Thermal and electrochemical C-X activation (X = Cl, Br, I) by the strong Lewis acid Pd3(dppm)3(Co)2+ cluster and its catalytic applications

Article abstract of DOI:10.1021/ja0297786

The stoichiometric and catalytic activations of alkyl halides and acid chlorides by the unsatured Pd₃(dppm)₃(CO)²⁺ cluster (Pd₃²⁺) are investigated in detail. A series of alkyl halides (R-X; R = t-Bu, Et, Pr, Bu, allyl; X = Cl, Br, I) react slowly with Pd₃²⁺ to form the corresponding Pd₃(X)⁺ adduct and "R⁺". This activation can proceed much faster if it is electrochemically induced via the formation of the paramagnetic species Pd₃⁺. The latter is the first confidently identified paramagnetic Pd cluster. The kinetic constants extracted from the evolution of the UV-vis spectra for the thermal activation, as well as the amount of electricity to bring the activation to completion for the electrochemically induced reactions, correlate the relative C-X bond strength and the steric factors. The highly reactive "R⁺" species has been trapped using phenol to afford the corresponding ether. On the other hand, the acid chlorides react rapidly with Pd₃²⁺ where no induction is necessary. The analysis of the cyclic voltammograms (CV) establishes that a dissociative mechanism operates (RCOCl → RCO⁺ + Cl^-; R = t-Bu, Ph) prior to Cl^- scavenging by the Pd₃²⁺ species. For the other acid chlorides (R = n-C₆H₁₃, Me₂CH, Et, Me, Pr), a second associative process (Pd₃²⁺ + RCOCl → Pd₃²⁺Cl(CO)(R)) is seen. Addition of Cu(NCMe)₄⁺ or Ag⁺ leads to the abstraction of Cl^- from Pd₃(Cl)⁺ to form Pd₃²⁺ and the insoluble MCl materials (M = Cu, Ag) allowing to regenerate the starting stating unsaturated cluster, where the precipitation of MX drives the reaction. By using a copper anode, the quasi-quantitative catalytic generation of the acylium ion ("RCO⁺") operates cleanly and rapidly. The trapping of "RCO⁺" with PF₆^- or BF₄^- leads to the corresponding acid fluorides and, with an alcohol (R′OH), to the corresponding ester catalytically, under mild conditions. Attempts were made to trap the key intermediates "Pd₃(Cl)⁺...M⁺" (M⁺ = Cu⁺, Ag⁺), which was successfully performed for Pd₃(ClAg)²⁺, as characterized by ³¹P NMR, IR, and FAB mass spectrometry. During the course of this investigation, the rare case of PF₆^- hydrolysis has been observed, where the product PF₂O₂^- anion is observed in the complex Pd₃(PF₂O₂)⁺, where the substrate is well-located inside the cavity formed by the dppm-Ph groups above the unsatured face of the Pd₃²⁺ center. This work shows that Pd₃²⁺ is a stronger Lewis acid in CH₂Cl₂ and THF than AlCl₃, Ag⁺, Cu⁺, and Tl⁺.

Full text of DOI:10.1021/ja0297786

Published on Web 04/10/2003
Thermal and Electrochemical C-X Activation (X ) Cl, Br, I) by
the Strong Lewis Acid Pd3(dppm)3(CO)2+ Cluster and Its
Catalytic Applications
Fre´de´ric Lemaˆıtre,1a,b Dominique Lucas,*,1a Katherine Groison,1b Philippe Richard,1a
Yves Mugnier,1a and Pierre D. Harvey*,1b
Contribution from the Laboratoire de Synthe`se et d’Electrosynthe`se Organome´talliques, CNRS
UMR 5632, Faculte´ des Sciences Gabriel, UniVersite´ de Bourgogne, 6 BouleVard Gabriel,
21000 Dijon, France and the De´partement de Chimie, UniVersite´ de Sherbrooke,
Sherbrooke, Que´bec, Canada J1K 2R1
Received December 17, 2002; E-mail: p.harvey@USherbrooke.ca; lucasd@u-bourgogne.fr
Abstract: The stoichiometric and catalytic activations of alkyl halides and acid chlorides by the unsatured
Pd3(dppm)3(CO)2+ cluster (Pd32+) are investigated in detail. A series of alkyl halides (R-X; R ) t-Bu, Et,
Pr, Bu, allyl; X ) Cl, Br, I) react slowly with Pd32+ to form the corresponding Pd3(X)+ adduct and “R+”. This
activation can proceed much faster if it is electrochemically induced via the formation of the paramagnetic
species Pd3+. The latter is the first confidently identified paramagnetic Pd cluster. The kinetic constants
extracted from the evolution of the UV-vis spectra for the thermal activation, as well as the amount of
electricity to bring the activation to completion for the electrochemically induced reactions, correlate the
relative C-X bond strength and the steric factors. The highly reactive “R+” species has been trapped using
2+
phenol to afford the corresponding ether. On the other hand, the acid chlorides react rapidly with Pd3
where no induction is necessary. The analysis of the cyclic voltammograms (CV) establishes that a
dissociative mechanism operates (RCOCl f RCO+ + Cl-; R ) t-Bu, Ph) prior to Cl- scavenging by the
Pd32+ species. For the other acid chlorides (R ) n-C6H13, Me2CH, Et, Me, Pr), a second associative process
(Pd32+ + RCOCl f Pd32+.....Cl(CO)(R)) is seen. Addition of Cu(NCMe)4+ or Ag+ leads to the abstraction of
Cl- from Pd3(Cl)+ to form Pd3 and the insoluble MCl materials (M ) Cu, Ag) allowing to regenerate the
2+
starting unsaturated cluster, where the precipitation of MX drives the reaction. By using a copper anode,
the quasi-quantitative catalytic generation of the acylium ion (“RCO+”) operates cleanly and rapidly. The
trapping of “RCO+” with PF6- or BF4- leads to the corresponding acid fluorides and, with an alcohol (R′OH),
to the corresponding ester catalytically, under mild conditions. Attempts were made to trap the key
intermediates "Pd3(Cl)+...M+" (M+ ) Cu+, Ag+), which was successfully performed for Pd3(ClAg)2+, as
characterized by 31P NMR, IR, and FAB mass spectrometry. During the course of this investigation, the
rare case of PF6- hydrolysis has been observed, where the product PF2O2- anion is observed in the complex
Pd3(PF2O2)+, where the substrate is well-located inside the cavity formed by the dppm-Ph groups above
the unsatured face of the Pd32+ center. This work shows that Pd32+ is a stronger Lewis acid in CH2Cl2 and
THF than AlCl3, Ag+, Cu+, and Tl+.
impressive, ranging from the simple nucleophilic substitution3
to the Heck reaction.4 Among the common methodologies
known for C-X bond activation, the use of Lewis acids such
as AlCl3, Ag+, Cu+, and Tl+ ions are popular and convenient.5
Recently, we reported the stoichiometric C-X bond activation
(X ) Br, I) of alkyl halides by the unsaturated 42-electron
cluster Pd3(dppm)3(CO)2+ (Pd32+).6 The originality stems, in
part, from the fact that this process operates inside a cavity
Introduction.
The C-X bond activation (X ) Cl, Br, I) is one of the most
basic processes for the functionalization of organic molecules.2
Its importance and the number of applications are simply
(1) (a) Universite´ de Bourgogne. (b) Universite´ de Sherbrooke.
(2) (a) Kulawiec, R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89. (b)
Stille, J. K.; Kriesler, S. Y. L. Acc. Chem. Res. 1977, 10, 434. (c) Kochi,
J. K. Organometallic Mechanisms and Cata1ysis; Academic Press: New
York, 1978; p 156. (d) Abel, E. W.; Stone, F. G. A.; Wilkinson, G.
ComprehensiVe Organometallic Chemistry II; Pergamon: Oxford, U.K.,
1995; Vol. 12, p 161. (e) Collman, J. P.; Hegedus, L. S.; Norton, J. R.
Principles and Applications of Organotransition Metal Chemistry; Uni-
versity Science Books: Mill Valley, CA, 1987. (f) Cornils, B.; Hermann,
W. A. Applied Homogeneous Catalysis with Organometallic Compounds;
Wiley: Weinheim, Germany, 1999. (g) Tsuji, J. Transition Metal Reagents
and Catalysts: InnoVations in Organic Synthesis; Wiley: Chichester, U.K.,
2000.
(3) (a) Trost, B. ComprehensiVe Organic Synthesis; Pergamon Press: New
York, 1991; Vol. 4, Chapter 2. (b) Stock, L. M. Aromatic Substitution
Reactions; Prentice Hall: Englewood Cliffs, NJ, 1968. (c) Clark, J. H.;
Wails, D.; Bastock, T. W. Aromatic Fluorination; CRC Press: Boca Raton,
FL, 1996. (d) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry;
Plenum Press: New York, 1990; Vol. A, Chapter 5, p 257.
(4) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009 and
references therein.
9
10.1021/ja0297786 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 5511-5522
5511

