Heterodimetallic palladium(II) complexes with bidentate (N,S) or terdentate (C,N,S)- ferrocenyl ligands. the effect of the ligand donor atoms on the regioselectivity of the allylic alkylation of cinnamyl acetate

Article abstract of DOI:10.1021/om060647d

The synthesis and characterization of [Pd(η³-1-R ¹-C₃H₄){FcCH=N-(C₆H ₄-2SMe)}][PF₆] {R¹ = H (2a) or Ph (3a)} are described. A comparative study of the catalytic activity of (a) the mixtures containing [Pd(η³-C₃H₅)(μ-Cl)] ₂ and its parent ligand (1a) or H₂N-C₆H ₄-2SMe (4) and (b) the palladacycles [Pd-{[(η-C₅H ₃)-CH=N-(C₆H₄-2SMe)]Fe(η⁵-C ₅H₅)}Cl] (5a), [Pd{(2-CH₂-4,6-Me ₂C₆H₂)-CH=N-(C₆H₄-2SMe)} Cl] (5b) in the allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate with sodium diethyl 2-methylmalonate is also reported.

Full text of DOI:10.1021/om060647d

Organometallics 2007, 26, 571-576
571
Heterodimetallic Palladium(II) Complexes with Bidentate (N,S) or
Terdentate (C,N,S)- Ferrocenyl Ligands. The Effect of the Ligand
Donor Atoms on the Regioselectivity of the Allylic Alkylation of
Cinnamyl Acetate
Concepcio´n Lo´pez,*,† Sonia Pe´rez,† Xavier Solans,‡ Merce` Font-Bard´ıa,‡ Anna Roig,§
Elies Molins,§ Piet W.N.M. van Leeuwen, and Gino P.F. van Strijdonck
Departament de Qu´ımica Inorga`nica, Facultat de Qu´ımica, UniVersitat de Barcelona, Mart´ı i Franque`s
1-11, 08028-Barcelona, Spain, Departament de Cristal‚lografia, Mineralogia i Dipo`sits Minerals, Facultat
de Geologia, UniVersitat de Barcelona, Mart´ı i Franque`s s/n, 08028-Barcelona, Spain, Institut de Cie`ncia
de Materials de Barcelona (ICMAB-CSIC), Campus de la UniVersitat Auto`noma de Barcelona,
08193-Bellaterra, Spain, and Van’t Hoff Institute for Molecular Sciences, UniVersity of Amsterdam,
Nieuwe Achtergracht 166, 1018-WV Amsterdam, The Netherlands
ReceiVed July 18, 2006
The synthesis and characterization of [Pd(η3-1-R1-C3H4){FcCHdN-(C6H4-2SMe)}][PF6] {R1 ) H
(2a) or Ph (3a)} are described. A comparative study of the catalytic activity of (a) the mixtures containing
[Pd(η3-C3H5)(µ-Cl)]2 and its parent ligand (1a) or H2N-C6H4-2SMe (4) and (b) the palladacycles [Pd-
{[(η5-C5H3)-CHdN-(C6H4-2SMe)]Fe(η5-C5H5)}Cl] (5a), [Pd{(2-CH2-4,6-Me2C6H2)-CHdN-(C6H4-
2SMe)}Cl] (5b) in the allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate with sodium diethyl
2-methylmalonate is also reported.
Scheme 1. Mechanism for the Palladium-Catalyzed Allylic
Substitutions Using Soft Nucleophiles {S ) Solvent, L and L′
) Monodentate Ligands or (L,L′) ) Bidentate Ligand, LG
) Leaving Group, Nu- ) Nucleophile}
Introduction
Transition metal complexes containing ferrocene derivatives
as ligands have attracted great interest in recent years.1,2 In
compounds of this kind, the presence of two proximal metal
centers with different environments, oxidation numbers, and spin
states may influence their mutual cooperation in a wide variety
of processes, including their electrochemical behavior or even
their reactivity.1-3 Among all the examples reported so far,
palladium(II) derivatives are particularly relevant due to their
physical or chemical properties and their utility in several
fields,4-10 such as homogeneous catalysis.1,10
Palladium(II)-catalyzed allylic substitution is one of the most
common procedures used in organic synthesis to achieve the
* Corresponding author. E-mail: conchi.lopez@qi.ub.es. Tel: (34) 93-
403-91-34. Fax: (34)-93-490-77-25.
† Departament de Qu´ımica Inorga`nica, Universitat de Barcelona.
‡ Departament de Cristal‚lografia, Mineralogia i Dipo`sits Minerals,
Universitat de Barcelona.
§ Institut de Cie`ncia de Materials de Barcelona (ICMAB-CSIC).
Van’t Hoff Institute for Molecular Sciences.
formation of C-C bonds.11,12 In this sort of processes the
catalytic precursor is a complex containing a (η3-C3H5) group
(1) Togni, A., Hayashi, T., Eds. Ferrocenes. Homogeneous Catalysis.
Organic Synthesis and Materials Science; VCH: Weinheim, Germany, 1995.
(2) For recent applications of this type of compounds see for instance
the articles included in the special issue devoted to ferrocene derivatives
published in: J. Organomet. Chem. 2001, 637-639.
(6) Wason, R. W.; McGrouther, K.; Ranatunge-Bandarage, P. R. R.;
Robinson, B. H.; Simpson, J. Appl. Organomet. Chem. 1999, 13, 163-
174.
