Mechanistic studies of catalytic olefin dimerization reactions using electrophilic η3-allyl-palladium(II) complexes

Article abstract of DOI:10.1021/ja9602354

The dimerization of olefins by well-defined cationic η³-allyl-palladium complexes of the type [(C₃H₅)Pd(L)(PR₃)]⁺[BAr'₄]^- (Ar' = [3,5-C₆H₃(CF₃)

Full text of DOI:10.1021/ja9602354

J. Am. Chem. Soc. 1996, 118, 6225-6234
6225
Mechanistic Studies of Catalytic Olefin Dimerization Reactions
Using Electrophilic η3-Allyl-Palladium(II) Complexes
Geraldine M. DiRenzo, Peter. S. White, and Maurice Brookhart*
Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
ReceiVed January 23, 1996X
Abstract: The dimerization of olefins by well-defined cationic η3-allyl-palladium complexes of the type
[(C3H5)Pd(L)(PR3)]+[BAr′4]- (Ar′ ) [3,5-C6H3(CF3)2]; L ) OEt2, H2O; R ) cyclohexyl (Cy), n-butyl (nBu)) has
been studied. These complexes react with ethylene or methyl acrylate at -80 °C with loss of L to form the η2-
olefin complexes [(C3H5)Pd(η2-olefin)(PR3)]+[BAr′4]- (olefin ) H2CdCH2, CH2dCHC(O)OCH3). Upon warming,
allyl-olefin coupling occurs. The dimerization of ethylene occurs rapidly at 0 °C with an observable ethyl-ethylene
intermediate [(C2H5)Pd(C2H4)2(PCy3)]+[BAr′4]-. Methyl acrylate reacts to form a stable acrylate chelate complex,
[(CH3O(O)CCH2CH2)Pd(CH2dCHC(O)OCH3)(PR3)]+[BAr′4]-, which is the catalyst resting state for methyl acrylate
dimerization which occurs at room temperature to give predominantly trans-dimethyl-2-hexenedioate.
Introduction
the unsaturated, linear diesters which can be converted to adipic
acid by hydrogenation and hydrolysis. Since acrylates are
produced from oxidation of propene, this route provides an entry
into adipic acid based on a C3 feedstock which contrasts with
the currently practiced methods based on C6 feedstocks.1,2
Transition metal-catalyzed dimerization of functionalized
olefins represents an attractive route to inexpensive difunctional
monomers useful for polymer synthesis via condensation
techniques. The regioselective tail-to-tail dimerization of methyl
acrylate has been extensively examined as an alternative route
to adipic acid, one of the monomers used in the production of
Nylon 6,6.1-29 As shown in eq 1, tail-to-tail dimerization yields
X Abstract published in AdVance ACS Abstracts, June 1, 1996.
(1) Parshall, G. W.; Ittel, S. D. In Homogeneous Catalysis, 2nd ed.;
Wiley-Interscience: New York, 1992; p 242.
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Acrylate dimerization catalysts based on Rh, Ru, Pd, and Ni
have all been reported.3-29 The most efficient catalyst systems
and those best understood mechanistically are cationic Rh(III)
catalysts derived from protonation of bis-ethylene complexes
of the type C5Me5Rh(C2H4)2, (η5-1,2,3-trimethylindenyl)Rh-
(C2H4)2, and (η5-1,2,3,4,5,6,7-heptamethylindenyl)Rh(C2H4)2.
The resting state of the C5Me5RhIII-based catalyst was shown
to be the rhodium(III)-chelate complex 3 illustrated below, with
the turnover-limiting step being the migratory insertion reaction
of 3 with exclusive tail-to-tail coupling occurring.9,11,12
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1986, 37, 53.
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390, 1987.
Extensive work on the economically more attractive Pd(II)-
based systems has been reported. Four catalyst precursors,
(C6H5CN)2PdCl2, Pd(CH3CN)42+, (η3-CH2CRCH2)PdL2, and
Pd(AcAc)2, have been investigated. Early work by Hazeldine17
on (C6H5CN)2PdCl2 revealed dimerization activity at 113 °C
(27) Wilke, G.; Sperling, K.; Stehling, L. U. S. Patent 4 594 447, 1986.
(28) Wilke, G. Angew. Chem. Int. Ed. Engl. 1988, 27, 185.
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metallics 1995, 14, 2565.
(25) Guibert, I.; Neibecker, D.; Tkatchenko, I. J. Chem. Soc., Chem.
Commun. 1989, 1850.
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S0002-7863(96)00235-1 CCC: $12.00 © 1996 American Chemical Society

6226 J. Am. Chem. Soc., Vol. 118, No. 26, 1996
DiRenzo et al.
in pure methyl acrylate (60 turnovers, approximately 90%
selectivity for linear dimers). Oehme and co-workers18 discov-
ered that activation of (C6H5CN)2PdCl2 with AgBF4/benzo-
quinone resulted in a longer lived catalyst (340 turnovers)
operating at room temperature while Nugent and McKinney5
examined the influence of various Lewis and protic acid
additives on the catalytic activity of this system. Nugent later
reported a mild, highly selective route for the dimerization of
methyl acrylate using the electrophilic, dicationic [Pd-
Scheme 1. Synthesis of Precatalysts [(C3H5)Pd(OEt2)-
n
(PR3)]+[BAr′4]- (7a,b: R ) Cy (a) Bu (b))
-
(CH3CN)4]2+[BF4]2 and LiBF4 (150 turnovers, 40 °C).19,20
Tkatchenko and co-workers24,30,31 have examined η3-allyl-
Pd(II) precursors for methyl acrylate dimerization. Typically,
(η3-allyl)Pd(COD)+ salts were treated with phosphines in
CH2Cl2 to generate active catalyst systems. Bu3P was found
to be particularly effective; addition of 0.5 equiv to (η3-CH2-
CCH3CH2)Pd(COD)+ resulted in an active catalyst system (300
turnovers, 20 h, 80 °C) with high selectivity for linear dimers.
Methyl acrylate dimerization was also achieved by protonation
of Pd(AcAc)2 with HBF4 in CH2Cl2.25,31
Scheme 2 In Situ Generation of Isomeric
[(C3H5)Pd(C2H4)(PCy3)]+[BAr′4]- (8r and 8â)
Several possible mechanisms have been suggested for these
Pd(II)-catalyzed acrylate dimerizations including an oxidative
addition pathway through a vinyl hydride intermediate,23
a
Cossee-Arlman insertion mechanism functioning through a
palladium hydride intermediate,17 and carbon-carbon bond
coupling through formation of a Pd(II) metallacycle.25 No
thorough mechanistic studies have been carried out which firmly
established the mechanistic details of these closely related Pd(II)
catalytic systems. Thus, it was the goal of this work to prepare
well-defined Pd(II)-catalyst precursors and examine working
catalyst systems by NMR spectroscopy in order to establish the
mechanism of acrylate dimerization.
We report here the synthesis of monocationic η3-allyl-
palladium(II) complexes of the type (η3-C3H5)Pd(PR3)(OEt2)+-
(BAr′4)- (Ar′ ) [3,5-(CF3)2C6H3]) and use of these complexes
for both catalytic ethylene and methyl acrylate dimerization and
low-temperature NMR studies which lend insight into the details
of the catalytic cycle for these dimerization reactions.
plexes and increases their solubility, whereas traditional coun-
terions, such as [PF6]- and [BF4]-, often yield insoluble
complexes that can decompose via halide ion abstraction.35 The
use of the noncoordinating counterion, together with the high
purity of the crystalline tricyclohexylphosphine complex, en-
abled the detailed mechanism of reactions of these complexes
with ethylene and methyl acrylate to be investigated by low-
temperature NMR spectroscopy.
2. Reactivity of the η3-Allyl Complex [(C3H5)Pd(OEt2)-
(PCy3)]+[BAr′4]- (7a) with Ethylene. Spectroscopic Detec-
tion of Reaction Intermediates. A. Initial Observation of
Catalytic Ethylene Dimerization. Treatment of a CD2Cl2
solution of the η3-allyl-palladium ether complex [(C3H5)Pd-
(OEt2)(PCy3)]+[BAr′4]- (7a) at -10 °C with excess ethylene
results in catalytic ethylene dimerization to form butenes. The
major butene isomer present was identified by 1H NMR
spectroscopy as trans-2-butene (cis:trans, ca. 1:6). Having
demonstrated that 7a dimerizes ethylene, a detailed analysis of
the reaction mechanism was performed by monitoring this
reaction by low temperature NMR spectroscopy. Two key
intermediates in the catalytic formation of butenes have been
observed. The characterization of these intermediates and the
details of the initiation process and the subsequent dimerization
reaction are described below.
