Once cleaved C-C bond was reformed: Reversible C-C bond cleavage of dihydroindenyltitanium complexes

Article abstract of DOI:10.1021/ja805352z

The once cleaved carbon-carbon bond of the Cp moiety in 2 was recombined in indene products. Aslo, we propose a novel mechanism for the cleavage of the carbon-carbon bond of the Cp moiety. Copyright

Full text of DOI:10.1021/ja805352z

Published on Web 10/25/2008
Once Cleaved C-C Bond Was Reformed: Reversible C-C Bond Cleavage of
Dihydroindenyltitanium Complexes
,†
†
†
‡
†
Tamotsu Takahashi,* Zhiyi Song, Yi-Fang Hsieh, Kiyohiko Nakajima, and Ken-ichiro Kanno
Catalysis Research Center and Graduate School of Life Sciences, 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 July 10, 2008; E-mail: Tamotsu@cat.hokudai.ac.jp
Carbon-carbon bond cleavage of organic molecules has been a
1-4
challenging target in organic chemistry.
Rosenthal et al. have
reported that 1a was converted into 2a by coupling of a Cp moiety
2
and a diene moiety of 1a. Recently we have converted 2a into Et
group migrated compound 3a by oxidation and proved that the
carbon-carbon bond of the original Cp moiety was cleaved in 3a by
C labeled experiment as shown in eq 1. It was unusual that cyclic
1
3
13
C, respectively. C NMR spectra clearly showed the original
five Cp carbons were linearly lined-up in the dihydroindenyl moi-
13
3a
1
3
13
ety of 2a- C and 2b- C.
five carbons of the cyclopentadienyl ligand of 1a were converted into
linearly aligned five carbons in 2a and 3a. It is reasonable to assume
that there is an intermediate obtained by coupling of a Cp ligand and
a diene moiety without carbon-carbon bond cleavage and then its
rearrangement proceeds by the carbon-carbon bond cleavage as shown
in eq 2. Rosenthal et al. demonstrated that a titanacyclopentadiene
1
3
When the complex 2b- C was treated with azobenzene at 60
1
3
°
C for 6 h, surprisingly, 4,5,6,7-tetraethylindene 5b- C was
1
3
obtained in 50% yield along with the formation of 3b- C as a
1
3
byproduct in 40% yield. C NMR study revealed that five carbons
1
3
originally from a Cp ligand were recombined as a cycle in 5b- C.
At lower temperature such as 25 °C, the ethyl group transfer
compound 3b was the major product. With increase the temperature,
the yield of 5b increased to 50%. Rosenthal complex 2a was also
converted into 4,7-diethyl-5,6-tetramethyleneindene 5a in 39% yield
as shown in eq 4. Compounds 5c,d were obtained in 45-61% yields.
prepared from a pyridyl group-substituted alkyne afforded a dihy-
droindenyl titanium complex without C-C bond cleavage of the Cp
moiety. It is due to the stabilization of the intermediate-type complex
5
by coordination of the pyridyl group to titanium metal center.
Very recently we observed the existence of the intermediate from
the same starting material 1a. Chlorodihydroindene derivative
without carbon-carbon bond cleavage of the Cp moiety 4a was
6
13
obtained by the reaction of 1a with TiCl
4
.
C labeled experiment
1
3
showed the five carbons from a Cp ligand of 1a- C were in the
1
3
cyclic five-membered ring in 4a- C. However, it is still unusual
for the mechanism for the formation of the linearly aligned five
carbons in 2 or 3 from the cyclic five carbons of the intermediate.
In this paper, we would like to report that the once cleaved
carbon-carbon bond of the Cp moiety in 2 was reformed in the
indene products as shown eq 3 and also propose a novel mechanism
for the cleavage of the carbon-carbon bond of the Cp moiety.
First we carried out verification of the linearly aligned five
It is interesting to note that direct treatment of 1a-d with
azobenzene also gave 5a-d in 72-95% yields at 50 °C for 8 h.
