Quantitative evaluation of the factors contributing to the copper effect in the stille reaction

Article abstract of DOI:10.1021/om020896b

The relative importance of the factors contributing to the accelerating effect of CuI on [PdL₄]-catalyzed couplings of R¹I and R²SnBu₃ (copper effect) has been quantitatively evaluated in THF for R¹ = 3,5-C₆Cl₂F₃; R² = vinyl, C₆H₄-4-OMe; L = AsPh₃, PPh₃, using spectroscopic and kinetic methods. The ¹⁹F NMR kinetic data show that the rate enhancement produced by addition of CuI is strongly related with the autoretardation effect intrinsic to [PdL₄] catalysts and is almost independent of the organotin reagent (vinyl, aryl). The autoretardation is due to the release of 2 equiv of L during the oxidation of [PdL₄] to yield trans-[PdR¹IL₂], which is the species undergoing transmetalation. CuI does not promote the dissociation of L from trans-[PdR¹IL₂], but it captures part of the free neutral ligand L and therefore mitigates the autoretardation produced by the presence of free L on the rate-determining associative transmetalation. In the conditions studied (Pd:Cu = 1:2; T = 322.6 K; THF as solvent), for L = AsPh₃ the CuI added captures about 25% of the free AsPh₃ and the copper effect compensates only ca. 1% of the autoretardation, whereas for L = PPh₃ the CuI captures about 99% of the free PPh₃ and the compensation is about 30%. This remarkable variation is caused by the combined effect of two independent factors: (i) The catalyst [Pd(PPh₃)₄] is more autoretarded than [Pd(AsPh₃)₄]; and (ii) CuI is a more effective scavenger for PPh₃ than for AsPh₃.

Full text of DOI:10.1021/om020896b

Organometallics 2003, 22, 1305-1309
1305
Qu a n tita tive Eva lu a tion of th e F a ctor s Con tr ibu tin g to
th e “Cop p er Effect” in th e Stille Rea ction
Arturo L. Casado and Pablo Espinet*
Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
E-47005 Valladolid, Spain
Received October 29, 2002
The relative importance of the factors contributing to the accelerating effect of CuI on
[PdL4]-catalyzed couplings of R1I and R2SnBu3 (copper effect) has been quantitatively
evaluated in THF for R1 ) 3,5-C6Cl2F3; R2 ) vinyl, C6H4-4-OMe; L ) AsPh3, PPh3, using
spectroscopic and kinetic methods. The 19F NMR kinetic data show that the rate enhancement
produced by addition of CuI is strongly related with the “autoretardation” effect intrinsic to
[PdL4] catalysts and is almost independent of the organotin reagent (vinyl, aryl). The
“autoretardation” is due to the release of 2 equiv of L during the oxidation of [PdL4] to yield
trans-[PdR1IL2], which is the species undergoing transmetalation. CuI does not promote the
dissociation of L from trans-[PdR1IL2], but it captures part of the free neutral ligand L and
therefore mitigates the autoretardation produced by the presence of free L on the rate-
determining associative transmetalation. In the conditions studied (Pd:Cu ) 1:2; T ) 322.6
K; THF as solvent), for L ) AsPh3 the CuI added captures about 25% of the free AsPh3 and
the copper effect compensates only ca. 1% of the autoretardation, whereas for L ) PPh3 the
CuI captures about 99% of the free PPh3 and the compensation is about 30%. This remarkable
variation is caused by the combined effect of two independent factors: (i) The catalyst [Pd-
(PPh3)4] is more autoretarded than [Pd(AsPh3)4]; and (ii) CuI is a more effective scavenger
for PPh3 than for AsPh3.
Sch em e 1
In tr od u ction
The Stille reaction (Pd-catalyzed coupling between
organotins and organic electrophiles)1 is complex and
can follow different pathways depending on a number
of factors such as the kind of electrophile, the solvent,
the ancillary ligand, and the addition or not of LiCl. We
have recently contributed to a better understanding of
these factors, providing the frame of a dual mechanism
shown in Scheme 1.2-4
The previous mechanistic conception implied predis-
sociation of L and rate-limiting transmetalation on
trans-[PdR1XL] or trans-[PdR1XL(solvent)] (X ) ha-
lide).5 Three important novelties in the modern concep-
tion are as follows: (i) The transmetalation is not always
the rate-determining step (rds). (ii) The transmetalation
is an associative ligand substitution on intermediates
trans-[PdR1YL2] involving a pentacoordinate transition
state TS(A) or TS(B), and this has stereochemical
consequences on the kind of Pd isomer produced after
the transmetalation. (iii) For transmetalations of chiral
R-carbons the outcome of the reaction can be retention
(upper cycle) or inversion (lower cycle) of configuration,
a kind of dual result reported in the literature,6,7
difficult to accommodate in the old dissociative model.
