A mechanistic study on modern palladium catalyst precursors as new gateways to Pd(0) in cationic Heck reactions

Article abstract of DOI:10.1016/j.tet.2007.11.111

Electrospray ionization mass spectrometry (ESI-MS) was used as a means to directly identify catalytic cationic organopalladium species in ligand-controlled Heck reactions involving electron-rich olefins and different Pd-sources. In these high-temperature Heck arylations, the oxidative addition intermediates were observed as bidentate ligand chelated cationic aryl palladium species, suggesting that the used ligand attaches to the metal center at the very beginning of the catalytic cycle. This was also in agreement with the obtained regioisomeric profile of the isolated products. The investigation supports the standard Pd(0)/Pd(II) Heck mechanism and provides further insight regarding the conceivable composition of fundamental Pd(II) intermediates in an ongoing Heck reaction.

Full text of DOI:10.1016/j.tet.2007.11.111

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 1808e1812
www.elsevier.com/locate/tet
A mechanistic study on modern palladium catalyst precursors as new
gateways to Pd(0) in cationic Heck reactions
a
b
a
a,
Andreas Svennebring , Per J.R. Sj o¨ berg , Mats Larhed , Peter Nilsson *
a
Organic Pharmaceutical Chemistry, Department of Medicinal Chemistry, Uppsala Biomedical Center, Uppsala University,
PO Box 574, SE-751 23 Uppsala, Sweden
Department of Physical and Analytical Chemistry, Uppsala Biomedical Center, Uppsala University, PO Box 599, SE-751 23 Uppsala, Sweden
b
Received 22 August 2007; received in revised form 13 November 2007; accepted 29 November 2007
Available online 8 December 2007
Abstract
Electrospray ionization mass spectrometry (ESI-MS) was used as a means to directly identify catalytic cationic organopalladium species in
ligand-controlled Heck reactions involving electron-rich olefins and different Pd-sources. In these high-temperature Heck arylations, the oxida-
tive addition intermediates were observed as bidentate ligand chelated cationic aryl palladium species, suggesting that the used ligand attaches to
the metal center at the very beginning of the catalytic cycle. This was also in agreement with the obtained regioisomeric profile of the isolated
products. The investigation supports the standard Pd(0)/Pd(II) Heck mechanism and provides further insight regarding the conceivable compo-
sition of fundamental Pd(II) intermediates in an ongoing Heck reaction.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: ESI-MS; Heck; Palladium; Microwave
1
. Introduction
Herrmann and Beller suggested early on that the use of palla-
1
dacycles (e.g., 3a ) might provide an alternative catalytic
0
1
1,12
Since its disclosure in the early seventies, the palladium(0)-
catalyzed Heck reaction has enjoyed a considerable amount of
interest in synthetic organic chemistry.
cycle, proceeding through a Pd(II)/Pd(IV) redox cycle.
A mechanistic rational for this cycle was later proposed by
1
e4
13
Shaw. The good thermal stability and high recovery of the
Originally, Pd(II)
salts were used as the precatalyst which were, under standard
Heck reaction conditions, believed to be rapidly reduced in
situ to form Pd(0). The essential aryl/alkenyl palladium(II)
intermediate was thereafter generated from Pd(0) and an or-
palladacycle after completion of the reaction, along with the
lack of a distinct reducing agent, supported this assumption.
However, several reports on the existence of highly active sol-
uble palladium(0) colloids (nanoparticles) has established the
function of palladacycles as plain Pd(0) reservoirs in high-
3
,4
ganic halide by an oxidative addition process. The recent
decade has seen the introduction of a multitude of new palla-
dium formulations/complexes to be used as precatalysts in pal-
ladium(0)-catalyzed coupling reactions. Of clear importance
1
4,15
temperature applications like Heck and Suzuki couplings.
Unfortunately, very little is known about the actual structures
of catalytically active PdL complexes generated from modern
n
5
,6
16
and highly sophisticated Pd-precursors. In addition, there
are the different classes of palladacycles and palladium
7
,8
9
carbenes as well as various polymer supported palladium
sources. In parallel, a debate regarding the true nature of the
catalytically active palladium species has been enduring.
have been few attempts to elucidate the composition of the
active species present in the Heck cycle. Published studies
1
7
have been limited to the use of electrochemistry, anionic
1
detection (ESI-MS, EXAFS) together with detailed kinetic
8
1
9e21
22,23
studies,
and catalyst poisoning experiments.
