Organometallics 1995, 14, 1818-1826
1818
Rates and Mechanism of the Formation of Zerovalent Palladium Complexes from Mixtures of Pd(OAc)2and Tertiary Phosphines and Their Reactivity in Oxidative Additions Christian Amatore,* Emmanuelle Carre, Anny Jutand," and Mohamed Amine M'Barkii Dkpartement dP Chimie, Ecole Normale Supkrieure, URA CNRS 1679,24 Rue Lhomond, 75231 Paris Ckdex 5, France Received November 3, 1994@ Mixtures of Pd(OAc)2 and tertiary phosphines spontaneously afford palladium(0) complexes. Kinetic investigations demonstrate that this reaction proceeds from the complex Pd(OAc)2(PR3)2via an inner-sphere reduction which is the rate-determining step of the overall reaction. The phosphine is thus oxidized to the corresponding phosphine oxide. The formation of the palladium(0) complex is sensitive to electronic and steric factors. The more the triarylphosphine is substituted by electron-withdrawing groups, the faster the reaction. The palladium( 0 )complex thus formed reacts with phenyl iodide via a n oxidation addition, and this reaction is faster when the phosphine is more electron-rich.
Introduction We have reported in a previous paper' that catalytic systems commonly used in H e ~ kreactions, ~,~ consisting of Pd(OAc)z(PPh& or a mixture of Pd(0Ac)z and nPPh3 ( n 1 21, spontaneously and quantitatively generate i n situ a zerovalent palladium complex able to activate aryl iodides via an oxidative addition. Indeed, we demonstrated by means of analytical techniques such as 31P NMR spectroscopy and cyclic voltammetry that triphenylphosphine was able to reduce Pd(0Ac)z into a zerovalent palladium complex, from the complex Pd-
* To whom any correspondence should be addressed. Present address: Universite Ibn Tofail, Facult6 des Sciences, BP 133, 14000 Kenitra, Morocco. @Abstract published in Advance ACS Abstracts, March 15, 1995. (1)Amatore, C.; Jutand, A,; MBarki, M. A. Organometallics 1992, 11, 3009. (2) For rewiews, see: (a) Heck, R. F. Acc. Chem. Res. 1979,12,146. (b) Tsuji, J. Organic Syntheses via Palladium Compounds; Springer Verlag: New York, 1980. (c) Kumada, M. Pure Appl. Chem. 1980,52, 669. (d) Negishi, E. I. Acc. Chem. Res. 1982,15,340. (e) Heck, R. F. Org. React. 1982,27, 345. (0 Heck, R. F. Palladium Reagents in Organic Syntheses; Academic Press: New York, 1985. (g) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986,25,508. (3) ( a ) Yamane, T.; Kikukawa, K.; Takagi, M.; Matsuda, T. Tetrahedron 1973,29,955.(b) Dieck, H. A,; Heck, R. F. J . A m . Chem. SOC. 1974,96, 1133. (c) Dieck, H. A.; Heck, R. F. J . Org. Chem. 1975,40, 1083. (d) Patel, B. A,; Heck, R. F. J . Org. Chem. 1978,43,3898. (e) Tsuji, J.; Yamakawa, T.; Kaito, M.; Mandai, T. Tetrahedron Lett. 1978, 2075. (0Ziegler, C. B.; Heck, R. F. J . Org. Chem. 1978,43,2941. (g) Patel, B. A,; Dickerson, J. E.; Heck, R. F. J . Org. Chem. 1978,43,5018. (h) Shimizu, I.; Yamada, T.; Tsuji, J. Tetrahedron Lett. 1980,21, 3199. (i) Johnson, P. Y.; Wen, J . Q. J. Org. Chem. 1981,46,2767. ti) Kim, J.-I. I.; Patel, B. A.; Heck, R. F. J . Org. Chem. 1981,46, 1067. (k) Hallberg, A,; Westerlund, C. Chem. Lett. 1982,1993. (1) Fiaud, J . C.; Aribi-Zouioueche, L. Tetrahedron Lett. 1982,23,5279. (m) Mori, M.; Oda, I.; Ban, Y. Tetrahedron Lett. 1982,23, 5315. ( n ) OConnor, J . M.; Stallman, B. J.; Clark, W. G . ;Shu, A. Y. L.; Spada, R. E.; Stevenson, T. M.; Dieck, H. A. J . Org. Chem. 1983,48,807. (0)Backvall, J. E.; Nordberg, R. E.; Vlgberg, J. Tetrahedron Lett. 1983,24, 411. (p) Cacchi, S.; Morera, E.; Ortar, G. Tetrahedron Lett. 1984,25,2271. (q) Mori, M.; Kanda, N.; Oda, I.; Ban, Y.Tetrahedron 1985,23,5465. ( r ) Karabelas, K.; Westerlund, C.; Hallberg, A. J . Org. Chem. 1985,50, 3896. (SI Andersson, C. M.; Karabelas, K.; Hallberg, A. J. Org. Chem. 1985, 50, 3891. (t) Mori, M.; Kimura, M.; Uozumi, Y.; Ban, Y. Tetrahedron Lett. 1985,26,5947. (u) Andersson, C. M.; Hallberg, A. J . Org. Chem. 1987,52,3529. (v)Nilsson, K.; Hallberg, A. Acta Chem. Scand. B 1987,41,569. (w) Nilsson, K.; Hallberg, A. Acta Chem. Scand. 1990,44,288. (x) Andersson, C. M.; Larsson, J.; Hallberg, A. J . Org. Chem. 1990,52,5757.
