Mechanistic study on the cross-coupling of alkynyl stannanes with aryl iodides catalyzed by η2-(dimethyl fumarate)palladium(0) complexes with iminophosphine ligands

Article abstract of DOI:10.1039/b300020f

The reactions of [Pd(η²-dmfu)(P-N)] [dmfu = dimethyl fumarate; P-N = 2-(PPh₂)C₆H₄-1-CH=NR, R = C₆H₄OMe-4 (1a), CHMe₂ (2a)] and [Pd(η²-dmfu)(P-N)₂] with IC₆H ₄CF₃-4, ISnBu₃ and PhC≡SnBu₃ have been studied under pseudo-first-order conditions. The oxidative addition of IC₆H₄CF₃-4 yields [PdI(C₆H ₄CF₃-4)(P-N)] (1b or 2b). No reaction takes place with PhC≡CSnBu₃ and also with ISnBu₃ in the presence of an excess of PhC≡CSnBu₃. In the presence of fumaronitrile (fn), 1b and 2b undergo transmetalation by PhC≡CSnBu₃ followed by fast reductive elimination to yield [Pd(η²-fn)(P-N)]. The same reaction sequence occurs for the system [PdI(C₆H₄CF ₃-4)(P-N)]/P-N (1: 1 molar ratio) to give [Pd(η²-fn) (P-N)₂]. The palladium(0) complexes are active catalysts in the cross-coupling of PhC≡CSnBu₃ with aryl iodides ArI (Ar = C ₆H₄CF₃-4, Ph). The catalytic efficiency depends on the complex: [Pd(η²-dmfu)(P-N)₂] > [Pd(η²-dmfu)(P-N)], and on the substituent R: C₆H ₄OMe-4 > CHMe₂. The reactivity and spectroscopic data suggest a catalytic cycle involving initial oxidative addition of ArI to a palladium(0) species, followed by transmetalation of the product and by fast reductive elimination to regenerate the starting palladium(0) compound. For [Pd(η²-dmfu)(P-N)] as catalyst, the oxidative addition is the rate-determining step, while for [Pd(η²-dmfu)(P-N)₂] the oxidative addition and the transmetalation steps occur at comparable rate. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b300020f

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Mechanistic study on the cross-coupling of alkynyl stannanes with
aryl iodides catalyzed by ꢀ2-(dimethyl fumarate)palladium(0)
complexes with iminophosphine ligands
Bruno Crociani,*a Simonetta Antonaroli,a Valentina Beghetto,b Ugo Matteolib and
Alberto Scrivantib
a Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma “Tor Vergata”,
Via della Ricerca Scientifica, 00133 Roma, Italy
b Dipartimento di Chimica, Università di Venezia, Dorsoduro 2137, 30123 Venezia, Italy
Received 2nd January 2003, Accepted 1st April 2003
First published as an Advance Article on the web 15th April 2003
The reactions of [Pd(η2-dmfu)(P–N)] [dmfu = dimethyl fumarate; P–N = 2-(PPh )C H -1-CH᎐NR, R = C H OMe-4
᎐
2
6
4
6
4
2
᎐
(1a), CHMe (2a)] and [Pd(η -dmfu)(P–N) ] with IC H CF -4, ISnBu and PhC᎐CSnBu have been studied under
᎐
2
2
6
4
3
3
3
pseudo-first-order conditions. The oxidative addition of IC6H4CF3-4 yields [PdI(C6H4CF3-4)(P–N)] (1b or 2b). No
᎐
᎐
reaction takes place with PhC᎐CSnBu and also with ISnBu in the presence of an excess of PhC᎐CSnBu . In the
᎐
᎐
3
3
3
᎐
presence of fumaronitrile (fn), 1b and 2b undergo transmetalation by PhC᎐CSnBu followed by fast reductive
᎐
3
elimination to yield [Pd(η2-fn)(P–N)]. The same reaction sequence occurs for the system [PdI(C6H4CF3-4)(P–N)]/P–N
(1 : 1 molar ratio) to give [Pd(η2-fn)(P–N)2]. The palladium(0) complexes are active catalysts in the cross-coupling
᎐
of PhC᎐CSnBu with aryl iodides ArI (Ar = C H CF -4, Ph). The catalytic efficiency depends on the complex:
᎐
3
6
4
3
[Pd(η2-dmfu)(P–N)2] > [Pd(η2-dmfu)(P–N)], and on the substituent R: C6H4OMe-4 > CHMe2. The reactivity and
spectroscopic data suggest a catalytic cycle involving initial oxidative addition of ArI to a palladium(0) species,
followed by transmetalation of the product and by fast reductive elimination to regenerate the starting palladium(0)
compound. For [Pd(η2-dmfu)(P–N)] as catalyst, the oxidative addition is the rate-determining step, while for
[Pd(η2-dmfu)(P–N)2] the oxidative addition and the transmetalation steps occur at comparable rate.
᎐
Ph )C H }(CO) MoPd(C᎐CPh)(PPh )], was recently reported
Introduction
᎐
2
5
2
3
3
to proceed via formation of a reactive solvent-coordinate
species at low initial concentration of the starting complex,
according to eqn. (1):6
The palladium-catalyzed cross-coupling of stannanes with
organic electrophiles (Stille reaction) is a widely used synthetic
method for the formation of carbon–carbon bonds.1 The com-
monly accepted picture of the catalytic cycle involves an initial
oxidative addition of the organic electrophile to a palladium(0)
species followed by transmetalation, trans-to-cis isomerization
if trans-diorganopalladium() intermediates are formed, and
eventually by reductive elimination of the coupling product.1,2
The rate-determining step of the cycle can be either the oxid-
ative addition or the transmetalation or even the reductive
elimination.1c,2,3 Although detailed studies have been carried
out for long time,4 the mechanism of the process is still the
subject of intense research for the complexity of the reactions
involved, which can be variously influenced by the nature of the
ligand, the solvent, the reacting organostannane and carbon
electrophile, and by the presence of additives such as LiCl or
the free ligand.
(1)
On the other hand, also the complex trans-[PdIPh(AsPh3)2] was
found by Amatore to be involved in a slow equilibrium with the
species [PdIPh(solvent)(AsPh3)] (solvent = CDCl3, dmf ) upon
release of AsPh3,7 in agreement with previous kinetic invest-
igations by Farina.1b,4e In a very recent report, however, the
concentration of [PdXR(solvent)(AsPh3)] (X = Cl, I; R = Ph,
C6Cl2F3) was found to be negligible in the presence of free
arsine (for a mixture AsPh3/[PdXR(AsPh3)2] in a 2 : 1 molar
ratio), indicating that under catalytic conditions the predom-
inant pathway involves the associative reaction of the stannane
with the complex [PdXR(AsPh3)2].8
So far, mainly palladium complexes with monodentate
phosphine or arsine ligands have been used in such catalytic
reactions. Bidentate chelating diphosphines usually slow down
the reaction rate,4e,9 although improved rates have also been
observed.10 A few reports dealing with cross-coupling reactions
catalyzed by palladium complexes with bidentate nitrogen
ligands have been published.11 More recently, the main inter-
mediates, namely [PdAr(CH᎐CH )(dppe)] and [Pd(η2-CH ᎐
In recent papers,5 Espinet has proposed the mechanism
described in Scheme 1 for the coupling of ArX (Ar = pentahalo-
phenyl; X = halide, triflate) with RSnBu3 (R = vinyl).
Two transmetalation pathways may be present: step (a)
through an associative SE2 “cyclic” mechanism and step (b)
through an associative “open” mechanism. The SE2 cyclic step
(a) is much favoured in weakly coordinating solvents of low
polarity when X is a good bridging ligand such as a halide. It
leads to a cis highly reactive species which undergoes an
immediate reductive elimination, and is strongly retarded by the
presence of free ligand L. The SE2 open step (b) is favoured
when LЈ is a ligand or a polar coordinating solvent, both
lacking bridging ability, and when X is a good-leaving and
poorly coordinating anionic ligand without bridging ability.