The
three-phase test has been used to differentiate between homo-
geneous and heterogeneous catalysts.
*
Corresponding author. Tel.: þ46 8 524 831 34; fax: þ46 8 524 831 01.
E-mail address: peter@orgfarm.uu.se (P. Nilsson).
2
4,25
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.111

A. Svennebring et al. / Tetrahedron 64 (2008) 1808e1812
1809
We have recently reported on the use of ESI-MS to monitor
cationic reaction intermediates in a Pd(II)-catalyzed and ligand-
N
L
L
L
2
6,27
Pd
5a-b
Pd
6a-b
promoted oxidative Heck reaction.
These studies indicate
O
Ar
Ar
a strong chelating effect from bidentate ligands (1,3-bis-(diphe-
nylphosphino)propane (dppp), and 2,9-dimethyl-1,10-phenan-
throline (dmphen)). Thus, we decided to utilize ESI-MS
detection with the addition of chelating ligands to search for
mechanistic information regarding Pd(0)-catalyzed Heck reac-
tions. Herein we present an investigation on the ability of selected
catalyst precursors to act as Pd(0)-sources in ligand-controlled
Heck reactions with electron-rich alkenes proceedingvia charged
intermediates.
a
O-n-Bu
b
Scheme 3. Cationic p-complexes affording internal a-arylation (a) and termi-
nal b-arylation (b); L¼neutral ligand.
L
L
L
heat
L
L
L
PdLn
ArOTf
1a-b
[
Pd]
5a-b
Pd (II)
L
Ar
(
0)
Scheme 4. Mechanistic hypothesis; L¼neutral ligand.
2
. Results and discussion
of non-chelating 1a with both phosphopalladacycle 3a and with
3
6
classic Pd(OAc)2. Performing the reactions with 2a, under mi-
crowave high-temperature conditions, using standard Pd(OAc)
and Pd dba based systems did not give any substantial differ-
2
.1. Preparative results from vinyl ether Heck arylations
Electron-rich heteroatom-substituted linear olefins (e.g.,
2
2
3
ence in the expected regioselectivity outcome but only in conver-
3
4
vinyl ether 1a) often furnish low regioselectivities under stan-
dard (neutral) reaction conditions. To overcome the poor
regioselectivity with non-functionalized acyclic electron-rich
sion and yield (Table 1). Thus, olefin 1a underwent internal
a-arylation with dppp (4a), but only non-selective arylation
under chloride containing neutral conditions (entries 5 and 7).
With olefin 1b, b-arylation predominated, delivering 6b un-
less a bidentate ligand was added. Using cationic conditions
and 3a, an excellent a-selectivity was experienced with both
2
8,29
olefins in Heck chemistry,
chelating ligands and cationic reaction conditions to obtain in-
Cabri introduced the use of
3
0,31
ternal (a)-substitutions of vinyl ethers.
Furthermore, aryl-
3
7
ation of the ArePd presenting N,N-dimethylamino substituted
vinyl ether 1b has been shown to result in the terminal b-prod-
uct in the presence of monodentate ligands, and in the internal
olefins, providing 5 and the corresponding methyl ketone
7a (via direct hydrolysis). Thus supporting a reaction mecha-
nism similar to traditional catalysts, such as Pd(OAc)
Pd (dba) (comparing entries 20,21 with 3,4 and 13,14). Simi-
and
2
3
2e34
product with bidentate ligands.
1
Accordingly, we selected
a and 1b as model substrates in our studies (Scheme 1).
2
3
larly, in the presence of dppp both the charcoal-supported
catalyst (Pd/C) and the recently marketed encapsulated poly-
urea-supported palladium catalysts (Pd EnCat 30) showed
complete a-selectivity (entries 28, 33 and 34, 38 and 39). In
contrast, olefin 1a failed to give any product with Pd/C. A pos-
sible explanation might be that olefin 1b, via the dimethyl-
amino group, can more efficiently trap the presumably very
low concentration of solubilized palladium, furnishing
OR
OTf
OR
[
Pd]
OR
additives
+
+
β
α
10% aq DMF
MW, 150 °C
1
R=n-Bu (1a)
R=C2H4NMe2 (1b) 2a
h
5a-b
α)
6a-b
(
(βΕ + βΖ)
Scheme 1. Preparative model reaction using different Pd-sources and additives
(ligands 4aee or LiCl).