A R T I C L E S
Lemaˆıtre et al.
°C for at least 2 days. Bu4NBF4, Bu4NClO4, and NaBPh4 were
purchased from Fluka (puriss p.a. for electrochemical grade) and used
as received.
Apparatus. The UV-vis spectra were acquired on a Hewlett-
Packard (HP 8452A) diode array spectrophotometer. NMR spectra were
measured on a Bruker WM 300 spectrophotometer (1H NMR, 300.15
MHz; 31P NMR, 121.497 MHz). The reference was the residual
nondeuterated solvent. The chemical shifts are reported with respect
to TMS (1H NMR) and H3PO4 (31P NMR). The GC-MS data were
collected on a Hewlett-Packard 6890 Series apparatus. EPR measure-
ments were carried out on a Bruker ESP 300 spectrometer; field
calibration was made with DPPH (g ) 2.0037). Mass spectra were
obtained with a Kratos Concept 32S spectrometer in the LSIMS mode
(matrix ) m-nitrobenzyl alcohol).
Electrochemical Experiments. All manipulations were per-
formed using Schlenk techniques in an atmosphere of dry oxygen-free
argon gas. The supporting electrolyte was degassed under vacuum
before use and then solubilized at a concentration of 0.2 M. For cyclic
voltammetry experiments, the concentration of the analyte was nearly
10-3 M. Voltammetric analyses were carried out in a standard three-
electrode cell with a Tacussel PJT24-1 potentiostat connected to a
waveform generator Tacussel GSTP4. The reference electrode was a
saturated calomel electrode (SCE) separated from the solution by a
sintered glass disk. The auxiliary electrode was a platinum wire. For
all voltammetric measurements, the working electrode was a vitreous
carbon electrode (φ ) 3 mm). In these conditions, when operating in
THF, the formal potential for the ferrocene+/- couple is found to be
+0.56 V versus SCE. The controlled potential electrolysis was
performed with an Amel 552 potentiostat coupled with an Amel 721
electronic integrator. High scale electrolyses were performed in a cell
with three compartments separated with fritted glasses of medium
porosity. A carbon gauze was used as the working electrode, a platinum
plate, as the counter-electrode, and a saturated calomel electrode, as
the reference electrode.
formed by the 6-dppm phenyl groups, above the Pd3 triangular
frame. The host-guest behavior for neutral species have been
thoroughly investigated.7 However, the C-X bond cleavage
requires an electrochemical induction for the [Pd3(CF3CO2)](CF3-
2+
CO2) species, provoking a change in oxidation state of Pd3
,
from +2/3 to +1/3. This change in the cluster charge induces a
greater lability of the CF3CO2 ion, which is trapped in the
-
cavity and attracted electrostatically by the positively charged
+
Pd3 center. Then, Pd3 abstracts X- from the R-X molecule.
During the course of this research, we noticed that some R-X
2+
molecules slowly react with Pd3 without the need of an
electrochemical induction.
We now wish to report a full account on the reactivity of
R-X molecules (X ) Cl, Br, I; R ) alkyl) and acid chlorides
(R ) alkyl, Ph) with Pd32+ on a mechanistic point of view and
intermediate elucidation. In addition, an effort has been made
to render this process catalytic and find new applications, notably
for the electrosynthesis of acid fluorides and esters. The
2+
regeneration of the Pd3 catalyst from the Pd3(X)+ species is
also presented.
Experimental Section
High scale electrolyses with acid chlorides were performed with a
copper plate as the working electrode (anode), a platinum plate as the
counter-electrode (cathode), and a saturated calomel electrode as the
reference electrode, each one being separated from the others in a three-
compartment cell (except the last experiment of Table 7 as described
in the text).
Materials. [Pd3(dppm)3(CO)](PF6)2, [Pd3(dppm)3(CO)(Cl)](PF6), and
[Pd3(dppm)3(CO)(CF3CO2)](CF3CO2) have been prepared according to
literature procedure.8 Dichloromethane was distilled under Ar over P2O5,
and tetrahydrofuran (THF) was distilled under Ar over Na (Aldrich).
The acid chlorides and alcohols were purchased from Aldrich and used
as received. The Bu4NPF6 salt was synthesized by mixing stoichiometric
amounts of Bu4NOH (40% in water) and HPF6 (60% in water). After
filtration, the salt was recrystallized twice in ethanol and dried at 80
Typical Procedures. Conversion of Acid Chlorides into Esters:
Preparation of Ethyl Benzoate. The [Pd3(dppm)3(CO)](PF6)2 cluster
(15.8 mg, 8.82 × 10-3 mmol, 1 mol %/mol of acyl chloride), ethanol
(51 µL, 0.87 mmol), and benzoyl chloride (100 µL, 0.86 mmol) were
added to the anodic compartment of the cell containing 15 mL of a 0.2
M solution of Bu4NClO4 in CH2Cl2. The cathodic compartment and
the reference electrode compartment were filled with the Bu4NClO4-
CH2Cl2 solution. The potential of the copper anode was set to +0.65
V versus SCE. The electrolysis was stopped after the current had
dropped to less than 0.5 mA. After filtration of the mixture, the solvent
was evaporated and the residue was extracted with ether (3 × 5 mL).
The internal standard method was used to measure the GC yield of the
ester product. The latter was identified by comparison of the GC-MS
spectra and GC retention times to those of available authentic samples.
Fluorination of Acid Chlorides. The procedure is the same as
described previously except that no alcohol is added and that Bu4NPF6
or Bu4NBF4 is used instead of Bu4NClO4.
(5) (a) Olah, G. A. Friedel-Crafts Chemistry; Wiley: New York, 1973. (b)
Olah, G. A.; Krishnamurti, R.; Prakash, G. K. S. In ComprehensiVe Organic
Synthesis; Trost, B. M., Ed.; Pergamon Press: Oxford, 1991; Vol. 3, Chapter
1.8, p 293. (c) Roberts, R. M.; Khalaf, A. A. Friedel-Crafts Alkylation
Chemistry; Marcel Dekker: New York, 1984. (d) Klunder, J. M.; Posner,
G. H. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Elmsford, New York, 1991; Vol. 3, p 207. (e) Ingold, C.
K. Structure and Mechanism in Organic Chemistry; Cornell University
Press: Ithaca, New York, 1969. (f) Hartshorn, S. R. Aliphatic Nucleophilic
Substitution; Cambridge University Press: London, 1973; p 20. (g) Lee,
A. G. The Chemistry of Thallium; Elsevier: New York, 1971. (h) Herr, R.
W.; Wieland, D. M.; Johnson, C. R. J. Am. Chem. Soc. 1970, 92, 3813. (i)
Johnson, C. R.; Herr, R. W.; Wieland, D. M. J. Org. Chem. 1973, 38,
4263. (j) Johnson, C. R.; Dutra, G. A. J. Am. Chem. Soc. 1973, 95, 7783.
(k) Lipshutz, B. H.; Wilhelm, R. S. J. Am. Chem. Soc. 1982, 104, 4696.
Ashby, E. C.; Coleman, D. J. Org. Chem. 1987, 52, 4554. (l) Bertz, S. H.;
Dabbagh, G.; Mujsce, A. M. J. Am. Chem. Soc. 1991, 113, 631.
(6) Brevet, D.; Lucas, D.; Cattey, H.; Lemaˆıtre, F.; Mugnier, Y.; Harvey, P.
D. J. Am. Chem. Soc. 2001, 123, 4340.
Synthesis of [Pd3(dppm)3(CO)(ClAg)](PF6)2. To a solution of
[Pd3(dppm)3(CO)(Cl)](PF6) (0.210 g, 0.125 mmol) in acetone (15 mL)
was added AgPF6 (7.52 mg, 0.137 mmol). The brown reaction mix-
ture was allowed to stir at room temperature and became red. After
20 min, 3 mL of water were added and the solution was concentrated
by the rotating evaporator. The resulting brown precipitate was isolated
by filtration, washed 3 times with water, and dried with ether. Yield:
(7) (a) Harvey, P. D.; Crozet, M.; Aye, K. T. Can. J. Chem. 1995, 73, 123. (b)
Harvey, P. D.; Hubig, S.; Ziegler, T. Inorg. Chem. 1994, 33, 3700. (c)
Harvey, P. D.; Provencher, R.; Gagnon, J.; Zhang, T.; Fortin, D.; Hierso,
K.; Drouin, M.; Socol, S. M. Can. J. Chem. 1996, 74, 2268. (d) Provencher,
R.; Aye, K. T.; Drouin, M.; Gagnon, J.; Boudreault, N.; Harvey, P. D.
Inorg. Chem. 1994, 33, 3689.
(8) (a) Puddephatt, R. J.; Manojlovic-Muir, L.; Muir, K. W. Polyhedron 1990,
9, 2767 and references therein. (b) Manojlovic-Muir, L.; Muir, K. W.;
Lloyd, B. R.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 1983, 1336.
(c) Lloyd, R. J.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt, R. J.
Organometallics 1993, 12, 1231. (d) Manojlovic-Muir, L.; Muir K. W.;
Lloyd, B. R.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 1985, 536.
97%. 31P NMR: δ(H3PO4 ) ) -4.65 ppm (s). IR: ν(CO) ) 1820 cm-1
MS (FAB): m/z (fragment) ) 659 (Pd2(dppm)(Cl)(CO)); 734.7
.
9
5512 J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003

