Mechanism of the palladium-catalyzed metal-carbon bond formation. A dual pathway for the transmetalation step

Article abstract of DOI:10.1021/ja011644p

The mechanism of the transmetalation step in the metal-carbon bond-formation process catalyzed by palladium complexes has been studied by spectroscopic and kinetic methods. The reaction of properly designed model complexes [η⁵-(1-Ph₂

Full text of DOI:10.1021/ja011644p

Published on Web 01/19/2002
Mechanism of the Palladium-Catalyzed Metal-Carbon Bond
Formation. A Dual Pathway for the Transmetalation Step
Antonella Ricci, Francesco Angelucci, Mauro Bassetti,* and Claudio Lo Sterzo*,†
Contribution from the Centro C.N.R. di Studio sui Meccanismi di Reazione,
Dipartimento di Chimica, UniVersita` “La Sapienza”, Box 34 - Roma 62, Piazzale Aldo Moro, 5
00185 Roma, Italy
Received July 6, 2001
Abstract: The mechanism of the transmetalation step in the metal-carbon bond-formation process catalyzed
by palladium complexes has been studied by spectroscopic and kinetic methods. The reaction of properly
designed model complexes [η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoPd(PR3)I (3, R ) Ph; 15, R ) Bu; 16, R )
Me), resulting from oxidative addition of a Mo-I moiety to a palladium center, with aryltributyltinacetylides
Bu3Sn-CtC-(p-XC6H4) (11a, X ) H; 11b, X ) Cl) yields the products of transmetalation [η5-(1-Ph2-
P-2,4-Ph2)C5H2](CO)3MoPd(PR3)-CtC-(p-XC6H4) (5a,b). The reaction, which shows a strong dependence
on the nature of the phosphine ligand PR3 (Ph > Bu > Me) and less so on the nature of the p-substituent
X group, proceeds through two competing pathways, depending on the initial concentration of substrate.
At high [3] (=10-2 M), the transmetalation proceeds through an intermediate species (12) formed by the
interaction of complex 3 with 11a. This associative complex accumulates in the presence of added PPh3
and has been characterized spectroscopically. At low [3] (=10-4 M), the reaction rate shows an inverse
dependence on the concentration of the complex. This is due to the formation of a solvent-coordinate
species (13), in which PPh3 has been substituted by a dimethylformamide (DMF) molecule, as shown by
UV-vis and 31P NMR spectroscopy. Values of kobs depend on the concentration and nature of the
aryltributyltinacetylides, in agreement with the existence of a kinetically detectable intermediate. A dimeric
iodide bridged complex [{η5-(1-Ph2P-2,4-Ph2)C5H2}(CO)3MoPdI]2 (14) has been obtained during attempts
at isolating 13, which changes quantitatively into 13 upon dissolution in DMF and reacts with 11a to give
the transmetalation product.
work,4 other authors have used this procedure to obtain simple
metal acetylide complexes,5 metallaacetylide dimers,6 oligomers,
Introduction
In the past few years most of our activity has been devoted
to the study of the palladium-catalyzed metal-carbon (acetylide)
bond formation.1 This is an unprecedented feature of palladium
that we discovered serendipitously while using the Stille
reaction2 to assemble bis(cyclopentadienyl)ethynyl-framed bi-
metallic complexes.3 This new transformation consists of the
zerovalent palladium-promoted coupling of metal halide (M-
I) complexes (M ) Fe, Ru, W, Mo) with trialkyltinacetylide
compounds (R3Sn-CtC-R′) (R ) Me, Bu; R′ ) H, alkyl,
aryl) to form metal acetylide derivatives (M-CtC-R′).4 The
mild reaction conditions, the effectiveness of the transformation,
and the considerable importance of the products have made this
new procedure a useful synthetic tool. Following our seminal
and polymers4,7 (Scheme 1).
With regard to the mechanistic features of the overall process,
we have looked for analogies with the palladium-catalyzed
carbon-carbon (C-C) coupling reaction between organic
electrophiles and organostannanes. This is known as the Stille
reaction and is an extremely valuable tool in organic chemistry.2
It is characterized by a sequence of oxidatiVe addition/
transmetalation/trans-to-cis isomerization/reductiVe elimination
steps which are represented in Figure 1.8 The detailed mecha-
nism of this process has been studied for long time and is still
a matter of intense debate,2b due to its intrinsic complexity.
(4) (a) Crescenzi, R.; Lo Sterzo, C. Organometallics 1992, 11, 4301. (b) Viola,
E.; Lo Sterzo, C.; Crescenzi, R.; Frachey, G. J. Organomet. Chem. 1995,
493, 55. (c) Viola, E.; Lo Sterzo, C.; Crescenzi, R.; Frachey, G. J.
Organomet. Chem. 1995, 493, C9. (d) Viola, E.; Lo Sterzo, C.; Trezzi, F.
Organometallics 1996, 15, 4352. (e) Buttinelli, A.; Viola, E.; Antonelli,
E.; Lo Sterzo, C. Organometallics 1998, 17, 2574.
(5) (a) Chandrasekharam, M.; Chang, S.-T.; Liang, K.-W.; Li, W.-T.; Liu, R.-
S. Tetrahedron Lett. 1998, 39, 643. (b) Cotton, F. A.; Stiriba, S.-A.;
Yokochi, A. J. Organomet. Chem. 2000, 595, 300.
(6) (a) Hartbaum, C.; Roth, G.; Fischer, H. Chem. Ber. 1997, 130, 479. (b)
Hartbaum, C.; Fischer, H. Chem. Ber. 1997, 130, 1063. (c) Hartbaum, C.;
Fischer, H. J. Organomet. Chem. 1999, 578, 186.
(7) (a) Antonelli, E.; Rosi, P.; Lo Sterzo, C.; Viola, E. J. Organomet. Chem.
1999, 578, 210. (b) Altamura, P.; Giardina, G.; Lo Sterzo, C.; Russo, M.
V. Organometallics 2001, 21, 4360.
† E-mail: Claudio.Losterzo@uniroma1.it.
(1) Lo Sterzo, C. J. Chem. Soc., Dalton Trans. 1992, 1989.
(2) (a) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (b) Mitchell,
T. N. In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang,
P. J., Eds.; Wiley-VCH: Weinheim, Germany, 1998; p 167. (c) Farina,
V.; Krishnamurthy, V.; Scott, W. J. The Stille Reaction; John Wiley &
Sons: New York, 1998.
(3) (a) Lo Sterzo, C.; Miller, M. M.; Stille, J. K. Organometallics 1989, 8,
2331. (b) Lo Sterzo, C.; Stille, J. K. Organometallics 1990, 9, 687. (c) Lo
Sterzo, C.; Bocelli, G. J. Chem. Soc., Dalton Trans. 1991, 1881. (d) Lo
Sterzo, C. Organometallics 1990, 9, 3185. (e) Lo Sterzo, C.; Bandoli, G.;
Dolmella, A. J. Chem. Soc., Dalton Trans. 1992, 697.
9
1060 VOL. 124, NO. 6, 2002 J. AM. CHEM. SOC.
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Palladium-Catalyzed Metal−Carbon Bond Formation
A R T I C L E S
cycle by 31P NMR, due to the presence of the two phosphorus
centers in complexes 3-8.9b,c These analyses have indicated
that each step of the catalytic cycle outlined in Figure 2 is indeed
composed by different elementary processes.
We report here a detailed mechanistic investigation of the
transmetalation step of the metal-carbon bond-formation
process. In Pd-catalyzed carbon-carbon coupling (Figure 1),
the rate-determining event of the cycle has been proposed to
be either the oxidative addition, the transmetalation, or the
reductive elimination reaction,8b,10,11 depending on the substrate
and reaction conditions. While a number of accurate studies
have been dedicated to the oxidative addition and reductive
elimination processes,12 the transmetalation step has received
much less attention and is not well understood.8c On the basis
of spectroscopic and kinetic studies, Farina13 and Hartwig14 have
independently postulated that the transmetalation process is
regulated by the rate of ligand dissociation (-L) from the
intermediate b of Scheme 2. Upon screening the effect of a
large variety of ligands of different donicity13 and stannanes of
different nucleophilicity14 on the rate of transmetalation, they
have ruled out that the reaction proceeds via a classical
associative process.15,19 In particular, it was shown that the
reactions of complexes containing ligands of poor donicity, such
as AsPh3 which dissociates easily from the Pd(II) intermediate
b, are much faster (>103) than those of complexes with good
donor ligands (i.e., PPh3).13 Following ligand dissociation, the
solvent-coordinated complex c interacts with the olefinic
stannane to form an intermediate π-complex (d), which then
evolves into the coupled product e. It was presumed that
couplings involving alkynylstannanes proceed through formation
of analogous intermediate π-complexes.