33
improved conversion and yield (c.f. entries 27 and 28).
The oxidatively more stable di-nitrogen ligand dmphen
(4b) provided full a-selectivity using 1a, but failed to fully
displace the internal nitrogen of olefin 1b in chelating the pal-
ladium metal, resulting in poor regioselectivity (entries 9 and
To unambiguously assign the metal oxidation state during
catalysis we utilized external mono- and bidentate ligands as
mechanistic tools (Scheme 2).
10, 25 and 26). Control experiments, using LiCl and/or
a monodentate ligand, PPh (4c), afforded regioisomeric mix-
3
PPh3
4c
tures regardless of catalyst precursor confirming that an exter-
nal bidentate ligand is necessary to obtain regioselective
Ph2P
PPh2
P(o-Tol)3
4d
2
Ph P
PPh2
N
N
4a
4b
4e
38
reactions. Employing amino palladacycle 3b and pincer
3
9,40
Scheme 2. Mono- and bidentate ligands employed in the study.
complex 3c
gave a preparative outcome perfectly consis-
tent with Pd(0)-catalysis under both cationic and neutral con-
ditions (entries 40e46). Overall, bidentate ligands gave more
productive reactions. A result, which is in agreement with the
hypothesis that electron-rich alkenes (good s-donors) react
more smoothly under cationic conditions for electronic rea-
sons. The emerging picture from the preparative investigation
reveals that none of the investigated Pd-sources influenced the
regioisomeric outcome. Instead it was the choice between neu-
tral or cationic conditions, which indicated that a normal
Pd(0)/Pd(II)-cycle was prevailing.
If the arylation of butyl vinyl ether (1a) with 1-naphthyl tri-
flate 2a using a palladium source and dppp/dmphen as preca-
talytic system would produce a selective a-arylation, this
would support the hypothesis that liberated Pd(0) was trapped
by chelating bidentate ligands (Schemes 3 and 4). In addition,
if a Pd(II)/Pd(IV)-pathway was active, and not the more
widely recognized Pd(0)/Pd(II)-route, some difference in
regioselectivity would be expected. Moreover, Herrmann has
reported a similar distribution of isomers in the Heck arylation
3
5

1810
A. Svennebring et al. / Tetrahedron 64 (2008) 1808e1812
Table 1
Microwave-assisted reactions employing different Pd-catalyst precursors and
additives for the reaction with 2a as depicted in Scheme 1
species. This so-called ion-fishing method provides the ability
to detect, directly from the reaction cocktail, existing interme-
41
diates due to the very mild desolvation technique involved.
a
Yield (a/b
No
Palladium source
Pd(OAc)
Additive
Olefin
E
/b
Z
)
ESI-MS analysis of a reaction using the conditions in entry
ꢀ
1
2
3
4
5
6
7
8
9
2
d
d
4a
4a
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1b
1a
1b
1a
1b
1b
1a
1b
1a
1b
1a
1b
1a
1b
1b
1b
1a
1b
1a
1b
1b
1a
1b
6:7:3
20, Table 1 at 120 C revealed three important signals exhib-
iting the characteristic Pd isotopic distribution (Ia, m/z¼821;
0:22:30
87:0:0
62:0:0
106
Ib, m/z¼1057; Ic, m/z¼1073; based on
Pd) (Scheme 5
and entry 1, Table 2). The cation complex corresponding to
Ia was subjected to MSeMS analysis and showed the colli-
sion-induced elimination of tri-o-tolyl phosphine (4d) and
dppp (4a). Interestingly, adding 4d together with 4a and
LiCl, 4c
LiCl, 4c
LiCl
LiCl
4b
32:12:4
11:52:21
3:6:4
6:13:53
39:0:0
Pd(OAc) again produced intermediate Ia along with mono-oxi-
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
4b
d
42:19:15
17:17:9
0:29:24
71:0:0
dated class Ic complex (Table 2, entry 2). Substitution of the
ꢀ
Pd
2
(dba)
3
d
4a
arylating agent to 4-cyanophenyl triflate 2b allowed, at 70 C,
mainly detection of the analogous intermediate of type Ic
4a
66:0:0
(
m/z¼1048) with 4a and Pd(OAc) (entry 3). Exchanging
2
LiCl, 4c
LiCl, 4c
LiCl
d
19:10:3
8:43:18
6:15:57
17:17:9
0:29:24
71:0:0
dppp with the commercially available and related (2S,4S)-(ꢁ)-
4
,4-bis(diphenylphosphino)pentane ligand (4e) generated
2
2
the corresponding cation analogous to class Ic (m/z¼1104)
d
4a
using Pd(OAc) (Table 2, entry 4).
