Formation of vinylidenes from internal alkynes at a cyclotriphosphato ruthenium complex

Article abstract of DOI:10.1021/ja8080567

A ruthenium cyclotriphosphato (P₃O₉^3-) complex with a labile MeOH ligand can affect the vinylidene rearrangement of general internal alkynes via the 1,2-migration of alkyl, aryl, and acyl groups. This provides the first internal alkyne-to-vinylidene isomerization with high generality. Several intermediary η²-alkyne complexes could be isolated and were successfully transformed into the corresponding vinylidene complexes. The reaction mechanism is also discussed on the basis of a kinetic study and migratory aptitude of alkyl, aryl, and acyl groups; the present reaction proceeds via an intramolecular process and is viewed as an uncommon electrophilic rearrangement. Copyright

Full text of DOI:10.1021/ja8080567

Published on Web 11/19/2008
Formation of Vinylidenes from Internal Alkynes at a Cyclotriphosphato
Ruthenium Complex
Yousuke Ikeda, Takafumi Yamaguchi, Keiichiro Kanao, Kazuhiro Kimura, Sou Kamimura,
Yuichiro Mutoh, Yoshiaki Tanabe, and Youichi Ishii*
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo UniVersity, Kasuga, Bunkyo-ku,
Tokyo 112-8551, Japan
Received October 13, 2008; E-mail: yo-ishii@kc.chuo-u.ac.jp
Terminal alkynes are readily converted into the corresponding
vinylidenes at transition metal complexes by a direct 1,2-hydrogen
shift, C-H oxidative addition-1,3-hydrogen shift, sequential
protonation-deprotonation, and other mechanisms.1 This tautomer-
ization is now recognized as the key step in many metal-promoted
or -catalyzed transformations of alkynes.2 Heteroatom-substituted
alkynes such as silyl-,3 stannyl-,4 thio-,5 and iodoalkynes6 are also
known to undergo similar rearrangement. In contrast, migration of
carbon substituents of internal alkynes has been limited to very
few rearrangements of acylalkynes,7 although the reverse process,
i.e., the conversion of disubstituted vinylidenes to the η2-internal
alkynes, has been described in the literature.8 In the course of our
study on the transition metal cyclophosphato complexes which are
structurally related to the hydroxyapatite-supported metal catalysts,9
we have revealed that this class of organometallic complexes
exhibits unique structural and chemical properties based on the
circular array of P-O groups as an effective σ-donor set.10 Now
we have found that a ruthenium cyclotriphosphato (P3O93-) complex
with a labile MeOH ligand can affect the vinylidene rearrangement
of general internal alkynes via the 1,2-migration of alkyl, aryl, and
acyl groups.
tively, and the RusCRsCꢀ bond angle of 178.4(3)° are comparable
to common Ru(II)-vinylidene complexes.12 Similar reactions were
also observed with other internal alkynes such as 4b-4g, providing
the first example of alkyne-to-vinylidene rearrangement of general
internal alkynes. At present, the role of the P3O9 ligand is not fully
understood, but we consider that its σ-donor character favors the
formation of a π-acidic vinylidene ligand. It should be noted that
the rate of the reaction is strongly dependent on the substituents;
the reactions of 4a, 4b, and 4g are relatively slow and take a long
time (3 days) to go to completion, while those of 4c, 4e, and 4f are
completed within 3-6 h under the same conditions (70 °C).
Intermediate complexes could not be isolated in any of these
reactions.
Scheme 1
Coordination chemistry of ruthenium P3O9 complexes has not
been explored extensively, but photochemical ligand substitution
of the benzene complex (PPN)[Ru(P3O9)(C6H6)] (1; PPN )
(Ph3P)2N+)11 was found to be a versatile entry to reactive ruthenium
species. Thus, UV-irradiation of 1 in MeOH-CH2Cl2 in the
presence of dppe (dppe ) Ph2PCH2CH2PPh2; 1.2 equiv) led to
isolation of the MeOH complex (PPN)[Ru(P3O9)(MeOH)(dppe)]
(2) in 71% yield as orange crystals. An X-ray diffraction study of
2·3MeOH has confirmed its molecular structure with an octa-
hedral geometry in which three facial coordination sites are
occupied by the axial oxygen atoms of the P3O9 ligand (see
Supporting Information). The MeOH ligand in 2 is highly labile
and readily replaced by N2 in CH2Cl2 to form (PPN)[Ru(P3O9)-