Results and Discussion
1. Synthesis of the η3-Allyl-Palladium(II) Precatalysts
[(C3H5)Pd(OEt2)(L)]+[BAr′4]- (7: L ) PCy3 (a), PBu3 (b)).
The dimeric complex [(C3H5)PdCl]2 (4)32 reacts with PCy3 or
PBu3 via chloride bridge-splitting to give the monomeric
complexes [(C3H5)Pd(Cl)(PR3)] (5a,b).33 Treatment of 5a,b
with CH3MgBr gives [(C3H5)Pd(CH3)(PR3)] (6a,b).34 Complex
6a (R ) Cy) and complex 6b (R ) nBu) are isolated as a pure
crystalline solid and as an oil, respectively. Protonation of
complexes 6a,b with the oxonium acid, (Et2O)2H+(BAr′4)- (Ar′
) 3,5-C6H3(CF3)2), gives complexes 7a,b in good yields
(Scheme 1).
Complexes 7a,b are highly electrophilic and the diethyl ether
ligands are quite labile. In the presence of even traces of water,
the ether ligand is readily displaced and thus complexes
[(C3H5)Pd(OEt2)(PR3)]+[BAr′4]- (7a,b), which are isolated or
are observed in solution, often contain small amounts of the
corresponding aquo complexes. The noncoordinating counterion
[3,5-C6H3(CF3)2]4B- stabilizes these highly electrophilic com-
B. Formation, Low-Temperature Spectroscopic Charac-
terization, and Dynamic Behavior of [(C3H5)Pd(C2H4)-
(PCy3)]+[BAr′4]- (8). Exposure of the η3-allyl-palladium ether
complex [(C3H5)Pd(OEt2)(PCy3)]+[BAr′4] (7a) to 40 equiv of
ethylene in CD2Cl2 at -90 °C results in the generation of two
new species in approximately a 1:1 ratio with 31P NMR signals
at 37.2 and 36.1 ppm (Scheme 2 and Figure 1a). Warming
this solution in the NMR probe results in broadening of these
signals, coalescence at -74 °C, and sharpening to a singlet at
-40 °C. Rate analysis (k ) π∆V/x2) at the coalescence
temperature (-74 °C) yields kex ) 300 s-1 with ∆Gq ) 9.3
kcal mol-1
.
(30) Grenouillet, P.; Neibecker, D.; Tkatchenko, I. Organometallics 1984,
3, 1130.
(31) Grenouillet, P.; Neibecker, D.; Tkatchenko, I. U.S. Patent 4 889
949, 1989.
(32) Tatsuno, Y.; Yoshida, T. In Inorganic Synthesis; Shriver, D. F.,
Ed.; John Wiley & Sons: New York, 1979; p 220.
(33) Powell, J.; Shaw, B. L. Inorg. Phys. Theor. 1967, 1839.
(34) Hayashi, Y.; Matsumoto, K.; Nakamura, Y. J. Chem. Soc., Dalton
Trans. 1989, 1519.
1
On the basis of H NMR spectroscopy, these species are
proposed to be two isomers of the η3-allyl-palladium ethylene
adduct [(C3H5)Pd(C2H4)(PCy3)]+[BAr′4]- (8). At -20 °C a
multiplet centered at 4.4 ppm (see Figure 1b) that can be
(35) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,
3920.

Dimerization of Olefins
J. Am. Chem. Soc., Vol. 118, No. 26, 1996 6227
Scheme 4. The Two Enantiomers of
[(C3H5)Pd(PCy3)2]+[BAr′4]- (9)
sets of AA′BB′ signals for the ethylene hydrogens. Rapid allyl
rocking will result in coalesence of the 31P signals as well as
averaging the two sets of AA′BB′ patterns to a single AA′BB′
1
pattern as observed at -20 °C. The fact that the ethylene H
signals do not coalesce to a single resonance indicates that
enantiomers 8r, and 8r′ (and 8â, and 8â′) do not interconvert
on an NMR time scale. Such a process could have occurred
by the normal π f σ allyl mechanism,37-42 but the narrow 1H
line widths at -20 °C indicate such a process must have a free
energy of activation greater than ca. 14 kcal/mol.
Figure 1. Variable-temperature 31P (a) and 1H (b) NMR spectra of 8.
Scheme 3. Proposed Dynamic Behavior of
[(C3H5)Pd(C2H4)(PCy3)]+[BAr′4]- (8)
C. Dynamic Behavior of [(C3H5)Pd(PCy3)2]+[BAr′4]- (9).
To provide further support for the allyl rocking mechanism, the
cationic allyl complex [(C3H5)Pd(PCy3)2]+[BAr′4]- (9) was
examined in which the two cis ligands (PCy3) are identical. If
the allyl unit is unsymmetrically bound to Pd an allyl rocking
would result in a degenerate exchange between the two
enantiomers 9 and 9′ (Scheme 4). Complex 9 was prepared by
addition of 1 equiv of PCy3 to a CD2Cl2 solution of
[(C3H5)Pd(OEt2)(PCy3)]+[BAr′4]- (7a). The 31P NMR spectrum
at -82 °C showed nonequivalent 31P signals as an AB quartet
centered at 36.6 ppm (Jpp ) 29.3 Hz). Warming resulted in
broadening and coalescence of the 31P signals to a singlet above
-50 °C. As expected, at -20 °C the 1H NMR spectrum showed
only three signals for the five allyl hydrogens at δ 5.36 (m,
1H, H5), 4.47 (bs, 2H, H1 and H4), and 2.91 (bs, 2H, H2 and
H3).43
interpreted as an AA′BB′ pattern is observed, which corresponds
to the four hydrogens of the palladium-bound ethylene ligand.
In the presence of excess ethylene, these signals remain sharp,
indicating that no exchange of bound ethylene with free ethylene
is occurring on an NMR time scale at -20 °C. Three allyl
resonances appear at 5.32 (multiplet), 4.58 (broad doublet), and
2.41 (broad) ppm. The two remaining allyl resonances are
obscured by the resonances of Et2O and the tricyclohexylphos-
phine. The 1H NMR resonances of the bound ethylene exhibit
complex behavior between -20 and -90 °C (Figure 1b).
Clearly between -20 and -74 °C the chemical shift difference
between HA, HA′ and HB, HB′ decreases. Below -74 °C the
resonances again separate, and at -90 °C two very broad signals
are observed which we propose to be two overlapping sets of
AA′BB′ resonances (see below). While accurate rate data
cannot be extracted from these spectra, it is clear that this
dynamic process must be the same one responsible for the
temperature dependence of the 31P signals.
The dynamic behavior of 8 is proposed to occur through a
“rocking” motion of the allyl ligand which interconverts isomers
8r and 8â (Scheme 3). Solid-state structures of certain η3-allyl-
palladium(II) complexes support the idea that the allyl unit can
be rotated out of the square plane in such a way that the C1-
C2 of the allyl ligand is nearly aligned with the plane formed
by Pd and the remaining two ligands.36 In analogy with other
Pd(II) olefin complexes, we assume that ethylene rotation in
8r and 8â (which interconverts H1 with H1′ and H2 with H2′)
is always rapid on an NMR time scale. At low temperatures
two 31P signals will be observed for 8r and 8â as well as two
The rates for allyl-rocking are most conveniently measured
by line shape analysis of the 31P NMR resonances of PA and
PB. The experimental line shapes were compared to calculated
line shapes using the DNMR3 simulation program.44-47 Figure
2 illustrates the set of observed and calculated spectra for
complex 9. Using this technique, rate constants could be
(37) Powell, J.; Robinson, S. D.; Shaw, B. L. J. Chem. Soc., Chem.
Commun. 1965, 78.
(38) Ramey, K. C.; Statton, G. L. J. Am. Chem. Soc. 1966, 88, 4387.
(39) Lukas, J.; Coren, S.; Blom, J. E. J. Chem. Soc., Chem. Commun.
1969, 1303.
(40) Lippard, S. J.; Morehouse, S. M. J. Am. Chem. Soc. 1972, 94, 6949.
(41) van Leeuwen, P. W. N. M.; Praat, A. P. J. Chem. Soc., Chem.
Commun. 1970, 365.
(42) Cesarotti, E.; Grassi, M.; Prati, L.; Demartin, F. J. Organomet. Chem.
1989, 370, 407.
