Synthesis of novel heteroditopic carbene-pyridine palladium(II) chloro vinyl complexes. Comparative reactivity of different palladium vinyl derivatives toward transmetalation with alkynyl stannane

Article abstract of DOI:10.1016/j.inoche.2013.03.026

DeC-activated alkynes such as di-t-butyl-ethyne-dicarboxylate (DTA) or the more reactive dimethyl-ethyne-dicarboxylate (DMA) do not react with palladium(II) chloro methyl complexes bearing heteroditopic carbene-pyridine (C-N) as spectator ligands to give the corresponding vinyl derivatives. In order to prepare this type of derivatives we resorted to a dedicated synthetic protocol based on the displacement of labile pyridine-thioether ligands of vinyl palladium pyridine-thioeter species by the carbene-pyridine ligand of carbene-pyridine silver chloride complexes. However, the novel carbene-pyridine vinyl complexes obtained by such synthetic protocol, if compared with other palladium vinyl species with similar steric requirements, display a markedly reduced reactivity toward transmetalation with ethynyl-stannane.

Full text of DOI:10.1016/j.inoche.2013.03.026

Inorganic Chemistry Communications 32 (2013) 74–77
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis of novel heteroditopic carbene–pyridine palladium(II) chloro
vinyl complexes. Comparative reactivity of different palladium vinyl
derivatives toward transmetalation with alkynyl stannane
⁎
Luciano Canovese , Fabiano Visentin, Carlo Levi, Claudio Santo
Dept. Molecular Sciences and Nanosystems, University Ca' Foscari, Venice, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
DeC-activated alkynes such as di-t-butyl-ethyne-dicarboxylate (DTA) or the more reactive dimethyl-ethyne-
dicarboxylate (DMA) do not react with palladium(II) chloro methyl complexes bearing heteroditopic
carbene–pyridine (C–N) as spectator ligands to give the corresponding vinyl derivatives. In order to prepare
this type of derivatives we resorted to a dedicated synthetic protocol based on the displacement of labile
pyridine-thioether ligands of vinyl palladium pyridine-thioeter species by the carbene–pyridine ligand of
carbene–pyridine silver chloride complexes. However, the novel carbene–pyridine vinyl complexes obtained
by such synthetic protocol, if compared with other palladium vinyl species with similar steric requirements,
display a markedly reduced reactivity toward transmetalation with ethynyl-stannane.
Received 8 February 2013
Accepted 24 March 2013
Available online 29 March 2013
Keywords:
Heteroditopic carbene–pyridine ligands
Palladium(II) vinyl complexes
Alkyne insertion reactions
Transmetalation reactions
© 2013 Elsevier B.V. All rights reserved.
The steric and electronic characteristics of spectator ligands play
an important role in determining the catalytic properties and in gen-
eral the reactivity of palladium complexes [1]. A catalytic cycle is usu-
ally represented as a series of chemical events in which the nature of
the spectator ligand might interfere differently at any different step of
the overall process and in the catalyst decay [2]. Among different
types of spectator ligands used in catalysis, nitrogen heterocyclic
carbenes (NHCs) have acquired an increasing importance thanks to
the stability to air and moisture that these ligands impart to their de-
rivatives. Consequently the complexes with NHC as ancillary ligands
are often preferred to those bearing phosphine [3].
The insertion of unsaturated molecules through the Pd\C bond rep-
resents an important step in cross-coupling [4] and co-polymerization
reactions [5]. In the past we have been involved in the study of insertion
of alkynes in methyl complexes of Pd(II) bearing pyridylthioether as an-
cillary ligands giving vinyl derivatives [6]. We therefore decided to in-
vestigate the possibility of obtaining new palladium vinyl species with
NHC as spectator ligands. A potential problem affecting such a synthesis
might however emerge because of the mutual cis position of the NHC li-
gands and the vinyl groups which might undergo reductive elimination
with consequent decomposition of the complexes [7,8]. The use of che-
lating NHC ligands reduces such a risk since the strain imposed by che-
lation should contrast the overlapping of orbitals and consequently the
decay of the compounds [9].
Thus, we have first synthesized the imidazolium salts A, B, and C
which were obtained according to published methods as bromide de-
rivatives [10] whereas D, was synthesized by means of a two step pro-
cedure consisting in the methylation of 2-hydroxy-6-methyl pyridine
by methanesulphonyl chloride followed by nucleophilic attack of the
specific imidazole on 2-chloro-6-methyl pyridine in the presence of
KBr in MeCN. Since the palladium precursors used in this study were
chloride derivatives, in order to avoid possible transhalogenation prod-
ucts we have converted the bromide into the corresponding chloride
salts by means of an ionic exchange resin DOVEX 21 KCL [11]. The chlo-
ride silver complexes 1 were obtained by reacting Ag2O with the appro-
priate imidazolium chloride according to the method of Elsevier et al.
[12] (see Scheme 1).
The imidazolium salt D and the silver derivatives 1 were character-
ized by, IR, 1H and 13C NMR spectroscopy (see Supplementary material).
In particular the formation of the silver compounds was apparent since
the broad signal due the acidic C2H proton of imidazolium salt at ca.
10 ppm in the 1H NMR spectra disappears whereas 13C NMR experi-
ments display the signals due to the carbene carbon coordinated to
the metal at ca. 180 ppm.
Furthermore, we have synthesized the complex 2D by reacting the
silver carbene derivative 1D with the complex [Pd(COD)(Me)Cl] [13]
in order to exploit its supposed high reactivity in promoting the alkyne
insertion and consequently the direct synthesis of the vinyl species,
thanks to the activating presence of a methyl substituent in position 6
of the pyridine ring [6]. Formation of complex 2D is clearly detected
from precipitation of AgCl and its NMR spectrum displaying all the sig-
nals attributable to the carbene ligand together with those related to the
protons of the methyl group coordinated to palladium at ca. 0.8 ppm.
⁎
Corresponding author. Tel.: +39 041 2348571; fax: +39 041 2348517.
E-mail address: cano@unive.it (L. Canovese).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.03.026

