Factors affecting the oxidative addition of aryl electrophiles to 1,1′-bis(diphenylphosphino)ferrocenepalladium(η2-methyl acrylate), an isolable Pd[0] alkene complex

Article abstract of DOI:10.1021/om9906417

The title complex 3 has been synthesized and characterized by X-ray crystallography. It undergoes oxidative addition with aryl triflates and iodides by a nondissociative mechanism in THF. The Heck addition product methyl cinnamate was detected at traces from the reaction with phenyl triflate, and from iodobenzene the direct oxidative addition product was characterized. The addition of halides or acetate salts to the system led to modest effects on the rate of oxidative addition of aryl triflates, but resulted in neutral arylpalladium(II) complexes. Addition of lithium or europium salts enhanced the reaction rate, the effect being most pronounced with Eu(OTf)₃, possibly through promotion of a competing dissociative pathway.

Full text of DOI:10.1021/om9906417

Organometallics 1999, 18, 5367-5374
5367
F a ctor s Affectin g th e Oxid a tive Ad d ition
of Ar yl Electr op h iles to
1,1′-Bis(d ip h en ylp h osp h in o)fer r ocen ep a lla d iu m (η2-m eth yl
a cr yla te), a n Isola ble P d [0] Alk en e Com p lex
Anny J utand,*,† King Kuok (Mimi) Hii,‡,§ Mark Thornton-Pett, and
J ohn M. Brown‡
De´partement de Chimie, Ecole Normale Supe´rieure, UMR CNRS 8640, 24 Rue Lhomond,
75231 Paris Cedex 05, France, Dyson Perrins Laboratory, South Parks Rd.,
Oxford OX1 3QY, U.K., and School of Chemistry, University of Leeds, Woodhouse Lane,
Leeds LS2 9J T, U.K.
Received August 9, 1999
The title complex 3 has been synthesized and characterized by X-ray crystallography. It
undergoes oxidative addition with aryl triflates and iodides by a nondissociative mechanism
in THF. The Heck addition product methyl cinnamate was detected at traces from the
reaction with phenyl triflate, and from iodobenzene the direct oxidative addition product
was characterized. The addition of halides or acetate salts to the system led to modest effects
on the rate of oxidative addition of aryl triflates, but resulted in neutral arylpalladium(II)
complexes. Addition of lithium or europium salts enhanced the reaction rate, the effect being
most pronounced with Eu(OTf)3, possibly through promotion of a competing dissociative
pathway.
Sch em e 1. Syn th etic Rou te to Com p lex 3 a n d Its
Role in th e Br ea k d ow n of a n Obser ved
In ter m ed ia te in Heck Ch em istr y
In tr od u ction
The mechanism of the Heck reaction continues to
excite interest and some controversy.1 In previous work
the nature of the reactive intermediates has been
considered by electrochemical techniques,2 by NMR, or
by mass spectrometry.3 In the course of this some
attention has been given to the reactivity of aryl triflates
toward zerovalent palladium complexes.4 In NMR stud-
ies that simulate the catalytic cycle but under stoichio-
metric conditions, breakdown of the Heck addition
product 1 in the presence of proton sponge and excess
alkene 2 leads to the alkenepalladium complex 1,1′-bis-
(diphenylphosphino)ferrocenepalladium(η2-methyl acry-
late), (dppf)Pd(MeA), 3 (Scheme 1).5 This can be sepa-
rately synthesized from the corresponding PdCl2 complex
4 and is moderately stable. Here we report an electro-
chemical study of complex 3 toward the oxidative
† De´partement de Chimie, Ecole Normale Supe´rieure.
‡ Dyson Perrins Laboratory.
addition of aryl electrophiles, both triflates and iodides.
This enables us to present a comparison between the
behavior of a stable Pd[0] alkene complex and other
potential catalyst precursors, such as Pd(PPh3)4,4 as well
as those generated in situ from phosphines and Pd(dba)2
(dba ) dibenzylidene acetone), reported in earlier work.6
Previous experiments with Pd(PPh3)4 had revealed that
chloride anions have an accelerating effect on the rate
of the oxidative addition of aryl triflates, as
§ Currently at Department of Chemistry, King’s College London,
Strand, London WC2R2LS, U.K.
University of Leeds.
(1) For possible intervention of palladium(IV) intermediates in
specific circumstances see: (a) Ohff, M.; Ohff, A.; Van der Boom, M.
E.; Milstein, D. J . Am. Chem. Soc. 1997, 119, 11687. (b) Shaw, B. L.
New J . Chem. 1998, 22, 77. (c) Shaw, B. L.; Perera, S. D.; Staley, E. A.
Chem. Commun. 1998, 1361.
(2) (a) Amatore, C.; J utand, A.; M’Barki, M. A. Organometallics
1992, 11, 3009. (b) Amatore, C.; Carre´, E.; J utand, A.; M’Barki, M. A.
Organometallics 1995, 14, 1818. (c) Amatore, C.; Carre´, E., J utand,
A.; M’Barki, M. A.; Meyer, G. Organometallics 1995, 14, 5602.
(3) (a) Brown, J . M.; Guiry, P. J . Inorg. Chim. Acta 1994, 220, 249.
(b) Brown, J . M.; Perez-Torrente, J . J .; Alcock, N. W.; Clase, H. J .
Organometallics 1995, 14, 207. (c) Brown, J . M.; Hii, K. K. Angew.
Chem., Int. Ed. Engl. 1996, 35, 657. (d) Hii, K. K.; Claridge, T.; Brown,
J . M. Angew. Chem., Int. Ed. Engl. 1997, 36, 984.
(6) (a) Amatore, C.; J utand, A.; Khalil, F.; M’Barki, M. A.; Mottier,
L. Organometallics 1993, 12, 3168. (b) Amatore, C.; Broeker, G.;
J utand, A.; Khalil, F. J . Am. Chem. Soc. 1997, 119, 5176. (c) Amatore,
C.; J utand, A.; Meyer, G. Inorg. Chim. Acta 1998, 273, 76. (d) Amatore,
C.; J utand, A.; Meyer, G.; Atmani, H.; Khalil, F.; Chahdi, F. O.
Organometallics 1998, 17, 2958.
