Selective head-to-tail coupling of methyl phenylpropynoate providing palladacycles with bidentate N-ligands

Article abstract of DOI:10.1016/j.ica.2005.06.058

The complexes (1,3-dicarbomethoxy-2,4-diphenylbuta-1,3-dien-1,4-diyl)(N,N) palladium(II), where N,N = 2,2′-bipyridyl (1), tetramethylethelenediamine (2), 1,10-phenanthroline (3) have been obtained by reaction of Pd(dba) ₂ with the respective bidentate N-ligand and two equivalents of methyl phenylpropynoate via a completely regioselective head-to-tail coupling of the asymmetric acetylenes. Such regioselectivity, especially in conjunction with the high yield, is very unusual in the formation of palladacycles and has so far only been observed for head-to-head or tail to tail coupling. The compounds 1-3 have been characterized by elemental analyses, NMR spectra and single crystal X-ray diffraction studies for 2 and 3. The X-ray crystal structures reveal pseudo square planar metal centers, the palladacycles and chelate rings are essentially planar.

Full text of DOI:10.1016/j.ica.2005.06.058

Inorganica Chimica Acta 359 (2006) 1773–1778
www.elsevier.com/locate/ica
Selective head-to-tail coupling of methyl phenylpropynoate providing
palladacycles with bidentate N-ligands
a
b
b
a
Alexandre Holuigue , Claude Sirlin , Michel Pfeffer , Kees Goubitz ,
a
a,
*
Jan Fraanje , Cornelis J. Elsevier
a
Van ꢀt Hoff Institute of Molecular Sciences, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
`
´ ´
Laboratoire de Syntheses Metallo-Induites, UMR CNRS 7513 Universite Louis Pasteur, 4, Rue Blaise Pascal, 67070 Strasbourg, France
b
Received 1 June 2005; accepted 23 June 2005
Available online 9 August 2005
Dedicated to Professor Dr. G. van Koten, in friendship and in recognition of his many outstanding contributions to organometallic chemistry.
Abstract
The complexes (1,3-dicarbomethoxy-2,4-diphenylbuta-1,3-dien-1,4-diyl)(N,N)palladium(II), where N,N = 2,20-bipyridyl (1), tetra-
methylethelenediamine (2), 1,10-phenanthroline (3) have been obtained by reaction of Pd(dba)2 with the respective bidentate N-ligand
and two equivalents of methyl phenylpropynoate via a completely regioselective head-to-tail coupling of the asymmetric acetylenes.
Such regioselectivity, especially in conjunction with the high yield, is very unusual in the formation of palladacycles and has so far only
been observed for head-to-head or tail to tail coupling. The compounds 1–3 have been characterized by elemental analyses, NMR spec-
tra and single crystal X-ray diffraction studies for 2 and 3. The X-ray crystal structures reveal pseudo square planar metal centers, the
palladacycles and chelate rings are essentially planar.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Palladacyclopentadiene; Regioselectivity; Bidentate N-ligands; Methyl phenylpropynoate; Crystal structure
1. Introduction
cases [4]. These compounds were found to be intermedi-
ates in the Pd(0)-catalyzed cyclotrimerization of acety-
lenes [5], dimerization-carbostannylation of alkynes [6]
and catalytic conversion of alkynes to conjugated
(Z,Z)-dienes (Scheme 1) [7].
In many instances, dissymmetric alkynes have been
employed, which give up to three isomeric metallacycles
(Scheme 2) and two isomeric benzene derivatives as the
trimerization products [8,9]. The catalytic three-compo-
nent synthesis of conjugated (Z,Z)-dienes appeared to
be limited to the use of symmetric (and electron-defi-
cient) alkynes; all attempts employing dissymmetric
alkynes were unsuccessful [4].
Metallacyclic compounds have attracted much recent
attention, especially those formed from unsaturated li-
gands such as alkenes and alkynes. Their interest stems
principally from studies concerning the mechanism of
transition metal catalyzed oligomerization reactions.
