The Pd3(dppm)3 (CO)2+ cluster: An efficient electrochemically assisted Lewis acid catalyst for the fluorination and alcoholysis of acyl chlorides

Article abstract of DOI:10.1021/jo026237q

The dicationic palladium cluster Pd₃(dppm)₃-(CO)²⁺ (dppm = bis(diphenylphosphino)methane) reacts with acid chlorides RCOCl (R = n-C₆H₁₃, t-Bu, Ph) to afford quantitatively the chloride adduct Pd₃(dppm)₃(CO)(Cl)⁺ and the acyl cation RCO⁺ as the organic counterpart. The dicationic reactive cluster can be reformed by electrolyzing the chloride complex with a copper anode leaving CuCl as a byproduct. The combination of these two reactions provides an electrocatalytic way to form the acylium from the acid chloride. Indeed, in CH₂Cl₂, 0.2 M NBu₄PF₆, or NBu₄BF₄, the electrolysis of the acid chloride in the presence of a catalytic amount of the cluster (1%) gives in good yields the acid fluoride RCOF, arising from the coupling of the acylium with a F^- issued from the fluorinated supporting electrolyte. Alternatively, in CH₂Cl₂ or 0.2 M NBu₄ClO₄, by operating with an alcohol R′OH as the nucleophile, the electrolysis gives the ester RC(O)OR′ as the only final product.

Full text of DOI:10.1021/jo026237q

Th e P d 3(d p p m )3(CO)2+ Clu ster : An Efficien t
Electr och em ica lly Assisted Lew is Acid
Ca ta lyst for th e F lu or in a tion a n d
Alcoh olysis of Acyl Ch lor id es
phile, to generate the corresponding nonsymmetric ether.
Pd3(dppm)3(CO)2+ + RX f
Pd3(dppm)3(CO)(X)+ + “R+” (1)
Fre´de´ric Lemaˆıtre,1a,b Dominique Lucas,*,1a
Yves Mugnier,1a and Pierre D. Harvey*,1b
i
n
t
R ) Me, Et, Pr, Bu, Bu, Bz, PhCH2CH2; X ) Br, I
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 J 1K 2R1
To render this process catalytic, one has to address the
2+
challenging problem of the regeneration of the Pd3
2+
species. To abstract the halide from the Pd3 center,
another Lewis acid must be used that can cleanly and
irreversibly eliminate these ions in a way that no side
or competitive reaction occurs.
2+
pharvey@courrier.usherb.ca;
dominique.lucas@u-bourgogne.fr
Acyl chlorides react more rapidly than RX with Pd3
and do not need electrochemical induction. This reaction
is the same as in the RX cases, except that, instead of a
carbonium, an acylium intermediate “RCO+” is gener-
ated. By trapping this cation with a selected nucleophile,
the reaction can be directed toward the formation of
desired products. This paper reports two applications:
the fluorination and the alcoholysis of acid chlorides.
Received J uly 24, 2002
Abstr a ct: The dicationic palladium cluster Pd3(dppm)3-
(CO)2+ (dppm ) bis(diphenylphosphino)methane) reacts with
acid chlorides RCOCl (R ) n-C6H13, t-Bu, Ph) to afford
quantitatively the chloride adduct Pd3(dppm)3(CO)(Cl)+ and
the acyl cation RCO+ as the organic counterpart. The
dicationic reactive cluster can be reformed by electrolyzing
the chloride complex with a copper anode leaving CuCl as a
byproduct. The combination of these two reactions provides
an electrocatalytic way to form the acylium from the acid
chloride. Indeed, in CH2Cl2, 0.2 M NBu4PF6, or NBu4BF4,
the electrolysis of the acid chloride in the presence of a
catalytic amount of the cluster (1%) gives in good yields the
acid fluoride RCOF, arising from the coupling of the acylium
with a F- issued from the fluorinated supporting electrolyte.