(43) A reviewer has suggested that the dynamic behavior we ascribe to
allyl rocking may be due to interconversion of two conformational isomers
of the tricyclohexylphosphine ligand. This cannot be ruled out for the mono-
tricyclohexylphosphine complexes. In the case of the bis-tricyclohexylphos-
phine complex 9, only two 31P resonances are observed under slow exchange
conditions. These signals are coupled to one another showing they belong
to a single isomer undergoing a degenerate isomerization which interconverts
the two 31P sites. This is consistent with the allyl rocking proposal (9 h
9′). If two conformations of the tricyclohexylphosphine ligand were possible,
then three isomers would be expected for 9. Only one isomer of the three
would exhibit inequivalent phosphorus nuclei. It seems unlikely that this
would be the sole isomer detectable. Thus we favor allyl rocking to account
for the dynamics of 9 and, by analogy, for the dynamics of the other allyl
complexes.
(44) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press: New
York, 1982.
(45) Allerhand, A.; Gutowsky, H. S.; Jonas, J.; Meinzer, R. A. J. Am.
Chem. Soc. 1966, 88, 3185.
(46) Gordon, J. G.; Holm, R. H. J. Am. Chem. Soc. 1970, 92, 5319.
(47) Eaton, S. S.; Hutchinson, J. R.; Holm, R. H.; Muetterties, E. L. J.
Am. Chem. Soc. 1972, 94, 6411.
(36) Musco, A.; Pontellini, R.; Grassi, M.; Sironi, A.; Meille, S. V.;
Ruegger, H.; Ammann, C.; Pregosin, P. S. Organometallics 1988, 7, 2130.

6228 J. Am. Chem. Soc., Vol. 118, No. 26, 1996
DiRenzo et al.
Scheme 5. Proposed Mechanistic Scheme for Catalytic
Butene Formation
Figure 2. Calculated (a) and observed (b) variable-temperature 31P
NMR spectra for 9.
results in nearly complete disappearance of the signal at 5.41
ppm for free ethylene. Correspondingly, irradiating the free
ethylene signal results in complete disappearance of the 3.58
ppm signal. This result indicates that the 3.58 ppm resonance
belongs to a new palladium-ethylene complex in which the
coordinated ethylene is exchanging with free ethylene at -20
°C. While the structure of this species is uncertain, a reasonable
proposal is that this species corresponds to the ethyl-ethylene
complex, 11, which is the catalyst resting state in the dimer-
ization cycle.49,50 After complete dimerization of ethylene, the
3.58 ppm species disappears. Scheme 5 represents a tentative
proposal for the reaction of [(C3H5)Pd(OEt2)(PCy3)]+[BAr′4]-
(7a) with ethylene and entry into the catalytic ethylene dimer-
ization cycle.
Figure 3. The Eyring plot of the rate data for intramolecular allyl
rocking in complex 9.
3. Reactivity of the Allyl Ether Complexes [(C3H5)-
Pd(OEt2)(L)]+[BAr′4]- (7: L ,) PCy3 (a), PBu3) (b) with
Methyl Acrylate. Spectroscopic Detection and Study of
Reaction Intermediates. A. Initial Observation of Catalytic
Methyl Acrylate Dimerization. Exposure of the η3-allyl-
palladium ether complexes [(C3H5)Pd(OEt2)(PR3)]+[BAr4′]-,
(7a,b) to excess methyl acrylate results in catalytic methyl
acrylate dimerization. For example, treatment of 1 mL of
methyl acrylate (10 mmol) with 0.05 g (0.04 mmol) of the tri-
n-butylphosphine complex, [(C3H5)Pd(OEt2)(P(nBu3))]+[BAr4′]-
(7b) results in the formation of tail-to-tail methyl acrylate dimers
(200 turnovers) after 24 h at 60 °C. The reactivity of methyl
acrylate with complexes 7a and 7b has been studied by low-
temperature NMR spectroscopy. These studies, which reveal
the mechanistic details of the initiation step and the catalytic
cycle, are described below.
obtained over a 50 °C temperature range (-86 to -36 °C),
which permits calculation of ∆Hq and ∆Sq for this process. The
Eyring plot of the data for intramolecular allyl rocking in
complex 9 yields ∆Hq ) 9.1 kcal mol-1 and ∆Sq ) -8.8 eu
and is shown in Figure 3.
D. Spectroscopic Studies of the Dimerization Cycle. The
η3-allyl-palladium ethylene complex [(C3H5)Pd(C2H4)-
(PCy3)]+[BAr′4]- (8) is stable up to -20 °C in the presence of
excess ethylene. Warming above -20 °C results in dimerization
of ethylene to butenes (primarily trans-2-butene cis:trans, ca.
1:6). The major organometallic species remaining after ethylene
dimerization is the unreacted allyl ethylene complex, 8. This
observation implies that complex 8, in a slow step, undergoes
a transformation which generates an active catalyst for ethylene
dimerization which then rapidly dimerizes ethylene before
appreciable depletion of starting complex 8. Monitoring of this
solution by variable-temperature 1H and 31P NMR spectroscopy
yields additional information.
B. Low-Temperature NMR Spectroscopic Characteriza-
tion of the Acrylate Complex [(C3H5)Pd(CH2dCHC-
(O)OCH3)(PCy3)]+[BAr′4]- (13). Treatment of [(C3H5)Pd-
(OEt2)(PCy3)]+[BAr′4]- (7a) with 1 equiv of methyl acrylate
in CD2Cl2 at -100 °C results in the formation of four new
species with 31P signals at 38.5, 37.5, 37.3, and 37.2 ppm
At -20 °C a new 31P resonance grows in at 33.7 ppm at the
same time that ethylene dimerization to predominantly the trans-
2-butene isomer occurs. Its maximum intensity corresponds to
only about 25% of that of resonances for 8r, 8â. This species
presumably corresponds to the catalyst resting state for ethylene
(48) Very recently, the reaction of similar allyl-palladium complexes
with ethylene was directly observed: Mecking, S.; Keim, W. Organo-
metallics In press.
1
dimerization. In addition to the production of butenes, the H
(49) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137.
(50) There is no evidence that this species is a bis-ethylene complex.
However, since ethylene readily displaces diethyl ether in these cationic
species, we believe a bis-ethylene complex is more likely than a solvated
mono-ethylene complex.
NMR spectrum reveals formation of a small amount of a species
containing olefinic resonances (assigned as 1,4 pentadiene) along
with the appearance of a new broad singlet at 3.58 ppm in the
1H NMR spectrum.48 Saturation of the resonance at 3.58 ppm

Dimerization of Olefins
J. Am. Chem. Soc., Vol. 118, No. 26, 1996 6229
1
Scheme 6. The Eight Stereoisomers of
35.1 ppm. The H NMR spectrum of this species exhibits a
[(C3H5)Pd(η2-CH2dCHC(O)OCH3)(PCy3)]+[BAr′4]- (13)a
methoxy resonance at 3.63 ppm, olefinic resonances at δ 6.78
(m, 1H), 5.74 (d, 1H), and 4.85 (d, 1H), and upfield multiplets
at δ 2.98 (m, 1H), 2.45 (m, 2H), and 2.07 (m, partially obscured
by PCy3). This species is proposed to be the four-membered
chelate 14, the result of coupling the allyl unit to Câ of the
acrylate ligand. 1H NMR assignments are shown below. The
pattern of the resonances at 6.78, 5.74, and 4.85 ppm clearly
corresponds to a terminal olefinic unit -CH2-CHdCH2, but
the shifts are quite different from an uncomplexed R-olefin.
Decoupling experiments establish that H1 (δ 6.78) is coupled
to both H2 (δ 5.74, J12 ) 16.7 Hz) and H3 (δ 4.85, J13 ) 9.4
Hz), consistent with the proposed structure. H1 (δ 6.78) is also
coupled to the resonance at δ 2.45 and provides support for
this assignment as H6, H6′. The 1H band at δ 2.98 couples
only to the 2H, δ 2.07 band and suggests that δ 2.98 is H5 and
δ 2.07 corresponds to H7, H7′. An alternate structure for 14 is
the five-membered chelate complex 15 which could result from
â-hydride elimination and reinsertion in 14. However, 14 is
preferred based on the observation that H5 is coupled only to
H7, H7′, and not H6, H6′. Also, being R to a carbonyl group H7,
H7′ would be expected to appear further downfield than δ 2.07.
These results show that allyl-acrylate coupling occurs at -25
°C to initially give the four-membered chelate 14. The rate of
allyl-acrylate coupling was measured at -25 °C (k ) 1.1 ×
10-3 s-1, ∆Gq ) 17.8 kcal mol-1). A typical first-order rate
plot is shown in the Experimental Section.
a Allyl rocking is shown from left to right and olefin rotation is shown
from top to bottom. Sets 1 and 2 result from coordination of the two
enantiofaces of the η2-methyl acrylate ligand.
