Reversibility of 1,4-metal migration in Cp*RhIII and Cp*IrIII Complexes

Article abstract of DOI:10.1021/om5003232

The (2-ethenyl-4-methylphenyl)arylrhodium complex [CpRh{C₆H ₃-2-(C(Ph)=CHPh)-4-Me}(PMe₃)][BAr^F ₄], which is formed from the vinyl-to-aryl 1,4-Rh migration of the 2-(m-tolyl)ethenyl complex [CpRh{C(Ph)=C(C₆H₄Me-m)Ph} (PMe₃)][BAr^F₄], is isomerized under mild conditions to the regioisomeric 2-ethenylphenyl complex [CpRh{C ₆H₄-2-(C(C₆H₄Me-m)=CHPh)}(PMe ₃)][BAr^F₄] by way of a back-reaction to regenerate the 2-(m-tolyl)ethenyl complex intermediate followed by the rotation of the C=C bond and the second 1,4-Rh migration, providing experimental evidence for the reversibility of the vinyl-to-aryl 1,4-metal migration.

Full text of DOI:10.1021/om5003232

Communication
pubs.acs.org/Organometallics
Reversibility of 1,4-Metal Migration in Cp*RhIII and Cp*IrIII Complexes
Yousuke Ikeda,† Koichi Takano,† Maiko Waragai,‡ Shintaro Kodama,† Noriko Tsuchida,§ Keiko Takano,‡
and Youichi Ishii*,†
†Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo
112-8551, Japan
‡Department of Chemistry and Biochemistry, Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Otsuka,
Bunkyo-ku, Tokyo 112-8610, Japan
§Department of Liberal Arts, Faculty of Medicine, Saitama Medical University, 38 Morohongo, Moroyama-machi, Iruma-gun, Saitama
350-0495, Japan
S
* Supporting Information
ABSTRACT: The (2-ethenyl-4-methylphenyl)arylrhodium complex
[Cp*Rh{C6H3-2-(C(Ph)CHPh)-4-Me}(PMe3)][BArF ], which is
4
formed from the vinyl-to-aryl 1,4-Rh migration of the 2-(m-
tolyl)ethenyl complex [Cp*Rh{C(Ph)C(C6H4Me-m)Ph}-
(PMe3)][BArF ], is isomerized under mild conditions to the
4
regioisomeric 2-ethenylphenyl complex [Cp*Rh{C6H4-2-(C-
(C6H4Me-m)CHPh)}(PMe3)][BArF ] by way of a back-reaction
4
to regenerate the 2-(m-tolyl)ethenyl complex intermediate followed
by the rotation of the CC bond and the second 1,4-Rh migration,
providing experimental evidence for the reversibility of the vinyl-to-
aryl 1,4-metal migration.
substituted phenyl complexes such as [Cp*M(C6H4Me-m)-
ntramolecular remote metal migration has recently been
Ideveloped as a potential C−H activation method, and a (PMe3)]+ (M = Rh, Ir) and found potential reversibility of this
vinyl-to-aryl 1,4-metal migration reaction coupled with the vinyl
CC bond rotation.