+
C−
X Activation by the Pd3(dppm)3(CO)2 Cluster
A R T I C L E S
(Pd3(dppm)3); 981.6 (Pd2(dppm)2); 1087.5 (Pd3(dppm)2); 1122
(Pd3(dppm)2(Cl)); 1395.4 (Pd2(dppm)3(CO)); 1470.4 (Pd3(dppm)3);
1499.4 (Pd3(dppm)3(CO)); 1612.3 (Pd3(dppm)3(ClAg)).
Crystallization from acetone/toluene/hexane superimposed layers
leads to the isolation of two different crystalline phases, corresponding,
respectively, to [Pd3(dppm)3(CO)](PF6)2 and [Pd3(dppm)3(CO)(PO2F2)]-
(PF6).
Results and Discussion
Pd3(dppm)3(CO)+ Species. Figure 1 shows the RDE volta-
mmogram of Pd32+ (as a PF6- salt in THF/0.2M NaBPh4; trace
a) which exhibits two one-electron waves (based on coulo-
metry)9 at -0.29 V (A1) and -0.66 V (A2) versus SCE (-0.29
V (A1) and -0.54 V (A2) in THF/0.2M Bu4NPF6). The bulk
electrolysis at wave A1 generates a new product, which exhibits
the RDE voltammogram shown in trace b. This species is the
Pd3+ cluster. This same species can also be obtained by reducing
Pd32+ at wave A2 to generate Pd30 (31P NMR, δ(acetone-d6) )
12 ppm). Then, the addition of 1 equiv of Tl+ produces the
monocation Pd3+ and metallic Tl (reaction 3). The E1/20/+ of Tl
is -0.582 V versus SCE, which indicates that this oxidation
reaction is thermodynamically feasible.10 Similarly, the bulk
electrolysis at wave A2 where only 1 equiv of electricity is
passed through the solution generates Pd3+. Indeed, when
Figure 1. RDE voltammogram of 0.84 mM Pd3(PF6)2, in THF. (a, dotted
line) Alone; (b, solid line) after reduction with 0.2 M NaBPh4. Initial
potential: +0.2 V. Scan rate: 20 mV s-1
.
0
applying this potential, Pd3 is generated at the electrode. But
the latter is unstable in the presence of unreduced Pd32+, due to
+
a fast comproponation equilibrium, strongly in favor of Pd3
(reaction 5).
The three electrochemical methods are summarized in eqs
1-5: Method 1
Pd32+ + 1e- f Pd3
(wave A1)
(1)
+
Method 2
Pd32+ + 2e- f Pd3
(wave A2)
(2)
0
Pd30 + Tl+ f Pd3+ + Tl(m)
Method 3
(potentiostat off) (3)
0
Pd32+ + 1e- f 1/2Pd32+ + 1/2Pd3 (wave A2)
(4)
1/2Pd32+ + 1/2Pd30 f Pd3
(potentiostat off) (5)
+
+
Figure 2. Experimental EPR spectrum of Pd3 in THF at 293 K (top);
+
experimental EPR spectrum of Pd3 in THF at 100 K (bottom).
Method 3 is preferred for a practical purpose: (i) It does not
need the addition of an oxidation reagent as in method 2. (ii)
Compared to method 1, by setting the potential at the potential
of wave A2 or lower, the duration of the electrolysis is
significantly shortened.
expectedly red-shifted in comparison with the more oxidized
2+
Pd3 (ν(CO) ) 1835 cm-1) and blue-shifted with respect to
the more reduced Pd30 (ν(CO) ) 1761 cm-1),9 due to the back-
bonding effect.
For characterization purposes, this same 43-electron species
can be cleanly chemically prepared using NaBPh4 as reducing
agent (eq 6):11
The isotropic (at 293 K) and anisotropic (at 100 K) EPR
spectra for Pd3 in THF were measured (Figure 2 as an
+
example). The former exhibits a septet with a relative intensity
approaching 1:6:15:20:15:6:1, along with a weaker structure.
These are due to coupling with six equivalent P atoms (S ) 1/2;
natural abundance 31P 100%), hence demonstrating the presence
of delocalization. This room-temperature spectrum also exhibits
weaker signals beside each of the stronger ones of the septet
Pd32+ + NaBPh4 f Pd3+ + Na+ + 1/2Ph-Ph + BPh3 (6)
where biphenyl is identified by GC-MS. The IR spectrum of
Pd3 exhibits a ν(CO) absorption at 1785 cm-1, which is
+
5
units due to coupling with 105Pd (S ) /2; natural abundance
(9) Gauthron, I.; Mugnier, Y.; Hierso, K.; Harvey, P. D. Can. J. Chem. 1997,
75, 1182.
22.2%). The following EPR parameters have been extracted: g
) 2.065, A(31P) ) 75.8 × 10-4 cm-1, A(105Pd) ) 7.6 × 10-4
cm-1 at 293 K; g// ) 2.059, g ) 2.078, A//(31P) ) 88.8 ×
(10) CRC Handbook of Chemistry and Physics, 64th ed.; CRC Press.: Boca
Raton, FL, 1983; pp D-159.
(11) Strauss, S. H. Chem. ReV. 1993, 93, 927.
9
J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003 5513

A R T I C L E S
Lemaˆıtre et al.
Table 1. Consumed Q for Electrochemically Induced RX
10-4 cm-1, A (31P) ) 85.0 × 10-4 cm-1 at 100 K. The relative
magnitude of the A constants (A(31P) . A(105Pd)) is due to the
difference in the gyromagnetic ratio.12 The presence of coupling
with the Pd and P atoms is completely consistent with the atomic
contributions of the SOMO (a2). Indeed, previous EHMO
Activation
molar ratio
RX/clustera
Q
entry
RX
(F/mol of cluster)
1
2
H2CdCHsCH2I
CH3I
1.1
10
thermal
0.18
2+
computations on the Pd3 species indicated that this MO is
1.1
1.1
1.1
1.1
1.1
1.1
1.2
1.1
1.5
1.1
1.1
1.1
1.1
1.1
10
0.39
0.45
0.72
0.20
0.16
0.81
0.70
1.01
0.86
no reactivity
0.80
no reactivity
0.65
no reactivity
2
2
primarily composed of in-plane Pd d orbitals (dxy, dx -y with
minor px and py contributions) and some phosphorus components
(py and px lone pairs), forming Pd-Pd and Pd-P antibonding
MOs.13
3
4
5
6
7
CH2I2
C2H5I
PhCH2I
(CH3)3CI
n-BuI
PhCH2CH2I
PhI
(CH3)3CBr
n-BuBr
PhCH2Br
Me2CHBr
H2CdCHsCH2Br
n-BuCl
8
9
Previous DFT calculations on geometry optimization pre-
dicted that the Pd-Pd distances are 2.592, 2.734, and 3.008 Å
10
11
12
13
14
15
for Pd32+, Pd3+, and Pd3 , respectively.9 The former value is in
0
agreement with X-ray data (2.604(30) Å),7d,14 while the latter
one is consistent with the d10-d10-d10 electronic configuration
and the distance found in the zerovalent Pd2(dppm)3, 2.959(2)
+
Å.15 The value of 2.734 Å for the Pd3 cluster indicates not
16
17
H2CdCHsCH2Cl
t-BuCl
1.1
1.1
10
0.80
no reactivity
only weak Pd-Pd bonding but also an increase in cavity size
above the Pd3 plane, suggesting that a larger substrate can
penetrate inside this pocket.16
a All experiments started from [Pd3(dppm)3(CO)(CF3CO2)](CF3CO2);
Carbon-Halogen Bond Activation with Electrochemical
Induction. The Pd32+, as a CF3CO2- salt, is unreactive toward
most R-X substrates (X ) Cl, Br, I) in stoichiometric quantities.
However, the reaction can be electrochemically induced to form
quantitatively the highly stable inorganic product Pd3(X)+ as
identified by the comparison of the 31P NMR and CV data of
authentic samples.17
however, for several RX, we have verified that Q does not significantly
-
-
change when replacing PF6 by CF3CO2
.
+
the formation of PhCH2F witnesses the intermediacy of PhCH2
,
which abstracts an F- from the supporting electrolyte anion,
PF6-. Supplemental evidence for the species “R+” is provided
by trapping with phenol, which gives the corresponding ether
R-O-Ph and H+.
6,18
Interestingly, as shown in Table 1, the amount of electricity
necessary to complete the conversion, as measured by coulom-
etry (electrolysis at -0.48 V vs SCE), is not stoichiometric,
being intermediate between 0 and 1 F/mol of cluster and
depending upon the nature and relative excess of R-X (see
the following).
To explain the formation of R• and R+, one can conceive
that two reactions compete, one leading to the radical and the
other to the carbocation according to
Pd3(L)+ + R-X f Pd3(X)+ + L- + R+
(7)
(8)
To elucidate the organic coproducts derivated from the alkyl
part of R-X, a series of experiments has been designed using
benzyl halides PhCH2X (X ) Br, I), which are potentially apt
to form low-volatility GC/MS analyzable products. Starting from
PhCH2Br (experiment 12 in Table 1, Q ) 0.80 F/mol of cluster),
we observed that the electrolyzed solution analysis provided
evidence for the formation of PhCH2CH2Ph (roughly estimated
yield, 50%), PhCH3 (not quantified as its peak is not fully
resolved from that of the solvent), and PhCH2F (traces).
This set of products demonstrates that both the radical R•
and the carbocation R+ are involved in the overall process.
Indeed, PhCH2CH2Ph must arise from homocoupling of PhCH2
and PhCH3 from solvent hydrogen abstraction [THF has been
reported earlier to act as a hydrogen donor]. On the other hand,
Pd3(L)+ + R-X + e- f Pd3(X)+ + L- + R•
- 19
where L- ) CF3CO2
.
As observed experimentally, the number of injected e- is less
than 1 per molecule of cluster (see Table 1), being directly
related to its own part of reaction 7 in the consumption of
Pd3(L)+. In accordance with our assumption, when PhCH2I is
used instead of PhCH2Br with a considerably lower amount of
electricity (0.2 against 0.8 F/mol, see Table 1), GPC/SM analysis
shows that PhCH2F becomes the major product (yield, 86%)
and PhCH2CH2Ph, the minor one (only traces detected).
•
Finally, the simultaneous appliance of reactions 7 and 8 can
be explained by the fully detailed mechanism shown in Scheme
1. After the reduction step a, Pd3(L)0 dissociates into Pd3+ and
L- (step b). Previous studies have shown that, in the reduced
(12) The NMR frequencies in MHz at a 7.0463 T field are 121.442 for 31P and
13.728 for 105Pd in Drago, R. S. Physical Methods for Chemists, 2nd ed.
Saunders College Publishing: Montreal, 1992. These data are also consistent
with those found for Rh2(O2CEt)4(PPh3)2+ as an example: A//(31P) ) 205
× 10-4 cm-1; A (31P) ) 152 × 10-4 cm-1; A//(103Rh) ) 13 × 10-4 cm-1
;
A (103Rh) ) not resolved in Kawamura, T.; Fukamachi, K.; Sowa, T.;
Hayashida, S.; Yonezawa, T. J. Am. Chem. Soc. 1981, 103, 364.
(13) Harvey, P. D.; Provencher, R. Inorg. Chem. 1993, 32, 61.
(14) Provencher, R.; Harvey, P. D. Inorg. Chem. 1996, 35, 2113.
(15) Kirss, R. U.; Eisenberg, R. Inorg. Chem. 1989, 28, 3372.
(16) Zhang, T.; Drouin, M.; Harvey, P. D. J. Chem. Soc., Chem. Commun. 1996,
877.
(18) As a representative example, Me3CI was reacted with Pd3(CF3CO2)+ in a
1:1 ratio in a THF solution containing 0.2 M Bu4NPF6 as supporting
electrolyte and PhOH (in the same molar amount as Me3CI). After
electrolysis, the solution was evaporated and the residue was extracted with
Et2O. The Ph-O-CMe3 ether coupling product is detected in low yield
by GC/MS (isolated yield ) 25%). The low yield is due to inefficient
trapping of the unstable carbocation for this stoichiometric reaction.
(19) As stated previously, the starting material Pd3(CF3CO2)+ exists as a host-
(17) The complexes have been prepared according to literature methods,8 and
the 31P NMR and electrochemical data are as follow: δ (acetone-d6) )
-
-
guest system where the CF3CO2 substrate is held by Pd32+.....CF3CO2
-1.25 (PF6-), -7.01 (CF3CO2-), -6.53 (Cl-), -6.14 (Br-), and -6.40
electronic interactions inside the cavity based upon X-ray data.7d The
binding constant found for carboxylates are relatively weak with respect
to anion species but strong with respect to neutral organic molecules.
2+/0
ppm (I-); E1/2
) -0.77 V (Cl-), -0.68 V (Br-), and -0.77 V (I-) in
THF solutions containing 0.2 M Bu4NPF6.
9
5514 J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003