Warming the solution of 14 to 15 °C results in elimination
of CH2dCHCH2CHdCHCO2CH3 and the formation of a new
palladium species. The assignment of the elimination product
as methyl 2,5-hexadieneoate is based on comparison of the 1H
NMR data with known values.51 Particularly characteristic is
the shift and pattern of the γ-methylene protons (H3), a triplet
of quartets at δ 2.96, due to vicinal coupling with H2, H4 (6.5
Hz), and long-range allylic coupling to H1, H5, H6 (ca. 1.5
Hz). The first-order rate constant for elimination of
CH2dCHCH2CHdCHCO2CH3 was determined to be 5.7 ×
10-4 s-1 at 15 °C (∆Gq ) 21.0 kcal mol-1).
The new Pd species is assigned as the five-membered acrylate
chelate complex [(η2-CH2dCHCO2CH3)Pd(CH2CH2C(O)OCH3)-
(PCy3)]+[BAr′4]- (16) based on 1H, 1H{31P}, 13C, and 31P NMR
spectroscopy. A single 31P band is observed at 49.5 ppm. The
1H NMR shows a single methoxy resonance at δ 3.77 (s, 3H)
and diagnostic of the chelate a 2H resonance at δ 2.72 (td, JHP
) 2.1 Hz) assigned to HA and a 2H resonance at δ 1.77 (td,
JHP ) 2.4 Hz) assigned to HB. There is only a single broad
resonance for methyl acrylate which suggests that free methyl
acrylate is in rapid equilibrium with bound methyl acrylate. This
was verified by cooling the solution to -60 °C which results
in the appearance of signals for free methyl acrylate (δ 6.38,
6.13, and 5.81) as well as a second set for the bound methyl
acrylate in 16 (δ 4.58, 4.31, and 4.12). The chelate structure is
further confirmed by 13C NMR data (20 °C) which show CR at
(approximate ratios, 1.5:2.5:1:2). Warming this solution results
in broadening of the 31P resonances, coalescence of two pairs
of signals at ca. -70 °C, and sharpening to two singlets at 38.3
and 37.7 ppm at -50 °C. We postulate that these species are
cationic η3-allyl-palladium complexes of the type [(C3H5)Pd-
(η2-CH2dCHC(O)OCH3)(PCy3)]+[BAr′4]- (13). Eight isomers
of complex 13 could possibly be observed if both allyl rocking
and methyl acrylate rotation were slow on the NMR time scale
(see Scheme 6).
Based on experiments described above, it is reasonable to
assume that at -100 °C allyl rocking is slow but acrylate
rotation is rapid. This would lead to the appearance of four
31P signals as is observed for the sets 13A h 13B, 13A′ h
13B′, 13C h 13D, 13C′ h 13D′. At higher temperatures rapid
allyl rocking would lead to interconversion of the isomers of
set 1, 13A, 13B, 13A′, 13B′, and, likewise, of set 2, 13C, 13D,
13C′, 13D′, and observation of two 31P signals. Averaging of
the two sets would have to occur by migration of Pd from one
enantioface of methyl acrylate to the other or from one
enantioface to the other of the allyl ligand. These processes
apparently have higher activation energies than the methyl
acrylate rotation and allyl rocking as supported by the dynamic
behavior of 8.
The isomeric η3-allyl-palladium acrylate complexes
[(C3H5)Pd(CH2dCHC(O)OCH3)(PCy3)]+[BAr′4]- (13) are stable
up to -30 °C in the presence of excess methyl acrylate.
Warming the solution in the NMR probe to -25 °C results in
the formation of a single new species with a 31P resonance at
(51) Sheffy, F. K.; Godschalx, J. P.; Stille, J. K. J. Am. Chem. Soc. 1984,
106, 4833.

6230 J. Am. Chem. Soc., Vol. 118, No. 26, 1996
DiRenzo et al.
Scheme 7. Mechanism for Catalytic Methyl Acrylate
Dimerization
Figure 4. ORTEP diagram for [(H2O)Pd(CH2CH2C(O)OCH)-
(PCy3)]+[BAr′4]- (18).
Table 1. Selected Interatomic Distances (in Å) for
[(H2O)Pd(CH2CH2C(O)OCH)(PCy3)]+ [BAr′4]- (18)
Pd-P
2.2353(24)
2.019(7)
2.837(6)
2.822(6)
2.150(4)
2.176(4)
P-C(11)
1.838(6)
1.832(7)
1.846(6)
1.531(8)
1.475(9)
1.243(8)
δ 40.4, Câ at δ 15.2, -OCH3 at δ 56.6, and the chelated
carbonyl resonance at δ 240.6, a typical shift for such carbonyl
groups.52
Pd-C(1)
Pd-C(2)
Pd-C(3)
Pd-O(4)
Pd-O(9)
P-C(21)
P-C(31)
C(1)-C(2)
C(2)-C(3)
C(3)-O(4)
Table 2. Selected Bond Angles (in deg) for
[(H2O)Pd(CH2CH2C(O)OCH)(PCy3)]+[BAr′4]- (18)
P(1)-Pd-C(1)
P(1)-Pd-O(9)
O(4)-Pd-O(9)
C(1)-Pd-O(4)
P(1)-Pd-O(4)
92.72(18)
96.92(13)
88.39(17)
82.04(21)
174.06(12)
C(1)-Pd-O(9)
Pd-P(1)-C(11)
Pd-P(1)-C(21)
Pd-P(1)-C(31)
Pd-O(4)-C(3)
170.30(20)
114.91(21)
111.34(20)
107.72(21)
109.6(4)
and angles are provided in Tables 1 and 2, respectively. A
distorted square-planar geometry exists about the Pd(II) center
with the carbonyl group chelated trans to tricyclohexylphosphine
(Pd-O, 2.150(4) Å).
Holding the solution of 16 at 20 °C in the presence of methyl
acrylate results in slow production of methyl acrylate dimers
(predominantly trans-dimethyl 2-hexenedioate). No other pal-
ladium species appear. The expected palladium hydride inter-
mediates in the reaction steps 14 f 16 and 17 f 16 are not
observable under the reaction conditions. Thus, we assign 16
as the catalyst resting state. Scheme 7 summarizes the catalyst
activation process and the overall catalytic cycle. The geometry
assigned to 16 places the η2-acrylate ligand cis to Câ of the
chelate in the necessary geometry for migratory insertion.
However, results described in the next section suggest that the
alternative trans geometry must be considered.
If the η2-acrylate in 16 occupies the same coordination site
as H2O in 18, then the catalyst resting state observed may
actually be 16′ (with acrylate trans to Câ) rather than 16. Since
carbon-carbon bond formation requires a cis arrangement of
the η2-bound olefin and the alkyl group, an isomerization of
complex 16′ from a trans geometry to a cis geometry would be
necessary prior to alkyl migration and production of trans-
dimethyl 2-hexenedioate.53 This is illustrated in Scheme 8.
The palladium-acrylate chelate complex 18 (L ) H2O, Et2O)
shows identical coupling patterns but slightly different chemical
shifts for the two methylene signals of the chelate (HA: δ 2.75
for 18, 2.72 for 16; HB: δ 1.96 for 18, 1.77 for 16). Treatment
of 18 at 25 °C in CH2Cl2 with excess methyl acrylate
C. Isolation and Crystallographic Characterization of a
Palladium Chelate Complex from the Catalytic Reaction.
To further verify the catalyst resting state as the chelate 16, a
derivative of this chelate has been isolated directly from the
catalyst solution and structurally characterized by X-ray crystal-
lography. Six equivalents of methyl acrylate were added to a
dichloromethane solution of complex 7a at -80 °C. The
reaction was warmed to 0 °C and stirred for 3 h. The solvent
was removed in vacuo, leaving a yellow oil. Single crystals
were grown from a concentrated solution of Et2O and hexane
at -30 °C. 1H NMR spectroscopy of this material reveals it to
be the chelate complex 18 isolated as a mixture of the diethyl
ether and aquo adducts. The crystal selected for X-ray analysis
turned out to be the aquo complex. The ORTEP diagram of
the aquo complex is shown in Figure 4 and selected bond lengths
(53) We cannot rule out the presence of both 16 and 16′ in rapid
equilibrium at 25 °C. Associative exchange of bound and free acrylate is
rapid on the NMR time scale at 25 °C (see above); the five coordinate
intermediate responsible for associative exchange thus could provide a
mechanism for very rapid interconversion of 16 and 16′.