The formation of alkyl transferred product 3a-d was not observed
in these cases. First, the yields of 5a-d were much higher than
the reaction from 2a-d. Second, alkyl transferred byproduct 3a-d
were not formed. Therefore, the formation of 5a-d from 1a-d
did not go though the complexes 2a-d.
1
3
6,3b
carbons in 2a and 2b by C labeling experiments.
The
1
3
13
13
complexes 2a- C and 2b- C were prepared from 1a- C and 1b-
Investigation of the reaction rates from 1c to 2c and from 1c to
5c revealed very interesting result (Scheme 1). Both reactions were
found to exhibit first order kinetic behavior and the both rates were
†
Hokkaido University, and SORST.
Aichi University of Education.
‡
1
5236 9 J. AM. CHEM. SOC. 2008, 130, 15236–15237
10.1021/ja805352z CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Reaction Path from 1c to 2c and 5c
after converting to Diels-Alder product with TCNE. This clearly
shows that in the intermediate 6e, the methyl group occupied C2
position of the dihydroindene moiety. After heating, 1e was converted
into 2e. The position of Me group was checked after hydrolysis. X-ray
analysis of Diels-Alder reaction product 9e of the hydrolysis product
8e with TCNE revealed that the methyl group occupied the C2 position
of dihydroindene 2e and 8e. According to the nine-membered ring
mechanism shown in Scheme 2, the methyl substituent should occupy
C3 position as shown in parentheses in Scheme 4. However, the methyl
Scheme 4. Mechanism for Transformation between 2e and 6e via
Metathesis
-
2
-2
(
5
1.03 ( 0.04) × 10 min-1 and (1.03 ( 0.09) × 10 min-1 at
0 °C, respectively. Even at the different temperatures such as 60
and 70 °C, the reaction rates for both reactions were almost the
-
2
same, for example, at 60 °C, (3.01 ( 0.10) × 10 and (3.02 (
-
2
-1
-2
0
(
.16) × 10 min and at 70 °C, (8.16 ( 0.24) × 10 and (8.73
-2
-1
0.43) × 10 min , respectively. Activation energy values for
both reactions were 95.2 ( 7.2 kJ/mol and 99.3 ( 8.8 kJ/mol,
respectively. In the case of the reaction from 2c to 5c, the reaction
also obeyed the first order rule but the reaction rate was (2.96 (
-
2
-1
0
.46) × 10 min at 50 °C. The reaction from 2c to 5c was
faster than that from 1c to 5c.
group occupied C2 position in 2e. This clearly shows that the
mechanism via the cyclic nine-membered ring is not consistent.
Me group occupied at the second carbon of linearly aligned five
carbons in 2e. This indicates that the carbon-carbon bond of the
bridge-head carbon and C3 carbon in 6e should be cleaved as shown
in Scheme 4. This bond is a part of the titanacyclobutane moiety
which can be converted into the titanium carbene moiety and the
olefin moiety by metathesis. The metathesis cleavage of the bond
provides 10 containing titanium carbene moiety. The complex 11
is the same as 10. Pentadienyl migration of titanium on six-
membered ring gives 12. The titanium carbine and one olefin
coupling affords again titanacyclobutane moiety in 13. Changing
the postion of Ti in 13 gives 2e. This mechanism via metathesis
can explain all the results we obtained. Therefore, we propose a
novel reversible mechanism via metathesis of titanacyclobutane
moiety of 6e and 2e as shown in Scheme 4.
This result suggests that the reaction from 1c to 2c and that
from 1c to 5c have the same intermediate 6, and the path from 1c
to the intermediate 6c is the rate determining step.
This result strongly suggests that the complexes 2 and intermediate 6
are in equilibrium. Higher stability of 2 over 6 may be due to the number
of substituents of diene moiety which coordinates to Ti. Hydrogen transfer
from 6 to azobenzene and aromatization afford indene 5.