(3) (a) Long, N. L. Angew. Chem., Int. Ed. Engl. 1995, 34, 21-38. (b)
Astruc, D. Electron Transfer and Radical Processes in Transition Metal
Chemistry, VCH: New York, 1995; Chapter 2. (c) Beer, P. D.; Smith, D.
K. Progress in Inorganic Chemistry; Karlin, K. D., Ed.; John Wiley: New
York, 1999; Vol. 46, p 66. (d) Boyd, P. D. W.; Burrell, A. K.; Campbell,
W. M.; Cocks, P. A.; Gordon, K. C.; Jameson, G. B.; Officer, D. L.; Zhao,
Z. Chem. Commun. 1999, 637-638.
(7) (a) Pe´rez, S.; Lo´pez, C.; Caubet, A.; Bosque, R.; Solans, X.; Font-
Bard´ıa, M.; Roig, A.; Molins, E. Organometallics 2004, 23, 224-236. (b)
Pe´rez, S.; Lo´pez, C.; Caubet, A.; Solans, X.; Font-Bard´ıa, M. J. Organomet.
Chem. 2004, 689, 3184-3196.
(8) Lo´pez, C.; Bosque, R.; Sainz, D.; Solans, X.; Font-Bard´ıa, M.
Organometallics 1997, 16, 3261-3266.
(9) Pe´rez, S.; Lo´pez, C.; Caubet, A.; Pawełczyk, A.; Solans, X.; Font-
Bard´ıa, M. Organometallics 2003, 22, 2396-2408, and references therein.
(10) For the latest advances in the utility of palladium(II) complexes
containing a σ[Pd-C(sp2,ferrocene)] bond in homogeneous catalysis see
for instance: (a) Anderson, C. E.; Donde, Y.; Douglas, C. J.; Overman, L.
E. J. Org. Chem. 2005, 70, 648-657. (b) Moyano, A.; Rosol, M.; Moreno,
R. M.; Lo´pez, C.; Maestro, M. A. Angew. Chem., Int. Ed. 2005, 44, 1865-
1869.
(4) Cornils, B., Herrmann, W. A., Eds. Applied Homogeneous Catalysis
with Organometallic Compounds, 2nd ed.; Wiley-VCH: Weinheim, Ger-
many, 2002.
(5) (a) Koridze, A. A.; Kuklin, S. A.; Sheloumov, A. M.; Dolgushin, F.
M.; Lagunova, V. Y.; Petukhova, I. I.; Ezernitskaya, M. G.; Peregudov, A.
S.; Petrovskii, P. V.; Vorontsov, E. V.; Baya, M.; Poli, R. Organometallics
2004, 23, 4585-4593. (b) Mart´ınez-AÄ lvarez, R.; Go´mez-Gallego, M.;
Ferna´ndez, I.; Manchen˜o, M. J.; Sierra, M. A. Organometallics. 2004, 23,
4647-4654.
(11) Ojima, I. Catalysis Asymmetric Synthesis, 2nd ed.; Wiley-VCH: New
York, 2000.
10.1021/om060647d CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/22/2006

572 Organometallics, Vol. 26, No. 3, 2007
Lo´pez et al.
Scheme 2. Allylic Alkylation of an Asymmetric π-allyl Complex {where LG ) leaving group, L and L′ ) monodentate ligands
or (L,L′) ) bidentate ligand, S ) solvent, and Nu- ) nucleophile}
and either one bidentate or two monodentate ligands. Due to
the relevance of this catalytic process,11,12 many papers have
focused on studying the influence of the nature of the donor
atoms of bidentate organic ligands on the enantioselectivity of
the reaction.13 According to the mechanism accepted for the
catalytic allylic alkylation process with soft nucleophiles
(Scheme 1), when the substrates used have R2 ) R3, the
palladium complex formed in step (b) has a symmetrically
substituted allyl ligand and the nucleophilic attack produces the
formation of a chiral product. However, if R1 * R2, the
intermediates formed in step (b) are nonsymmetrically substi-
tuted palladium(II) allyl complexes, regiocontrol becomes an
issue prior to enantiocontrol,14 and three different products can
be formed: the nonchiral E and Z linear products and the chiral
branched derivative (Scheme 2).
Many papers have focused on studying the use of ferrocene
derivatives or their palladium(II) complexes in enantioselective
allylic alkylation.13,15 However, the effect of the nature of the
donor atoms of the ferrocenyl ligands on the regioselectivity
has not been studied so intesively. Most of the articles deal with
bidentate (P,E) ligands (E ) P′, N, or S),13,16 and recently Kim
et al.17 described the catalytic activity of the two palladium(II)
complexes shown in Figure 1 on the allylic alkylation of
cinnamyl acetate. In view of this, and due to the increasing
interest in palladium(II) complexes derived from thioet-
hers,16,18,19 we were prompted to prepare and characterize [Pd-
(η3-1-R1-C3H4){FcCHdN-(C6H4-2SMe)}][PF6] {R1 ) H (2a)
or Ph (3a)}. A comparative study of the catalytic activity of
2a, its parent ligand 1a, [(2,4,6-Me3C6H2)-CHdN-(C6H4-
2SMe)] (1b), the amine H2N-C6H4-2SMe (4), and the palla-
dacycles 5a7a and 5b18a (Figure 2) in the allylic alkylation of
(E)-3-phenyl-2-propenyl (cinnamyl) acetate with sodium diethyl
2-methylmalonate is also reported.