(52) Brookhart, M.; Rix, F. C.; DeSimone, J. M. J. Am. Chem. Soc. 1992,
114, 5894.

Dimerization of Olefins
J. Am. Chem. Soc., Vol. 118, No. 26, 1996 6231
results in displacement of H2O/Et2O and formation of 16 (or
16′) which at 25 °C catalytically produces trans-dimethyl
Scheme 8
2-hexenedioate with a turnover frequency of 2.2 day-1
.
4. Reactivity of the η3-Allyl-palladium Complex
[(C3H5)Pd(OEt2)(PnBu3)]+[BAr′4]- (7b) with Methyl Acry-
late. Spectroscopic Detection and Study of Reaction Inter-
mediates. Low -Temperature NMR Spectroscopic Studies.
Treatment of [(C3H5)Pd(OEt2)(PnBu3)]+[BAr′4]- (7b) with 1
equiv of methyl acrylate in CD2Cl2 results in the formation of
η2-acrylate complex 19 which exhibits two 31P signals at -80
°C (13.5 and 13.0 ppm) in approximately a 2:1 ratio. We
assume the 31P spectrum of 19 at -80 °C is analogous to the
spectrum of 13 at -60 °C and corresponds to rapid equilibration
among isomers of set 1 and isomers of set 2 (see above) and
the observation of only two 31P signals. Warming this solution
in the NMR probe results in broadening these signals, coales-
cence at -30 °C, and sharpening to a singlet at 11.4 ppm at
-10 °C. Line shape analysis using the slow exchange ap-
16.54 [(C3H5)Pd(CH2CH2C(O)OCH3)(PnBu3)]+[BAr′4]]- (21)
dimerizes methyl acrylate at 60 °C to predominantly trans-
dimethyl 2-hexenedioate (200 turnovers, 24 h).
proximation (k ) π∆ω) yields k1 ) 30 s-1 with ∆Gq ) 12.5
1
kcal mol-1 and k-1 ) 14 s-1 with ∆Gq-1 ) 12.9 kcal mol-1 at
-40 °C. Consistent with the 31P results, the 1H NMR spectrum
at -80 °C indicates the formation of two sets of bound acrylate
resonances. Three of the six resonances are clearly visible at
5.12, 4.93, and 4.80 ppm. The three remaining resonances are
observed at approximately 5.6, 5.4, and 4.6 ppm and are
particularly obscured by the allyl resonances.
Conclusions
The observations described here clearly show that a Cossee-
Arlman type migratory insertion mechanism operates in the
methyl acrylate dimerization cycle for the Pd(II) complexes
studied here. It seems quite likely that a similar mechanism
applies to all previously identified Pd(II)-methyl acrylate
dimerization catalysts. Furthermore, the catalyst resting state
in these Pd(II) systems, a chelate, η2-acrylate complex, is quite
similar to the catalyst resting state, 3, previously established
for the cationic Cp*RhIII-based system.
These observations, coupled with previous studies, provide
a rationale for the highly regioselective tail-to-tail coupling
normally observed for metal-catalyzed acrylate dimerization.
It is well-established that migratory insertion of acrylate
complexes of the general structure 22 (R ) alkyl) occur in a
highly regioselective 2,1 fashion to yield a species with the ester
group R to the metal center. A chelate structure is observed in
the cases where the first insertion product can be detected.55
The allyl acrylate complex [(C3H5)Pd(CH2dCHC(O)-
OCH3)(PnBu3)]+[BAr′4]- (19) is stable up to -20 °C in the
presence of excess methyl acrylate. Warming the solution in
the NMR probe results in the formation of a new species with
31P resonance at 18.6 ppm at -10 °C. On the basis of
comparison with the 1H NMR spectra of complex 14, this new
species, 20, is assigned as the PnBu3 analog of the four-
membered-chelate intermediate 14, which results from allyl
coupling to the â-carbon of the bound methyl acrylate ligand.
Complex 20 shows similar coupling patterns to complex 14 but
slightly different chemical shifts are observed. 1H NMR (300
MHz, -10 °C, CD2Cl2) δ 7.72 (s, 8H, Ar′); 7.56 (s, 4H, Ar′);
6.78 (m, 1H, H1); 5.62 (d, 1H, H2); 3.64 (s, 3H, OCH3); 4.80
(d, 1H, H3); 2.90 (m, 1H, H5).
Several factors may dictate such regioselectivity including
the polarity of the double bond induced by the ester group, steric
effects, and participation of the carbonyl oxygen in stabilizing
the vacant coordination site which develops during migratory
insertion.56 In the case of hydride migration (R ) H) the
insertion reaction is reVersible and, while kinetic 2,1 insertion
is likely preferred,57 reversibility allows for rapid formation of
Warming the solution to 0 °C results in the elimination of
CH2dCHCH2CHdCHCO2CH3 and the formation of a new
species with 1H resonances at 2.75 ppm (triplet-of-doublets) and
1.85 (triplet, partially obscured by the tricyclohexyl region). The
(54) The NMR analysis of the sequence 19 f 20 f 21 is more
complicated than the analogous transformations for the tricyclohexyl
phosphine complexes 13 f 14 f 16. The rate of 19 f 20 is only slightly
faster than the 20 f 21 conversion and thus 20 never appears without
substantial amounts of either 19 and/or 21 present.
(55) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996,
118, 267.
(56) Hauptman, E.; Brookhart, M.; Calabrese, J. C.; Fagan, P. J.
Organometallics 1994, 13, 774.
(57) Grant, B.; Brookhart, M. Unpublished results.
n
new species is assigned as 21, the Bu3 analogue of the five-
1
membered acrylate chelate complex 16, based on H NMR
comparisons with complex 16 and broad band phosphorous
decoupling, which reduces the triplet-of-doublet pattern at 2.75
ppm to a triplet. After 2 h approximately a 1:1 ratio of 20:21
is observed which indicates that the elimination of methyl 2,5-
hexadieneoate from 20 occurs at a much slower rate than from

6232 J. Am. Chem. Soc., Vol. 118, No. 26, 1996
DiRenzo et al.
Scheme 9. A General Scheme for Selective Tail-to-Tail
the thermodynamically more stable five-membered chelate. In
support of this idea we have observed rapid isomerization of a
four-membered chelate to a five-membered chelate at -80 °C,
in related Pd(II) R-diimine systems.55
Acrylate Dimerization
Based on these observations, the tail-to-tail selectivity can
be explained using the following general scheme (Scheme 9).
The metal hydride, [M]-H, generated in the catalytic cycle
reacts with methyl acrylate to yield the more stable five-
membered chelate 25 (the four-membered chelate, 24, is likely
the kinetically favored insertion product). Binding of methyl
acrylate to 25 leads to the η2-acrylate complex 26. Species 26
then undergoes irreversible 2,1 insertion via migration of C2 to
Câ which results in tail-to-tail coupling. Small amounts of head-
to-tail dimer are sometimes observed;28 these isomers most likely
result from trapping and insertion of a four-membered chelate
of type 24.
A second general feature of acrylate dimerization catalysts
which operate by this mechanism is also apparent. The chelate
occupies two coordination sites and the carbonyl oxygen is
unable to be displaced by the weakly donating η2-acrylate ligand.
Thus, an additional coordination site cis to the [M]-C2 bond
must be available for coordination of acrylate to allow migratory
insertion from the chelate. Consistent with this hypothesis,
Pd(II) complexes possessing bidentate ligands such as phenan-
throline are ineffective acrylate dimerization catalysts due to
the unavailability of the third coordination site:
analyses were performed by Oneida Research Services, Inc. In
reporting chemical shifts, allyl carbons and hydrogens are labeled as
follows:
[(C3H5)PdCl]2 (4). This compound was prepared as previously
reported.32 1H NMR (300 MHz, 20 °C, CD2Cl2) δ 5.44 (m, 1H, H5);
4.05 (d, 2H, H1 and H4); 2.97 (d, 2H, H2 and H3). 13C NMR (75.4
MHz, -80 °C, CD2Cl2) δ 111.5 (C2); 63.1 (C1 and C3).
[(C3H5)Pd(Cl)(PCy3)] (5a). The following procedure is a modifica-
tion of that reported by Shaw.33 A Schlenk flask was charged with
0.50 g (1.37 mmol) of [Pd(C3H5)Cl]2 and 0.766 g (2.73 mmol) of PCy3.