+
C−
X Activation by the Pd3(dppm)3(CO)2 Cluster
A R T I C L E S
Scheme 1
concentration increases, Q decreases which is consistent with
the increased rate of reactivity.
+
Since the paramagnetic Pd3 species is a potential reducing
agent, quenching experiments were also investigated. The
electrochemical generation of Pd3+ followed by the addition of
1 equiv of ferricinium (Fc+) with no applied potential leads to
2+
the rapid formation of Pd3 and ferrocene (Fc), as identified
by RDE voltammetry, both in the presence and in the absence
of R-X. The reactivity is explained by an outer-sphere electron
transfer according to
Pd3+ + Fc+ f Pd32+ + Fc0
(9)
The Fc+/Fc couple is +0.56 V versus SCE in our experiments.
As stated in Table 1, Q varies as I < Br < Cl where most
RsCl substrates are found unreactive. For instance the allyl
halides (H2CdCHsCH2X) require 0 (thermal), 0.65, and 0.80
F/mol for X ) I, Br, and Cl, respectively. Similarly, the n-BuX
substrates are unreactive for X ) Br and Cl, and Q ) 0.81
F/mol for X ) I. This trend is consistent with the relative
CsX bond strength.21 The rare example of activated R-Cl
state, the binding of L- is no more stabilizing.20 So, the R-X
substrate is able to penetrate into the empty cavity of the Pd3
+
cluster to form the host-guest complex Pd3(RX)+ (step c).
Hence, there are two reaction possibilities: (i) the activated R-X
undergoes a C-X homolytic cleavage to form the radical R•
and Pd3(X)+ (step d); (ii) alternatively, a solution outer-sphere
electron transfer (SET) occurs between Pd3(RX)+ and Pd3(L)+,
which gives Pd3(RX)2+ and Pd3(L)0 (step e). The latter is able
to enter in a new catalytic cycle, and the former forms the
carbocation through the heterolytic cleavage of the C-X bond
(step f). There is no direct evidence (e.g., spectroscopic proof)
of the extremely reactive Pd3(RX)2+. However, the same
intermediate type is demonstrated to exist and to react strictly
substrates (i.e., H2CdCHsCH2Cl) stems from the charge
+
stabilization by resonance in H2CdCHsCH2
.
While a correlation between Q and the C-X bond strength
is obvious such as for i-PrBr (no reactivity), n-BuBr (no
reactivity), PhCH2Br (Q ) 0.80 F/mol, 1.1 equiv), and t-BuBr
(Q ) 0.86 F/mol, 1.5 equiv), for instance, other comparisons
are not so direct. Indeed, Q for MeI is significantly smaller than
Q for RCH2CH2I (R ) H, Et, Ph), the latter values being similar.
In fact, the smaller MeI substrate interacts more efficienly with
2+
in the same way within the reactivity of Pd3 onto acid
+
the Pd3 center inside the phenyl-dppm cavity than the longer
chlorides (see the following).
RCH2CH2I substrates. In addition, the Q value for t-BuI is small
despite its size and reflects the relative stability of the carboca-
tion Me3C+ and the reduced probability of a back reaction
(R+ + X- f R-X; inductive effect). This observation is also
corroborated by the PhI datum, where no charge stabilization
is effective. Indeed Q ) 1.01 F/mol, a maximum value where
the turnover number of 1 is obtained (i.e., stoichiometric). In
this case, the carbocation Ph+ is a strong Lewis acid, and a
back reaction is very efficient.
To demonstrate the reasonability of steps c and d of Scheme
1, a slight excess of PhCH2Br was added to a pure solution of
Pd3+: an immediate reaction took place, leading quantitatively
to Pd3(Br)+ and to PCH2CH2Ph (44%) and PhCH3, with no R+
forming product, that is, PhCH2F.
Reaction 7 is made of steps b + c + e. The ET step a initiates
the catalytic sequence, allowing for the quick elimination of
L-, which is blocking the cluster reactive site.19 In that way,
the reaction with R-X is rendered faster. We have found that
reaction 7 can operate solely, without electrochemical induction,
but over longer periods of time (typically, 1 day is necessary
for the reaction between the cluster and PhCH2Br to be
completed, in comparison with 1 h for the electrocatalyzed
method); accordingly, in the uncatalyzed conditions, PhCH2F
is the only organic product observed (yield, 90%).
Thermally Induced R-X Activation. On some occasions,
the Pd3(CF3CO2)+ species exhibits slow thermal R-X activa-
tion. The time scale for the thermal activation ranges from hours
to days under stoichiometric conditions, in comparison with ∼15
min in the electrochemical induced studies. This reactivity
becomes faster as the quantity of substrate becomes more
important. This is due to the host-guest behavior of the
CF3CO2- anion, which can be displaced by a higher amount of
substrate, allowing substrate......Pd32+ interactions without elec-
trochemical induction. The thermal reactivity can be monitored
by UV-vis spectroscopy where the starting inorganic material,
Pd32+, and the final product, Pd3(X)+, exhibit λmax at 496 or
By injecting a small amount of electricity (0.04 F/mol for
1.1 equiv of substrate MeI for instance and stopping the
electrolysis before the current reaches zero), we observe that
the R-X activation reaction goes to completion after 1 h. As a
result, the turnover number for this specific example becomes
24 (i.e., (1-0.04)/0.04) in comparison with 1.5 (i.e., (1-0.39)/
0.39) for the more rapid exhaustive nonstop electrolysis (Table
1, entry 2). So, the turnover number is a function of the initial
amount of injected electricity. In addition, as the substrate
-
-
484 nm (PF6 and CF3CO2 as counteranions) and 464 nm,
for X- ) Cl-, Br-, and I-, respectively. Figure 3 shows a typical
example of the progression of the UV-vis spectra with time
during an R-X activation reaction (R-X ) n-BuI).
(20) Lemaˆıtre, F.; Brevet, D.; Lucas, D.; Vallat, A.; Mugnier, Y.; Harvey, P.
(21) March, J. AdVanced Organic Chemistry; John Wiley and Sons: New York,
D. Inorg. Chem. 2002, 41, 2368.
1992; p 24.
9
J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003 5515

