Palladium-assisted regioselective olefin insertion into and βhydrogen elimination of hydrogen-molybdenum and -tungsten bonds. Synthesis and reactions of heterodinuclear hydrido complexes of palladium and platinum with molybdenum and tungsten

Article abstract of DOI:10.1021/om0506872

Smooth reversible olefin insertion of ethyl acrylate, acrylonitrile, ethylene, dimethyl fumarate, and dimethyl maleate into MHCp(CO)₂ (M = Mo (1a), W (1b)) has been catalyzed by Pd(PPh₃)₄ (4) or Pd(ethyl acrylate)(dppe) (8

Full text of DOI:10.1021/om0506872

© Copyright 2006
Volume 25, Number 2, January 16, 2006
American Chemical Society
Communications
Palladium-Assisted Regioselective Olefin Insertion into and
â-Hydrogen Elimination of Hydrogen-Molybdenum and -Tungsten
Bonds. Synthesis and Reactions of Heterodinuclear Hydrido
Complexes of Palladium and Platinum with Molybdenum and
Tungsten
Ayako Kuramoto, Kouhei Nakanishi, Tatsuya Kawabata, Nobuyuki Komine,
Masafumi Hirano, and Sanshiro Komiya*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo UniVersity of Agriculture and
Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
ReceiVed August 9, 2005
Summary: Smooth reVersible olefin insertion of ethyl acrylate,
acrylonitrile, ethylene, dimethyl fumarate, and dimethyl maleate
into MHCp(CO)3 (M ) Mo (1a), W (1b)) has been catalyzed
by Pd(PPh3)4 (4) or Pd(ethyl acrylate)(dppe) (8) at room
temperature.
alkynes with a strong electron-withdrawing group can insert.4
Insertion and its reverse, â-hydrogen elimination processes,
generally require a coplanar transition state or intermediate
consisting of a metal hydride and a CdC double bond.5 We
recently reported enhanced â-hydrogen elimination in a het-
erodinuclear ethylplatinum-molybdenum complex, (dppe)EtPt-
MoCp(CO)3 (2), giving the corresponding hydridoplatinum-
molybdenum complex (dppe)HPt-MoCp(CO)3 (3a), which
further reacts with alkynes having an electron-withdrawing
group to cause facile reductive elimination to give MoHCp-
(CO)3 and Pt(alkyne)(dppe).6 When a terminal alkyne is
employed, the succeeding Markovnikov addition of MoHCp-
(CO)3 to the coordinated alkyne takes place to give the new
heterodinuclear µ-alkenyl complexes (dppe)(µ-CH2dCR)Pt-
MoCp(µ-CO)(CO). These facts prompted us to investigate the
Transition metal-hydride addition to olefins (so-called olefin
insertion) is one of the most well-established, important,
fundamental reactions in organometallic catalysis.1 Such a
process is frequently facile and reversible, as observed in
transition-metal-catalyzed olefin isomerization2 as well as in
disproportionation of the alkyl ligands in dialkylplatinum(II)
complexes.3 However, for some transition-metal hydrides such
as the molybdenum and tungsten hydrides MHCp(CO)3 (M )
Mo (1a), W (1b)), olefin insertion did not take place, though
* To whom correspondence should be addressed. E-mail:
komiya@cc.tuat.ac.jp.
(3) (a) Whitesides, G. M.; Gaasch, J. F.; Stedronsky, E. R. J. Am. Chem.
Soc. 1972, 94, 6521. (b) Whitesides, G. M. Pure Appl. Chem. 1981, 53,
287. (c) McCarthy, T. J.; Nuzzo, R. G.; Whitesides, G. M. J. Am. Chem.
Soc. 1981, 103, 3396. (d) Nuzzo, R. G.; McCarthy, T. J.; Whitesides, G.
M. J. Am. Chem. Soc. 1981, 103, 3404. (e) Komiya, S.; Morimoto, Y.;
Yamamoto, A.; Yamamoto, T. Organometallics 1982, 1, 1528.
(4) Scordia, H.; Kergoat, R.; Kubicki, M. M.; Guerchais, J. E.; L’Haridon,
P. Organometallics 1983, 2, 1681.
(1) (a) Applied Homogeneous Catalysis with Organometllic Compounds;
Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim, Germany, 1996;
Vols. 1 and 2. (b) Homogeneous Transition Metal Catalyzed Reactions;
Moser, W. R., Slocum, D. W., Eds.; Adv. Chem. Ser. 230; American
Chemical Society: Washington, DC, 1992. (c) Beller, M.; Bolm, C.