In contrast to this mechanism where the transmetalation steps
proceed without any prior dissociation (or solvent displace-
ment) of the ligand L in the intermediates trans-[PdXArL2],
the transmetalation of [{η5-(1-Ph2P)(2,4-Ph2)C5H2}(CO)3-
᎐
᎐
2
2
CHAr)(dppe)], have been observed and characterized in the
coupling of pentahaloaryl triflates (ArX) with CH ᎐CHSnBu
catalyzed by [PdXAr(dppe)] [dppe = 1,2-bis(diphenylphos-
᎐
2
3
phino)ethane)].12
In a comparative study on the catalytic activity of the
systems [PdCl (η3-C3H5)]2/L–LЈ (1 : 2 molar ratio) [L–LЈ =
5
᎐
MoPdI(PPh )] by PhC᎐CSnBu , leading to [{η -(1-Ph P)(2,4-
2-(PPh )C H -1-CH᎐NC H Ph,
2-(PPh2)C6H4-1-CH2NMe2,
᎐
᎐
3
3
2
2
6
4
2
4
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
2194

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Scheme 1 L = PPh3, AsPh3; LЈ = ligand or solvent.
1,3-(PPh2)2C3H6], Shirakawa noticed that the iminophosphine
system has the higher catalytic efficiency in the coupling of
13
᎐
(2)
4-(trifluoromethyl)iodobenzene with PhC᎐CSnBu . From
᎐
3
the detection of the complexes [PdI(SnBu3)(L–LЈ)] and
᎐
[Pd(C᎐CPh)(SnBu )(L–LЈ)] [L–LЈ
=
2-(PPh )C H -1-CH᎐
᎐
2 6 4
The iminophosphine ligands and the zerovalent complexes
᎐
3
NC2H4Ph] in the 31P NMR spectra of the reaction mixture, a
[Pd(η2-dmfu)(P–N)] used as catalysts (or catalyst precursors)
new catalytic cycle was proposed involving the initial oxidative
are shown in Scheme 3.
᎐
addition of PhC᎐CSnBu to a palladium(0) species Pd(L–LЈ)
᎐
3
generated in situ (Scheme 2).
Scheme 3
The choice of these ligands stems from the fact that the imino
nitrogen substituents R, a para-substituted aryl group in 1 and
a secondary alkyl group in 2, have comparable steric require-
ments but different electronic properties. Where possible, the
reactions involved in the single steps of the cycles in Scheme 1
and Scheme 2 have been examined in thf (or thf-d8) at 25 ЊC,
under pseudo-first-order conditions generally using a palladium
complex/reactant molar ratio of 1 : 10 either in the absence or
in the presence of one equivalent of added iminophosphine.
An initial palladium complex concentration of 2 × 10Ϫ2 mol
dmϪ3 was used. The progress was monitored by IR spectro-
Scheme 2
Following our studies on the catalytic properties of the
zerovalent complexes [Pd(η2-dmfu)(P–N)] [dmfu = dimethyl
fumarate; P–N = 2-(PPh )C H -1-CH᎐NR, R = alkyl and aryl
᎐
2
6
4
group],14 which can be easily prepared and stored without
appreciable decomposition in the presence of air,15 we have
recently reported that these complexes are quite active catalysts
(or catalyst precursors) in the coupling of organostannanes
with aryl halides.16 The catalytic efficiency is retained for
prolonged time and increases considerably on going from alkyl
to aryl N–R substituents, and also in the presence of the free
iminophosphine for a P–N/[Pd(η2-dmfu)(P–N)] molar ratio ≥ 1.
These observations prompted us to carry out a mechanistic
investigation in order to ascertain (i) the actual catalytic cycle
of the reaction and (ii) the origin of the long life-time of the
catalyst.
1
scopy in the range 2400–1500 cmϪ1, and by H and 31P NMR
spectroscopy.
Reactions in the absence of added iminophosphine
Since the complexes [Pd(η2-dmfu)(P–N)] undergo oxidative
addition of organic halides and also olefin substitution by
π-accepting unsaturated ligands,15 we have examined the
possible reactions of 1a and 2a with the reactants and products
of the cross-coupling catalysis [eqn. (2)].
Oxidative additions and olefin substitutions. The occurrence
of these reactions can be easily detected in the IR spectra
as they involve the formation of free dmfu characterized by a
ν(C᎐O) band at 1726 cmϪ1 (cf. the corresponding ν(C᎐O) band
Results and discussion
The cross-coupling of 4-(trifluoromethyl)iodobenzene with
tributyl(phenylethynyl)tin was chosen as a model reaction for
the mechanistic study [eqn. (2)]:
᎐
᎐
at 1687 cmϪ1 for 1a and at 1688 cmϪ1 for 2a). The reactions
studied are summarized in Scheme 4.
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
2195

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Scheme 4
Scheme 5
The reaction with a ten-fold excess of IC6H4CF3-4 goes to
completion in ca. 40 min for 1a, whereas it is much slower for
2a (ca. 15% progress in 40 min). The products 1b and 2b were
isolated and characterized (see Experimental). Although their
Transmetalation and reductive elimination. As is generally
accepted for palladium complexes containing chelating bi-
dentate ligands,4d,12,13 the transmetalation of [PdI(C6H4CF3-
᎐
4)(P–N)] (1b or 2b) with PhC᎐CSnBu leads to a labile inter-
᎐
3
1
᎐
coordination geometry cannot be assessed from the H and
mediate [Pd(C᎐CPh)(C H CF -4)(P–N)]. The subsequent
᎐
6
4
3
31P NMR data, we propose a configuration with the aryl ligand
trans to the imino nitrogen on the basis of trans influence
considerations, the aryl group and the phosphorus atom being
the strongest donors,17 and X-ray structural analyses of related
complexes [PdX(R)(L–LЈ)] (X = Cl, I; R = alkyl or aryl group;
L–LЈ = aminophosphine or iminophosphine).18
reductive elimination of PhC᎐CC H CF -4 generates a co-
᎐
᎐
6
4
3
ordinatively unsaturated palladium(0) transient, Pd(P–N),
which may undergo different reactions with the species present
in solution, such as η2-coordination of unsaturated ligands4d,12
13
᎐
or oxidative addition of ISnBu3 and/or PhC᎐CSnBu . We
᎐
3
have therefore examined two different types of reactions as
No reaction is observed to occur between 1a or 2a and a
reported in Scheme 5.
The reaction of 1b or 2b with PhC᎐CSnBu was first studied
᎐
᎐
᎐
ten-fold excess of PhC᎐CSnBu or PhC᎐CC H CF -4. In the
᎐
᎐
᎐
3
6
4
3
3
case of 1a, only a slight decomposition of the complex takes
place after prolonged time (3–5 h). This result shows that these
alkynes are unable to displace the η2-bound olefin and also
that the alkynylstannane does not oxidatively add to [Pd(η2-
dmfu)(P–N)].
The complex 1a reacts with a ten-fold excess of ISnBu3 at
a lower rate than with IC6H4CF3-4. After 2 h, the 31P NMR
spectrum of the reaction mixture shows the presence of at
least four products [δ(P31) signals as singlets at 40.5, 32.8, 31.1
and 15.1 ppm] which could not be isolated and characterized.