A R T I C L E S
Lemaˆıtre et al.
2+
Figure 3. UV-vis spectral changes during the reaction between Pd3 (2.78 × 10-5 M) and 1000 equiv of BuI in THF.
Table 2. Rate Constants for the R-X Activation by Pd3(CF3CO2)+
Experimentally, it has been found that the reaction rate (V)
can be expressed as in eqs 10 and 11 (details provided in the
Supporting Information):
in a 1000:1 Ratio
106
×
(min-
k*
1
RX
)
2+
-
t-BuI
n-BuI
n-PrI
EtI
instantaneous
1.48
V ) k[RX][Pd3
]
(counteranion ) PF6 ) (10)
3.02
[RX][Pd3(L)+]
[L-]
5.52
-
V ) k*
(counteranion ) L- ) CF3CO2 )
MeI
6.40
t-BuBr
1.35
H2CdCHsCH2I
H2CdCHsCH2Br
H2CdCHsCH2Cl
instantaneous
2.75
(11)
1.73
-
This result is consistent with the fact that CF3CO2 blocks
the cavity, whereas, with PF6-, it can be considered free to
access for the R-X substrate.16,19
When a large excess of R-X is used (typically, a 1000-fold
excess), eqs 10 and 11 can be simplified to
2+
Table 3. Rate Constants for the R-X Activation by Pd3 in a
1000:1 Ratio
k
1
RX
(L mol-1 min-
)
2+
-
t-BuI
n-PrI
MeI
instantaneous
0.842
V ) k[RX]0[Pd3
]
(counteranion ) PF6 ) (12)
0.853
[RX]0[Pd3(L)+]
[L-]
n-BuI
0.50
V ) k*
Table 4. Rate Constants for the t-BuI Stoichiometric Activation by
(counteranion ) L- ) CF3CO2 ) (13)
-
Pd3(L)+
experimental
conditions
103
(min-
×
k*
where [RX]0 is the concentration at t ) 0.
From eqs 12 and 13, one can demonstrate that the rate
constant k or k* can be obtained according to (Supporting
Information)
1
L-
)
-
CF3CO2-
CF3CO2-
CF3CO2-
CF3CO2
5.24
5.34
5.49
4.75
+1 equiv Ag+
+0.2 M (TBA)PF6
+1 equiv Tl+
2+
[Pd3
[Pd3
]
]
L-
k (mol L-1 min-
)
1
1
-
ln
) -kt
(counteranion ) PF6 ) (14)
2+
-
[RX]0
PF6
instantaneous
[Pd3(L)+]0
[RX]0
[Pd3(L)+]
[Pd3(L)+]0
[Pd3(L)+]
Many substrates react so fast at this level of [RX] that k
cannot be measured. This is the case for t-BuI where instanta-
neous reactions are observed upon mixing the Pd32+ and R-X
solutions, for ratios 1:1000 and 1:100. However, for 1:10 with
Pd3(CF3CO2)+, the reaction becomes slow enough for analysis.
The t-BuI substrate has been investigated in a little more detail
in the stoichiometric conditions (i.e., for a 1:1 ratio). While it
still reacts spontaneously with Pd3(PF6)+ (i.e., the cavity is
available), the slower reaction for the Pd3(CF3CO2)+ case allows
(Table 4) the rate constant k* to be extracted from (see
Supporting Information)
2 ln
+ 1 -
) -k*t
+
(
)
[Pd3(L) ]0
-
(counteranion ) L- ) CF3CO2 ) (15)
The inset of Figure 3 shows an example of such an analysis,
and the data are compared in Tables 2 and 3. Numerous
important trends are noticed. First, the rates vary as Cl < Br <
I for a given substrate, illustrating further the effect of the C-X
bond strength on the rates. Finally in the n-alkane series, the
rate also varies according to n-BuI < n-PrI < EtI < MeI,
suggesting a small effect of steric hindrance on the rates, while
the C-I bond strength remains relatively constant along this
group. This trend was also observed with the Q values extracted
in the electrochemically induced activation.
[Pd3(L)+]
[Pd3(L)+]0
[Pd3(L)+]
[Pd3(L)+]
ln
+ 2
0 - 2 ) k*t
(16)
9
5516 J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003

+
C−
X Activation by the Pd3(dppm)3(CO)2 Cluster
A R T I C L E S
The addition of 0.2 M Bu4NPF6 (typical concentration in the
electrochemical activation) or 1 equiv of Ag+ or Tl+ does not
change greatly the rate within the experimental uncertainty.
These observations indicate that the R-X activations are not
influenced by the presence of the supporting electrolyte and
that Pd32+ must be a much stronger Lewis acid than Ag+ (under
stoichiometric conditions).
Acid Chloride Activation. In a stoichiometric amount,
RCOCl reacts with Pd3(L)+ (L- ) CF3CO2-, PF6-) without
electrochemical induction. This reactivity has been monitored
by CV and 31P NMR. Thus, the presence of Pd3(Cl)+ has been
clearly demonstrated as the sole inorganic product. As a typical
example, Figure 4 shows the progression of the CV of a
Pd3(CF3CO2)+ solution in the presence of 1 equiv of RCOCl
(R ) t-Bu) where the electrochemical system of the initial
cluster (A/A′) is gradually replaced by that of the chloride adduct
(B/B′). So, the reaction proceeds according to
Pd3(L)+ + RCOCl f Pd3(Cl)+ + L- + “RCO+” (17)
R ) t-Bu, Ph, CH3, Me2CH, n-C6H13
Figure 4. Cyclic voltammogram evolution of Pd3(CF3CO2)+ (0.5 mM)
with 1 equiv of t-BuCOCl in THF with 0.2 M NBu4PF6. Initial potential:
0 V. Scan rate: 100 mV s-1
.
L- ) CF3CO2 , PF6
-
-
the dissociative mechanism is applied, the theorical rate law is
as follows (Supporting Information):
The presence of the “RCO+” intermediate (R ) Ph for instance)
has been demonstrated by trapping it with anisole to form the
corresponding p-methoxybenzophenone according to
k2′k3′[Pd32+][RCOCl]
d[RCOCl]
V ) -
)
(24)
k-2′[RCO+] + k3′[Pd3
]
2+
dt
At the beginning of the reaction, k-2′[RCO+] , k3′[Pd32+] and
the rate can be expressed as
The low amount of the starting material used does not allow
deduction of the chemical yield precisely in stoichiometric
conditions.
V ) k2′[RCOCl]
(25)
The rate constant k2′ can be obtained according to
Two mechanisms are possible. The associative mechanism
2+
involves a host-guest intermediate between the Pd3 cluster
[RCOCl]
and the nondissociated RCOCl molecule. This mechanism is
described by the following equations:
ln
) -k2′t
(26)
[RCOCl]0
Pd3(L)+ yk1z Pd32+ + L-
(19)
(20)
With both clusters, from an analysis of the time-resolved CV,
the linearity of the graphs ln([RCOCl]/[RCOCl]0) versus time
(for R ) Ph, t-Bu) indicates a dissociative mechanism (see, as
an example, Figure 5b). The data were also analyzed assuming
the associative mechanism, but did not lead to a linear behavior
with the corresponding equations (Figure 5a). The case of
pivaloyl chloride is shown (Figure 6) for Pd3(L)+, where L- )
CF3CO2-, PF6-. After a certain time, the data deviate from
k-1
Pd32+ + RCOCl yk2z Pd32+‚‚‚‚‚‚Cl(CO)R
k-2
k3
9
Pd32+‚‚‚‚‚‚Cl(CO)R
8 Pd3(Cl)+ + “RCO+” (21)
This mechanism is essentially the same as that observed in the
alkyl halide cases. However, a dissociative mechanism must
also be considered, due to the weaker bonding C-Cl interactions
in the RCOCl molecules. The equations are as follows:
′
linearity, since the approximation k-2′[RCO+] , k3 [Pd32+] no
longer holds. The range of linearity is greater in the PF6- graph
than that of CF3CO2-. Indeed, in the latter case, [Pd32+] is
controlled in part by the Pd3(CF3CO2)+ dissociation equilibrium
(19):
RCOCl yk2′z “RCO+” + Cl-
(22)
(23)
k-2
′
k1[Pd3(L)+]
k-1[L-] + k′3[Cl-]
[Pd32+] )
(27)
k3′
Pd32+ + Cl- 98 Pd3(Cl)+
2+
where the Pd3 is also generated according to eq 19. In the
case where L- ) PF6-, the Pd3(L)+ cluster is completely
dissociated in solution and eq 19 does not apply. Thus, when
In fact, at any time, [Pd32+] is lower than in the PF6- case, and
in this way, the previous approximation is valid upon a shorter
period of reaction time.
9
J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003 5517

