuterium atom during
11678
9
J. AM. CHEM. SOC. 2007, 129, 11678-11679
10.1021/ja074180p CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Reaction of Titanacyclopentadienes 1a with Metal
Halides Affording 8a
Scheme 1. Possible Reaction Mechanism for the Formation of 8
from 1
metal
chloride
a
entry
n equiv
yield /%
1
2
3
4
5
6
7
TiCl4
TiCl4
TiCl4
CuCl2
ZnCl2
ZrCl4
TaCl5
1
4
8
4
4
4
4
5
70
99
11
0
This is a strong evidence of the formation of complex 4 without
carbon-carbon bond cleavage from titanacyclopentadiene 1 with
usual alkyl substituents in addition to all previously described
products on the basis of complex 3 with carbon-carbon bond
cleavage.1
0
0
,4a
a
NMR yield. Conditions: 0 °C, 1 h.
Scheme 1 shows a plausible mechanism of the indene formation
reaction. At first, coordination mode of the Cp ring is changed from
5
3
4
η - to η -fashion, caused by coordination of the Cl atom of TiCl .
Diels-Alder reaction or stepwise coupling between the slipped Cp
ring and butadienyl moiety of the titanacycle occurs to produce
the corresponding dihydroindenyl complex 4. It is known that the
titanium metal center in the type 4 complex occupied the endo-
position as an allyltitanium species having an interaction with the
diene moiety. This means that the Cl anion attacks from the back
side of the π-allyltitanium of the five-membered ring giving 8. This
is also consistent with the configuration of the Cl atom shown in
X-ray structure of 9.
Figure 1. X-ray crystal structure of 9.
Table 2. Formation of 1-Chloro-4,5,6,7-tetrasubstituted
In conclusion, titanacyclopentadienes reacted with TiCl
-chlorotetraalkyldihydroindene derivatives 8 without carbon-
carbon bond cleavage of the Cp ligand. This is in sharp contrast to
the reaction using the same starting complexes, which affords 2
with carbon-carbon bond cleavage of the Cp ligand.
4
to give
1
Dihydroindene Derivatives 8 from Titanacyclopentadienes 1
We must await further investigation to elucidate how the carbon-
carbon bond is cleaved in the coupling reaction of a Cp ligand and
the diene moiety of titanacyclopentadienes 1.
Supporting Information Available: Experimental procedure,
13
spectra data and NMR spectra for 5a-e, 5a-d , 5a- C, 6a, 7a, 8a-e,
4
and 9. Crystallographic data for 9. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(
(
(
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a
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(
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(
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(
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(
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(
(
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