Alternatively, in CH2Cl2 or 0.2 M NBu4ClO4, by operating
with an alcohol R′OH as the nucleophile, the electrolysis
gives the ester RC(O)OR′ as the only final product.
2+
Ca ta lysis Design . The Pd3 cluster reacts with the
acid chlorides RC(O)Cl (R ) Ph, n-C6H13, t-Bu) to gener-
ate quantitatively Pd3Cl+ 6 product (eq 2):
Pd3(dppm)3(CO)2+ + RC(O)Cl f
Pd3(dppm)3(CO)(Cl)+ + “RCO+” (2)
The stoechiometric addition of Cu(NCMe)4+, a reagent
commonly employed for Cl- abstraction,7 provides an
opportunity to regenerate Pd32+ and to render reaction 2
catalytic. Indeed, this reaction proceeds quantitatively
(reaction 3):
The importance of palladium in hetero- and homoge-
neous catalysis is well-known.2 However, electrocatalytic
Pd3(dppm)3(CO)(Cl)+ + Cu(CH3CN)4+ f
Pd3(dppm)3(CO)2+ + CuCl(s) + 4CH3CN (3)
systems are rather uncommon. Recently, it was shown
2+ 3
that the great affinity of Pd3(dppm)3(CO)2+ (Pd3
)
The latter requires the use of Cu(NCMe)4+, which
needs to be synthesized at a prior stage. Instead, in a
more practical way, reaction 3 can also proceed quanti-
tatively, but in one step by means of electrolysis of the
Pd3Cl+ solution with a copper anode as the working
electrode (polarized at +0.65 V versus SCE). Coulometric
toward halide ions promotes the heterolytic cleavage of
the C-X (X ) Br, I) bond of alkyl halides (eq 1).4 The
detailed analysis of the mechanism implies an electro-
chemical induction leading to an electron-transfer chain
reaction. The driving force for this reaction is the great
stability of the Pd3(dppm)3(CO)(X)+ inorganic product
(Pd3X+).5 Hence, the formation of “R+” provides an
opportunity to seek applications. As an example, the
latter species can be trapped with phenol, as a nucleo-
measurements indicate that 1 F/mol of Pd3Cl+ is required
2+
to regenerate Pd3
:
(5) (a) The binding constants have been measured spectroscopically
(X ) I, Br, Cl) and electrochemically (X ) I) and were found to be very
large.5b,c (b) Harvey, P. D.; Hierso, K.; Braunstein, P.; Morise, X. Inorg.
Chim. Acta 1996, 250, 337. (c) Lemaˆıtre, F.; Brevet, D.; Lucas, D.;
Vallat, A.; Mugnier, Y.; Harvey, P. D. Inorg. Chem. 2002, 41, 2368.
(6) (a) The Pd3Cl+ was identified from its 31P NMR spectrum and
cyclic voltammogram (CV) by comparison with an authentic sample.6b
(b) The complex Pd3Cl+ has been prepared according to the literature
* To whom correspondence should be addressed. (P.D.H.) Tel: (819)
821-2005. Fax: (819) 821-8017. (D.L.) Tel and Fax: (33) 3 80 39 60 65.
(1) (a) Universite´ de Bourgogne. (b) Universite´ de Sherbrooke.
(2) For recent published works, see: (a) Tsuji, J . Palladium Reagents
amd Catalysts; Wiley: New York, 1995. (b) Moiseev, I. I.; Vargaftik,
M. N. New J . Chem. 1998, 22, 1217. (c) Beletskaya, I. P.; Cheprakov,
A. V. Chem. Rev. 2000, 100, 3009. (d) Shmid, G. Clusters and Collo¨ıds;
VCH: Weinheim, 1994. (e) Moiseev, I. I.; Vargaftik, M. N. In Catalysis
by Di- and Ploynuclear Metal Cluster Complexes; Adams, R. D., Cotton,
F. A., Eds.; Wiley-VCH: New York, 1998; Chapter 12 p 395.
(3) Puddephatt, R. J .; Manojlovic-Muir, L.; Muir, K. W. Polyhedron
1990, 9, 2767 and references therein.
method;3 the 31P NMR and electrochemical data are as follows:
δ
2+/0
(acetone-d6) ) -6.53 ppm and E1/2
M of Bu4NPF6).