A R T I C L E S
Lemaˆıtre et al.
2+
2+
0
Figure 5. First-order plot for the reactivity of Pd3 ([Pd3
]
) 0.8 mM) with 1 equiv of t-BuCOCl (b) or Me2CHCOCl (c); variation of ln([RCOCl]/
[RCOCl]0) as a function of time. Second-order plot for the reactivity of Pd3 ([Pd32+] ) 0.8 mM) with 1 equiv of t-BuCOCl (a) or Me2CHCOCl (d);
variation of (1/[Pd32+]0)-(1/([Pd32+]) as a function of time.
2+
Table 5. Solvent Effect on the Kinetic Constant for the Reaction
between Pd3(CF3CO2)+ and t-BuCOCl
k
′
2
1
solvent
(min-
)
THF
CH2Cl2
acetone
0.038
0.057
0.253
The solvent effect on the rate (Table 5) indicates an increase
in rate with the polarity. These results support the dissociative
mechanism.22
For the RCOCl substrates where R is an alkyl group (R )
Me, Et, Pr, Me2CH, n-C6H13), the kinetic data show rather that
an associative mechanism operates. In this case, the general
equation for the rate of reaction is (Supporting Information):
k2k3[Pd32+][RCOCl]
V )
) k[Pd32+][RCOCl] (28)
k-2 + k3
In the simplest case (counterion ) PF6-; mechanism made of
steps 20 and 21) and under stoichiometric conditions (ratio
[Pd32+]0/[RCOCl]0 ) 1/1), integration of eq 28 leads to
(Supporting Information)
1
1
-
) kt
(29)
Figure 6. First-order plot for the reactivity of Pd3(CF3CO2)+ ([Pd3(CF3-
CO2)+]0 ) 8 × 10-4 mol/L) with 1 equiv of t-BuCOCl. Variation of ln-
([RCOCl]/[RCOCl]0) as a function of time (top). First-order plot for the
reactivity of Pd32+ ([Pd32+]0 ) 8 × 10-4 mol/L) with 1 equiv of t-BuCOCl.
Variation of ln([RCOCl]/[RCOCl]0) as a function of time (bottom).
2+
2+
[Pd3
]
[Pd3 ]
0
The corresponding plot for the above listed R groups (see
Figure 5d for example) exhibits linearity, and the observed rate
constant varies according to n-C6H13 < I-Pr < Pr < Et < Me
(Table 6). If a dissociative process is assumed, the inductive
electron donation from R suggests that n-C6H13 should dissociate
more easily than Me, and so on. Accordingly, the reaction with
the former must be faster. The rates indicate the reverse,
suggesting that the larger the R group, the slower the reaction
is, in agreement with a sterically sensitive associative process.
Design of a Catalytic Cycle. To render the RCOCl activation
2+
catalytic, one has to regenerate the starting Pd3 catalyst by
(22) March, J. AdVanced Organic Chemistry; John Wiley and Sons: New York,
1992; p 450.
abstracting the chloride cleanly and irreversibly. The obvious
9
5518 J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003

+
C−
X Activation by the Pd3(dppm)3(CO)2 Cluster
A R T I C L E S
2+
the Pd32+ catalyst. As a blank experiment, these same reactions
were investigated in the absence of the Pd32+ species. In these
cases, no current and no reactivity was detected. All in all, the
Pd3 cluster acts as a halide transfer agent from the organic
molecule to the electrode surface.
Table 6. Rate Constants for the Reaction between Pd3 and
RCOCl in the Associative Mode
1
RCOCl
k (L mol-1 min-
)
2+
MeCOCl
3448
1609
683
2279
460
Me(CH2)2COCl
Me2CHCOCl
EtCOCl
Hence the overall reaction can be written as follows:
CH3(CH2)5COCl
2+
RCOCl + Cu(s) - e- Pd3 (cat.)8 RCO+ + CuCl(s) (33)
Scheme 2
A typical catalytic conversion is now described. When the
2+
-
Pd3 cluster is introduced (as a PF6 salt) into a solution
containing an excess of 100 molar equiv of RCOCl where R )
Ph, t-Bu, or n-C6H13, polarizing the copper electrode generates
a strong current. This current drops to zero when a quantity of
electricity nearly equivalent to the amount of RCOCl initially
introduced has passed. The weight of CuCl precipitate collected
gave a yield approaching 100%. This catalysis using an Ag
anode also allows the generation catalytically of the acylium
“RCO+” ion but was not investigated in this work. The
interactions between Pd3(X)+ and Ag+ (X- ) Cl-, Br-, I-)
are described in the following.
options are the precipitations of CuCl and AgCl by using the
corresponding M(I) salts. The use of Cu(I) species to abstract
Cl- ions from polynuclear complexes is not uncommon.23
Indeed, the Cl- abstraction proceeds quantitatively according
to
Applications. Knowing the strong electrophilic nature of the
“RCO+” species, we believe this catalytic system can be of great
utility in acylation reactions. Two synthetic applications are now
presented. These are the synthesis of fluoroacids25 and conver-
sions of alcohols in esters via O-acylation.26 These selected
reactions are based upon the following justifications.
Acid fluorides find numerous important applications in
organic synthesis.25,27,28 Generally, acid fluorides are prepared
from halide exchange with the corresponding acid chloride29
with the use of a fluorinating agent such as KF,30 KF/HF,31
HF,32 SbF3,33 BrF3,34 and ZrF2.35 These reactions require high
-
-
where the counteranion is PF6 or BF4
.
The precipitation of CuCl and AgCl constitutes the driving
force of the reaction. The cost of the Ag+ reagent, however,
(25) (a) Carpino, L. A.; Sadat-Aalaee, H. G.; Chao, H. G.; Deselms, R. H. J.
Am. Chem. Soc. 1990, 112, 9651. (b) Bertho, J. N.; Loffet, A.; Pinel, C.;
Reuther, F.; Sennyey, G. Tetrahedron Lett. 1991, 32, 1303. (c) Wenschuh,
H.; Beyermann, M.; Krause, E.; Brudel, M.; Winter, R.; Schu¨mann, M.;
Carpino, L. A.; Bienert, M. J. Org. Chem. 1994, 59, 3275.
+
motivates a cheaper alternative. In addition, the Cu(NCMe)4
material requires its synthesis24 prior to use. Instead, in a more
practical way, this Cl- abstraction reaction (eqs 30 and 31) can
also proceed quantitatively in one step by means of an
electrolysis using a copper anode. Indeed, the electrolysis of a
Pd3(Cl)+ solution in CH2Cl2 with a copper anode as a working
electrode operating at +0.65 V versus SCE leads to the desired
(26) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry, 3rd ed.; Plenum
Press, 1990; Vol. I, p 475.
(27) (a) Ramig, K.; Kudzma, L. V.; Lessor, R. A.; Rozov, L. A. J. Fluorine
Chem. 1999, 94, 1 and the references therein. (b) Hudlicky, M.; Pavlath,
A. E. Chemistry of Organic Fluorine Compounds II: A Critical ReView;
American Chemical Society: Washington, DC, 1995.
(28) (a) Wagner, R.; Wiedel, B.; Gu¨nther, W.; Go¨rls, H.; Anders, E. Eur. J.
Org. Chem. 1999, 2383. (b) Hyatt, J. A.; Raynolds, P. W. J. Org. Chem.
1984, 49, 384.
2+
reactivity (where the Pd3 product is identified by cyclic
voltammetry and 31P NMR):
(29) (a) Hasek, W. R.; Smith, W. C.; Engelhardt, V. A. J. Am. Chem. Soc. 1960,
82, 543. (b) Bloshchitsa, F. A.; Burmakov, A. I.; Kunshenko, B. V.;
Alekseeva, L. A.; Yagupol’skii, L. M. J. Org. Chem. USSR (English
Translation) 1985, 21, 1286. (c) Ritter, S. K.; Hill, B. K.; Odian, M. A.;
Dai, J.; Noftle, R. E.; Gard, G. L. J. Fluorine Chem. 1999, 93, 73.
(30) (a) Saunders, S. J. Chem. Soc. 1948, 1778. (b) Haszeldine, N. J. Chem.
Soc. 1959, 1084. (c) Pittman, A. G.; Sharp, D. L. J. Org. Chem. 1966, 31,
2316. (d) 18-crown-6 complex: Liotta, C. L.; Harris, H. P. J. Am. Chem.
Soc. 1974, 96, 2250. (e) Ishikawa, N.; Kitazume, T.; Yamazaki, T.;
Mochida, Y.; Tatsuno, T. Chem. Lett. 1981, 761. (f) Tordeux, M.;
Wakselman, C. Synth. Commun. 1982, 12, 513. (g) Clark, J. H.; Hyde, A.
J.; Smith, D. K. J. Chem. Soc., Chem. Commun. 1986, 10, 791. (h) Liu,
H.; Wang, P.; Sun, P. J. Fluorine Chem. 1989, 43, 429. (i) Krespan, C. G.;
Dixon, D. A. J. Org. Chem. 1991, 56, 3915.
Coulometric measurements indicate that 1 F/mol of Pd3(Cl)+
(nexp ) 1.06 F/mol) is required to regenerate Pd32+. When these
equations are combined, a catalytic cycle can be now described
2+
in Scheme 2. The Pd3 catalyst acts as a strong Lewis acid
(31) (a) Olah G. A.; Kuhn, S.; Beke, S. Chem. Ber. 1956, 89, 862. (b) Miller,
J.; Ying, O.-L. J. Chem. Soc., Perkin Trans. 2, 1985, 325.
which abstracts Cl- from RCOCl to generate the very reactive
acylium “RCO+” intermediate. The inorganic product Pd3(Cl)+
diffuses toward the polarized copper anode, and a Cl- ion
transfer occurs to form CuCl at the surface, hence regenerating
(32) (a) Young, D. J. Org. Chem. 1959, 24, 1021. (b) Olah, G. A.; Welch, J.
T.; Vankar, Y. D.; Nojima, M.; Kerekes, I.; Olah, J. A. J. Org. Chem.
1979, 44, 3872. (c) Abe, T.; Hayashi, E.; Baba, H.; Nagase, S. J. Fluorine
Chem. 1984, 25, 419.
(33) Meerwein, H.; Borner, P.; Fuchs, O.; Sasse, H. J.; Schrodt, H.; Spille, J.
Chem. Ber. 1956, 89, 2060.
(23) (a) Braunstein, P.; Luke, A. New J. Chem. 1988, 12, 429. (b) Braunstein,
P.; Luke, M. A.; Tiripicchio, A.; Camellini, M. Angew. Chem., Int. Ed.
Engl. 1987, 26, 768.
(34) Rozen, S.; Ben-David, I. J. Fluorine Chem. 1996, 76, 145.
(35) (a) Blicke, F. F. J. Am. Chem. Soc. 1924, 46, 1516. (b) Swain, S. J. Am.
Chem. Soc. 1953, 75, 246. (c) Nenajdenko, V. G.; Lebedev, M. V.;
Belenkova, E. S. Tetrahedron Lett. 1995, 36, 6317.
(24) Kubas, G. J. Inorg. Synth. 1979, 19, 90.
9
J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003 5519

