Formation of benzofurans in a stoichiometric annulation reaction between stable Pallada (II) Cycles hypervalent vinyl- and alkynyl(phenyl)iodonium salts

Article abstract of DOI:10.1016/j.jorganchem.2007.11.034

Stable pallada(II)cycles featuring Csp²-Pd and Csp³-Pd bonds reacted with vinyl- and alkynyl(phenyl)iodonium salts to generate two new geminal carbon-carbon bonds to the terminal carbon of the vinyl and alkynyl substituents providing benzofuran and dihydrobenzofuran heterocycles. The new annulation process was rationalized by the involvement of Pd(IV) intermediates arising via an initial oxidative addition of hypervalent iodonium electrophiles to the Pd(II) center. Reaction monitoring via low temperature ¹H NMR spectroscopy was performed, and organopalladium(II) intermediates featuring a new Csp²-Csp² or Csp²-Csp bond were isolated and characterized, providing insights into the regiochemical course of the proposed mechanistic pathway.

Full text of DOI:10.1016/j.jorganchem.2007.11.034

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 567–573
www.elsevier.com/locate/jorganchem
Note
Formation of benzofurans in a stoichiometric annulation
reaction between stable pallada(II)cycles and hypervalent
vinyl- and alkynyl(phenyl)iodonium salts
Piyali Datta Chaudhuri, Ruiyun Guo, Helena C. Malinakova *
Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, KS 66045-7582, United States
Received 4 October 2007; received in revised form 19 November 2007; accepted 19 November 2007
Available online 23 November 2007
Abstract
Stable pallada(II)cycles featuring Csp2–Pd and Csp3–Pd bonds reacted with vinyl- and alkynyl(phenyl)iodonium salts to generate two
new geminal carbon–carbon bonds to the terminal carbon of the vinyl and alkynyl substituents providing benzofuran and
dihydrobenzofuran heterocycles. The new annulation process was rationalized by the involvement of Pd(IV) intermediates arising via
1
an initial oxidative addition of hypervalent iodonium electrophiles to the Pd(II) center. Reaction monitoring via low temperature H
NMR spectroscopy was performed, and organopalladium(II) intermediates featuring a new Csp2–Csp2 or Csp2–Csp bond were isolated
and characterized, providing insights into the regiochemical course of the proposed mechanistic pathway.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Palladacycles; Iodonium salts; Annulation reactions; Heterocycles
1. Introduction
[4]. Furthermore, we have shown that the Pd(IV) complex
II underwent a synthetically productive cascade process
involving reductive elimination followed by an intramolec-
ular Heck reaction to afford benzoxepine and benzopyran
heterocycles via the formation of two new C–C bonds
(Fig. 1) [4]. We envisioned that highly reactive vinyl- and
alkynyl(phenyl)iodonium salts could also operate as
oxidants for Pd(II) in pallada(II)cycles I generating a
Pd(IV) intermediate III, which would underwent reductive
elimination and subsequently the Heck reaction, giving rise
to analogous annulation sequences (Fig. 2). The reports on
systematic exploration of the reactivity of hypervalent iod-
onium electrophiles with palladium(II) complexes seeking
new pathways for the carbon–carbon bond formation
remain rare [5]. An impressive catalytic arylation reaction
has been proposed to proceed via Pd(IV) complexes featur-
ing two carbon–palladium bonds arising via oxidation of
cyclopalladated Pd(II) complexes with aryl iodonium tetra-
fluoroborates [2b]. Studies in the stoichiometric mode
yielded spectroscopic evidence for the formation of hexaco-
ordinate Pd(IV) complexes with three carbon–palladium
Palladium-catalyzed C–C bond-forming reactions
involving hypervalent iodonium salts have become popular
synthetic tools [1]. The traditional processes rely on cata-
lytic cycles featuring palladium in oxidation states Pd(0)
and Pd(II) [1]. However, recent developments have sug-
gested that reactions involving Pd(IV) intermediates could
give rise to synthetically attractive transformations [2]. Pal-
ladium(IV) complexes arising from palladium(II) com-
plexes via oxidative addition of certain electrophiles have
been previously detected spectroscopically, isolated, and
in some cases characterized by X-ray crystallography [3].
Our most recent studies on the oxidation of stable
pallada(II)cycles with allyl bromide electrophiles culmi-
nated in a successful isolation and full characterization
including X-ray crystallographic data of a hexacoordinate
Pd(IV) complex II featuring three carbon–palladium bonds
*
Corresponding author.