Figure 1. Catalytic cycle of the Pd-promoted carbon-carbon (C-C) bond-
formation process.
Scheme 1
With the use of properly designed model substrates [η5-(1-
Ph2P-2,4-Ph2)C5H2](CO)3MI (1, M ) Mo; 2, M ) W), bearing
a chelating diphenylphosphine sidearm on a cyclopentadienyl
ring, it has been possible to isolate and characterize the products
of the stoichiometric reaction between the metal-iodide com-
plexes 1 and 2 and zerovalent palladium9 (Figure 2). The
(10) (a) Negishi, E.; Takahashi, T.; Baba, S.; Van Horn, D. E.; Okukado, N. J.
Am. Chem. Soc. 1987, 109, 2293. (b) Farina, V. In ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon: Oxford, 1995; Vol. 12, p 200.
(11) Casado, A.; Espinet, P.; Gallego, A. M. J. Am. Chem. Soc. 2000, 122,
11771 and references therein.
(12) (a) Stang, P. J.; Kowalski, M. H.; Schiavelli, M. D.; Longford, D. J. Am.
Chem. Soc. 1989, 111, 3347. (b) Brown, J. M.; Cooley, N. A. Organome-
tallics 1990, 9, 353. (c) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991,
113, 9585. (d) Farina, V.; Krishnan, B.; Marshall, D. R.; Roth, G. P. J.
Org. Chem. 1993, 58, 5434. (e) Amatore, C.; Carre´, E.; Jutand, A.; Tanaka,
H.; Ren, Q.; Torii, S. Chem.-Eur. J. 1996, 2, 957. (f) Amatore, C.; Broeker,
G.; Jutand, A.; Khalil, F. J. Am. Chem. Soc. 1997, 119, 5176. (g) Gillie,
A.; Stille, J. K. J. Am. Chem. Soc. 1980, 102, 4933. (h) Tatsumi, K.;
Hoffmann, R.; Yamamoto, A.; Stille, J. K. Bull. Chem. Soc. Jpn. 1981,
54, 1857.
(13) Farina, V.; Krishnan, B.; Marshall, D. R.; Roth, G. P. J. Am. Chem. Soc.
1991, 113, 9585.
(14) (a) Louie, J.; Hartwig, J. F. J. Am. Chem. Soc. 1995, 117, 11598. (b)
Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046.
(15) If occurring through a classic substitution process, transmetalation should
proceed via an associative mechanism in which the stannane (incoming
group) and the halide (leaving group) are simultaneously bound at the apical
positions of a penta-coordinate intermediate.16,17 Recent studies also
provided a theoretical background for this transformation.18
complexes [η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MPd(PPh3)I (3, M
) Mo; 4, M ) W) are the product of oxidative addition of the
M-I moiety of 1 or 2 to the palladium center. The subsequent
reaction of 3 and 4 with trialkyltinacetylides Bu3Sn-CtC-R
affords the corresponding transmetalated complexes [η5-(1-
Ph2P-2,4-Ph2)C5H2](CO)3MPd(PPh3)(CtCR) (5, M ) Mo; 6,
M ) W), which are further converted into complexes 7 and 8,
respectively, by trans-to-cis isomerization. The final products
of the catalytic metal-carbon coupling process [η5-(1-Ph2P-
2,4-Ph2)C5H2](CO)3M(CtCR) (9, M ) Mo; 10, M ) W) are
then generated by reductiVe elimination on palladium (Figure
2).9c Therefore, an entire catalytic cycle, from metal halides 1
and 2 to metal acetylides 9 and 10, has been examined by
analysis of each of the consecutive reaction steps, which are
closely related to the typical sequence of the Pd-catalyzed C-C
bond-formation process.2,8
During these studies, it has been possible to detect the
existence of transient species in the various steps of the catalytic
(16) Cattalini, L. In Progress in Inorganic Chemistry; Lippard, S., Ed.; Wiley:
New York, 1970; Vol. 13, pp 263-327.
(17) Cross, R. J. Ligand Substitution Reactions of Square-Planar Molecules;
The Royal Society of Chemistry: London, 1985.
(8) (a) Mateo, C.; Cardenas, D. J.; Fernandez-Rivas, C.; Echavarren, A. M.
Chem.-Eur. J. 1996, 2, 1596. (b) Amatore, C.; Jutand, A.; Suarez, A. J.
Am. Chem. Soc. 1993, 115, 9531. (c) Casado, A. L.; Espinet, P.
Organometallics 1998, 17, 954.
(9) (a) Spadoni, L.; Lo Sterzo, C.; Crescenzi, R.; Frachey, G. Organometallics
1995, 14, 3149. (b) Cianfriglia, P.; Narducci, V.; Lo Sterzo, C.; Viola, E.;
Bocelli, G.; Kodenkandath, T. A. Organometallics 1996, 15, 5220. (c)
Tollis, S.; Narducci, V.; Cianfriglia, P.; Lo Sterzo, C.; Viola, E. Organo-
metallics 1998, 17, 2388.
(18) Frankcombe, K. E.; Cavell, K. J.; Yates, B. F.; Knott, R. B. Organometallics
1997, 16, 3199 and references therein.
(19) In a more recent study about the accelerating effect of Cu(I) salts in the
Stille coupling, while remarking about the previously proposed “predis-
sociation” mechanism, Farina postulates that the transmetalation of a newly
formed organocopper derivative, after a Sn/Cu exchange, with the oxidative
addition intermediate may occur through an associative mechanism (via a
penta-coordinate intermediate).15,20
9
J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002 1061

A R T I C L E S
Ricci et al.
Figure 2. Catalytic cycle of the Pd-promoted metal-carbon (M-C) bond-formation process.
Scheme 2
In a series of recent papers,11,21,22 Espinet has proposed that
the transmetalation reaction in Pd-catalyzed coupling of aryl
electrophiles R1-X (X ) halides, triflates) with organotin
reagents R2SnBu3 (R2 ) vinyl) occurs via two different
associative pathways, named “SE2 cyclic” and “SE2 open”
(Scheme 3).23
When the cyclic mechanism cannot operate, because a
coordinating solvent or the absence of halide prevents formation
of the Pd-X-Sn bridge, transmetalation occurs via a “SE2
open” mechanism. This proceeds through either neutral (f, path
b) or cationic species (l, path c, S ) L or solvent), which are
involved in dynamic equilibria affected by the nature of X, L,
the solvent, the temperature, or the presence of added LiCl (in
the case of X ) OTf). The process is an associative substitution
reaction, in which the R carbon of R2 is the entering ligand,
and the Pd complex is the electrophile. Depending on the trans
effect of R1 and on the other ligands, the open transition state
may form by a X-for-R2 or a L-for-R2 replacement at the Pd
center, which leads either to the cis (n) or the trans (o)
transmetalated intermediates. When the trans-to-cis isomeriza-
tion is slow, the intermediate o may be observed before
evolution toward the final product of coupling R1-R2. These
studies, based on kinetic, spectroscopic, and stereochemical
evidences, offer a comprehensive view of the Stille reaction
and support an associatiVe model based on an L-for-R2
substitution as the rate-determining step of the transmetalation
reaction (in contrast with the proposal of a dissociatiVe rate-
determining step).13,14
The SE2 cyclic mechanism, in solvents of moderate polarity
and weakly coordinating, is characterized by the presence of a
bridging leaving group X (halide), which assists the R2-for-L
substitution on palladium (path a). This process leads to the
three-coordinate T-shaped intermediate i, from which the
coupling product R1-R2 is rapidly formed without need of
further dissociation or isomerization steps. The coupling is
strongly retarded by free L, and the straightforward formation
of the product R1-R2 does not allow the detection of intermedi-
ates after transmetalation.
(20) Farina, V.; Kapadia, S.; Krishnan, B.; Wang, C.; Liebeskind, L. S. J. Org.
Chem. 1994, 59, 5905 and references therein.
(21) Casado, A. L.; Espinet, P. J. Am. Chem. Soc. 1998, 120, 8978.
(22) Casado, A. L.; Espinet, P.; Gallego, A. M.; Martinez-Ilarduya, J. M. Chem.
