Structure and reactivity of trans-bis[2-(2-chloroethyl)pyridine]palladium chloride (1). A study on the elimination reaction of 1 and 2-(2-chloroethyl) pyridine induced by quinuclidine in acetonitrile

Article abstract of DOI:10.1021/jo051324e

The trans-bis[2-(2-chloroethyl)pyridine]palladium chloride (1) has been prepared and structurally characterized by X-ray spectroscopy and computational study. The X-ray structure of 1 is consistent with the trans isomer (with respect to Pd). The NMR spectrum and the computational study are in agreement with an equilibrium in CD₃CN solution between two isomers of the trans structure. The reaction of the palladium complex with quinuclidine in CH₃CN, at 25 °C, leads to competing elimination and displacement reactions with formation of vinylpyridine and chloroethylpyridine in a ratio of 1.5:1. However, the rate constant for formation of uncoordinated (vinyl)pyridine monitored by HPLC (k;_Q^HPLC = 2.3 × 10^-3 M^-1 s^-1) is nearly 3 times slower than a rate constant monitored spectrophotometrically (k_Q = 6.5 × 10^-3 M^-1 s^-1). This suggests that the initial product of elimination is a palladium complex of vinylpyridine and that displacement from this complex is partially rate determining in the formation of the uncoordinated product. A study by UV spectroscopy at λ = 295 nm of trans-bis[2-(2-chloroethyl)pyridine-d2]palladium chloride with quinuclidine (Q) has shown the presence of a significant primary kinetic isotope effect, k _Q(H)/k_Q(D) = 1.8, for the elimination reaction within the Pd complex, 1. The second-order rate constant for the β-elimination reaction from 2-(2-chloroethyl)pyridine induced by quinuclidine in CH ₃CN at 25 °C is k_Q^FREE = 6.2 × 10^-6 M^-1 s^-1. It can be observed as a significant activation (about 3 orders of magnitude) of the β-elimination reaction within the complex 1 with respect to the free 2-(2-chloroethyl) pyridine. The possible mechanism in agreement with these results is discussed.

Full text of DOI:10.1021/jo051324e

Structure and Reactivity of
trans-Bis[2-(2-chloroethyl)pyridine]palladium Chloride (1). A
Study on the Elimination Reaction of 1 and
-(2-Chloroethyl)pyridine Induced by Quinuclidine in Acetonitrile
2
,†
†
‡
†
†
Sergio Alunni,* Gianfranco Bellachioma, Eric Clot, Tiziana Del Giacco, Laura Ottavi, and
†
Daniele Zuccaccia
Dipartimento di Chimica, Universit a` degli Studi di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy,
and LSDSMS (UMR 5636 CNRS), case courrier 14, Institut Gerhardt, Universit e´ Montpellier 2,
F-34095 Montpellier Cedex 5, France
alunnis@unipg.it
Received June 28, 2005
The trans-bis[2-(2-chloroethyl)pyridine]palladium chloride (1) has been prepared and structurally
characterized by X-ray spectroscopy and computational study. The X-ray structure of 1 is consistent
with the trans isomer (with respect to Pd). The NMR spectrum and the computational study are
in agreement with an equilibrium in CD
The reaction of the palladium complex with quinuclidine in CH
3
CN solution between two isomers of the trans structure.
CN, at 25 °C, leads to competing
3
elimination and displacement reactions with formation of vinylpyridine and chloroethylpyridine
in a ratio of 1.5:1. However, the rate constant for formation of uncoordinated (vinyl)pyridine
HPLC
-3
-1 -1
monitored by HPLC (kQ
) 2.3 × 10
M
s ) is nearly 3 times slower than a rate constant
-
3
-1 -1
monitored spectrophotometrically (k
of elimination is a palladium complex of vinylpyridine and that displacement from this complex is
partially rate determining in the formation of the uncoordinated product. A study by UV spectroscopy
Q
) 6.5 × 10
M
s ). This suggests that the initial product
at λ ) 295 nm of trans-bis[2-(2-chloroethyl)pyridine-d
has shown the presence of a significant primary kinetic isotope effect, k
elimination reaction within the Pd complex, 1. The second-order rate constant for the â-elimination
2
]palladium chloride with quinuclidine (Q)
Q
(H)/k (D) ) 1.8, for the
Q
FREE
reaction from 2-(2-chloroethyl)pyridine induced by quinuclidine in CH
3
CN at 25 °C is kQ
) 6.2
-
6
-1 -1
×
10
M
s . It can be observed as a significant activation (about 3 orders of magnitude) of the
â-elimination reaction within the complex 1 with respect to the free 2-(2-chloroethyl)pyridine. The
possible mechanism in agreement with these results is discussed.