The synthesis of palladacyclopentadienes bearing
electron withdrawing substituents has been reported
previously [1]. Contrary to other metals, where usually
no metallacycles have been observed [2,3], palladium
permits mechanistic investigations due to the lower reac-
tivity of palladium (g2-alkyne) complexes and pallada-
cycles that have been observed in this and in previous
The regiochemical outcome of the reactions involving
dissymmetric alkynes is difficult to predict a priori, due
to the two possible, often rather similar, orientations
of the incoming alkyne during the insertion into the
*
Corresponding author.
E-mail address: elsevier@science.uva.nl (C.J. Elsevier).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.06.058

1774
A. Holuigue et al. / Inorganica Chimica Acta 359 (2006) 1773–1778
R'X
Sn(CH )
E
Ar
N
Ar
N
Ar
N
E
E
E
E
E
E
E
E
3 4
E
E
R'
E
E
E
E
+
Pd
Pd
Pd
Me
DMF 650C
THF R.T.
N
N
N
E
R'X = Ph-CH2Br, MeI or PhI
E = -COOMe or -CF3
Ar
Ar
Ar
Ar
N
Ar
Ar
N
N
N
N
or
with Ar =
=
N
N
N
Ar
Ar
Ar
Ar-BIAN
Ar-BIP
bpy
Scheme 1.
R1
R2
R1
according to the literature procedures. Methyl phenyl-
propynoate, dimethyl-2-butynedioate (dmbd), 2,20-
bipyridyl,1,10-phenanthroline were obtained commer-
cially and were used without further purification.
N,N-tetramethylethylenediamine (tmeda) was obtained
commercially and was distilled prior to use.
R2
R2
R1
R1
R2
R1
L
L
L
L
L
M
M
M
L
R1
R2
R2
I
III
II
Scheme 2.
2.2. Synthesis of pallada-2,4-bis(carbomethoxy)-3,
5-diphenylcyclopentadiene-bipyridyl (1)
M(g2-alkyne) likely intermediate. We endeavoured to
study the feasibility and selectivity, if any, of the forma-
tion of palladacyclopentadienes starting from dissym-
metric alkynes. In this contribution, we describe our
results of the regioselective formation of head-to-tail
palladacycles involving methyl phenylpropynoate
(PhAC„CACO2CH3).
A suspension of Pd(dba)2 (200 mg, 0.35 mmol), 2,20-
bipyridyl was stirred at room temperature for 10 min
in THF (20 mL). Then methyl phenylpropynoate
(134 lL, 0.90 mmol) was added to the solution. After
2 h, the solvent of the resulting orange solution was re-
moved in vacuo and the product was washed with
diethyl ether (4 · 50 mL). The orange product was dis-
solved in dichloromethane and filtered over celite filter
aid in order to remove the metallic palladium. The sol-
vent was again removed in vacuo, yielding 145 mg
(0.25 mmol, 72%) of yellow orange product.
2. Experimental
2.1. General procedures
1. Anal. Calc. for C30H24N2O4Pd: C, 61.81; H, 4.15;
N, 4.81. Found: C, 61.64; H, 4.22; N, 4.88%.
All reactions were performed in Schlenck tubes under
nitrogen unless otherwise specified. Solvents were dried
and distilled under nitrogen prior to use: diethyl ether,
n-hexane and tetrahydrofuran over sodium, dichloro-
methane over calcium hydride.
1H NMR (CDCl3, 500 MHz): d 8.80 (d, 1H, H6,
3J = 5.5), 7.99 (m, 2H, H4 and H60), 7.95 (d, 1H,
3
H3, J = 8.0), 7.82 (dd, 1H, H50, 3J = 7.6, 3J = 7.5),
3
0
7.50 (dd, 1H, H5, J = 5.3), 7.41 (dd, 2H, Ho ,
3J = 7.0), 7.35 (dd, 2H, Ho, 3J = 7.0), 7.27–7.16 (m,
6H, Hm,m and Hp,p ), 7.02 (dd, 1H, H4 , 3J = 6.5),
1
0
0
0
All NMR spectra were recorded at 298 K. The H
3
and 13C {1H} NMR were recorded on a Bruker AMX
300 spectrometer (300.13 and 75.18 MHz, respectively).
The chemical shifts are referenced to TMS via the resid-
ual solvent peaks. In the cases of 1 and 3 the assign-
ments of the 1H NMR and 13C {1H} NMR signals
were made with the aid of COSY and HETCOR. Chem-
ical shifts (d) and coupling constants (J) are expressed in
ppm and in Hertz (Hz), respectively.
