An expedient variant of heck reaction of alkenyl nonaflates: Homogeneous ligand-free palladium catalysis at room temperature

Article abstract of DOI:10.1002/adsc.200600388

A mechanistic study on the ligand-free room-temperature Heck reaction of alkenyl nonafluorobutanesulfonates (nonaflates) is described. Kinetic data obtained from poisoning experiments, centrifugation and variation of catalyst loading consistently provide

Full text of DOI:10.1002/adsc.200600388

COMMUNICATIONS
DOI: 10.1002/adsc.200600388
An Expedient Variant of Heck Reaction of Alkenyl Nonaflates:
Homogeneous Ligand-Free Palladium Catalysis at Room
Temperature
a,b
b,
c,
Michael A. K. Vogel, Christian B. W. Stark, * and Ilya M. Lyapkalo *
a
Institute ofChemical & Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833
Freie Universität Berlin, Institut für Chemie und Biochemie, Takustr. 3, 14195 Berlin, Germany;
b
e-mail: stark@chemie.fu-berlin.de
Institute ofOrganic Chemistry and Biochemistry, Academy ofSciences ofthe Czech Republic, Flemingovo n. 2, 166 10
c
Prague 6, Czech Republic
Fax : (+420)-220-183-578; phone: (+420)-220-183-420; e-mail: ilya.lyapkalo@uochb.cas.cz
Received: August 1, 2006; Revised: February 19, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
[
4]
Abstract: A mechanistic study on the ligand-free
room-temperature Heck reaction ofalkenyl nona-
fluorobutanesulfonates (nonaflates) is described.
Kinetic data obtained from poisoning experiments,
centrifugation and variation of catalyst loading con-
sistently provide evidence for a homogeneous palla-
dium catalysis unprecedented in Heck chemistry.
The Heck reaction of alkenyl perfluorobutanesulfo-
nates represents a remarkably robust, active and ef-
ficient catalytic system generally applicable to the
coupling with a broad range ofterminal ole if ns in-
cluding non-activated ones under ambient condi-
tions. It features insensitivity towards atmospheric
oxygen and moisture, furnishing uniformly high
yields ofthe anticipated coupling products without
the necessity to purify commercial reagents and sol-
vents.
deactivates the catalyst, in particular, when aryl io-
[
5]
dides are used as substrates. From this standpoint, it
is ofno surprise that the af stest versions ofHeck re-
action known so far are described for ligand-free sys-
À
À
tems containing free halides (Cl or Br added as
quaternary ammonium salts) which are believed to
[
1d]
stabilize Pd(0)-species.
Later on, it was shown that intermediary Pd nano-
particles as colloids are likely to be the true catalytic
species in such systems. Owing to the recent ad-
vancements in the mechanistic study and design of
robust and efficient low Pd-loading systems, the
ligand-free Heck reaction became an emerging trend
with particular promise for industrial applications.
On the other hand, it was demonstrated in a number
[
6]
[7]
[8]
[
7a,9]
ofwell-documented cases that palladacycles
and
are pre-catalysts and not
the actual active species catalyzing the Heck reaction
[
10]
pincer Pd(II) complexes
[
11]
Keywords: alkenyl nonaflates; catalyst poisoning;
Heck reaction; homogeneous catalysis; kinetics;
ligand-free catalysis
ofaryl halides as it was believed earlier.
ling evidence based on kinetic studies
tative poisoning experiments was obtained that the
above complexes decompose under the reaction con-
ditions to generate the actual catalytic species, Pd
nanoparticles.
Compel-
and quanti-
[
10,12]
[
10]
The above breakthroughs in understanding the
nature ofcatalysis in the Heck chemistry ofaryl hal-
monly known as the Heck reaction, represents one of ides and a lack ofmechanistic insight in the under-
Coupling ofol ei fn s with aryl/alkenyl halides, com-
[
13]
[
1]
the basic tools in contemporary organic synthesis.