Introduction
ring (Scheme 1). The proton activating factor (PAF) is
defined as the ratio of the rate constants for the nitrogen
6
In previous studies,1 we have shown that a large
activation of the â-elimination reactions, in systems
activated by a pyridine ring, can be observed because of
electrophilic catalysis at the nitrogen atom of the pyridine
-5
protonated substrate and the nitrogen unprotonated
substrate, and the value of PAF for the acetohydroxam-
2
ate-induced â-elimination reaction with 2-(2-chloroeth-
5
5
yl)pyridine, 2, is 1.38 × 10 . It was shown that methy-
lation of the pyridine ring in the same systems consistently
produces large activation of the process (MethylAF ) 6.88
†
Universit a` degli Studi di Perugia.
‡
Universit e´ Montpellier.
5
2+
(
1) Alunni, S.; Conti, A.; Palmizio Errico, R. J. Chem. Soc., Perkin
× 10 ). Activation by Zn is also large, for substrate N-[2-
Trans. 2 2000, 453.
(
(
2) Alunni, S.; Busti, A. J. Chem. Soc., Perkin Trans. 2 2001, 778.
3) Alunni, S.; Laureti, V.; Ottavi, L.; Ruzziconi, R. J. Org. Chem.
(6) (a) Stewart, R.; Srinivasan, R. Acc. Chem. Res. 1978, 11, 271.
(b) McCann, G. M.; More O’Ferrall, R. A.; Walsh, S. M. J. Chem. Soc.,
Perkin Trans. 2 1997, 2761. (c) Eustace, S. J.; McCann, G. M.; More
O’Ferrall, R. A.; Murphy, M. G.; Murray, B. A.; Walsh, S. M. J. Phys.
Org. Chem. 1998, 11, 519.
2
2
003, 68, 718.
(
4) Alunni, S.; Conti, A.; Palmizio Errico, R. Res. Chem. Intermed.
001, 27, 653.
5) Alunni, S.; Ottavi, L. J. Org. Chem. 2004, 69, 2272.
(
10.1021/jo051324e CCC: $30.25 © 2005 American Chemical Society
10688
J. Org. Chem. 2005, 70, 10688-10692
Published on Web 11/18/2005

Reaction of 1 and 2-(2-Chloroethyl)pyridine by Q
SCHEME 1
SCHEME 2
7
(
2-pyridyl)ethyl]quinuclidinium, with the metal activat-
4
ing factor (MetAF) being 8.1 × 10 .
In this work, we have studied the comparison in
reactivity between free 2 and Pd-bonded 2 within the Pd
planar square complex trans-bis[2-(2-chloroethyl)pyridine]-
palladium chloride, 1 (Scheme 2); the structure of 1 has
been determined by X-ray spectroscopy, NMR study, and
theoretical calculations (see Experimental Section).
Chemistry of the Pd complex is important in several
8
,9
applied fields, and the quantification of the activation
of the â-elimination reaction within a Pd complex adds
relevant information to our previous studies.
FIGURE 1. B3PW91 optimized geometries of the three
isomers, 1tt, 1tc, and 1ct, and of the transition state, TS1tt-
1
tc, connecting 1tt and 1tc. Selected geometrical parameters
Results and Discussion
(distances in Å, angles in degrees, experimental values in
parentheses) for 1tt: Pd-N ) 2.038 (2.029), Pd-Cl ) 2.329
A. Structural Characterization. The X-ray structure
of 1 is consistent with the trans isomer (with respect to
Pd; Scheme 2). However, a solution of 1 in CD CN
3
presents an NMR spectrum consistent with the presence
of an equilibrium between two isomers in a ratio of 1:0.8
(
2.298), Cl-Pd-N ) 89.9 (88.8), and C-N-Pd-Cl ) 79.4
(70.6).