(4) J utand, A.; Mosleh, A. Organometallics 1995, 14, 1810.
(5) Hii, K. K.; Brown J . M. Manuscript in preparation.
10.1021/om9906417 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 11/17/1999

5368 Organometallics, Vol. 18, No. 25, 1999
J utand et al.
Ta ble 1. In ter a tom ic Dista n ces (Å) w ith esd ’s in
P a r en th eses
Fe(1)-C(6)
Fe(1)-C(5)
Fe(1)-C(10)
Fe(1)-C(3)
Fe(1)-C(8)
C(1)-C(2)
2.040(2)
2.042(2)
2.056(2)
2.064(2)
2.066(2)
1.437(3)
1.821(2)
1.424(3)
1.431(3)
1.808(2)
1.420(3)
1.831(2)
1.395(3)
1.393(3)
1.388(3)
1.395(3)
1.391(3)
1.383(3)
1.839(2)
1.398(3)
1.389(3)
1.387(3)
1.394(3)
1.395(3)
1.389(4)
2.3234(5)
2.122(2)
1.414(3)
0.96(3)
Fe(1)-C(7)
Fe(1)-C(1)
Fe(1)-C(2)
Fe(1)-C(4)
Fe(1)-C(9)
C(1)-C(5)
2.042(2)
2.048(2)
2.059(2)
2.065(2)
2.069(2)
1.437(3)
1.425(3)
1.426(3)
1.445(3)
1.424(3)
1.417(3)
1.831(2)
1.403(3)
1.386(3)
1.389(3)
1.401(3)
1.389(3)
1.391(3)
1.839(2)
1.400(3)
1.396(3)
1.394(3)
1.394(3)
1.390(4)
1.393(3)
2.3089(5)
2.160(2)
1.00(3)
C(1)-P(1)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(6)-C(10)
C(6)-P(2)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
P(1)-C(11)
C(11)-C(16)
C(13)-C(14)
C(15)-C(16)
C(21)-C(22)
C(23)-C(24)
C(25)-C(26)
P(2)-C(31)
C(31)-C(32)
C(33)-C(34)
C(35)-C(36)
C(41)-C(42)
C(43)-C(44)
C(45)-C(46)
Pd(1)-P(2)
Pd(1)-C(52)
C(51)-H(51a)
C(52)-C(53)
C(53)-O(53)
O(54)-C(54)
P(1)-C(21)
C(11)-C(12)
C(12)-C(13)
C(14)-C(15)
C(21)-C(26)
C(22)-C(23)
C(24)-C(25)
P(2)-C(41)
C(31)-C(36)
C(32)-C(33)
C(34)-C(35)
C(41)-C(46)
C(42)-C(43)
C(44)-C(45)
Pd(1)-P(1)
Pd(1)-C(51)
C(51)-C(52)
C(51)-H(51b)
C(52)-H(52)
C(53)-O(54)
F igu r e 1. X-ray structure of complex 3, with numbering
corresponding to the assignments in Tables 1 and 2.
1.467(3)
1.212(3)
1.443(3)
0.92(3)
1.362(3)
well as on the structure of the complex formed in this
reaction.4 Due to current interest in the role of potential
promoters7 for catalytic Heck reactions, we initiated a
study of the effect of polar addends on the oxidative
addition of complex 3 with aryl triflates.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of Com p lex 3.
In situ reduction of diphosphinepalladium dihalides
with cyclooctatetraene dianion gives an η2-COT complex
which may be used as a precursor to other alkene
complexes.8 It is possible to prepare the complex (dppf)-
Pd(MeA), 3, in this way, and it proved to be reasonably
stable in the solid state. In solution the 31P NMR is a
closely spaced AB quartet (17.3, 21.4 ppm, J PP ) 5 Hz),
reasonably sharp at ambient temperature. Rapid inter-
molecular exchange (kexchange ) ca. 103 M-1 s-1 at 20
F igu r e 2. Cyclic voltammetry of (dppf)Pd(MeA), 3 (2 mmol
dm-3), in THF (containing n-Bu4NBF4, 0.3 mol dm-3) at a
steady gold disk electrode (i.d. ) 2 mm) with a scan rate
of 0.2 V s-1, at 20 °C.
P-Pd-P plane. In general the bond lengths and dis-
tances are unexceptional.9
Ar ylp a lla d iu m (II) Com p lexes F or m ed in th e
Oxid a tive Ad d ition of Ar yl Tr ifla tes to Com p lex
3. Cyclic voltammetry performed on a solution of
complex 3 (2 mmol dm-3) in THF (containing n-Bu4-
NBF4, 0.3 mol dm-3) exhibited two successive oxidation
peaks at EpO1 ) +0.73 V (bielectronic, irreversible) and
°C) was demonstrated by severe line broadening of 31
P
resonances observed on sequential addition of methyl
acrylate to the solution. The isolated complex does not
show a sufficiently stable parent ion to allow satisfactory
MS characterization by FAB or electrospray methods.
Ep ) +1.41 V vs SCE (monoelectronic, reversible)
O2
(Figure 2).
Crystallization of a sample from toluene/hexane in the
presence of excess methyl acrylate gave pale orange
plates. The X-ray structure is represented in Figure 1,
and selected bond lengths and angles are shown in
Tables 1 and 2. The palladium complex is monomeric
with a square planar geometry around palladium and
with the alkene η2-coordinated to the metal in the
Addition of iodobenzene resulted in gradual conver-
sion into a new complex (eq 1), which displayed two new
oxidation peaks at +0.94 V (irreversible) and +1.16 V
(reversible). This species and its 31P NMR spectrum
(Table 3) were found to be identical to those of an
(9) For some prior examples of 16-electron Pd alkene complexes see:
(a) Hodgson, M.; Parker, D.; Taylor, R. J .; Ferguson, G. J . Chem. Soc.,
Chem. Commun. 1987, 1309. (b) Benn, R.; Betz, P.; Goddard, R.; J olly,
P. W.; Kokel, N.; Kruger, C.; Topalovic, I. Z. Naturforsch. B 1991, 46,
1395. (c) Herrmann, W. A.; Thiel, W. R.; Brossmer, C.; O¨ fele, K.;
Priermeier, T.; Scherer, W. J . Organomet. Chem. 1993, 461, 51. (d)
Fawcett, J .; Kemmitt, R.; Russell, D. R.; Serindag, O. J . Organomet.