The existing vast realm ofthe Heck chemistry can
conventionally be subdivided to ligand-assisted and nature ofPd-catalysis fo r the latter substrates.
ligand-free catalysis. Ligands at palladium play a dif- Herein, we report on the nature ofthe catalyst and
ligated Heck reaction ofaryl and alkenyl per lf uoroal-
[14]
kanesulfonates
prompted us to investigate the
ferent role depending on the application. While the li- the remarkable efficiency of our ligand- and additive-
gation is crucially important for enantioselective var- free system enabling room-temperature Heck cou-
[
2]
[15]
iants ofthe Heck reaction or activation ofotherwise
pling ofreadily available alkenyl nonal fa tes
with
[
3]
unreactive aryl chlorides, it often tends to deterio- electron-withdrawing, electron-rich as well as inacti-
rate the desired coupling, causes side reactions and vated alkenes. Our primary objective within the scope
Adv. Synth. Catal. 2007, 349, 1019 – 1024
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1019

Michael A. K. Vogel et al.
COMMUNICATIONS
Scheme 1. A model room-temperature Heck reaction.
[
7]
ofthis publication was to establish whether the reac-
zene studied earlier. In the latter case, the reaction
tion is effected by heterogeneous or homogeneous stops at an early stage when a high Pd loading is em-
catalysis.
As a test case the reaction ofcyclopentenyl nona-
ployed because aggregation leading to Pd clusters of
low catalytic activity outruns the desired Heck cou-
[
7,18]
flate (1a) with methyl acrylate (2a) was chosen pling.
Scheme 1). The model reaction was carried out obtained using our protocol with nonaflate 1a as a
under optimized conditions at ambient temperature substrate (Figure 1). It manifests a steady increase of
A significantly different kinetic profile was
(
[
16]
(
20–248C) with Et N as a base in DMF resulting in the reaction rate with increasing catalyst-to-substrate
3
clean and complete conversion ofthe starting nona-
ratio even at high Pd concentrations. This result can
flate 1a to the 1,3-diene ester 3a within 8–9 h. The be regarded as (negative) evidence for a kinetically
catalytic system employed features excellent reprodu- stable homogeneous catalytic species lacking cluster-
cibility, robustness and insensitivity towards atmos- ing.
pheric oxygen and moisture. Reduced catalytic load-
To gain further insight into the catalyst identity we
ing (0.5 mol%) appreciably slows down the room monitored the model reaction kinetics in the presence
temperature reaction. Thus, in a typical example, it ofdi fe rent amounts ofthiophene (Figure 2) and PPh
3
required 6 days to attain 88% conversion. However, a (Figure 3).
subsequent gentle heating at 508C rendered a com-
It has been generally accepted that quantitative ki-
plete conversion within 3 h indicating that the ob- netic data provide the most compelling evidence for
[
19]
served slow-down effect is not due to the irreversible the identity ofthe true catalyst.
deactivation ofthe catalyst. Noteworthy, the reaction quantitative studies, well-established catalyst poisons
mixtures remain essentially homogeneous during the such as PPh , CS or thiophene taken in amounts
In a number of
3
2
entire course ofthe reactions regardless ofthe
amount ofPd taken and in spite ofthe absence ofPd-
colloid stabilizers.
!1 equiv. (hereinafter, the amounts of the additives
are expressed in equiv. relative to the Pd catalyst) or
a large excess of metallic mercury were found to ef-
[
17]
The first indirect evidence for the homogeneous fectively quell the catalytic activity thus proving the
nature ofthe catalytic system studied herein comes
from the comparative analysis of conversion rate as a Heck reactions ofaryl halides.
function of catalyst loading in the model Heck reac-
heterogeneous nature ofthe Pd(0) catalysts in the
[
10,20]
For this reason, we
tion ofnona lf ate 1a (Figure 1) vs. that ofbromoben-
Figure 2. Kinetic profiles of the model reaction (see
Scheme 1) in the presence of different amounts of thiophene
(from 0.1 to 2 equivs.), at 208C.
Figure 1. Effect of palladium catalyst loading on the conver-
sion rate ofthe model reaction (see Scheme 1), at 24 8C.