be prohibitive. The most stable structure, 1tt, corre-
sponds to the geometry obtained in the X-ray crystal-
lography study. The agreement between the calculated
and experimental geometrical parameters is very good
(25 °C). We assume that this mixture is related to two
rotamers of the trans structure (with respect to Pd), in-
volving the chloroethyl system on the same side (trans-
cis) or on the opposite side (trans-trans). In fact, an NMR
study excludes the possibility of the cis-trans isomer
from the NOE analysis (lack of dipolar interactions
(
see caption of Figure 1). The rotamer 1tc with trans
pyridyl rings but with cis chloroethyl chains is computed
to be less stable by 1.8 kcal/mol, in very good agreement
with the experimental value determined from NMR
spectroscopy. As expected, the cis geometry of the two
pyridyl rings is computed to be less stable and the isomer
2
between CH Cl hydrogens and the 5,6-aromatic hydro-
gens of the pyridine ring). The cis-cis isomer should be
1
ct lies at 13.6 kcal/mol above 1tt, ruling out any
sterically hindered. The presence of the two rotamers is
in agreement with similar reported10 compounds. An H-
1
presence of the latter in solution.
q
The transition state, TS1tt-1tc, connecting the two
trans isomers has been optimized, and it presents a
perpendicular pyridyl ring (Figure 1). The transition-
state vector consists of the rotation of this pyridyl ring,
and the calculated enthalpy of this transition state (TS)
EXSY study between 5 and 30 °C gave values of ∆H )
q
1
3.7 kcal/mol and ∆S ) 12 kcal/mol K for the conversion
of the two isomers. The ∆H is 0.9 kcal/mol.
0
To further confirm the nature of the two isomers
observed in solution, DFT (B3PW91) calculations on 1
have been carried out. Three of the four possible geom-
etries, defined by the respective positions of the pyridine
rings (cis-trans) and by the relative position of the
chloroethyl chains (cis-trans), were considered (Figure
(
14.3 kcal/mol) is in excellent agreement with the experi-
mental value of 13.7 kcal/mol. The calculations are thus
in full agreement with the description of the observed
complexes in solution as the two rotamers of the trans
isomer.
1
). The cis-cis geometry was not calculated because the
B. Reaction Products and Kinetic Study. To
identify the possible products of the reaction between 1
and quinuclidine, Q, in CD
4.5 mg of 1 in 0.6 mL of CD
react at 25 °C, and by recording the H NMR spectrum,
it was possible to observe the disappearance of the signals
steric strain between the two chloroethyl chains would
3
CN, an NMR tube containing
CN and 0.1 M Q was left to
(
7) Alunni, S.; Del Giacco, T.; De Maria, P.; Fifi, G.; Fontana, A.;
Ottavi, L.; Tesei, I. J. Org. Chem. 2004, 69, 3276.
8) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metal, 3rd ed.; John Wiley and Sons: New York, 2001.
9) (a) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry,
3
(
1
(
4
th ed; John Wiley and Sons: New York, 1980. (b) Atwood, J. D.
at δ ) 4.23-4.47 ppm related to the CH
2
-CH -Pd
2
Inorganic and Organometallic Reaction Mechanism, 2nd ed.; Wiley-
system or the disappearance of the related aromatic
signals. It was clear that the formation of 2-vinylpyridine
unbonded with Pd by the characteristic signals at δ )
VCH: New York, 1997.
(
10) Boixassa, A.; Pons, J.; Virgili, A.; Solans, X.; Bardia, M. F.; Ros,
J. Inorg. Chim. Acta 2002, 340, 49.
J. Org. Chem, Vol. 70, No. 26, 2005 10689

Alunni et al.
1 2
and 2-(2-chloroethyl)pyridine-d in a
SCHEME 3
2-vinylpyridine-d
ratio of alkene/chloride 0.54:1.
The fact that a study of the initial rate of the elimina-
tion reaction from free 2-(2-chloroethyl)pyridine (in CH
CN at 25 °C induced by Q) gave a value of the second-
3
-
FREE
-6
-1
-1
order rate constant, kQ
) 6.2 × 10
M
s , is
relevant. This value is significantly lower than the one
observed for the elimination reaction from 1, and this
result allows excluding that the process from free 2-(2-
chloroethyl)pyridine can be significant in the elimination
reaction from 1.
The mechanistic interpretation of these results is
complicated by the presence of two molecules of 2 (2-(2-
chloroethyl)pyridine) bonded to Pd. The mechanism
proposed is shown in Scheme 3; in fact, the intermediate
5
could give a second elimination or an exchange reaction
with Q. The overall system is therefore complicated, but
presumably, the reactivity of 5 in the elimination or
substitution reactions is similar to that of 1.