E-mail address: hmalina@ku.edu (H.C. Malinakova).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.11.034

568
P.D. Chaudhuri et al. / Journal of Organometallic Chemistry 693 (2008) 567–573
NMR afforded spectroscopic evidence consistent with the
COOEt
L
H
O
L
involvement of the proposed Pd(IV) intermediate. Results
reported herein provide a stoichiometric proof of concept
for the application of vinyl- and alkynyl hypervalent iodo-
nium salts in potentially synthetically useful cascade car-
bon–carbon bond-forming processes.
H
Br
Pd
L
L
MeOOC
H
Br
H
Pd
O
H
oxidative
addition
COOEt
COOMe
X-ray crystallography
II
I
2. Results and discussion
reductive
elimination
L-L=bipyridine
Stable palladacycles 1a–b (Table 1) featuring either
bipyridine or N,N0-dicyclohexylethylenenediimine ligands
were prepared from the corresponding o-iodophenoxy ace-
tates by established methods.[6] The treatment of pallada-
cycle 1a with 1-octenyl(phenyl)iodonium tetrafluoroborate
at room temperature afforded benzofurans 2a (48%) and 3
(9%) (entry 1, Table 1). Thus, the anticipated formation
of two new C–C bonds to the carbon C1 of the 1-octenyl
substituent indeed occurred, and was accompanied by aro-
matization via double-bond migration, as well as an unex-
pected decarboxylation leading to the benzofuran 3.
b-Hydride elimination followed by readdition of the H–Pd(II)
complex [7], and a base- (path A) or nucleophile-induced
(path B) fragmentation causing a formal elimination of
the ester group activated by association with the cationic
Pd(II) center in intermediate V, could account for the
MeOOC
H
Br
O
L
COOMe
L
Pd
intramolecular
Heck reaction
H
O
COOEt
COOEt
Fig. 1. Prior study reported by us (see Ref. [4]).
H
R
H
R
I
X
Ph
L
L
L
L
R
Pd
O
Pd
O
X
or
Ph
H
H
L
L
COOEt
COOEt
H
n-hexyl
H
L
L
I
Pd
n-hexyl
Pd
X
O
III
O
I
3
H
HPd(II)
elimination/
readdition
reductive
elimination
B (path A)
O
O
O
oxidative
addition
O
or
V
Nu (path B)
R
decarboxylation [8]. We observed that careful solvent puri-
fication (treatment with basic alumina and CaH2) was an
important factor in limiting the extent of the decarboxyl-
ation. Notably, the sterically demanding bisimine ligand
in palladacycle 1b completely foiled the decarboxylation
providing benzofuran 2a as a single product in 74% yield
(entry 2, Table 1). We reason that the process reported in
L
intramolecular
Heck reaction
R
Pd
L
H
X
O
COOEt
COOEt
IV
O
Fig. 2. Proposed annulation with iodine (III) salts.
bonds from the oxidation of pallada(II)cycles with aryl and
alkynyl iodonium salts [5]. However, no synthetically pro-
ductive carbon–carbon bond forming processes of these
complexes have been described. Herein we report a study
on the outcome of stoichiometric reactions between stable
pallada(II)cycles featuring a Csp2–Pd bond and a Csp3–Pd
bond with vinyl- and alkynyl(phenyl)iodonium salts. As
anticipated according to our prior work [4] and literature
reports [2,5], the treatment of palladacycles I with vinyl-
or alkynyl(phenyl)iodonium salts afforded benzofuran or
dihydrobenzofuran derivatives, realizing the formation of
two new carbon–carbon bonds. Under controlled condi-
tions, organopalladium(II) intermediates IV featuring a
new Csp2–Csp2 or Csp2–Csp bond were isolated and char-
acterized, providing insights into the regiochemical course
of the reductive elimination step of the proposed mechanis-
Table 1
Annulations with an unactivated vinyl iodine(III) salt
n-hexyl
n-heptyl
COOEt
L
L
I
O
Pd
O
BF4
Ph
2a
3
COOEt
1,2-dichloroethane
rt, 16 h
+
n-heptyl
H
1a: L-L = bipyridine
1b: L-L = N,N-dicyclohexylethylenebisimine
O
Entry
Substrate
Product (% yield)
1
2
1a
1b
2a (48)a
2a (74)b
3 (9)a
Not formed
a
The solvent was dried over basic alumina and distilled from CaH2.