Commun. 2001, 339.
(23) The possibility of a transmetalation process proceeding via a cyclic and/or
an open transition state was first envisaged by Stille.24
9
1062 J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002

Palladium-Catalyzed Metal−Carbon Bond Formation
A R T I C L E S
Scheme 3 a
a Path a: X ) halide, nonpolar, noncoordinating solvents. Paths b, c: X ) halide, OTf; S ) THF, PhCl, CH2Cl2, HMPA, NMP.
In this paper, we report the results of our studies on the
transmetalation step in the palladium-catalyzed metal-carbon
(M-C) coupling process. On the basis of (i) the spectroscopic
observation of an adduct formed between the organostannane
and the oxidative addition complex, (ii) the isolation (in a
halobridged dimeric form) of an intermediate species resulting
from ligand dissociation, and (iii) the analysis of consistent
kinetic data, it is shown that the transmetalation reaction
proceeds by two different mechanisms, which operate depending
on the initial concentration of the oxidative addition complex.
Close analogies between the catalytic cycles of Pd-catalyzed
carbon-carbon bond (Figure 1) and metal-carbon bond (Figure
2) formation become evident from comparison of our results
with those reported in Farina’s,13 Hartwig’s,14 and Espin-
et’s11,21,22 studies.
the simultaneous increase of the signals of the product (Figure
3, path a, red and blue, respectively).29 In the reaction of 3 with
a 10-fold excess of 11a, two new narrow doublets (J = 10 Hz)
appear along with the peaks of substrate and product, indicating
the formation of a transient species (Figure 3, path b, green).
The fact that this intermediate complex is observed only in the
presence of a large excess of the reagent suggests that it is an
adduct between complex 3 and the tinacetylide. This species
appears during the time of conversion of 3 into 5 and is not
detected at the end of the reaction. The strong variation of the
31P NMR coupling constants, from ca. 450 Hz of complexes 3
or 5 to 10 Hz (see expansion in path b), indicates that the
phosphorus ligands are still bound to the palladium center in
the intermediate, while a severe change of the coordination
geometry occurs during the transmetalation. On the basis of
these facts, a tentative description of the intermediate complex
is shown in Figure 3 as the five-coordinate structure 12.
The proposal that the species 12 is an associative complex is
supported by various considerations. (i) The tinacetylide com-
pounds exhibit Lewis acid character and are prone to become
five-coordinate, as in the case of intramolecular coordination
of heteroligands.30 (ii) The iodine atom of 3 offers an unshared
electron pair to the Lewis acid center and may contribute to
the association via a bridging halide.31 (iii) π-Complexation
between Pd and the acetylenic group is well known.32 (iv)
Finally, the palladium center of complex 3 suffers a severe
distortion from the ideal square-planar geometry, as it was
Results and Discussion
The transmetalation reaction of this study is a transformation
by which an arylethynyl group is transferred from tin to
palladium.25 The molybdenum complex 3, that is, the product
of the oxidative addition step of the catalytic cycle in Figure 2,
has been used as substrate to study in detail the transmetalation
process. The transformation of 3 into product 5 has been
monitored by 31P NMR spectroscopy, since both complexes bear
two nonequivalent phosphorus moieties bound to palladium
which exhibits two doublets (J = 450 Hz) characteristic of trans
configuration around a square-planar metal center (Figure 3).
Different features were observed depending on the relative
molar ratio between 3 and the tinacetylides 11a,b. Upon
treatment of 3 with a 3-fold excess of (phenylethynyl)tributyltin
(11a), in DMF at 25 °C, a sequence of spectra shows the
progressive decrease of the signals of the starting material and
(27) Hegedus L. S. In Organometallics in Synthesis; Schlosser, M., Ed.; John
Wiley & Sons: Chichester, England, 1994; p 415.
(28) Mateo, C.; Fernandez-Rivas, C.; Echavarren, A.; Cardenas, D. J. Organo-
metallics 1997, 16, 1997.
(29) Reagents were loaded in a 5 mm NMR tube and dissolved in DMF (0.4
mL). DMF-d7 (0.01 mL) was added to the solution to provide the lock
signal.
(30) (a) Davies, A. G. Organotin Chemistry; VCH Verlagsgesellschaft: Wein-
heim, Germany, 1997; p 134. (b) Petrosyan, V. S.; Yashina, N. S.; Reutov,
O. A. In AdVances in Organometallic Chemistry; Stone, F. G. A., West,
R., Eds.; Academic Press: New York, 1976; Vol. 14, p 63. (c) Yasuda,
M.; Chiba, K.; Baba, A. J. Am. Chem. Soc. 2000, 122, 7549.
(31) Purcell, K. F.; Kotz, J. C. Inorganic Chemistry; Holt-Saunders International
Editions: Hong Kong, 1987; p 205.
(24) Labadie, J. W.; Stille, J. K. J. Am. Chem. Soc. 1983, 105, 6129.
(25) The driving force of this metathesis reaction is provided by the difference
in electronegativity of the two metals (Sn more electropositive than Pd)26
as well as by the formation of the stable Bu3SnI.27,28
(26) Tsuji, J. Palladium Reagents and Catalysts; John Wiley: England, 1996;
p 17.
9
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A R T I C L E S
Ricci et al.
Figure 3. Transmetalation of complex 3 with tributyl(phenylethylnyl)tin (11a) followed by 31P NMR spectroscopy. Path a: reaction of 3 with 3 equiv of
11a. Path b: reaction of 3 with 10 equiv of 11a.
determined by X-ray structural analysis,9b which makes penta-
coordination a favorable process. Further support to our proposal
comes from the structural analogy between 12 and the activated
complex g of Scheme 3,11,21 as well as from the structure of an
associative intermediate observed by Cotter33 in the course of
a transmetalation reaction.
Kinetic and Spectroscopic Investigation of the Trans-
metalation Reaction by 31P NMR: The Observation of Two
Alternative Mechanisms. Reactions were carried out in DMF
at 25 °C using complex 3 in the concentration range (1.5-4.5)
× 10-2 M and at least a 10-fold excess of (phenylethynyl)-
tributyltin (11a), as to work under pseudo first-order condi-
tions.29 Disappearance of the substrate 3, formation of the
transmetalated product 5, as well as of the intermediate 12 versus
Figure 4. Reaction profile for the transmetalation step of complex 3 (0.045
time are shown in Figure 4. The concentration of the intermedi-
ate species has already reached the highest value in the first
spectrum after mixing. This indicates that the rate of formation
of 12 is higher than the eventual back reaction or its transforma-
tion into 5. Consumption of starting material was complete
within 30 min and followed a first-order behavior. The analysis
according to eq 1 yields a value of observed rate constant kobs
) 0.0021 s-1 ([3] ) 0.045 M, [11a] ) 0.47 M), and the
corresponding plot of ln[3] versus time is linear. Values of kobs
M) with 11a (0.47 M) in DMF-d7 at 25 °C, obtained from the 31P NMR
signals of 3 (b, δ 22.0 ppm), 12 (O, δ 29.0 ppm), and 5 (], δ 30.5 ppm).
The concentration values of the intermediate species 12 are only an estimate
judged from peak intensities.
dependence on the concentration of the organostannane (entries
2-6, Table 1).
To evaluate the role of the triphenylphosphine ligand in the
formation of the associative complex 12 and in the overall
transmetalation process, measurements were performed in the
presence of added PPh3 (entry 9). The addition of PPh3 (0.13
M) to the reaction of 3 (0.016 M) with 11a (0.36 M) retards
the consumption of the substrate and, to a greater extent, the
ct ) c∞ + (c0 - c∞) exp(-kobst)
(1)
for the disappearance of 3 under different conditions are reported
in Table 1. At constant [3] ) 0.023 M, kobs increases upon
increasing the concentration of (phenylethynyl)tributyltin, from
0.0026 s-1 (0.19 M) to 0.0042 s-1 (0.72 M), indicating rate
(32) Trebbe, R.; Schager, F.; Goddard, R.; Po¨rschke, K.-R. Organometallics
2000, 19, 521.
(33) Cotter, W. D.; Barbour, L.; McNamara, K. L.; Hechter, R.; Lachicotte, R.
