Reversibility of disubstituted vinylidene-internal alkyne isomerization at cationic ruthenium and iron complexes

Article abstract of DOI:10.1021/om101109c

The cationic disubstituted vinylideneruthenium complexes [CpRu{=C=C(Ph)R}(dppe)][BAr^F₄], which are prepared directly from internal alkynes via 1,2-migration of carbon substituents, are shown to undergo disubstituted vinylidene-to-internal alkyne isomerization on reaction with a monophosphine. The observation provides the first examples in which the reversible conversion between internal alkynes and disubstituted vinylidenes is experimentally confirmed, and the kinetic and mechnistic investigations indicated that an uncommon electrophilic 1,2-migration of the carbon substituents takes place in the internal alkyne-disubstituted vinylidene interconversion.

Full text of DOI:10.1021/om101109c

204 Organometallics 2011, 30, 204–207
DOI: 10.1021/om101109c
Reversibility of Disubstituted Vinylidene-Internal Alkyne Isomerization
at Cationic Ruthenium and Iron Complexes
Yuichiro Mutoh,* Kohei Imai, Yusuke Kimura, Yousuke Ikeda, and Youichi Ishii*
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27, Kasuga,
Bunkyo-ku, Tokyo 112-8551, Japan
Received November 26, 2010
Summary: The cationic disubstituted vinylideneruthenium
complexes [CpRu{dCdC(Ph)R}(dppe)][BArF4], which are
prepared directly from internal alkynes via 1,2-migration of
carbon substituents, are shown to undergo disubstituted vinyl-
idene-to-internal alkyne isomerization on reaction with a
monophosphine. The observation provides the first examples
in which the reversible conversion between internal alkynes and
disubstituted vinylidenes is experimentally confirmed, and the
kinetic and mechnistic investigations indicated that an uncom-
mon electrophilic 1,2-migration of the carbon substituents
takes place in the internal alkyne-disubstituted vinylidene
interconversion.
iodo-substituted6 alkynes have appeared in the literature.
Very recently, we have demonstrated that several iron and
ruthenium complexes can affect the vinylidene rearrange-
ment of a wide variety of internal alkynes to give disubsti-
tuted vinylidene complexes such as [CpRu{dCdC(Ph)Ar}-
(dppe)][BArF4] (1; Cp = η5-C5H5, dppe = Ph2PCH2CH2-
PPh2, ArF=3,5-(CF3)2C6H3).7 We have also disclosed that
this transformation involves an uncommon electrophilic 1,2-
migration of carbon substituents at the intermediary η2-alkyne
complexes. Meanwhile, the reverse process, i.e., the conversion
of vinylidenes to alkynes, is well documented for the mono-
substituted vinylidene complexes, especially the [(η5-C9H7)Ru-
(PPh3)2] system.8 However, similar conversions of η1-vinylidenes
with two carbon substituents into the η2 internal alkynes
have scarcely been explored. The only example reported so
far is the irreversible conversion of the η1-vinylidene in
[CpFe(CO)2{dCdC(Ph)Me}](OTf) (Tf=SO2CF3) into the
corresponding η2-alkyne complex,9 although the reversible
migration of a silyl group was observed in the (β,β-bis(silyl)-
vinylidene)chromium complex [Cr(η6-C6Me6)(CO)2{dCdC-
(SiMe3)2}]3b and (β-silyl-β-arylvinylidene)rhodium complex
trans-[RhCl{dCdC(ferrocenyl)SiMe3}(P-i-Pr3)2].3c Now, we
have experimentally demonstrated for the first time that the
transformation between alkynes and vinylidenes via carbon
substituent migration is potentially reversible (Scheme 1).
It is well recognized that terminal alkynes are readily and
reversibly interconvertible with monosubstituted vinylidenes
at transition-metal complexes, and this rearrangement has in
fact been utilized in many metal-promoted or -catalyzed
transformations of alkynes as the key step.1 In contrast,
prior to our study, migration of carbon substituents in internal
alkynes leading to the disubstituted vinylidenes was limited
to very few rearrangements of acylalkynes,2 although
similar rearrangements of silyl-,3 stannyl-,4 sulfenyl-,5 and
*To whom correspondence should be addressed. E-mail:
ymutoh@kc.chuo-u.ac.jp (Y.M.); yo-ishii@kc.chuo-u.ac.jp (Y.I.).