(OAc)2(PPh&, triphenylphosphine being oxidized to triphenylphosphine oxide. Data a t hand were consistent with the following tentative mechanism:l Pd(OAc)p + 2PPh3 PhsP,
,OAC pdll)
a Pd(OAc)*(PPh&
(1)
slow
"Pd0(PPh3)(OAc)-"+ AcO-PPh3+
(2)
AcO' L'PPh3 2"Pdo(PPh3)(OAc)-" 3[Pdo(PPh3)(OA~)]22-
(3)
In the presence of excess p h o ~ p h i n e : ~ "Pd0(PPh3)(OAc)-" + 2PPh3
5Pdo(PPh3)3(OAc)-
(4)
+
0276-733319512314-1818$09.00/0
The formation of triphenylphosphine oxide could be interpreted as follows: AcO
AcO-PPhz+
% f'PPh3 +H'
H - V
-
AcOH
+ O=PPh3
or
We wish to extend these preliminary results by reporting further kinetic data and investigating the effect of various tertiary phosphines. Also we report data on the reactivity of the resulting palladium(0) complexes in oxidative addition with phenyl iodide.
Results and Discussion a. Kinetic Data on the Formation of Zerovalent Palladium from Pd(0Ac)z+ nPPh (n =- 2) in DMF. As we have already reported,l mixtures of Pd(OAc12 and nPPhs ( n 1 2) immediately afforded a bivalent palladium complex Pd(OAc)z(PPh&, detected and characterized by its reduction peak in cyclic voltammetry at EPRed = -1.38 V us SCE in DMF. This system led spontaneously but slowly to a palladium(0) complex 0 1995 American Chemical Society
Zeroualent Palladium Complexes
Organometallics, Vol. 14,No. 4,1995 1819
b
a
*
a:
f
3
v
+
r
C
2
+ +*
10
L
3
0
v v
v
v
v
VV
vv ,
,
0 0
I
I
. v
;iV
2
1
3
time (h) Figure 1. Variation of the plateau currents at a rotating gold disk electrode (4 = 2 mm; u = 0.02 V s-l; w = 105 rad-s-1) of the system Pd(0Ac)z and PPh3 as a function of time in DMF (nBmNBF4,0.3 M) at 25 "C. (a) Variation of the reduction current at a potential of -1.30 V of the divalent palladium complex formed in situ from the mixture Pd(OAc)z(2 mM) and PPh3 (20 mM) (0).Variation of the oxidation current at a potential of $0.4 V of the zerovalent palladium generated from the mixture Pd(OAc)z(2 mM) and PPh3 (8 mM) (+I, PPh3 (10 mM) (v),and PPh3 (20 mM) (+ and 0). (b) Variation of the oxidation current at a potential of +0.4 V of the zerovalent palladium generated from the mixture PPh3 (20 mM) and Pd(0Ac)z (2 mM) (+), Pd(0Ac)z (1 mM) (v), and Pd(0Ac)z (1 mM) (+I (same as v but a i P d ( 0 ) is plotted to facilitate the comparison because of the concentration change. detected by its oxidation peak in cyclic voltammetry at EPox = f0.054 V us SCE in DMF. The palladium(0) oxidation peak current increased as a function of time a t the expense of the reduction peak current of Pd(OAc)2(PPh3)2. Due to the instability of the palladium(0) complex generated from the mixture of Pd(0Ac)a and 2PPh3, kinetic investigations were achieved in the presence of excess triphenylphosphine ( n > 2). In those cases a saturated stable complex, Pdo(PPh3)3(0Ac)-,was obtained4 and detected by its oxidation peak. Kinetic investigation of this system could be achieved using steady state voltammetry, performed at a rotating disk e1ectrode.l The electrode 'was polarized at a potential of +0.4 V in DMF, on the plateau of the oxidation wave of the zerovalent palladium, and the resulting current was recorded as a function of time. The same technique was used to monitor the disappearance of the starting divalent complex Pd(OAc)2(PPh&, upon polarization on the plateau current of its reduction wave at -1.30 V in DMF. From the curves presented in Figure la, we can observe that the rate of formation of the palladium(0) complex is not affected by the triphenylphosphine concentration, demonstrating that the formation of the palladium(0) complex is zero order in triphenylphosphine. The half life time t112 for the generation of the palladium(0) complex was independent of the initial Pd(0Ac)z concentration ([Pd(OAc)zl= 2 mM, t1/2 = 44 min; [Pd(OAc)zl = 1 mM, t112 = 47 min). This identity was even more striking when the variation of the palladium(0) concentration was plotted as a function of time (Figure lb). Indeed when the data for [Pd(OAc)zl = 1 mM (v)were multiplied by a factor of 2 to account for the change of concentration, the resulting plot (+I became exactly superimposable on that obtained for a n initial concentration of [Pd(OAc)zl = 2 mM (+I. This (4) Evidence for the ligation of the palladium(0) complex by a n acetate anion and its consequences on the rate of oxidative addition will be published later: Amatore, C.; Carre, E.; Jutand, A.; M'Barki, M. A.; Meyer, G . Unpublished results (1994),manuscript in preparation.
-Q 20 3
v
+ C
9 L
10
3
0
0 0
2
1
time
3
(h!
