Insertion reactions of allenes giving vinyl complexes

Article abstract of DOI:10.1021/om0506825

Treatment of OsHCl(PPh₃)₃ with allenes CH ₂=C=CHR at room temperature in benzene produced the vinyl complexes OsCl(C(CH₃)=CHR)(CH₂=C=CHR)-(PPh₃) ₂, instead of η³-allyl

Full text of DOI:10.1021/om0506825

4896
Organometallics 2005, 24, 4896-4898
Insertion Reactions of Allenes Giving Vinyl Complexes
Peng Xue, Jun Zhu, Herman S. Y. Hung, Ian D. Williams, Zhenyang Lin,* and
Guochen Jia*
Department of Chemistry and Open Laboratory of Chirotechnology of the Institute of
Molecular Technology for Drug Discovery and Synthesis, The Hong Kong University of Science
and Technology, Clear Water Bay, Kowloon, Hong Kong
Received August 6, 2005
Scheme 1
Summary: Treatment of OsHCl(PPh3)3 with allenes
CH2dCdCHR at room temperature in benzene produced
the vinyl complexes OsCl(C(CH3)dCHR)(CH2dCdCHR)-
(PPh3)2, instead of η3-allyl complexes as normally ob-
served. DFT calculations show that the formation of the
vinyl complex is favored kinetically.
There has been great interest in transition-metal-
catalyzed reactions of allenes.1,2 Insertion of allenes into
M-R bonds represents one of the most important
fundamental steps in catalytic reactions of allenes. In
principle, allenes can undergo insertion reactions with
LnM-R to give either transition-metal allyl complexes
A or vinyl complexes B, as illustrated in eq 1. The
plexes are now well documented. Although catalytic
reactions involving insertion of allenes into M-R bonds
to give vinyl intermediates were occasionally proposed,6
stoichiometric insertion reactions of allenes with well-
defined LnM-R to give vinyl complexes cannot be found
in the literature, to the best of our knowledge. In this
work, we wish to report the first examples of formation
of vinyl complexes from allene insertion reactions.
insertion of allenes into M-R bonds to give allyl
intermediates has been frequently invoked in the mech-
anisms of transition-metal-catalyzed reactions of al-
lenes. In supporting the proposed mechanisms, stoichio-
metric insertion reactions of allenes with LnM-R (M )
Pd(II) or Pt(II),3 Rh(I),4 Ru(II);5 R ) halide, H, acyl, allyl,
vinyl, alkyl, aryl) to give transition-metal allyl com-
Treatment of OsHCl(PPh3)3 (1)7 with CH2dCdCHC-
Me3 at room temperature in benzene produced OsCl(C-
(CH3)dCHCMe3)(CH2dCdCHCMe3)(PPh3)2 (2) (Scheme
1). The structure of 2 has been confirmed by a single-
crystal X-ray diffraction study.8 As shown in Figure 1,
complex 2 contains an η2-CH2dCdCHCMe3 ligand and
the vinyl ligand MeCdCHCMe3, which interacts with
the osmium center agostically through one of the
hydrogen atoms of the R-methyl group. In agreement
(1) (a) Ma, S. Acc. Chem. Res. 2003, 36, 701. (b) Sydnes, L. K. Chem.
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Kadowaki, S.; Takagi, K. Organometallics 2004, 23, 4116.
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C.; Bandoli, G.; Maini, L. Organometallics 2003, 22, 3230. (b) Yagyu,
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20, 1087. (d) Canovese, L.; Visentin, F.; Chessa, G.; Uguagliati, P.;
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J. H.; Vrieze, K.; van Leeuwen, P. W. N. M.; Veldman, N.; Spek, A. L.
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Soc. 1997, 119, 10028. (g) Groen, J. H.; Elsevier, C. J.; Vrieze, K.;
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K.; Yamamoto, T. Organometallics 1998, 17, 3044. (c) Osakada, K.;
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1
with the solid-state structure, the H NMR spectrum
showed the agostic methyl proton signal at -0.84 ppm.
The chemical shift is unusually upfield for a typical CH3
attached to an sp2-hybridized carbon but is close to that
of the methyl group with an agostic interaction in [IrPh-
(C(CHdCHCMe3)dCHCMe3)(PMe3)3]PF6 (-0.7 ppm).9
Complex 2 represents a rare example of â-agostic vinyl
(5) (a) Xue, P.; Bi, S.; Sung, H. H. Y.; Williams, I. D.; Lin, Z.; Jia, G.
Organometallics 2004, 23, 4735. (b) Sasabe, H.; Nakanishi, S.; Takata,
T. Inorg. Chem. Commun. 2003, 6, 1140. (c) Sasabe, H.; Nakanishi,
S.; Takata, T. Inorg. Chem. Commun. 2002, 5, 177. (d) Nakanishi, S.;
Sasabe, H.; Takata, T. Chem. Lett. 2000, 1058. (e) Hill, A. F.; Ho, C.
T.; Wilton-Ely, D. E. T. Chem. Commun. 1997, 2207.
(6) (a) Fu, C.; Ma, S. Org. Lett. 2005, 7, 1605. (b) Arredondo, V. M.;
McDonald, F. E.; Marks, T. J. Organometallics 1999, 18, 1949. (c)
Arredondo, V. M.; Tian, S.; McDonald, F. E.; Marks, T. J. J. Am. Chem.
Soc. 1999, 121, 3633-3639.
(7) Ferrando, G.; Caulton, K. G. Inorg. Chem. 1999, 38, 4168.
(8) See the Supporting Information for experimental and spectro-
scopic details of 2-6, for X-ray studies of 2 and 5, and for computational
details.
(9) Yi, C. S.; Liu, N. Organometallics 1998, 17, 3158.
10.1021/om0506825 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 09/15/2005