(1) Stille, J . K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508-523.
(2) Casado, A. L.; Espinet, P. J . Am. Chem. Soc. 1998, 120, 8978-
8985.
(3) Casado, A. L.; Espinet, P.; Gallego, A. M. J . Am. Chem. Soc. 2000,
122, 11771-11782.
(4) Casado, A. L.; Espinet, P. Organometallics 1998, 17, 954-959.
(5) Farina, V.; Roth, G. P. Adv. Metalorg. Chem. 1996, 5, 1-53.
(6) Labadie, J . W.; Stille, J . K. J . Am. Chem. Soc. 1983, 105, 6129-
6137.
According to our previous results, the rds in the
conditions we are concerned with in this article (Y ) I,
solvent ) THF) is the transmetalation, while the
oxidative addition is fast. The reaction follows the upper
(7) Ye, J .; Bath, R. K.; Falk, J . R. J . Am. Chem. Soc. 1994, 116,
1-5.
10.1021/om020896b CCC: $25.00 © 2003 American Chemical Society
Publication on Web 02/15/2003

1306 Organometallics, Vol. 22, No. 6, 2003
Casado and Espinet
Sch em e 2
and this is why addition of CuI produces minimal rate
accelerations.
The modern associative scheme can accommodate this
explanation easily by changing the concept “L predis-
sociation” for “L associative substitution” on a square
planar Pd(II), since the associative transmetalation is
equally retarded by free ligand. A conceptual difference
is that scavenging free ligand in the dissociative scheme
implies increasing the concentration of species supposed
to undergo transmetalation ([PdR1IL(S)]), whereas in
the associative mechanism it does not imply change in
concentration of the species actually undergoing trans-
metalation (trans-[PdR1IL2]).8 In the associative scheme
the two obvious factors contributing to the different
behavior of PPh3 and AsPh3 are the different values of
k1 and k-1 in eq 1 for the two ligands and also the
different affinity of Cu(I) for them. We thought it should
be interesting to try to quantify the effect and do it
separately for the two factors, taking advantage of our
experience in the use of stable palladium fluoroaryl
derivatives for kinetic studies and the facility of obser-
vation associated with 19F NMR spectroscopy. The
results are presented here.
Sch em e 3
cycle via a cyclic transition state TS(A). The transmeta-
lation produces an R2-for-L substitution, releasing 1 mol
of the leaving ligand L and leading to a cis-PdR1R2
arrangement in the product. Since the reaction rate is
determined by the transmetalation step (Scheme 2), it
shows an inverse dependence on [L] (eq 1), which can
mislead one to the conclusion of a transmetalation
involving a dissociative preequilibrium, as discussed
elsewhere.8
The reader should be aware that the effect studied
in this paper is not the only possible role of copper salts.
In their initial study Farina and Liebeskind already
proposed that in very polar solvents a Sn/Cu transmeta-
lation was taking place (i.e., organocopper species were
participating in the reaction), a proposal recently de-
velopped into an effective coupling system for sterically
congested substrates.16 In fact copper-mediated cross-
coupling of organostannanes with organic electrophiles
in mild conditions (usually in polar solvents) is now a
useful synthetic method.18-23
k1k2[PdR1IL2]
robs ) kobs[R2SnBu3] )
[R2SnBu3] (1)
k-1[L] + k2
Resu lts a n d Discu ssion
A remarkable phenomenon in Stille couplings is the
effect of the addition of CuI or other Cu(I) salts, which
are known to accelerate with variable success the
couplings catalyzed by [PdL4].5,7,9-16 This so-called “cop-
per effect” has been studied before by the groups of
Farina and Liebeskind.17 Within the frame of the
predissociation mechanism for the transmetalation,
accepted at that moment, they concluded that the role
of CuI in front of “strong” ligands such as PPh3 is to
scavenge free ligand. The latter is detrimental for the
transmetalation because it inhibits dissociation (Scheme
3). It was also suggested that for “soft” ligands (such as
AsPh3) ligand dissociation from Pd(II) is not a problem,
Ca ta lytic Activity of tr a n s-[P d (C6Cl2F 3)IL2] (L )
P P h 3, AsP h 3) w ith ou t Cu I. It has been established
before that the transmetalation step is rate determining
for the catalytic coupling of aryl or vinyl stannanes and
aryl iodide in THF.2,5,9 The coupling of C6Cl2F3I (1) with
R2SnBu3 (R2 ) vinyl, 2a ; 4-anisyl, 2b) catalyzed by [Pd-
(PPh3)4] does not occur at a detectable rate: Although
the initial oxidative addition gives [Pd(C6Cl2F3)I(PPh3)2]
+ 2 PPh3, the subsequent transmetalation step does not
proceed in the presence of such excess of free PPh3.