5
.47 (d), 6.24 (d), and 6.84 (dd) ppm. It was also possible
to observe the presence of 2-(2-chloroethyl)pyridine un-
bonded to Pd by comparison with the spectrum of a pure
Step I is the elimination reaction of HCl by a direct
attack of Q on 1; this step must be partially rate
determining owing to the presence of the primary deu-
sample (δ ) 4.00 ppm (t); CH -Cl). After 2.5 h, the molar
2
ratio of 2-vinylpyridine/2-(2-chloroethyl)pyridine, calcu-
lated by integration of signals, was 1.5:1, and from the
reaction, 60% of 2-vinylpyridine and 40% of 2-(2-chloro-
ethyl)pyridine were produced. The UV kinetic profiles
showed a leveling-off after 2.5 h; however, a slow decrease
of the amount of 2 was observed after this time, but this
process was too slow to affect reactions of the coordinated
species.
terium isotope effect, k
isotope effect that refers to the two overall rate constants
(H) and k (D); the term k (H)/k (D) for the elimination
Q Q
(H)/k (D) ) 1.8 (this is an overall
k
Q
Q
Q
Q
reaction should be calculated from the proper rate
constants, but the complexity of this system does not
allow the complete dissection of the various parameters).
Step II is the release of free 2-vinylpyridine, and this step
must also be partially rate determining because the
The reactivity of the system 1 + Q in CH
3
CN, at 25
°
C, was evaluated by following spectrophotometrically the
Q
second-order rate constant k obtained by following the
increase in absorbance at λ ) 295 nm due to products
formation. The reaction followed good pseudo-first-order
kinetics (excess of Q) up to at least 80% of the reaction.
This result shows that the two rotamers of 1 have very
similar reactivities or are rapidly interconverted on the
time scale of the reaction. The average value of the
formation of 2-vinylpyridine by HPLC is lower than the
one obtained following product formation spectrophoto-
metrically at λ ) 295 nm. It is possible that at λ ) 295
nm there is the absorption of both the Pd-bonded and
free 2-vinylpyridine. In fact, an experiment following an
-
3
-1 -1
3
NMR tube reaction with 1 and Q in CD CN, at 25 °C, at
second-order rate constant k
Q
is 6.5 × 10
M
s . A
various times, showed, together with the formation of the
signals related to free 2-vinylpyridine, the formation of
signals at σ ) 6 ppm that can be related to the vinylic
hydrogens of Pd-bonded 2-vinylpyridine. This signal
disappears during the reaction. We have not determined
the structures of the products containing Pd (probably
kinetic study by HPLC (UV-detector; λ ) 280 nm),
following the formation of 2-vinylpyridine (pseudo-first-
HPLC
order; [Q] ) 0.1-0.17 M), gave a value of k
) 2.3 ×
Q
-
3
-1 -1
1
0
M
Q
s . The difference between k values obtained
with the two different techniques could be explained
considering the mechanism proposed below; following the
reaction by UV spectroscopy at λ ) 295 nm, both the Pd-
bonded and free 2-vinylpyridine can be observed, whereas
by HPLC, only free 2-vinylpyridine is detected. It follows
that there must be a sequence of steps, and the expulsion
of 2-vinylpyridine from 5 (see Scheme 3) is partially rate
determining. A full kinetic analysis is not possible, owing
to the probable variations of the rate constants of the
various possible intermediates in the reaction paths.
To check if C-H bond breaking for the elimination
reaction from 1 is rate determining, the substrate 2-(2-
2 2
PdCl Q ) because we have focused the study on the
formation of 2-vinylpyridine (by elimination reaction) or
free 2-(2-chloroethyl)pyridine.