Results and Discusion
The reaction of [FcCHdN-(C6H4-2SMe)] (1a)7a with the
corresponding [Pd(η3-1-R1-C3H4)(µ-Cl)]2 (R1 ) H or Ph)
complex in a 2:1 molar ratio and an excess of K[PF6] gave [Pd-
(η3-1-R1-C3H4){FcCHdN-(C6H4-2SMe)}][PF6] {R1 ) H (2a)
or Ph (3a)} (Scheme 3). Compounds 2a and 3a are garnet red
and air-stable solids at 298 K. Their IR spectra showed the
absorption due to the [PF6]- anion20 and a sharp and intense
band due to the stretching of the imine group. The shift of this
band to lower frequencies is consistent with the binding of the
ligand to palladium(II) through the imine nitrogen.7,9,18a,19
The crystal structure of 2a contains equimolar amounts of
[Pd(η3-C3H5){FcCHdN-(C6H4-2SMe)}]+ and [PF6]-. The
palladium(II) is bound to the two heteroatoms of the ferrocenyl
ligand and to the allyl group in a η3-fashion. The five-membered
chelate ring has an envelope-like conformation in which the
palladium(II) is out of the main plane defined by the atoms N,
C(12), C(17), and S. The Pd-N and Pd-S bond lengths agree
with those of related palladium(II) complexes with (N,S)
ligands.7a,18,21,22 The differences between the Pd-C(19) and the
Pd-C(21) bond distances {2.14(2) and 2.11(2) Å, respectively}
do not exceed 3σ, but the N-Pd-C(19) bond angle [105.8-
(5)°] is larger than that of S-Pd-C(21) [101.5(5)°]. A similar
variation has been found in [Pd(η3-1,3-Ph2C3H3){(2-Cl-C6H4)-
CHdN-CH(CHMe2)-CH2SPh}](ClO4),22 which also contains
a (N,S) bidentate ligand. The imine adopts the anti-E conforma-
tion, and the C(11)-N bond length is similar to the value
reported for most ferrocenylimines.7a,19,21,23 The methyl group
of the -SMe moiety, the central “-CH-” unit of the allyl
group, and the “Fe(η5-C5H5)” unit are located on the same side
of the coordination plane of the palladium(II).
The crystal structure of 3a‚H2O is formed by two nonequiva-
lent units of [Pd(η3-1-Ph-C3H4){FcCHdN-(C6H4-2SMe)}]+
(I and II, respectively), [PF6]- anions, and H2O molecules. In
I and II (Figure 3), 1a binds to the Pd(II) through the N and S
atoms. For this distribution of groups, the hydrogen atoms on
the ortho site of the C5H4 ring and one of the -CH2- moiety,
in a cis-arrangement to the nitrogen, are close {distances
between these atoms: 2.44 (in I) and 2.77 Å (in II)}.
In cations I and II (a) the sulfur is in a cis-arrangement to
the substituted carbon of the allyl unit; (b) the phenyl ring of
the allyl ligand is in a syn-position in relation to the central
hydrogen of this group; and (c) the five-membered chelates have
an envelope-like conformation.
(12) (a) Trost, B. M. J. Org. Chem. 2004, 69, 5813-5837. (b) Colacot,
T. J. Chem. ReV. 2003, 103, 3101-3118. (c) Helmchen, G.; Ernst, M.;
Paradies, G. Pure Appl. Chem. 2004, 76, 495-505.
(13) (a) van Haaren, R. J.; Goubitz, K.; Fraanje, J.; van Strijdonck, G.
P. F.; Oevering, H.; Coussens, B.; Reek, J. N. H.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M. Inorg. Chem. 2001, 40, 3363-3372. (b) You, S. L.;
Hou, X. L.; Dai, L. X.; Yu, Y. H.; Xia, W. J. Org. Chem. 2002, 67, 4684-
4695. (c) Zhang, W.; Xu, Q.; Shi, M. Tetrahedron Asymmetry 2004, 15,
3161-3169. (d) Faller, J. W.; Wilt, J. C. Organometallics 2005, 24, 5076-
5083.
(14) (a) Oosterom, G. E.; van Haaren, R. J.; Reek, J. N. H.; Kamer, P.
C. J.; van Leeuwen, P. W. N. M. Chem. Commun. 1999, 1119-1120. (b)
Franco, D.; Go´mez, M.; Jime´nez, F.; Muller, G.; Rocamora, M.; Maestro,
M. A.; Mah´ıa, J. Organometallics 2004, 23, 3197-3209.
(15) For the latest advances in this area see for instance: (a) Zheng, W.
H.; Sun, N.; Hou, X. L. Org. Lett. 2005, 7, 5151-5154. (b) Seitzberg, J.
G.; Dissing, C.; Søtofte, I.; Norrby, P. O.; Johannsen, M. J. Org. Chem.
2005, 70, 8332-8337.
In 3a the presence of the phenyl produces a larger twist of
the C5H4 ring in relation to the Cipso-CHdN- moiety {angles
between planes ) 29.7° (for I) and 23.1° (for II)} than for 2a
(where the angle is 20.3°). A similar distortion has also been
found in [Pd(η3-1,3-Ph2C3H3){(2-Cl-C6H4)-CHdN-CH-
(16) Koning, B.; Meetsma, A.; Kellogg, R. M. J. Org. Chem. 1998, 63,
5533-5540.