Transition Metals for Organic Synthesis: Building Blocks and Fine
Chemicals; Wiley-VCH: Weinheim, Germany, 1998; Vols. 1 and 2.
(2) (a) Parshal, G. W.; Ittel, S. Homogeneous Catalysis, 2nd ed.; Wiley-
Interscience: New York, 1992. (b) Chaloner, P. A. Handbook of Coordina-
tion Catalysis in Organic Chemistry; Butterworths: London, 1986.
(5) Espinet, P.; Albe´niz, A. C. In Fundamentals of Molecular Catalysis;
Kurosawa, H., Yamamoto, A., Eds.; Elsevier: Amsterdam, 2003; pp 293-
372.
10.1021/om0506872 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 12/14/2005

312 Organometallics, Vol. 25, No. 2, 2006
Communications
enhancement effect of platinum-group metal complexes toward
olefin insertion of the stable molybdenum- or tungsten-
hydrogen bond. In this paper, we wish to report palladium-
catalyzed olefin insertion into molybdenum- and tungsten-
hydrogen bonds, giving the corresponding organomolybdenum
and -tungsten complexes. Synthesis and related reactions of these
heterodinuclear hydrido complexes of palladium and platinum
with molybdenum and tungsten complexes are also described.
When the hydridomolybdenum complex 1a (0.064 M) in
benzene was treated with an approximately equimolar amount
of ethyl acrylate (0.071 M) in the presence of 5 mol % of the
zerovalent palladium complex Pd(PPh3)4 (4) at room temper-
ature, instant olefin insertion took place to give Mo[CH(CO2-
Et)Me]Cp(CO)3 (5a) in 82% yield (based on 1a) in 10 min (eq
1).
Figure 1. Molecular structure of (dppe)HPt-MoCp(CO)3 (3a). All
hydrogen atoms and the incorporated C6H6 molecule are omitted
for clarity. Ellipsoids represent 50% probability.
equilibrium constants (Keq ) [5a]/[1a][ethyl acrylate]) estimated
from the NMR spectra of the reaction mixture of 1a and ethyl
acrylate in C6D6 at 20 °C was 2.0 × 102 M-1, which was
approximately the same as the Keq value of 1.9 × 102 M-1
obtained from the solution of 5a in the presence of 4.
To shed some light on the reaction mechanism, deuterium
labeling experiments were carried out. When WDCp(CO)3 (1b-
d) was employed in this reaction, an extensive regioselective
H-D exchange reaction of the deuterium in 1b-d with only
geminal methylene protons of free ethyl acrylate took place
immediately, indicating that facile and reversible Markovnikov
addition of W-D to ethyl acrylate takes place before the
Acrylonitrile and ethylene also instantly gave the correspond-
ing insertion products in 100 and 91% yields under similar
conditions, respectively. However, insertion of 1,2-disubstituted
olefins such as dimethyl fumarate and dimethyl maleate was
relatively slow and took a few hours to complete, giving 59
and 79% yields, respectively. No insertion took place for
substituted alkenes such as styrene, propylene, and 1-butene,
where extensive isomerization of the unreacted 1-butene to a
mixture of cis- and trans-2-butene was observed,7 suggesting
that rapid and reversible insertion of 1-butene into the Mo-H
bond is actually taking place. The olefin-coordinated complex
Pd(ethyl acrylate)(dppe) (8) also catalyzed the insertion, but the
catalytic activity of 8 was lower than that of 4. In fact, 8 did
not catalyze insertion of dimethyl fumarate and dimethyl
maleate. Olefin insertion into the tungsten-hydrogen bond in
1b was also achieved under similar reaction conditions, though
the apparent insertion of 1b was much slower than that of
molybdenum hydride 1a. For example, after treatment of 1b
with acrylonitrile and ethyl acrylate in the presence of 4 (5 mol
%) for 10 min at room temperature, the corresponding insertion
products were obtained only in ca. 20% yields, respectively,
but the reactions were complete in 5 h.8 It should be noted that
no insertion took place in the reaction of these hydrides with
these olefins in the absence of Pd complexes as well as in the
presence of only ligands such as PPh3 and dppe.
formation of product 5b before the rate-determining step.10
A
dinuclear alkylpalladium-tungsten intermediate may be smoothly
formed in equilibrium, and slow reductive elimination from this
intermediate would give 5b (vide infra).