Thus, to compare the ligand influence on the kinetic
behavior of PPh3- and AsPh3-based catalysts, we have
studied the coupling catalyzed by trans-[Pd(C6Cl2F3)I-
(PPh3)2] (3) in THF in the presence of various smaller
proportions of added PPh3. The reactions were clean,
and no byproducts were observed. The products C6-
Cl2F3-R2 (R2 ) vinyl, 5a ; 4-anisyl, 5b) were formed in
up to 95% yield (eq 2). The decay of [1] (monitored by
19F NMR) followed first-order kinetics.
(8) Casares, J . A.; Espinet, P.; Salas, G. Chem. Eur. J . 2002, 8,
4843-4853.
(9) Farina, V.; Krishnan, B. J . Am. Chem. Soc. 1991, 113, 9585-
9595.
(10) Liebeskind, L. S.; Fengl, R. W. J . Org. Chem. 1990, 55, 5359-
5364.
(11) Hobbs, F. W., J r. J . Org. Chem. 1989, 54, 3420-3422.
(12) Louie, J .; Hartwing J . F. J . Am. Chem. Soc. 1995, 117, 11598-
1599.
(13) Liebeskind, L. S.; Riesinger, S. W. J . Org. Chem. 1993, 58, 408-
cat
413.
2
2
C Cl F l
(14) Go´mez Bengoa, E.; Echavarren, A. M. J . Org. Chem. 1991, 56,
3497-3501.
+
(3 or 4)8
(2)
R SnBu3
R C6Cl2F3 + lSnBu3
6
2
3
1
2
5
(15) Nicolaou, K. C.; Sato, M.: Miller, N. D.; Gunzner, J . L.; Renaud,
J .; Untersteller, E. Angew. Chem., Int. Ed. Engl. 1996, 35, 889-891.
(16) Han, X.; Stoltz, B. M.; Corey, E. J . J . Am. Chem. Soc. 1999,
121, 7600-7605.
(18) Piers, E.; Wong, T. J . Org. Chem. 1993, 58, 3609-3610.
(19) Ye, J .; Bath, R. K.; Falk, J . R. J . Am. Chem. Soc. 1994, 116,
1-5.
(17) Farina, V.; Kapadia, S.; Krishnan, B.; Wang, C.; Liebeskind,
L. S. J . Org. Chem. 1994, 59, 5905-5911.

“Copper Effect” in the Stille Reaction
Organometallics, Vol. 22, No. 6, 2003 1307
Ta ble 1. F ir st-Or d er Ra te Con sta n ts, kobs, for th e
Cou p lin g of C6Cl2F 3I (1) a n d Sn (vin yl)Bu 3 (2a )
Ca ta lyzed by tr a n s-[P d (C6Cl2F 3)I(P P h 3)2] (3) in
THF a
T/K
[PPh3]added/10-3 mol L-1
k
obs/10-5 s-1
328.5 ( 0.2
322.6
322.6
322.6
322.6
317.1
311.3
305.9
299.9
0
1.44 ( 0.04
0.273 ( 0.009
0.233 ( 0.009
0.42 ( 0.02
0.164 ( 0.005
0.065 ( 0.002
0.92 ( 0.03
0.99 ( 0.04
0.685 ( 0.010
0.386 ( 0.018
0.247 ( 0.008
0.139 ( 0.006
0.099 ( 0.006
0
0
0
0
0
0
-1
295.8
F igu r e 1. kobs vs [L]added plots for the couplings of C6-
Cl2F3I (1, 0.2 mol L-1) and Sn(vinyl)Bu3 (2a , 0.2 mol L-1
)
a
[1]0 ) [2a ]0 ) (2.000 ( 0.017) × 10-1 mol L-1, [3] ) (1.00 (
0.03) × 10-2 mol L-1. Data for the reaction catalyzed by trans-
[Pd(C6Cl2F3)I(AsPh3)2] (4) are reported in ref 2.