(17) Co, T. T.; Paek, S. W.; Shim, S. C.; Cho, C. S.; Kim, T. J.; Choi,
D. W.; Kang, S. O.; Jeong, J. H. Organometallics 2003, 22, 1475-1482.
(18) For recent work on organometallic Pd(II) complexes containing
organic ligands with thioether groups: (a) Lo´pez, C.; Pe´rez, S.; Solans, X.;
Font-Bard´ıa, M. J. Organomet. Chem. 2002, 650, 258-267. (b) Masdeu-
Bulto´, A. M.; Die´guez, M.; Martin, E.; Go´mez, M. Coord. Chem. ReV.
2003, 242, 159-201.
(20) Nakamoto, K. IR and Raman Spectra of Inorganic and Coordination
Compounds, 5th ed.; John Wiley & Sons: New York, 1997.
(21) Allen, T. H. Acta Crystallogr. 2002, B58, 380-388.
(22) Adams, H.; Anderson, J. C.; Cubbon, R.; James, D. S.; Mathias, J.
P. J. Org. Chem. 1999, 64, 8256-8262.
(23) Lo´pez, C.; Bosque, R.; Solans, X.; Font-Bard´ıa, M. New J. Chem.
1996, 20, 1285-1292.
(19) Lo´pez, C.; Caubet, A.; Bosque, R.; Solans, X.; Font-Bard´ıa, M. J.
Organomet. Chem. 2002, 645, 146-151.

Heterodimetallic Palladium(II) Complexes
Organometallics, Vol. 26, No. 3, 2007 573
dynamic processes between different isomeric forms. Several
isomers of the cation of 2a could in principle be expected
(Scheme 4), due to (a) the prochirality of the sulfur atom; (b)
the presence of the allyl group; and (c) the asymmetry of the
(N,S) ferrocenyl ligand. The methyl group of the -SMe moiety
and the central carbon of the allyl group (Câ) could be located
on the same side of the coordination plane of the palladium(II)
(A and D in Scheme 4) or on opposite sides (B and C in Scheme
4).18 Thus, the two pairs of cations (A, C) and (B, D) differ in
the configuration of the sulfur center. In addition, it should be
noted that the imine may adopt the anti-E or the syn-Z
Figure 1. Palladium(II)-allyl complexes containing (N,O) ferro-
cenyl ligands used as precursors in allylic alkylation processes.17
conformation. Moreover, if the free rotation around the Cipso
-
Cimine bond of the ligand is inhibited, different rotameric species
could also be present in solution. This should be especially
relevant at low temperatures.
The 1H NMR spectrum of 2a in acetone-d6 at 203 K suggested
the presence of two isomers (2aI and 2aII) of [Pd(η3-C3H5)-
{FcCHdN-(C6H4-2SMe)}]+ in solution in a molar ratio 10.0:
2.2. The {1H-1H}-NOESY spectrum showed (a) that the ligand
adopted the anti-E conformation in 2aI and 2aII and (b) a NOE
interaction between the -SMe protons and the Hγ of 2aI.
anti
This is only possible if they are on the same side of the
coordination plane of the Pd(II).
Figure 2. Ligands and cyclopalladated complexes used in this
The differences detected in the chemical shifts of the signals
due to 2aI and 2aII are very similar to those reported for the
two isomers (endo and exo) of [Pd(η3-C3H5)(L,L′)]+ (Figure
4).16 On this basis, we tentatively postulate that 2aI and 2aII
may differ in the conformation of the allyl group. Furthermore,
the {1H-1H}-NOESY spectrum of 2a showed interchange peaks
between the signals of the methyl groups of 2aI and 2aII,
suggesting that this process mainly affects the Me units of the
two isomers. Dynamic processes involving the cleavage of the
σ(M-SMe) bond have also been reported for [M{[(η5-C5H3)-
CHdN-(C6H4-2SMe)]Fe(η5-C5H5)}(PPh3)]Cl (M ) Pd or
Pt).7b
work.
a
Scheme 3
a (i) In acetone followed by the addition of [Pd(η3-1-R1-C3H4)(µ-
Cl)]2 {in a 1a:Pd(II) molar ratio of 1} and a slight excess (20%) of
K[PF6].
Due to the asymmetry of the (η3-1-Ph-C3H4) unit and the
presence of two different donor atoms (N and S) on 1a, the
number of possible isomers of 3a is greater than for 2a. For 3a
the substituted carbon of the allyl group (Cγ) could be in a cis-
or in a trans-arrangement to the sulfur and the phenyl group
attached to Cγ may be located in the syn- or anti-position.
NMR spectra of 3a in acetone-d6 at 193 K suggested the
coexistence of four isomers (3aI-3aIV, respectively) in molar
ratios of 10.0:1.5:1.6:0.4. {1H-1H}-NOESY experiments indi-
cated that in 3aI the imine had the anti-E conformation and
that the phenyl ring bound to Cγ was in the syn-position. The
upfield shift of the signals due to the SMe units of 3aI and 3aII,
compared with the values of 2aI and 2aII, is similar to that
reported for [M{[(η5-C5H3)-CHdN-(C6H4-2SMe)]Fe(η5-
C5H5)}(PPh3)]Cl (M ) Pd or Pt),7b in which the ligand acts as
a terdentate group and the PPh3 is in a cis-arrangement to the
sulfur atom. These findings suggest that the Cγ and S atoms
were in a cis-arrangement.