J. J. Am. Chem. Soc. 1998, 120, 11016.
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Palladium-Catalyzed Metal−Carbon Bond Formation
A R T I C L E S
Table 1. Values of kobs for the Reaction of Complex 3 with
Bu3SnCtCPh (11a), Obtained by 31P NMR in DMF/DMF-d7 at 25
°C
entry
3 (M)
11a (M)
kobs (s-1
)
1
2
3
4
5
6
7
8
9
0.45
0.47
0.19
0.23
0.33
0.68
0.72
0.72
0.16
0.0021
0.0026
0.0026
0.0036
0.0041
0.0042
too fast
too fast
very slow
0.023
0.023
0.023
0.023
0.023
0.015
0.015
0.016
0.36 (+PPh3 0.13 M)
Figure 5. 31P NMR spectrum recorded during transmetalation of 3 with
11a in the presence of a 10-fold excess of PPh3. Accumulation of the
intermediate 12.
conversion of the intermediate 12 into 5. As a consequence,
the species 12 accumulates in solution, where it is observed as
the main component (Figure 5).
This experiment suggests that triphenylphosphine must
temporarily dissociate from complex 12 (and re-enter in a
subsequent step) to allow formation of 5 (vide infra). The
dissociation step becomes rate limiting in the overall transmeta-
lation reaction.
A rate dependence was observed on the initial concentration
of complex 3. At the same concentration of (phenylethynyl)-
tributyltin (0.72 M) used in the experiment of entry 6, or lower
(0.16 M), and [3] ) 0.015 M, rather than 0.023 M, the reaction
becomes too fast to be followed by 31P NMR spectroscopy
(entries 7, 8). This inverse dependence of the reaction rate on
the initial concentration of 3 is also evident from comparison
of the kobs value of entry 1 with those of entries 4 and 5, where
higher rates correspond to lower [3]. These results suggest that
the true species which undergoes transmetalation is generated
from 3 upon dilution. 31P NMR spectra obtained upon increased
dilution of 3, in DMF, are shown in Figure 6, where traces a,
b, and c correspond to different concentrations of the complex
(a ) 0.029 M, b ) 0.0019 M, c ) 0.0010 M). The clean doublet
of doublets of the two nonequivalent phosphorus atoms in a
trans configuration around palladium is displayed in trace a. In
the more dilute sample b, a new peak appears at δ 51.9 ppm.
This becomes the main species in solution c. The change from
the doublet of doublets, relative to structure 3, into a singlet
accounts for the formation of a new species in which the two
phosphorus atoms are no longer interacting. These experimental
observations are in agreement with an equilibrium, described
by eq 2,
Figure 6. 31P NMR spectra of complex 3 at different concentrations in
DMF/DMF-d7. Trace a, [3] ) 0.029 M; trace b, [3] ) 0.0019 M; trace c,
[3] ) 0.0010 M; trace d, solution of trace c after addition of excess PPh3.
Scheme 4
for the inverse dependence of kobs on the initial concentration
of 3. In diluted solutions, complex 13 is the main species and
may react faster than 3 with the tin reagent, because of its higher
electrophilic character.35 The reactivity of the solvent-coordinate
species 13 was investigated by addition of (phenylethynyl)-
tributyltin (11a) to the solution of trace b. The signal at δ 51.9
ppm immediately disappears upon addition, and this is followed
by a slower change of 3 into 5.
The lability of the triphenylphosphine ligand in complex 3
was confirmed by its fast and complete replacement upon
treatment of 3 with tributylphosphine.36The course of the
transmetalation reactions of complexes bearing different L, with
cR2
1 - R
K )
(2)
which involves a substitution of ligand (PPh3) by solvent (DMF)
to form the species 13, as represented in Scheme 4. The signal
of free triphenylphosphine is not detectable under these condi-
tions.34
Trace d is the spectrum obtained upon addition of an excess
of triphenylphosphine to the solution of trace c (Figure 6).
Disappearance of the signal at δ 51.9 ppm and formation of
the original doublet of doublets of complex 3 account for the
reverse reaction of Scheme 4. The formation of 13 accounts
ppm, appeared in progressively diluted samples of 3. By comparison with
an authentic sample, this new peak was identified as being due to
triphenylphosphine oxide, which formed by adventitious oxidation of free
PPh3 released from 3.
(35) Substitution of a good donor ligand such as PPh3 by a solvent molecule is
expected to increase the electrophilic character of palladium and hence to
increase its reactivity.
(34) According to the dissociative equilibrium presented in Scheme 4, the 31P
NMR spectra a-c of Figure 7 should reveal the presence of free
triphenylphosphine. It is possible that the rate of exchange of triphenylphos-
phine with the solvent on palladium is too fast with respect to the NMR
time scale. Occasionally, a peak at δ 26.1 ppm, along with that at δ 51.9
(36) The 31P NMR spectrum of a solution of 3 in DMF/DMF-d7 in the presence
of PBu3 showed immediate and complete exchange of the phosphine ligand,
as indicated by the replacement of the signals of 3 (trace a of Figure 7)
with a new doublet of doublets (δ 35.1, d; -0.45, d; J ) 468 Hz).
9
J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002 1065

A R T I C L E S
Ricci et al.
Figure 7. 31P NMR evidence of the formation of the solvent-coordinate species 13 by reaction of complex 1 with Pd2(dba)3, and subsequent conversion of
13 into the oxidative addition intermediate 3 by reaction with PPh3.
respect to 3, gave further information on the role of the
phosphine ligand.37 At room temperature, both the tributyl- and
trimethyl-phosphine derivatives 15 and 16 remain inert in the
presence of an excess of (phenylethynyl)tributyltin. The onset
of the transmetalation reaction is observed only at 65 °C, and
several hours are required for completion. PBu3 and PMe3 are
good donor ligands which have a smaller cone angle and a
higher dissociation energy than does PPh3.37
The identification of the associative complex 12 and the
existence of the solvent-coordinate species 13 (isolated in the
halobridged dimeric form 14) in diluted solutions indicate that
the transmetalation reaction of complex 3 occurs by two distinct
pathways. In this respect, the system of this study shows a close
analogy with the dual pathway scheme found by Espinet for
the transmetalation step of the Pd-catalyzed C-C bond-
formation process (Scheme 3).
Kinetic and Spectroscopic Measurements by UV-Vis
Spectrophotometry. UV-vis spectrophotometry has been used
as a complementary technique to obtain more information on
this complex phenomenon, in particular to observe the process
in the low concentration range of substrate, where 13 is most
likely the dominant or only species in solution. A series of UV-
vis spectra of complex 3 in DMF were obtained in the
concentration range 2.9 × 10-5 - 1.0 × 10-2 M. The intensity
of a shoulder absorption with λmax ) 470 nm increases linearly
We attempted the direct synthesis of the solvent-coordinate
species 13. A preliminary investigation was carried out by
following the 31P NMR spectra of the reaction of 1 with 1 equiv
of Pd2(dba)3 (dba ) 1,5-diphenyl-1,4-pentadiene-3-one) in DMF
(Figure 7). The singlet peak of 1 at δ -18.9 ppm (trace a) is
replaced by a new peak at δ 51.9 ppm (trace b). By addition of
PPh3 to this mixture, the latter is immediately and completely
converted into the doublet of doublets characteristic of 3 (trace
c). This experiment shows that the solvent-coordinate species
13 (δ 51.9 ppm) can be either obtained from 3 by loss of PPh3
(Scheme 4) or prepared independently by reaction of 1 with a
zerovalent palladium complex in a coordinating solvent, in the
absence of phosphorus ligands. The synthesis of 13 was then
attempted on a preparative scale. Despite the complete formation
of 13 observed by 31P NMR, the workup of the reaction afforded
instead the dimeric halobridged complex [η5-(1-Ph2P-2,4-Ph2)-
C5H2](CO)3MoPd(µ-I)2PdMo(CO)3[η5-(1-Ph2P-2,4-Ph2)C5H2]
(14).38
(38) During the preparation of this manuscript, a complex closely related to 14
was characterized by X-ray structural analysis.39 The structure shows the
existence of a halo-bridged dimeric form of two coordinatively unsaturated
palladium moieties.
(37) Luo, L.; Nolan, S. P. Organometallics 1994, 13, 4781.
9
1066 J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002

Palladium-Catalyzed Metal−Carbon Bond Formation
A R T I C L E S
Scheme 5
coupling between the two species,42 and one involves rear-
rangement of the substrate in the first step followed by attack
of the reagent.43 Discrimination between the two alternative
mechanisms is possible by varying the nature of the tinacetylide,
which is expected to influence the limiting value of kobs in case
of rate-determining transformation of a molecular complex.