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Wiley-VCH: Weinheim, Germany, 2008. (b) Bruce, M. I. Chem. Rev. 1991,
91, 197–257. (c) Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999, 32, 311–
323. (d) Valerga, P.; Puerta, M. C. Coord. Chem. Rev. 1999, 193-195, 977–
1025. (e) Wakatsuki, Y. J. Organomet. Chem. 2004, 689, 4092–4109. (f)
Katayama, H.; Ozawa, F. Coord. Chem. Rev. 2004, 248, 1703–1715. (g)
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pubs.acs.org/Organometallics
Published on Web 12/20/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 30, No. 2, 2011 205
Table 1. Reaction of Disubstituted Vinylidene Complexes 1
with La
Scheme 2
Scheme 3
entry
1
R
L
time
1 h
2
GC yield (%)
1
2
3
4
5
6
7
8
1a C6H4OMe-p
1b C6H4Me-p
1c C6H5
1d C6H5Cl-p
1e C6H4CO2Et-p PPh3
1f Me
1g COPh
1c C6H5
1c C6H5
1c C6H5
1c C6H5
PPh3
2a
2b
2c
2d
2e
95
99
91
51
81
21
91
92
PPh3
PPh3
PPh3
4 h
23 h
22 h
137 h
34 days 2f
3 h
PPh3
PPh3
2g
2c
2c
2c
PMe2Phb 23 h
b
9
10
11
PMe3
P(OPh)3
MeCNc
19 h
23 h
24 h
89
99
no reacn
a Conditions: [1]0 = 10.0 mM, [L]0 = 12.0 mM, in toluene, at 100 °C.
disubstituted vinylidenes were stable in MeCN (entry 11),
whereas the indenylruthenium disubstituted vinylidene com-
b [L]0 = 20.0 mM. c Neat, at reflux temperature.
plex [(η5-C9H7)Ru{dCdC(Ph)C6H4OMe-p}(dppe)][BArF
]
4
(4) reacted with MeCN to give 2a and the Ru-NCMe com-
plex [(η5-C9H7)Ru(dppe)(NCMe)][BArF ] (5) (Scheme 2).10
Since 1 and 4 are directly obtained by the reaction of
Scheme 1
4
[CpRuCl(dppe)] and [(η5-C9H7)RuCl(dppe)] with internal
alkynes 2 in the presence of NaBArF 7b these observations
,
4
provide the first examples in which the reversible conversion
between internal alkynes and disubstituted vinylidenes has
been experimentally confirmed.
Not only the ruthenium complexes 1 and 4 but also the
disubstituted vinylideneiron complex [CpFe(dCdC(Ph)C6H4-
OMe-p)(dppe)][BArF4] (6),7b which can be derived directly
from [CpFeCl(dppe)] and 2a in the presence of NaBArF
,
undergoes vinylidene-to-alkyne rearrangement on treatment
4
with PPh3 in toluene at 100 °C to give alkyne 2a and the PPh3-
The migratory aptitude of the alkyl and aryl substituents in
this rearrangement has been investigated.
ligated complex [CpFe(dppe)(PPh3)][BArF ] (7) (Scheme 3). It
4
should be noted that only the disubstituted vinylidene-to-
alkyne rearrangement was observed at [CpFe(CO)2]þ, as
already mentioned,9 while [CpFe(dppe)]þ can promote
both conversions.