Figure 2. Variation of the plateau currents at a rotating gold disk electrode (4 = 2 mm; u = 0.02 Vs-l; w = 105 rad s-l) of the system Pd(0Ac)z (2 mM) and PPh3 (20 mM) as a function of time in DMF (nBu4NBF4, 0.3 M) at 25 "C, variation of the reduction current at a potential of -1.30 V of the divalent palladium complex (v),variation of the oxidation current at a potential of H . 4 V of the zerovalent palladium (01, sum of the experimental reduction and oxidation currents (+), and difference between the theoretical sum of the reduction and oxidation currents (22.07 PA, horizontal solid lines) and the experimental one (0,dashed line). The solid lines are the theoretical predictions according to reactions 2 and 4 with k l = 4.2 x s-l and kZ' = 1.8 10-3 s-1.
demonstrates that the formation of the palladium(0) complex is first order in palladium(I1) and thus that the palladium(0) complex is spontaneously produced from the bivalent complex Pd(OAc)z(PPh& by an intramolecular reaction. The sum of the oxidation current of the palladium(0) complex and the reduction current of Pd(OAc)z(PPh3)2 was not constant as a function of time (See Figure 2) evidencing the formation of a n intermediate complex (dashed curve in Figure 2) prior the formation of the final complex Pdo(PPh&(OAc)-. The rate of disappear-
1820 Organometallics, Vol. 14, No. 4, 1995
Amatore et al.
b
a
C
\ I
i
-2.0
0.0
0.5
1
.o
-2.0 1.5
‘
0.0
0.5
1
.o
1.5
time (h)
time (hi
Figure 3. Kinetics of the disappearance of the palladium(I1)complex and of the formation of the palladium(0) complex from the mixture Pd(0Ac)z (2 mM) and PPh3 (20 mM) in DMF at 25 “C: (a) Variation of the reduction current of the palladium(I1) complex, ln(i/io) as a function of time; (b) variation of the oxidation current of the palladium(0) complex, ln((iIlm- i)/ilim)as a function of time. ance of the complex Pd(OAc)z(PPh&, represented by the plot of the variation of ln(i/io)p,jcII)(i: reduction current at -1.3 V at t; io at t = 0) as a function of time (Figure 3a), follows a kinetic law first order in palladium(I1) s-l at 25 “C.Howwith a rate constant k l = 4.1 x ever, the rate of formation of the complex PdO(PPh3)s(0Ac)- does not follow a simple first order kinetic law, since the plot of the variation of h((iiim - i)/ilim)Pd(O) (2, oxidation current at +0.4 V at t ; ilim, limit of i at infinite time) as a function of time (Figure 3b) does not afford a straight line with zero intercept. These data are indicative of the formation of an intermediate complex prior the formation of the final complex Pdo(PPh3)3(0Ac)-,according to the following mechanism:
kl
Pd(OAc),(PPh,), A “Pdo(PPh3)(OA~)-” AcO-PPh,+
+
(2)
B
+
“Pdo(PPh3)(OAc)-” 2PPh3
B
k2
Pd0(PPh3),(OAc)- (4) C The formation of the final product Pdo(PPh3)3(0Ac)-(C) obeys the following kinetic law5 (see Experimental Section): [Cl = [AI, - [Alo(k’2e-k1t - kle-k’2t)/(k’2- k,)
k’, = k,[PPh312 When long times are considered, this equation simplifies into ln(([Al, - [Cl)/[AlO)= ln(k’,/(k’, - k,)) - k,t = ln((ilim- i)/ilim) From the slope of the straight line obtained at long (5) Benson, S. W. The Foundations of Chemical Kinetics; McGrawHill Book Co. Inc.: New York, Toronto, London, 1960;p 33.
times (Figure 3b) we get k1 = 4.2 x loF4s-l, a value which is close to that of the rate constant found for the s-l (Figdisappearance of Pd(OAc)2(PPh&, 4.1 x ure 3a). From the intercept we can calculate 12’2 = 1.8 x s-l and k2 = 4.5 M-2 s-l. The fact that k’2 > k1 further establishes our above claim that the intramolecular step 2 is the rate-determining step of the overall reaction 2 + 4.l Moreover such a larger value of k’2 explains why most of the data in Figure l a concerning the formation of the palladium(0) are almost superimposed when the phosphine concentration varies. Indeed, the result of varying the phosphine concentration amounts only in a slight shifi of the data near the origin of time. Since k’2 increases like [PPh3I2, this effect rapidly cancels at large phosphine concentration. From the experimental values of kl and k’2, the values of the concentrations [Bl and [Cl may be determined at any time as well as the theoretical curves for the disappearance of the palladium(I1) complex and for the formation of the palladium(0) complex. The theoretical curves, represented in solid lines in Figure 2, fit the experimental data. These results confirm that a palladium(0) complex is formed directly from Pd(0Ac)n(PPh3)2 by an inner-sphere reduction and that this reaction 2 is the rate-determining step. The final palladium(0) complex Pdo(PPh3)3(0Ac)-is then formed by a faster reaction 4 from the original palladium(0) complex obtained in reaction 2 with triphenylphosphine. Influence of Water on the Rate of Formation of the Palladium(0) Complex from the Mixture Pd(0Ac)z + 1OPPh3 in DMF. Just aRer our former paper concerning triphenylphosphine,l Osawa and Hayashi have reported that the complex Pd(0Ac)zcombined with 3 equiv of the ligand Binap afforded spontaneously and quantitatively a zerovalent palladium complex Pdo(Binap)~and Binap oxide.6 Their reaction was performed in the presence of known amounts of added water, the presence of which was found by these authors to be essential for the quantitative formation of the palladium(0) complex. Using solvents without water resulted in low conversion in palladium(0) complex, while the conversion and the rate of formation of the palladium(0) complex were enhanced by the presence (6)Ozawa, F.; Kubo, A.; Hayashi, T. Chem. Lett. 1992, 2177
Zerovalent Palladium Complexes
Organometallics, Vol. 14,No.4,1995 1821