The first elimination reaction of HCl must occur within
the 1 complex because the rate constant for the â-elim-
ination induced by Q from 2-(2-chloroethyl)pyridine is
FREE
Q
-6
-1 -1
k
) 6.2 × 10
M
s . This rate constant is much
lower than the one observed with 1, so it can be excluded
from the reaction of free 2-(2-chloroethyl)pyridine. The
second-order rate constant (spectrophotometrically de-
termined at λ ) 295 nm) is about 1000 times larger than
that of free 2-(2-chloroethyl)pyridine in the same reaction
chloroethyl)pyridine-d
lated Pd complex was prepared (trans-bis[2-(2-chloroethyl)-
pyridine-d ]palladium chloride, 4) following the same
2
, 3, was synthesized and the re-
2
conditions (Q/CH
3
CN, 25 °C). This activation can be
compared with that by protonation of the nitrogen atom
of the pyridine ring of 2-(2-chloroethyl)pyridine (CH
2
procedure as that used for the undeuterated system 1. A
kinetic study, following spectrophotometrically at λ ) 295
nm the products formation from 4 in pseudo-first-order
conditions ([Q] ) 0.05-0.2 M), gave a second-order rate
3
-
-
3 2
CONHO /CH CONHOH, H O, µ ) 1 M KCl, 50 °C), with
5
the proton activating factor (PAF) being 1.38 × 10 . Also,
-
3
-1 -1
5
constant k
the products of the reaction from 4 + Q (0.1 M) in CD
CN, at 25 °C, showed, after 5 h, the formation of
Q
(D) ) 3.6 × 10
M
s . An NMR study of
we have previously reported the activation for the same
3
-
process by methylation of the N atom, with the methyl
5
-
activating factor (MethylAF) being 6.88 × 10 (OH /H
2
O,
10690 J. Org. Chem., Vol. 70, No. 26, 2005

Reaction of 1 and 2-(2-Chloroethyl)pyridine by Q
SCHEME 4
tion to give 2-(hydroxyethyl)pyridine-d
2
. The percent of deu-
teration was 97%, assigned by NMR and gas-mass analysis.
2
-(2-Chloroethyl)pyridine-d (3) was prepared from 6
2
2
according to a previously described procedure. The percent
of deuteration was 97%.
2
trans-Bis[2-(2-chloroethyl)pyridine-d ]palladium chlo-
ride (4) was prepared as described previously for complex 1.
The percent of deuteration was 97%.
NMR Measurements. A. T1 Measurements. The longi-
1
tudinal relaxation times for the H nuclei were measured by
the standard inversion recovery method on solutions of
7
µ ) 1 M KCl, 25 °C). We have also showed that in a
complex trans-bis[2-(2-chloroethyl)pyridine]palladium chloride
related system with a leaving group quinuclidine (N-[2-
2
in methylene chloride-d at 302.6 K. A total of 15 experiments,
2-pyridyl)ethyl]quinuclidinium) the activation with Zn2+
by metal ion interaction with the N atom of the pyridine
having a relaxation delay ranging from 0.001 to 20 s, were
acquired, each of them consisting of 16 scans. The total
relaxation delay between two consecutive scans was 40 s. All
single experiments were Fourier transformed. Frequency
domain spectra were processed with a standard T1/T2 software
package available on spectrometers to extract the relaxation
(
(
4
-
ring) is 8.1 × 10 (OH /H
2
O, µ ) 1 M KCl, 50 °C).
It should be considered that the mechanism of the
elimination reaction from 1 can be E2 concerted or E1cb;
a
parameters. Important T1 values are 4.0 s for H1 , 4.0 s for
in fact, it is not possible to distinguish between these two
b
b
possibilities in this case. In previous studies,1
,2,5,7
H1 , 3.1 s for H3, 3.0 s for H4, 2.9 s for H2, 1.4 s for H7 , 1.4
a a b
it was
s for H7 , and 1.1 s for H6 and H6 .
shown that systems where the pyridine ring is protonated
or interacts with a metal ion react by the E1cb mecha-
nism.
It should be finally noted that the mechanism of
Scheme 3 is related to two parallel processes, so the
calculation for the second-order rate constant should take
this fact into account. However, in this case, the second-
order rate constant would be a combination of constants
so we prefer to evaluate the overall activation due to Pd
complexation.