(CHMe2)-CH2SPh}](ClO4),22 where the 2-chlorophenyl ring
is out of conjugation with the imine unit by ca. 40.9°.
In order to clarify whether the nature of the substituent on
the allyl group could affect the electronic environment of the
Fe(II) center, 57Fe Mo¨ssbauer studies were performed. In both
cases, the spectra consisted of a single quadrupole doublet, thus
indicating that there was a single iron site.24 It is well-known
that in ferrocene derivatives the presence of electron-donating
groups produces an increase in the quadrupole splitting (∆Eq)
relative to that of ferrocene, whereas electron-withdrawing
substituents have the opposite effect.25 For 2a and 3a, the ∆Eq24
was smaller than that of Fc-H26 (∆Eq ) 2.41 mm/s at 80 K),
thus indicating that the “Pd(η3-1-R1-C3H4){-CHdN-(C6H4-
2SMe)}” moieties have a somewhat stronger electron-withdraw-
ing effect. The differences between the ∆Eq values for 2a and
3a24 indicate that the replacement of the hydrogen in 2a by a
phenyl in 3a increases the electron-pulling ability of the
substituent bound to the ferrocenyl unit.
1
The comparison of the H NMR at 298 and 223 K in CD2-
Cl2 and acetone-d6 suggests that the isomeric processes and
interconversion rates were strongly dependent on the solvent.27
The {1H-1H}-NOESY spectrum of 3a in CD2Cl2 at 188 K
indicated that the ligand had the anti-E conformation in 3aI-
3aIII. For 3aIII and 3aIV the resonances of the Me protons
appeared at lower fields than for 3aI and 3aII. This can be
1
The H NMR spectrum of 2a in acetone-d6 at 298 K had
broad signals. This could be indicative of the existence of
(24) Iron-57 Mo¨ssbauer hyperfine parameters {isomer shift (i.s.), quad-
rupole splitting (∆Eq), and full-width at half-heigth (Γ) in mm/s} at 80 K
for 2a: i.s. ) 0.488(2), ∆Eq ) 2.224(4), Γ ) 0.234(6) and for 3a: i.s. )
0.480(2), ∆Eq ) 2.218(4), Γ ) 0.237(6).
(25) Houlton, A.; Miller, J. R.; Silver, J.; Jassim, N.; Ahmet, M. J.; Axon,
T. L.; Bloor, D.; Cross, G. H. Inorg. Chim. Acta 1993, 205, 67-70, and
references therein.
(27) The 1H NMR spectrum of 3a at 223 K in acetone-d6 showed
extremely broad bands, while in CD2Cl2 3aI-3aIV coexisted in molar ratios
) 10:8.4:8.3:5.4. At 298 K two sets of signals (of relative intensities 10:
1.3 (in acetone-d6) and 10:9.6 (in CD2Cl2) were observed.
(26) Houlton, A.; Bishop, P. T.; Roberts, R. M. G.; Silver, J.; Herberhold,
M. J. Organomet. Chem. 1989, 364, 381-389.

574 Organometallics, Vol. 26, No. 3, 2007
Lo´pez et al.
Figure 3. ORTEP plots of the two nonequivalent cations (I and II) found in the crystal structure of [Pd(η3-1-Ph-C3H4){FcCHdN-
(C6H4-2SMe)}][PF6], 3a·H2O. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)-N(1),
2.118(9); Pd(1)-S(1), 2.351(4); Pd(1)-C(19), 2.128(14); Pd(1)-C(20), 2.121(14); Pd(1)-C(21), 2.127(13); C(11)-N(1), 1.270(12); Pd-
(2)-N(2), 2.078(10); Pd(2)-S(2), 2.367(4); Pd(2)-C(119), 2.159(16); Pd(2)-C(120), 2.088(14); Pd(2)-C(121), 2.184(16); C(110)-C(111),
1.508(13); C(111)-N(2), 1.217(13); N(1)-Pd(1)-S(1), 83.0(2); N(1)-Pd(1)-C(19), 105.6(5); N(1)-Pd(1)-C(21), 168.3(5); C(20)-Pd-
(1)-C(21), 35.1(5); N(2)-Pd(2)-S(2), 81.0(3); N(2)-Pd(2)-C(119), 104.7(5); N(2)-Pd(2)-C(121), 161.7(5); C(120)-Pd(2)-C(121),
37.5(5).
Scheme 4. Schematic View of Some of the Isomeric Forms
Expected for the Cation of Compound 2aa
Figure 4. Palladium(II) allyl complex described by Kellogg et al.16
i
(with R ) Pr).
the methyl protons of the -SMe unit suggests that the four
isomeric forms of 3a interchange in solution.
In a first attempt to clarify the proclivity of the coordinated
allyl group to undergo nucleophilic attacks, 3a was treated with
an excess of sodium diethyl 2-methylmalonate in THF at 298
K (stoichiometric reaction). The reaction was instantaneous and
gave, after workup, a mixture of the linear trans-E derivative
(6) and the branched compound (7) in a molar ratio of 6:7 )
96.3:3.7.