) -0.77 V vs SCE (in THF 0.2
(7) (a) Kubas, G. J . Inorg. Synth. 1979, 19, 90. (b) Braunstein, P.;
Luke, M. A. New J . Chem. 1988, 12, 429. (c) Braunstein, P.; Luke, M.
A.; Tiripicchio, A.; Camellini, M. Angew. Chem., Int. Ed. Eng. 1987,
26, 768.
(4) Brevet, D.; Lucas, D.; Cattey, H.; Lemaˆıtre, F.; Mugnier, Y.;
Harvey, P. D. J . Am. Chem. Soc. 2001, 123, 4340.
10.1021/jo026237q CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/10/2002
J . Org. Chem. 2002, 67, 7537-7540
7537

Pd3(dppm)3(CO)(Cl)+ + Cu - e- f
Pd3(dppm)3(CO)2+ + CuCl(s) (4)
SCHEME 1
Combining reactions 2 and 4 provides the catalytic
cycle depicted in Scheme 1, which results in the selective
formation of the acylium cation from the acyl chloride.
The overall reaction can be written as follows (eq 5),
where the precipitation of CuCl constitutes the driving
force of the whole process:
2+
RC(O)Cl + Cu(s) - e- Pd3 (cat.)8 “RCO+” + CuCl(s) (5)
As a typical example, when the Pd32+ is introduced into
the solution together with an excess of acid chloride (100
molar equiv), using various derivatives (such as R ) Ph,
t-Bu, or n-C6H13), polarizing the copper electrode gener-
ates a strong current, which indicates an increase of
electron flow. The latter drops to zero after the quantity
of current passed has nearly reached the amount of acid
chloride initially introduced. The CuCl produced through-
out the electrolysis was quantified, giving a quantitative
yield with respect to the electrolyzed acid chloride.8 This
catalytic system can be of great utility in acylation
reactions. Two synthetic applications are selected: the
synthesis of fluoro acid,9 and conversion of alcohols in
esters via O-acylation.10
F lu or in a tion . These compounds find numerous im-
portant applications in organic synthesis.9,11 Generally,
acid fluorides are prepared from halide exchange with
the corresponding acid chloride12 with the use of a
fluorinating agent such as KF,13 KF/HF,14 HF,15 SbF3,16
BrF3,17 and ZrF2.18 These reactions require high temper-
TABLE 1. F lu or in a tion (En tr ies 1-4) a n d Alcoh olysis
2+
(En tr ies 5-12) of Acid Ch lor id es Ca ta lyzed by P d 3 (1%
Mol) u n d er Oxid a tion w ith a Cop p er An od ea
acid
chloride
Qb
nucleophile (F/mol) yieldc (%) yieldd (%)
chemical
faradic
entry
1
2
3
4
5
6
7
8
9
PhCOCl
PhCOCl
t-BuCOCl
n-C6H13COCl Bu4NPF6
PhCOCl
PhCOCl
t-BuCOCl
n-C6H13COCl EtOH
PhCOCl
Bu4NPF6
Bu4NBF4
Bu4NBF4
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
100
96
98
97
95
93
94
91
99
95
MeOH
EtOH
EtOH
i-PrOH
10 PhCOCl
11 PhCOCl
12 PhCOCl
sec-BuOH
t-BuOH
t-BuOH
0e
78f
0e
96f
a
See the general procedure in the Experimental Section.