The corresponding reaction with 2a is much slower and leads
to a mixture of several products. On the basis of the course of
the oxidative addition of ISnBu3 to palladium(0) substrates,13
these reactions presumably involve the initial and slow form-
ation of labile derivatives [PdI(SnBu3)(P–N)] which readily
decompose in solution.
in the presence of an activated olefin of enhanced π-accepting
properties, such as fumaronitrile (fn), since the products [Pd(η2-
fn)(P–N)] (1c, 2c), independently prepared,15 do not react with
᎐
either ISnBu or PhC᎐CSnBu . As indicated by the IR and the
᎐
3
3
multinuclear (1H, 31P, 119Sn) NMR spectra of the mixtures
᎐
[PdI(C H CF -4)(P–N)]/PhC᎐CSnBu /fn (1 : 10 : 1.5 molar
᎐
6
4
3
3
ratio), the reaction yields smoothly and quantitatively the
2
Ϫ1
products [Pd(η -fn)(P–N)] [1c: ν(C᎐N) at 2202 cm and δ(31P)
᎐
᎐
at 23.0 ppm; 2c: ν(C᎐N) at 2195 cm and δ(31P) at 22.3 ppm],
Ϫ1
᎐
᎐
ISnBu3 [δ(119Sn) at 79.5 ppm] and PhC᎐CC H CF -4 [ν(C᎐C) at
᎐
᎐
᎐
᎐
6
4
3
2220 cmϪ1], no intermediate or side-product being detected.
᎐
This implies that the reductive elimination of PhC᎐CC H CF -
4 from the proposed intermediate [Pd(C᎐CPh)(C H CF -4)-
᎐
6
4
3
᎐
᎐
6
4
3
(P–N)] is much faster than the initial transmetalation step. The
overall rate depends also on the iminophosphine nitrogen sub-
stituent in the order R = C6H4OMe-4 > CHMe2 as estimated by
the completion times of ca. 15 min for 1b and of ca. 70 min for
2b. Quite similar results are obtained in the presence of a less
No reaction of 1a or 2a with ISnBu3 is observed in the
᎐
presence of an excess of PhC᎐CSnBu when the reactants
᎐
3
2
᎐
[Pd(η -dmfu)(P–N)]/ISnBu /PhC᎐CSnBu are mixed together
π-accepting olefin such as dmfu. As an example, the reaction of
᎐
3
3
᎐
in a 1 : 5 : 10 molar ratio.
2b with PhC᎐CSnBu and dmfu (1 : 10 : 1.5 molar ratio) goes to
᎐
3
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
2196

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Scheme 6
completion in ca. 65 min and yields quantitatively the product
derivative 2a upon addition of dmfu, in agreement with the
observed lack of reaction of 1a or 2a with ISnBu3 in the pres-
2a [ν(C᎐O) at 1688 cmϪ1 and δ(31P) at 22.9 ppm] along with
᎐
᎐
᎐
ISnBu and PhC᎐CC H CF -4. No side-product or intermedi-
ence of an excess of PhC᎐CSnBu .
᎐
᎐
3
6
᎐
4
3
3
ate of the type [Pd(C᎐CPh)(C H CF -4)(P–N)] is observed dur-
᎐
6
4
3
ing the course of the reaction, indicating that also in this case
the reductive elimination is faster than the transmetalation step.
Reactions in the presence of added iminophosphine
Like the analogous derivatives [Pd(η2-fn)(P–N)],15 the com-
plexes 1a, 2a react with one equivalent of iminophosphine
according to eqn. (3):
The complexes [PdI(C6H4CF3-4)(P–N)] were then reacted
᎐
with an excess of PhC᎐CSnBu (Pd/Sn 1 : 11 molar ratio)
᎐
3
without any added olefin. For 1b, a fast reaction took place,
followed by precipitation of a red–brown solid which prevented
any spectroscopic analysis of the mixture. For 2b, the reaction
was somewhat slower but no precipitation was observed. After
completion (ca. 70 min), two products were formed in a ratio of
3.2 : 1. The major product is characterized by the following
(3)
NMR spectral data: a δ(N᎐CH) doublet at 8.45 ppm [J(PH) 2.7
᎐
Hz], a δ(CHMe2) septet at 4.77 ppm, a δ(31P) singlet at 33.2
ppm, and a δ(119Sn) doublet at Ϫ7.8 ppm [J(PSn) 25 Hz]. For
the minor product, the corresponding NMR signals consist of
Although being slow (on the NMR time scale) and much
shifted to the right (>95%), such an equilibrium prevents the
isolation of 1f and 2f as pure compounds. However, these
products can be conveniently characterized in solution by
spectroscopic methods (see Experimental). Both complexes
contain two P-monodentate iminophosphines as indicated by
the close proximity of the imino proton signals to those of the
a δ(N᎐CH) doublet at 8.33 ppm [J(PH) 3.2 Hz], a δ(CHMe )
᎐
2
septet at 5.32 ppm, a δ(31P) singlet at 39.5 ppm, and a δ(119Sn)
doublet at 35.5 ppm [J(PSn) 123 Hz]. The 119Sn NMR spectrum
shows also a singlet at 79.5 ppm due to ISnBu3. The observed
NMR spectral data are very close to those reported for the
᎐
labile derivatives [Pd(C᎐CPh)(SnBu )(P–N)] and [PdI(SnBu )-
᎐
3
3
free ligands [δ(N᎐CH) at 9.19 ppm in 1f and at 9.32 ppm in 1;
(P–N)] [P–N = 2-(PPh )C H -1-CH᎐NC H Ph], obtained in
᎐
᎐
2
6
4
2
4
᎐
δ(N᎐CH) at 9.34 ppm in 2f and at 9.02 ppm in 2], and an
solution from the oxidative addition of PhC᎐CSnBu and
᎐
᎐
3
η2-bound olefin as indicated by the low-frequency shift of
ISnBu3, respectively, to a palladium(0) species generated in situ
by reacting the mixture [PdCl(η3-C3H5)]2/P–N (1 : 2 molar
ratio) with NaCH(CO2Me)2.13 Accordingly, the major product
is formulated as complex 2d and the minor one as complex 2e,
which originate from the corresponding reactions of Pd(P–N)
35–40 cmϪ1 (relative to free dmfu) for the ν(C᎐O) bands and by
᎐
the considerable shielding of 2.8–3.0 ppm (relative to free dmfu)
for the olefin protons, which are detected as an AAЈ multiplet of
an AAЈXXЈ spin system.
formed in the reductive elimination of the transmetalated
᎐
intermediate [Pd(C᎐CPh)(C H CF -4)(P–N)] (P–N
=
2).
Oxidative additions and olefin substitutions. These reactions
are carried out on complexes 1f and 2f prepared in solution
[eqn. (3)] and are summarized in Scheme 6.
᎐
6
4
3
Furthermore, the absorption at 2088 cmϪ1 in the IR spectrum
᎐
of the reaction mixture is assigned to a C᎐C stretching
᎐
᎐
vibration of 2d as it falls in the range typical of the ν(C᎐C)
The oxidative addition of 4-(trifluoromethyl)iodobenzene to
1f or 2f occurs at higher rates than the corresponding reaction
with 1a or 2a (Scheme 4). For an initial Pd/IC6H4CF3-4 molar
ratio of 1 : 10, the completion times are of ca. 20 min for 1f and
of ca. 50 min for 2f. The reaction with 2f yields the product 2b
along with the free iminophosphine 2, whereas the reaction
with 1f yields an equilibrium mixture of 1g, 1b and the free
iminophosphine 1. The same equilibrium mixture can be
obtained from the reaction of 1b with one equivalent of the
ligand 1.