(N2)(dppe)] (3), which exhibits a νNtN band at 2154 cm-1 in
the IR spectrum. On dissolution in MeOH under argon, 3 is
quickly converted back to 2.
When 2 was allowed to react with 1-phenyl-1-propyne (4a, 2.6
equiv) in C2H4Cl2 at 70 °C for 3 days, the vinylidene complex
(PPN)[Ru(P3O9)(dCdC(Ph)Me)(dppe)] (5a) was obtained in 85%
yield as green crystals (Scheme 1; see Supporting Information for
experimental details). Complex 5a exhibits 13C{1H} NMR signals
On the other hand, treatment of 2 with alkynes 4h-4k (4h: R1
) R2 ) CO2Me; 4i: R1 ) R2 ) CO2Et; 4j: R1 ) Me, R2 ) CO2Et;
4k: R1 ) Et, R2 ) CO2Et) at 50 °C resulted in the formation of the
η2-alkyne complexes (PPN)[Ru(P3O9)(R1CtCR2)(dppe)] (6h-6k)
in 66-85% isolated yields, which were characterized by the
13C{1H} NMR signals at δ 72-87 (coordinated CtC) and the IR
absorption at 1920-1950 cm-1 (νCtC) as well as by a crystal-
lographic study of 6j. These alkyne complexes were further
transformed into the corresponding vinylidene complexes 5h-5k
in 54-85% isolated yields either by heating in C2H4Cl2 (up to 5
days) or more effectively by UV-irradiation at room temperature.3a,d
2
at δ 369.0 (t, JPC ) 20 Hz) and 118.2 (s) characteristic of the R
and ꢀ carbons of a vinylidene ligand, respectively. The molecular
structure of 5a·2MeOH has been established unambiguously by
an X-ray diffraction study to confirm that either Me or Ph migration
took place to form the disubstituted vinylidene ligand (see Sup-
porting Information). The metrical features including the RusCR
and CRsCꢀ bond distances of 1.842 (4) and 1.277(5) Å, respec-
9
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10.1021/ja8080567 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Obviously, in these cases, the vinylidene rearrangement is much
slower than the alkyne coordination.
vinylidene rearrangement is not parallel with the migratory aptitude
of the substituents. Especially in the series of 1-aryl-2-phenylacety-
lenes, the orders of reactivity and migratory aptitude are opposite
to each other. We consider the rate of the vinylidene formation is
strongly controlled by the stability of the intermediate alkyne
complexes rather than the migratory aptitude of each substituent.
In conclusion, we have developed the first internal alkyne-to-
vinylidene isomerization with high generality by using ruthenium
P3O9 complex 2 and determined the migratory aptitude of alkyl,
aryl, and acyl groups. Detailed mechanisms and synthetic applica-
tions of this reaction are now under investigation.
A preliminary kinetic study on the conversion of 6k to 5k at 70
°C in the presence of excess 4k by means of 31P{1H} NMR clearly
indicated that the reaction obeys first-order kinetics with an apparent
rate constant (k) of 3.09 × 10-5 s-1. This result demonstrated that
the reaction proceeds via an intramolecular process, which is further
supported by the fact that crossover of the alkyne substituents was
not observed in the reactions with unsymmetric alkynes, especially
4f. On the other hand, judging from the normal νNtN value of 3,13
the oxidative addition of an internal alkyne to the 16e
[RuII(P3O9)(dppe)]- fragment to form the (alkyl/aryl)(alkynyl)
complex seems unlikely. Therefore we presume that the present
vinylidene rearrangement involves the 1,2-alkyl/aryl shift of the
intermediary alkyne complex. Theoretical studies on the terminal
alkyne-to-vinylidene rearrangement have also suggested that the
1,2-hydrogen shift mechanism is most commonly operative with a
Ru(II) center.14
Acknowledgment. This work was supported by Grants-in-Aid
for Scientific Research (20037060) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Supporting Information Available: Experimental details and
crystallographic data for 2·3MeOH, 5a·2MeOH, and 6j·2C2H4Cl2 in
CIF format. This material is available free of charge via the Internet at
http://pubs.acs.org.
To gain deeper insight into the reaction mechanism, the migratory
aptitude of alkyl, aryl, and acyl groups has been investigated by
13
using 13C-enriched alkynes PhCt CR (4-13C, 25.9% 13C). Migra-
References
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a Reaction conditions: C2H4Cl2, 70 °C.
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