Stoichiometric reactions of the zerovalent platinum complexes
Pt(CH2dCHR)(dppe) (R ) Ph (9), CO2Et (10)) with the
hydridomolybdenum complex MoHCp(CO)3 (1a) were carried
out in benzene at room temperature. They smoothly gave the
heterodinuclear hydridoplatinum complex (dppe)HPt-MoCp-
(CO)3 (3a)6a in high yields with liberation of the corresponding
olefin (eq 2). Similar reactions with the hydridotungsten complex
1b also gave the corresponding heterodinuclear hydridoplatinum
complex (dppe)HPt-WCp(CO)3 (3b).6a
Another important fact is that the reactions are reversi-
ble, since the alkylmolybdenum product 5a, which was inde-
pendently prepared by the metathetical reaction of MeSO3CH-
(CO2Et)Me with Na[MoCp(CO)3],9 smoothly gave 1a and
CH2dCHCO2Et in 39 and 44% yields, respectively, in 12 min
in the presence of 5 mol % of 4 under the same reaction
conditions with a low concentration of 5a (0.031 M). The
Complexes 3a and 3b were characterized by 1H and 31P{1H}
NMR and IR spectroscopy as well as by X-ray structure
analysis.11 Figure 1 gives an ORTEP drawing of 3a, showing
(10) Reaction of WDCp(CO)3 (90% D, 0.023 mmol, 0.034 M) with ethyl
acrylate (0.023 mmol, 0.034 M) in C6D6 at room temperature immediately
caused H-D exchange between W-D and geminal methylene protons of
ethyl acrylate, giving WDCp(CO)3 (17% D, 0.017 mmol, 74%, 6 min),
and the alkyltungsten complex 5b-d was formed in 21% yield. The D content
in free ethyl acrylate (0.017 mmol, 77%) could not be estimated, due to
extensive broadening of these signals. However, the final alkyltungsten
complex 5b-d contained 30% D (79% yield) in the R-methyl group but no
deuterium in the methine proton, which is confirmed by both 1H and 2D
NMR. The result indicates that only the geminal two protons in ethyl acrylate
were rapidly exchanged, but not the vicinal proton. The smaller deuterium
content in tungsten hydride 1b as compared to the statistical value (30%)
may be due to the equilibrium isotope effect of these species: Churchill,
D. G.; Janak, K. E.; Wittenberg, J. S.; Parkin, G. J. Am. Chem. Soc. 2003,
125, 1403.
(6) (a) Fukuoka, A.; Sugiura, T.; Yasuda, T.; Taguchi, T.; Hirano, M.;
Komiya, S. Chem. Lett. 1997, 329. (b) Yasuda, T.; Fukuoka, A.; Hirano,
M.; Komiya, S. Chem. Lett. 1998, 29.
(7) 1-Butene was catalytically isomerized to 2-butenes in C6D6 for 6 h
at 20 °C to give a butene mixture with the ratio 1-butene:cis-2-butene:
trans-2-butene ) 1:5:18.
(8) Reactions of 1b with acrylonitrile and ethyl acrylate (10 min, 20 °C,
C6D6) gave 18 and 24% yields of the corresponding insertion products,
respectively. After 5 h, the yields reached 100 and 72%.
(9) Burkhardt, E. R.; Doney, J. J.; Bergman, R. G.; Heathcock, C. H. J.
Am. Chem. Soc. 1987, 109, 2022.

Communications
Organometallics, Vol. 25, No. 2, 2006 313
Scheme 1
formation of a Pt-Mo single bond. The geometry around Pt in
3a is essentially square planar, though the hydride was not
clearly found in the differential Fourier map, and the Mo has a
four-legged piano-stool configuration.12 The Pt-Mo bond
distance (2.786(1) Å) is significantly shorter than those for
(Ph3P)2HPt-MoCp(CO)3 (2.839(1) Å)13 and (dppe)EtPt-MoCp-
(CO)3 (2) (2.912(3) and 2.934(3) Å),14 suggesting weaker steric
repulsion between the small hydride and the Mo moiety.