of an increasing amount of water. In our case, all reactions were performed in the absence of purposely added water but the amount of residual water in carefully distilled DMF had not been determined. However, we found that the presence of purposely added water (10 equiv) did not affect the rate of the formation of the palladium(0) complex (kl = 3.9 x s-l). It means that if residual water was involved in the mechanism of the formation of zerovalent palladium, this reaction would take place after the rate-determining step, viz. after the intramolecular reduction step 2. After complete conversion of Pd(OAc)2(PPh3)2 to Pdo(PPh3)3(0Ac)-, a reduction peak was detected in cyclic voltammetry at -1.61 V in DMF. Addition of 1 equiv of acetic acid resulted in the same reduction peak with a double magnitude, demonstrating that acetic acid was formed in a quantitative yield during the conversion of Pd(OAc)z(PPh& to Pdo(PPh3)3(0Ac)-. Since the resulting palladium(0) is ligated by one acetate: 1equiv of acetic acid can only be produced by reaction 5. This implies that, in this experiment, the role of water (viz. the residual water in our experiments) is to convert an intermediate phosphonium salt into triphenylphosphine ~ and oxide (eq 5) as in the Mitsunobu r e a ~ t i o n .Osawa Hayashi' results6 could then be interpreted by considering that, at least under their conditions, the intramolecular step 2 is a reversible reaction that could be shifted t o the formation of the palladium(0) complex by reaction of water with the postulated phosphonium salt, according to a reaction similar to reaction 5. Influence of a Trialkylamine on the Rate of Formation of the Palladium(0)Complex from the Mixture Pd(OAc12 + 10PPh3, in DMF. Before we1 and, later, Hayashi's group6 demonstrated that the triphenylphosphine was able to reduce the palladium acetate into a palladium(0) complex, it was very often suggested that a trialkylamine, which is always present and essential in order for the Heck r e a ~ t i o nto~ proceed, ,~ was the reductant able to reduce the palladium acetate to palladium(0),8 especially when the Heck reactions were performed in the absence of any p h o ~ p h i n e . ~ Investigation by cyclic voltammetry of the Pd(OAc12 salt, alone or in the presence of triethylamine (2 equiv), afforded badly resolved reduction peaks. In the oxidation range, no oxidation peaks could be detected except that of the triethylamine at +OB9 V. However, in the absence of any authentic sample of the postulated palladium(0) complex formed in the case where the triethylamine could reduce the palladium acetate, it is hazardous at this stage to conclude whether triethylamine is able or not to reduce Pd(0Ac)z under our experimental conditions.10 It is why we have investigated the kinetics of the formation of the palladium(0) complex from the mixture Pd(0Ac)z 50 equiv of NEt3
+
(7) Pautard-Cooper, A.; Slayton A. E. J . Org. Chem. 1989,54,2485. ( 8 ) Collman, J. P.; Hegedus, L. S.; Norton, J. R.: Finke, R. G. Principles and Applications of Organotransition Metal Chemistry, 2nd ed.; University Science Books: Mill Valley, CA, 1987; p 725. (9) Ziegler, C. B., Jr.; Heck, R. F. J . Org. Chem. 1978,43, 2941. (10) Were the amine able to reduce the palladium acetate, it would result in the formation of palladium metal in the absence of any ligand able to stabilize the palladium(0). This metallic palladium could not be of course detected by cyclic voltammetry. It was reported that triethylamine partially reduces complexes such as PdC12(RCN)( R = Ph or Me) into metallic palladium. See: McCrindle. R.; Ferguson, G.; Arsenault, G. J.; McAllees, A. J.; Stephenson, D. K. J. Chem. Res. Synop. 1984, 360.
L
\
Y '= L
i
2.9
3.3
10317
3.5
( ~ - 1 1
Figure 4. Arrhenius plot for the disappearance of the palladium(I1) complex (0) and for the formation of the palladium(0) complex ( 0 )from the mixture Pd(0Ac)z (2 mM) and PPh3 (20 mM) in DMF (k,s-1). in the presence of 10 equiv of PPh3 in order that a stable palladium(0) complex might be detected. A similar study as described above showed that, in the presence or in the absence of the amine, palladium(0)was formed with an identical rate constant of K 1 = 4.0 x s-l at 25 "C. So it is inferred that were triethylamine able to reduce the palladium acetate, this reaction would be much slower than the reduction by triphenylphosphine. When the reaction was performed in the presence of triethylamine, the reduction peak of acetic acid was of course not detected because of its reaction with the amine. Influence of the Temperature on the Rate of Formation of the Palladium(0)Complex from the Mixture Pd(0Ac)z 10PPh3 in DMF. The rate constant of disappearance of Pd(OAc)2(PPh3)2 (calculated as in Figure 3a) and that of formation of the palladium(0) complex (calculated as in Figure 3b by considering the limiting straight line at long times) from the mixture Pd(0Ac)z (2 mM) and PPh3 (10 equiv) were determined a t various temperatures in the range 1560 "C, in DMF. The results showed that the rate of formation of the palladium(0) complex increases with the temperature (eg. t l l 2 = 5620 s at 15 "C and t l i 2 = 106 s at 60 "C). The rate of disappearance of the complex Pd(OAc)z(PPh& also increases with the temperature. An Arrhenius plot for both reactions (disappearance of Pd(I1) and formation of Pd(0)) afforded a common straight line (Figure 4) and allowed an evalu= 65 k J mol-l ation of the activation parameters: and AS* = -94 J mo1-l K-l for reaction 2. The fact that either considering the rate of disappearance of the palladium(I1) complex or that of the formation of the palladium(0) complex affords similar values of hHs and AS* further supports that the elemental reaction 2 that produces palladium(0) from Pd(0Ac)z(PPhd2 is the rate-determining step of the overall reaction. The negative value found for the entropy means that the palladium(I1) complex of the transition state is more organized than the original complex Pd(OAc)2(PPh3)2. It is therefore reasonable to propose that the palladium(0) complex is produced by a sort of reductive elimination occurring in the inner sphere of the palladium(I1) complex (reaction 2). All these results clarify and confirm the mechanism we have already proposed (reactions 1, 2, 4, and 51.l
+
Amatore et al.