B. EXSY and NOE Measurements. Quantitative EXSY
2 2
and NOE measurements were carried out in CD Cl because
of the higher solubility of complex 1 in this solvent; however,
qualitative measurements in CD CN gave similar results. The
3
11
1
H-NOESY NMR experiments were acquired by the standard
12
three-pulse sequence or by the PFG version. The number of
transients was 64, the number of data points was 256, and
1
the “nt” values were 48. Quantitative H-NOESY NMR experi-
ments were carried out with a relaxation delay of 12 s and a
1
mixing time of 0.4 s. The H-NOESY NMR experiments were
conducted at different temperatures in the range 278.9-302.6
K. The auto- and cross-peak volumes were determined using
XWinNMR Bruker software after phase and baseline correc-
tions in both dimensions. The volume uncertainty was esti-
mated by determining the volume of noise signals in a “blank
space” of the spectrum. The use of 2D-NOESY for chemical
Experimental Section
Materials. Quinuclidine, CH
O, DMSO-d , and CD CN were commercial materials. 2-(2-
Chloroethyl)pyridine, 2, was prepared according to a previously
3 2 4
CN (HPLC grade), Na PdCl ,
D
2
6
3
11
kinetics was first proposed by Jeneer and Ernst and was
called 2D-EXSY (exchange spectroscopy). 2D-EXSY spectra
arise from noncoherent magnetization transfer that can take
place by exchange of nuclei between nonequivalent sites and
avoids the cross relaxation between sites with different
resonance frequencies. 2D-EXSY can measure a rate constant,
2
1
described procedure. One- and two-dimensional H NMR
spectra were recorded on 200 and 400 MHz spectrometers.
Referencing is relative to TMS.
trans-Bis[2-(2-chloroethyl)pyridine]palladium Chlo-
ride (1). A mixture of Na
chloroethyl)pyridine (0.3 g, 2.19 mmol), and acetone (20 mL)
was left to react under nitrogen and magnetic stirring for 15
h. The solvent was evaporated under reduced pressure, and
-
1
2
-1 13
2
PdCl
4
(0.31 g, 1.05 mmol), 2-(2-
k, on the order of 10 -10 s
.
The upper limit is governed
- ω ), and the lower
). For an
by the difference in chemical shift (k , ω
a
b
limit is dictated by the relaxation time T
1
(k ∼ 1/T
1
uncoupled system of spins A and B, with the simplification of
the residue was washed several times with CH
Cl
washed with n-hexane and crystallized from CH
2
Cl
2
. The CH
2
-
equal spin-lattice relaxation time, the rate constant kobs is
2
solution was taken to dryness, and the solid residue was
related to the mixing time τ
m
by eq 1:
2
Cl
2
/n-hexane
1
at room temperature. Mp: 195-199 °C dec. H NMR (CD
Cl
2
-
r + 1
r - 1
, 302.6 K, 400.13 MHz, J in Hz, Scheme 4): δ 4.27 (t,
ln(
) ) kobsτm
(1)
2
3
a
a
a
3
b
b
b
J
J
J
H6 -H7 ) 6.8, H6 ), 4.32 (t, JH6 -H7 ) 6.8, H6 ), 4.40 (t,
a b b
a 3 b
3
3
H7 -H6a ) 6.8, H7 ), 4.50 (t, JH7 -H6 ) 6.8, H7 ), 7.38 (ddd,
3
4
3
In eq 1, the term r is related to the volumes of the cross and
diagonal signal (I) and to the molar fraction of the species A
and B (X) by eq 2:
H2-H1 ) JH2-H3 ) 7.7, JH2-H4 ) 1.4, H2), 7.47 (d, JH4-H3
)
.8, H4), 7.84 (ddd, JH3-H2 ) 3JH3-H4 ) 7.7, JH3-H1 ) 1.4, H3),
3
4
7
8
3
b
b
3
a
a
.94 (d, JH1 -H2 ) 7.7, H1 ), 9.10 (d, JH1 -H2 ) 7.7, H1 ). Anal.
Calcd for C14
16 2 4
H N Cl Pd: C, 36.51; H, 3.50; N, 6.08. Found:
C, 36.35; H, 3.44; N, 6.03.
4X X (I + I )
a
b
aa
bb
r )
- (X - X )
(2)
To obtain the thermodynamic parameter of the equilibrium
a
b
(
Iab + Iab)
1
between the two isomers, H NMR experiments were conducted
at different temperatures in the range 278.9-302.6 K.
Because the system is in equilibrium (A a B), it is possible to
determine the direct ka-b and the reverse kb-a rate constants
by eqs 3 and 4:
The values of the equilibrium constants at different tem-
2
78.9
291.0
peratures are K
) 1.428 ( 0.001, K
) 1.333 ( 0.001,
3
02.6
and K
) 1.266 ( 0.001. The enthalpy energy for the
equilibrium was derived by plotting K as a function of 1/T,
and the value is ∆Hab° ) 0.86 ( 0.2 kcal/mol.
ka-b ) Xbkobs
(3)
(4)
2
-(2-Hydroxyethyl)pyridine-d
2
(6). 2-(2-Hydroxyethyl)-
(40:60, v/v) and 0.67 M
kb-a ) Xak
obs
pyridine dissolved in D
2
O/DSMO-d
6
-
OD was kept at 58 °C, and the deuteration, to form 6, was
checked by NMR. After 288 h, the solution was poured in DCl/
(
11) Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem.