Analogous treatment of the olefin-coordinated zerovalent
palladium complex 8 with 1 equiv of 1a in C6D6 for 1 h also
gave a heterodinuclear hydridopalladium complex, (dppe)HPd-
MoCp(CO)3 (11),15 in 44% yield (NMR), but in this reaction
the insertion product 5a was always a contaminant (9%). 11
can be isolated in 45% yield from a similar reaction mixture in
THF by adding a large excess amount of hexane at -40 °C (eq
3).
noted that, in the reaction of the Pd complex, a succeeding
Markovnikov addition of the metal hydride to ethyl acrylate
also took place to give the insertion product 5a (60%), whereas
no such reaction is observed for the Pt case (eqs 4 and 5).
1
The variable-temperature 31P{1H} and H NMR studies of
11 showed dynamic behavior due to characteristic cis site
exchange of the hydride and Mo moiety,16 in contrast to the
case for the stereochemically rigid platinum analogue 3a.
When (dppe)HPt-MoCp(CO)3 (3a) was treated with 2 equiv
of ethyl acrylate at room temperature, hydride migration took
place to give the starting hydridomolybdenum complex 1a and
zerovalent platinum complex 10 in ca. 10% yields. On the other
hand, reaction of the heterodinuclear palladium analogue 11 with
2 equiv of ethyl acrylate caused immediate reductive elimination
of the hydridomolybdenum complex 1a in 67% yield. These
data indicate the reversibility of this addition-elimina-
tion process of the Mo-H bond to Pt or Pd. It should also be
(11) Crystallographic data of 3a‚C6H6: C40H36MoO3P2Pt, FW ) 917.70,
monoclinic, P21/n (No. 14), a ) 15.403(6) Å, b ) 10.846(5) Å, c ) 22.757-
(5) Å, â ) 105.33(2)°, V ) 3666(2) Å3, Z ) 4, Dcalcd ) 1.662 g cm-3
4435 unique reflections with I > 3σ(I). R (Rw) ) 0.099 (0.147).
,
(12) Unfortunately, the hydride was not clearly found in the X-ray
analysis of 3a, but Pt, Mo, P1, and P2 are in the plane and deviated from
their least-squares plane within 0.17 Å, and the P1-Pt-P2 and P2-Pt-
Mo bond angles are 84.9(2) and 112.0(1)°, suggesting that the Pt has
essentially square-planar geometry with a putative hydride ligand. The
dihedral angle of the two Pt-C(O)-Mo planes is 53.5°, suggesting a four-
legged piano-stool structure at Mo.
To confirm the reversibility of the stoichiometric reactions,
the following three reactions were performed at room temper-
ature: (a) the reaction of the dinuclear hydridopalladium
complex 11 with ethyl acrylate, (b) the reaction of the zerovalent
palladium complex 8 with molybdenum hydride 1a, and (c) the
reaction of the zerovalent palladium complex Pd(styrene)-
(dppe), having a weakly coordinated styrene ligand, with the
(1-(ethoxycarbonyl)ethyl)molybdenum complex 5a. These reac-
tions all gave essentially mixtures containing all these species
in the same ratio at room temperature in a few hours. Though
an accurate ratio of the products was difficult to estimate, due
to line broadening of the signals by fast olefin exchange at room
temperature, the ratio 1a:5a:8:11 was roughly 2:2:4:3 by NMR.
The result cleanly indicates the equilibration of all these
complexes under these conditions.
(13) Bars, O.; Braunstein, P.; Geoffroy, G. L.; Metz, B. Organometallics
1986, 5, 2021.
(14) Single crystals of 2, which was prepared by the metathetical reaction
of PtEt(NO3)(dppe) with Na[MoCp(CO)3] according to our previous
method,6a were obtained from cold benzene/hexane solution. Crystal-
lographic data of 2: C35H34MoO3P2Pt, FW ) 855.63, monoclinic, P21/n
(No. 14), a ) 14.755(9) Å, b ) 26.58(1) Å, c ) 20.482(7) Å, â ) 99.16-
(4)°, V ) 7930(6) Å3, Z ) 8, Dcalcd ) 1.433 g cm-3, 5647 unique reflections
with I > 3σ(I). R (Rw) ) 0.073 (0.114). The unit cell contained two
crystallographically independent molecules.