᎐
bands for alkynylpalladium() complexes.19 The complexes 2d
and 2e are in equilibrium with each other as indicated by the
dependence of the 2d/2e molar ratio on the relative concen-
tration of PhC᎐CSnBu and ISnBu . The equilibrium shifts in
favour of 2d when the concentration of PhC᎐CSnBu is
increased: upon addition of five equivalents of PhC᎐CSnBu to
the equilibrium mixture resulting from the reaction of 2b with
PhC᎐CSnBu (1 : 11), the molar ratio 2d/2e changes from 3.2 : 1
to 5.0 : 1. Conversely, the equilibrium shifts in favour of 2e
when the concentration of ISnBu3 is increased: upon addition
of four equivalents of ISnBu3 to the equilibrium mixture result-
᎐
᎐
3
3
᎐
᎐
3
᎐
᎐
3
᎐
᎐
3
The complex 1g was characterized in solution as containing
two trans P-monodentate iminophosphine ligands from its
NMR data: δ(31P) as a rather broad singlet at 19.6 ppm and
᎐
ing from the reaction of 2b with PhC᎐CSnBu (1 : 11), the
᎐
3
molar ratio 2d/2e changes from 3.2 : 1 to 0.16 : 1. When
dimethyl fumarate is added to the reaction mixtures (Pd/dmfu =
1 : 1.5), both products 2d and 2e disappear with concomitant
formation of the complex 2a.20 These results indicate that a
complex equilibrium system is present in the above mixtures
which shifts completely towards the more stable palladium(0)
δ(N᎐CH) as a rather broad signal at 9.15 ppm (cf. the corre-
᎐
sponding proton resonance at 9.32 ppm in the free ligand 1 and
at 8.52 ppm in the complex 1b where the iminophosphine is
P,N-chelate to the central metal). The broadness of the NMR
signals of 1g, as compared to the sharpness of the related
signals of 1b and 1 in the equilibrium mixture, indicates that the
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
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Scheme 7
Table
1
Cross-coupling of iodobenzene with tributyl(phenyl-
equilibrium is slow on the NMR time scale, and that the
complex 1g has some fluxional behaviour in solution. It is to be
noted that a similar equilibrium does not occur for complex 2b
under comparable experimental conditions, in agreement with
the better ligating properties of the imino nitrogen carrying the
electron-donating isopropyl substituent.
ethynyl)tin: catalytic data at 50 ЊCa
Conversion (%) at different times
Run
Catalyst
1 h
2 h
4 h
24 h
In the 31P NMR spectra of the reaction mixtures, however,
a weak singlet in the range 28.5–29.5 ppm is detected, which
is assigned to a small amount of the phosphonium salt
1
2
3
4
1a
50
31
89
46
65
50
91
72
84
59
95
79
94
82
99
90
2a
1a/1b
2a/2b
[PPh (C H CF -4)(C H -2-CH᎐NR)]I formed in the reaction
᎐
2
6
4
3
6
4
a
3
᎐
of IC6H4CF3-4 with P–N and/or in the interaction of
[PdI(C6H4CF3-4)(P–N)] with P–N.21
Solvent: thf (5 cm ); PhI (1.35 mmol); PhC᎐CSnBu3 (1.35 mmol);
᎐
palladium complex (0.0065 mmol); substrate/Pd = 200/1 (mol/mol).
b Palladium complex/ligand = 1/1 (mol/mol).
As in the case of complexes 1a and 2a, the η2-bound dmfu in
᎐
1f and 2f is not displaced by alkynes such as PhC᎐CSnBu or
᎐
3
᎐
PhC᎐CC H CF -4. On the other hand, the lack of oxidative
addition with PhC᎐CSnBu rules out the catalytic cycle of
Scheme 2 for the cross-coupling reaction 2 when the systems
᎐
6
4
3
times are of ca. 30 min for 1b/1 and of ca. 75 min for 2b/2. If
compared with the corresponding reactions with 1b and 2b in
the absence of added iminophosphine, one can see that the
overall rate decreases for the 1b/1 system, whereas it remains
almost unchanged for 2b/2. Such a retarding effect is to be
related to the occurrence of the equilibrium with the species 1g
in the initial mixture 1b/1 (1 : 1), as confirmed by the fact
that when the reaction is carried out with a 1b/1 molar ratio of
1 : 2, other things being equal, the completion time increases to
40 min.
᎐
᎐
3
[Pd(η2-dmfu)(P–N)]/P–N are used as catalysts.16
No reaction is observed to occur between 1f or 2f and ISnBu3
if an excess of PhC᎐CSnBu is present (1f or 2f/ISnBu3/
PhC᎐CSnBu molar ratio of 1 : 5 : 10) even though the
complexes 1f and 2f react with ISnBu3 at higher rates than 1a
and 2a, yielding a mixture of unidentified products. It is likely
that also in this case the initial oxidative addition of ISnBu3
is followed by extensive decomposition which prevents the
characterization of the products.
᎐
᎐
3
᎐
᎐
3
Catalytic reactions
Transmetalation and reductive elimination. The complexes
[PdI(C6H4CF3-4)(P–N)] (1b or 2b) in the presence of 1 equiv-
alent of the corresponding iminophosphine P–N (1 or 2) and
Two sets of catalytic experiments have been examined. In the
first set, the reaction 2 was carried out with a catalytic amount
(5%) of the palladium complex at 25 ЊC in thf (or thf-d8). The
reaction progress was monitored by IR spectroscopy. 31P NMR
spectra of the reaction mixtures were also run at different times
in order to identify the palladium complexes involved in the
rate-determining steps. In the second set, the cross-coupling of
1.5 equivalents of fumaronitrile react with a ten-fold excess of
᎐
PhC᎐CSnBu as shown in Scheme 7.
᎐
3
᎐
In addition to ISnBu and PhC᎐CC H CF -4, the products
᎐
3
6
4
3
consist of an equilibrium mixture of the complexes [Pd(η2-fn)-
(P–N)2], [Pd(η2-fn)(P–N)] and of the free ligand P–N. For
iodobenzene with PhC᎐CSnBu was carried out with a catalytic
᎐
᎐
3
1
P–N = 1, the equilibrium is rather fast (on the H NMR time
amount (0.5%) of the palladium complex at 50 ЊC in thf. The
reaction progress was monitored by GLC analysis of the
mixture at different times. The results of the latter experiments
are listed in Table 1.
scale) and almost completely shifted towards the product 1h, as
previously reported.15 Thus, in thf-d8 at 25 ЊC, the imino proton
and the olefin protons resonate as broad signals at 9.0 and 3.0
ppm, respectively, while in the 31P NMR spectrum only a singlet
at 20.8 ppm is observed, no signals due to 1c and 1 being
detected. For P–N = 2, the equilibrium is also rather fast (on the
NMR time scale) but it involves comparable amounts of the
Cross-coupling reactions catalyzed by [Pd(ꢀ2-dmfu)(P–N)].