6.98 (d, 1H, H30, J = 5.0), 3.42 (s, 3H, OCH3), 3.17
(s, 3H, OCH3). 13C NMR (CDCl3, 75.48 MHz): d
0
0
177.1 (CO), 175.5 (CO ), 166.5 (Cb), 156.9 (Ca ),
155.4 and 155.2 (2C, C2 and C20), 151.9 (C3), 151.6
(C6 ), 149.0 (Cb ), 147.5 (Ci ), 146.9 (Ca), 140.1 (Ci),
139.3 (C6), 138.8 (C5), 128.2, 128.0, 127.8 and 127.6
0
0
0
0
0
(4 · 2C, 2Co, 2Co , 2Cm and 2Cm ), 126.5 and 126.4
0
0
(2C, Cp and Cp ), 126.0 (C5 ), 125.6 (C4), 122.4 and
122.0 (2C, C30 and C40), 51.0 and 50.8 (2C, OCH3
The starting materials Pd(dba)2 [10] (dba = dibenzy-
lidenacetone), 4-MeOC6H4-bian [11,12] were prepared
and OCH03).

A. Holuigue et al. / Inorganica Chimica Acta 359 (2006) 1773–1778
1775
2.3. Synthesis of pallada-2,4-bis(carbomethoxy)-3,
5-diphenylcyclopentadiene-tmeda (2)
direct methods and refined against |F|. Hydrogen atoms
were introduced as fixed contributors. For all computa-
tions, the Nonius OpenMoleN package was used [13].
The procedure was the same as for 1: Pd(dba)2
(200 mg, 0.35 mmol), tmeda (60 lL, 0.40 mmol) and
methyl phenylpropynoate (140 lL, 0.95 mmol) gave 2
after 3 h reaction and washing with diethyl ether
(2 · 50 mL) 85 mg (0.16 mmol, 45%) of yellow product.
2. Anal. Calc. for C26H32N2O4Pd Æ 0.5CH2Cl2: C,
54.38;H,5.68;N,4.79.Found:C,54.79;H,5.62;N,5.19%.
1H NMR (CDCl3, 300 MHz): d 7.17 (m, 10H, ar),
3.32 (s, 3H, OCH3), 2.98 (s, 3H, OCH3), 2.59 (s, 6H,
2 · CH3), 2.43 (s, 4H, 2 · CH2), 1.95 (s, 6H, 2 · CH3).
2.6. X-ray crystallographic structure determination of 3
A crystal of 3 with dimensions 0.15 · 0.30 · 0.35 mm
approximately was used for data collection on an Enraf-
Nonius CAD-4 diffractometer with graphite-monochro-
mated Cu Ka radiation and x–2h scan. A total of 5350
unique reflections was measured within the range
ꢀ14 6 h 6 14, ꢀ15 6 k 6 0, 0 6 l 6 22. Of these, 4605
were above the significance level of 4r(Fobs) and were
treated as observed. The range of (sinh)/k was 0.049–
13C NMR (CDCl3, 75.48 MHz): d 176.2 (CO), 173.0
0
˚
0
0
(CO ), 166.5 (Cb), 155.8 (Ca ), 148.5 (Ca), 148.3 (Cb ),
0.626 A (4.3 6 h 6 74.7ꢁ). Two reference reflections
ꢀꢀ
0
0
148.0 (Ci ), 140.4 (Ci), 127.7 (8C, 2 Co, 2Co , 2 Cm and
ð½311ꢁ; ½103ꢁÞ were measured hourly and showed no de-
0
0
2 Cm ), 126.3 and 125.6 (2C, Cp and Cp ), 62.0 and
61.3 (2C, CH2 and CH20 ), 51.0 and 50.5 (2C, OCH3
and OCH03), 48.8 and 48.7 (4C, 2 · CH3 and 2 · CH03).
crease during the 91 h collecting time. Unit-cell parame-
ters were refined by a least squares fitting procedure
using 23 reflections with 40.21 6 2h 6 44.69. Correc-
tions for Lorentz and polarization effects were applied.