catalyzed by trans-[Pd(C6Cl2F3)IL2] (0.01 mol L-1) in THF
at 322.6 K: (a) L ) AsPh3; (b) L ) PPh3. Values for L )
AsPh3 have been taken from ref 2.
The kobs values found for the different concentrations
of added PPh3 are given in Table 1. The results are
compared with those obtained previously using trans-
[Pd(C6Cl2F3)I(AsPh3)2] (4).2 An analysis of the data
reveals two major kinetic differences between the two
catalysts:
(ii) The coupling retardation by addition of L is very
swift for 3 and temperate for 4. In both cases, the
reaction is minus first-order in the concentration of
-1
added L. The linear plots kobs vs [L]added are shown in
Figure 1. The slopes are (1.54 ( 0.13) × 109 L s mol-1
for 3 and (4.31 ( 0.17) × 106 L s mol-1 for 4. That is,
the coupling catalyzed by 3 is retarded by L over 2
orders of magnitude more quickly than that catalyzed
by 4.
(i) The reaction catalyzed by complex 4 is about 40
times faster than using 3. Since the entropic and steric
factors should be very similar,24 the variation in rates
must be mainly due to differences in activation en-
thalpy. In effect, an Eyring treatment of kobs in the range
295-330 K yields ∆Hq ) 65.1 ( 1.4 kJ mol-1 and ∆Sq
) -139 ( 5 J K-1 mol-1 for the reaction catalyzed by 3,
compared to ∆Hq ) 50 ( 2 kJ mol-1 and ∆Sq ) -155 (
7 J K-1 mol-1 for the reaction catalyzed by 4.2 Both
activation entropy values are in the range of those found
for other associative ligand substitutions in Pd(II)
complexes.25 This supports an associative transmeta-
lation mechanism involving a pentacoordinated palla-
dium(II) transition state, as proposed in Scheme 1
(upper cycle). The difference in activation enthalpy for
3 and 4 reflects two main contributions: (a) Complex
trans-[PdR1I(AsPh3)2] is more electrophilic than trans-
[PdR1I(PPh3)2] (because PPh3 is a stronger donor); and
(b) the elongation (and the eventual scission) of the bond
to the leaving ligand, Pd-L, is easier for L ) AsPh3,
because the Pd-P bond is stronger than Pd-As. We
have not found data for these bond energies in the
literature, but the expected effect is that if Pd-PPh3 is
the stronger bond, it should be preferred to Pd-AsPh3.
This is a well-documented fact: [PdR2(AsPh3)2] and
[PdRX(AsPh3)2] complexes are general precursors for
complexes with many other L ligands (including L )
PPh3) by associative ligand substitution reactions,26
whereas PPh3 is hardly displaced by other ligands from
similar PPh3 complexes.
Ca ta lytic Activity of tr a n s-[P d (C6Cl2F 3)IL2] (L )
P P h 3, AsP h 3) w ith 2 Cu I + 2 L. The kinetic effect of
CuI in these systems was estimated from the measure-
ments described now. First, we determined the observed
rate constant k′obs for eq 2 using as catalyst a mixture
of the Pd complex (3 or 4) and 2 equiv of L per Pd (a
mixture complex:L ) 1:2 should be catalytically equiva-
lent to the system produced once the oxidative addition
has taken place in a catalysis with [PdL4]).27 On the
other hand, we measured the rate constant k′′obs for eq
2 using as catalyst a mixture (3 or 4):L:CuI ) 1:2:2 (this
catalytic mixture is equivalent to a mixture of [PdL4]
and 2 equiv of CuI). These observed rate constants are
compared with the observed rate constants measured
in the absence of added L or CuI (k0obs, taken from Table
1 and from ref 2). The results are shown in Table 2. In
the last column of Table 2 we collect the value of the
quotient (k′′obs - k′obs)/(k0 - k′obs). Since the autore-
obs
tardation is a consequence of the release of 2 equiv of L
when [PdL4] enters the catalytic cycle and is trans-
formed into trans-[PdR1IL2], (k′′obs - k′obs)/(k0 - k′obs
)
obs
is an estimation of the ratio between the rate enhance-
ment achieved by addition of CuI to couplings catalyzed
by [PdL4] (k′′obs - k′obs) and the autoretardation effect
in these catalyses (k0
- k′obs). In other words the
obs
quotient estimates the fraction of autoretardation com-
pensated by CuI. For L ) PPh3 the addition of CuI
compensates about 30% of the autoretardation, whereas
for L ) AsPh3, the compensation is only about 1%. For
L ) PPh3 the effect is almost independent of the
organotin reagent, suggesting that the involvement of
organocopper species in the rate enhancement is insig-
nificant, as expected in low polar solvents and with
strong ligands from the study by Farina and Liebes-
(20) Takeda, T.; Matsunaga, K. I.; Kabasawa, Y.; Fukiwara, T.