A series of kinetic measurements performed using Bu3SnCt
C (p-Cl-C6H4) (11b) as the transmetalating agent also exhibit
a saturation behavior, as observed for Bu3SnCtCPh, and yield
a different extrapolated value of kobs (0.0070 s-1) (Figure 8).
These data are therefore described by eq 3,
Kk2[11]
kobs
)
(3)
1 + K[11]
Figure 8. Plot of kobs vs alkynylstannane concentration for the reactions
of 3 (1.67 × 10-4 M) with 11a (]) and 11b (b).
in which K represents the equilibrium constant for formation
of an associative complex between 3 and the tin reagent and k2
is the rate constant of the transmetalation step, according to
Scheme 5. The fitting between eq 3 and the experimental points
is also shown in Figure 8. These results indicate that the
transmetalation reaction occurs through a multistep mechanism
characterized by a fast pre-equilibrium between substrate and
tinacetylide to form an associative intermediate species, which
slowly evolves toward the product. The kinetic analysis yields
the values of the equilibrium constant between complex 3 and
each of the tinacetylide species, which are K ) 376 M-1 for
Bu3SnCtCPh and K ) 430 M-1 for Bu3SnCtC(p-Cl-C6H4).
The p-substituent in the aryl ring of the tinacetylide affects the
rate of the transmetalation step (k2) but does not influence the
pre-equilibrium (K).
Rate Dependence on the Concentration of Complex 3. Rate
measurements of the reaction with Bu3SnCtCPh (1.14 × 10-2
M) were obtained using different solutions of complex 3 in DMF
(in the range 10-5 - 10-3 M). A plot of kobs versus [3] shows
the inverse dependence of the rate constant on the initial
concentration of the complex.
These results, in analogy with the 31P NMR experiments,
suggest that the transmetalation reaction proceeds via the
solvent-coordinated species 13 of Scheme 4. The higher
reactivity of complex 13 with respect to the phosphine-
substituted complex 3 may be due to the higher electrophilic
character of the palladium center when coordinated by a poor
donor ligand such as DMF. As a consequence, the transmeta-
lation rate increases with increasing dilution of 3, which shifts
the equilibrium of Scheme 4 versus 13.
with concentration only up to [3] = 1 × 10-3 M, while a
downward deviation occurs at higher concentrations. This is
indicative of a change in the nature of 3 with concentration, in
analogy with the 31P NMR studies.
The UV-vis spectra of the starting material 3 and of product
5 were recorded in DMF solutions at identical concentrations
(1.7 × 10-4 M) to find the optimal conditions for kinetic
measurements. Complex 3 is characterized by an intense band
with λmax ) 400 nm (ꢀ ) 15 117 M-1 cm-1), and complex 5 is
characterized by a shoulder of smaller intensity (ꢀ ) 4 586 M-1
cm-1) at the same wavelength.
The reaction of 3 (1.67 × 10-4 M) with Bu3SnCtCPh (2.14
× 10-3 M) in DMF was monitored at 400 nm (25 °C). The
process is characterized by a rapid decay of absorbance, which
follows a first-order behavior (kobs ) 0.010 s-1). This is followed
by a slower increase, which is due to the reductive elimination
reaction of complex 5, favored by the excess of tinacetylide.9c,40
Effect of Concentration and Nature of the Organostan-
nane. Kinetic measurements were obtained at different con-
centrations of Bu3SnCtCPh, and [3] ) 1.67 × 10-4 M, using
at least a 10-fold molar excess of stannane to ensure pseudo
first-order conditions. A plot of kobs values versus [11a] shows
a saturation behavior (Figure 8) where the rate constants tend
toward a limiting value (k ) 0.023 s-1). This is typical of a
reaction proceeding through formation of kinetically detectable
intermediates. Among the possible mechanisms which describe
this phenomenon, one involves the formation of a molecular
complex of substrate and reagent followed by rate-determining
The halobridged dimeric complex 14, which was obtained
in the attempt to synthesize and isolate complex 13, represents
(39) Ricci, A.; Angelucci, F.; Masi, D.; Bianchini, C.; Lo Sterzo, C., manuscript
in preparation.
(40) To confirm this sequence, the reaction of 3 (0.015 M) with Bu3SnCtCPh
(0.17 M) in DMF was monitored by 31P NMR spectroscopy. Following
the fast disappearance of the doublet of doublets due to 3 and its replacement
by the signals of 5, a new peak at δ -20.2 ppm due to the reductive
elimination product 9 appears and slowly rises in intensity with time.41
a useful precursor of the DMF-substituted reactive species.44
A
simple 31P NMR experiment showed the connection between
complexes 14 and 13. The addition of DMF to a solution of
pure 14 in CDCl3 (δ 50.1 ppm) causes the appearance of a new
signal at δ 50.8 ppm, which increases in intensity, with respect
(42) Jenks, W. P. Catalysis in Chemistry and Enzimology; McGraw-Hill: New
York, 1969; p 572.
(43) Butler, I. S.; Basolo, F.; Pearson, R. G. Inorg. Chem. 1967, 6, 2074.
(44) The structure and the reactive behavior of species 13 and 14 are similar to
those of a dimeric halobridged oxidative addition intermediate previously
described by Hartwig,14b which was isolated as a dimer, but found to react
in the catalytic cycle as a monomer.
(41) In our former 31P NMR studies, forcing conditions (80 °C) were necessary
to induce the reductive elimination process ([3] ) ∼0.1 M).9c In the present
work, complex 3 (0.015 M) spontaneously evolves toward reductive
elimination. This is also an indication that the rate of reductive elimination
may show an inverse dependence on the initial concentration of 3.
9
J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002 1067

A R T I C L E S
Ricci et al.
Scheme 6
Scheme 7
to that of the halobridged dimeric complex, upon further addition
of DMF.45 This new peak is due to the formation of the solvent-
coordinated species 13, which forms by attack of DMF to the
halobridged dimer 14 (Scheme 6).
The interaction of DMF with complex 14 was studied further
by UV-vis spectroscopy. Figure 9 shows the spectra of
solutions of 14 at identical concentrations (3.1 × 10-5 M) in
CH2Cl2 and in DMF. The different patterns of the two spectra
indicate that different species exist in the two solvents, in
particular that the dimeric complex 14 exists as such in the
noncoordinating solvent CH2Cl2, while the solvent-coordinate
species 13 forms in the donor solvent DMF.46
Further 31P NMR experiments have shown the involvement
of complex 13 in the catalytic cycle of the palladium catalyzed
metal-carbon bond-formation process. After dissolving complex
14 in DMF, the appearance of the 31P NMR signal at 51.9 ppm
indicates the immediate formation of the solvent-coordinated
species 13. This signal disappears upon addition of an 8-fold
excess of Bu3SnCtCPh, while two peaks of equal intensity are
found at δ 1.1 and 2.2 ppm. Subsequent addition of PPh3
produced a slow conversion of these signals into the typical
doublet of doublets of the transmetalated product 5. This
experiment indicates that the PPh3-deficient species 13 under-
goes transmetalation upon treatment with tinacetylides as well
as it does with the corresponding PPh3-coordinate complex 3.
In fact, the new species characterized by the signals at δ 1.1
and 2.2 ppm can be envisaged as the transmetalated intermediate
17, which converts into the PPh3-coordinate complex 5 upon
addition of the phosphine (Scheme 7). When the PPh3-deficient
species 17 is warmed overnight at 60 °C, the total conversion
into the reductive elimination product 9 is achieved.47 Therefore,
the intermediate complex 17 can transform directly into the
reductive elimination product, also in the absence of PPh3.
Kinetic experiments were performed on the reaction of Bu3-
SnCtCPh with complex 14 in DMF, which generates 13 in
situ. Values of kobs identical within experimental error were
(45) Complex 14 dissolved in pure DMF is converted quantitatively into 13,
which shows a peak at δ 51.9 ppm in the 31P NMR spectrum. The slight
difference found for the chemical shift of 13 generated by addition of DMF
into a CDCl3 solution of 14 may be due to a solvent effect.
(46) The intense color of the CH2Cl2 solution with respect to the DMF solution
and the band at λmax ) 420 nm might be due to a higher conjugation in 14
than that in 13. Moreover, the spectrum profile of 13 obtained by dissolving
14 in DMF is the same as the spectra of 13 generated by increased dilution
of 3 in DMF.