D
2
O and extracted with Et
2
2
O. The organic layer was dried over
Phys. 1979, 71, 4546.
Na SO
4
, and then the solvent was removed by rotary evapora-
(12) Wagner, R.; Berger, S. J. Magn. Reson., Ser. A 1996, 123, 119.
J. Org. Chem, Vol. 70, No. 26, 2005 10691

Alunni et al.
The rate constants kobs, kab, and kba could be obtained by
applying directly eqs 1, 3, and 4, respectively. The rate
constants of exchange between the two isomers for complex
trans-bis[2-(2-chloroethyl)pyridine]palladium chloride in me-
In the complex, the palladium center is coplanar with the four
coordinating atoms (the N-Pd-N and the Cl-Pd-Cl angles
are 180°). The X-ray diffraction spectrum of the complex
corroborates the presence of a unic confomer in the solid state,
in which the two carbon chains are in an anti disposition.
CCDC 244649 contains the supplementary crystallographic
data for this paper.
thylene chloride-d
2
at 278.9, 291.0, and 302.6 K were deter-
a b
mined by detecting the H1 and H1 resonances. The values
2
78.9
of the rate constants at different temperatures are kobs
)
2
78.9
278.9
291.0
1
.6 ( 0.1, kab
) 0.7 ( 0.1, kba
) 0.9 ( 0.1, kobs
) 4.7
The atomic coordinates for these structures have been
deposited with the Cambridge Crystallographic Data Centre.
These data can be obtained free of charge via www.ccdc.ca-
m.ac.uk/data_request/cif, by email at data_request@
ccdc.cam.ac.uk, or by contacting the Director, The Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, U.K.; fax: +44 1223 336033.
2
91.0
291.0
302.6
(
0.3, kab
) 2.0 ( 0.3, kba
) 2.7 ( 0.3, kobs
) 9.5 (
3
02.6
302.6
-1
0.5, kab
) 4.2 ( 0.5, and kba
) 5.3 ( 0.5 s . The
thermodynamic parameters of activation for the exchange
q
process were derived from Eyring plots and are ∆Hab ) 14 (
q
q
1
kcal/mol, ∆Sab ) 12 ( 4 kcal/mol K, ∆Hba ) 13 ( 1 kcal/
q
mol, and ∆Sba ) 10 ( 4 kcal/mol K.
Kinetic Studies. Kinetics of the reaction from 1 with
Computational Details. All calculations were performed
1
8
quinuclidine in CH
3
CN at 25 °C was followed spectrophoto-
with the Gaussian 98 set of programs within the framework
19 20 21
metrically at λ ) 295 nm. The concentration of 1 was [1] ) 1
of hybrid DFT B3PW91. The palladium and chlorine
-
4
-4
×
10 - 2 × 10 M, and [Q] ) 0.05-0.2 M. The pseudo-first-
atoms were represented by the relativistic effective core
potential (RECP) from the Stuttgart group and their associated
-
1
order rate constant kobs (s ) was calculated as the slope of a
2
2
22
plot, ln(A
∞
- A
0
)/(A
∞
- A
t
), against time. Good linearity was
basis set and augmented by an f (Pd) or a d (Cl) polarization
23
observed up to at least 80% of the reaction. The kinetics of 1
function. A 6-31G(d,p) basis set was used for all the remain-
ing atoms of the molecules studied (C, H, and N). The geometry
optimizations were performed without any symmetry con-
straints, and the nature of the stationary point was confirmed
to be a minimum by analytical frequency calculations. The
energy differences between the optimized structures were
enthalpy values, as obtained after frequency calculations with
Gaussian 98.
with quinuclidine was also followed by HPLC in isocratic
3 2
conditions, eluent CH CN/ H O 80:20, by monitoring the
2
-vinylpyridine formed with a UV detector at 280 nm. Kinetics
of the elimination reaction from 2-(2-chloroethyl)pyridine, 2,
3
with quinuclidine in CH CN at 25 °C was followed by initial
2
rates, monitoring the formation of 2-vinylpyridine by UV
spectroscopy at λ ) 278 nm.