2
O
O
4a
66:0:0
4:17:5
Pd
LiCl, 4c
LiCl
LiCl
4b
P
o-Tol o-Tol
15:18:4
13:11:42
72:0:0
2
dppp
Ph2P
+ dppp
2
3a
(o-Tol) P
Pd(0)dppp
2
Ia
3
a
-(o-Tol)3P
Pd
PPh2
4b
4a
50:16:10
0:0:0
41:0:0
Ph P
2
Pd/C
4a
Ph2 Ar
P
LiCl, 4c
LiCl, 4c
LiCl
d
4a
4a
27:14:3
8:38: 23
Pd PPh
2
Ib
(II)
Ph2P
OR
Ar
b
9:10:13
1a-b
+
EnCat 30
0:11:49
82:0:0
70:0:0
12:9:3
9:36:11
0:13:50
67:0:0
71:0:0
O
Ph2P
Ph2 Ar
P
5
a-b
Pd PPh
2
Ic
(II)
Ph2P
LiCl, 4c
LiCl, 4c
LiCl
4a
Scheme 5. Catalytic Heck reaction pathway supported by detected ESI-MS
signals; Ar¼1-naphthyl (2a) or 4-CNePh (2b).
EnCat Pd(0)
4a
Two experiments were performed to further establish the
composition of metal containing Ia and Ic, which included
N
O
40
41
42
43
d
4a
1b
1a
1b
1b
0:21:28
82:0:0
63:0:0
Pd
O
4a
LiCl
2
5:13:40
Table 2
Detected cationic intermediates from ESI-MS analysis using various substrates
and catalysts for the microwave-assisted Heck coupling
3
b
4
4
4
4
5
6
O
O
PPh2
4a
4a
1a
1b
1a
67:0:0
68:0:0
27:16:4
O
a
Pd
O
LiCl, 4c
Entry
Catalyst
Reaction
time (min)
Aryl
triflate
Ligand
Detected
intermediate
CF3
PPh2
3c
1
2
3
4
5
6
7
8
9
a
3a
10
10
2a
2a
2b
2b
2b
2b
2b
2b
2b
4a
Ia, Ib, Ic
Ia, Ic
Ic
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
4d, 4a
4a
4e
a
Conditions: 2a (0.4 mmol), 1 (1.0 mmol), DMF (1.8 mL), water (0.2 mL),
additive (4a (7.5 mol %) or 4b (7.5 mol %) or 4c (7.5 mol %)þLiCl (1 mmol),
or LiCl (4 mmol)þtriethylamine (0.5 mmol), and palladium source (2.5 mol %
b
20
5
5
5
Ic
Pd EnCat 30
4a
4a
Ic
ꢀ
Pd) were mixed and microwave heated at 150 C for 1 h in a sealed vessel. The
products were isolated after column purification. The a-product 5 was isolated
as 1-naphthyl methyl ketone 7a after acidic hydrolysis (>95% by GCeMS).
3a
3a
3b
3c
Ia, Ic
Ia
b
20
4a
4a
10
5
IIa, Ic
Ic
b
Reaction time (3 h).
4a
Conditions: 2 (0.4 mmol), 1a (1.0 mmol), DMF (1.8 mL), water (0.2 mL),
(0.5 mmol), and palladium source (2.5 mol % Pd)
2
.2. ESI-MS investigation
ligand (7.5 mol %), K CO
were mixed and heated at 120 C in a sealed vessel. After the indicated time an
aliquot was taken, diluted, and injected to the ESI-MS.