A R T I C L E S
Lemaˆıtre et al.
Table 7. Fluorination (Entries 1-4) and Alcoholysis (Entries
temperature which is rather inconvenient for polyfunctional
systems. No catalyst is known for such a reaction.
In parallel, the conversion of alcohols into esters is of
fundamental importance in organic synthesis.26 The acylating
agent may be the acid chloride or the corresponding anhydride
as shown by eq 34:
2+
5-12) of Acid Chlorides Catalyzed by Pd3 (1% mol) under
Oxidation with a Copper Anodef
acid
Qa
chemical
yield (%)
faradaic
yield (%)
entry
chloride
nucleophile
(F/mol)
1
2
3
4
5
6
7
8
9
PhCOCl
PhCOCl
t-BuCOCl
n-C6H13COCl
PhCOCl
Bu4NPF6
Bu4NBF4
Bu4NBF4
Bu4NPF6
MeOH
EtOH
EtOH
EtOH
i-PrOH
sec-BuOH
t-BuOH
t-BuOH
0.98
0.90
0.88
0.89
0.88
0.84
0.85
0.93
0.81
0.84
0.86
0.81
98
86
86
86
84
78
80
85
80
80
0d
100
96
98
97
95
93
94
91
99
95
0d
RC(O)Y + R′(OH) f RC(O)OR′ + HY
Y ) Cl, OC(O)R
(34)
PhCOCl
t-BuCOCl
n-C6H13COCl
PhCOCl
PhCOCl
PhCOCl
The most popular catalyst used for this reaction is 4-(dim-
ethylamino)pyridine (DMAP), which needs the presence of a
base, generally a tertiary amine.36 The drawback is that the
quantity of required catalyst is comparatively great (5-10%)
instead of the 1% commonly used and the amine cocatalyst is
in excess. In addition, DMAP is uneffective toward sterically
hindered alcohols. Occasionally, high temperature, long reaction
time, and excess of acyl reagents are necessary. This situation
has led over the past 10 years to the development of various
catalysts, including TaCl5,37 TMSOTf,38 Sc(OTf)3,39 Bu3P,40
CoCl2,41 Montmorillonite K-10 and KSF,42 and inorganic solids
such as alumine43 and zinc.44
10
11
12
PhCOCl
78e
96e
a Determined per mol of RCOCl, after the current had dropped to zero;
the nonstoichiometric amount of electricity (relative to the quantity of acid
chloride) can be explained by the passivation of the copper anode which
appears at the end of the electrolysis, recovered with cuprous chloride.
b Determined by GC/MS (internal standard method). c Faradaic yield )
chemical yield/Q. d No more ester was found when the same experiment
was conducted with excess alcohol (10 equiv relative to PhCOCl).
e Conducted in an undivided cell. f See the general procedure in the
Experimental Section.
The acid fluorides are prepared by using a fluorinated
supporting electrolyte, that is, Bu4NPF6 or Bu4NBF4. Indeed,
the catalytic conversion into acid fluoride operates readily and
quantitatively from the corresponding acid chloride according
to
Similarly, the rapid and mild conversion of alcohols into esters
proceeds by the same way with the exception that Bu4NClO4
is used as a nonreactive supporting electrolyte (the data are
compared in Table 7):
2+
Pd3 (1%)
RCOCl + R′OH + Cu(s) - e-
8
2+
Pd3 (1%)
CH2Cl2
RCOCl + F- + Cu(s) - e-
8
0.2 M Bu4NPF6 or Bu4NBF4
CH2Cl2
0.2 M Bu4NPF6 or Bu4NBF4
RCO2R′+ CuCl(s) + H+ (36)
RCOF + CuCl(s) (35)
The reactions are found to be almost quantitative (faradaic yields
close to 100%). Again the nature of R or the nucleophile (here
aliphatic alcohols) does not play an important role on the
efficiency of the reactions, even with sterically encumbered
reagents such as pivaloyl chloride or tert-butyl alcohol.
Because reaction 36 releases H+, a secondary process occurs
such as the catalytic dehydration of the alcohols, as illustrated
below:47
The reaction is quasi-quantitative in faradaic yields, and high
chemical yields are obtained according to GC-MS findings
(Table 7), for all the acid chlorides (R ) Ph, t-Bu, n-C6H13)
and "F-" donors. The proposed mechanism involves the
abstraction of an F- from the PF6- or BF4- anion by the strongly
electrophilic “RCO+” ion. Previous studies have demonstrated
the ability of both anions to act as sources of F- to fluorinate
electrophilic centers, such as either organometallic45 or even
carbocationic groups.46
(36) (a) Hofle, G.; Steglich, W.; Vorbruggen, H. Angew. Chem., Int. Ed. Engl.
1978, 17, 569. (b) Scriven, E. F. V. Chem. Soc. ReV. 1983, 12, 129.
(37) Chandrasekhar, S.; Ramachander, T.; Takhi, M. Tetrahedron Lett. 1998,
39, 3263.
(38) (a) Procopiou, P. A.; Baugh, S. P. D.; Flack, S. S.; Inglis, G. G. A. Chem.
Commun. 1996, 2625. (b) Procopiou, P. A.; Baugh, S. P. D.; Flack, S. S.;
Inglis, G. G. A. J. Org. Chem. 1998, 63, 2342.
(39) (a) Ishihara, K.; Kubota, M.; Kurihara, H.; Yamamoto, H. J. Am. Chem.
Soc. 1995, 117, 4413. (b) Ishihara, K.; Kubota, M.; Kurihara, H.;
Yamamoto, H. J. Org. Chem. 1996, 61, 4560. (c) Ishihara, K.; Kubota,
M.; Yamamoto, H. Synlett 1996, 265. (d) Barett, A. G. M.; Braddock, D.
C. Chem. Commun. 1997, 351. (e) Zhao, H.; Pendri, A.; Greenwald, R. B.
J. Org. Chem. 1998, 63, 7559.
(40) (a) Vedejs, E.; Diver, S. T. J. Am. Chem. Soc. 1993, 115, 3358. (b) Vedejs,
E.; Benett, N. S.; Conn, L. M.; Diver, S. T.; Gingras, M.; Oliver, P. A.;
Peterson, M. J. J. Org. Chem. 1993, 58, 7286.
(41) Iqbal, J.; Srivastava, R. R. J. Org. Chem. 1992, 57, 2001.
(42) (a) Li, A.-X.; Li, T.-S.; Ding, T.-H. Chem. Commun. 1997, 1389. (b)
Bhaskar, P. M.; Loganathan, D. Tetrahedron Lett. 1998, 39, 2215.
(43) Nagasawa, K.; Yoshitake, S.; Amiya, T.; Ito, K. Synth. Commun. 1990,
20, 2033.
(44) Yadav, J. S.; Reddy, G. S.; Suinivas, D.; Himabindu, K. Synth. Commun.
1998, 28, 2337.
This undesired reactivity makes the ester preparation more
unfavorable. To solve this problem, the use of a platinum
cathode in the same compartment forces the reduction of H+
as soon it is formed, thus giving the ester as the sole product
(Table 7, entry 12). On the other hand, when the electrolysis
proceeds in a double-compartment cell, the reaction does not
produce the ester but rather the alkene and the acid (Table 7,
entry 11).
Pd3(ClAg)2+ Intermediate. These processes are catalytic
because the Cl- ion is successfully and cleanly abstracted from
the stable Pd3(Cl)+ inorganic product. The key intermediates
in the catalytic cycle are the Pd3(ClM)2+ species (M ) Cu, Ag).
These unstable species in solution have been investigated and
(45) (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem. Soc. 1986,
108, 1718. (b) Bochmann, M.; Wilson, L. M. Organometallics 1987, 6,
2556. (c) Gorrell, I. B.; Parkin, G. Inorg. Chem. 1990, 29, 2452.
(46) Swain, C. R.; Rogers, R. J. J. Am. Chem. Soc. 1975, 97, 799.
(47) Kaiser, E. M.; Woodruff, R. A. J. Org. Chem. 1970, 35, 1198.
9
5520 J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003