Figure 9. UV-vis spectra (250-600 nm) of complex 14 (3.1 × 10-5 M)
in CH2Cl2 (-EnDash) and in DMF (-‚‚-‚‚).
(47) Angelucci, F.; Ricci, A.; Lo Sterzo, C.; Masi, D.; Bianchini, C.; Bocelli,
G., manuscript in preparation.
9
1068 J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002

Palladium-Catalyzed Metal−Carbon Bond Formation
A R T I C L E S
Scheme 8. Full Reaction Scheme for the Transmetalation of 3 with 11aa
a Path a: reaction via the metallacycle intermediate 12. Path b: reaction via the solvent-coordinate species 13.
Table 2. Values of kobs for the Reaction of 14 with 11a, Obtained
strongly retarded by addition of an excess of PPh3, which favors
the accumulation of 12.49
by UV-Vis Spectroscopy in DMF at 25 °C
entry
14 (M)
11a (M)
kobs (s-1
)
When transmetalation is performed using 3 in lower concen-
trations (=10-4 M) (path b of Scheme 8), the reaction proceeds
through formation of a highly reactive solvent-coordinate species
(13), in which the palladium center has gained a stronger
electrophilic character, due to the substitution of a good donor
ligand (PPh3) by DMF. Since a competing exchange between
PPh3 and DMF accounts for the equilibrium between 3 and 13,
formation of the latter species is favored in dilute solutions,
and, as a consequence, the reaction rate is inversely dependent
on the initial concentration of complex 3. In contrast, when the
1
2
3
4
3.4 × 10-5
1.7 × 10-5
3.4 × 10-5
1.7 × 10-5
2.7 × 10-3
2.7 × 10-3
6.8 × 10-3
6.8 × 10-3
0.034
0.036
0.057
0.056
obtained using two DMF solutions of 14 (3.4 × 10-5 and 1.7
× 10-5 M; [Bu3SnCtCPh] ) 2.7 × 10-3 M; Table 2, entries
1 and 2). The reaction rate constants increased upon increasing
the concentration of Bu3SnCtCPh (6.8 × 10-3 M)48 and were
again identical at two different concentrations of 14 (entries 3
and 4). Therefore, the transmetalation rate is independent of
the initial concentration of 14, in contrast with the behavior of
the PPh3-substituted complex 3. In this case, de-coordination
of PPh3 to form complex 13 precedes the transmetalation
process.
PPh3-deficient halobridged dimeric complex [{η5-(1-Ph2P-2,4-
Ph2)C5H2}(CO)3MoPdI]2 (14) is dissolved in DMF, it is
converted immediately and quantitatively into the solvent-
coordinated species 13. Under these conditions, the transmeta-
lation rate is independent of the initial concentration of 14, due
to the absence of PPh3.
Conclusions
The transmetalation in the reaction of 13 occurs through an
association complex with the organostannane (18), which has
been kinetically detected. It is evident that path a, involving
the formation of the metallacycle complex 12, closely resembles
the “SE2 cyclic” mechanism proposed by Espinet11,21,22 for the
transmetalation step in the coupling of aryl halides with
organostannanes (path a of Scheme 3). However, it should be
taken into account that the key species g of the “SE2 cyclic”
mechanism of Scheme 3 is an activated complex, while the
metallacycle complex 12 of this work is a true intermediate
The overall mechanistic picture which comes out of this work
is described in Scheme 8. We have shown that the transmeta-
lation reaction of the oxidative addition complex [η5-(1-Ph2-
P-2,4-Ph2)C5H2](CO)3MoPd(PPh3)I (3) with tributyltinary-
lacetylides proceeds by two pathways (paths a and b), depending
on the initial concentration of 3.
In concentrated solutions of complex 3 (=10-2 M), trans-
metalation occurs through the formation of the intermediate
species 12, which is an associative complex between 3 and the
organostannane (path a of Scheme 8). The essential feature of
12 is the cyclic structure resulting from the simultaneous
interaction of the palladium and iodine atoms of 3 with the tin
atom and the acetylenic moiety of the organostannane. Evolution
of complex 12 toward the product of transmetalation 5 is
(49) Although a significant amount of 12 is formed in solution (path b in Figure
3) and, in the presence of an excess of PPh3 (∼8:1) and Bu3SnCtCPh
(∼20:1), 12 represents the main component in solution (Figure 7), its
characterization is mainly based on 31P NMR evidence. Attempts to obtain
13C NMR and 1H NMR data of 12 are frustrated by the overwhelming
intensities of signals of PPh3 and Bu3SnCtCPh that must be used in large
excess to maximize its formation. However, a FT-IR analysis of the reaction
mixture under similar conditions shows a band at ν ) 2105 cm-1, which
may account for π-coordination of the alkyne at Pd (ν (Bu3SnCtCPh) )
2132 cm-1).
(48) In entry 1, the [Bu3SnCtCPh]/[13] ratio is ca. 80, while in entry 3 the
relative ratio is 200.
9
J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002 1069

A R T I C L E S
Ricci et al.
which can accumulate in solution as the main reaction compo-
nent. However, the transmetalation mechanism of path b in
Scheme 8, involving the formation of the highly reactive solvent-
coordinate species 13, is related to Espinet’s “SE2 open”
mechanism, which occurs when the cyclic mechanism is
disfavored by reaction conditions (paths b and c of Scheme 3).
In this respect, it is worth mentioning that the intermediate 13
is an analogous species of the solvent-coordinate complex c
proposed by Farina (Scheme 2).20 In fact, a preliminary L-for-S
(PPh3 vs DMF) exchange occurs in complex 3 to form 13,
followed by an I-for-R2 exchange, most likely preceded by an
initial π-coordination of the alkynyl stannane on palladium. This
sequence of events follows closely the one proposed by Farina
(paths a-d, Scheme 2). Therefore, the results found in this work
are characterized by clear analogies with mechanistic studies
of the carbon-carbon coupling process and offer a unifying
view of previous contradictory interpretations,11,13,14,21,22 with
the understanding that different systems have been investigated.
In conclusion, this work confirms our early supposition of
similar mechanistic features between the palladium-catalyzed
metal-carbon (M-C) bond-formation and the carbon-carbon
(C-C) bond-formation processes.9,50 In this respect, because
of the fortunate design of the model substrates 1 and 2, and the
easy access to the stable, but still reactive, oxidative addition
and transmetalated derivatives 3-6, further investigations on
this palladium-catalyzed process can be foreseen. Such studies
may clarify the factors affecting the metal-carbon bond-
formation reaction, as well as shed light on unclear aspects of
the carbon-carbon coupling itself, a phenomenon of paramount
importance in synthetic organic chemistry.
C5H2](CO)3MoPd(PPh3)CtCPh (5) was prepared by adapting our
previously described procedure for the p-NO2 derivative.9b
Pd2(dba)3 (Aldrich), PPh3 (Aldrich), PBu3 (Aldrich), PMe3 (1 M in
THF, Aldrich), and Bu3Sn-CtC-Ph (Aldrich) were used as received.
Legend for 13C NMR spectra:
[η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoPd(PBu3)I (15). A 100 mL
Schlenk flask was loaded with [η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoI (1)‚
0.4CH2Cl2 (0.50 g, 0.67 mmol) and Pd2(dba)3 (0.34 g, 0.37 mmol).
After three cycles of vacuum/argon, THF (20 mL) and PBu3 (0.17 mL,
0.68 mmol) were added, causing the formation of a deep red solution.
After stirring (1 h, room temperature), approximately 20 g of Celite
was added to the reaction mixture, and the solvent was removed under
vacuum. The residue was chromatographed on a silica column (40 cm
× 3 cm). Elution with hexane/dichloromethane 7/3 allowed the
separation of a pale yellow band, which was discarded. Further elution
with hexane/dichloromethane 6/4 yielded a red band, which was
collected to give, after removal of the solvent, 0.56 g (82%) of (15) as
a red solid. A sample suitable for microanalysis was obtained by
crystallization from THF/pentane (vapor diffusion) at room temperature.