The rate of reaction 2 catalyzed by 1a or 2a depends markedly
on the imino nitrogen substituent in the order R = C6H4OMe-4
> CHMe2 as indicated by the different times required to reach a
50% conversion: 115 min for 1a and 510 min for 2a. The greater
catalytic efficiency of 1a is also evident from a comparison of
the runs 1 and 2 in Table 1. On the basis of the reactivity results
reported above, a catalytic cycle is proposed in which the initial
oxidative addition of the aryl iodide to [Pd(η2-dmfu)(P–N)] is
the rate-determining step, as it is followed by a faster trans-
metalation of [PdI(C6H4CF3-4)(P–N)] and by an even faster
reductive elimination of the transmetalated intermediate to
yield the coupling product and regenerate the starting
palladium(0) complex. This is confirmed by the spectral data
of the reaction mixture. When reaction 2 is catalyzed by 2a, the
31P NMR spectra show that only complex 2a is present in the
mixture up to 50% conversion (Fig. 1b). When reaction 2 is
catalyzed by 1a, complex 1a is the predominant species in the
mixture up to 50% conversion (Fig. 1a) as it is accompanied by
1
three species 2h, 2c and 2. In the H NMR spectrum at 25 ЊC,
broad resonances are present at 9.1 ppm for the imino protons,
at 3.6 ppm for the isopropyl CHMe2 protons, and at 2,9 ppm for
the olefin protons, whereas, in the 31P NMR spectrum, three
broad singlets appear at 22.3 ppm (2c), 19.5 ppm (2h) and
Ϫ11.9 ppm (2). At lower temperatures, however, the equi-
librium shifts toward the left in favour of the complex 2h.22
During the course of the reactions, the 31P NMR spectra
show also the presence of a weak singlet in the range 28.5–29.5
ppm (assigned to the phosphonium salt [PPh2(C6H4CF3-4)-
(C H -2-CH᎐NR)]I),21 the intensity of which remains
᎐
6
4
practically constant throughout, but give no evidence for
transmetalated intermediates. This result indicates that also in
the presence of added iminophosphine the initial trans-
metalation step is followed by a much faster reductive
elimination to yield the palladium(0) products. The completion
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
2198

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metalation step involving the mixture [PdI(C6H4CF3-4)(P–N)]/
P–N (1 : 1 molar ratio) occur at comparable rates, as suggested
by the comparable completion times. As a matter of fact, in the
31P NMR spectra of the reaction mixture with the catalytic
system 2a/2 (up to 50% conversion) comparable amounts of the
complex 2b (24.1 ppm), 2f (19.7 ppm) and of the free ligand
2 (Ϫ11.9 ppm) are detected, while in those with the catalytic
system 1a/1 a small amount of the complex 1g is also detected
at 19.6 ppm (Fig. 2). In both cases, the 31P NMR spectra show
also the presence of a singlet in the range 28.5–29.5 ppm,
assigned to the phosphonium salt [PPh2(C6H4CF3-4)(C6H4-
2-CH᎐NR)]I.21
᎐
Fig. 2 31P NMR spectra of the reaction mixture (at ca. 30%
᎐
conversion) of the coupling of IC6H4CF3-4 with PhC᎐CSnBu3 in thf in
᎐
Fig. 1 31P NMR spectra of the reaction mixture (at ca. 40%
the presence of the system 1a/1 (1 : 1 molar ratio). * signal assigned to the
phosphonium salt [PPh2(C6H4CF3-4)(C6H4-2-CH᎐NC6H4OCH3-4]I.
᎐
conversion) of the coupling of IC6H4CF3-4 with PhC᎐CSnBu3 in thf:
᎐
᎐
(a) in the presence of complex 1a; (b) in the presence of complex 2a.
Consistently, in the IR spectra a substantial amount of the
free dmfu (liberated in the oxidative addition step leading to
[PdI(C6H4CF3-4)(P–N)]) is present together with the complex
[Pd(η2-dmfu)(P–N)2]. Thus, the higher catalytic activity of
the system 1a/1 results from the fact that both the oxidative
addition and transmetalation steps are faster in 1a/1 than in
2a/2.
a small amount of 1b and trace amounts of decomposition
products.
Consistently, a weak ν(C᎐O) band of the free dmfu at 1726
᎐
cmϪ1 is detected in the IR spectra along with a strong ν(C᎐O)
᎐
band of 1a at 1687 cmϪ1. The presence of 1b is due to the fact
᎐
that its transmetalation with PhC᎐CSnBu is not exceedingly
᎐
3
faster than the oxidative addition of IC6H4CF3-4 to 1a (see the
completion times for these reactions reported above).
The greater catalytic efficiency of the systems 1a/1 and 2a/2
compared to that of 1a and 2a, respectively, and the increasing
The rate-determining oxidative addition accounts for the
different catalytic activity of the complexes 1a and 2a, the
oxidative addition of IC6H4CF3-4 to 1a being much faster than
that to 2a, and also for the solvent effect observed in the cross-
reaction rate on going from 2a/2 to 1a/1 are also observed in the
᎐
cross-coupling of iodobenzene with PhC᎐CSnBu at 50 ЊC
᎐
3
(runs 3 and 4 of Table 1).
16
᎐
coupling of iodobenzene with PhC᎐CSnBu catalyzed by 1a.
᎐
3
The increasing rate in solvents of increasing polarity can be
related to an increased rate of the oxidative addition step, as
reported by Farina for similar reactions with [Pd(AsPh3)4].4f
Conclusion
From the observed reactivity and catalytic data it can be
concluded that the cross-coupling of aryl iodides with alkynyl
stannanes in the presence of the zerovalent derivatives [Pd(η2-
dmfu)(P–N)] or [Pd(η2-dmfu)(P–N)2] proceeds through a
catalytic cycle where the initial oxidative addition of the aryl
iodide to the palladium(0) complex is followed by transmetal-
ation of the product and by fast reductive elimination to
regenerate the palladium(0) species. The increasing catalytic
activity on going from [Pd(η2-dmfu)(P–N)] to [Pd(η2-dmfu)-
(P–N)2] is due to the increasing rate of the oxidative addition
step, which is rate-determining for [Pd(η2-dmfu)(P–N)]. For
[Pd(η2-dmfu)(P–N)2] the oxidative addition and the trans-
metalation steps proceed at comparable rates. The greater
catalytic efficiency of the complexes with P–N = 1 (R =
C6H4OMe-4) compared to that of the corresponding complexes
Cross-coupling reactions catalyzed by [Pd(ꢀ2-dmfu)(P–N)]/
P–N (1 : 1 molar ratio). With the systems 1a/1 or 2a/2 the
reaction 2 proceeds at a faster rate than with the complexes 1a
or 2a alone. Also in this case, the rate depends on the imino
nitrogen substituent (R = C6H4OMe-4 > CHMe2). For 1a/1, a
50% conversion is reached after 75 min from the mixing of
the reactants, whereas for 2a/2 the time required is 290 min.
The catalytic cycle proposed for the systems 1a/1 and 2a/2 is
analogous to that proposed for the reaction 2 catalyzed by 1a
and 2a (see the Results and discussion). In the case of the
systems [Pd(η2-dmfu)(P–N)]/P–N, however, the oxidative
addition of IC6H4CF3-4 to [Pd η2-dmfu)(P–N)2] and the trans-
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
2199

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with P–N = 2 (R = CHMe2) results from the fact that both the
oxidative addition and transmetalation steps are faster for P–N
= 1 than for P–N = 2. A detailed kinetic investigation of these
reactions is now in progress.
90 min for 2a). The solvent was then removed at reduced
pressure, and the solid residue was exctracted with CH2Cl2
(2 × 20 cm3). The solution was filtered and evaporated to about
3 cm3. Upon addition of Et2O, the product precipitated as a
yellow (1b) or pale-yellow (2b) solid. Both compounds were
further purified by recrystallization from CH2Cl2/Et2O.
The prolonged catalytic activity of [Pd(η2-dmfu)(P–N)] and
[Pd(η2-dmfu)(P–N)2] can be explained by the lack of reaction
of these zerovalent complexes with ISnBu3 in the presence of
Complex 1b (0.580 g, 75%): found: C 51.45, H 3.33, N 1.80%
᎐
PhC᎐CSnBu .
– C33H26F3INOPPd requires C 51.22, H 3.39, N 1.81%; νmax/
᎐
3
4
cmϪ1 (C᎐N) 1612mw (Nujol); δ (thf-d8) 8.49 (1 H, d, J(PH)
᎐
H
2.2 Hz, N᎐CH), 7.06–7.00 (2 H, m, m-H of C H OMe-4),
6.94–6.88 (2 H, m, m-H of C6H4CF3-4), 3.95 (3 H, s, OCH3);
δP (thf-d8) 23.2 (s).