At the beginning, we believed that the formation of nine-
membered cyclic ligand 7 is reasonable to explain the transforma-
tion from the intermediate 6 to 2 as shown in Scheme 2. A complex
Scheme 2. Mechanism for Reversible Path for 2 and 6 via
Cyclononatetraenyl Moiety
Supporting Information Available: Experimental details and
spectra data for all new compounds (PDF), X-ray analysis data for 9e,
and Diels-Alder products of 4e with TCNE. This material is available
free of charge via the Internet at http://pubs.acs.org.
with cyclic nine-membered ring has been known and a nine-
membered cyclic ligand can be formed via cleavage of the bridge
carbon-carbon bond of dihydroindene moiety.
To confirm the possibility of this mechanism, we carried out the
reaction using methyl-substituted cyclopentadienyltitanacyclopentadiene
References
7
,8
(1) For recent reviews for C-C bond cleavage with metallocene complexes,
see: Takahashi, T.; Kanno, K. Metallocene in Regio- and Stereoselective
Synthesis. In Topics in Organometallic Chemistry; Takahashi, T., Ed.;
Springer: Berlin, 2005; Vol. 8, p 217.
1
e. As shown in Scheme 3, first, 1e was treated with azobenzene and
(2) For C-C bond cleavage reactions of Cp ligand, see ref 2, 3, and 4. Tillack,
A.; Baumann, W.; Lefeber, O. C.; Spannenberg, A.; Kempe, R.; Rosenthal,
U. J. Organomet. Chem. 1996, 520, 187–193.
Scheme 3. Transformation of Me-Substituted
Titanacyclopentadiene 1e
(
3) (a) Takahashi, T.; Kuzuba, Y.; Kong, F.; Nakajima, K.; Xi, Z. J. Am. Chem. Soc.
005, 127, 17188. (b) Xi, Z.; Sato, K.; Gao, Y.; Lu, J.; Takahashi, T. J. Am.
2
Chem. Soc. 2003, 125, 9568–9569. For the theoretical study on 3b, see
also: (c) Suresh, C. H.; Koga, N. Organometallics 2006, 25, 1924–1937.
4) Carbon-carbon bond cleavage of Cp ligand on Co complexes and others,
see: (a) Dzwinniel, T. L.; Stryker, J. M. J. Am. Chem. Soc. 2004, 126, 9184.
(
(
b) Dzwinniel, T. L.; Etkin, N.; Stryker, J. M. J. Am. Chem. Soc. 1999, 121,
1
5
0640–10641. (c) Crowe, W. E.; Vu, A. T. J. Am. Chem. Soc. 1996, 118,
508–5509. (d) Gleiter, R.; Wittwer, W. Chem. Ber. 1994, 127, 1797. (e)
Giolando, D. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 1984, 106, 6455.
5) Rosenthal, U.; Lefeber, C.; Arndt, P.; Tillack, A.; Baumann, W.; Kempe,
R.; Burlakov, V. V. J. Organomet. Chem. 1995, 503, 221. See also Thomas,
D.; Peulecke, N.; Burlakov, V. V.; Heller, B.; Baumann, W.; Spannenberg,
A.; Kempe, R.; Rosenthal, U. Z. Anorg. Allg. Chem. 1998, 624, 919–924.
6) Takahashi, T.; Song, Z.; Sato, K.; Kuzuba, Y.; Nakajima, K.; Kanno, K.
J. Am. Chem. Soc. 2007, 129, 11678–11679.
7) Titanacyclopentdienes were prepared by our method, see : Sato, K.; Nishihara,
Y.; Huo, S.; Xi, Z.; Takahashi, T. J. Organomet. Chem. 2001, 633, 18–26.
8) Wilson, A. M.; Waldman, T. E.; Rheingold, A. L.; Ernst, R. D. J. Am. Chem.
Soc. 1992, 114, 6252–6254.
(
(
(
(
2
-methylindene 5e was obtained. In addition, 1e was also treated with
TiCl and the corresponding chlorodihydroindene 4e was obtained.
The position of the Me group in 4e was verified by X-ray analysis
4
JA805352Z
J. AM. CHEM. SOC. 9 VOL. 130, NO. 46, 2008 15237