X-ray Crystallography. A single crystal of trans-bis[2-(2-
chloroethyl)pyridine]palladium chloride, 1, suitable for X-ray
diffraction (a yellow block with approximate dimensions of 0.20
Acknowledgment. The authors thank the Minis-
tero dell’Istruzione, dell’Universit a` e della Ricerca
(MIUR), for financial support.
×
0.10 × 0.06 mm), was obtained as described previously. Data
were collected on a (CCD areal) diffractometer using Mo KR
graphite monochromated radiation (λ ) 0.71073 Å), and ω
scans and the frame data were acquired with the CRYSALIS
Note Added after ASAP Publication. The ratio of
vinylpyridine and chloroethylpyridine was incorrect in
the Abstract in the version published ASAP November
18, 2005; the corrected version was published ASAP
November 22, 2005.
(
CCD 169) software. The crystal to detector distance was 65.77
mm. The frames were processed using the CRYSALIS (RED
69) software to give the hkl file, corrected for scan speed,
1
background, Lorentz, and polarization effects. Standard reflec-
tions, measured periodically, showed no apparent variation in
intensity during data collection, and so, no correction for
crystal decomposition was necessary. The data were collected
Supporting Information Available: X-ray crystallo-
graphic information including a CIF table for complex 1. This
material is available free of charge via the Internet at
http://pubs.acs.org.
1
4
for absorption using the SADABS program.
The Laue symmetry was determined to be 2/m. The dimen-
-
3
JO051324E
sions of the cell yielded a calculated density of 1.78 g cm (Z
)
2 and fw ) 460.49) that was confirmed by the experimental
-3
(15) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Gagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. Sir97: a new tool for crystal structure determination and refine-
ment. J. Appl. Crystallogr. 1999, 32, 115.
value of 1.80(3) g cm . The observed systematic absences were
1
consistent with the monoclinic space group P2 /n (No. 14). The
data were collected at room temperature. The lattice param-
eters found were a ) 7.641 (5), b ) 14.915 (5), c ) 7.965 (5) Å,
(
16) Sheldrick, G. M. SHELXL-97: A program for crystal structure
3
â ) 108.52 (5), and V ) 860.7 (9) Å . Data were collected to
refinement, release 92-2; University of G o¨ ttingen: G o¨ ttingen, Germany,
2Θ
max of 56.68° in the index range -7 e h e 9, -19 e k e 19,
1997.
(
(
17) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
and -10 e l e 10 with a total of 5243 collected reflections, of
which 185 were rejected, and after merging, 1989 were unique
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.11; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(
R(int) ) 0.0263).
The structure was solved by the direct method using the
1
5
Sir97 program and refined by the full-matrix least-squares
2
16
17
method F using SHELXL-97, WinGX version. All non-
hydrogen atoms were refined anisotropically. The hydrogen
atoms were added at the calculated positions and refined using
a riding model.
The final cycle of full-matrix least-squares refinement
2
against |F| was based on 1591 observed reflections [F
0
>
4
σ(F
unweighted and weighted agreement factors of R ) 0.0377,
) 0.0584, and GOF ) 1.079.
0
)] and 100 variable parameters and converged with
(
19) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Perdew, J.
R
w
P.; Wang, Y. Phys. Rev. B 1992, 45, 13244.
(20) Andrae, D.; H a¨ ussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123.
The crystal structure is a monomeric molecule containing
Pd(II) coordinated, in a square planar environment, by two
trans chlorides and two nitrogen atoms in the pyridine rings.
(21) Bergner, A.; Dolg, M.; K u¨ chle, W.; Stoll, H.; Preuss, H. Mol.
Phys. 1993, 30, 1431.
(
22) Ehlers, A. W.; B o¨ hme, M.; Dapprich, S.; Gobbi, A.; H o¨ llwarth,
(
(
13) Perrin, C. L.; Dwyer, T. J. Chem. Rev. 1990, 90, 935.
A.; Jonas, V.; K o¨ hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking,
G. Chem. Phys. Lett. 1993, 208, 111.
14) SADABS: Area-Detector Absorption Correction; Siemens In-
dustrial Automation, Inc.: Madison, WI, 1996.
(23) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
10692 J. Org. Chem., Vol. 70, No. 26, 2005