1020
asc.wiley-vch.de
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1019 – 1024

An Expedient Variant ofHeck Reaction ofAlkenyl Nona fl ates
COMMUNICATIONS
effect on our reaction system. Whereas the Heck re-
actions catalyzed by Pd colloids are instantaneously
and completely suppressed after addition of mercu-
[
10]
ry, no such dramatic effect was observed under our
conditions. As shown in Figure 4, a treatment with
[
24]
3
00 equivs. Hg(0) resulted in an appreciable slow-
down effect. However, no irreversible loss of catalytic
activity was observed, and the reaction did reach
complete conversion.
Finally, centrifugation (14500 rpm, 25 min) of the
slightly turbid reaction mixture at 55% completion
(
after 2.5 h) resulted in the formation of a tiny
amount ofa very i fn e dark-brown precipitate. The
residue proved to be catalytically inactive after expo-
sure to the freshly added reactants for 24 h at room
temperature. The reaction rate in the clear homoge-
neous reaction mixture matched well that ofthe re fe r-
ence reaction indicating that the true catalytic species
fully remains in solution (Figure 4, cf. curves b and c).
To test the generality ofthis ligand- rf ee cross-cou-
pling protocol, a range ofsubstrates was submitted to
the optimized reaction conditions. Gratifyingly, a
Figure 3. Kinetic profiles of the model reaction (see
Scheme 1) in the presence ofPPh 3 (0.5 and 1 equiv.), at
248C.
number ofalkenyl nonal fa tes
ketones were found to react with terminal alkenes 2
Scheme 2) to give the expected cross-coupling prod-
1 derived from cyclic
(
ucts in high yields (Table 1).
While selectivities follow typically observed
[
1h]
trends, we were surprised to learn that the catalytic
system is active enough to effect the room-tempera-
ture Heck coupling even with poorly activated (sty-
rene, 2e) or non-activated (1-hexene, 2f) olefins.
Attempts to extend our ligand-free system to aryl
nonaflates met with no success so far. Thus exposure
Figure 4. Kinetic profiles of the model reaction (see ofphenyl nona lf ate with methyl acrylate 2a for 4 d at
Scheme 1) subjected to Hg poisoning (a) or centrifugation
room temperature or heating for 2 h at 508C did not
furnish a detectable amount of product with the cou-
pling reactants remaining intact.
(
b), at 248C.
At this stage, we would refrain from speculating on
decided to apply these poisoning methods to the the structure ofthe catalyst in the Heck reaction of
model reaction ofnona fl ate 1a (Scheme 1). alkenyl nonaflates. As a working hypothesis, we sug-
The poisoning experiments (Figure 2 and Figure 3) gest that Pd(0) exists in kinetically stable form, PdLn
clearly demonstrate that the heterogeneous catalyst in dynamic equilibrium with the reaction components
poisons, PPh and thiophene, added in amounts ofup
(L=1,3-diene 3a or olefin 2a or DMF or Et N) play-
3
3
to 2.0 equivs. have minimal to no effect on the out- ing a role ofweak ligands readily coordinating and
come and the rate of the standard Heck reaction of dissociating and thus enabling an easy access ofsub-
nonaflate 1a. These observations dramatically contrast strate to the coordinating sphere ofPd(0)-centre.
[
10b]
results obtained by Eberhard
who observed that With all likelihood, low-nucleophilic nonaflate anion
!
1 equiv. ofPPh 3 or thiophene is sufficient to fully accumulated during the reaction course does not co-
suppress the activity ofthe heterogeneous Pd catalyst
apparently resulting from decomposition of pincer affect the catalyst activity.
ordinate to the catalytic Pd-species and thus does not
[30]
Pd(II) complexes. On the other hand, the lack ofpoi-
soning with the above additives can be regarded as an tems designed for Heck couplings of aryl halides,
In conclusion, unlike the relevant ligand-free sys-
[6,7]
evidence ofa kinetically stable fo rm ofthe homogene- the room-temperature Heck reactions ofalkenyl non-
ous Pd(0) catalytic species throughout the entire aflates described herein do not indicate signs of heter-
[
21,22]
course ofthe reaction.
ogeneous (colloidal Pd) catalysis. The catalytic system
Despite certain limitations on the applicability of features remarkable activity, endurance and insensi-
[
23]
Hg(0) poisoning tests, we decided to investigate its tivity to the established heterogeneous catalyst poi-
Adv. Synth. Catal. 2007, 349, 1019 – 1024
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1021

Michael A. K. Vogel et al.