The reaction flask was cooled to -78 °C. Ether (50 mL) was added
and the suspension was warmed slightly to give a yellow solution, which
was cooled to -78 °C and stirred for 1 h, at which time a beige powder
precipitated. The solvent was reduced in vacuo. The reaction mixture
was filtered and the product was washed with Et2O. An off-white solid
was recovered (1.08 g, 85%). 1H NMR (300 MHz, -20 °C, CD2Cl2)
δ 5.50 (m, 1H, H5); 4.55 (q, 1H, H1); 3.58 (dd, 1H, H2); 3.50 (t, 1H,
H4); 2.78 (d, 1H, H3). 13C NMR (75.4 MHz, 20 °C, CD2Cl2) δ (allylic)
116.5 (d, JCP ) 4.7 Hz, C2); 79.5 (d, JCP ) 28.8 Hz, C3); 51.4 (d, JCP
) 1.8, C1); (PCy3) 34.5 (d, JCP ) 18.9 Hz, C1); 27.9 (d, JCP ) 11.3 Hz,
C2); 30.4 (s, C3); 26.7 (s, C4). 31P NMR (121.5 MHz, 20 °C, CD2Cl2)
δ 41.2.
Similarly 27 is an extremely poor dimerization catalyst9,11,12 and
28 exhibits no dimerization activity:56
[(C3H5)Pd(Cl)(PnBu3)] (5b). The following procedure is a modi-
fication of that reported by Shaw.33 A Schlenk flask was charged with
0.50 g (1.37 mmol) of [Pd(C3H5)Cl]2 and cooled to -78 °C. Diethyl
ether (50 mL) was added and the suspension was stirred. PnBu3 (2.73
mmol) was added dropwise and the suspension was warmed slightly
to give a yellow solution, which was cooled to -78 °C and stirred for
2 h, at which time a white solid precipitated. The reaction mixture
was filtered and the product was washed with hexane. A white solid
was recovered (1.05 g, 82%). 1H NMR (250 MHz, 20 °C, C6D6) δ
4.8 (m, 1H, H5); 4.45 (t, 1H, H1); 3.3 (dd, 1H, H2); 2.9 (bs, 1H, H4);
2.05 (bd, 1H, H3). 13C NMR (75.4 MHz, 20 °C, CD2Cl2) δ (allylic)
117.1 (d, JCP ) 5.3 Hz, C2); 78.7 (d, JCP ) 31.1 Hz, C3); 51.1 (bd, JCP
) 1.8 Hz, C1); (PnBu3) 26.7 (s, Cγ); 24.8 (d, JCP ) 1.2 Hz, Câ); 24.5
(d, JCP ) 8.8 Hz, CR); 13.8 (s, CH3). 31P NMR (121.5 MHz, 20 °C,
CD2Cl2) δ 15.1. Anal. Found: C, 47.28; H, 8.59. Calcd: C, 46.76;
H, 8.37.
These general observations provide guidelines for design of
other late-metal catalysts for dimerization of acrylates.
Experimental Section
General. All reactions were conducted under an atmosphere of dry,
oxygen-free nitrogen or argon using standard drybox or Schlenk
techniques. Air- and moisture-sensitive reagents were handled using
standard syringe techniques. Diethyl ether and n-hexane were freshly
distilled under a nitrogen atmosphere from sodium and benzophenone.
Dichloromethane-d2 was vacuum transferred from CaH2 and degassed
via several freeze-pump-thaw cycles prior to use. Methyl acrylate
was purchased from Aldrich and stored over 4 Å molecular sieves.
The following preparations were based on standard literature proce-
dures: [H+(Et2O)2(BAr4′)-],35 [(C3H5)PdCl]2,32 [(C3H5)Pd(Cl)(PCy3)],33
and [(C3H5)Pd(CH3)(PCy3)].34 1H, 13C, and 31P chemical shifts were
referenced to protonated residues of solvents or 85% H3PO4. Elemental
[(C3H5)Pd(CH3)(PCy3)] (6a). The following procedure is a modi-
fication of that reported by Nakamura.34 A Schlenk flask was charged
with 0.400 g (0.86 mmol) of [(C3H5)Pd(Cl)(PCy3)] (5a) and Et2O (15
mL) was added to give a suspension. The reaction mixture was cooled
to -78 °C and 288 µL of a 3 M solution of MeMgBr in Et2O was

Dimerization of Olefins
J. Am. Chem. Soc., Vol. 118, No. 26, 1996 6233
added dropwise. The solution was stirred at -78 °C for 1 h and the
solvent was removed in vacuo. The white solid was dissolved in hexane
and filtered. The solution was reduced in vacuo and the flask was
cooled to -30 °C. White crystals were isolated (0.28 g, 73%). 1H
NMR (300 MHz, CD2Cl2) δ 4.99 (m, 1H, H5); 3.47 (dd, 1H, JHH ) 7.3
Hz, H4); 3.30 (t, 1H, H1); 2.38 (dd, 1H, JHH ) 12.9 Hz, H2); 2.33 (d,
1H, JHH ) 13.2 Hz, H3); 0.03 (d, 3H, JHP ) 4.7 Hz, CH3). 13C NMR
(75.4 MHz, CD2Cl2) δ (allylic) 117.2 (d, JCP ) 4.2 Hz, C2); 60.7 (s,
C1); 56.0 (d, JCP ) 36.8 Hz, C3); (PCy3) 35.3 (d, JCP ) 17.4 Hz, C1);
30.6 (d, JCP ) 13.4 Hz, C2) 28.1 (d, JCP ) 10.6 Hz, C3); 27.0 (s, C4);
-17.9 (d, JCP ) 13.2 Hz, CH3). 31P NMR (121.5 MHz, 20 °C, CD2Cl2)
δ 42.2.
[(C3H5)Pd(CH3)(PnBu3)] (6b). The following procedure is a
modification of that reported by Nakamura.34 A Schlenk flask was
charged with 0.866 g (2.25 mmol) of [(C3H5)Pd(Cl)(nBu3)] (5b) and
Et2O (40 mL) was added to give a suspension. The reaction mixture
was cooled to -78 °C and 750 µL of a 3 M solution of MeMgBr in
Et2O was added dropwise. The solution was stirred at -78 °C for 3
h. The resulting pale green solution was filtered, leaving a white solid
(MgBrCl). The solution was reduced in vacuo and filtered again to
remove additional MgBrCl. The solvent was then removed in vacuo
and a yellow oil was isolated (1.430 g, 52%). 1H NMR (250 MHz,
CDCl3) δ 5.0 (m, 1H, H5); 3.45 (m, 2H); 2.45 (dd, 1H); 2.3 (d, 1H);
0.05 (d, 3H, CH3).
[(C3H5)Pd(OEt2)(PCy3)]+[BAr′4]- (7a: Ar′ ) [3,5-C6H3(CF3)2]).
A Schlenk flask was charged with 0.26 g (0.58 mmol) of [(C3H5)Pd-
(CH3)(PCy3)] (6a) and 0.59 g (0.58 mmol) of [H+(OEt2)2(BAr′4)-]. The
reaction flask was cooled to -78 °C and Et2O (10 mL) was added.
The reaction was warmed slightly to form a clear solution, which was
cooled to -78 °C and stirred for 1 h. The solvent was reduced in
vacuo to precipitate a beige powder, which was washed with hexane
and isolated (0.58 g, 73%). In solution, an ether complex and an aquo
complex of 7a are observed in approximately a 7:3 ratio, respectively.
1H NMR (300 MHz, -20 °C, CD2Cl2) δ 7.72 (s, 8H, Ar′); 7.56 (s, 4H,
Ar′), 5.79 (m, 1H, H5); 5.08 (bt, 1H, H1); 4.01 (dd, 1H, H2); 3.80 (b,
1H, H4); 3.10 (bd, 1H, H3). 13C NMR (75.4 MHz, -20 °C, CD2Cl2)
[(H3CO(O)CCH2CH2)Pd(PCy3)(L)]+[BAr′4]- (18: Ar′ ) 3,5-
C6H3(CF3)2; L ) OEt2, OH2). A Schlenk flask was charged with
0.290 g (0.21 mmol) of [(C3H5)Pd(OEt2)(PCy3)]+[BAr′4]- (7a). The
reaction flask was cooled to -78 °C and 10 mL of CH2Cl2 was added.