Chem. Lett. 1995, 771-772.
(21) Allred, G. D.; Liebeskind, L. S. J . Am. Chem. Soc. 1996, 118,
2748-2749.
(22) Linderman, R. J .; Siedlecki, J . M. J . Org. Chem. 1996, 61,
6492-6493.
(23) Kang, S.-K.; J im, J .-S.; Choi, S.-C. J . Org. Chem. 1997, 62,
4208-4209.
(24) The cone angles of the two ligands are 145° for PPh3 and 142°
for AsPh3: Tolman, C. A. Chem. Rev. 1977, 77, 313-348.
(25) Minniti, D. Inorg. Chem. 1994, 33, 2631-2634.
(26) (a) Uso´n, R.; Fornie´s, J .; Gonzalo J . Organomet. Chem. 1976,
104, 253-258. (b) Uso´n, R.; Fornie´s, J .; Gonzalo, S.; Mart´ınez, F.;
Navarro, R. Rev. Acad. Ci. Zaragoza 1977, 32, 75-84.
(27) This procedure facilitates the kinetic study because it obviates
the induction period for catalysts activation.

1308 Organometallics, Vol. 22, No. 6, 2003
Casado and Espinet
Ta ble 2. Deter m in a tion of th e Cop p er Effect in Cou p lin gs of C6Cl2F 3I (1) w ith R2Sn Bu 3 (2a ,b) Ca ta lyzed by
tr a n s-[P d (C6Cl2F 3)IL2] (3, 4)a
R2
L
k0obs/10-5 s-1
k′obs/10-5 s-1
k′′obs/10-5 s-1
(k′′obs - k′obs)/(k0obs - k′obs
)
vinyl
vinyl
aryl
AsPh3
PPh3
AsPh3
PPh3
33.7 ( 0.4
1.12 ( 0.03
1.60 ( 0.03
0.015 ( 0.001
0.314 ( 0.014
0.002 ( 0.016
0.30 ( 0.04
0.99 ( 0.04
1.94 ( 0.02
0.054 ( 0.002
≈0
0.118 ( 0.02
≈0
0.311 ( 0.005
0.122 ( 0.02
0.016 ( 0.002
aryl
[1]0 ) [2]0 ) (2.000 ( 0.017) × 10-1 mol L-1, [3] or [4] ) (1.00 ( 0.03) × 10-2 mol L-1, THF, 322.6 K. See definition of k0obs, k′obs, and
a
k′′obs in the text.
kind.17,28 For L ) AsPh3 the values measured are small
and the error is significant, precluding to be conclusive
in this respect.
and CuI, to yield [CuI(PPh3)],30 is fast compared to the
transmetalation in the Stille cycle. A second spectrum
recorded after addition of an additional 1 equiv of PPh3
(CuI:PPh3 ) 1:2) showed only one broad signal at -3.17
ppm. This signal became sharper and shifted toward
the position of free PPh3 upon successive additions of
PPh3 to the mixture (δ ) -1.00 ppm for Cu:PPh3 ) 1:4;
δ ) -0.38 ppm for Cu:PPh3 ) 1:7). From these results,
it seems that CuI in THF can take more than one PPh3
and form adducts [CuI(PPh3)n] or [CuI(PPh3)n(THF)m].31,32
Moreover, the observation of only one broad signal
suggests a fast exchange between free and coordinated
PPh3 (eq 4).33
In separate experiments, complexes 3 and 4 did not
react noticeably with CuI under the experimental condi-
tions used in the catalytic couplings (1 × 10-2 mol L-1
,
THF, 322 K). No new Pd species were observed by 19F
or 31P NMR. Therefore, CuI is not able to extract L from
trans-[PdR1IL2] (L ) PPh3, AsPh3) to a perceptible
extent.29 However, it captures efficiently free neutral
ligand L and mitigates the intrinsic autoretardation of
this catalytic system (eq 3).