The crystal structure of 3a showed that the substituted carbon
of the allyl unit (Cγ) was in a cis-arrangement to the sulfur and
the phenyl was in a syn-position. This arrangement is the same
a
The wavy lines indicate that the “Fe(η5-C5H5)” moieties may be
located in an upper (or in a lower) plane in reference to that defined
as that found for 3aI. Thus, the formation of 6 arises from the
attack of the nucleophile on CR, which is in a trans-arrangement
to the sulfur in 3aI.
by the C5H4 ring of the ferrocenyl unit.
explained if we assume that in 3aIII and 3aIV the Cγ carbon is
in a cis-arrangement to the nitrogen, while in 3aI and 3aII these
atoms are in a trans-disposition.
There is increasing interest in the study of the utility of
ferrocene derivatives with two different donor atoms and their
Pd(II) complexes in the allylic alkylation and in elucidating the
effects induced by these heteroatoms or the ferrocenyl unit on
the regio- and stereoselectivity of the process.14a,17,18b Therefore,
we decided to study the alkylation of cinnamyl acetate with
sodium diethyl 2-methylmalonate using catalytic amounts of
1a, 1b18a (Figure 2) or 2a (Table 1, entries I-IV). In all cases
the formation of 6 is strongly preferred over that of the branched
product (7). The use of 1b compared to 1a results in a slight
decrease in the regioselectivity (entry II). When 2a was used
The variations detected in the δ(SMe) values for 3aI and 3aII
could be explained in terms of the relative arrangement between
the phenyl and the Me units. Molecular models revealed that if
the Câ-Hâ bond and SMe unit are on the same side of the
coordination plane, the phenyl ring (which is in a syn-position)
should be close to the Me protons. NMR experiments indicated
that in 3aII the Ph and the Me groups are on the same side of
the coordination plane, while in 3aI they are on opposite sides.
The existence of cross-peaks between the four singlets due to

Heterodimetallic Palladium(II) Complexes
Organometallics, Vol. 26, No. 3, 2007 575
Table 1. Results of the Catalytic Allylic Alkylation of Cinnamyl Acetate with Sodium Diethyl 2-Methylmalonatea
t
conversion
(%)
molar ratiob
entry
[Pd]
(h)
6:7
I
II
1a and [Pd( η3-C3H5)(µ-Cl)]2
20
20
3
20
20
44
44
44
100.0
100.0
90.5
100.0
87.8
98.0
90.5
90.7
96.5:3.5
95.8:4.2
94.8:5.2
93.4:6.6
99.0:1.0
98.9:1.1
98.6:1.4
98.8:1.2
1b and [Pd(η3-C3H5)(µ-Cl)]2
III
IV
V
VI
VII
VIII
2a
2a
4 and [Pd(η3-C3H5)(µ-Cl)]2
4 and [Pd(η3-C3H5)(µ-Cl)]2
5ac
5bc
a Experimental conditions: mixtures containing 2.5 × 10-3 mmol of [Pd(η3-C3H5)(µ-Cl)]2 and 5.0 × 10-3 mmol of ligand 1a, 1b, or 4 (entries I, II, V,
and VI) or 5.0 × 10-3 mmol of 2a, 5a, or 5b (entries III, IV, VII, and VIII); 0.5 mmol of the allylic substrate, 1 mmol of sodium diethyl 2-methylmalonate,
b
c
THF (5 mL), and decane (0.258 mmol) at 298 K. Determined by GC. See text.
(Table 1, entries III and IV), the regioselectivity decreased in
comparison to that obtained in entry I.
effects derived from the presence of the bulky (ferrocenyl or
mesityl) groups bound to the nitrogen in 1 or 2a when compared
with 4.
In order to ascertain the influence of the electronic properties
of the nitrogen in this process, a parallel study was carried out
using H2N-C6H4-2SMe (4). In this case, the regioselectivity
(Table 1, entries V and VI) was a bit higher but the process
was slower than that of 1a or 1b.
There may be several reasons for the differences in the
regioselectivity of experiments I-III presented in Table 1.
Previous studies on the reactivity of imines with Pd(II) or Pt-
(II) have shown that in the presence of a base these imines may
hydrolyze, giving the aldehyde and the amine.7a,28 Since these
catalytic studies are performed in basic medium, the formation
of small amounts of 4 cannot be ruled out. The presence of
traces of 4 in the reaction medium would produce a slight
increase in the molar ratio 6:7.
The results presented here have shown the potential utility
of compounds 1a, 1b, 2a, 4, 5a, and 5b as precursors in the
allylic alkylation of cinnamyl acetate. In general terms, for 1a,
1b, and 2a the reactions are faster than for 4. However, the
regioselectivity toward the linear trans-E product decreases.
Cyclopalladated complexes have been used in several catalytic
processes (i.e., Heck, Suzuki, Stille, Sonogashira reactions, aza-
Claisen rearangements, etc.),10,11,30 but their utility in allylic
alkylations has not been studied in detail. Only two cyclopal-
ladated complexes with pincer [P,C,E] ligands have been
tested.30,31 Therefore, the results obtained with 5 constitute the
first step toward further work in this area. Such work could
provide additional information on the role of the palladacycles
in this catalytic process and on the influence of the nature of
the donor atoms of the terdentate ligand on the regioselectivity
and the rate of these reactions. We realize that the metallacycles
may not be present in the active catalytic species. However,
this is also true for the metalated Suzuki and Heck catalysts,
for which the cyclometalated species are excellent precursors
or perhaps resting states.