When the disubstituted vinylidene complex [CpRu{dCdC-
(Ph)C6H4OMe-p}(dppe)][BArF4] (1a) was allowed to react
with PPh3 (1.2 equiv) in toluene at 100 °C for 1 h, a color
change from red to pale yellow was observed, and the 31P-
{1H} NMR analysis of the solution indicated the complete
consumption of 1a. GC analysis of the reaction mixture
showed liberation of the internal alkyne PhCtCC6H4OMe-p
(2a) from 1a in 83% yield. The organometallic product
The kinetic aspects of the reaction were revealed by following
the conversion of the disubstituted vinylidene complex 1c
into the phosphine complex 3 by means of 31P{1H} NMR
spectroscopy at 100 °C. No intermediate species was de-
tected, and the reaction could be treated as first order in the
concentration of 1c. As given in Table 2, the observed rate
constant (kobs) was estimated as [4.65(6)] ꢀ 10-5 s-1 in the
presence of 1.2 equiv of PPh3 (entry 1), and this value was not
affected at this temperature either by the concentration of
added PPh3 (entries 1-4) or by that of added 2c (entry 5). In
addition, essentially the same kobs values were obtained even
[CpRu(dppe)(PPh3)][BArF ] (3) was isolated in good yield as
4
pale yellow crystals, which was confirmed by spectroscopic and
elemental analysis as well as by an X-ray study (structure not
shown). As summarized in Table 1, diarylalkynes 2b-e with
various substituents at the para position were also obtained in
good yields from the (aryl)(phenyl)-substituted vinylidenes
(entries 2-5), where the electron-donating substituents en-
hanced the reaction rate. β-Alkyl and β-acyl groups were
tolerated in the reaction (entries 6 and 7), but the conversion
of 1f was much slower than for diarylvinylidene complexes.
Similar reactions with other tertiary phosphines such as
PMe2Ph, PMe3, and P(OPh)3 proceeded smoothly to give
the corresponding alkyne and phosphine-ligated complexes
[CpRu(dppe)(P)][BArF4] (P = PMe2Ph, PMe3, P(OPh)3)
(entries 8-10). In the absence of a phosphine ligand,
(10) Although we must await further investigation, this observation
may be attributable to the so-called “indenyl effect”: (a) Hartwig, J. F. In
Organotransition Metal Chemistry: From Bonding to Catalysis; Univer-
sity Science Books: Sausalito, CA, 2010; pp 250-253. (b) O'Connor, J. M.;
Casey, C. P. Chem. Rev. 1987, 87, 307–318. (c) Veiros, L. F. Organome-
tallics 2000, 19, 3127–3136. (d) Zargarian, D. Coord. Chem. Rev. 2002,
~
233-234, 157–176. (e) Calhorda, M, J.; Romao, C. C.; Veiros, L. F. Chem.
Eur. J. 2002, 8, 868–875.

206 Organometallics, Vol. 30, No. 2, 2011
Mutoh et al.
Table 3. Migratory Aptitude in the Reaction of 1-13C with PPh3
a
Table 2. Effect of Concentration and Types of Phosphine on the
First-Order Rate Constants kobs for the Conversion of 1c into 3a
entry
L
[L]0 (mM)
105kobs (s-1
)
R
time
1 h
R migration/Ph migrationb
1
2
3
4
5b
6
7
8
PPh3
PPh3
PPh3
PPh3
PPh3
PMe2Ph
PMe3
P(OPh)3
10.3
26.3
41.6
83.1
25.2
23.2
26.0
25.9
4.65(6)
4.61(5)
4.60(4)
4.67(5)
4.67(7)
4.67(8)
4.68(6)
4.68(5)
C6H4OMe-p
C6H4Me-p
C6H4Cl-p
C6H4CO2Et-p
Me
3/97
4 h
25/75
50/50
83/13
15/85
>99/<1
44 h
137 h
34 days
3 h
COPh
a The reactions were performed as shown in Table 1. The asterisks
represent 13C-enriched carbon atoms. 2-13
C were obtained in
a Reaction conditions: [1c]0 = 8.47 ( 0.12 mM, [2c]0 = 0 mM, in
toluene, at 100 °C for 10 h (ca. 80% conversion of 1c). b [2c]0 = 25.2 mM.
8-86% GC yields and 3 in 68-89% isolated yields. b The ratios were
estimated by 13C{1H} NMR analysis. See the Supporting Information
for details.
indicates an almost linear correlation between σP and log[(Ar
migration)/(Ph migration)] (F = 2.8).