᎐
6
4
Experimental
General
Complex 2b (0.504 g, 71%): found: C 49.30, H 3.60, N 2.00%
1H, 31P and 119Sn NMR spectra were recorded on a Bruker
AM400 spectrometer operating at 400.13, 161.98 and 149.21
MHz, respectively. Chemical shifts are reported in ppm down-
– C29H26F3INPPd requires C 49.07, 3.69, N 1.97%; νmax/cmϪ1
(C᎐N) 1620m (Nujol); δ (thf-d ) 8.45 (1 H, s, N᎐CH),
᎐
᎐
H
8
6.95–6.89 (2 H, m, m-H of C6H4CF3-4), 5.75 (1 H, spt, 3J(HH)
6.6 Hz, CHMe2), 1.36 (6 H, d, 3J(HH) 6.6 Hz, CH3); δP (thf-d8)
24.1 (s).
1
field from SiMe4 for H, from H3PO4 as external standard for
31P, and from SnMe4 as external standard for 119Sn. The spectra
were recorded at 25 ЊC except when noted. IR spectra were
carried out using a Perkin-Elmer 983G spectrophotometer. All
the reactions and catalytic experiments were carried out under
᎐
Reactions of [PdI(C H CF -4)(P–N)] (2b) with PhC᎐CSnBu .
᎐
6
4
3
3
In the presence of fn. For IR measurements, the stannane
N2. The solvent thf was distilled from sodium benzophenone.
᎐
PhC᎐CSnBu (0.35 ml, 1 mmol) was added to a solution of 2b
᎐
3
᎐
The compounds PhC᎐CSnBu , ISnBu , dimethyl fumarate and
᎐
3
3
(71.0 mg, 0.1 mmol) and fn (11.7 mg, 0.15 mmol) in thf (5 ml),
in a round-bottom flask thermostatted at 25 ЊC. The progress of
the reaction was monitored by IR spectra of the mixture at
different times in the range 2400–2000 cmϪ1, following the
fumaronitrile are commercially available and were used without
further purification. The aryl iodides IC6H4CF3-4 and PhI were
distilled at reduced pressure before use. The iminophosphine 1
and the complexes 1a and 1c were prepared as reported in the
increase in intensity of the ν(C᎐N) band of 2c at 2195 cmϪ1. For
᎐
literature.15 The alkyne PhC᎐CC H CF -4 was prepared by a
᎐
᎐
᎐
6
4
3
31P and Sn NMR measurements, PhC᎐CSnBu (0.14 ml, 0.4
119
᎐
published method.13
᎐
3
mmol) was added to a solution of 2b (28.4 mg, 0.04 mmol) and
fn (4.7 mg, 0.06 mmol) in 2 ml of a mixture of thf/thf-d8 (3 : 1
v/v). The progress of the reaction was monitored by 31P NMR
spectra of the solution at different times, following the increase
in intensity of the 2c singlet at 22.3 ppm and the concomitant
decrease in intensity of the 2b singlet at 24.1 ppm. No other
signal was detected during the course of the reaction. When
the reaction was complete, the 119Sn NMR spectrum showed a
signal at 79.5 ppm due to ISnBu3.
Synthesis of N-(2-(diphenylphosphino)benzylidene)(isopropyl)-
amine (2). The 2-(diphenylphosphino)benzaldehyde23 (4.35 g,
15 mmol) and isopropylamine (2.22 g, 37.5 mmol) were
dissolved in 160 cm3 of the solvent mixture MeOH/CH2Cl2
(3 : 1 v/v). When the condensation was complete (3 h at room
temperature), the reaction mixture was worked up as described
15
for the preparation of 1 to give the product as a pale-yellow
microcrystalline solid (4.13 g, 83%); found: C 79.92, H 6.60, N
4.26% – C22H22NP requires C 79.73, H 6.69, N 4.23%; νmax/cmϪ1
In the presence of dmfu. The reactions were carried out under
the same experimental conditions as above, using dmfu (21.6
mg, 0.15 mmol, for IR measurements and 8.6 mg, 0.06 mmol,
for 31P NMR measurements). The reaction progress was
monitored by IR spectra at different times in the range 2300–
4
(C᎐N) 1629s (thf ); δ (thf-d8) 9.01 (1 H, d, J(PH) 4.8 Hz,
᎐
H
3
N᎐CH), 3.52 (1 H, spt, J(HH) 6.3 Hz, CHMe ), 1.20 (6 H, d,
᎐
2
3J(HH) 6.3 Hz, CH3); δP (thf-d8) Ϫ11.9 (s).
1600 cmϪ1, following the increase in intensity of the ν(C᎐O)
Synthesis of [Pd(ꢀ2-ol)(P–N)] [ol ؍ dmfu (2a), fn (2c)]. The
complexes 2a and 2c were prepared from the reaction of
[Pd(η3-C3H5)(P–N)]BF4 (P–N = 2, 1 mmol) with a moderate
excess of diethylamine (0.266 g, 2.5 mmol) in the presence of
the appropriate olefin (1.2 mmol) as described in the literature
for related compounds.15
᎐
band of 2a at 1688 cmϪ1 and of the ν(C᎐C) band of
᎐
᎐
PhC᎐CC H CF -4 at 2220 cmϪ1. The 31P NMR spectra at
᎐
᎐
6
4
3
different times showed the presence of only a singlet of increas-
ing intensity at 22.9 ppm (2a) and a singlet of decreasing
intensity at 24.1 ppm (2b).
In the absence of olefin. For IR measurements, the reaction
was carried out under similar experimental conditions as
Complex 2a (0.471 g, 81%): found: C 57.50, H 5.10, N 2.42%
– C28H30NO4PPd requires C 57.79, H 5.20, N 2.41%; νmax/cmϪ1
(C᎐O) 1688s, (C᎐N) 1622mw (thf ); δ (thf-d8) 8.57 (1 H, d,
᎐
described above, by adding PhC᎐CSnBu (0.385 ml, 1.1 mmol)
᎐
3
᎐
᎐
᎐
H
to the solution of 2b (71.0 mg, 0.1 mmol). For 31P and 119Sn
NMR measurements, the stannane (58 µl, 0.165 mmol) was
added to a solution of 2b (10.6 mg, 0.015 mmol) in thf-d8
(0.75 ml). In the IR spectra, we observed a weak band of
3
3
4J(PH) 3.4 Hz, N᎐CH), 4.18 (1 H, dd, J(HH) 9.9 Hz, J(PH)
2.7 Hz, olefin CH trans to N), 3.86–3.78 (overlapping signals,
2 H, dd, 3J(HH) = 3J(PH) 10.0 Hz; olefin CH trans to P, and spt,
3J(HH) 6.5 Hz; CHMe2), 3.65 (3 H, s, OCH3), 3.24 (3 H, s,
OCH3), 1.46 (3 H, d, 3J(HH) 6.5 Hz, CH3), 1.44 (3 H, d, 3J(HH)
6.5 Hz, CH3); δP (thf-d8) 22.9 (s).
increasing intensity at 2220 cmϪ1 [ν(C᎐C) of PhC᎐CC H CF -4]
᎐
᎐
᎐
᎐
6
4
3
᎐
and a medium–weak band at 2088 cmϪ1 assigned to a ν(C᎐C)
᎐
vibration of 2d, the intensity of which initially increased and
then slowly decreased to a constant value. In the 31P NMR
spectra, only a singlet at 33.2 ppm (2d) and a singlet at 39.5 ppm
(2e) were detected, along with a singlet at 24.1 ppm (2b) which
decreased progressively in intensity and eventually disappeared.