Chem. 1995, 486, 171. (e) Goddard, R.; Hopp, G.; J olly, P. W.; Kruger,
C.; Mynott, R.; Wirtz, C. J . Organomet. Chem. 1995, 486, 163.
(7) For anionic addends influencing Heck reactions see: (a) Cabri,
W.; Candiani, I. Acc. Chem. Res. 1995, 28, 2, and references therein.
(b) Overman, L. E.; Poon, D. J . Angew. Chem., Int. Ed. Engl. 1997,
36, 518.
(8) (a) Brown, J . M.; Cooley, N. A. Organometallics 1990, 9, 353.
(b) Schager, F.; Haack, K. J .; Mynott, R.; Rufinska, A.; Porschke, K.
R. Organometallics 1998, 17, 807.

Oxidative Addition of Aryl Electrophiles
Organometallics, Vol. 18, No. 25, 1999 5369
Ta ble 2. An gles betw een In ter a tom ic Vector s (d eg)
w ith esd ’s in P a r en th eses
In contrast, addition of aryl triflates such as 4-NO2-
C6H4-OTf, 3,5-CF3-C6H3-OTf, or PhOTf to complex
3 resulted in an electrolyte that did not show any
identifiable 31P NMR signals, even though it displayed
a new reversible oxidation peak at +1.3 V in each case.
This was ascribed to the instability of the resulting
cationic arylpalladium triflate complex (eq 2), which
decomposes at room temperature.3c
C(6)-Fe(1)-C(7)
C(7)-Fe(1)-C(5)
C(7)-Fe(1)-C(1)
C(6)-Fe(1)-C(10)
C(5)-Fe(1)-C(10)
C(6)-Fe(1)-C(2)
C(5)-Fe(1)-C(2)
C(10)-Fe(1)-C(2)
C(7)-Fe(1)-C(3)
C(1)-Fe(1)-C(3)
C(2)-Fe(1)-C(3)
C(7)-Fe(1)-C(4)
C(1)-Fe(1)-C(4)
C(2)-Fe(1)-C(4)
C(6)-Fe(1)-C(8)
C(5)-Fe(1)-C(8)
C(10)-Fe(1)-C(8)
C(3)-Fe(1)-C(8)
C(6)-Fe(1)-C(9)
C(5)-Fe(1)-C(9)
C(10)-Fe(1)-C(9)
C(3)-Fe(1)-C(9)
C(8)-Fe(1)-C(9)
C(2)-C(1)-C(5)
C(5)-C(1)-P(1)
C(5)-C(1)-Fe(1)
C(3)-C(2)-C(1)
C(1)-C(2)-Fe(1)
C(4)-C(3)-Fe(1)
C(3)-C(4)-C(5)
C(5)-C(4)-Fe(1)
C(4)-C(5)-Fe(1)
C(10)-C(6)-C(7)
C(7)-C(6)-P(2)
C(7)-C(6)-Fe(1)
C(8)-C(7)-C(6)
C(6)-C(7)-Fe(1)
C(9)-C(8)-Fe(1)
C(10)-C(9)-C(8)
C(8)-C(9)-Fe(1)
C(9)-C(10)-Fe(1)
C(1)-P(1)-C(21)
C(21)-P(1)-C(11)
C(21)-P(1)-Pd(1)
41.47(8) C(6)-Fe(1)-C(5)
140.78(8) C(6)-Fe(1)-C(1)
111.37(8) C(5)-Fe(1)-C(1)
40.89(8) C(7)-Fe(1)-C(10)
111.54(8) C(1)-Fe(1)-C(10)
137.90(8) C(7)-Fe(1)-C(2)
68.67(8) C(1)-Fe(1)-C(2)
178.79(8) C(6)-Fe(1)-C(3)
137.43(8) C(5)-Fe(1)-C(3)
68.65(8) C(10)-Fe(1)-C(3)
40.42(8) C(6)-Fe(1)-C(4)
177.74(8) C(5)-Fe(1)-C(4)
68.69(8) C(10)-Fe(1)-C(4)
68.07(8) C(3)-Fe(1)-C(4)
68.86(8) C(7)-Fe(1)-C(8)
178.47(8) C(1)-Fe(1)-C(8)
67.89(8) C(2)-Fe(1)-C(8)
111.08(8) C(4)-Fe(1)-C(8)
68.48(8) C(7)-Fe(1)-C(9)
138.53(8) C(1)-Fe(1)-C(9)
40.19(8) C(2)-Fe(1)-C(9)
112.66(8) C(4)-Fe(1)-C(9)
40.16(9) C(2)-C(1)-P(1)
111.72(8)
110.23(8)
41.15(7)
68.64(8)
138.45(8)
110.41(8)
40.97(7)
178.28(8)
68.39(8)
140.79(8)
140.76(8)
40.64(8)
112.91(8)
40.35(8)
40.55(8)
140.19(8)
111.93(8)
138.06(8)
68.09(8)
178.59(8)
140.38(8)
111.90(8)
126.43(14)
69.94(11)
125.54(10)
69.98(11)
108.2(2)
69.59(11)
69.82(11)
108.2(2)
69.63(11)
28.0(2)
(dppf)Pd(MeA) + ArOTf f
ArPd(dppf)+ + TfO- + MeA (2)
A parallel 1H NMR experiment10 carried out in THF-d8
with PhOTf using similar concentrations showed the
disappearance of the complex 3 over 18 h, with concur-
rent release of methyl acrylate 2. Due however to the
relatively low concentration of the methyl acrylate, only
2% of methyl cinnamate 5, the formal product of a Heck
reaction, was observed.