trans-[Pd(C6Cl2F3)IL2] + 2 L + 2 CuI h
trans-[Pd(C6Cl2F3)IL2] + 2 [CuIL] (3)
[CuI(PPh3)n-1(THT)m] + PPh3 h
[CuI(PPh3)n(THT)m-1] + THT (4)
Competition experiments were carried out to further
compare the affinity of CuI for PPh3 versus AsPh3.
Treating a 0.02 mol L-1 sample of PPh3 with 1 equiv of
[CuI(AsPh3)] (6) (THF, 322 K), the 31P NMR signal for
free PPh3 quickly vanished, suggesting that all the PPh3
was trapped by CuI (eq 5). The peak for free PPh3 was
not recovered after addition of 50 equiv of AsPh3,
revealing that the coordinated PPh3 was not substituted
significantly in the presence of that huge excess of
arsine and suggesting that the affinity of CuI is more
than 50-fold higher for PPh3 than for AsPh3. This
behavior is consistent with the estimations drawn from
the kinetic data discussed before.
Based on this model, an estimation of the concentra-
tion of free ligand L in a catalytic mixture containing
(3 or 4):L:CuI ) 1:2:2 can be made from the kinetic data
entering the value of k′′obs from Table 2 into the kinetic
plot in Figure 1. For 4, the concentration of free AsPh3
is 1.5 × 10-2 mol L-1. This means that CuI captures
ca. 25% of the AsPh3 released after the oxidative
addition on [Pd(AsPh3)4]. In the case of 3, the concen-
tration of free PPh3 is 2.1 × 10-4 mol L-1; that is, CuI
captures ca. 99% of the PPh3 released from [Pd(PPh3)4]
by oxidative addition of IR.1
31P NMR Exp er im en ts on Cu I/L System s. The
question arises whether the capture of L by CuI is fast
enough compared to the transmetalation. The dynamics
of [CuIL]/CuI/L systems was studied independently by
31P NMR under conditions similar to those used in
catalytic couplings. The spectrum of a 0.02 mol L-1
solution of PPh3 (THF, 322 K) showed a sharp singlet
at -0.30 ppm. This peak disappeared immediately upon
addition of 1 equiv of CuI (the 31P NMR spectrum of an
original sample of [CuI(PPh3)] (5) is also flat). This
experiment indicates that the reaction between PPh3
[CuI(AsPh3)] + PPh3 f [CuI(PPh3)n(AsPh3)m] (5)
Con clu sion
CuI does not promote the dissociation of L from trans-
[PdR1IL2], but it captures part of the free neutral ligand
L released during the oxidation of [PdL4] that yields the
species actually undergoing transmetalation, trans-
[PdR1IL2], plus 2 L. Therefore CuI mitigates the “au-
toretardation” produced by the presence of free L on the
rate-determining associative transmetalation. In the
conditions studied (Pd:Cu ) 1:2; THT as solvent), for L
(28) This is also supported by the fact that the Stille couplings in
Table 2, carried out with Cu(I) species, were very clean and no
byproducts were detected. As an exception, ca. 5% mol of (C6Cl2F3)-
SnBu3 (2c) was formed when the coupling between 1 with 2a (R2
)
(30) It has been reported that UV spectra of [CuIL] in THF are
concentration dependent, suggesting the existence in solution of several
[CuIL]n species with different nuclearity: Reichle, W. T. Inorg. Chim.
Acta 1971, 5, 325-332.
(31) For instance, the X-ray structure of [Cu2I2(AsPh3)3] has been
reported: Eller, P. G.; Kubas, G. J .; Ryan, R. R. Inorg. Chem. 1977,
10, 2454-462.
vinyl) was catalyzed by trans-[Pd(C6Cl2F3)I(PPh3)2] (3), CuI, and PPh3.