1822 Organometallics, Vol. 14, No. 4, 1995
Table 1. Reduction and Oxidation Peak Potentials of the P~(OAC)~((PZ-C&)~P)~ and Pdo((PZ-C6H4)3P)3(OAc)Complexes
--
-0.5
-1.5
+0.5
-0.5
Z
o Hammett
CF1 CI F H CH3 CHiO
+OS4 +0.227 +0.062 0 -0.170 -0.268
Pd(OAc)?((pZ-ChHd)jP)? Pdn((pZ-ChH4)3P)3(OAc)D R e d . V v s SCE" D o x .V vs SCE"
- 1.205 n.d. - 1.335 - 1.395 - 1.425 - 1.505
+OS45 +0.25~ +0.205 +0.055 -0.035 -0.105
' I Determined at 0.2 V s-I at a stationary gold disk electrode (@ = 0.5 mm) in DMF containing nBu4NBF4 (0.3 M), at 25 "C.
E, volts vs SCE
Figure 5. (a) Cyclic voltammetry of Pd(OAc)z(@-CF3C6H4)3P)z generated from Pd(0Ac)z (2 mM) and @-CF3C6H4)3P(20 mM) in DMF (nBu4NBF4, 0.3 M) at a stationary gold disk electrode (4 = 0.5 mm) with a scan rate of 0.2 V s-l. The cathodic scan, from -0.4 to -1.7 V, has been performed as a function of time: (-) 6, (. 12, (- -) 19 mn. Only forward scans are shown for 12 and 19 mn for simplification. (b) Cyclic voltammetry of the palladium(0) complex generated in situ from Pd(0Ac)z (2 mM) and (pCF&6H4)3P (20 mM), in DMF (0.3 M nBmNBF4) at the same electrode, with a scan rate of 0.2 V s-l. The anodic scan, from -0.6 to +1V, has been performed as a function of time: (-1 9, 13, ( - . - - ) 21 mn. .e)
0 .
(..a)
In the presence of at least four phosphine ligands, a stable complex could be accumulated: Pd(OAc),
+ 4 PPh, + H,O - PdO(PPh,),(OAc)- + AcOH + O=PPh3 + H+ (7)
b. Kinetic Data on the Formation of Zerovalent Palladium from Pd(0Ac)zand a Series of Tertiary Phosphines in DMF. The formation of palladium(0) complexes from mixtures of Pd(0Ac)n and tertiary phosphinesll was first investigated by considering triarylphosphines para-substituted by electron-withdrawing or -donating groups Z: @Z-C6H4)3P. The reaction was investigated with mixtures of Pd(0Ac)z and 10 equiv of the triarylphosphines in order to obtain a stable palladium(0) complex (reaction 6). The behavior of these systems was similar to that previously observed for triphenylphosphine. Indeed, whatever the triarylphosphine, the mixture of Pd(0Ac)z and 10 equiv of (pZ-Cs&)# in DMF, immediately afforded a bivalent palladium complex Pd(OAc)e((pZC S H ~ ) ~detected P ) ~ by its reduction peak (see Figure 5a and Table 1). As expected, the resulting complexes were more easily reduced when the triarylphosphines were substituted by an electron-withdrawing group (see Table 1). The resulting palladium(I1) complexes were not stable and led spontaneously to species detected by their oxidation peak (see Figure 5b and Table 1). Their oxidation peak current increased with time concomitantly with a decrease of the reduction peak current of the divalent palladium complexes Pd(OAc)z(@Z-C&)3P)2 (Figure 5a,b). After addition of iodobenzene, the oxidation peaks were no more observed, evidencing that the species that was formed from spontaneous evolution of the complex P ~ ( O A C ) ~ ( ( ~ Z - C ~was H ~a) ~zerovalent P)~ palladium complex able to react with iodobenzene by (11) The influence of various phosphines on the Heck reaction has been investigated. See ref 9.
a n oxidative addition. As expected, the resulting zerovalent complexes Pd0((pZ-CsH4)3P)3(0Ac)-were more easily oxidized when the triarylphosphines were substituted by a n electron donor group (Table 1). A kinetic study similar to that mentioned above for triphenylphosphine allowed the determination of the rate constant of the formation of the palladium(0) complex, K l Z . We observed that the rate of this reaction was faster when the triarylphosphine was substituted by an electronwithdrawing group. In order to obtain reasonable reaction times in the case of electron-donating substituents and to allow comparison between phosphines, all reactions were performed at 60 "C. A plot of the variation of log(KIZ/klH)as a function of the (T Hammett constants showed that the rates of formation of palladium(0) complexes from P~(OAC)Z((PZ-CSH&P)~ follow a Hammett correlation with a positive value of e = f 0 . 9 (Figure 6a). These results show that the formation of the palladium(0) complex according to reaction 2' is favored Ar3P,
,OAc pdll)
ki
"PdO(PAr3)(0Ac)-" + AcO-PAr3+
(2')
AcO' 4c'PAr3
when the electronic density on the phosphorous is low (electron-withdrawing group) i.e. when it is prompt to be attacked by the acetate leading to the reductive elimination-like step. However it should be mentioned that, at 25 "C, the determination of e, made in the case of electron-withdrawing groups, led to e = f2.4 (Figure 6b). This shows that reaction 2' proceeds through a transition state, in which the stabilizing effects due t o phosphine electronic properties become less effective at higher temperatures. Results concerning other tertiary phosphines such as PPhnMe, PMezPh, and P B u P are more difficult to rationalize when compared with triphenylphosphine. In this series, we can observe from Figure 7a,b that no correlation is found between the rate of formation kl of the palladium(0) complex with either electronic factors (as evaluated from pK values13) or steric ones (as evaluated from cone angles el4)when both parameters vary. However, when only electronic factors vary (such as in para-substituted triarylphosphines), a good correlation is obtained (Figure 6). This suggests that when both factors are varied, the rate constant is sensitive t o (12) ( a )Mandai, T.; Matsumoto, T.; Tsuji, 3. Tetrahedron Lett. 1993, 34,2513. (b)Ono, K.; Fugami, K.; Tanaka, S.; Tamaru, Y. Tetrahedron Lett. 1994, 35, 4133. (13)Zizelman, P. M.; Amatore, C.; Kochi, J. K. J . Am. Chem. SOC. 1984, 106, 3771. (14)Tolman, C. A. Chem. Rev. 1977, 77, 313.