After completion (ca. 70 min), the 119Sn NMR spectrum
showed a singlet at 79.5 ppm (ISnBu3), a doublet at Ϫ 7.8 ppm
(2d) and a doublet at 35.5 ppm (2e). When the reactions were
complete, addition of dmfu ( 21.6 mg, 0.15 mmol, to the IR
solution and 3.3 mg, 0.023 mmol, to the 31P NMR solution)
caused the disappearance of the 2d and 2e signals and the
Complex 2c (0.480 g, 93%): found: C 60.62, H 4.75, N 8.10%
– C26H24N3PPd requires C 60.53, H 4.70, N 8.15%; νmax/cmϪ1
᎐
(C᎐N) 2197s, (C᎐N) 1623m (Nujol); δ (thf-d8) 8.66 (1 H, d,
᎐
᎐
H
4J(PH) 3.3 Hz, N᎐CH), 3.88 (1 H, spt, 3J(HH) 6.4 Hz, CHMe ),
᎐
2
3.33 (1 H, dd, 3J(HH) 9.5 Hz, 3J(PH) 3.4 Hz, olefin CH trans to
3
3
N), 2.90 (1 H, dd, J(HH) = J(PH) 9.6 Hz, olefin CH trans
to P), 1.50 (3 H, d, 3J(HH) 6.4 Hz, CH3), 1.47 (3 H, d, 3J(HH)
6.4 Hz, CH3); δP (thf-d8) 22.3 (s).
Synthesis of [PdI(C6H4CF3–4)(P–N)] (1b, 2b). The complex
1a (0.646 g, 1 mmol) or 2a (0.582 g, 1 mmol) and the aryl iodide
IC6H4CF3-4 (0.410 g, 1.5 mmol) were dissolved in toluene
(80 cm3). The mixture was heated at 70 ЊC ( 40 min for 1a, or
concomitant appearance of the characteristic signals of 2a
1
[ν(C᎐O) at 1688 cmϪ1 and δ(31P) at 22.9 ppm]. For H NMR
᎐
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
2200

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᎐
pp. 167–202; ( f ) V. Farina, V. Krishnamurthy and V. J. Scott, The
Stille Reaction, Wiley, New York, 1998.
2 C. Amatore, A. Jutand and A. Suarez, J. Am. Chem. Soc., 1993, 115,
9531–9541; C. Mateo, D. J. Cardenas, C. Fernandez-Rivas and
A. M. Echavarren, Chem. Eur. J., 1996, 2, 1596–1606.
3 E. Negishi, T. Takahashi, S. Baba, D. E. Van Horn and N. Okukado,
J. Am. Chem. Soc., 1987, 109, 2393–2401.
measurements, PhC᎐CSnBu (77 µl, 0.22 mmol) was added to a
᎐
3
solution of 2b (14.2 mg, 0.02 mmol) in thf-d8 (0.5 ml). After
completion of the reaction, the 2d/2e molar ratio was estimated
by integration of the CHMe2 septets at 4.77 ppm (2d) and at
5.32 ppm (2e).
Synthesis and characterization of [Pd(ꢀ2-dmfu)(P–N)2] (1f, 2f)
in solution. The complexes 1f or 2f are formed almost quanti-
tatively in the reaction of [Pd(η2-dmfu)(P–N)] (1a or 2a) with
1 equivalent of P–N (1 or 2) in thf. For NMR measurements,
the complex 1a (11.6 mg, 0.018 mmol) or 2a (10.5 mg, 0.018
mmol) and the ligand 1 (7.1 mg, 0.018 mmol) or 2 (6.0 mg,
0.018 mmol) were dissolved in thf-d8 (0.6 cm3). For IR meas-
urements, the complex 1a (12.9 mg, 0.02 mmol) or 2a (11.6 mg,
0.02 mmol) and the ligand 1 (7.9 mg, 0.02 mmol) or 2 (6.6 mg,
0.02 mmol) were dissolved in thf (1 cm3).
4 (a) A. Gillie and J. K. Stille, J. Am. Chem. Soc., 1980, 102, 4933–
4941; (b) K. Tatsumi, R. Hoffmann, A. Yamamoto and J. K. Stille,
Bull. Chem. Soc. Jpn., 1981, 54, 1857–1867; (c) P. J. Stang, M. H.
Kowalski, M. D. Schiavelli and D. J. Longford, J. Am. Chem. Soc.,
1989, 111, 3347–3356; (d ) J. M. Brown and N. A. Cooley,
Organometallics, 1990, 9, 353–359; (e) V. Farina and B. Krishnan,
J. Am. Chem. Soc., 1991, 113, 9585–9595; ( f ) V. Farina, B.
Krishnan, D. R. Marshall and G. P. Roth, J. Org. Chem., 1993, 58,
5434–5444; (g) J. Louie and J. F. Hartwig, J. Am. Chem. Soc., 1995,
117, 11598–11599; (h) C. Amatore, E. Carré, A. Jutand, H. Tanaka,
Q. Ren and S. Torii, Chem. Eur. J., 1996, 2, 957–966; (i) C. Amatore,
G. Broeker, A. Jutand and F. Khalil, J. Am. Chem. Soc., 1997, 119,
5176–5185; (j) J. F. Hartwig, Angew. Chem., Int. Ed., 1998, 37,
2046–2067.
Complex 1f: νmax/cmϪ1 (C᎐O) 1690s, (C᎐N) 1614mw; δH 9.19
᎐
᎐
(2 H, d, 4J(PH) 3.0 Hz, N᎐CH), 4.21 (2 H, m, olefin CH), 3.90
᎐
5 A. L. Casado and P. Espinet, J. Am. Chem. Soc., 1998, 120,
8978–8995; A. L. Casado, P. Espinet and A. M. Gallego, J. Am.
Chem. Soc., 2000, 122, 11771–11782.
6 A. Ricci, F. Angelucci, M. Bassetti and C. Lo Sterzo, J. Am. Chem.
Soc., 2002, 124, 1060–1071.
7 C. Amatore, A. Bucaille, A. Fuxa, A. Jutand, G. Meyer and A. N.
Ntepe, Chem. Eur. J., 2001, 7, 2134–2142.
(6 H, s, OCH3); δP 21.3 (s).
Complex 2f: νmax/cmϪ1 (C᎐O) 1687s, (C᎐N) 1629m; δ 9.34
᎐
᎐
H
(2 H, d, 4J(PH) Hz, N᎐CH), 3.98 (2 H, m, olefin CH), 3.51 (2 H,
᎐
3
3
spt, J(HH) 6.2 Hz, CHMe2), 1.29 (6 H, d, J(HH) 6.2 Hz,
CH3), 0.95 (6 H, d, 3J(HH) 6.2 Hz, CH3); δP 19.7 (s).
8 J. A. Casares, P. Espinet and G. Salas, Chem. Eur. J., 2002, 8,
4844–4853.
9 W. J. Scott and J. K. Stille, J. Am. Chem. Soc., 1986, 108, 3033–3040.
10 N. Tamayo, A. M. Echavarren, M. C. Paredes, F. Farina and
P. Noheda, Tetrahedron Lett., 1990, 31, 5189–5192.
11 R. Sustmann, J. Lau and M. Zipp, Tetrahedron Lett., 1986, 27,
5207–5210; M. E. Wright and C. K. Lowe-Ma, Organometallics,
1990, 9, 347–352; R. van Asselt and C. J. Elsevier, Organometallics,
1992, 11, 1999–2001.
12 A. L. Casado, P. Espinet, A. M. Gallego and J. M. Martìnez-
Ilarduya, Chem. Commun., 2001, 339–340.
Catalytic experiments
Reaction 2 was carried out at 25 ЊC in thf under nitrogen, with
a palladium complex concentration of 1 × 10Ϫ2 mol dmϪ3
᎐
and with an initial complex/IC H CF -4/PhC᎐CSnBu molar
ratio of 1 : 20 : 20. The reaction progress was monitored by
IR spectroscopy (in the absorbance mode), following the
᎐
6
4
3
3
᎐
᎐
increasing intensity of the ν(C᎐C) band of PhC᎐CC H CF -4
᎐
᎐
6
4
3
at 2220 cmϪ1 and the concomitant decreasing intensity of the
ν(C᎐C) band of PhC᎐CSnBu at 2134 cmϪ1, the conversion at
᎐
᎐
᎐
᎐
3
13 E. Shirakawa and T. Hiyama, J. Organomet. Chem., 1999, 576,
different times being estimated from calibration curves. The
details of reaction 2 catalyzed by 1a are reported as an example.