1H NMR (CDCl3): δH 8.42-8.36 (m), 7.68-7.61 (m), 7.55-7.51 (m),
7.26-7.24 (m), 7.21-7.17 (m), 7.02-6.90 (m), 6.82-6.75 (m), 6.15
(t, 1H, J ) 2.1 Hz, Cp-H), 4.57 (t, 1H, J ) 2.1 Hz, Cp-H), 2.22-
2.16 (m, 6H, P(CH2CH2CH2CH3)3), 1.59-1.52 (m, 6H, P(CH2CH2-
CH2CH3)3), 1.47-1.40 (m, 6H, P(CH2CH2CH2CH3)3), 0.93 (t, 9H, J
) 7.1 Hz, P(CH2CH2CH2CH3)3). 31P NMR (CDCl3): δP 35.13 (d, J )
469 Hz, -PPh2), -1.93 (d, J ) 469 Hz, PBu3). 13C NMR (CDCl3):
δC 236.2, 225.5, 225.3 (CO), 137.4-125.6 (Ph), 120.0 (d, JC-P ) 9.8
Hz, C2), 110.9 (d, JC-P ) 7.3 Hz, C4), 95.1 (d, JC-P ) 6.1 Hz, C3),
90.2 (d, JC-P ) 7.3 Hz, C5), 58.8 (d, JC-P ) 36.6 Hz, C1), 26.7 (-CH2-
CH2CH2CH3), 24.7 (d, JC-P ) 12.2 Hz, -CH2CH2CH2CH3), 24.4 (d,
JC-P ) 2.4 Hz, -CH2CH2CH2CH3), 13.8 (-CH2CH2CH2CH3). FT-IR
(CH2Cl2, cm-1): 1959.8 (s), 1868.5 (s) (νCO). Anal. Calcd for C44H49-
IMoO3P2Pd: C, 51.96; H, 4.86. Found: C, 52.02; H, 4.88. MS (15 V,
ESP+) ) 932 (M+ - 3CO).
Experimental Section
General Procedures. Elemental analyses were performed by the
Servizio Microanalisi of the Dipartimento di Chimica, Universita` di
Roma “La Sapienza”. IR spectra were recorded on a FT-IR Nicolet
510 instrument in the solvent subtraction mode, using a 0.1 mm CaF2
cell. UV-visible spectra were measured on a Perkin-Elmer Lambda
18 spectrophotometer. The kinetic measurements were performed on
a Hewlett-Packard Vectra XM spectrophotometer. 1H, 13C, and 31P NMR
spectra were recorded on a Bruker AC300P spectrometer at 300, 75,
and 121 MHz, respectively. Chemical shifts (ppm) are reported in δ
[η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoPd(PMe3)I (16). A Schlenk
flask was loaded with [η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoI (1)‚0.4CH2-
Cl2 (0.31 g, 0.41 mmol) and Pd2(dba)3 (0.21 g, 0.23 mmol). After three
cycles of vacuum/argon, the addition of THF (20 mL) and PMe3 (1 M
in THF, 0.4 mL, 0.82 mmol) produced a dark solution with yellow
reflections. The mixture was stirred for 30 min, and then solvent and
excess PMe3 were removed by vacuum transfer. The resulting solid
was dissolved in CH2Cl2, adsorbed on Celite, and chromatographed
on a silica column (40 cm × 3 cm). Elution with hexane/dichlo-
romethane 7/3 separated a pale yellow band, which was discarded.
Further elution with hexane/dichloromethane 1/1 gave an orange band,
which was eluted and collected. Evaporation to dryness left 0.32 g
(88%) of (16) as an orange solid. The product was crystallized from
THF/pentane (vapor diffusion) at room temperature. 1H NMR
(CDCl3): δH 8.36-8.30 (m), 7.76-7.70 (m), 7.58-7.48 (m), 7.27-
7.24 (m), 7.22-7.15 (m), 7.02-6.96 (m), 6.93-6.89 (m), 6.79-6.77
(m), 6.18 (t, 1H, J ) 2.1 Hz, Cp-H), 4.60 (t, 1H, J ) 2.1 Hz, Cp-H),
1
values relative to Me4Si; for H NMR, CHCl3 (δ 7.24) or DMF (δ
2.90), and for 13C NMR, CDCl3 (δ 77.0) were used as internal standards.
The 31P NMR chemical shifts are relative to 85% H3PO4. Mass spectra
were obtained on a Fisons Instruments VG-Platform Benchtop LC-
MS (positive ion electrospray, ESP+) spectrometer. Solvents, including
those used for chromatography, were thoroughly degassed before use.
Chromatographic separations were performed with 70-230 mesh silica
gel (Merck).
All manipulations were carried out under an atmosphere of argon
with Schlenk type equipment on a dual manifold/argon vacuum system.
Liquids were transferred by syringe or cannula. THF and Et2O were
distilled from sodium-potassium alloy; DMF was distilled from CaH2
under reduced pressure.
51
Bu3SnNEt2 [η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoI (1),9b [η5-(1-
1.73 (dd, 9H, J1
) 9.5 Hz, J2
) 3.0 Hz, P(CH3)3). 31P NMR
P-H
P-H
Ph2P-2,4-Ph2)C5H2](CO)3MoPd(PPh3)I (3),9b and HCtC(p-Cl-C6H4)52
were prepared according to published procedures. [η5-(1-Ph2P-2,4-Ph2)-
(CDCl3): δP 33.96 (d, J ) 502 Hz, -PPh2), -15.19 (d, J ) 502 Hz,
P(CH3)3). 13C NMR (CDCl3): δC 235.9, 225.6, 224.9 (CO), 137.0-
125.6 (Ph), 119.8 (d, JC-P ) 9.0 Hz, C2), 111.0 (s, C4), 94.7 (d, JC-P
) 5.4 Hz, C3), 90.0 (d, JC-P ) 12.6 Hz, C5), 59.9 (d, JC-P ) 37.7 Hz,
C1), 16.1 (d, JC-P ) 23.3 Hz, -CH3). FT-IR (CH2Cl2, cm-1): 1959.1
(50) Because the analogy between the two Pd-catalyzed processes is legitimate,
it is worth mentioning that our early reports on the preparation of the
transmetalated complex 5 and other similar derivatives9b,c indeed represent
the first examples of observation and characterization of a transmetalated
intermediate in a Stille-like catalytic cycle. Overlooking these papers, other
authors have later claimed their reports as unprecedented.11,12f,32
(51) Jones, K.; Lappert, M. F. J. Chem. Soc. 1965, 1944.
(52) Jacobs, T. L. Organic Reactions; John Wiley: New York, 1949; Vol. 5.
9
1070 J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002

Palladium-Catalyzed Metal−Carbon Bond Formation
A R T I C L E S
) 2.4 Hz, JHP ) 3.3 Hz, Cp-H), 4.59 (t, 1H, J ) 2.4 Hz, Cp-H). 31
(s), 1865.1 (s) (νCO). Anal. Calcd for C35H31IMoO3P2Pd: C, 47.19; H,
3.51. Found: C, 47.31; H, 3.53. MS (15 V, ESP+) ) 806 (M+ - 3CO).
P
NMR (CDCl3): δP 50.16. 13C NMR (CDCl3): δC 237.7, 224.7, 223.7
(CO), 136.9-125.1 (Ph), 119.4 (d, JC-P ) 9.0 Hz, C2), 110.5 (d, JC-P
) 9.0 Hz, C4), 95.2 (d, JC-P ) 5.4 Hz, C3), 89.2 (d, JC-P ) 12.6 Hz,
C5), 57.7 (d, JC-P ) 46.7 Hz, C1). FT-IR (DMF, cm-1): 1966 (s), 1895
(sh), 1876 (s) (νCO). Anal. Calcd for C32H22IMoO3PPd: C, 47.17; H,
2.72. Found: C, 46.97; H, 2.74. MS (15 V, ESP+) ) 818 (M + H)+.
Kinetic Measurements. Manipulations were carried out under argon.
DMF was degassed and distilled from CaH2 under reduced pressure.
The kinetic experiments were performed by NMR spectroscopy and
UV-visible spectrophotometry, under pseudo first-order conditions,
using at least a 10-fold excess of tributyltinacetylides with respect to
complexes 3 or 14. The temperature in the NMR probe was determined
from the chemical shift difference between the OH and CH2 signals of
a solution of ethylene glycol containing 20% DMSO-d6. The NMR
tube (5 mm) was charged with complex 3 (10-30 mg), 400 µL of
DMF, and 100 µL of DMF-d7, and Bu3Sn-CtC-Ph was added by
syringe. Spectra were collected immediately, using a macro sequence.
The rate of disappearance of the complex 3 was followed by recording
the intensity value of the signal at δ 22.0 ppm. First-order rate constants
(kobs) were obtained by exponential fitting of [3] versus time, using a
nonlinear least-squares regression program. The kinetic runs were
reproducible to within 10%.