The solvent was dried over molecular sieves.
1
b
tic pathway. Reaction monitoring via low temperature H

P.D. Chaudhuri et al. / Journal of Organometallic Chemistry 693 (2008) 567–573
569
Table 1 indeed proceeds according to the pathway outlined
in Fig. 2. The reported ability of aryl- and alkynyl iodonium
salts to oxidize pallada(II)cycles [5], as well as the compe-
tency of palladacycles I to generate Pd(IV) intermediates
with allylic electrophiles [4] provide an indirect support
for this proposition. Although a pathway invoking trans-
metallation between two Pd(II) centers has been proposed
as a possible alternative to processes involving Pd(IV) inter-
mediates, and was supported by recent computational stud-
ies [9], its operation in the stoichiometric process described
herein appears unlikely since only one Pd(II) complex is
present in appreciable concentration [10].
To assess the generality of the annulation protocol,
functionalized vinyl(phenyl)iodonium tetrafluoroborates
bearing phenyl or 4-methyl-1-pentyne-1-yl substituents R1
(Table 2) were prepared [11] and reacted with palladacycle
1a. Although the functionalized iodonium salts apparently
oxidized the palladacycles even faster than their unfunc-
tionalized counterparts, the anticipated heterocyclic prod-
ucts did not form. Aiming to facilitate the migratory
insertion step (vide supra), appropriate additives were
sought. Indeed, benzofuran 2b (54% yield, entry 1, Table
2, R1 = Ph) was obtained via a one pot/two-step protocol,
employing elevated temperatures and PPh3 (2.2 equiv.),
TEA (4.0 equiv.) and (n-Bu)4NCl (3.0 equiv.) in the second
step [12]. The treatment of palladacycle 1a with the alkyne-
containing (R1 = 4-methyl-1-pentyne-1-yl)vinyl(phenyl)iod-
onium tetrafluoroborate under these conditions afforded
benzofuran 2c (20%) along with a single diastereomer of
a 2,3-dihydrobenzofuran 4 (35%) (entry 2, Table 2). Con-
trol experiments on the reaction reported in entry 2 (Table
2) had shown that only traces, or no product formation
occurred in the absence of either PPh3 or TEA, whereas
benzofuran 2c (35%) was isolated exclusively from an
experiment performed in the absence of n-Bu4NCl. Con-
ceivably, the (r-propargyl)Pd(II) complex arising via
migratory insertion is stabilized by interconversion to
(r-allenyl)Pd(II) species [13] allowing for protonolysis of
the C–Pd bond to afford benzofuran 4.
Finally, an alkynyl iodonium salt, 4-methyl-1-pentyne-
1-yl(phenyl)iodonium tetrafluoroborate [11] was tested
for its utility in the model annulation reaction with pallada-
cycle 1a (Table 3). However, a rapid consumption of the
palladacycle 1a presumably by oxidative addition was not
followed by the formation of heterocyclic products. Seek-
ing to activate the alkyne functionality towards carbomet-
allation, Lewis acids (AlCl3 or CuI) [14] were employed in
the second stage of a one pot/two-step protocol (Table 3).
Indeed, the anticipated benzofurans 2d (76%, entry 1,
Table 3) or 2d (26%) and 5 (53%, entry 2, Table 3) were
obtained using AlCl3 and CuI additives, respectively.