Zerovalent Palladium Complexes
Organometallics, Vol. 14, No. 4, 1995 1823
b
a 0.5
-
.
I-
1
0.2
-J-
Y
v
-
-0.1
-0.4
-0.6
-0.4 -0.2 0.0 0.2 D
0.0
0.6
0.4
Hammett
0.2
0.6
0.4
D Hammett
Figure 6, Hammett plot for the rate of the formation of the palladium(0)complex generated from the mixture Pd(0Ac)z (2 mM) and a series of para-substituted triarylphosphines (p-Z-CeH&P (20 mM) in DMF ( k l , s-l): (a)At 60 "C (e = +0.9; r = 0.973); (b) at 25 "C (e = +2.4; r = 0.998).
-0.5
-0.5
PPh,Me
-
0
-1.0 .
-
0
PPh,Me -
0
,-
?
x
v
- 1.0
-1.5.
PMe2Ph
5 Y
0
0 -
0 -2.0
.
PPh,
PBu,
0
0
4
-2.5
-1.5.
PMe,Ph 0
-2.0 .
PBU,
PPh,
0
0
-2.5
cone angle
PK
Figure 7. Variation of the rate of formation, k~ (s-l), of the palladium(0)complex from mixtures of P ~ ( O A C(2) mM) ~ and a series of tertiaryphosphines (20 mM) in DMF at 60 "C: (a) As a function of the pK of the phosphine; (b) as a function of the cone angle of the phosphine.
both variations. However, a simple linear relationship of the kind1, log12, = A + B p K + C8
(8)
does not hold any better. This is evidenced in Figure 7a,b by the nonlinear variations for the series PPh3, PPhzMe, and PMezPh. Indeed, in this series, 0 and pK values correlate linearly ( r = 0.99):13
8 = 167.3 - 7.lpK
(9)
Were a two-parameter linear equation such as that in (9) to hold, owing t o the fortuitous relationship in (9), one would obtain a linear correlation with either pK or 0 values for the series PPh3, PPhzMe, and PMezPh. Taking into account the fact that when 0 is constant (Figure 61, a good correlation is observed with electronic factors, it appears that the absence of correlation observed in Figure 7a,b stems from the fact that cone angle parameter is involved through nonlinear contributions. In practice, an extremely good correlation ( r = 0.999) is obtained by considering a quadratic involvement of the cone angle: log 12, = 1.00 - 0.35pK - 8.7 x
- 129)2 (10)
Such a quadratic dependance on cone angle suggests that two opposing effects are involved when the steric
parameters are modified. This may be rationalized by the following interpretation: when 0 increases, steric constraint builds up in the Pd(OAc)z(PR& complex thus favoring its reorganization to a nonplanar geometry. This would favor transition t o the activated complex involved in reaction 2'. Conversely, when 8 is too large, it is probable that intramolecular attack on the phosphine by the acetate is disfavored. Due to these antagonist effects, one should observe an optimum value of steric parameters. Equation 10 indicates that this optimum value is close t o a cone angle of 129". c. Kinetic Data on the Oxidative Addition of Phenyl Iodide with Palladium(0)Complexes Generated from Pd(0Ac)z and Tertiary Phosphines, in DMF. Reactivity of the Palladium(0) Complex Generated from Pd(0Ac)z and 1OPPb. The oxidation peak (EP = f0.054 V us SCE) of the complex Pdo(PPh3)3(0Ac)-,generated from the mixture of Pd2 mM in DMF, combined with 10 equiv of (OAC)~, triphenylphosphine, disappeared in the presence of phenyl iodide, demonstrating that the oxidative addition in eq 11 took place, the final product of the reaction PdO(PPh,),(OAc)-
+ PhI kapp PhPd(OAc)(PPh,), + I- + PPh, (11)
being PhPd(OAc)(PPh&. 1,4 Therefore the reactivity of
Amatore et al.
1824 Organometallics, Vol. 14,No. 4,1995
b
a 2o
I
b b b
50
0
100
150
200
250
time is)
0
20
60
40
80
100
120
time (s)
Figure 8. Oxidative addition of phenyl iodide (2 mM) with the palladium(0) complex quantitatively generated in situ from Pd(0Ac)z (2 mM) and PPh3 (20 mM), in DMF (nBu4NBF4, 0.3 M) at 25 "C: (a) Variation of the oxidation plateau current ipd(0) at f0.2 V, at a rotating gold disk electrode (4 = 2 mm; u = 0.02 Vs-'; w = 105 rad s-l) in the presence of phenyl iodide (2 mM) as a function of time; (b) variation of idi as a function of time.