169–178 and references therein.
14 A. Scrivanti, U. Matteoli, V. Beghetto, S. Antonaroli, R. Scarpelli
and B. Crociani, J. Mol. Catal. A, 2001, 170, 51–56.
15 S. Antonaroli and B. Crociani, J. Organomet. Chem., 1998, 560,
137–146.
16 A. Scrivanti, U. Matteoli, V. Beghetto, S. Antonaroli and B.
Crociani, Tetrahedron, 2002, 58, 6881–6886.
17 T. G. Appleton, H. C. Clark and L. E. Manzer, Coord. Chem. Rev.,
1973, 10, 335–422.
18 G. P. C. M. Dekker, A. Buijs, C. J. Elsevier, K. Vrieze, P. W. N. M.
van Leeuwen, W. J. J. Smeets, A. L. Spek, Y. F. Wang and
C. H. Stam, Organometallics, 1992, 11, 1937–1948; R. E. Rülke,
V. E. Kaasjager, P. Wehman, C. J. Elsevier, P. W. N. M. van Leeuwen,
K. Vrieze, J. Fraanje, K. Goubitz and A. L. Spek, Organometallics,
1996, 15, 3022–3031; L. Crociani, G. Bandoli, A. Dolmella,
M. Basato and B. Corain, Eur. J. Inorg. Chem., 1998, 1811–1820.
19 P. M. Maitlis, P. Espinet and M. J. H. Russell, in Comprehensive
Organometallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and
E. W. Abel, Pergamon, Oxford, 1982, vol. 6, ch. 38.4, pp. 305.
3
᎐
The reactants, IC H CF -4 (0.151 cm , 1.0 mmol) and PhC᎐
᎐
6
4
3
CSnBu3 (0.370 cm3, 1.0 mmol), and the complex 1a (32.3 mg,
0.05 mmol) were dissolved in thf (5 cm3) under nitrogen in
a two-necked round-bottom flask, which was kept in a
thermostat at 25 ЊC. Samples of the reaction mixture were
taken at different times for IR spectra.
The cross-coupling of iodobenzene with PhC᎐CSnBu was
᎐
᎐
3
carried out in a jacketed glass reactor equipped with a reflux
condenser, a side arm with stopcock for freeze–thaw cycles and
with a threaded side port with rubber septum and cap for
syringe sampling. The details for run 1 of Table 1 are reported
as an example. The reactor was charged under argon with thf
3
᎐
(5 cm ), PhC᎐CSnBu (0.510 g, 1.3 mmol), iodobenzene
᎐
3
(0.270 g, 1.3 mmol) and the complex 1a (4.2 mg, 6.5 × 10Ϫ3
mmol). The reactor was then heated at 50 ЊC by circulating
a thermostatic fluid through the outer jacket. Samples of the
reaction mixture were taken at different times, cooled to
room temperature and analysed by GLC to determine substrate
conversion and product yield.
20 Alternatively, the products of the reaction of 2b with an excess of
᎐
PhC᎐CSnBu3 may be formulated either as an equilibrium mixture
᎐
2
᎐
of the two isomers [Pd(η -PhC᎐CSnBu3)(P–N)], resulting from
᎐
different orientations of the asymmetric alkyne relative to the P–N
ligand in an essentially planar structure of the type:
Acknowledgements
Financial support by Ministero Italiano dell’Istruzione,
2
᎐
dell’Università
e
della Ricerca Scientifica is gratefully
or as an equilibrium mixture of one isomer [Pd(η -PhC᎐CSnBu3)-
᎐
(P–N)] with the oxidative addition derivative 2d. In both cases, the
formation of 2a upon addition of dmfu would occur by
displacement of the coordinated alkyne. These equilibria, however,
acknowledged.
appear to be hardly influenced by the concentrations of
References and notes
᎐
PhC᎐CSnBu3 or ISnBu3. On the other hand, the IR spectra of the
᎐
᎐
1 (a) J. K. Stille, Angew. Chem., Int. Ed., 1986, 25, 508–524; (b) T. N.
Mitchell, Synthesis, 1992, 803–815; (c) V. Farina, in Comprehensive
Organometallic Chemistry II, ed. E. W. Abel, F. G. A. Stone and
G. Wilkinson, Pergamon, Oxford, 1995, vol. 12, ch. 3.4, pp. 161–240;
(d ) V. Farina and G. P. Roth, Adv. Met.-Org. Chem., 1996, 5, 1–53;
(e) T. N. Mitchell, in Metal-Catalyzed Cross-Coupling Reactions,
ed. F. Diederich and P. J. Stang, Wiley-VCH, Weinheim, 1998,
reaction mixture show no absorption attributable to a ν(C᎐C)
᎐
vibration of the η2-bound alkyne in the range 2000–1700 cmϪ1. For
structural and IR data of palladium(0) complexes with η2-bound
alkynes, see: P. M. Maitlis, P. Espinet and M. J. H. Russell, in
Comprehensive Organometallic Chemistry, ed. G. Wilkinson, F. G. A.
Stone and E. W. Abel, Pergamon, Oxford, 1982, vol. 6, ch. 38.5.2,
pp. 353–356.
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
2201

View Article Online
21 A weak singlet in the range 28.5–29.5 ppm appears also in the 31P
NMR spectra of the mixtures 1 or 2/IC6H4CF3-4 (1 : 20 molar ratio)
in thf-d8 after 2–3 h at 25 ЊC. On the other hand, formation of
phosphonium salts has been reported to occur in palladium-
catalyzed reaction involving oxidative addition of aryl halides to
palladium(0)–phosphine complexes, see: C. B. Ziegler, Jr and
R. F. Heck, J. Org. Chem., 1978, 43, 2941–2946. A convenient
preparation of tetraarylphosphonium iodides is based on the
reaction of ArI with PAr3 catalyzed by [Pd(PPh3)4] or Pd(OAc)2, see:
T. Migita, T. Nagai, K. Kiughi and M. Kosugi, Bull. Chem. Soc.
Jpn., 1983, 56, 2869–2870 . However, any attempt to isolate the
IC6H4CF3-4 with an equimolar amount of P–N, in the presence of a
catalytic amount (1%) of [Pd(η2-dmfu)(P–N)] or [PdI(C6H4CF3-4)-
(P–N)], was unsuccessful even though for the reaction in refluxing
thf the 31P NMR spectra at different times showed a singlet of
increasing intensity in the range 28.5–29.5 ppm.
22 At Ϫ35 ЊC, the equilibrium of Scheme 7 is almost completely shifted
towards the complex 2h, characterized by the following 1H NMR
spectrum: δH (thf-d8) 9.13 (2 H, s, N᎐CH), 3.44 (2 H, spt, 3J(HH) 5.8
᎐
Hz, CHMe2), 2.94 (2 H, m, olefin CH), 1.25 (6 H, d, 3J(HH) 5.8 Hz,
CH3), 1.00 (6 H, d, 3J(HH) 5.8 Hz, CH3).
23 J. E. Hoots, T. B. Rauchfuss and D. A. Wrobleski, Inorg. Synth.,
1982, 21, 175–179.
salt [PPh2(C6H4CF3-4)(C6H4-2-CH᎐NR)]I from the reaction of
᎐
D a l t o n T r a n s . , 2 0 0 3 , 2 1 9 4 – 2 2 0 2
2202