Methyl acrylate (114 µL, 1.26 mmol) was added. The solution was
warmed to 0 °C and stirred for 3 h. The solvent was removed in vacuo,
leaving a yellow oil. The oil was dissolved in a minimum amount of
Et2O/hexane and cooled to -30 °C. Pale yellow crystals (0.90 g, 30%)
were isolated. 1H NMR (300 MHz, 20 °C, CD2Cl2) δ 7.72 (s, 8H,
Ar′); 7.56 (s, 4H, Ar′); 3.87 (s, 3H, OCH3); 2.75 (td, JHP ) 2.1 Hz,
2H); 1.96 (td, JHP ) 2.4 Hz, 2H); 1.0-2.0 (m, cyclohexyl). 13C NMR
(75.4 MHz, 20 °C, CD2Cl2) δ 240.6 (CdO), 56.6 (OCH3), 40.4 (Câ),
15.2 (CR); (BAr4′) 163.1 (q, C1), 136.1 (C2), 130.0 (m, C3), 126.0 (q,
CF3), 118.8 (C4); (PCy3) 35.3 (d, C1), 31.4 (C2), 28.7 (d, C3), 27.4
(C4). 31P NMR (121.5 MHz, 20 °C, CD2Cl2) δ 49.54, 46.07. Anal.
Found: C, 49.32; H, 4.43. Calcd: C, 49.36; H, 4.43.
X-ray Structural Analysis of [(H2O)(H3CO(O)CCH2CH2)-
Pd(PCy3)]+[BAr′4]- (18). A single crystal of 18 (0.20 × 0.20 × 0.40
mm) was grown from a concentrated solution of Et2O and hexane at
-30 °C. The crystal was triclinic (P1, No. 2) with the following cell
dimensions determined from 50 reflections (λ(Mo) ) 0.71073 Å): a
) 12.816(7) Å, b ) 14.879(15) Å, c ) 18.537(9) Å; R ) 79.10(6)°,
â ) 89.28(4)°, γ ) 85.72(6)°; z ) 2; V ) 3461(4) Å3; Fw ) 1503.40
(PdC62H74BF24O5P); Dc ) 1.442 g m-3
.
Data were collected at -120 °C on a Rigaku diffractometer with a
graphite monochromator using Mo KR radiation. A total of 9559 data
were collected of which 6187 unique reflections with I g 2.5σ(I) were
observed (5° e 2θ e 45°; maximum h, k, l ) 13, 16, 19; data octants
) +/-h, +k, +/-l; ω scan method; scan speed ) 2 deg min-1). Three
standards were collected every 100 reflections with no significant
change in intensity. An absorption correction was applied using the
ψ scan method58 with a range of transmission factors of 0.85 to 0.94.
The structure was solved using direct methods. The asymmetric
unit consists of one ion pair in a general position. Hydrogen atoms
were idealized with C-H ) 0.96 Å. The structure was refined by
full-matrix least squares and corrected for anamolous dispersion. There
are 847 parameters (data to parameter ratio of 7.30); final R ) 0.055
(Rw ) 0.063). The error of fit was 1.78 with a maximum ∆σ-1 of
0.021. The deepest hole was -0.93 e Å-3 and the highest peak was
1.52 e Å-3. Selected interatomic distances and angles are summarized
in Tables 1 and 2, respectively.
δ (BAr′4) 161.7 (q, JCB ) 49.2, C1); 134.7 (s, C2); 128.7 (qq, JCF
)
32.2, C3); 124.5 (q, 272.1, CF3); 117.5 (bt, C4); (PCy3) 34.3 (d, 21.2,
C1); 30.1 (d, 16.1, C2); 27.4 (d, 1.7, C3); 26.0 (s, C4); (allylic) 120.3
(b, C2); 86.8 (d, JCP ) 22.9 Hz, C3); (Et2O) 66.9 (b, CH2CH3); 15.2 (s,
CH2CH3). 31P NMR (121.5 MHz, 20 °C, CD2Cl2) δ 39.7 (Et2O
complex); δ 41.3 (H2O complex). Anal. Found: C, 50.30; H, 4.35.
Calcd: C, 50.15; H, 4.43.
[(C3H5)Pd(OEt2)(PnBu3)]+[BAr′4]- (7b: Ar′ ) [3,5-C6H3(CF3)]).
A Schlenk flask was charged with 0.616 g (1.69 mmol) of
[(C3H5)Pd(CH3)(PnBu3)] (6b). The reaction flask was cooled to -78
°C and Et2O (10 mL) was added. To the yellow palladium solution
was added 1.71 g (1.69 mmol) of [H+(OEt2)2(BAr′4)-]. The resulting
yellow solution was stirred at -78 °C for 1 h. The reaction was filtered,
leaving a small amount of insoluble material. Hexane was added to
the filtrate to facilitate precipitation. The solvent was then reduced in
Measurement of the Rate of Interconversion between [(C3H5)-
(PCy3)Pd(C2H4)]+[BAr′4]- 8r and 8â. The sample was prepared in
a drybox by charging the palladium complex [(C3H5)Pd(OEt2)-
(PCy3)]+[BAr′4]- (7a) (12 mg, 0.009 mmol) and CD2Cl2 into an NMR
tube at ambient temperature. The NMR tube was cooled to -78 °C
under an argon atmosphere and ethylene was introduced via syringe
(excess ethylene was removed by purging the solution with argon for
approximately 30 min). The NMR tube was introduced into a precooled
(-90 °C) NMR probe and gradually warmed. 1H and 31P NMR spectra
were recorded. The rate of interconversion of 8r and 8â at the
coalescent temperature (-74 °C, Bruker AMX-300 spectrometer) is
calculated from the frequency seperation of the two PCy3 resonances
in the slow-exchange limit, k ) π(VΑ - VΒ)/x2 (k ) 300 s-1 at -74
°C). The free energy of activation obtained from the Eyring equation
vacuo and the resulting yellow solution was cooled to -30 °C.
A
yellow powder was isolated (0.320 g, 15%). 1H NMR resonances (300
MHz, -80 °C, CD2Cl2) δ 7.72 (s, 8H, Ar′); 7.56 (s, 4H, Ar′); 5.65 (m,
1H, H5); 4.75 (bt, 1H, H1); 3.75 (dd, 1H, H2); 2.6 (bd, 1H). 31P NMR
(121.5 MHz, -80 °C, CD2Cl2) δ 16 (Et2O complex); 11 (H2O complex).
is 9.3 kcal mol-1
37.04.
.
31P NMR (121.5 MHz, -90 °C, CD2Cl2) δ 35.93,
Measurement of the Rate of Interconversion (Allyl Rocking)
[(C3H5)Pd(PCy3)2]+[BAr′4]- (9: Ar′ ) 3,5-C6H3(CF3)2). A Schlenk
flask was charged with 100 mg (0.073 mmol) of
[(C3H5)Pd(OEt2)(PCy3)]+[BAr′4]- (7a) and 21 mg (0.073 mmol) of
PCy3. The reaction flask was cooled to -78 °C and 5 mL of CH2Cl2
was added. The solution was warmed to -40 °C and stirred for 3 h.
The solvent was removed in vacuo, leaving a yellow oil, which was
dissolved in a minimum amount of Et2O/hexane and cooled to -78
°C. A white solid (0.06 g, 52%) was isolated. 1H NMR (300 MHz,
-20 °C, CD2Cl2) δ 7.72 (s, 8H, Ar′); 7.56 (s, 4H, Ar′); 5.36 (m, 1H,
H5); 4.47 (bs, 2H, H1 and H4); 2.91 (bs, 2H, H2 and H3). 13C NMR
(75.4 MHz, -20 °C, CD2Cl2) δ 119.5 (t, C2); 70.6 (t, C1 and C3); (BAr4′)
163.1 (q, C1), 136.1 (C2), 130.0 (m, C3), 126.0 (q, CF3), 118.8 (C4);
(PCy3) 36.0 (b, C1), 30.5 (C2), 27.6 (d, C3), 26.2 (C4). 31P NMR (121.5
MHz, -90 °C, CD2Cl2) δ 36.5 (dd).
between [(C3H5)(PCy3)Pd(C2H4)]+[BAr′4]- 9 and 9′. The sample was
prepared in
a drybox by charging the palladium complex
[(C3H5)Pd(PCy3)2]+[BAr′4]- (9) (10 mg, 0.006 mmol) and CD2Cl2 into
an NMR tube at ambient temperature. The NMR tube was introduced
into a precooled (-90 °C) NMR probe and gradually warmed. 31P
NMR spectra were recorded. Line broadening experiments were carried
out for 9 and 9′ beginning at a temperature where the complexes were
static (the phosphorus signals appeared as an AB quartet) to a
temperature where the complexes were dynamic (the phosphorus signals
appeared as a broad singlet). Experimental line shapes were compared
to line shapes calculated using the DNMR3 program43-46 (for an
(58) Gabe, E. J.; Le Page, Y. L.; Charland, J. P.; Lee, F. L.; White, P.