This reaction is very slow (25 h at 322 K in THF), and we have observed
sometimes the formation of this byproduct in other slow reactions in
the absence of CuI (P. Espinet and Ana Gallego, unpublished results.
See also ref 3). Due to this observation, the participation of organo-
copper derivatives to a small extent cannot be fully excluded in this
reaction.
(32) Tetracoordinated Cu(I) complexes are common. See: Hattaway,
B. J . Copper, section 53.3. In Comprehensive Coordination Chemistry;
Wilkinson, G., Gillard, R. D., McCleverty, J . A., Eds.; Pergamon
Press: Oxford, U.K., 1987; Vol. 5.
(29) No new product was observed by 19F and 31P{1H} NMR after
mixing trans-[Pd(C6Cl2F3)IL2] (L ) PPh3, AsPh3) and CuI at 50 °C in
THF. In the case of L extraction by CuI or in the case of spontaneous
predissociation (see ref 8), the known dimer [Pd2(C6Cl2F3)2(µ-I)2L2]
should be formed. For L ) PPh3, traces of the dimer, along with OPPh3,
were detected due to oxidation by air, but these were formed in similar
amounts in a control experiment in the absence of CuI.
(33) The 63Cu quadrupolar relaxation mechanism may also contrib-
ute to the fast relaxation of phosphorus nuclei, giving broad signals:
Pregosin, P. S., Ed. Transition Metal Nuclear Magnetic Resonance;
Elsevier: Amsterdam, 1991.

“Copper Effect” in the Stille Reaction
Organometallics, Vol. 22, No. 6, 2003 1309
CuI (100 mg, 0.525 mmol) in the same solvent (10 mL) under
N2. After a few seconds complex 6 started to precipitate. The
mixture was stirred for 1 h, and the solid was filtered, washed
with MeCN, and vacuum-dried (yield 225 mg, 87%): IR (KBr)
1482 (s), 1434 (vs), 1079 (s), 741 (vs), 694 (vs), 470 (s). Anal.
Calcd for C18H15AsCuI: C, 43.53; H, 3.04. Found: C, 43.35;
H, 2.97. Compound 5 was similarly prepared from PPh3 (yield
97%): IR (KBr) 1479 (s), 1434 (vs), 1097 (vs), 748 (vs), 695
(vs), 521 (vs), 503 (s). Anal. Calcd for C18H15CuIP: C, 47.75;
H, 3.34. Found: C, 47.48; H, 3.23.
Kin et ic E xp er im en t s on P d -Ca t a lyzed Cou p lin gs of
C6Cl2F 3I w ith Or ga n otr ibu tyltin s; Rea ction s of Or ga n o-
p a lla d iu m (II), Or ga n ot r ib u t ylt in s, a n d [Cu IL] Com -
p lexes. These NMR experiments were carried out as described
previously for similar cases.2
) AsPh3 the CuI added captures about 25% of the free
AsPh3 and the copper effect compensates only ca. 1% of
the autoretardation. For L ) PPh3 the CuI captures
about 99% of the free PPh3 and the compensation is
about 30%. This variation is caused by the combined
effect of two independent factors: (i) The catalyst [Pd-
(PPh3)4] is more autoretarded than [Pd(AsPh3)4]; and
(ii) CuI is a more effective scavenger for released PPh3
than for released AsPh3. It is of little use to add CuI
when L ) AsPh3, at least for the purpose of rate
acceleration. The coupling of aryl iodides and stannanes
catalyzed by [PdL4] is faster for L ) AsPh3, even in the
absence of CuI, than for L ) PPh3 with CuI added.
Exp er im en ta l Section
Ack n ow led gm en t. Financial support by the Direc-
cio´n General de Investigacio´n (BQU2001-2015) and the
J unta de Castilla y Leo´n (VA17/00B) is gratefully
acknowledged.
The preparation of C6Cl2F3I (1), R2SnBu3 (R2 ) vinyl, 2a ;
4-anisyl, 2b), trans-[Pd(C6Cl2F3)I(AsPh3)2] (4), and C6Cl2F3-
R2 (R2 ) vinyl, 5a ; 4-anisyl, 5b) is reported in ref 2.
Cu IL (L ) P P h 3, 5; AsP h 3, 6). A solution of AsPh3 (161
mg, 0.526 mmol) in MeCN (2 mL) was added to a solution of
OM020896B