Communications
Organometallics, Vol. 24, No. 21, 2005 4897
Figure 2. X-ray structure of 5. Selected bond distances
(Å) and angles (deg): Os(1)-Cl(1), 2.4511(7); Os(1)-P(1),
2.3801(8); Os(1)-(P2), 2.4142(8); Os(1)-C(3), 2.405(3);
Os(1)-C(4), 2.416(3); Os(1)-C(12), 2.078(3); Os(1)-O(11),
2.144(2); C(3)-C(4), 1.367(5); C(2)-C(3), 1.336(5); O(11)-
C(10), 1.261(4); C(10)-C(11), 1.416(5); C(11)-C(12), 1.363(5);
C(12)-C(13), 1.504(4); C(2)-C(3)-C(4), 142.6(3).
Figure 1. X-ray structure of 2. Selected bond distances
(Å) and angles (deg): Os(1)-Cl(1), 2.4570(6); Os(1)-P(1),
2.3780(6); Os(1)-(P2), 2.3646(6); Os(1)-C(1), 2.501(2);
Os(1)-C(2), 2.026(2); Os(1)-C(11), 2.122(2); Os(1)-C(12),
2.044(2); C(1)-C(2), 1.498(3); C(2)-C(3), 1.332(4); C(11)-
C(12), 1.428(3); C(12)-C(13), 1.321(3); C(11)-C(12)-C(13),
142.9(2).
it slowly isomerized to complex 5, in which the vinyl
ligand is chelated to osmium though a C atom and an
O atom. The structure of 5 has been confirmed by X-ray
diffraction (Figure 2). The solution NMR data are fully
consistent with the solid-state structure.
complexes.10 â-Agostic vinyl complexes are interesting,
as they can be regarded as the resting states for the
interconversion of allene and vinyl complexes.11
The vinyl ligand MeCdCHCMe3 in 2 is derived from
insertion of CH2dCdCHCMe3 into an Os-H bond. A
number of allene insertion reactions have been reported,
for example, in the reactions of allenes with complexes
such as [MR(sol)(L-L)]+ (M ) Pt, Pd; L ) nitrogen, or
sulfur or phosphorus donors),3 RhH(L)(PPh3)3 (L ) CO,
PPh3),4 RuHCl(CO)(PPh3)3,5a and RuH(NO)(PPh3)3.5b In
all these reported reactions, the isolated products are
allyl complexes. Thus, the formation of complex 2 from
the reaction of 1 with CH2dCdCHCMe3 is interesting
and unexpected, as it represents the first example of
formation of vinyl complexes from allene insertion
reactions.
To see if the reactions are general, we have studied
the reactions of OsHCl(PPh3)3 with other allenes. Reac-
tion of OsHCl(PPh3)3 with CH2dCdCHPh produced the
analogous complex OsCl(C(CH3)dCHPh)(CH2dCdCH-
Ph)(PPh3)2 (3), which was previously obtained from the
reaction of OsHCl(PPh3)3 with excess PhCtCCH3.12
Similarly, reaction of OsHCl(PPh3)3 with CH2dCdCH-
CO2Et produced the insertion product OsCl(C(CH3)dCH-
CO2Et)(CH2dCdCHCO2Et)(PPh3)2 (4) (Scheme 1). The
structure of 4 must be similar to that of 2, as inferred
from its NMR spectroscopic data. It is found that
complex 4 is unstable in solution. At room temperature,
The reactions described above demonstrate that al-
lenes with either electron-donating or -withdrawing
groups undergo insertion reactions with OsHCl(PPh3)3
at the less substituted CdC bond to give vinyl complexes
instead of η3-allyl complexes, as normally observed. To
understand why allyl complexes were not produced in
the reactions of OsHCl(PPh3)3, we have studied the
formation of vinyl versus η3-allyl complexes from the
model reaction of OsHCl(PH3)3 with H2CdCdCH2 by
density functional theory calculations.8 Figure 3 shows
the potential energy profiles for the two insertion
pathways leading to the formation of the vinyl (PR-
vinyl) and allyl (PR-allyl) model complexes. The
results indicate that the formation of the vinyl complex
is kinetically favorable. Thermodynamically, the vinyl
complex is less stable than the η3-allyl complex. The
very high barrier for the reverse process, PR-vinyl f
PC-vinyl, prevents the formation of the thermodynami-
cally favored η3-allyl (PR-allyl) complex. Therefore,
once PR-vinyl is formed, it is stable enough to be
isolated. The important implication of the results is that,
although vinyl complexes may be in general thermody-
namically less stable than η3-allyl complexes (in view
of the dominant allyl complexes in the literature),
formation of vinyl complexes can still be kinetically
possible. OsHCl(PPh3)3 provides an opportunity for such
a possibility to be realized. To check the reliability of
the relative energies presented in Figure 3, which were
evaluated on the basis of the gas-phase model calcula-
tions, we examined the solvation effects by doing single-
point solvation energy calculations with the polarizable
continuum models (PCM).8 The results of the PCM
(10) Selnau, H. E.; Merola, J. S. J. Am. Chem. Soc. 1991, 113, 4008.
(11) (a) Schafer, M.; Wolf, J.; Werner, H. Organometallics 2004, 23,
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tallics 1999, 18, 4810. (c) Chin, C. S.; Maeng, W.; Chong, D.; Won, G.;
Lee, B.; Park, Y. J.; Shin, J. M. Organometallics 1999, 18, 2210. (d)
Brinkmann, P. H. P.; Luinstra, G. A.; Saenz, A. J. Am. Chem. Soc.
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19, 3466.