Absorption correction was performed with the program
PLATON [14] following the method of North et al. [15]
using W-scans of five reflections, with coefficients in
the range 0.673–0.984. The structure was solved by the
PATTY option of the DIRDIF-99 program system [16].
The hydrogen atoms were calculated and kept fixed
2.4. Synthesis of pallada-2,4-bis(carbomethoxy)-3,
5-diphenylcyclopentadiene-phenanthroline (3)
The procedure was the sameas for 1: Pd(dba)2 (307 mg,
0.53 mmol), 1,10-phenanthroline (96 mg, 0.53 mmol) and
phenyl methylpropynoate (200 lL, 1.36 mmol) gave 3
after 1.5 h reaction and washing with diethyl ether
(2 · 50 mL) 195 mg (0.32 mmol, 61%) of yellow product.
3. Anal. Calc. for C32H24N2O4Pd: C, 63.32; H, 3.99;
N, 4.62. Found: C, 63.26; H, 4.08; N, 4.55%.
2
˚
at their calculated positions with U = 0.1 A . Full-
matrix least-squares refinement on F, anisotropic for
the non-hydrogen atoms and isotropic for the hydrogen
atoms, converged to R = 0.080, Rw = 0.091, (D/r)max
0.03, S = 1.04. A weighting scheme w = [13 + 0.01
=
*
1H NMR (CDCl3, 500 MHz): d 9.10 (d, 1H, H6,
3
3J = 5.0), 8.45 (d, 1H, H4, J = 8.0), 8.29 (d, 1H, H60,
(r(Fobs))2 + 0.01/(r(Fobs))]ꢀ1 was used. The secondary
isotropic extinction coefficient [17,18] refined to
g = 1794(117). A final difference Fourier map revealed
3J = 8.0), 7.88 (s, 2H, H7 and H70), 7.80 (dd, 1H, H5,
3
3
3
0
J = 7.8, J = 5.3), 7.41 (d, 2H, Ho , J = 6.5), 7.39 (d,
2H, Ho, J = 7.0), 7.30 (dd, 1H, H50, 3J = 7.8, 3J =
a
residꢀu3al electron density between ꢀ3.57 and
3
˚
0
0
5.3), 7.27–7.15 (m, 6H, Hm,m and Hp,p ), 7.08 (d, 1H,
H40, 3J = 5.0), 3.46 (s, 3H, OCH3), 3.18 (s, 3H,
OCH3). 13C NMR (CDCl3, 75.48 MHz): d 177.0 (CO),
2.23 e A in the vicinity of the heavy atom and O2
and C26. Scattering factors were taken from Cromer
and Mann [19,20]. The anomalous scattering of Pd
was taken into account [21]. All calculations were
performed with XTAL3.7 [22], unless stated otherwise.
Atoms O2 and C26 are highly anisotropic compared
to the other atoms. This is maybe due to positional dis-
order in that OCH3 moiety; no attempts were made to
quantify this disorder.
0
0
0
175.2 (CO ), 166.4 (Cb), 156.7 (Ca ), 152.0 (C4 ), 151.6
0
0
(C6), 149.0 (Cb ), 147.8 (Ci ), 147.1 (Ca), 146.2 (2C, C2
and C20), 140.2 (Ci), 138.4 (C4), 137.9 (C60), 130.0 and
129.7 (2C, C3 and C3 ), 130.9–125.9 (14C, 2 Co, 2 Co ,
2 Cm 2 Cm , Cp, Cp, C5, C5 , C7, and C7 ), 51.0 and
50.8 (2C, OCH3 and OCH03).