At the completion of reactions with 1-n-octene-
yl(phenyl)iodonium tetrafluoroborates (Table 1), palla-
dium was recovered as a black precipitate of Pd(0). In
contrast, mixtures of poorly soluble complexes of palla-
dium were isolated from reactions described in Tables 2
and 3. Although our efforts at isolating the proposed
Pd(IV) complexes III were unsuccessful, palladium(II)
complexes 6 and 7 presumably arising from Pd(IV) inter-
mediates of type III (e.g. structure 8) via reductive elimina-
tion were isolated in good yields (82% and 88%,
respectively) and fully characterized (Scheme 1). The
results indicate that reductive elimination from the pre-
sumed Pd(IV) complexes favors the formation of Csp2–
Csp2 and Csp2–Csp bonds over Csp3–Csp2 and Csp3–Csp
bond, respectively [15]. Aiming to provide spectroscopic
evidence for the formation of the putative Pd(IV)
intermediate 8, in situ monitoring of the reaction between
Table 2
Annulations with an functionalized vinyl iodine(III) salt
R1
COOEt
R1
O
2b : R1 = Ph
I
N
N
2c : R1 = CCCH2CH(CH3)2
Pd
O
BF4
Ph
COOEt
+
Table 3
(i) 1,2-dichloroethane
temp/time
(ii) PPh3 (2.2 equiv), n-
Bu4NCl (3.0 equiv),
TEA (4.0 equiv)
55oC/18 h
Annulations with an alkynyl iodine(III) salt
1a
COOEt
O
4
COOEt
I
Ph
O
O
Entry R1
1
Temperature (°C)/time (h) Product (% yield)
N
N
BF4
2d
Pd
O
25/2
2b (54)
COOEt
(i) 1,2-dichloroethane
0 oC/ 3 h
(ii) Lewis Acid (4.0
equiv)
H
1a
4 (35)a
COOEt
2
25/8
2c (20)
5
rt/16 h
Entry
Lewis acid
Product (% yield)
1
2
AlCl3
Cul
2d (76)
2d (26)
a
Inconclusive NOE analysis and J coupling data did not allow for an
5 (53)
unequivocal assignment of the relative stereochemistry.

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P.D. Chaudhuri et al. / Journal of Organometallic Chemistry 693 (2008) 567–573
in novel Pd-catalyzed cascade reactions. The stoichiometric
n-hexyl
model study described herein provides a conceptual guid-
ance for future design of novel Pd-mediated annulation
protocols.
L-L
I
N
N
I
BF4
BF4
Pd
O
Ph
Ph
COOEt
dichloromethane
0 oC/5 h
dichloromethane
0 oC/3 h
3. Experimental
1a
Unless otherwise indicated, all NMR data were collected
at room temperature in CDCl3 with internal CHCl3 or
SiMe4 as the references (d 7.26 ppm for 1H and
77.00 ppm for 13C in the case of CHCl3 and 0.00 ppm for
1H and 13C in the case of TMS). IR spectra were measured
as thin films on salt (NaCl) plates. Melting points are
uncorrected and were taken in open capillary tubes. MS
were measured under electrospray ionization (ES+) condi-
tions and fast atom bombardment ionization (FAB) condi-
tions. Analytical thin-layer chromatography (TLC) was
carried out on commercial Merck silica gel 60 plates,
250 lm thickness, with fluorescent indicator (F-254) or
stained with aqueous KMnO4 solution. Column chroma-
tography was performed with 32–63 lm silica gel
(Sorbent).
n-pentyl
H
Hc
H
N
N
Hd
He
Pd
Hf
COOCHaHbCH3
O
BF4
8 (1H NMR monitoring, Figure 3)
n-hexyl
L
L
L
L
Pd
BF4
COOEt
Pd
BF4
COOEt
O
O
4. Materials
7 (88%)
6 (82%)
Tetrahydrofuran (THF) was freshly distilled from
sodium/benzophenone. 1,2-Dichloroethane, dichlorometh-
ane and benzene were kept over 3 A (8–12 mesh) molecular
Scheme 1. Isolation of palladium(II) complexes.
˚
palladacycle 1a and (E)-1-octenyl(phenyl)iodonium tetra-
sieves under an atmosphere of dry argon; other solvents
were used as received. Unless otherwise specified, all reac-
tions were carried out under an atmosphere of dry argon
in oven-dried (at least 6 h at 140 °C) glassware. The prepa-
ration and characterization of palladacycles (1a, 1b) was
reported in our previous work [6]. Other materials were
used as received from commercial suppliers. The vinyl-
and alkynyl(phenyl) iodonium salts were prepared accord-
ing to methods reported in the literature [11].
fluoroborate in 1:1.1 molar ratio was performed utilizing
1
low temperature H NMR (400 MHz) spectroscopy [16].
Experiments conducted at ꢀ10 °C indicated an immediate
formation of a 1:1 mixture of complex 6 with a new orga-
nopalladium intermediate distinct from the palladacycle
1a. A complete clean conversion of the new intermediate,
tentatively assigned the structure of organopalladium(IV)
complex 8 (Scheme 1), into complex 6 occurred within
1 h at ꢀ10 °C [17]. Subsequently, the temperature for the
reaction monitoring experiment was lowered to ꢀ50 °C
(Fig. 3). Under these conditions, palladacycle 1a (trace a,
Fig. 3) was again immediately converted into an intermedi-
ate identical to the complex detected at ꢀ10 °C, and dis-
tinct from the complex 6 (trace d, Fig. 3). The putative
complex 8 proved to be stable in solution at temperatures
4.1. A representative procedure for the experiments reported
in Table 1
Entry 2: Ethyl 3-n-heptyl-2-benzofurancarboxylate (2a).