The oxidative addition of aryl triflates to complex 3
performed in the presence of anions (Cl-, I-, or AcO-)
led to clean formation of neutral arylpalladium(II)
complexes (eq 3). These were found to have oxidation
peaks and 31P NMR signals identical to independently
prepared ArPdCl(dppf), ArPdI(dppf), and ArPd(OAc)-
(dppf) complexes11 (Tables 3, 4). Analogous neutral
complexes had also been obtained in the related reaction
with Pd(PPh3)4.4
107.2(2)
126.4(2)
C(2)-C(1)-Fe(1)
P(1)-C(1)-Fe(1)
69.22(11) C(3)-C(2)-Fe(1)
108.2(2) C(4)-C(3)-C(2)
69.09(11) C(2)-C(3)-Fe(1)
69.83(11) C(3)-C(4)-Fe(1)
108.1(2)
C(4)-C(5)-C(1)
68.84(11) C(1)-C(5)-Fe(1)
70.52(11) C(10)-C(6)-P(2)1
106.9(2)
125.1(2)
C(10)-C(6)-Fe(1)
P(2)-C(6)-Fe(1)
70.16(11)
126.51(10)
70.63(11)
108.1(2)
68.81(11)
69.39(11)
108.5(2)
68.95(1
69.35(11) C(8)-C(7)-Fe(1)
108.0(2) C(9)-C(8)-C(7)
{(dppf)Pd(MeA) + X-} + ArOTf f
X ) Cl-, I-, AcO-
69.18(11) C(7)-C(8)-Fe(1)
70.04(12) C(10)-C(9)-Fe(1)
ArPdX(dppf) + MeA + TfO- (3)
108.5(2)
C(9)-C(10)-C(6)
69.80(12) C(6)-C(10)-Fe(1)
70.42(12) C(1)-P(1)-C(11)
99.01(9) C(1)-P(1)-Pd(1)
104.13(9) C(11)-P(1)-Pd(1)
112.28(7) C(12)-C(11)-P(1)
100.04(9)
120.80(7)
117.75(7)
119.4(2)
120.7(2)
120.0(2)
120.5(2)
116.9(2)
120.2(2)
119.8(2)
120.7(2)
100.82(9)
116.41(7)
118.73(7)
122.4(2)
120.6(2)
119.6(2)
120.3(2)
122.2(2)
120.5(2)
119.8(2)
120.6(2)
107.37(7)
147.94(7)
104.48(2)
118(2)
On the other hand, the nature of the cationic compo-
nent of the added salt (Li+, Eu3+) had no effect on the
structure of the complex formed in the oxidative addi-
tion of aryl triflates.
C(12)-C(11)-C(16) 118.7(2)
C(16)-C(11)-P(1) 122.0(2)
C(14)-C(13)-C(12) 120.0(2)
C(14)-C(15)-C(16) 120.1(2)
C(26)-C(21)-C(22) 118.8(2)
C(22)-C(21)-P(1)
C(24)-C(23)-C(22) 120.3(2)
C(24)-C(25)-C(26) 120.1(2)
C(6)-P(2)-C(41)
C(41)-P(2)-C(31)
C(41)-P(2)-Pd(1)
C(36)-C(31)-C(32) 118.9(2)
C(32)-C(31)-P(2) 118.6(2)
C(13)-C(12)-C(11)
C(13)-C(14)-C(15)
C(15)-C(16)-C(11)
C(26)-C(21)-P(1)
C(23)-C(22)-C(21)
C(25)-C(24)-C(23)
C(25)-C(26)-C(21)
C(6)-P(2)-C(31)
Th e Ra te a n d Mech a n ism of th e Oxid a tive Ad -
d ition of Ar yl Electr op h iles to Com p lex 3. The
kinetics of the oxidative addition of complex 3 to aryl
triflates and iodides was monitored by electrochemical
techniques by recording the decay of its first oxidation
peak O1 (the current being proportional to the concen-
tration of the palladium(0) complex). Previous work4,6
has shown that amperometry is the most convenient
technique to monitor the kinetics of oxidative additions
of palladium(0) complexes. This involves polarization of
a rotating disk electrode at +0.6 V (first oxidation wave
O1) in a solution of complex 3. The decay of the oxidation
current of (dppf)Pd(MeA) was subsequently recorded as
a function of time in the presence of an excess of the
aryl triflate (Figure 3a). The oxidation current of
complex 3 dropped to zero, indicating that the products
formed in the oxidative addition were not electroactive
at +0.6 V (see Table 3). A plot of ln([Pd0]/[Pd0]0) versus
time resulted in a straight line (Figure 3b).
124.0(2)
101.69(9) C(6)-P(2)-Pd(1)
102.61(9) C(31)-P(2)-Pd(1)
114.11(7) C(36)-C(31)-P(2)
C(33)-C(32)-C(31)
C(35)-C(34)-C(33)
C(35)-C(36)-C(31)
C(46)-C(41)-P(2)
C(41)-C(42)-C(43)
C(45)-C(44)-C(43)
C(45)-C(46)-C(41)
C(51)-Pd(1)-P(2)
C(32)-C(33)-C(34) 120.1(2)
C(34)-C(35)-C(36) 120.4(2)
C(46)-C(41)-C(42) 119.0(2)
C(42)-C(41)-P(2)
118.7(2)
C(44)-C(43)-C(42) 120.0(2)
C(44)-C(45)-C(46) 120.1(2)
C(51)-Pd(1)-C(52)
C(52)-Pd(1)-P(2)
C(52)-Pd(1)-P(1)
C(52)-C(51)-Pd(1)
38.55(9) C(51)-Pd(1)-P(1)
145.93(6) P(2)-Pd(1)-P(1)
109.51(6) C(52)-C(51)-H(51a)
72.16(12) C(52)-C(51)-H(51b)
119(2)
Pd(1)-C(51)-H(51a 110(2)
Pd(1)-C(51)-H(51b 108(2)
C(51)-C(52)-C(53) 121.0(2)
H(51a)-C(51)-H(51b) 118(2)
C(51)-C(52)-Pd(1)
C(51)-C(52)-H(52)
C(53)-C(52)-Pd(1) 108.39(14) Pd(1)-C(52)-H(52)
69.28(12)
123(2)
108(2)
C(53)-C(52)-H(52) 114(2)
O(53)-C(53)-O(54) 121.9(2)
O(54)-C(53)-C(52) 111.7(2)
O(53)-C(53)-C(52)
C(53)-O(54)-C(54)
126.5(2)
114.8(2)
(10) We thank Dr. T. D. W. Claridge for help with this experiment.