0
0
0
0
0
0
p'
p'
p'
m'
m'
m'
o'
5'
5'
m'
m'
o'
m'
4'
3'
6'
o'
o'
4'
3'
6'
o'
i'
i'
o'
i'
COOMe'
COOMe'
COOMe'
α'
α'
α'
N
N
N
N
3. Results and discussion
7'
7
β'
β'
β'
2'
2
2'
2
Pd
Pd
o
o
Pd
o
β
β
β
m
m
m
3
4
i
i
i
N
N
3
4
o
p
o
p
o
p
MeOOC
MeOOC
MeOOC
6
3.1. Synthesis and identification
6
m
m
m
5
5
1
2
3
Reaction of Pd(dba)2 with the respective bidentate
N,N ligand: tmeda, 2,20-bipyridyl or 1,10-phenanthro-
line in the presence of 2 equivalents of methyl phen-
ylpropynoate, gave the pure compounds 1–3 as unique
regioisomers. The palladacycles 1–3 were synthesized
in good yield (35–70%) via a completely regioselective
2.5. X-ray crystallographic structure determination of 2
Reflections were collected on a Kappa CCD diffrac-
tometer using Mo Ka graphite-monochromated radia-
˚
tion (k = 0.71073 A). The structure was solved using

1776
A. Holuigue et al. / Inorganica Chimica Acta 359 (2006) 1773–1778
head-to-tail coupling of the two dissymmetric alkynes
involved (Scheme 3). The typical reaction of Pd(dba)2
with 2,20-bipyridyl and methyl phenylpropynoate leads
to reproducible results. The new compounds are air-sta-
ble solids which are very soluble in chloroform and
dichloromethane. They have been analyzed by elemental
analysis or mass spectrometry and by 1H and 13C NMR
spectroscopy in solution. The formation of compounds
1–3 was usually instantaneous. In none of the cases stud-
ied could p-acetylenic intermediates be observed, which
is probably due to the weak p-acceptor character of the
ancillary ligands. It has been reported that (L)2Pd0(g2-
alkyne) complexes are more stable with increasing
p-accepting capacity of the ligands L [23,24].
3.2. X-ray crystal structures of 2 and 3
The adopted numbering schemes of the molecular
structures of 2 and 3 are depicted in Figs. 1 and 2,
selected bond distances, bond angles and torsion angles
have been compiled in Table 1.
Both complexes 2 and 3 are characterized by a
distorted square-planar coordination around the metal
center. The palladacycle itself and the chelate ring are
essentially planar as indicated in 3 by the small devia-
tions from the least-squares planes, which are 0.01(1)
˚
and 0.02(1) A for C16 and C13, respectively. The distor-
tion from square planarity is reflected by the dihedral
angle between these planes of 21.6(3)ꢁ. It is normal for
palladacyclopentadiene complexes, e.g., for (bpy)pal-
lada-2,3,4,5-tetrakis(carbomethoxy)-2,4-cyclopentadiene
and its phenyl-bip analogue [4] it was 15.6(4)ꢁ and
27.1(4)ꢁ, respectively. This is caused by an intermediate
interaction of the phenanthroline, as compared to the
bpy and phenyl-bip, with the ester group and the phenyl
group on the palladacycle. The bite angle N12–Pd–N1
The most characteristic chemical shifts of all com-
plexes are the ones that correspond to the methoxycar-
bonyl groups. They can act as a probe for the
geometry of compounds 1–3, i.e., symmetric complexes
(of types I and II) show one peak for the methoxycarbo-
nyls and the asymmetric one (type III) shows two reso-
nances for the methoxycarbonyls. In the present cases,
two signals due to the methoxycarbonyl groups are ob-
1
served around 3.4 ppm in the H NMR spectra. A low-
C24
frequency shift of about 0.4 ppm due to anisotropic
shielding by the phenyl groups of the metallacycle is ob-
served relatively to the pallada-2,3,4,5-tetrakis(carbome-
thoxy)-cyclopentadienes [25]. Therefore, we wondered
whether it was a 50/50 mixture of complexes of types I
and II or only the type III. The structure of compounds
1–3 was established on the basis of the anisochronicity
of the protons located on two inequivalent halves of
the N,N-ligand, which show mutual coupling. Further-
more, we observe only one set of resonances for each
complex, proving that we are dealing with a single com-
pound (type III) in each case and not with a mixture of
regioisomers I and II.
X-ray crystal structure determinations were per-
formed on pallada-2,4-bis(carbomethoxy)-3,5-bis(phe-
nyl)-cyclopentadiene-tmeda 2 and its phenanthroline
analogue 3, which corroborated that their molecular
structures are of type III. In most of the cases, metalla-
cyclopentadienes similar to the ones presented are
supposed to be intermediates in cyclotrimerization
process, but just a few of them have been isolated and
characterized so far.