A solution of (E)-1-octenyl(phenyl)iodonium tetrafluoro-
borate (0.063 g, 0.15 mmol) in 1,2-dichloroethane
(1.0 mL) was cannulated to a solution of palladacycle 1b
(0.056 g, 0.11 mmol) in 1,2-dichloroethane (2.0 mL). The
orange solution was stirred for 16 h at r.t. The resulting
black mixture was filtered over celite separating the Pd(0)
precipitate, solvents were removed under reduced pressure,
and the crude product was purified by preparative TLC
over silica eluting with hexane:EtOAc (6:1) mixtures to
afford benzofuran 2a (0.023 g, 74%) as a colorless oil:
Rf = 0.7 (silica gel, EtOAc/hexanes 1:6); IR (neat, cmꢀ1):
2956, 2927, 2856, 1712 (CO), 1595; 1H NMR (CDCl3,
400 MHz) o 7.58 (d, J = 6.2 Hz, 1H), 7.50 (d, J = 6.7 Hz,
1H), 7.38 (td, J = 5.8 Hz, J = 0.96 Hz, 1H), 7.21 (t, J =
6.4 Hz, 1H), 4.41 (q, J = 5.6 Hz, 2H), 3.01 (t, J = 6.2 Hz,
1
ꢀ50 to 40 °C, and an H NMR spectra of an essentially
pure (contains phenyl iodide) [18] complex 8 was recorded
(trace b, Fig. 3). The first signals indicating the conversion
of complex 8 into the ‘‘open form” complex 6 were detected
following the treatment of the reaction mixture for 10 min
at ꢀ30 °C (trace c, Fig. 3) [16]. The proposed structure of
the intermediate 8 is further supported by the assignments
1
of the characteristic signals observed in H NMR spectra
of the intermediate (trace b in Fig. 3) to structure 8 in
Scheme 1.
In conclusion, the mild reaction conditions permitting
the initial C–C bond formation (Scheme 1) highlight the
considerable potential of hypervalent iodine(III) reagents

P.D. Chaudhuri et al. / Journal of Organometallic Chemistry 693 (2008) 567–573
571
(a) palladacycle 1a (- 50 oC)
(b) reaction mixture treated at –50 - -40 oC for 1.5 h, intermediate 8 (with PhI)
Hc
Hf
He
Hb
Ha
Hd
(c) reaction mixture treated for additional 10 min at – 30 oC
(d) complex 6 (-50 oC) (with PhI)
10
9
8
7
6
5
4
3
2
1
Fig. 3. Low temperature 1H NMR (400 MHz, CDCl3) monitoring of the reaction between palladacycle 1a and (E)-1-octenyl(phenyl)iodonium
tetrafluoroborate.
2H), 1.65 (p, J = 5.9 Hz, 2H), 1.41 (t, J = 5.7 Hz, 3H),
1.37–1.15 (m, 8H), 0.81 (t, J = 5.4 Hz, 3H); 13C NMR
(CDCl3, 125 MHz) o 159.4, 153.4, 139.6, 129.5, 127.5,
126.6, 122.0, 120.3, 111.3, 60.1, 30.9, 28.7, 28.3, 28.1,
23.2, 21.6, 13.4, 13.1; HRMS (ESI TOF) Calc. for
C18H25O3 [M + H+]: 289.1804. Found: 289.1807.
gel, EtOAc/hexanes 1:6); IR (neat, cmꢀ1): 3028, 2966,
1712 (CO), 1294, 1267; 1H NMR (CDCl3, 400 MHz) o
7.48 (d, J = 6.7 Hz, 1H), 7.42 (d, J = 6.3 Hz, 1H), 7.34
(td, J = 4.9 Hz, J = 1.0 Hz, 1H), 7.23–7.10 (m, 6H), 4.42
(s, 2H), 4.38 (q, J = 5.6 Hz, 2H), 1.31 (t, J = 5.7 Hz, 3H);
13C NMR (CDCl3, 125 MHz) o 159.4, 153.5, 140.1,
137.9, 127.5 (2C), 127.4 (2C), 127.3, 127.1, 126.7, 125.3,
122.3, 120.7, 111.2, 60.3, 29.1, 13.4; HRMS (ESI TOF)
Calc. for C18H17O3 [M + H+]: 281.1178. Found: 281.1184.