+
C−
X Activation by the Pd3(dppm)3(CO)2 Cluster
A R T I C L E S
Figure 7. ORTEP drawing of [Pd3(dppm)3(CO)(PF2O2)](PF6). Toluene. The thermal ellipsoids are shown with 30% probability. The toluene solvation
molecule and the H atoms are not shown for clarity.
tentatively isolated when possible. The Pd3(X)+ species reacts
evidence of π-arene complexation as well.49 In view of these
studies, we tentatively propose for Pd3(ClAg)2+ a bonding
arrangement with the Ag(I) bonded to the Cl atom coordinated
to the trimetallic unit, with possibly a stabilizing contribution
of the phenyl groups of the dppm,49 but this hypothesis would
need to be supported by a structure determination.
with Cu(NCMe)4 and Ag+ as stated for X- ) Cl- in eqs 31
+
and 32, according to
Unfortunately, attempts to obtain single crystals suitable for
X-ray structure determination stubbornly failed. The long period
of time necessary to grow crystals leads to the slow precipitation
of AgCl, and two types of crystals have been isolated. One has
been identified as the Pd32+ starting material. In the second one,
the compound can be formulated as [Pd3(dppm)3(CO)(PO2F2)]-
(PF6), which shows that, during the course of this study, an
where X- ) Cl- and Br- (with I-, no reaction is seen within
a day) and
-
interesting side reaction occurred between the counteranion PF6
where X- ) Cl-, Br-, and I-.
These reactions have been monitored by 31P NMR where the
starting and final products are clearly observed.17 All the
reactions are rapid where the signal of the desired Pd3(XM)2+
and the residual H2O in the solvents (acetone/toluene/hexane)
to form the PF2O2- anion as demonstrated from crystallography
(Figure 7). This hydrolysis reaction is relatively scarce, as only
a few other examples in the case of palladium complexes exist
to our knowledge.50 The authors of this original paper explained
the formation of this PF2O2- anion from a complex scheme of
reactions involving species such as POF4-, SiF4 (from the SiO2),
and SiF62-, according to
products is not observed, except for one case, Pd3(ClAg)2+
.
Indeed, this species exhibits a distinct 31P signal at -4.65 ppm
(s) which is intermediate between -1.25 (Pd32+) and -6.53
ppm (Pd3(Cl)+). In addition, the solid-state IR spectrum shows
2+
a ν(CO) peak at 1820 cm-1, which differs from ν(CO) in Pd3
H2O + PF6- f POF4- + 2HF
SiO2 + 4HF h SiF4 + 2H2O
(40)
(41)
(1835 cm-1)9 and Pd3(Cl)+ (1830 cm-1). Finally, the FAB mass
spectrum shows clear evidence for this species (details of the
peak assignments are provided in the Supporting Information).
The largest fragment is at the 1612.3 mass unit which corre-
sponds to the fragment "Pd3(dppm)3(ClAg)" after the loss of
CO. Bianchini et al. reported the structure of a complex which
had some similarities with the Pd3(ClAg)2+ complex.48 Indeed,
Bianchini’s compound, formulated as [(PP3)RuH(η1-ClTl)](PF6)
(PP3 ) P(CH2CH2PPh2)3) and containing TlCl as a ligand, was
obtained by reaction of Tl+ with the corresponding chloride
complex. Structurally, the Tl-Cl-Ru fragment is observed,
where the Cl atom behaves as a bridging ligand. It is interesting
to note that Tl+...phenyl interactions are also observed in this
complex. This feature brings in the idea that π-systems stabilize
metal cations. In relevance with this work, some calix[4]arenes
have also been shown to act as hosts for the ion Ag+, with
2-
SiF4 + 2F- f SiF6
(42)
(43)
POF4- + H2O f PO2F2- + 2HF
In this structure, the PF2O2- anion is located inside the cavity.
Due to size, this anion is likely favored to penetrate the cavity
over the larger PF6 anion,7d and one may speculate that this
-
interaction could drive the reaction. The interactions between
the PF2O2- anion and the Pd32+ center are electrostatic in nature,
as the O....Pd distances are 2.718 and 2.930 Å. The values
compare favorably to those reported for the related cluster
[Pd3(dppm)3(CO)(CF3CO2)](PF6).acetone (2.75(2) and 2.68(2)
(49) For example, see: Xu, W.; Puddephatt, R. J. Organometallics 1994, 13,
3054.
(50) Fernandez-Galan, R.; Manzano, B. R.; Otero, A.; Lanfranchi, M.; Pelling-
helli, M. A. Inorg. Chem. 1994, 33, 2309.
(48) Bianchini, C.; Masi, D.; Linn, K.; Mealli, C.; Peruzzini, M.; Anobini, F.
Inorg. Chem. 1992, 31, 4036.
9
J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003 5521

A R T I C L E S
Lemaˆıtre et al.
Table 8. Comparison of the Pd3(dppm)3(CO)2+ Skeletonsa
Cl- abstraction by the Pd3 cluster. In comparison with the
acid chlorides, allyl chlorides, and alkyl chlorides (here no
reactivity), the rates of abstraction vary as AlCl4- > RCOCl >
allyl-Cl . R-Cl and are consistent with the relative A-Cl
bond strength (A ) atom).
2+
d(Pd−Pd)
d(Pd
−P)
d(Pd
−C)
d(CdO)
Å
Å
Å
Å
ref
[Pd3(PF2O2)]PF6
[Pd3(CF3CO2)]PF6
[Pd3(Cl)]BF4
2.612
2.595
2.595
2.613
2.322
2.311
2.312
2.318
2.104
2.08
2.104
2.104
1.17
1.17
1.15
1.16
this work
7d
7c
7d
[Pd3](PF6)2
Conclusion
a Averaged values.
This work illustrated the C-X bond activation properties of
the Pd3 unsaturated cluster. Both bond strength and host-
2+
Å). The bond length and angles (Supporting Information) are
normal and compare favorably to those other Pd3(dppm)3(CO)2+
species investigated in our laboratory (Table 8).51 The com-
parison indicates that there are practically no changes in structure
parameters upon host-guest interactions within the experimental
uncertainties. This list includes the Pd3(Cl)+ species for which
the binding constant is very large.16,52 This behavior may reflect
guest (guest sizes versus cavity dimension) interactions influence
both the rate of reactivity and the mechanism. This species
shows a better Lewis acid behavior than that “traditional” Lewis
acids such as AlCl3. The preparations of ethers, esters, and acid
fluorides show the great potential for organic synthesis,
particularly when the reaction is rendered catalytic. The elucida-
tion of the intermediate Pd3(ClAg)2+ is very interesting. The
use of such a Lewis acid opens the door to other applications
such as catalytic nucleophilic substitution with inversion of
configuration (SN2) assisted by a transition metal and regiose-
lective Friedel-Crafts reactions, operating within a constrained
area above the Pd32+ plane. Further research will be published
in due course.
2+
the soft-hard Lewis acid/base affinity properties of the Pd3
centers and donors.
Attempts to abstract X- from Pd3(X)+ (X- ) Cl-, Br-, I-)
with Tl+ and abstract Cl- from Pd3(Cl)+ with AlCl3 have been
made in CH2Cl2 and THF. No reaction was observed. On the
other hand, the abstraction of Cl- from AlCl4- by the unsatur-
ated Pd32+ species operates rapidly and quantitatively according
to
Acknowledgment. P.D.H. thanks the NSERC (Natural Sci-
ences and Engineering Research Council) and FCAR (Fonds
Concerte´s pour l′Avancement de la Recherche) for funding.
Y.M. is grateful to the CNRS (Centre National de la Recherche
Scientifique) and Conseil Re´gional de Bourgogne for funds.
Pd32+ + AlCl4- f Pd3(Cl)+ + “AlCl3”
(44)
This result illustrates the stronger Lewis acidity of the Pd32+ in
these solvents. Based upon this work’s results, this reaction must
proceed by a host-guest interaction between the AlCl4- anion
2+
Supporting Information Available: Demonstration and in-
tegration of kinetic rate laws and their relations to mechanisms,
peak assignments of the Pd3(dppm)3(CO)(ClAg)2+ FAB mass
spectrum, tables giving crystal data and details of the structure
determination, atomic coordinates, bond distances, bond angles,
anisotropic thermal parameters, and hydrogen atom positions
for [Pd3(dppm)3(CO)(PF2O2)](PF6). This material is available
free of charge via the Internet at http://pubs.acs.org.
and the Pd3 cluster similar to that indicated by the X-ray
structure of the Pd3(PF2O2)+ species discussed previously,
forming a "Pd3....Cl....AlCl3+" intermediate, followed by a rapid
(51) (a) The term “host-guest“ complex is generally reserved for reversible
systems.51b The halide adducts Pd3(X)+ do not exhibit such reversibility,
and therefore, the use of this term is inappropriate in this case, even if the
strongly coordinated X- ions are located inside the cavity. (b) Connors,
K. A. Binding Constants: The Measurements of Molecular Complex
Stability; J. Wiley and Sons: New York, 1987.
(52) Harvey, P. D.; Hierso, K.; Braunstein, P.; Morise, X. Inorg. Chim. Acta
1996, 250, 337.
JA0297786
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5522 J. AM. CHEM. SOC. VOL. 125, NO. 18, 2003