COMMUNICATIONS
Scheme 2. Ligand-free room-temperature Heck reactions of alkenyl nonaflates 1 with terminal alkenes 2.
Table 1. Reaction conditions and yields ofthe products
and 4 according to Scheme 2.
3
ExperimentalSection
GeneralProcedure for the Syntheses of Dienes 3
from Alkenyl Nonaflates 1
Entry Reaction Time Products 3/4
+ 2 (ratio,%:%)
Yield
[a]
1
[%]
DMF (1 mL), Et N (202 mg, 2.00 mmol), alkenyl nonaflate 1
[
14f]
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1a + 2a 11 h (E)-3a
1a + 2b 16 h 3b, E/Z=79:21
1a + 2c 13 h (E)-3c
94
92
90
86
86
84
98
98
98
93
94
92
87
94
91
(
1.00 mmol), alkene 2 (1.30 mmol), and Pd
A
H
R
U
G
0
.05 mmol) were subsequently added to a screw-cap vial
[25]
equipped with magnetic stirring bar. The reaction mixture
was stirred for the designated amount of time (see Table 1)
at ambient temperature. It was then subjected to aqueous
work-up (pentane/water). The combined organic layers were
washed with brine and dried with Na SO . The solvent was
1a + 2d 18 h (E)-3d/4d (6:94)
[14f]
1a + 2e 13 h (E)-3e[ /4e (93:7)
26]
[b]
1a + 2f 15 h (E)-3f /4f (56:31)
1b + 2a 24 h (E)-3g
2
4
1b + 2e 24 h (E)-3h/4h (84:16)
removed under reduced pressure and the residue purified by
either column chromatography (silica gel) or Kugelrohr dis-
tillation, to give pure dienes 3.
[
27]
1c + 2c 14 h (E)-3i
0
1
2
3
4
5
1d + 2a 13 h (E)-3j [
28]
14f]
1d + 2e 15 h (E)-3k
[
1e + 2a 13 h (E)-3l
1e + 2b 14 h 3m
[
14e]
E29/]Z=(67:33)
[
1e + 2c 13 h (E)-3n
1e + 2d 37 h 4o
Acknowledgements
[
[
a]
Isolated yields.
Contains 13 mol% ofthe 1,4-diene product, 1-hex-2-
enylcyclopentene.
Financial support by the Agency for Science Technology and
Research (A*STAR Singapore) is most gratefully acknowl-
edged. We are indebted to Bayer AG for the generous dona-
tion of nonafluorobutane-1-sulfonyl fluoride.
b]
References
sons. These results suggest that the coupling reactions
are catalyzed by homogeneous Pd(0)-complex(es) of
yet unidentified structure. The study on the structure
ofthe catalyst and the Pd-intermediates is currently
in progress.
[1] a) G. D. Daves, A. Hallberg, Chem. Rev. 1989, 89,
1433–1445; b) R. F. Heck, in: Comprehensive Organic
Synthesis, Vol. 4, (Eds.: B. M. Trost, I. Fleming, M. F.
Semmelhack), Pergamon Press, Oxford, 1991, pp. 833–
8
1
1
64; c) A. De Meijere, F. E. Meyer, Angew. Chem.
994, 106, 2473–2506; Angew. Chem. Int. Ed. Engl.
994, 33, 2379–2411; d) T. Jeffery, in: Advances in
From the synthetic viewpoint, the protocol de-
scribed herein is simple, robust and generally applica-
ble to the coupling ofstable and easy-to-handle al-
Metal-Organic Chemistry, Vol. 5, (Ed.: L. S. Liebes-
[15]
kenyl nonaflates
with electron-withdrawing, elec-
kind), JAI Press Inc, Greenwich, CT, 1996; pp 153–
tron-rich as well as non-activated alkenes.
2
60; e) S. Bräse, A. de Meijere, in: Metal-Catalyzed
Cross-Coupling Reactions, (Eds.: F. Diederich, P. J.