A 2 mM solution of complex 3 in THF-d8 containing 0.3 M electrolyte
and 50 equiv of phenyl triflate was examined by 1H NMR spectroscopy
at 500 MHz. The disappearance of the signals corresponding to complex
3 was monitored, and these were integrated against the emergence of
new peaks corresponding to methyl cinnamate and methyl acrylate.
(11) ArPdI(dppf) was prepared according to procedures described
in a previous reference.3a The corresponding acetate and chloride
complexes were obtained by substitution of the anionic ligand at -60
°C, with AgOAc and AgBF4/n-Bu4NCl, respectively.
authentic sample of the known neutral complex PhPdI-
(dppf).3a
(dppf)Pd(MeA) + ArI f ArPdI(dppf) + MeA (1)

5370 Organometallics, Vol. 18, No. 25, 1999
J utand et al.
Ta ble 3. Oxid a tion P ea k P oten tia l of th e Ar ylp a lla d iu m (II) Com p lexes F or m ed in th e Oxid a tive Ad d ition
of P d (d p p f)(MA) to Ar yl Tr ifla tes a n d Iod id es a s a F u n ction of th e Ad d en d
addend
(equiv/Pd)
Ep
Ep
ox
ox
entry
ArX
(V vs SCE)a
(V vs SCE)a
1
2
3
4
5
6
7
8
9
4-NO2-C6H4-OTf
none
+1.34
3.4 n-Bu4NCl
3.4 n-Bu4NOAc
LiBF4
Zn(BF4)2
Eu(tfc)3
none
3.4 n-Bu4NCl
none
+1.21, ArPdCl(dppf)
+1.11, ArPd(OAc)(dppf)
+1.33
+1.33
+1.32
+1.34
+1.14
3,5-CF3-C6H3-OTf
+1.20, ArPdCl(dppf)
Ph-OTf
nd
10
11
12
13
3.4 n-Bu4NCl
3.4 n-Bu4NI
none
+1.15, PhPdCl(dppf)
+0.94, PhPdI(dppf)
+0.94, PhPdI(dppf)
+0.82, +0.97
PhI
3 Eu(OTf)3
a
Potentials at a steady gold electrode (i.d. 2 mm) with a scan rate of 0.2 V s-1 at 20 °C.
reactive than (dppf)Pd(dba), its lower concentration
means that both complexes react with PhI in parallel,
resulting in a reaction order that differs from one in PhI
(i.e., kapp ) a + b[PhI]).6b
(dppf)Pd(dba) a Pd(dppf) + dba
(5)
To investigate whether a similar equilibrium operates
in our present system (eq 6 in Scheme 2) and to
determine the effective reactive species, the kinetics of
the oxidative addition was also probed with the more
reactive iodobenzene.
A plot of ln([Pd0]/[Pd0]0) as a function of time afforded
a series of straight lines whose slopes (kapp) were found
to vary linearly with the iodobenzene concentration with
a zero intercept (Figure 4a) (ln([Pd0]/[Pd0]0) ) -kappt )
-k[PhI]t). This means that the reaction order in PhI is
unity and implies that only one palladium(0) complex,
either complex 3 or Pd(dppf), is reactive in the oxidative
addition. Since the kinetic law, ln([Pd0]/[Pd0]0) - [Pd0]/
[Pd0]0 + 1 ) -kappt, was not obeyed, as would be
expected if Pd(dppf) were the sole reactive species, this
requires that complex 3 is the only reactive species in
the oxidative addition (Scheme 2). Further proof was
provided when no significant decelerating effect was
observed when the reaction was carried out in the
presence of added methyl acrylate.12
F igu r e 3. Oxidative addition of 4-NO2-C6H4-OTf (100
mmol dm-3) to (dppf)Pd(MeA), 3 (2 mmol dm-3), in THF
(containing n-Bu4NBF4, 0.3 mol dm-3) at 40 °C. (a) Varia-
tion of [Pd]/[Pd]0 as a function of time ([Pd]/[Pd]0 ) i/i0: i,
oxidation current of O1 at time t; i0, initial oxidation current
of O1). (b) Variation of ln([Pd]/[Pd]0) as a function of time.
The surprising lack of participation of Pd(dppf) in our
present system suggests that (i) its concentration in
equilibrium 6 is considerably smaller than in equilib-
rium 5 and/or (ii) complex 3 is more reactive per se than
(dppf)Pd(dba). The presence of one free dba for the in
situ case prevented any more rigorous comparison of the
two experiments.
ln([Pd0]/[Pd0]0) ) -kapp
t
(4)
[Pd0]/[Pd0]0 ) i/i0, where i is the oxidation current of
O1 at time t and i0 is the initial oxidation current of O1.
The apparent rate constant kapp of the oxidative
addition (eq 4) was given by the slope (Table 4). As was
observed for palladium(0) complexes ligated by mono-
dentate phosphines,4 the oxidative addition of (dppf)-
Pd(MeA) proceeds faster when the aryl triflate is
substituted by electron-withdrawing groups; and aryl
triflates are less reactive than the corresponding iodides
(entries 10 and 13, Table 4).
For a similar (diphosphine)Pd(alkene) complex, gen-
erated in situ from a mixture of Pd(dba)2 and 1 equiv
of dppf, a dynamic exchange was found to exist between
(dppf)Pd(dba) and a less ligated complex (eq 5).6b
Although the 14-electron Pd(dppf) complex is more
The kinetic law, expressed as ln([Pd]/[Pd]0) ) -k[ArX]t,
allows the determination of the rate constant k of the
oxidative addition of complex 3 to aryl triflates and
iodides (Table 4). When compared to another well-
investigated palladium(0) complexes, complex 3 is con-
siderably less reactive toward iodobenzene than
(12) When cyclic voltammetry was performed at low scan rate (0.2
V s-1) on
a solution of (dppf)Pd(MeA) directly in reduction, the
reduction peak of methyl acrylate at -2.2 V vs SCE was not detected.
This means that equilibrium 6 has not shifted significantly to the right-
hand side despite continuous reduction of MeA in the diffusion layer
(CE mechanism13). The equilibrium is thus considerably in favor of
(dppf)Pd(MeA), and the complex is not very labile, in accord with NMR
observations (ref 10).