number of catalytic reactions involving this process have been
reported during the past decade.1 Among these reactions, 1,4-
Rh(I)2 and Pd(II)3 migrations are most commonly observed,4
where theoretical studies suggested that the Rh(I) center
migrates via the successive C−H oxidative addition−reductive
elimination mechanism,2i while Pd(II) migrates via some
distinct mechanisms.1b,5 However, the reversibility of these
reactions still remains to be clarified. For the aryl-to-aryl 1,4-
migtration, there have been obtained experimental evidences
that equilibrium between two arylmetal species is attained
under certain conditions.6 In contrast, the reversibility of the
vinyl-to-aryl 1,4-migration has not been well understood,3b
although this process is commonly seen in the 1,4-metal
migrations. This is partially because most of the vinyl-to-aryl
1,4-metal migration reactions give rise to exclusive formation of
the aryl complex, but not an equilibrium mixture of vinyl and
aryl species.7
When [Cp*RhCl(C6H4Me-m)(PMe3)] (1) was treated with
PhCCMe (5 equiv) in the presence of NaBArF (1.1 equiv;
4
ArF = 3,5-(CF3)2C6H3) at 25 °C for a few minutes in 1,2-
dichloroethane (C2H4Cl2), the reaction mixture turned
immediately from orange to dark red. The 31P{1H} NMR
spectra of the resultant mixture revealed the formation of a new
1
complex which showed a doublet at δ −2.19 (d, JRhP = 159
Hz). Recrystallization of this dark red solution afforded the (2-
ethenylphenyl)rhodium complex [Cp*Rh{C6H3-2-(C(Me)
CHPh)-4-Me}(PMe3)][BArF ] (2a) with a Rh−(vinyl CH)
4
agostic interaction in 96% yield as the sole product (Scheme 1).
Complex 2a was characterized by means of spectroscopic
measurements and a preliminary X-ray diffraction study
1
(Supporting Information). In the H NMR of 2a, the vinyl
CH signal appeared as a triplet with a high-field shift at δ 0.84
1
2
(t, JRhP = JPH = 10.0 Hz), indicating the presence of a Rh−
(vinyl CH) agostic interaction.9 The structure of 2a clearly
demonstrates that the reaction of 1 with PhCCMe proceeds
through alkyne insertion into the Rh−Ph bond with syn
stereochemistry followed by selective 1,4-metal migration to
the 6-position of the m-C6H4Me substituent; the CH3 group act
as an efficient steric hindrance to prevent the migration to the
In the course of our investigation into the activation of
internal alkynes,8 we have recently demonstrated that the
vinylrhodium complexes [Cp*Rh(CR1CR2Ph)(PR3)]-
[BArF ] (R = Ph, Me; R1, R2 = alkyl, aryl), which are formed
4
from the reaction of [Cp*Rh(Ph)(PR3)][BArF ] with R1C
4
CR2, are transformed to the vinylaryl complexes [Cp*Rh-
{C6H4-2-(CR2CHR1)}(PR3)][BArF ] by 1,4-migration of
4
the Rh(III) center under mild conditions.9 It is noteworthy
that this reaction is the first example of a 1,4-Rh(III) migration
reaction.10 In this study, we turned our attention to meta-
Received: March 25, 2014
Published: April 28, 2014
© 2014 American Chemical Society
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Organometallics
Communication
Scheme 1. Reaction of 1 with Internal Alkynes and NaBArF
Scheme 3. Reversibility of the 1,4-Rh(III) Migration
4
Scheme 2. Reaction of 1 with PhCCPh and NaBArF
4
Scheme 4. Reaction of 4 with PhCCPh and NaBArF4 at 25
°C
Scheme 5. Isomerization of 5 to 6′ at 50 °C
Figure 1. 31P{1H} NMR spectra of the reaction of 1 with PhCCPh
and NaBArF at 25 and 50 °C in C2H4Cl2.
4
Figure 3. Variable-temperature 31P{1H} NMR spectra of the CDCl3
solution of 8.
Figure 2. ORTEP drawings of 2c (left) and 2c′ (right). The anionic
part and hydrogen atoms except for H1 are omitted for clarity.
Selected interatomic distances (Å) are as follows. 2c: Rh1−C1,
2.529(4); Rh1−H1, 1.817; C1−C2, 1.358(7). 2c′, Rh1−C1, 2.494(6);
Rh1−H1, 1.771; C1−C2, 1.363(8).
in 85% yield, appeared in a lower-field region (δ 1.74 (m)).
This result suggests that 2b has the structure with an η2-bound
alkene moiety rather than an agostic vinyl CH, which was
supported by a preliminary X-ray diffraction study.