Stang), Wiley-VCH, New York, 1998, pp 99–166;
1022
asc.wiley-vch.de
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1019 – 1024

An Expedient Variant ofHeck Reaction ofAlkenyl Nona fl ates
COMMUNICATIONS
f) J. T. Link, L. E. Overman, in: Metal-Catalyzed Cross-
Coupling Reactions, (Eds.: F. Diederich, P. J. Stang),
Wiley-VCH, New York, 1998, pp 231–269; g) M. T.
Reetz, in: Transition Metal Catalyzed Reactions, (Eds.:
S. G. Davies, S.-I. Murahashi), Blackwell Sci., Oxford,
C. W. Jones, Adv. Synth. Catal. 2006, 348, 609–679;
noteworthy, in the very recent account, a leaching of
homogeneous Pd-species from Pd colloids in the
ligand-free Heck reaction was proven by a molecular
membrane technique: b) M. B. Thathagar, J. E. ten
Elshof, G. Rothenberg, Angew. Chem. 2006, 118, 2952–
2956; Angew. Chem. Int. Ed. 2006, 45, 2886–2890.
1
999, pp 207–224; h) I. P. Beletskaya, A. V. Cheprakov,
Chem. Rev. 2000, 100, 3009–3066; i) M. Beller, A.
Zapf, T. H. Riermeier, in: Transition Metals for Organic [14] Heck reactions of alkenyl triflates (refs. a–d) and nona-
nd
Synthesis, Vol. 1, 2 edn, (Eds.: M. Beller, C. Bolm),
Wiley-VCH, Weinheim, 2004, pp 271–305; j) N.
Miyaura, Adv. Synth. Catal. 2004, 346, 1522–1523.
2] M. Shibasaki, E. M. Vogl, T. Ohshima, Adv. Synth.
Catal. 2004, 346, 1533–1552.
flates (refs. e–g) are often carried out without phos-
phine added: a) A. Arcadi, E. Bernocchi, S. Cacchi, L.
Caglioti, F. Marinelli, Tetrahedron Lett. 1990, 31, 2463–
2466; b) S. Cacchi, G. Fabrizi, R. C. Larock, P. Pace, V.
Reddy, Synlett 1998, 888–890; c) R. C. Larock, H.
Yang, P. Pace, K. Narayanan, C. E. Russell, S. Cacchi,
G. Fabrizi, Tetrahedron 1998, 54, 7343–7356; d) T.
Flessner, V. Ludwig, H. Siebeneicher, E. Winterfeldt,
Synthesis 2002, 1373–1378; e) M. Webel, H.-U. Reissig,
Synlett 1997, 1141–1142; f) I. M. Lyapkalo, M. Webel,
H.-U. Reissig, Eur. J. Org. Chem. 2002, 3646–3658;
g) I. M. Lyapkalo, J. Hçgermeier, H.-U. Reissig, Tetra-
[
[
3] a) A. F. Littke, G. C. Fu, J. Am. Chem. Soc. 2001, 123,
6
2
4
989–7000; b) A. F. Littke, G. C. Fu, Angew. Chem.
002, 114, 4350–4386; Angew. Chem. Int. Ed. 2002, 41,
176–4211.
[
4] a) W. A. Herrmann, C. Broßmer, K. Öfele, M. Beller,
H. Fischer, J. Organomet. Chem. 1995, 491, C1-C4;
b) K. Nishi, Y. Narukawa, H. Onoue, Tetrahedron Lett.
1
996, 37, 2987–2988; c) A. Bråthe, L.-L. Gundersen, F.
hedron 2004, 60, 7721–7729; a negative impact ofPPh
on the reaction outcome was noticed earlier, see ref.
3
4b].
[
Rise, A. B. Eriksen, A. V. Vollsnes, L. Wang, Tetrahe-
dron 1999, 55, 211–228.
[15] A versatile methodology has been developed that ena-
bles an easy one-step access to alkenyl nonaflates from
enolizable carbonyl compounds and a commercially
available convenient technical product, nonafluorobu-
tane-1-sulfonyl fluoride (NfF); a) via trimethylsilyl enol
ethers: I. M. Lyapkalo, M. Webel, H.-U. Reissig, Eur. J.
Org. Chem. 2002, 1015–1025; b) via Li-enolates: I. M.