Oxidative Addition of Aryl Electrophiles
Organometallics, Vol. 18, No. 25, 1999 5371
Ta ble 4. Oxid a tive Ad d ition of (d p p f)P d (MeA)a to Ar yl Tr ifla tes a n d Iod id es in THF : Role of Ad d en d s
addend
(equiv/Pd)
104 kapp
(s-1
103
k
31P NMR
δ ppm [J PP Hz]
b
entry
ArX
[ArX]/[Pd]
T (°C)
)
(M-1 s-1
5.9
)
1
2
4-NO2-C6H4-OTf
50
50
40
40
none
n-Bu4NCl
(3.4)
n-Bu4NOAc
(3.4)
5.9
4.6
c
ArPdCl(dppf)
11.7; 31.4 [30]
ArPd(OAc)(dppf)
11.9; 31.5 [30]
c
3
4
5
6
7
50
50
50
50
50
40
40
40
40
40
6.4
8.1
9.2
7.3
LiBF4
(1)
LiBF4
d
c
c
c
(20)
Eu(hfc)3
(3)
Eu(OTf)3
d
46
(3)
8
9
3,5-CF3-C6H3-OTf
50
50
25
25
none
n-Bu4NCl
(3)
none
n-Bu4NCl
(3)
n-Bu4NI
(3)
none
1.2
0.8
1.2
1.1
c
ArPdCl(dppf)
12.3; 32.0 [29]
c
PhPdCl(dppf)
10.2; 31.3 [32]
PhPdI(dppf)
8.2; 26.4 [35]
PhPdI(dppf)
c
10
11
PhOTf
100
100
25
25
2.3
1.0
12
100
25
nd
13
14
PhI
50
50
25
25
5.8
53.8
5.8
d
Eu(OTf)3
(3)
a
b
d
[(dppf)Pd(MeA)] ) 2 mmol dm-3
.
For the definition of k, see Scheme 2. c No identifiable signals. Premixed with ArX.
Sch em e 2. Oxid a tive Ad d ition of Ar yl Electr op h iles to Com p lex 3
Pd(PPh3)4.6a The ratio of the two rate constants is
weaker Lewis acidic Eu(hfc)3 was found to be less
effective (entries 1, 6, 7 in Table 4). LiBF4 had little
effect, even when present at high concentration (entries
1, 4, 5 in Table 4). The observed acceleration by Eu-
(OTf)3 in the oxidative addition probably arises from the
complexation of the Lewis acid with ligated methyl
acrylate. This promotes the dissociation of the alkene
ligand from palladium(0), causing a shift in the equi-
librium 6 (Scheme 2) to the more reactive 14-electron
Pd(dppf) complex, thus enhancing the rate of the
subsequent oxidative addition step. When 3 equiv of Eu-
(OTf)3 was added to complex 3, slow decomposition
occurred, presumably via the same 14-electron complex
Pd(dppf).16
approximately 1:4300, but the corresponding ratio for
phenyl triflate is ca. 1:5.4
Addition of 3 equiv of Cl- has a slight decelerating
effect on the rate of the oxidative addition to aryl
triflates (entries 1,2; 8,9; 10,11, Table 4). The close
vicinity of the oxidation peaks of complex 3 and Cl-
hampers investigations at higher chloride concentra-
tions. The opposite effect was observed for Pd(PPh3)4,
where Pd(PPh3)2Cl- species are involved in promoting
the rate of oxidative addition.4,14 This implies that Cl-
binds less strongly to Pd(dppf) than to Pd(PPh3)2, even
though trace amounts of the anionic species Pd(dppf)Cl-
have been detected in a solution mixture of complex 3
and n-Bu4NCl by electrospray mass spectrometry.15
Addition of Eu(OTf)3 has a strong accelerating effect
(ca. 10-fold) on the rate of the oxidative addition of both
aryl triflates and aryl halides to complex 3 (entries 1,7
and 13,14 in Table 4, Figure 4b). The hexacoordinated,
(15) We thank Dr. R. T. Aplin for help with this experiment. A
reaction mixture generated from complex 3 and 4.5 equiv of n-Bu4-
NCl in methanol was introduced directly into the electrospray mass
spectrometer operating in negative ion mode at room temperature, with
methanol as the carrier solvent and a cone voltage of 30 V. A species
corresponding to the ion (dppf)PdCl- was observed at m/z 695, 697
(
106Pd). No clear-cut evidence was obtained for the existence of any
(13) Bard, A. J .; Faulkner, L. R. Electrochemical Methods; Wiley:
New York, 1980; pp 136-143.
(14) Amatore, C.; Azzabi, M.; J utand, A. J . Am. Chem. Soc. 1991,
113, 8375.
related P2PdX anion (P2 ) dppf) under comparable conditions.
(16) The rate of decomposition was less than that of the oxidative
addition; nevertheless the cation and the aryl derivative were added
to the (dppf)Pd(MeA) solution as a premixed solution.

5372 Organometallics, Vol. 18, No. 25, 1999
J utand et al.
Ta ble 5. Resu lts of Cou p lin g betw een Meth yl
Acr yla te a n d 4-Nitr op h en yl Tr ifla te 5, Usin g
Com p lex 3 a s Ca ta lysta
entry added species
time/h
% conversion ArOTf
6:7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
none
40
17
17
64
20
40
71
64
71
94
71
120
41
120
100
94
100
34
100
100
16
45
94
100
100
97
32:68
0:100
24:76
16:84
14:86
37:63
100:0
100:0
90:10
100:0
100:0
20:80
34:66
54:46
Sc(OTf)3
Yb(OTf)3
Cu(OTf)3
Sm(OTf)3
Eu(OTf)3
CeCl3
LiCl
EuCl3
CuCl2
HfOCl2
LiF
CsF
SnF4
96
91
a
All experiments were carried out in THF at 65 °C, using 3
equiv of triethylamine as base and 5 mol % of addend where
applicable. The conversion of 4-nitrophenyl triflate and product
distribution were monitored by GC.