Scheme 4
The above order of the migratory aptitude is opposite to
that of common nucleophilic rearrangements in organic
chemistry, where the migrating group moves with its bond-
ing electrons.11 The derived F value denotes substantial
development of negative charge in the transition state,
reflecting that an electron-withdrawing group enhances
migratory aptitude. These tendencies are also observed for
internal alkyne-disubstituted vinylidene isomerization at an
anionic Ru(P3O9) complex7a and cationic CpRu and CpFe
complexes.7b Therefore, we suggest that the η2 internal
alkyne-η1 disubstituted vinylidene isomerization proceeds
through an uncommon electrophilic 1,2-shift of the carbon
substituents. In this context, it should be mentioned that the
mechanism of vinylidene rearrangement of terminal alkynes
at ruthenium complexes still remains a subject of discussion;
migration of a proton is proposed for the rearrangement of
[RuCl2(HCtCH)(PH3)2],12 while it has been suggested that
migration of a hydride rather than a proton could be
involved in the η1 monosubstituted vinylidene to η2-1-alkyne
rearrangement at the indenylruthenium complex [(η5-
C9H7)Ru(dCdCHAr)(PPh3)2][PF6].8g
It should also be noted that the reactivity of disubstituted
vinylidenes 1 for the present rearrangement is not controlled
by the migratory aptitude of the substituents; 1a with a
C6H4OMe-p substituent, which shows the lowest migratory
aptitude, is the fastest to react. Probably the stability of the
vinylidene and η2-alkyne complexes is increased by an
electron-withdrawing group. A similar tendency was ob-
served for the internal alkyne to vinylidene isomerization
at [(P3O9)Ru]-, [CpRu]þ, and [CpFe]þ complexes.7
when different phosphines were used (entries 2 and 6-8). On
the basis of these observations, the present reaction is
deduced to be a two-step successive reaction involving the
intramolecular isomerization of the starting disubstituted
vinylidene complex (1V) to the η2 internal alkyne intermediate
(1A) followed by ligand substitution to give 3a (Scheme 4).
Judging from the coordinatively saturated nature of 1A, the
latter process is considered to proceed by a dissociative
mechanism, but the zero-order dependence on the concen-
tration of added phosphine and internal alkyne indicates that
the isomerization process from 1V to 1A is the rate-determin-
ing step. This view agrees with the fact that the concentration
of 1A remains lower than the observation limit by 31P{1H}
NMR.
To gain further insight into the isomerization mechanism,
we next investigated the migratory aptitude of the substitu-
ents in disubstituted vinylidene complexes 1 using 13C-
enriched vinylidene complexes 1-13C (Table 3). Migrations
of the R group in 1-13Cr and phenyl group in 1-13Cβ (denoted
as R migration and Ph migration, respectively) give rise to
the internal alkynes 2-13CR, where the carbon atom next to
the R group is labeled with 13C. Similarly, Ph migration from
1-13Cβ and R migration from 1-13Cr leads to the formation
of 2-13CPh with the 13C-enriched carbon atom next to the
phenyl group. Detailed 13C{1H} NMR analyses disclosed the
ratio of R migration to Ph migration as shown in the table,
where the migratory aptitude is in the order COPh >
C6H4CO2Et-p>C6H4Cl-p/C6H5>C6H4Me-p>Me>C6H4-
OMe-p. In this regard, the order obtained for Ph/Me migra-
tion is in accordance with Ph migration at [CpFe(CO)2-
{dCdC(Ph)Me}](OTf) rather than Me migration.9 Hammett
analysis of the relative migratory aptitude of aryl groups
In summary, we have demonstrated the first reversible
interconversion between internal alkynes and disubstituted
vinylidenes at cationic ruthenium and iron complexes.
(11) Smith, M. B.; March, J. In March’s Advanced Organic Chem-
istry: Reactions, Mechanisms, and Structure, 6th ed.; Wiley: Hoboken, NJ,
2007; pp 1568-1570.
(12) Wakatsuki, Y.; Koga, N.; Yamazaki, H.; Morokuma, K. J. Am.
Chem. Soc. 1994, 116, 8105–8111.

Communication
Organometallics, Vol. 30, No. 2, 2011 207
The kinetics and migratory aptitude revealed that an un-
common electrophilic 1,2-shift of carbon substituents takes
place in the present isomerization. Further studies on de-
tailed mechanisms and synthetic applications of this reaction
will be reported in due course.
Areas (No. 20037060, “Chemistry of Concerto Catal-
ysis”) from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
Supporting Information Available: Text, figures, and CIF files
giving experimental procedures and crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. This work was financially supported
by a Grant-in-Aid for Scientific Research on Priority