(15) Physical and spectroscopic data for 11‚0.5THF are as follows. Mp:
125-128 °C dec. Molar electric conductivity Λ (THF, -21 °C) ) 0.49 S
cm2 mol-1. Anal. Found: C, 55.77; H, 5.13. Calcd for C39H40MoO3.5P2Pd:
C, 56.50; H, 4.86. IR (KBr): ν(CO) 1722, 1856, 1928 cm-1
.
31P{1H} NMR
(toluene-d8, -50 °C, 121.6 Hz): δ 25.1 (d, 2JPP ) 28 Hz), 26.5 (d, 2JPP
)
A tentative reaction mechanism for this palladium complex
28 Hz). 1H NMR (toluene-d8, -70 °C, 300.4 Hz): δ -4.64 (dd, JPH
80.5, 9.0 Hz, 1H, PdH), 2.3-2.4 (m, 4H, dppe CH2), 5.25 (s, 5H, Cp),
7.40-7.65 (m, 12H, m, p-Ph), 7.70-7.95 (m, 8H, o-Ph).
)
2
catalyzed insertion is considered. At first, 4 and Pd(ethyl
17
acrylate)(PPh3)2 are in facile equilibrium in the presence of
(16) When the temperature was lowered to -70 °C, the center signal of
the triplet at δ -5.00 (2JPH ) 36.3 Hz) assignable to the hydride gradually
broadened and collapsed, and then new signals appeared at the outsides of
the remaining two sharp signals to give a doublet of doublets.15 The present
feature is interpreted by the facile site exchange process of the hydrido and
Mo moieties at Pd, as is known for the hydrido(silyl)platinum complexes:
Danny, C.; Simon, B. D.; Sarah, L. H.; Iman, G. K.; Robin, N. P.; Sylviane,
S.-E.; Philippa, L. T. Organometallics 2004, 23, 5744.
ethyl acrylate. Addition of the hydridomolybdenum or -tungsten
complex 1a or 1b to this solution also joins in this facile
equilibrium via a putative alkylpalladium-molybdenum com-
plex, (1-(ethoxycarbonyl)ethyl)palladium-molybdenum or -tung-
sten (12) (Scheme 1). Such an intermediate is likely formed by
(1) nucleophilic addition of the hydride to the coordinated ethyl

314 Organometallics, Vol. 25, No. 2, 2006
Communications
acrylate, (2) oxidative addition of the hydride to the olefin
complex followed by insertion, or (3) initial oxidative addition
of the hydride to 4, giving a hydridopalladium complex followed
by olefin insertion. These fast and reversible processes are
suggested by deuterium scrambling between free ethyl acrylate
and tungsten deuteride. The following rate-limiting step should
involve slow but reversible reductive elimination of 12 to give
the insertion products, slowly reaching the total equilibration,
though the mechanism is not clear at present.18
A detailed investigation including kinetics is required to
clarify the mechanism. Despite this consideration, the present
communication provides a new route for olefin insertion into
stable transition-metal hydrides.
(17) Takahashi, S.; Hagiwara, N. Nippon Kagaku Kaishi 1967, 88, 1306;
Chem. Abstr. 1968, 69, 27514g.
(18) A brief kinetic study showed that the initial rate increased on
increasing the concentrations of the tungsten hydride 1b (0.038-0.19 M)
and catalyst 4 (0.000 38-0.0019 M, 1-5 mol %) but was independent of
the ethyl acrylate concentration (0.038-0.18 M). Although slow reductive
elimination of the alkylpalladium-tungsten intermediate associated with
1b after facile and reversible insertion of olefin into the Pd-H bond is one
of the possible processes, the reaction seems to be actually more
complicated, since the added triphenylphosphine significantly retarded the
reaction but the enhancement effect (up to 30%) of the added olefin (0.036-
0.79 M) on the rate was not significant even under the reaction conditions
in the presence of triphenylphosphine (0.019 M). In addition, unfortunately,
major palladium active species under these catalytic conditions could not
be characterized by NMR due to extensive broadening. Further detailed
kinetic studies to clarify the mechanism are in progress.
Acknowledgment. This work was financially supported by
a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
Supporting Information Available: CIF files and text and
tables giving crystallographic data for 3a and 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0506872