2-position.2c,g On the other hand, the vinyl CH signal of
Interestingly, the reaction of 1 with PhCCPh showed a
[Cp*Rh{C6H3-2-(C(Et)CHEt)-4-Me}(PMe3)][BArF ]
different behavior. When 1 was allowed to react with PhC
4
(2b), which was formed from the reaction of 1 with EtCCEt
CPh (5 equiv) in the presence of NaBArF at 25 °C for 1 h in
4
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Organometallics
Communication
migration from the vinyl to the ortho position of the m-tolyl
group. At 50 °C, on the other hand, 2c is isomerized to the
more thermodynamically favored regioisomeric complex 2c′ by
way of a back-reaction to regenerate 3 followed by the rotation
of the CC bond13 and the second 1,4-Rh migration to the
ortho position of the Ph group (Scheme 3). With respect to the
reversibility of aryl-to-aryl 1,4-metal migration, Gallagher and
Larock reported that a Pd(II) center migrates reversibly
between two ortho positions of an unsymmetric biphenyl to
attain equilibrium.6 In the case of vinyl-to-aryl 1,4-metal
migration, however, the reaction usually goes to competition
to give the arylmetal product, and the reversibility of this
process has not been clarified.3b The findings shown above
provide an experimental evidence for this problem.
Scheme 6. Interconversion of the Solution State of Complex
8
To confirm the above mechanism, additional experiments
were performed with the more sterically demanding 3,5-
xylylrhodium complex [Cp*RhCl(C6H3-3,5-Me2)(PMe3)] (4).
The reaction of 4 with NaBArF4 and PhCCPh at 25 °C for 3
h in C2H4Cl2 afforded the 2-(3,5-xylyl)ethenyl complex
[Cp*Rh{C(Ph)C(Ph)C6H3-3,5-Me2}(PMe3)][BArF ] (5)
4
with a Rh−(o-CH of 3,5-xylyl) agostic interaction in 76%
yield as the sole product, and the corresponding 1,4-migration
product [Cp*Rh{C6H3-2-(C(Ph)CHPh)-4,6-Me2}(PMe3)]-
1
[BArF ] (6) has not been observed (Scheme 4). In the H
4
NMR of 5, ortho protons of the 3,5-xylyl substituent exhibit a
remarkable high-field shift (δ 5.77 (br, 2H)),9,11 indicating the
3,5-xylyl substituent is located cis to the rhodium atom and has
an agostic interaction with the metal center. When the
temperature was maintained at 50 °C in C2H4Cl2, 5 slowly
isomerized to the 2-ethenylphenyl complex [Cp*Rh{C6H4-2-
C2H4Cl2, the 31P{1H} NMR spectrum of the resultant dark red
reaction mixture exhibited two distinct doublet signals at δ
−2.92 (d, 1JRhP = 156.3 Hz) and −5.11 (d, 1JRhP = 172.7 Hz) in
the intensity ratio of 1:0.2 with the disappearance of the signal
ascribed to 1 (Scheme 2, Figure 1). This reaction reached
equilibrium at this stage, and no further change was observed
with longer reaction time in 31P{1H} NMR. The major doublet
is assigned to the (2-ethenyl-4-methylphenyl)rhodium complex
(C(C6H3-3,5-Me2)CHPh)}(PMe3)][BArF ] (6′) without
4
forming any other observable intermediate (Scheme 5). After
90 h, complex 6′ was isolated in 83% yield and characterized by
1
[Cp*Rh{C6H3-2-(C(Ph)CHPh)-4-Me}(PMe3)][BArF ]
its H NMR exhibiting the vinyl CH signal as a triplet with a
4
1
2
high-field shift at δ 0.36 (t, JRhP = JPH = 11.5 Hz) and the
equivalence of the two 3,5-xylyl Me groups (δ 2.28, s, 6H). In
this case, 1,4-migration of the metal center to the 3,5-xylyl
group is sterically unfavored and therefore takes place only after
the rotation of the CC bond.14
(2c), whereas the minor doublet is attributable to the 2-(m-