.o
o \
0.0
' .c
29
3
'1
10'
-
33
c.4
35
f c ' ,
Figure 9. Arrhenius plot for the oxidative addition of phenyl iodide with the palladium(0) complex generated from Pd(0Ac)z (2 mM) and PPh3 (20 mM), in DMF (nBu4NBF4, 0.3 M) (K,,,, M-' s-').
the zerovalent palladium complex with phenyl iodide could be monitored by amperometry. A kinetic investigation of this oxidative addition was carried out using steady state cyclic voltammetry, performed at a rotating disk electrode, as the analytical technique.lJ5 After complete conversion of the palladium(I1) complex to the palladium(0) complex, the electrode was polarized at a potential of +0.20 V on the plateau of the oxidation wave of the palladium(0) complex, and the resulting oxidation current was recorded as a function of time, after addition of 1 equiv of phenyl iodide. A typical curve is represented in Figure 8a. A plot of the ratio idi as a function of time results in a straight line (idi= -k,,,cot + 1; Figure 8b) demonstrating that the oxidative addition is first order in palladium(0) and in phenyl iodide. The value of the apparent rate constant of the oxidative addition could then be deduced from the slope of the straight line represented in Figure 8b and was found to be kapp= 12 M-l s-l, at 25 "C. The reactivity of phenyl iodide with the palladium(0) complex was investigated at various temperatures in the range 15-60 "C. An Arrhenius plot afforded a straight line (Figure 9) and allowed the determination of the activation parameters: AW = 60 f 7 k J mol-I and AS* = -1 f 1 J mol-I K-l. These values have to be compared to that found for the oxidative addition of
'0.C CJ
3.4
'3.8
barme:!
Figure 10. Hammett plot for the oxidative addition of phenyl iodide with the palladium(0) complex generated from mixtures of Pd(0Ac)z (2 mM) and a series of parasubstituted triarylphosphines (p-Z-CeH4)3P (20 mM) in DMF (nBu4NBF4, 0.3 M) at 25 "C (e = -2.8; r = 0.951).
PhI with Pdo(PPh3)4, in the presence of 50 equiv of PPhg,lbaAV = 77 k J mol-l and AS*= +14 J mol-l K-l, and to that of Pd(OI(PPh314,in the presence of 100 equiv of chloride anions,15bAV = 95 k J m o t 1 and AS* = +lo1 J mol-l K-l. The values of A.W are comparable for the three systems. However the values of AS* are very different. The high value AS* = +lo1 J mol-' K-I found in the case of the oxidative addition of Pdo(PPh3)4 in the presence of chloride anions was interpreted by the cleavage of two bonds (uiz. Pd-C1 and Pd-PPh3) from the complex Pdo(PPh3)3C1-during the oxidative addition (eq 12).15bThe small value of AS* found in the
+
-
PhPdI(PPh,),
+ C1- + PPh3
AS* = +lo1 J mol-' K-'
(12)
Pdo(PPh3)3C1- PhI
present paper seems to indicate that cleavage of only one bond occurred, despite the presence of an acetate anion ligated to the palladium(0) in P~O(PP~&(OAC)-.~ A tentative explanation could be the formation of a transient pentacoordinated anionic a-arylpalladium complex in which the palladium(I1) would remain li(15)( a ) Fauvarque, J. F.; Pfluger, F.; Troupel, M. J. Organonet. Chem. 1981,208,419.(b) Amatore, C.;Azzabi, M.; Jutand, A. J.Am. Chem. SOC.1991,113, 8375.
Organometallics, Vol. 14, No. 4, 1995 1825
Zerovalent Palladium Complexes
b
a 2.8
PBU, A
8
Y0
2.4
0 -
2 .o
2
8
6
4
10
120
130
140
150
cone angle
PK
Figure 11. Variation of the rate of the oxidative addition, kapp(M-l s-l), of phenyl iodide with the palladium(0) complex generated from mixtures of Pd(0Ac)z (2 mM) and a series of tertiary phosphines (20 mM) in DMF at 60 "C: (a) As a function of the pK of the phosphine (slope = +0.10; r = 0.995); (b) as a function of the cone angle of the phosphine (slope = -0.015; r = 0.999, except for PBud. gated by an acetate anion.16 Pdo(PPh3),(OAc)-
+ PhI -
PhPdI(PPh,),(OAc)-
+ PPh3 (13)
AS* = -1 J mol-' K-' PhPdI(PPh,),(OAc)-
-
PhPd(OAc)(PPh,),
+ I-
(14)
This hypothesis is indeed supported by the fact that the final product of the oxidation addition is not PhPdI(PPh& but PhPd(oA~)(PPhs)e.~,~ Reactivity of the Palladium(0)Complex Generated from Pd(0Ac)zand lop&. The same experiments were performed with triarylphosphines substituted in the para position by electron-withdrawing or -donating groups, 2. A plot of the variation of log(k,p,z/ K a p p ) as a function of the 0 Hammett constants showed that the oxidative addition follows a Hammett correlation with e = -2.8 (Figure 10). As expected, the negative value of e indicates that the oxidative addition of phenyl iodide with Pd0((pZ-C6H4)3P)3(OAc)-is faster when the aryl group of the triarylphosphine is substituted by an electron-donating group, in which case the nucleophily of the palladium(0) complex is enhanced. Reactivity of the Palladium(0)Complex Generated from Pd(0Ac)zand 10PR3. The apparent rate constant of the oxidative addition with phenyl iodide has been determined for different tertiary phosphines. The logarithm of the apparent rate constant varied linearly with the pK of the phosphine (Figure l l a ) . As expected, the oxidative addition of phenyl iodide with the palladium(0)complex is favored when the ligand is more basic. No correlation was found when considering the cone angle (Figure l l b ) . However, in the series PPh3, PPhnMe, and PMezPh, since 8 and pK values correlate (see eq 9),13 a good correlation was found for the cone angle (Figure l l b ) ; the higher the cone angle of the phosphine, the lower the reactivity of the palladium(0) complex. (16) Anionic pentacoordinated arylpalladium complexes ArPdI(PPh&Cl- have been reported as intermediates in oxidative additions. See: Amatore, C.; Jutand, A.; Suarez, A. J . Am. Chem. SOC.1993,115, 9531.
