Coming Back to the Starting Position of Carbons Traveling in Organic Molecules on Titanium: Merry-Go-Round Reaction

Article abstract of DOI:10.1021/acs.organomet.8b00876

Traveling of carbon atoms in organic molecules was successfully achieved. The six-membered ring of dihydroindenyl moiety on titanium rotated based on the five-membered ring like a merry-go-round. Two carbons at the bridgehead of the dihydroindenyl moiety

Full text of DOI:10.1021/acs.organomet.8b00876

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Coming Back to the Starting Position of Carbons Traveling in
Organic Molecules on Titanium: Merry-Go-Round Reaction
†
‡
†
,†
Masayoshi Bando, Kiyohiko Nakajima, Zhiyi Song, and Tamotsu Takahashi*
†
Institute for Catalysis, Hokkaido University, Kita 21, Nishi 10, Sapporo 001-0021, Japan
‡
Department of Chemistry, Aichi University of Education, Igaya, Kariya, Aichi 448-8542, Japan
*
S Supporting Information
ABSTRACT: Traveling of carbon atoms in organic molecules was
successfully achieved. The six-membered ring of dihydroindenyl moiety
on titanium rotated based on the five-membered ring like a merry-go-
round. Two carbons at the bridgehead of the dihydroindenyl moiety
and the other three carbons of the five-membered ring were monitored
1
3
by C-labeled experiments. The two carbons at the bridgehead moved
to the farthest positions with the rotation of the six-membered ring.
The intermediate where the two carbons were located at the farthest
positions was isolated and fully characterized. Treatment of the
intermediate with azobenzene made the two carbons come back to the
starting positions. There is another viewpoint on this reaction. When
the six-membered ring is fixed and the movement of three carbons of
the five-membered ring of the dihydroindenyl moiety on titanium is
focused, it was found that those three carbons moved around the six-membered ring and came back to the starting points.
changed step-by-step in an organic molecule as in compounds
−4. To achieve this challenge, each transformation consists of
raveling of carbon atoms in organic molecules is a new
challenge in organic chemistry. The general scheme of the
1
T
carbon−carbon bond-cleaving and carbon−carbon bond-
forming processes. Although C−C bond cleavage has been
well studied over the past three decades, there is no report on the
traveling and coming back to the starting point of carbon atoms
in the same type of carbon skeleton to the best of our
reaction is shown in Scheme 1a. The position of carbons is
a
Scheme 1. Traveling of Carbon Atoms in Organic Molecules
1
−21
knowledge.
When compounds 1−4 are in equilibrium, a
target molecule should be designed as the most stable
compound among 1−4. In other words, traveling of carbons
in organic molecules is controlled by the stability of the
molecules.
Herein we report a rotation of a six-membered ring of
dihydroindenyl moiety on titanium based on the five-membered
ring like a merry-go-round (Scheme 1b). Two carbons at
bridgehead and three carbons of the five-membered ring were
1
3
monitored by C-labeled experiments. We focused on two
carbons at the bridgehead positions of the dihydroindenyl
moiety on titanium. These two carbons moved to the farthest
positions with the rotation of the six-membered ring by the effect
of substituents (R) and came back to the starting positions by
the effect of a reactant. The intermediate where the two carbons
moved to the farthest positions was isolated and fully
characterized.
We have previously reported traveling of two carbons from
the starting complex 5 to 6 and finally to complex 7 by the effect
a
22−24
(
a) General scheme of traveling of carbon atoms in organic
of a ligand, as shown in Scheme 2.
Complexes 5 and 6 have
molecules. (b) Traveling of two carbons along with a rotation of the
six-membered ring in dihydroindenyl moiety on titanium (merry-go-
round reaction).
Received: December 4, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00876
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
1
Scheme 2. Traveling of Two Carbons in Dihydroindenyl
Molecules on Titanium through Titanacyclobutane Rings
stable than complexes 5−7, we focused on substituents R ,
which occupied the bridgehead positions in complexes 6 and 7
Scheme 2). When R is a bulky substituent such as a
1
(
trimethylsilyl group, it enhances its steric hindrance on the
quaternary carbon atom and destabilizes complexes 6 and 7 as
compared with 5 and 9. When complex 9 is compared with
complex 5, its diene moiety coordinating titanium is
disubstituted diene in complex 9, whereas it is tetrasubstituted
diene in complex 5. Hence, it is presumed that complex 9 is the
most stable among complexes 5−7 and 9. Moreover, the
trimethylsilyl group stabilizes the α position of organotitanium
complexes. Then, we designed the titanacyclopentadienes 10a
and 10b having trimethylsilyl groups on α-carbons for providing
dihydroindenyltitanium complexes 9.
As shown in Scheme 3a, when a toluene solution of complex
0a, which was prepared from 1,7-bis(trimethylsilyl)-1,6-
1
been reported and isolated from titanacyclopentadienes by
17,25
Rosenthal et al. as a pioneer work.
In the reaction from
a
Scheme 3
complex 6 to complex 7, aminopyridine stabilized complex 7
through its coordination to titanium, and the following
protonolysis with NH group provided dihydroindene 8. To
2
continue further traveling of the carbons in organic molecules,
the undesired protonation step to remove titanium should be
avoided. In addition, a more stable titanium complex should be
designed to the destination of the two carbons.
We have previously proposed a reaction mechanism for the
traveling of two carbons from complex 5 to 6 (Supporting
22,23
Information (SI), page S32).
Key steps for the traveling are
the metathesis-type carbon−carbon bond cleavage and
reformation in the same type of the core structure,
dihydroindenyl moiety, through the titanacyclobutane ring. As
we have already reported, complexes 5 and 6 were in
a(a) Synthesis of the starting complex 5a from complex 10a. (b) 13C-