S. J. Appl. Crystallogr. 1989, 22, 384.

6234 J. Am. Chem. Soc., Vol. 118, No. 26, 1996
DiRenzo et al.
chelate complex 16 were observed. For best signal resolution the NMR
probe was warmed to 25 °C. 1H NMR (300 MHz, 25 °C, CD2Cl2) δ
7.72 (s, 8H, Ar′); 7.56 (s, 4H, Ar′); 3.78 (s, 3H, OCH3); 2.72 (td, JHP
) 2.1 Hz, 2H); 1.77 (td, JHP ) 2.4 Hz, 2H); 1.0-2.0 (m, cyclohexyl).
31P NMR (121.5 MHz, 20 °C, CD2Cl2) δ 49.5.
Spectroscopic Characterization of Catalytic Intermediates during
Dimerization of Methyl Acrylate with [(C3H5)Pd(OEt2)-
(PnBu3)]+[BAr′4]- (7b). In a typical experiment, the sample was
prepared in
a drybox by charging the palladium complex
[(C3H5)Pd(OEt2)(PnBu3)]+[BAr′4]- (7b) (10 mg, 0.008 mmol) and
CD2Cl2 into an NMR tube at ambient temperature. The NMR tube
was cooled to -78 °C under an argon atmosphere and methyl acrylate
was introduced via syringe. The NMR tube was introduced into a
precooled (-80 °C) NMR probe. Two isomers of complex 19 were
immediately generated (31P NMR (121.5 MHz, -100 °C, CD2Cl2) δ
13.5 and 13.0). The rate of interconversion of these isomers at the
coalescent temperature (-40 °C, Bruker AMX-300 spectrometer) is
calculated from line shape analysis of the two PnBu3 resonances using
Figure 5. The -ln[(A0 - B)]/A0 versus time data for allyl acrylate
coupling of complex 13.
illustration see Figure 4 in the Results and Discussion section). This
dynamic NMR fitting program gave values of rate constants (k) which
were used to calculate free energies of activation using the Eyring
equation: 5 s-1 at -82 °C, ∆Gq ) 10.4; 8 s-1 at -77 °C, ∆Gq ) 10.5;
16 s-1 at -72 °C, ∆Gq ) 10.5; 29 s-1 at -67 °C, ∆Gq ) 10.5; 53 s-1
at -61 °C, ∆Gq ) 10.6; 81 s-1 at -56 °C, ∆Gq ) 10.7; 222 s-1 at
-45 °C, ∆Gq ) 10.8; 405 s-1 at -39 °C, ∆Gq ) 10.8.
the slow-exchange approximation, k ) π∆ω (k1 ) 30 s-1 and k-1
)
14 s-1). The free energies of activation obtained from the Eyring
equation are ∆Gq1 ) 12.5 kcal mol-1 and ∆Gq-1 ) 12.9 kcal mol-1 at
-40 °C. The NMR probe was gradually warmed to -10 °C, at which
temperature the four-membered chelate complex 20 was observed. 1H
NMR (300 MHz, -10 °C, CD2Cl2) δ 7.72 (s, 8H, Ar′); 7.56 (s, 4H,
Ar′); 6.78 (m, 1H, H1); 5.62 (d, 1H, H2); 3.64 (s, 3H, OCH3); 4.80 (d,
1H, H3); 2.90 (m, 1H, H5). 31P NMR (121.5 MHz, -10 °C, CD2Cl2)
δ 18.6. The NMR probe was further warmed to 0 °C, at which
temperature methyl 2,5-hexadieneoate (see above) and the five-
membered acrylate chelate complex 21 were observed. 1H NMR (300
MHz, 0 °C, CD2Cl2) δ 7.72 (s, 8H, Ar′); 7.56 (s, 4H, Ar′); 2.75 (triplet-
of-doublets) (1.85, triplet, partially obscured by tricyclohexyl reso-
nances).
Spectroscopic Characterization of Catalytic Intermediates during
Dimerization of Ethylene with [(C3H5)Pd(OEt2)(PCy3)]+[BAr′4]-
(7a). In a typical experiment, the sample was prepared in a drybox by
charging the palladium complex [(C3H5)Pd(OEt2)(PCy3)]+[BAr′4]- (7a)
(10 mg, 0.007 mmol) and CD2Cl2 into an NMR tube at ambient
temperature. The NMR tube was cooled to -78 °C under an argon
atmosphere and ethylene was bubbled through the solution for 5 min.
The NMR tube was then purged with argon for 30 min to remove excess
ethylene and introduced into a precooled (-90 °C) NMR probe. Two
isomers of complex 13 were immediately generated. 31P NMR (121.5
MHz, -90 °C, CD2Cl2) δ 35.93, 37.04. Excess ethylene was added
via syringe. The NMR tube was reintroduced into the NMR probe
and gradually warmed to -20 °C, at which temperature a new species
is observed which we propose to be the ethyl-ethylene complex 11.
31P NMR (121.5 MHz, -20 °C, CD2Cl2) δ 38.5. 1H NMR (300 MHz,
-20 °C, CD2Cl2) δ 3.58 (bs). The NMR probe was further warmed to
-15 °C, at which temperature dimerization of ethylene to butenes
(primarily trans-2-butene cis:trans, ca. 1:6) was observed.
Spectroscopic Characterization of Catalytic Intermediates during
Dimerization of Methyl Acrylate with [(C3H5)Pd(OEt2)-
(PCy3)]+[BAr′4]- (7a). In a typical experiment, the sample was
prepared in a drybox by charging the palladium complex [(C3H5)Pd-
(OEt2)(PCy3)]+[BAr′4]- (7a) (10 mg, 0.007 mmol) and CD2Cl2 into an
NMR tube at ambient temperature. The NMR tube was cooled to -78
°C under an argon atmosphere and methyl acrylate was introduced via
syringe. The NMR tube was introduced into a precooled (-100 °C)
NMR probe. Four isomers of complex 13 were immediately generated
(31P NMR (121.5 MHz, -100 °C, CD2Cl2) δ 38.5, 37.5, 37.3, and 37.2).
The NMR probe was gradually warmed to -25 °C, at which
temperature the four-membered chelate complex 14 was observed. 1H
NMR (300 MHz, -25 °C, CD2Cl2) δ 7.72 (s, 8H, Ar′); 7.56 (s, 4H,
Ar′); 6.78 (m, 1H, H1); 5.74 (d, 1H, J12 ) 16.7 Hz); 4.85 (d, 1H, J13
) 9.4 Hz); 2.98 (m, 1H, H5); 2.45 (m, 2H, H6,6′); 2.07 (m, partially
obscured by cyclohexyl resonances). The rate of allyl acrylate coupling
Dimerization of Methyl Acrylate with [(C3H5)Pd(OEt2)-
(PnBu3)]+[BAr′4]- (7b). In a typical experiment, the sample was
prepared in a drybox by charging [(C3H5)Pd(OEt2)(PnBu3)]+[BAr′4]-
(7b) (50 mg, 0.04 mmol) to a medium-walled glass Schlenk flask fitted
with a Kontes high vacuum Teflon plug. The Schlenk flask was placed
under an argon atmosphere, methyl acrylate (stabilized with 1 wt %
3,5-di-tert-butyl-4-hydroxyanisole) was introduced via syringe (0.96
g, 11 mmol, 284 equiv), and the reaction mixture was heated (60 °C,
24 h). Quantitative organic analyses were obtained with an Hewlett
Packard 5890A gas chromatograph using a J & W Scientific DB-5
capillary column (30 M, 0.25-mm i.d., 0.25-mm film thickness), the
following temperature program (50 °C for 4 min; 50-250 °C at 5 deg
min-1), and flame ionization detection. The assignment of products is
based on comparison with an authentic sample of mixed diester products
which was obtained from a reaction solution via distillation.
Acknowledgment. This work was supported by the National
Science Foundation (CHE-8705534) and by E. I. du Pont de
Nemours & Co., Inc. We would like to especially thank
Elisabeth Hauptman and Chi-Deun Poon for helpful discussions.
Supporting Information Available: X-ray data for 18
including tables of fractional coordinates and isotropic thermal
parameters, interatomic distances, intramolecular angles, in-
tramolecular nonbonding distances, and intermolecular distances
(12 pages). Ordering information is given on any current
masthead page.
was measured at -25 °C (k ) 1.1 × 10-3 s-1, ∆Gq ) 17.8 kcal mol-1
)
by monitoring the appearance of H5 (2.98 ppm) of the four-membered
chelate intermediate 14 (a typical first-order plot is shown in Figure
5). The NMR probe was further warmed to -15 °C, at which
temperature methyl 2,5-hexadieneoate and the five-membered acrylate
JA9602354