Conclusion Mixtures of Pd(0Ac)z and tertiary phosphines afford spontaneously a palladium(0) complex and the corresponding phosphine oxide. The reaction proceeds via a reductive elimination performed on a palladium(I1) complex, Pd(OAc)2(PR3)2. This inner-sphere reduction is the rate-determining step of the overall reaction. The reaction seems to be general since it occurs with aromatic phosphines as well as with aliphatic phosphines. The rate of formation of the palladium(0) complex is higher when the triarylphosphine is substituted by a n electron-withdrawing group. The resulting palladium(0) complex Pdo(PR3)3(0Ac)-undergoes oxidative addition with phenyl iodide. The rate of this reaction is higher when the triarylphosphine is substituted by a n electron-donating group and when the phosphine is more basic. We also observed that mixtures of triphenylphosphine with other bivalent palladium salts such as Pd(0COCF3)2 and Pd(Acac)z (Acac = acetylacetonate) result in the formation of palladium(0) complexes and triphenylphosphine 0 ~ i d e . lWhen ~ the bivalent palladium does not contain an oxygenated ligand such as in the cationic complex Pd(PPh&(BF& stable palladium(0) complexes are spontaneously formed, provided water and triphenylphosphine are present in solution.18 It seems that the spontaneous reduction of bivalent palladium complexes to zerovalent palladium by triphenylphosphine is a general reaction that occurs as soon as the divalent palladium is combined with an oxygenated ligand such as N03-,19 A c O - , ~ ,CF~COZ-," ~ Acac-,17 oxide,lg or OH-.lWo Experimental Section All the experiments were performed under argon. Chemicals. DMF was distilled from calcium hydride. Iodobenzene and triethylamine were commercial and used after filtration on alumina. Pd(OAc)z, Pd(OCOCF3)s, Pd(17) As observed in preliminary studies involving different bivalent palladium salts with oxygenated and nonoxygenated anions: Amatore, C.; Jutand, A,; MBarki, M. A. Unpublished results (1994). (18)Amatore, C.; Jutand, A.; Medeiros, M. J. Unpublished results (19941, manuscript in preparation. (19) Malatesta, L.; Angoletta, M. J. J.Chem. SOC.1967,1186. (20)Grushin, V. V.; Alper, H. organometallics 1993,12, 1890.
Amatore et al.
1826 Organometallics, Vol. 14, No. 4, 1995 (Acac)Z, and triphenylphosphine were commercial (Janssen). The other trialkyl- and arylphosphines were also from a commercial source (Strem Chemicals). Pd(PPhdz(BF4)z was synthesized according t o a published procedure.21
Electrochemical Setup and Electrochemical Procedure for Cy :lic Voltammetry. Cyclic voltammetry was performed with a homemade potentiostatZ2and a wave-form generator, PAR Model 175. The cyclic voltammograms were recorded with a Nicolet 3091 digital oscilloscope. Experiments were carried out in a three-electrode cell connected to a Schlenk line. The cell was equipped with a double envelope in order to perform the reactions at different temperatures, using a Lauda M3 thermostat. The counter electrode was a platinum wire of ca. 1cm2apparent surface area; the reference was a saturated calomel electrode (Tacussel) separated from the solution by a bridge (3 mL) filled with a 0.3 M nBu4NBF4 solution in DMF. A 12-mL volume of DMF containing 0.3 M nBu4NBF4,was poured into the cell. A 5.4-mg amount (2 x M) of Pd(OAc)2was then added, followed by the suitable amount of the desired phosphine. The cyclic voltammetry was performed at a stationary disk electrode (a gold disk made from cross section of wire (4 = 0.5 mm) sealed into glass) at a scan rate of 0.2 V s-l. The kinetic measurements were performed by steady state cyclic voltammetry at a rotating disk electrode (a gold disk (4 = 2 mm) inserted into a Teflon holder, Tacussel ED1 65109) at a scan rate of 0.02 V s-l and an angular velocity of 105 rad (Tacussel controvit). The RDE potential was set on the plateau of the reduction wave of the bivalent palladium complex, and the reduction current was monitored as a function of time up to 100% conversion. In another set of experiments, the RDE was polarized on the plateau of the oxidation wave of the zerovalent palladium complex, and the oxidation current was monitored as a function of time. When the limit of the oxidation current was reached, the suitable amount of iodobenzene was added to the cell and the oxidation current recorded to follow the kinetics of oxidative addition with PhI.’J5
Derivation of Kinetic Laws Corresponding to the Mechanism Represented by Reactions 2 and 4.5 With the notations in eqs 2 and 4, one has (21) Bushnell, G . W.; Dixon, K. R. Can. J . Chem. 1972, 50, 3694. (22) Amatore, C.; Lefrou, C.; Pfluger, F.J . Electround. Chem. 1989, 270,43.
d[A]/dt = -kl[Al Straightforward integration affords [A] = [A], and d[Bl/dt = kl[Al - k‘,[Bl = kl[AlOe-k’t- k’,[Bl
k’, = k,[L12 d[BYdt + k’,[Bl = k,[AI,
The formation of the final product Pdo(PPh&(OAc)- (C) obeys the following kinetic law: d[Cl/dt = k‘,[Bl = k’z(e-kl‘ - e-k’2t)/(k1[A10)/(k’2 - k,) From where it follows that
Since at long times, k’< ([AI,
- [CI)/[Al,
>
kle-k‘2t,we get
%
(k’,e-k’t)/(k‘2- k,)
That is
Acknowledgment. This work has been supported in part by the Centre National de la Recherche Scientifique (CNRS, URA 1679, “Processus d’Activtion Mo16culaire”) and the Ministere de 1’Enseignement Sup6rieur et de la Recherche (Ecole Normale SupBrieure, D6partement de Chimie). OM940838T