labeling experiments: Traveling and coming back to the starting
position of two carbons in dihydroindenyl moiety on titanium.
2
3
equilibrium. Because complex 6 was more stable than complex
because of the less substituted diene coordinated to titanium
5
compared with complex 5, the equilibrium lay to the side of
complex 6.
heptadiyne, Mg, and Cp TiCl , was heated to 80 °C in the
2
2
As with complex 5, complex 6 also has the titanacyclobutane
moiety. Therefore, complex 6 is further converted to complex 7
if complex 7 is more stable than complex 6. The reaction shown
in Scheme 2 demonstrates the existence of complex 7; however,
aminopyridine not only stabilized complex 7 but also protonated
complex 7 to give the organic compound 8, which did not have
the titanium. Because there was no titanacyclobutane moiety in
presence of trimethylphosphine, the starting complex 5a was
obtained in 94% yield by a coupling reaction of a Cp ligand and
the diene moiety. The structure of 5a was verified by X-ray
analysis (SI, page S13). In this reaction, the desired complex 9
was not formed because the five-membered side ring made the
structure very rigid for the rearrangement.
When a more flexible six-membered side ring was used instead
of the five-membered ring, our expected complex 9b was
obtained in high yield (Scheme 3b). Complex 9b afforded single
crystals suitable for X-ray analysis. The crystal structure of 9b
(SI, page S15) indicated that two carbons originating from a Cp
ligand traveled to the longest distance positions (C(7) and C(8)
positions in the ORTEP drawing) of the dihydroindenyl moiety.
8
, the traveling of two carbon atoms in the organic molecule
stopped at this position. If the protonation does not occur for
complex 7, then further transformation can be expected because
complex 7 also has the titanacyclobutane moiety.
If our mechanism (SI, page S32) is correct, then we can expect
the formation of complex 9, where the two carbons move to the
longest distance from the starting position of complex 5
1
3
To confirm the positions of the traveling two carbons, a C-
1
3
(
9
Scheme 2). Because the mechanism is metathesis-like, complex
is also in equilibrium with other complexes 5−7. To make the
labeled experiment was carried out (Scheme 3b). The C NMR
13
13
spectrum of complex 9b- C revealed that two C-labeled
carbons were located at the opposite side of the five-membered
ring in the dihydroindenyl moiety. This result clearly showed
traveling of two carbons to the farthest positions, complex 9
must be more stable than complexes 5−7.
1
3
There are two possible methods to control the stability of such
dihydroindenyltitanium complexes. One is control by ligands.
The other is control by substituents. As shown in Scheme 2,
aminopyridine acted as a ligand but unfortunately removed
that two C-labeled carbons moved to the farthest position
from the starting position in complex 5.
Now two carbons traveled to the longest distance destination
from the starting point in the dihydroindenyl moiety on
titanium, as shown above. The next question is, “Can the two
carbons, which already traveled to the longest distance position
in the organic molecule, come back to the starting point?”.
Possessing a titanacyclobutane moiety, complex 9 is also in
equilibrium with other complexes 5−7 and has the potential for
the traveling of carbons from its position. The results shown in
titanium from complex 7 by the protonation with its NH group.
2
Although substituted aminopyridine, which had no hydrogen
atom on nitrogen, was used to avoid the protonolysis, the
reaction did not afford complex 7. Therefore, the substituent
control had to be chosen to move the two carbons to the longest
distance position as in complex 9. To make complex 9 more
B
DOI: 10.1021/acs.organomet.8b00876
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 3b revealed that complex 9b was the most stable among
complexes 5b−7b and 9b by the effect of silyl substituents. This
means that to move the two carbons back to the starting point in
complex 5, it is necessary that complex 5 should step out of the
equilibrium irreversibly by derivatization. Two hydrogen atoms
at the bridgehead carbons of complex 5 can be abstracted by
Accession Codes
CCDC 1879710−1879711 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
2
6
azobenzene to give an indene derivative 11. Because
complexes 6, 7, and 9 do not have the two hydrogen atoms
capable of being abstracted, we expected that the reaction of
complex 9 with azobenzene gave indene derivative 11 through
the formation of complex 5.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tamotsu@cat.hokudai.ac.jp.
■
1
3
13
A
C-labeling experiment of complex 9b- C with
ORCID
13
azobenzene provided indene derivative 11b- C in a good
yield, as we expected (Scheme 3b). The 13C NMR study
demonstrated that three carbons in the five-membered ring and
traveling two carbons were in the same five-membered ring in
Tamotsu Takahashi: 0000-0003-4728-6547
Notes
The authors declare no competing financial interest.
13
indene derivative 11b- C. This result clearly showed that
complex 9 was converted into complex 5 and then the two
hydrogen atoms of 5 were eliminated by azobenzene to produce
indene derivative 11. The two carbons, once moved to the
farthest positions as in complex 9 with the rotation of the six-
membered ring, came back to the starting point of complex 5.
The above results are summarized in Scheme 1b. The two
carbons starting from complex 5 traveled to the longest distance
position in complex 9 via complexes 6 and 7 because 9 was the
most stable complex among complexes 5−7 and 9 by the
substituent effect. The carbons that once moved to the longest
distance position, in turn, moved back to the starting position in
complex 5 with the effect of azobenzene. The hydrogen
elimination from complex 5 gave indene derivative 11, which
clearly showed that the two carbons came back to the starting
point. A different chemical drawing, as shown in Scheme 4,
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DOI: 10.1021/acs.organomet.8b00876
Organometallics XXXX, XXX, XXX−XXX