Determined per mol of RCOCl, after the current had dropped to
b
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
d
standard method). Faradic yield ) chemical yield/Q. e No more
(8) As a blank experiment, we have verified that no current was
detected when the Pd32+ was not present in solution, other conditions
being unchanged. All in all, Pd32+ acts as an halide-transfer agent from
the organic molecule to the electrode.
ester was found when the same experiment was conducted with
excess alcohol (10 equiv relative to PhCOCl). f Conducted in an
undivided cell.
(9) (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.
ature, which is rather inconvenient for polyfunctional
systems. No catalyst is known for such a reaction.
By using a fluorinated supporting electrolyte, i.e.,
Bu4NPF6 (Table 1, entries 1 and 4) or Bu4NBF4 (Table
1, entries 2 and 3), the acid chloride is readily and
quantitatively converted into the corresponding fluoride
(Table 1 and eq 6):
(10) Carey, F. A.; Sundberg, R. J . Advanced Organic Chemistry, 3rd
ed.; Plenum Press: New York, 1990; Vol. I, 475.
(11) (a) Ramig, K.; Kudzma, L. V.; Lessor, R. A.; Rozov, L. A. J .
Fluorine Chem. 1999, 94, 1 and references therein. (b) Hudlicky, M.;
Pavlath, A. E. Chemistry of Organic Fluorine Compounds II: A Critical
Review; American Chemical Society: Washington, DC, 1995.
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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 (Engl.
Trans.) 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.
(13) (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.
(14) (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.
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Welch, J . T.; Vankar, Y. D.; Nojima, M.; Kerekes, I.; Olah, J . A. J .
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S J . Fluorine Chem. 1984, 25, 419.
2+
cat.: Pd3 (1%)
R(C)Cl + “F-” + Cu - e-
8
CH2Cl2
0.2M Bu4NPF6 or Bu4NBF4
RC(O)F + CuCl(s) (6)
The proposed mechanism involves the abstraction of
-
-
a F- from the PF6 or BF4 anion by the strongly
electrophilic acylium ion.19
Alcoh olysis. The conversion of alcohols into esters is
of fundamental importance in organic synthesis.10 The
(17) Rozen, S.; Ben-David, I. J . Fluorine Chem. 1996, 76, 145.
(18) (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.
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Soc. 1986, 108, 1718. (b) Bochmann, M., Wilson, L. M. Organometallics
1987, 6, 2556. (c) Gorell, I. B., Parkin, G. Inorg. Chem. 1990, 29, 2452.
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Spille, J . Chem. Ber. 1956, 89, 2060.
7538 J . Org. Chem., Vol. 67, No. 21, 2002

acylating agent may be the acid chloride or the corre-
sponding anhydride, as shown by eq 7:
RC(O)Y + R′(OH)
Y ) Cl, OC(O)R
f RC(O)OR′ + HY
(7)
The most popular catalyst used for this reaction is
4-(dimethylamino)pyridine (DMAP), which needs the
presence of a base, generally a tertiary amine.20 The
drawback is that the quantity of required catalyst is
comparatively great (5-10%) instead of 1% commonly
used and the amine cocatalyst is in excess. In addition,
DMAP is uneffective toward sterically hindrered alco-
hols. Occasionally, high temperature, long reaction time,
and an excess of acyl reactants are necessary. This
situation has led over the past 10 years to the develop-
ment of various catalysts, including TaCl5,21 TMSOTf,22
Sc(OTf)3,23 Bu3P,24 CoCl2,25 Montmorillonite K-10, and
KSF,26 and inorganic solids such as alumine27 and zinc.28
By combining Bu4NClO4 as a nonreactive supporting
electrolyte and an alcohol as the nucleophile, our cata-
lytical method allows one to obtain in good yield the
corresponding disymmetric ester (Table 1 and eq 8):
F IGURE 1. Diagram of the electrochemical cell: (a) argon
inlet; (b) platinum counter-electrode; (c) tube with sintered
glass cap to separate the counter-electrode from the other
compartments; (d) saturated calomel reference (reference
electrode); (e) tube with sintered glass cap to isolate the
aqueous phase reference electrode; (f) electrical copper wire;
(g) copper plate (working electrode; anode).