C25
C23
C2
C26
C22
O3
C21
C1
C3
C19
C20
C7
N1
C18
O4
C17
C10
Pd1
C5
C4
N2
C16
C11
C15
C6
C8
C12
O2
C14
O1
C13
C9
Fig. 1. Crystal structure of 2. Ellipsoids are at the 30% probability
level; hydrogen atoms are omitted for clarity.
COOMe
N
N
Pd(dba)2
MeOOC
2
+
+
Pd
THF; 20 oC
N
N
MeOOC
1; bpy
2; tmeda =
3; phen
N
N
Scheme 3.

A. Holuigue et al. / Inorganica Chimica Acta 359 (2006) 1773–1778
1777
Table 2
C22
˚
Selected bond lengths (A), bond angles (ꢁ) and torsion angles (ꢁ) for 2
and 3
C23
C21
2
3
C10
C24
O1
Atoms
Distance
Atoms
Distance
2.143(8)
C9
C20
C11
N12
Pd1–N1
Pd1–N2
Pd1–C20
Pd1–C7
C3–C4
C20–C17
C17–C10
C10–C7
2.227(1) Pd–N12
2.229(6) Pd–N1
2.041(7) Pd–C15
2.032(7) Pd–C18
1.49(1)
1.36(1)
1.47(1)
1.36(1)
C19
C15
C26
2.194(7)
2.037(9)
1.997(9)
1.427(12)
1.390(13)
1.454(13)
1.386(13)
C8
C25
C7
C13
C14
C13–C14
C15–C16
C16–C17
C17–C18
C16
C17
O2
Pd
C6
C5
C28
N1
C18
C33
Atoms
Bond angle
80.9(2)
Atoms
Bond angle
78.1(3)
80.8(4)
C27
C4
N1–Pd1–N2
C20–Pd1–C7
N1–Pd1–C7
N1–Pd1–C20
N2–Pd1–C7
N2–Pd1–C20
Pd1–N1–C3
Pd1–N2–C4
Pd1–C20–C17
Pd1–C7–C10
C20–C17–C10
C7–C10–C17
N12–Pd–N1
C15–Pd–C18
N12–Pd–C18
N12–Pd–C15
N1–Pd–C18
N1–Pd–C15
Pd–N12–C13
Pd–N1–C14
Pd–C15–C16
Pd–C18–C17
C15–C16–C17
C18–C17–C16
C2
C29
79.3(3)
O4
C32
C3
179.5(3)
100.8(3)
98.9(3)
177.9(3)
104.4(5)
107.0(5)
115.0(5)
116.7(5)
115.9(6)
113.0(6)
172.3(3)
101.3(3)
102.2(3)
162.0(3)
113.3(6)
111.8(6)
112.7(6)
117.0(7)
117.2(8)
112.3(8)
O3
C30
C31
C34
Fig. 2. Crystal structure of 3. Ellipsoids are at the 30% probability
level; hydrogen atoms are omitted for clarity.
Table 1
Crystal data and structure refinement for compounds 2 and 3
Atoms
N1–C3–C4–N2
C20–C17–C10–C7
Torsion angle Atoms
ꢀ58.1(7)
Torsion angle
ꢀ2.3(13)
2
3
N12–C13–C14–N1
C15–C16–C17–C18
Empirical formula
2[C26H32N2O4Pd] Æ
CH2Cl2
585.44
C32H24N2O4Pd
2.7(9)
1.5(12)
O3–C18–C17–C10 ꢀ120.3(9)
O1–C25–C16–C17 ꢀ153.0(14)
Formula weight (g molꢀ1
Temperature (K)
)
606.96
295
O1–C8–C7–C10
ꢀ105.3(8)
O3–C33–C18–C17 65.9(13)
173
Crystal size (nm)
0.12 · 0.08 · 0.08
0.71073
monoclinic
P1c1
0.35 · 0.30 · 0.15
1.54180
monoclinic
P21/n
˚
Wavelength (A)
Crystal system
Space group
in metallacyclopentadiene formation involving unsym-
metric alkynes. Some bis(aryloxy)titanacyclopentadiene
complexes (Scheme 4; A) exhibit such regioselectivity
since only the head-to-tail coupled regioisomer is
formed [27]. A [cobalt(g5-cyclopentadienyl)(triphenyl-
phosphine) (acetylene)] complex B [8] yields one
regioisomer for the homocoupling product of 1-phenyl-
prop-1-yne, but it is important to notice that the homo-
coupling occurs via a tail to tail coupling. However, with
this cobalt complex, no regioselectivity was observed for
methyl phenylpropynoate as the alkyne.