tolyl)ethenyl complex [Cp*Rh{C(Ph)C(C6H4Me-m)Ph}-
(PMe3)][BArF ] (3). They were characterized by NMR analysis
4
1
of the mixture (see the Supporting Information). In the H
NMR, the vinyl CH signal of 2c appeared at δ 0.30 as a triplet
2
(1JRhH = JPH = 11.2 Hz, 1H), whereas the ortho proton of the
Finally, the reactivity of [Cp*IrCl(C6H4Me-m)(PMe3)] (7),
the iridium analogue of 1, was examined. When 7 was allowed
m-C6H4Me substituent of 3 was found at δ 4.46 as a broad
to react with PhCCPh and NaBArF at 25 °C for a few
triplet (1JRhH
=
3JHH = 5.2 Hz, 1H).9,11 Fortunately, the
4
minutes, the 31P{1H} NMR of the reaction mixture showed a
broad singlet at δ −35.3. Recrystallization of this solution
afforded dark red crystals, which was confirmed to be the (2-
ethenylphenyl)iridium complex [Cp*Ir{C6H4-2-(C(C6H4Me-
molecular structure of 2c has been determined unambiguously
by a single-crystal X-ray diffraction study (Figure 2, left).
A further reaction of 2c and 3 was observed at higher
temperatures (Scheme 2). When a C2H4Cl2 solution of 2c and
3 generated in situ was heated at 50 °C, 31P{1H} NMR showed
m)CHPh)}(PMe3)][BArF ] (8) by an X-ray diffraction study
4
1
(see the Supporting Information). The VT-NMR analysis of
the CDCl3 solution of 8 uncovered its dynamic behavior; the
31P{1H} NMR spectrum measured at −60 °C exhibited four
distinct sharp singlets at δ −34.7, −34.6, −34.4, and −32.8 in
the intensity ratio of 1.35:1:1:0.24, and these signals were
broadened with increasing temperature and finally averaged at
production of a new species at δ −3.30 (d, JRhP = 154.0 Hz)
which was attributed to [Cp*Rh{C6H4-2-(C(C6H4Me-m)
CHPh)}(PMe3)][BArF ] (2c′), the geometric isomer of 2c
4
with respect to the alkene CC bond. After 20 h, the reaction
reached equilibrium where 2c, 3, and 2c′ existed in the ratio of
1:0.2:2.4 (determined by 31P{1H} NMR, see Figure 1), and the
1H NMR of this mixture showed a new signal of the agostic
1
25 °C (Figure 3). In the H NMR spectra at −60 °C, three
1
2
signals diagnostic of the Ir−(vinyl CH) agostic interaction were
vinyl CH proton of 2c′ as a triplet at δ 0.36 (t, JRhH = JPH
=
observed at δ −0.25 (d, 2JPH = 13.2 Hz), −0.37 (d, 2JPH = 13.2
11.2 Hz). The structure of 2c′ was determined by an X-ray
diffraction study (Figure 2, right) to confirm that the rhodium
center migrated to the Ph group from the C6H4Me-m
substituent.12
2
Hz), and −0.42 (d, JPH = 12.6 Hz), and a high-field-shifted o-
aryl signal was also observed at δ 3.33 (br d, 3JHH = 4.6 Hz).15,16
Tentatively we assign these signals to those of 8, 8′ (the
rotamer of 8), 8″ (the regioisomer of 8), and the 2-(m-
tolyl)ethenyl complex [Cp*Ir{C(Ph)C(C6H4Me-m)Ph}-
These results manifest that the present 1,4-Rh(III) migration
is a reversible process. Complex 2c is formed as the kinetic
product at 25 °C through the insertion of PhCCPh into the
Rh−(C6H4Me-m) bond to give 3 followed by the 1,4-Rh
(PMe3)][BArF ] (9) by comparison of the NMR data with
4
those of 2c, 2c′, and 3. It is interesting to mention that iridium
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Organometallics
Communication
Chem. Soc. 2002, 124, 14326−14327. (c) Campo, M. A.; Zhang, H.;
Yao, T.; Ibdah, A.; McCulla, R. D.; Huang, Q.; Zhao, J.; Jenks, W. S.;
Larock, R. C. J. Am. Chem. Soc. 2007, 129, 6298−6307.