F igu r e 4. (a) Oxidative addition of iodobenzene to (dppf)-
Pd(MeA), 3 (2 mmol dm-3), in THF (containing n-Bu4NBF4,
0.3 mol dm-3) at 25 °C. Variation of the apparent rate
constant of the oxidative addition, kapp (s-1), as a function
of the iodobenzene concentration. Slope: k ) (5.8 ( 0.1) ×
10-3 M-1 s-1; r ) 0.998. (b) Oxidative addition of (dppf)-
Pd(MeA), 3 (2 mmol dm-3), at 25 °C to (O) PhI (0.1 mol
being attributed to hydride transfer from the R-carbon
of NEt3 to the Pd aryl cation and reductive elimina-
tion.17
Su m m a r y a n d Con clu sion s
dm-3) in the absence of Eu(OTf)3; (b) PhI (0.1 mol dm-3
)
In this work we have isolated and characterized a
stable palladium(0) P2Pd[alkene] complex, (dppf)Pd-
(MeA), 3. We have demonstrated an exclusive partition-
ing of this P2Pd[alkene] complex along the 16-electron
oxidative addition pathway with aryl derivatives, with-
out alkene dissociation. This is significantly different
from a similar set of experiments reported for (dppf)-
Pd(dba). Many of the recent studies involving Heck
reactions with high turnover employ methyl acrylate
and aryl halides, and it is important to recognize that,
just as in the case of dba, the alkene reactant (and
product) are electrophilic alkenes whose strong associa-
tion to Pd(0) may diminish the turnover rate by enforc-
ing a kinetically unfavorable mechanism of oxidative
addition.
A further point of interest lies in the relative reac-
tivities of Ph-I and Ph-OTf, differing here by a factor
of 5. In the related addition of the same electrophiles
to Pd(PPh3)4, around 104 greater discrimination is
observed in favor of Ph-I. This is reminiscent of the
wide variation in halide/tosylate rate ratios observed in
aliphatic solvolysis.18a,b There could also be a change in
the actual mechanism of the oxidative addition step as
a function of the leaving group and/or ligand. If the
comparable reactivity of the 16-electron Pd intermediate
(dppf)Pd(MeA) with PhI and PhOTf is the consequence
of a SNAr mechanism,18c then the greater difference in
reactivity for PhI and PhOTf observed with the 14-
and Eu(OTf)3 (6 mmol dm-3).
In summary, we have observed that the addition of
anions generally does not have a significant effect on
the rate of the oxidative addition of aryl triflates to
complex 3, but the nature of the resulting arylpalla-
dium(II) complex was modified. In contrast, addition of
certain cations operating as Lewis acids enhances the
rate of oxidative addition of aryl triflates to complex 3
without altering the cationic nature of the complex
formed by the reaction.
Rela ted Ca ta lytic Tu r n over Exp er im en ts. Ex-
periments were first carried out on the coupling reaction
between methyl acrylate and 4-nitrophenyl triflate,
using complex 3 as precatalyst. The results are sum-
marized in Table 5.
In the absence of addends, the reaction was fairly
sluggish with an unfavorable product distribution, ni-
trobenzene 7 being formed as a side product (entry 1).
The addition of lanthanide triflates (entries 2-6) in-
creases the turnover rate with a worsening in the
product distribution. Chloride salts (entries 7-11)
improve selectivities dramatically, but with a corre-
sponding reduction in turnover rates. Europium, copper,
and hafnium chlorides (entries 9, 10, and 11) perform
better than the alkali metal chlorides with respect to
the rate of product formation. Fluoride salts were also
used as addends, but again the product distribution was
biased toward the reduction product, with no apparent
enhancement in the reaction rate. Competition between
reduction and coupling of aryl triflates has previously
been observed by Cabri and co-workers, the reduction
(17) Cabri, W.; Candiani, I.; Debernardinis, S.; Francalanci, F.;
Penco, S.; Santi, R. J . Org. Chem. 1991, 56, 5796.
(18) (a) Kevill, D. N.; Abduljaber, M. H. J . Chem. Soc., Perkin Trans.
2 1995, 1985. (b) Liu, K. T.; Tang, C. S. J . Org. Chem. 1996, 61, 1523,
and references therein. (c) Fitton, P.; Rick, E. A. J . Organomet. Chem.
1971, 28, 287.

Oxidative Addition of Aryl Electrophiles
Organometallics, Vol. 18, No. 25, 1999 5373
Ta ble 6. Cr ysta llogr a p h ic Da ta for Com p lex 3
electron Pd(PPh3)2 implies more developed Pd-I bond-
ing at the TS in that case.
empirical formula
M
C38H34FeO2P2Pd
746.84
The accelerating effect of Eu(OTf)3 on the oxidative
addition rate of either PhI or PhOTf to (dppf)Pd(MeA)
is explicable if the carbonyl oxygen of coordinated
acrylate is the site of Eu association. This would both
increase the electrophilic character of the alkene and
facilitate its dissociation, providing a significant con-
centration of the 14-electron (dppf)Pd[0] species, which
then becomes the main channel for the oxidative addi-
tion.
The importance of anionic addends X- (X ) Cl, I, AcO)
in the oxidative addition of aryl triflates is reiterated.
Although there is no significant effect on the rate of the
oxidative addition, the formation of neutral ArPdX(dppf)
complexes may have important implications in the
subsequent alkene insertion of Heck reactions.2c,3b,7b
size (mm)
temp (K)
cryst syst
space group
unit cell dimens
a (Å)
0.26 × 0.10 × 0.07
180
monoclinic
P21/c
15.3313(1)
10.7804(1)
19.4659(2)
102.0370(6)
3146.54(5)
4
b (Å)
c (Å)
â (deg)
V (Å3)
Z
F
calc (Mg m-3
)
1.577
1.169
µ (mm-1
)
max, min transmn
F(000)
0.923, 0.751
1520
θ range (deg)
no. reflns collected
no. indep reflns
no. reflns with Fo > 2σ(Fo
Rint
1.36-26.0
77911
6179
5643
0.0132
2
2
)
Exp er im en ta l Section
no. of params
goodness of fit on F2
R1 (F2 > 2σF2)
wR (all data, F2)
411
1.035
0.0254
0.0674
31P NMR spectra were recorded on Bruker spectrometers
1
(101 and 202 MHz) using H3PO4 as an external reference. H
NMR spectra were recorded on a Bruker spectrometer (500
MHz) and referenced to TMS.
largest peak, hole (e Å-3
)
0.422, -0.826
Rea gen ts. All experiments were performed under argon.