It should be noted that 1a and 1b are also active in the
catalytic alkylation of other substrates such as cinnamyl chloride
or even rac-R-vinylbenzylacetate.29
There has been considerable recent interest in the utility of
palladacycles in homogeneous catalysis.10,11,30 For this reason,
and to evaluate whether the mode of binding of the ferrocenyl
ligand could affect the effectiveness of the palladium(II)
complex, the regioselectivity, or the rate of the catalytic process,
we also performed a parallel study with 5a and 5b (Figure 2).
Compounds 5 are also active in the allylic alkylation of the
cinnamyl acetate (Table 1, entries VII and VIII). These reactions
were slower than those of the catalytic systems formed by [Pd-
(η3-C3H5)(µ-Cl)]2 and 1a or 1b, and the molar ratios 6:7 were
somewhat higher. The yield was smaller (50%) for 5b than for
5a (76.4%).
Experimental Section
Material and Methods. Compounds 1a,b, 5a,b, and [Pd(η3-1-
R1-C3H4)(µ-Cl)]2 (R1 ) H or Ph) were synthetized as described
previously.7a,18a,32,33 Sodium diethyl 2-methylmalonate (0.5 M in
THF) was prepared from diethyl-2-methyl malonate and NaH in
THF at 273 K. The remaining reagents were obtained from Aldrich
and used as received. The solvents were distilled and dried before
use.34 Elemental analyses were carried out at the Serveis de
Recursos Cientifics i Te`cnics (Univ. Rovira i Virgili). FAB+ mass
spectra were performed at the Servei d’Espectrometria de Masses
(Univ. Barcelona) with a VG-Quattro Fisions instrument and using
All these observations suggest that the nature of the N-donor
atom {sp2 (in 1 or 2a) or sp3 (in 4)} plays a crucial role. This
could be due to several factors such as the different trans-
influences of the N(sp2) and N(sp3) donor atoms or the steric
(28) Lo´pez, C.; Caubet, A.; Pe´rez, S.; Solans, X.; Font-Bard´ıa, M. Chem.
Commun. 2004, 540-541, and references therein.
(29) For cinnamyl chloride: conversion ) 100% (t ) 20 h) and molar
ratios 6:7 ) 98.9:1.1 (for 1a) and 95.9:4.1 (for 1b). For rac-R-vinylben-
zylacetate: conversion (t ) 20 h) ) 61.2% (for 1a) and 51.7% (for 1b),
and the molar ratios of the final products linear:branched were 86.3:13.7
(for 1a) and 80.5:19.5 (for 1b).
(31) Wang, Z.; Eberhard, M. R.; Jensen, C. M.; Matsukawa, S.;
Yamamoto, Y. J. Organomet. Chem. 2003, 681, 189-195.
(32) Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic
Chemistry, A ComprehensiVe Laboratory Experience; John Wiley & Sons:
New York, 1991.
(33) Auburn, P. R.; Mackenzie, P. B.; Bosnich, B. J. Am. Chem. Soc.
1985, 107, 2033-2046.
(34) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, UK, 1996.
(30) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. ReV. 2005, 105, 2527-
2571, and references therein.

576 Organometallics, Vol. 26, No. 3, 2007
Lo´pez et al.
3-nitrobenzylalcohol as matrix. IR spectra were obtained with a
Nicolet 400-FTIR instrument using KBr pellets. Routine H and
mmol)}, cinnamyl acetate (0.5 mmol), and sodium diethyl 2-me-
thylmalonate (1.0 mmol). The reaction was monitored by taking
samples from the reaction mixture. Each aliquot was diluted with
Et2O, washed with water, and dried over MgSO4. Aliquots then
were analyzed by GC using decane (0.258 mmol) as the internal
standard.
1
13C{1H} NMR spectra were obtained with a Gemini-200 MHz and
a Bruker 250-DXR or a Mercury-400 instrument, respectively.
High-resolution 1H NMR spectra and the {1H-1H}-NOESY,
COSY, {1H-13C}-HSQC, and HMBC NMR experiments were
recorded with a Varian VRX-500 or a Bruker Advance-DMX 500
instrument at 298 K. The latter equipment was also used to perform
the VT-NMR experiments. The chemical shifts (δ) are given in
ppm and the coupling constants (J) in Hz. The product distribution
of the alkylation experiments was measured on an Interscience
Mega2 or Trace-DQS apparatus. The Interscience Mega2 was
equipped with a DB1 column, length 30 m, inner diameter 0.32
mm, a film thickness of 3.0 µm, and a flame ionization detector.
The Trace-DQS instrument had a HP-5 column (25 m in length,
0.5 µm film thickness, and 0.2 mm inner diameter) and was
equipped with an electron impact mass detector.