2
3
ꢀ
ESI-MS has become an increasingly valuable method for
investigating the composition of charged organometallic
b
ꢀ
70 C.

A. Svennebring et al. / Tetrahedron 64 (2008) 1808e1812
1811
the interpretation of the corresponding MSeMS fragmenta-
tions (entries 6 and 7). Employing Pd EnCat 30 as Pd source
with 2b gave a signal (m/z¼1048) in agreement with interme-
diate Ic, indicating the function of this polyurea encapsulated
Pd(II) as a source of plain Pd(0) in our high-temperature system
entry, either of 4a (0.03 mmol, 12.4 mg) or 4b (0.03 mmol,
6.2 mg) or 4c (0.03 mmol, 7.9 mg) and LiCl (1 mmol,
42 mg) or LiCl (4 mmol, 168 mg). To the mixture were added
DMF (1.8 mL), water (0.2 mL), 2a (0.4 mmol, 110 mg), Et N
3
(0.5 mmol, 70 mL), and either of 1a (1 mmol, 129 mL) or 1b
(1 mmol, 140 mL). The vial was sealed under air and the con-
tents were magnetically stirred and microwave heated for 1 h
(
produced the oxidative addition intermediate Ic, although at
entry 5). Using palladacycle 3a with the electron-deficient 2b,
ꢀ
ꢀ
lower temperature (70 C) only intermediate Ia was detectable.
This result implies that ligand exchange to furnish Ia should be
at 150 C. After cooling, the reaction mixture was extracted
(0.1 M NaOH, ether), washed (water), dried (K CO ), and
2
3
considered a facile process. Moreover, performing preparative
ꢀ
the solvent was removed under reduced pressure. The com-
pounds were purified by chromatography on aluminum oxide
(reactions starting from 1a, 0e20% ether/isohexane) or silica
(reactions starting from 1b, 0e20% ether/isohexane with 1%
reactions at 150 C over 15 min using 1a and 2b with 4a fur-
nished selectively 4-cyanoacetophenone 7b in 66% yield using
4
3
a, and 84% yield with Pd(OAc)2. Using the dimethylamino
3
palladacycle 3b as precursor produced two key signals with
m/z¼652 and 1048, corresponding to intermediates of type
IIa and Ic (entry 8). The depicted structures in Scheme 6
were further supported by MSeMS fragmentation analysis.
Et N in the mobile phases). Any contamination of remaining
3
2a in the product was removed by bulb-to-bulb distillation
ꢀ
(1.5 mmHg, 130 C).
4.2. Preparative reactions employing aryl triflate 2b
NPh2P
dppp
2b
3b
Pd
Ic
The following chemicals were added to a 5-mL process
vial: Pd(OAc) (0.01 mmol, 2.2 mg) or 3a (0.01 mmol [Pd],
Ph2P
2
IIa
4.7 mg), dppp (0.03 mmol, 12.4 mg), K CO (0.5 mmol,
2 3
66 mg), 2b (0.4 mmol, 100 mg), 10% aqueous DMF (2 mL),
1a (1 mmol, 129 mL) or 1b (1 mmol, 140 mL). The vial was
Scheme 6. Plausible reaction pathway using 3b, as indicated by detected
ESI-MS signals.
sealed under air and the contents were magnetically stirred
ꢀ
The pincer complex 3c did not give any detectable signal
from a ligand exchange complex similar to Ia, providing
only a Ic signal (entry 9).
Regrettably, no cationic complexes containing Pd-associ-
ated olefin 1a could be detected in this study, suggesting
that such complexes are either present in very low concentra-
tions, or just too unstable, and therefore not to be considered
as resting states for the catalytic cycle.
and microwave heated for 15 min at 150 C. After cooling
to ambient temperature, 12 M (aq) HCl (1 mL) was added
and the reaction mixture was magnetically stirred for 1 h,
diluted with water (20 mL), extracted with ether, washed
(water), dried (MgSO ), and the solvent was removed at re-
4
duced pressure. The crude product was purified by chromatog-
raphy on silica to give pure 7b.
4.3. Mass spectrometric investigations (Table 2)
3
. Conclusion
The following chemicals were added to a 5-mL process vial
according to entry: either of Pd(OAc) (0.01 mmol, 2.2 mg),
The findings from the ESI-MS analyses of ongoing
2
bidentate-controlled Heck reactions support the following
mechanistic hypothesis. The palladacycles/pincer undergoes
a structural cleavage upon metal coordination to the bidentate
3a (0.01 mmol [Pd], 4.7 mg), Pd EnCat 30 (0.01 mmol [Pd],
25.0 mg) or 3b (0.01 mmol [Pd], 3.0 mg), 3c (0.01 mmol
[Pd], 7.0 mg); either of 4a (0.03 mmol, 12.4 mg) or 4d
(0.01 mmol, 3.0 mg) or 4e (0.03 mmol, 13.3 mg). To the
same vial was added: K CO (0.5 mmol, 66 mg), DMF
4
4,45
ligand followed by a reduction to ‘Pd(0)’.