2+
cat.: Pd3
R(CO)Cl + R′OH + Cu - e-
(1%)8
CH2Cl2
produce the ester, presumably forming the alkene (Table
1, entry 11). This point will be investigated further.
In conclusion, both alcoholysis and fluorination operate
under particularly mild conditions (room temperature,
small quantity of catalyst, and no excess of reactant).
This important feature leaves hope that this method can
selectively transform sensitive and polyfunctional mol-
ecules. In addition, this method is also potentially useful
to any other acylation reaction. Indeed, recent experi-
ments have demonstrated that it is active in Friedel-
Crafts catalysis. Studies are currently in progress.
0.2 M Bu4NClO4
R(CO)OR′ + CuCl(s) + H+ (8)
The results are summarized in Table 1. With a wide
range of type of alcohols and acid chlorides, the reaction
is found to be in the range of 78-85% while the faradic
yield is close to 100%.
The yield is high, even with sterically encumbered
reactants such as pivaloyl chloride (Table 1, entry 7) or
tert-butyl alcohol (Table 1, entry 12). In these cases, the
reaction can be thwarted by a side reaction involving the
released proton in reaction 8 (eq 9).29
Exp er im en ta l Section
Ma ter ia ls. The [Pd3(dppm)3(CO)](PF6)2 complex has been
prepared according to the literature procedure.3 Dichloromethane
was distilled under Ar over P2O5. 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 °C for at least 2
days.
Electr och em ica l Exp er im en ts. All manipulations were
performed 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. High-scale30 electrolyses were performed with an Amel
552 potentiostat coupled with an Amel 721 electronic. A copper
plate was used 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 (Figure 1, except
the last experiment of Table 1 as described in the text).
Typ ica l P r oced u r es. Con ver sion of Acid Ch lor id es in to
Ester s: P r ep a r a tion of Eth yl Ben zoa te. The [Pd3(dppm)3-
To solve this problem, the use of a platinum cathode
in the same compartment prevents the undesired reac-
tion by forcing the reduction of H+ as soon as it is formed,
thus giving the ester as the sole product (Table 1, entry
12). On the other hand, when the electrolysis proceeds
in a double-compartment cell, the reaction does not
(20) (a) Hofle, G.; Steglich, W.; Vorbruggen, H. Angew. Chem., Int.
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12, 129.
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Am. Chem. Soc. 1995, 117, 4413. (b) Ishihara, K.; Kubota, M.;
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is nearly 10-3 M, whereas it is close to 10-1 M in high-scale electrolyses.
J . Org. Chem, Vol. 67, No. 21, 2002 7539

(CO)](PF6)2 cluster (15.8 mg, 8.82 × 10-3 mmol, 1 mol % per
mol of acyl chloride), ethanol (51 µL, 0.87 mmol), and benzoyl
chloride (100 µL, 0.86 mmol) were added to the anodic compart-
ment of the cell containing 15 mL of a 0.2 M solution of
Bu4NClO4 in CH2Cl2. The cathodic compartment and the refer-
ence electrode compartment were filled with the Bu4NClO4-
CH2Cl2 solution. The potential of the copper anode was set to
+0.65 V vs 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 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.
F lu or in a tion of Acid Ch lor id es. The procedure is the same
as described above, except that no alcohol is added and Bu4NPF6
or Bu4NBF4 is used instead of Bu4NClO4.
Ack n ow led gm en t. P.D.H. thanks NSERC (Natural
Sciences and Engineering Research Council) and FCAR
(Fonds Concerte´s pour l3Avancement de la Recherche)
for funding. Y.M. is grateful to CNRS (Center National
de la Recherche Scientifique) for funds.
J O026237Q
7540 J . Org. Chem., Vol. 67, No. 21, 2002