In the same way, the selective formation of a symmet-
rically substituted hafnacyclopentadiene (D), which pre-
sents the same regioisomer (tail to tail coupling), has
been reported [28]. Concerning the formation of palla-
dacycles from unsymmetric alkynes, selective head-to-
tail coupling is very unusual; only head to head or tail
to tail coupling has been observed. One example involv-
ing a symmetric palladacyclopentadiene supported by
N-aryldiazadienes, regioselectively formed in low yield,
via a head to head coupling due to the steric hindrance
of the isopropyl group of the N-aryldiazadienes ligand,
has been reported (C) [5]. The regioselectivity of the
cyclization process in such cases appears to be governed
by the steric factors of the substituents of the alkyne,
rather than by electronic factors. The regioselectivity
˚
a (A)
13.8537(1)
10.9551(1)
35.4177(2)
98.797(5)
5312.06(7)
2
11.3922(8)
12.4860(6)
18.318(4)
90.57(2)
2605.5(6)
4
5350
4605
with F > 4r(F)
353
1.04
˚
b (A)
˚
c (A)
b (ꢁ)
Volume (A )
3
˚
Z
Number of data measured 16264
Number of data
11432
with I > 3r(I)
1241
1.213
0.037
Number of variables
Goodness-of-fit on F
R
wR2
0.080
0.091
0.062
of 78.1(3)ꢁ is somewhat smaller than the similar bite an-
gle in the palladium bis(methoxycarbonyl) coordination
compound [Pd(phen)(CO2CH3)2] [26] (82.5(3)ꢁ), due to
a larger steric encumbrance of the palladacycle com-
pared to the two mutually cis carbomethoxy groups
(see Table 2).
3.3. Regioselectivity
To the best of our knowledge there are only a few
examples in the literature of comparable regioselectivity

1778
A. Holuigue et al. / Inorganica Chimica Acta 359 (2006) 1773–1778
But
Me3Si
Hf
H3C
H3C
COOEt
COOEt
CH3
CH3
CH3
CH3
Ar''O
Ar''O
N
N
Ti
Co
Pd
But
Ph3P
Me3Si
B
C
D
A
Scheme 4.
[9] N.E. Schore, Chem. Rev. 88 (1988) 1081.
in case of 1–3, which has been obtained by head-to-tail
coupling, has never been observed before.
[10] M.F. Rettig, P.M. Maitlis, Inorg. Synth. 17 (1977) 134.
[11] R. Van Asselt, C.J. Elsevier, W.J.J. Smeets, A.L. Spek, R.
Benedix, Recl. Trav. Chim. Pays-Bas 113 (1994) 88.
[12] M. Gasperini, F. Ragaini, S. Cenini, Organometallics 21 (2002)
2950.
[13] OpenMoleN, Interactive Intelligent Structure Solution, 1997,
Nonius B.V., Delft, The Netherlands.
[14] A.L. Spek, Acta Crystallogr., Sect. A 46 (1990) C-34.
[15] A.C.T. North, D.C. Phillips, F. Scott Mathews, Acta Crystallogr.,
Sect. A 26 (1968) 351.
[16] P.T. Beurskens, G. Beurskens, R. Gelder, S. De Garcia-Granda,
R.O. Gould, R. Israel, J.M.M. Smits, The DIRDIF-99 Program
System, Crystallography Laboratory, University of Nijmegen,
The Netherlands, 1999.
[17] W.H. Zachariasen, Acta Crystallogr., Sect. A 23 (1967) 558.
[18] A.C. Larson, The inclusion of secondary extinction in least-
squares refinement of crystal structures, in: F.R. Ahmed, S.R.
Hall, C.P. Huber (Eds.), Crystallographic Computing, Munksg-
aard, Copenhagen, 1969, pp. 291–294.
It is clear that we did not observe any product(s) that
stem from a homogeneous catalytic cyclotrimerization.