Lyapkalo, M. Webel, H.-U. Reissig, Synlett 2001, 1293–
1295; c) using P-bases: I. M. Lyapkalo, M. A. K. Vogel,
Angew. Chem. 2006, 118, 4124–4127; Angew. Chem.
Int. Ed. 2006, 45, 4019–4023.
[
[
5] L. S. Hegedus, Palladium in Organic Synthesis, in: Or-
ganometallics in Synthesis – a Manual, (Ed.: M.
Schlosser), John Wiley & Sons Ltd., Chichester, 1994, p
4
21.
6] a) M. T. Reetz, E. Westermann, Angew. Chem. 2000,
1
12, 170–173; Angew. Chem. Int. Ed. 2000, 39, 165–
1
68; for the first report on the use of preformed col-
loids in Heck couplings, see: b) M. Beller, H. Fischer,
K. Kühlein, C. P. Reisinger, W. A. Herrmann, J. Orga-
nomet. Chem. 1996, 520, 257–259.
[
[
[
7] a) M. T. Reetz, J. G. de Vries, Chem. Commun. 2004,
[16] Effects of solvent, auxiliary base and additives on the
reaction rate will be reported in a full account else-
where.
1
559–1563, and references cited therein; b) A. H. M.
de Vries, J. M. C. A. Mulders, J. H. M. Mommers,
H. J. W. Henderickx, J. G. de Vries, Org. Lett. 2003, 5,
[17] The absence ofcolloid stabilizers (typically quaternary
ammonium salts) in ligand-free Heck reactions of aryl
halides leads to rapid formation of Pd-black with con-
3
285–3288.
8] a) M. Beller, A. Zapf, in: Handbook of Organopalladi-
um Chemistry for Organic Synthesis, Vol. 1, (Eds.: E.-i.
Negishi, A. de Meijere), John Wiley & Sons, Chiches-
ter, 2002, pp 1209–1222; b) J. G. de Vries, Can. J.
Chem. 2001, 79, 1086–1092.
9] a) C. S. Consorti, F. R. Flores, J. Dupont, J. Am. Chem.
Soc. 2005, 127, 12054–12065; b) C. S. Consorti, M. L.
Zanini, S. Leal, G. Ebeling, J. Dupont, Org. Lett. 2003,
[1d,7b,9a].
comitant deterioration of the catalyst efficiency.
[18] As the aggregation process is higher order in Pd con-
centration than Heck coupling, by reasonable lowering
the catalyst-to-substrate ratio significant improvement
ofthe kinetics in af vour ofthe Heck reaction was ach-
[7b]
ieved.
[19] J. A. Widegren, R. G. Finke, J. Mol. Catal. A: Chem.
5
, 983–986; c) C. Rocaboy, J. A. Gladysz, New J.
2003, 198, 317–341.
Chem. 2003, 27, 39–49; d) C. Rocaboy, J. A. Gladysz,
Org. Lett. 2002, 4, 1993–1996.
10] a) W. J. Sommer, K. Yu, J. S. Sears, Y. Ji, X. Zheng,
R. J. Davis, C. David Sherrill, C. W. Jones, M. Weck,
Organometallics 2005, 24, 4351–4361; b) M. R. Eber-
hard, Org. Lett. 2004, 6, 2125–2128.
[20] In the heterogeneous catalyst, only a fraction of the
metal atoms on the surface is active. Therefore, (much)
less than 1 equiv. ofthe catalytic poison (PPh , CS or
[
[
3
2
thiophene) is sufficient to block the active sites. Com-
plete quenching ofthe catalytic activity with !1 equiv.
ofthe above poisons was regarded as a solid evidence
for the heterogeneous nature of the Pd(0)-catalyst in
Heck coupling ofaryl halides. Metallic mercury taken
in a large excess amalgamates the colloidal metallic Pd
thus deactivating it.
11] W. A. Herrmann, C. Broßmer, K. Öfele, C.-P. Reising-
er, T. Priermeier, M. Beller, H. Fischer, Angew. Chem.
1
1
995, 107, 1989–1992; Angew. Chem. Int. Ed. Engl.
995, 34, 1844–1847.
[
[
12] M. Nowotny, U. Hanefeld, H. van Koningsveld, T.