Crystallographic Data. For 2a: orthorhombic, space group
P212121, a ) 9.780(1) Å, b ) 10.513(1) Å, c ) 22.371(1) Å, R )
â ) γ ) 90°, V ) 2300.1(3) Å3, T ) 293(2) K, Z ) 4, Dc ) 1.813
g cm-3, µ ) 1.630 mm-1, MAR345 diffractometer, and 5433
reflections were collected {1712 were unique (Rint ) 0.0302)}. The
structure was solved by direct methods (SHELXS)35 and refined
by full-matrix least-squares (SHELX93).36 Final R indices: R1 )
0.0390 and wR2 ) 0.1059 {I >2σ(I)} and R1 ) 0.0441 and wR2
) 0.1334 (all data), and the Flack coefficient37 was 0.0(6). For
3a‚H2O: triclinic, space group P1h, a ) 10.075(1) Å, b ) 15.148-
(4) Å, c ) 19.533(7) Å, R ) 84.67(3)°, â ) 79.20(5)°, γ ) 83.55-
Synthesis of [Pd(η3-C3H5){FcCHdN-(C6H4-2SMe)}][PF6]
(2a). Ligand 1a (183 mg, 5.46 × 10-4 mol) and K[PF6] (120 mg,
6.52 × 10-4 mol) were added to a solution of [Pd(η3-C3H5)(µ-
Cl)]2 (100 mg, 2.73 × 10-4 mol) in acetone (20 mL). The mixture
was stirred at 298 K for 1 h and then filtered. The filtrate was
concentrated to dryness on a rotary evaporator. The garnet residue
was treated with CH2Cl2 (ca. 5 mL), and the undissolved materials
were removed by filtration and discarded. The filtrate was
concentrated to dryness on a rotary evaporator. The isolated solid
was dried in vacuum for 24 h and then collected (yield: 262 mg,
76%). Crystals of 2a were grown by slow evaporation of a solution
of 2a in a CH2Cl2/hexane (1:1) mixture.
(4)°, V ) 2902(3) Å3, T ) 293(2) K, Z ) 4, Dc ) 1.652 g cm-3
,
µ ) 1.307 mm-1, Enraf-Nonius-CAD4 diffractometer, and 16 857
reflections were collected and used (Rint ) 0.0028). The structure
was solved by direct methods (SHELXS)35 and refined by full-
matrix least-squares (SHELX97).38 Final R indices: R1 ) 0.0508
and wR2 ) 0.1243 {I >2σ(I)} and R1 ) 0.2259 and wR2 ) 0.1657
(all data).
57Fe Mossbauer Studies. Mo¨ssbauer spectra were recorded using
powdered solid samples. The samples were placed in liquid N2,
quenched to 80 K, and transferred to an Oxford Instrument cryostat.
Spectra were collected at 80 K using a constant acceleration
Mo¨ssbauer spectrometer with a 57Co/Rh source. The source was
moved via a triangular velocity wave, and the γ-counts were
collected in a 512 multichannel analyzer. The velocity calibration
was undertaken using a 25 µm thick metallic iron foil. The
Mo¨ssbauer spectral parameters are given to this standard at room
temperature.
Synthesis of [Pd(η3-1-Ph-C3H4){FcCHdN-(C6H4-2SMe)}]-
[PF6] (3a)‚H2O. This product was prepared according to the
procedure described for 2a, but [Pd(η3-1-Ph-C3H4)(µ-Cl)]2 (109
mg, 2.72 × 10-4 mol) was used as the starting material. In this
case, the filtrate obtained after the removal of the inorganic salts
was treated with Et2O. Slow evaporation of the solution produced
the formation of microcrystals of 3a. These were collected, air-
dried, and then dried in vacuum for 24 h (yield: 323 mg, 82%).
Alkylation Reactions. These studies were performed under
nitrogen. The stoichiometric alkylation reaction of 3a was performed
at 298 K by adding an excess of sodium diethyl-2-methylmalonate
(0.8 mL of a 0.5 M solution in THF) to a solution containing 3a
(80 mg, 1.1 × 10-4 mol). The reaction was instantaneous, and H2O
was added after 10 min. The reaction mixture was filtered over
Celite. The filtrate was then treated with Et2O (∼15 mL), and the
organic layer was washed three times with H2O. The organic phase
was dried over MgSO4, and the filtrate was concentrated to dryness
on a rotary evaporator. The residue was dissolved in a minimum
amount of Et2O and passed through a short SiO2 column (4.0 cm
× 0.6 cm). The band released was collected and concentrated to
Acknowledgment. This work was supported by the Minis-
terio de Ciencia y Tecnolog´ıa (Spain).
Supporting Information Available: Characterization data
{elemental analyses and spectroscopic data (MS, IR, and NMR
data)} for 2a and 3a, ORTEP plot of the cationic array of [Pd(η3-
C3H5){FcCHdN-(C6H4-2SMe)}][PF6] (2a) with a selection of
bond lengths and angles (Figure S1), 57Fe Mo¨ssbauer spectra of
solid samples of 2a and 3a‚H2O at 80 K (Figure S2), and full details
of crystallographic analyses of 2a and 3a‚H2O in CIF format. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
OM060647D
(35) Sheldrick, G.M. SHELXS, A computer program for determination
of crystal structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(36) Sheldrick, G.M. SHELX93, A computer program for determination
of crystal structures; University of Go¨ttingen: Go¨ttingen, Germany, 1994.
(37) Flack, H. D. Acta Crystallogr. 1983, A39, 876-881.
1
dryness. The oily residue contained {according to H NMR (500
MHz) and GC} 6 and 7 in a molar ratio 6:7 ) 96.3:3.7.
The catalytic reactions were performed at 298 K in THF (5 mL)
using 5.0 × 10-3 mmol of 2a, 5a, or 5b {or mixtures of [Pd(η3-
C3H5)(µ-Cl)]2 (2.5 × 10-3 mmol) and 1a, 1b, or 4 (5.0 × 10-3
(38) Sheldrick, G. M. SHELX97, A computer program for determination
of crystal structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.