[η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoPd(PPh3)CtCPh (5). A Schlenk
flask was loaded with [η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoPd(PPh3)I (3)
(0.52 g, 0.48 mmol), and three cycles of vacuum/argon were performed.
Upon addition of DMF (10 mL), a bright red solution was obtained.
Following dropwise addition of tributyl(phenylethynyl)tin (0.56 g, 1.36
mmol), the color changed to dark brown, and the mixture was stirred
for 1 h. The reaction mixture was transferred into a separatory funnel,
diluted with 50 mL of THF, and extracted with brine (6 × 50 mL).
The organic phase was dried over sodium sulfate, filtered, and
concentrated. Pentane was added to the solution, which was left
overnight at -28 °C, to allow the formation of a precipitate. This was
washed with pentane and dried in a vacuum, yielding 0.50 g (92%) of
product (5) as a brown solid. An analytical sample was obtained by
recrystallization from THF/pentane (vapor diffusion) at room temper-
1
ature. H NMR (CDCl3): δH 8.67-8.59 (m), 7.90-7.62 (m), 7.60-
7.48 (m), 7.45-7.29 (m), 7.27-7.22 (m), 7.21-7.07 (m), 7.05-6.97
(m), 6.94-6.72 (m), 6.31-6.25 (m), 6.12 (t, 1H, J ) 2.1 Hz, Cp-H),
4.74 (t, 1H, J ) 2.1 Hz, Cp-H). 31P NMR (CDCl3): δP 39.72 (d, J )
455 Hz, -PPh2), 28.22 (d, J ) 455 Hz, PPh3). 13C NMR (DMF-d7):
δC 234.1, 226.5, 225.4 (CO), 138.1-124.9 (Ph), 118.7 (d, JC-P ) 9.8
Hz, C2), 115.2 (d, JC-P ) 8.6 Hz, -CtC-Ph), 109.6 (d, JC-P ) 7.3
UV-Visible Kinetics. A series of stock solutions were prepared
by dissolving complex 3 (4.4 mg) or complex 14 (3.5 mg) in dry
degassed DMF in a 25 mL volumetric flask. In a typical run, a quartz
cuvette (1 cm path length) was loaded with 2 mL of a stock solution
and allowed to equilibrate at the appropriate temperature, before addition
of tributyltinacetylide. Bu3SnCtCPh was added (2-15 µL) as neat
liquid, while appropriate amounts (80-500 µL) of a solution of Bu3-
SnCtC(p-ClC6H4) in DMF (70.9 mg in 5 mL) were used. The decrease
in absorbance associated with the reaction was followed with time.
First-order rate constants (kobs) were obtained by fitting the exponential
dependence of absorbance versus time data using a nonlinear least-
squares regression program, which provides values of kobs and A∞.
Fittings of kobs to eq 3 to give K and k2 values were obtained with
nonlinear least-squares calculations carried out by the program Sigma
Plot. The reaction of 3 with Bu3SnCtCPh was followed at 400 nm
for [3] < 2.0 × 10-4 M, and at 490 nm for higher concentrations. The
reaction of 3 with Bu3SnCtC(p-Cl-C6H4) was followed at 420 nm,
and that of 14 with Bu3SnCtCPh at 410 nm. Single kinetic runs were
reproducible to within 5%, and the parameters K and k2 were
reproducible to within 12% on duplication of a full kinetic set.
Hz, C4), 107.2 (dd, J1
) 6.1 Hz, J2
) 28.5 Hz, -CtC-Ph),
C-P
C-P
94.4 (d, JC-P ) 6.1 Hz, C3), 89.4 (d, JC-P ) 11.2 Hz, C5), 55.1 (d,
JC-P ) 43.3 Hz, C1). FT-IR (CH2Cl2, cm-1): 2106 (w) (νCtC), 1949.8
(s), 1869.0 (m), 1842.1 (s) (νCO). Anal. Calcd for C58H42MoO3P2Pd:
C, 66.27; H, 4.03. Found: C, 66.54; H, 4.12. MS (15 V, ESP+) ) 967
(M+ - 3CO).
Bu3Sn-CtC-(C6H4)p-Cl (11b). A mixture of (diethylamino)-
tributylstannane (7.96 g, 0.022 mol) and p-clorophenylacetylene (2.50
g, 0.018 mol) was stirred at room temperature for 1 h. Following
removal in vacuo of the diethylamine formed during the reaction, a
pure product (6.90 g, 94%) was isolated, as a pale yellow oil, by
1
Kugelrohr distillation at 200 °C/10-4 mm Hg. H NMR (CDCl3): δH
7.35 (d, 2H, J ) 8.9 Hz, meta Ph), 7.23 (d, 2H, J ) 8.9 Hz, ortho Ph),
1.61 (p, 6H, J ) 8.6 Hz, CH3-CH2-CH2-CH2-Sn), 1.37 (s, 6H, J )
7.7 Hz, CH3-CH2-CH2-CH2-Sn), 1.05 (t, 6H, J ) 8.3 Hz, CH3-
CH2-CH2-CH2-Sn), 0.92 (t, 9H, J ) 7.1 Hz, CH3-CH2-CH2-CH2-
Sn). 13C NMR (CDCl3): δC 133.69, 133.07, 128.37, 122.51 (Ph), 108.71
(Ph-CtC-Sn), 94.63 (Ph-CtC-Sn), 28.87 (t, JCSn ) 11.3 Hz, CH3-
CH2-CH2-CH2-Sn), 26.93 (t, JCSn ) 29.4 Hz, CH3-CH2-CH2-
CH2-Sn), 13.65 (CH3-CH2-CH2-CH2-Sn), 11.12 (CH3-CH2-
CH2-CH2-Sn). FT-IR (CH2Cl2, cm-1): 2959 (s), 2924 (br), 2872 (m),
2854 (m), 2134 (w), 1848 (s), 1466 (m), 1377 (w), 1207 (w), 1094
(m). MS (15 V, ESP+) ) 449 (M + Na)+. HRMS (m/e): calcd for
C20H31ClSn (M+): 426.1136; found, 426.1084; calcd for C20H30ClSn
(M - H): 425.1058; found, 425.1060.
Acknowledgment. Thanks are given to CNR/RAS (Russian
Academy of Science, Moscow) bilateral agreement and to the
Ministero dell’Universita` e della Ricerca Scientifica e Tecno-
logica, Project “Cooperative Effects in Polyfunctional Organo-
metallic Systems” (COFIN 1999) for financial support. Thanks
are given to Dr. G. Giorgi (Universita` di Siena-Italy) for
obtaining the HRMS spectrum of 11b. This paper is dedicated
with respect and gratitude to Prof. Carlo Floriani, my esteemed
teacher and friend.
[η5-(1-Ph2P-2,4-Ph2)C5H2](CO)3MoPd(µ-I)2PdMo(CO)3[η5-(1-
Ph2P-2,4-Ph2)C5H2] (14). A Schlenk flask was loaded with [η5-(1-
Ph2P-2,4-Ph2)C5H2](CO)3MoI (1)‚0.4CH2Cl2 (0.20 g, 0.28 mmol) and
Pd2(dba)3 (0.14 g, 0.15 mmol). After three cycles of vacuum/argon, 20
mL of THF was added to the flask, and the resulting dark solution was
stirred for 30 min at room temperature. Celite (10 g) was added to the
reaction mixture, the solvent was removed under vacuum, and the
residue was placed on a silica column (30 cm × 3 cm). Elution with
hexane/dichloromethane 4/6 produced a dark red band, which was eluted
and collected. After removal of the solvent, l0.11 g (48%) of (14) as
a dark brown solid was obtained. An analytical sample was obtained
by crystallization from THF/pentane (vapor diffusion) at room tem-
Supporting Information Available: Figure showing plot of
ln[3] versus time for the reaction with 11a, figure showing [3]
versus time in the reaction with 11a in the presence and absence
of PPh3, figure showing UV-vis spectra of complexes 3 and 5
in DMF, figure showing plot of kobs versus [3] for the reaction
with 11a, figures showing 1H and 13C NMR spectra accounting
purity of 11b (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
1
perature. H NMR (CDCl3): δH 8.57-8.45 (m), 7.73-7.48 (m), 7.22
(s), 7.22-7.14 (m), 7.00-6.89 (m), 6.83-6.75 (m), 6.06 (q, 1H, JHH
JA011644P
9
J. AM. CHEM. SOC. VOL. 124, NO. 6, 2002 1071