Subsequent
ligand-exchange forms the crucial Pd(0)edppp (or dmphen)
complex. Oxidative addition and regioselective insertion de-
liver the internally arylated a-product according to the cationic
route depicted in Scheme 4. The Pd(II) catalyst Pd EnCat 30
2
3
(1.8 mL), water (0.2 mL) and either of 2a (0.4 mmol,
110 mg) or 2b (0.4 mmol, 100 mg). The reactions were micro-
ꢀ
wave or metal block heated in sealed vessels at 120 C for
5e20 min according to Table 2 after which a sample of
100 mL were taken from the hot reaction mixture and pipetted
show similar behavior as standard Pd(OAc) , both in prepara-
2
tive experiments and in ESI-MS investigations. Based on
ESI-MS detected charged intermediates and measured product
regioselectivities, we conclude that it is unlikely that the inves-
ꢀ
ꢀ
into 5 mL of cold (ꢁ78 C) methanol and stored at ꢁ78 C un-
til injection. Before injection, the samples were again diluted
ten times with methanol stored at ambient temperature. An
API IIIþ triple-quadrupole mass spectrometer equipped with
an articulated IonSpray interface was used in this study. The
samples were introduced by continuous infusion with the aid
of a syringe pump at a flow rate of 5 mL/min through a fused
silica capillary (50 mm inner diameter and 184 mm outer dia-
meter). The fused silica capillary was centered in a stainless
steel capillary counter assembly, which also served as the
4
4e46
tigated reactions are catalyzed via Pd(IV)-intermediates.
4
. Experimental
4
.1. General procedure for the preparative Table 1
The following chemicals were added to a 5-mL process
vial: palladium precatalyst (0.01 mmol [Pd]) according to

1812
A. Svennebring et al. / Tetrahedron 64 (2008) 1808e1812
ESI high voltage contact. The nebulizer gas flow was set to
0
preserved throughout the catalytic cycle, see: Solin, N.; Kjellgren, J.;
Szabo, K. J. Angew. Chem., Int. Ed. 2003, 42, 3656e3658.
6. Sabino, A. A.; Machado, A. H. L.; Correia, C. R. D.; Eberlin, M. N.
Angew. Chem., Int. Ed. 2004, 43, 2514e2518.
.5 L/min. The flow rate of dry nitrogen counter-current cur-
1
1
ꢀ
tain gas (heated to 60 C) was 1.2 L/min over the sampling or-
ifice. The mass spectrometric parameters were as follows: ion
spray voltage (ISV) 3500 V, interface plate voltage (IN) 650 V,
orifice lens voltage (OR) 50 V, and AC entrance rod (R0) 30 V.
Mass spectral data were typically recorded by scanning the
7. d’Orlye, F.; Jutand, A. Tetrahedron 2005, 61, 9670e9678.
18. de Vries, J. G. Dalton Trans. 2006, 421e429.
19. Rosner, T.; Pfaltz, A.; Blackmond, D. G. J. Am. Chem. Soc. 2001, 123,
4
621e4622.
0. Rosner, T.; Le Bars, J.; Pfaltz, A.; Blackmond, D. G. J. Am. Chem. Soc.
001, 123, 1848e1855.
2
2
1
00e1200 u region with a dwell time of 1e2 ms and a step
2
size of 0.1 u in multi channel acquisition mode (MCA summa-
tion of 10e20 scans). Mass scale calibration was performed
using polypropylene glycol solution (PPG). During the
MSeMS experiments the collision energy was set to 20 eV.
The collision gas was argon with 99.9999% purity. The colli-
1. Mathew, J. S.; Klussmann, M.; Iwamura, H.; Valera, F.; Futran, A.;
Emanuelsson, E. A. C.; Blackmond, D. G. J. Org. Chem. 2006, 71,
4711e4722.
2
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2
1
5
2
sion gas thickness was 2ꢂ10 molecules/cm .
2
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We would like to thank Knut and Alice Wallenberg’s Foun-
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ESI-MS analyses. Supplementary data associated with this
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