4898 Organometallics, Vol. 24, No. 21, 2005
Communications
Figure 3. Energy profiles for the two possible insertion pathways in the reaction of OsHCl(PH3)3 with allene. In all of the
structures, P stands for PH3. The relative free energies and reaction energies (in parentheses) are given in kcal/mol. In
calculating the relative energies, we included the energies calculated for free H2CdCdCH2 and PH3 whenever necessary
in order to make the energies comparable for all the species.
calculations show that the changes in the relative
energies are within 0.3 kcal/mol when the solvation
energies are included (see the Supporting Information
for more details).
We are now doing more calculations in order to gain
better insight into this important issue.
In summary, allenes undergo insertion reactions with
OsHCl(PPh3)3 to give vinyl complexes instead of η3-allyl
complexes. These are the first examples of formation of
vinyl complexes from allene insertion reactions with
well-defined metal complexes. The formation of the vinyl
complex was found to be kinetically favorable. From this
study, we see that it is possible to tune the properties
of LnM-R so that they undergo insertion reactions with
allenes to give vinyl complexes rather than allyl com-
plexes and that new metal catalysts can be designed to
catalyze insertion reactions of allenes via a vinyl
intermediate. We are currently studying other systems
to further understand factors controlling the formation
of vinyl or allyl complexes.
Figure 3 also shows the key structural features of the
calculated structures of the allene-coordinated precursor
complexes (PC-vinyl and PC-allyl) and the insertion
transition states (TS-vinyl and TS-allyl). The Os-H
bond distances in the two precursor complexes are
approximately the same. In both the two precursor
complexes, the distance between the metal center and
the central carbon of the η2-allene ligand is noticeably
shorter than that between the metal center and the
terminal π-bonded allenic carbon. The insertion reac-
tions involve the breaking of an allenic Os-C bond and
an Os-H bond and the formation of a new C-H bond.
From PC-vinyl to TS-vinyl, the weaker allenic Os-C
bond in PC-vinyl is the one being stretched. From PC-
allyl to TS-allyl, the stronger allenic Os-C bond in PC-
allyl is the one being stretched. Thus, the insertion
occurs more easily from the precursor complex PC-vinyl
leading to the formation of the vinyl product PR-vinyl
in comparison with that from PC-allyl. For many other
allene complexes, the bond between the metal center
and the central carbon of the η2-allene ligand was also
found to be generally shorter than that between the
metal center and the terminal π-bonded allenic carbon.3j
However, there has been no report of observing the
relevant vinyl products from their insertion reactions.
Acknowledgment. This work was supported by the
Hong Kong Research Grant Council (Project Nos. 602304,
601804) and the National Natural Science Foundation
of China through the Outstanding Young Investigator
Award Fund (Project No. 20429201).
Supporting Information Available: Text and tables
giving experimental procedures, characterization data, and
Cartesian coordinates of the calculated structures and CIF files
giving X-ray crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM0506825