Previously, apart from the case of dimethylbutynedioate
[29], this has never been observed for palladium(0)
either, except in the case of intramolecular process [30]
or cyclotrimerization of arynes [31]. In all of those cases,
only cyclo-cotrimerization has been observed, by intro-
ducing a third alkyne that is different from the one pres-
ent in the palladacycle, but such a reaction never
proceeded under catalytic conditions [1a,5].
The integrity of compounds 1–3 in the presence of
other alkynes [27] and their reactions with organic ha-
lides and organotin reagents will be the subject of future
investigations.
[19] D.T. Cromer, J.B. Mann, Acta Cryst. A24 (1968) 321–
324.
Acknowledgments
[20] D.T. Cromer, J.B. MannInternational Tables for X-ray Crystal-
lography, IV, Kynoch Press, Birmingham, 1974, p. 55.
[21] D.T. Cromer, D. Liberman, J. Chem. Phys. 53 (1970) 1891.
[22] S.R. Hall, D.J. Du Boulay, R. Olthof-Hazekamp (Eds.), XTAL3.7
System, University of Western Australia, Lamb, Perth, 2000.
[23] S. Ito, S. Hasegawa, Y. Takahashi, Y. Ishii, J. Organometal.
Chem. 73 (1974) 401.
Dr. A. de Cian and N. Kyritsakas (ULP Strasbourg)
are gratefully acknowledged for the X-ray diffraction
study of 2.
[24] R. Van Belzen, R.A. Klein, W.J.J. Smeets, A.L. Spek, R. Benedix,
C.J. Elsevier, Recl. Trav. Chim. Pays-Bas 115 (1996) 275.
[25] B. Milani, A. Scarel, E. Zangrando, G. Mestroni, C. Carfagna,
B. Binotti, Inorg. Chim. Acta 350 (2003) 592.
[26] R. Santi, A.M. Romano, R. Garrone, R. Millini, J. Organometal.
Chem. 566 (1998) 37.
References
[1] (a) K. Moseley, P.M. Maitlis, Chem. Commun. (1971) 1604;
(b) S. Ito, S. Hasegawa, Y. Takahashi, Y. Ishii, J. Chem. Soc.
Chem. Commun. (1972) 629.
[2] H. Yan, A.M. Beatty, T.P. Fehlner, Organometallics 21 (2002)
5029.
[27] J.E. Hill, G. Balaich, P.E. Fanwick, I.P. Rothwell, Organomet-
allics 12 (1993) 2911.
[3] E. Farnetti, N. Marsich, J. Organometal. Chem. 689 (2004) 14.
[4] R. Van Belzen, R.A. Klein, H. Kooijman, N. Veldman, A.L.
Spek, C.J. Elsevier, Organometallics 17 (1998) 1812.
[28] M.B. Sabade, M.F. Farona, E.A. Zarate, W.J. Youngs, J.
Organometal. Chem. 338 (1988) 347.
[29] P.M. Maitlis, Pure Appl. Chem. 30 (1972) 427.
[30] (a) A. Takeda, A. Ohno, I. Kadota, V. Gevorgyan, Y. Yamamoto,
J. Am. Chem. Soc. 119 (1997) 4547;
[5] H.T. Dieck, C. Munz, C. Muller, J. Organometal. Chem. 384
¨
(1990) 243.
[6] H. Yoshida, E. Shirakawa, Y. Nakao, Y. Honda, T. Hiyama,
Bull. Chem. Soc. Jpn. (2001) 637.
(b) Y. Yamamoto, A. Nagata, H. Nagata, Y. Ando, Y. Arikawa,
K. Tatsumi, K. Itoh, Chem. Eur. J. 9 (2003) 2469.
´
´
[7] R. Van Belzen, H. Hoffmann, C.J. Elsevier, Angew. Chem., Int.
Ed. Engl. 36 (1997) 1743.
[31] (a) D. Pena, S. Escudero, D. Perez, E. Guitian, L. Castedo,
˜
Angew. Chem., Int. Ed. 37 (1998) 2659;
´
´
[8] Y. Wakatsuki, O. Nomura, K. Kitaura, K. Morokuma, H.
Yamazaki, J. Am. Chem. Soc. 105 (1983) 1907.
(b) B. Iglesias, A. Cobas, D. Perez, E. Guitian, K.P.C. Vollhardt,
Org. Lett. 6 (2004) 3557.