complex 8 is rapidly interconverting among the vinylaryliridium
complexes 8, 8′, and 8″ and the vinyliridium complex 9
(Scheme 6) even at room temperature.17
In conclusion, we have demonstrated that the vinyl-to-aryl
1,4-metal migration is a potentially reversible process coupled
with the vinylic CC bond rotation by a detailed analysis of
the reaction between [Cp*M(C6H4Me-m)(PMe3)]+ (M = Rh,
Ir) and PhCCPh. Detailed mechanisms involving DFT
calculations and synthetic applications of this reaction are now
under investigation.
(7) In the vinyl-to-aryl 1,4-migration, deuterium labeling experiments
indicated that the metal center completely migrates to the ortho
position of the aryl group.2b,g
(8) (a) Ikeda, Y.; Yamaguchi, T.; Kanao, K.; Kimura, K.; Kamimura,
S.; Mutoh, Y.; Tanabe, Y.; Ishii, Y. J. Am. Chem. Soc. 2008, 130,
16856−16857. (b) Mutoh, Y.; Ikeda, Y.; Kimura, Y.; Ishii, Y. Chem.
Lett. 2009, 38, 534−535. (c) Mutoh, Y.; Imai, K.; Kimura, Y.; Ikeda,
Y.; Ishii, Y. Organometallics 2011, 30, 204−207. (d) Mutoh, Y.;
Kimura, Y.; Ikeda, Y.; Tsuchida, N.; Takano, K.; Ishii, Y. Organo-
matallics 2012, 31, 5150−5158. (e) Otsuka, M.; Tsuchida, N.; Ikeda,
Y.; Kimura, Y.; Mutoh, Y.; Ishii, Y.; Takano, K. J. Am. Chem. Soc. 2012,
134, 17746−17756. (f) Ikeda, Y.; Mutoh, Y.; Imai, K.; Tsuchida, N.;
Takano, K.; Ishii, Y. Organomatallics 2013, 32, 4353−4358.
(9) Ikeda, Y.; Takano, K.; Kodama, S.; Ishii, Y. Chem. Commun. 2013,
11104−11106.
(10) On the basis of preliminary density functional theory (DFT)
calculations, the mechanism of this 1,4-Rh(III) migration is considered
to be σ-complex-assisted metathesis (σ-CAM) rather than successive
oxidative addition/reductive elimination, the latter of which is the
common mechanism for the 1,4-Rh(I) migration (unpublished
results). For the σ-CAM process, see: (a) Perutz, R. N.; Sabo-
Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578−2592. (b) Balcells,
D.; Clot, E.; Eisenstein, O. Chem. Rev. 2010, 110, 749−823.
(11) (a) Vigaloc, A.; Uzan, O.; Shimon, L. J. W.; Ben-David, Y.;
Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 12539−
12544. (b) Sau, Y.-K.; Chan, K.-W.; Zhang, Q.-F.; Williams, I. D.;
Leung, W.-H. Organometallics 2007, 26, 6338−6345. (c) Frech, C. M.;
Shimon, L. J. W.; Milstein, D. Organometallics 2009, 28, 1900−1908.
(12) The m-C6H4Me substituent of complex 2c′ is considered to be
rotating at room temperature, and the two rotamers were in fact
observed by 1H and 31P{1H} NMR measurements at −60 °C (see the
Supporting Information).
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving experimental details
and characterization data for the compounds prepared in this
paper and crystallographic data for 2a, 2c, 2c′, and 8. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for Y.I.: yo-ishii@kc.chuo-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Y. Ikeda thanks the Japan Society for the Promotion of Science
(JSPS) Research Fellowships for Young Scientists.
■
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■
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1
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