4.2. A representative procedure for the experiments reported
in Table 2
Entry 1: Ethyl 3-benzyl-2-benzofurancarboxylate (2b).
To a solution of palladacycle 1a (0.064 g, 0.145 mmol) in
dichloroethane (2 mL) was added a solution of (E)-sty-
4.3. A representative procedure for the experiments reported
in Table 3
ryl(phenyl)iodonium
tetrafluoroborate
(0.083 g,
Entry 1: Ethyl 3-isopentyl-2-benzofurancarboxylate (2d).
To a solution of palladacycle 1a (0.064 g, 0.145 mmol) in
dichloroethane (3 mL) was added a solution of 4-methyl-1-
pentyne-1-yl(phenyl)iodonium tetrafluoroborate (0.075 g,
0.203 mmol) in dichloroethane (3 mL) at 0 °C under argon.
The solution immediately turned dark brown and was stir-
red at 0 °C for additional 3 h. Then, AlCl3 (0.058 g,
0.435 mmol) was added, and the mixture was gradually
warmed to rt and stirred for 16 h. The reaction mixture
was filtered over celite, and solvent was evaporated under
reduced pressure to afford the crude product, which was
0.203 mmol) in dichloroethane (1 mL). The reaction mix-
ture was stirred at rt for 2 h prior to the addition of n-
Bu4NCl (0.120 g, 0.435 mmol), PPh3(0.084 g, 0.32 mmol)
and Et3N (0.08 mL, 0.58 mmol). The resulting reddish-
brown suspension was heated at 55 °C for 18 h. The reac-
tion mixture was then cooled, filtered over celite, and the
solvent was removed under reduced pressure to afford the
crude product, which was purified by preparative TLC over
silica, eluting with hexane:EtOAc (6:1) to afford benzopy-
ran 2b (0.020 g, 54%) as a colorless oil: Rf = 0.55 (silica

572
P.D. Chaudhuri et al. / Journal of Organometallic Chemistry 693 (2008) 567–573
purified by preparative TLC over silica eluting with hex-
ane:EtOAc (6:1) to afford benzofuran 2d (0.030 g, 76%) as
a colorless oil: Rf = 0.60 (silica gel, EtOAc/hexanes 1:6);
IR (neat, cmꢀ1): 2956, 2929, 1712 (CO), 1299, 1064, 1027;
1H NMR (CDCl3, 400 MHz) o 7.59 (d, J = 6.3 Hz, 1H),
7.48 (d, J = 6.7 Hz, 1H), 7.36 (td, J = 4.9 Hz, J = 0.96 Hz,
1H), 7.22 (td, J = 5.7 Hz, J = 0.5 Hz, 1H), 4.39 (q,
J = 5.7 Hz, 2H), 3.01 (t, J = 6.5 Hz, 2H), 1.65-1.52 (m,
1H), 1.50–1.46 (m, 2H), 1.36 (t, J = 5.7 Hz, 3H), 0.92 (d,
J = 5.2 Hz, 6H); 13C NMR (CDCl3, 125 MHz) o 159.4,
153.5, 139.4, 129.6, 127.4, 126.6, 122.0, 120.2, 111.3, 60.1,
37.9, 27.2, 21.4 (2C), 21.2, 13.4; HRMS (ESI TOF) Calc.
for C16H21O3+ [M + H+]: 261.1491. Found: 261.1484.
J = 6.8 Hz, J = 6.4 Hz, 1H), 2.30 (dd, J = 6.8 Hz,
J = 6.5 Hz, 1H), 2.21–2.14 (m, 1H), 1.53 (t, J = 7.1 Hz,
3H), 1.14 (d, J = 6.5 Hz, 3H), 1.01 (d, J = 6.6 Hz, 3H);
13C NMR (CDCl3, 100 MHz) o 182.3, 163.5, 156.7,
152.9, 152.5, 150.6, 148.9, 141.4, 141.1, 129.8, 128.9,
128.4, 127.7, 127.6, 124.5, 123.5, 123.3, 122.0, 110.8,
85.3, 68.2, 45.3, 29.5, 23.8, 22.9, 13.9. HRMS (FAB) Calc.
for C26H27N2O3Pd [M+] 521.1056. Found: 521.1079.