[21] Despite gaining the evidence ofa leaching ofthe ho-
mogeneous Pd-species from Pd colloids, the entity of
the suggested Pd(0) catalyst upon completion ofthe
catalytic cycle is still obscure. Ifit remains homogene-
ous and does not re-cluster to the Pd-nanoparticles, it
Maschmeyer, Chem. Commun. 2000, 1877–1878.
13] Current state ofart in elucidation ofactive species in
Heck and Suzuki coupling reactions is summarized in
the recent review: a) N. T. S. Phan, M. Van Der Sluys,
Adv. Synth. Catal. 2007, 349, 1019 – 1024
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1023

Michael A. K. Vogel et al.
COMMUNICATIONS
should not be affected by the quantitative heterogene-
ous catalyst poisons. However no poisoning experi-
ments with PPh , CS or thiophene were reported.
stirring stopped, the Hg-phase was allowed to com-
pletely settle to the bottom, the clear supernatant or-
ganic phase was transferred into the new vial, and the
reaction kinetics was monitored further.
3
2
[
[
22] Indeed, a single-metal Pd(0)-complex efficiently cata-
lyzing a hydrogenation ofalkenes was regarded as a
truly homogeneous catalyst based on its insensitivity to-
wards poisoning with thiophene: R. van Asselt, C. J.
Elsevier, J. Mol. Catal. 1991, 65, L13–L19.
[25] P. W. Collins, A. F. Gasiecki, W. E. Perkins, G. W. Gulli-
kson, R. G. Bianchi, S. W. Kramer, J. S. Ng, E. E.
Yonan, L. Swenton, P. H. Jones, R. F. Bauer, J. Med.
Chem. 1990, 33, 2784–2793.
[19]
23] Widegren and Finke remarked that Hg(0)-poisoning
experiments are not universally applicable as Hg(0)
can cause side reactions and interact with some single-
metal complexes: a) G. M. Whitesides, M. Hackett,
R. L. Brainard, J. P. P. M. Lavalleye, A. F. Sowinski,
A. N. Izumi, S. S. Moore, D. W. Brown, E. M. Staudt,
Organometallics 1985, 4, 1819–1830; b) R. A. Jones,
F. M. Real, G. Wilkinson, A. M. R. Galas, M. B. Hurst-
house, J. Chem. Soc., Dalton Trans. 1981, 126–131; c) J.
Stein, L. N. Lewis, Y. Gao, R. A. Scott, J. Am. Chem.
[
[
[
[
[
26] M. El Hattab, G. Culioli, A. Ortalo-Magne, L. Piovetti,
S. E. Chitour, J. Essential Oil Res. 2002, 14, 422–424.
27] S. Cacchi, G. Fabrizi, F. Gasparrini, C. Villani, Synlett
1
999, 345–347.
28] I. U. Khand, P. L. Pauson, M. J. A. Habib, J. Chem. Res.
Synop. 1978, 348–349.
29] E. A. Braude, W. F. Forbes, E. A. Evans, J. Chem. Soc.
1
953, 2202–2207.
30] The influence of anionic nonaflate on the catalytic ac-
tivity would be masked by DMF and Et N which are
[22]
3
Soc. 1999, 121, 3693–3703; see also ref. While poi-
more nucleophilic, stronger coordinating, and are pres-
ent in the reaction mixture in greater excess. However,
for the ligand-free Heck reactions of aryl halides, a ki-
netic model was suggested by Dupont and co-workers
that accounts for the catalysis deactivation by forma-
soning ofthe heterogeneous Pd-catalysts is well-docu-
[10]
mented,
we could not find literature precedents of
single-metal Pd(0)-complexes that have been proven to
induce a true homogeneous catalysis and be not affect-
ed by treatment with Hg(0). For the recent discussions
mÀ
[9a]
[9a,13a]
tion ofPdX
n
, see ref. We thank one ofthe re fe rees
on Hg(0) poisoning ofHeck reaction see re fs .
24] Metallic mercury was added to a reaction mixture at
7% completion. It was vigorously stirred for 1 h, then
for the respective comment.
[
3
1024
asc.wiley-vch.de
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1019 – 1024