THF (Acros) was distilled from sodium-benzophenone. The
solvent was transferred to the cells according to standard
Schlenk procedures. The dppf and methyl acrylate ligands
were obtained commercially (Aldrich) and used without further
purification. Iodobenzene (Acros) was filtered on neutral
alumina and stored under argon. (dppf)PdCl2,19 Li2(COT),20
and aryl triflates21 were prepared according to literature
methods.
sample of (dppf)Pd(MeA) was then added. The cyclic voltam-
metry was performed at a steady gold disk electrode (i.d. ) 2
mm) with a scan rate of 0.2 V s-1
.
The kinetic measurements were performed by amperometry
at a rotating gold disk electrode (i.d. ) 2 mm, Tacussel EDI
65109) with an angular velocity of 105 rad s-1 (Tacussel
controvit). The RDE was polarized at +0.6 V on the first
oxidation wave O1 of (dppf)Pd(MeA). After addition of the
required amount of the aryl triflate or iodide, the oxidation
current was recorded as a function of time up to 100%
conversion. In another set of experiments, the desired amount
of addend was introduced before the addition of the aryl
derivatives (in the case of added anions) or together with the
aryl derivatives (in the case of added cations).
X-r a y str u ctu r a l a n a lysis of 3. Single yellow prismatic
crystals were obtained by diffusion of n-hexane into a CH2Cl2
solution at room temperature. Crystal data and details of data
collection and refinement are summarized in Table 6. All
crystallographic measurements were carried out at 180 K on
a Nonius Kappa CCD area-detector diffractometer using Mo
KR radiation (λ ) 0.71073 Å). The detector was positioned 25
mm from the crystal, and data were collected as follows:
“head-on” data (ø ) 0°) as 360 oscillation frames of 60 s
exposure time with 1° rotation in φ; “cusp” data (ø ) 90°) as
35 frames of 60 s exposure time and 1° ω rotations, collected
at each of four different φ settings. The package DENZO-
SMN22 was used for indexing, unit cell refinement, and data
integration and scaling. The data were corrected for Lorentz
and polarization effects using Scalepack.23 Subsequently an
empirical absorption correction based on redundant and sym-
metry-equivalent data was applied using the program SOR-
TAV.24 The structure was solved by direct methods (SHELXS-
97)25 and was developed by alternating cycles of least-squares
refinement (on F2) and Fourier difference syntheses (SHELXL-
97).25 All non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were con-
strained to idealized positions (using a riding model with free
rotation for methyl groups and fixed isotropic displacement
parameters).
Syn th esis of [(d p p f)P d (MeA)], 3. Li2(COT) (0.26 M in
diethyl ether, 2.7 mL, 0.7 mmol.) was added to a stirred
suspension of (dppf)PdCl2 (0.5 g, 0.7 mmol) in dry THF (20
mL) at -78 °C. The resulting deep red solution was treated
with methyl acrylate (4 mL), and the reaction mixture was
slowly warmed to room temperature, whereupon the solvent
was removed under reduced pressure. Methanol was added
to the residue, giving the required compound as a yellow solid.
Yield: 0.39 g, 77%. IR (KBr disc): ν(CtO) 1692 s cm-1 31P
.
(202 MHz, CD2Cl2): δ 17.3 (d, J PP ) 5 Hz), 21.4 (d, J PP 5 Hz).
1H (500 MHz): δ 7.8-7.3 (m, 20H, PPh) 4.45 (m, 2H, Cp); 4.36
(m, 2H, Cp); 4.26 (m, 2H, Cp); 4.01 (m, 2H, Cp); 3.85 (ddd 1H,
J PH 8.0 Hz, J HH 8.0, 11.0 Hz, Ha); 3.11 (s, 3H, CO2Me); 3.00
(dd 1H, J PH 8.0 Hz, J HH 8.0 Hz, Hb); 2.82 (dd, 1H, J PH 7.0 Hz,
J HH 11.0 Hz, Hc).
The alkene complex was successfully prepared in related
experiments where the Li2(COT) was replaced by NaBH4 in
THF/EtOH at -60 °C.
Electr och em ica l Setu p a n d P r oced u r e for Cyclic Vol-
ta m m etr y a n d Am p er om etr y. Cyclic voltammetry was
performed with a homemade potentiostat and a waveform
generator, Tacussel (GSTP4). The voltammograms were re-
corded on a Nicolet 3091 digital oscilloscope. Experiments were
carried out in a thermostated three-electrode cell connected
to a Schlenk line, under argon. The counter electrode was a
platinum wire of ca. 1 cm2 apparent 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 n-Bu4NBF4
solution in THF. A 20 mL sample of THF containing 0.3 M
n-Bu4NBF4 was poured into the cell. A 30 mg (2 mmol dm-3
)
(19) Hayashi, T.; Konishi, M.; Kobori, Y.; Kumada, M.; Higuchi, T.;
Hirotsu. K. J . Am. Chem. Soc. 1984, 106, 158.
(20) Katz, T.; Garratt, P. J . J . Am. Chem. Soc. 1964, 86, 4876.
(21) Hirota, K.; Isobe, Y.; Maki, Y. J . Chem. Soc., Perkin Trans. 1
1989, 2513.
(22) Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276, 307.
(23) Blessing, R. H. Acta Crystallogr., Sect A 1995, 51, 33.
(24) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(25) Sheldrick, G. M. SHELXL-97, Program for refinement of crystal
structures; University of Go¨ttingen, 1997.

5374 Organometallics, Vol. 18, No. 25, 1999
J utand et al.
Ack n ow led gm en t. The authors would like to thank
The Royal Society for a travelling grant between the
institutions and EPSRC for postdoctoral support [K.K.H.].
This work has also been supported in part by The
Centre National de la Recherche Scientifique (CNRS,
UMR 8640) and the Ministe`re de l′Education Nationale
et de la Recherche et de la Technologie (Ecole Normale
Supe´rieure).
OM9906417