Acknowledgements
We gratefully acknowledge financial support from the
National Science Foundation via CAREER Award
(CHE-0239123) to H. Malinakova.
4.4. General procedure for the synthesis of
palladium(II)complexes
{1-(Ethoxycarbonyl)-1-[2-(1-octene-1-yl)phenoxy]methyl}
(N,N0-2,20-dipyridyl)palladium tetrafluoroborate (6) and
{1-(Ethoxycarbonyl)-1-[2-(4-methyl-1-pentyne-1-yl)phen-
oxy]methyl}(N,N0-2,20-dipyridyl)palladium tetrafluorobo-
rate (7) reported in Scheme 1. To a solution of
palladacycle 1a (0.050 g, 0.11 mmol) in dichloromethane
(2 mL) at 0 °C was cannulated a solution of (E)-1-octe-
nyl(phenyl)iodonium tetrafluoroborate (0.050 g, 0.12
mmol) or 4-methyl-1-pentyne-1-yl(phenyl)iodonium tetra-
fluoroborate (0.056 g, 0.15 mmol) in dichloromethane
(1 mL). The yellow colored suspension was stirred at 0 °C
for 3 or 5 h. The suspension was filtered over celite and
the solid was washed with dichloromethane. The solvent
was removed under reduced pressure to afford an orange
oil, which was triturated with ether at ꢀ78 °C to afford
complex 6 (0.058 g, 82%) or complex 7 (0.058 g, 88%) as
orange or brown solids.
Appendix A. Supplementary material
Complete description of the synthesis and characteriza-
tion of all the compounds reported in the text above can be
found in the online version, at doi:10.1016/j.jorganchem.
2007.11.034.
References
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Analytical data for complex 6: IR (neat, cmꢀ1): 3467,
2966, 1739 (CO), 1298, 1203, 1145; 1H NMR (CDCl3,
400 MHz) o 8.87 (br s, 2H), 8.52 (d, J = 8.0 Hz, 2H),
8.22 (t, J = 7.9 Hz, 2H), 7.70 (d, J = 6.1 Hz, 2H), 7.20 (t,
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J = 7.8 Hz, 1H), 6.78-6.73 (m, 1H), 6.54 (d, J = 15.0 Hz,
1H), 5.57 (s, 1H), 4.35-4.27 (m, 2H), 2.39–2.22 (m, 2H),
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8.54-8.49 (m, 3H), 8.45 (d, J = 6.0 Hz, 1H), 8.30 (td,
J = 7.9 Hz, J = 1.4 Hz, 1H), 8.22 (td, J = 7.9 Hz,
J = 1.6 Hz, 1H), 7.75 (td, J = 5.3 Hz, J = 0.9 Hz, 1H),
7.64 (td, J = 5.7 Hz, J = 1.3 Hz, 1H), 7.40 (d,
J = 7.5 Hz, 1H), 7.18 (t, J = 7.1 Hz, 1H), 7.01 (t,
J = 6.8 Hz, 1H), 6.97 (d, J = 8.0 Hz, 1H), 6.50 (s, 1H),
4.84-4.78 (m, 1H), 4.73–4.64 (m, 1H), 2.71 (dd,
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[16] For a detailed description of the experimental protocols and results of
the low temperature in situ 1H NMR monitoring of the reaction of
palladacycle 1a with (E)-1-octenyl(phenyl)iodonium tetrafluoroborate
see p. S-41 in the Supporting Information.
[17] For the spectral traces recorded in the 1H NMR monitoring
experiments at ꢀ10 °C see the Supporting Information, p. S-
41.
[10] Furthermore, migratory insertion of the olefinic or alkyne groups in
the iodonium salts to palladacycles I could be ruled out as the initial
step, since our prior work (see Ref. [6d]) indicated that palla-
da(II)cycles I bearing bidentate ligand fail to react in this manner with
highly activated alkynes. A hypothetical electrophilic ipso substitu-
tion of the Csp2–Pd bond with the iodonium salts cannot be
unequivocally ruled out. However such pathway remains without a
precedent, and an increased steric hindrance introduced by the N,N0-
dicyclohexylethylenenediimine ligand in palladacycle 1b would be
expected to disfavor such event. For a discussion of general reactivity
patterns of vinyl iodonium salts, see: M. Fujita, W.H. Kim, Y.
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[18] Phenyl iodide arises from the (E)-1-octenyl(phenyl)iodonium
tetrafluoroborate.