L. Canovese et al. / Inorganic Chemistry Communications 32 (2013) 74–77
75
Scheme 1. Ligands and complexes synthesized and studied.
However, at variance with what we expected complex 2D does not
react either with the alkyne di-t-butyl-ethyne-dicarboxylate (DTA) or
with the more reactive dimethyl-ethyne-dicarboxylate (DMA) so that
the vinyl complexes 3 and 4 could not be obtained directly. On the
basis of our previous experience in the synthesis of Pd(0) carbene com-
plexes [13b], we took an alternative approach to complexes 3 and 4
based on the transmetalation between palladium vinyl complexes bear-
ing a labile ancillary ligand and the silver carbene derivatives. Thus, we
resorted to our long-standing protocol exploiting the peculiar reactivity
of palladium complexes bearing substituted pyridyl-thioethers as
spectator ligands. In particular, the complex [Pd(MeN-SPh)(Me)Cl]
(MeN-SPh = 2-methyl-6-phenylsulfanylmethyl-pyridine) displays a
remarkable reactivity toward Pd\C bond insertion due to the distortion
of the main coordination plane of the complex imposed by the methyl
group in position 2 of the pyridine ring [6]. It is noteworthy that
complex 2D does not show similar reactivity despite the presence of
the 2-methyl substituted pyridine in the ligand scaffold since in this
case the increased flexibility of the six-membered ring probably re-
duces the distortion. Thus, we have reacted the silver carbene com-
plexes 1 and the vinyl palladium derivatives of MeN-SPh synthesized
according to the literature [6] in anhydrous CH2Cl2 under strictly 1:1
stoichiometric conditions (Scheme 2).
In any case, the process went smoothly to completion with a reac-
tion time slightly dependent on the steric requirements of the li-
gands. Thus, methyl derivatives 1A and 1D react in a few minutes
whereas the reaction time involved in the case of mesityl and
di-isopropylphenyl complexes is about 1 h.
The 1H NMR spectra of complexes 3 and 4 are characterized by the
presence of two distinct signals (α and β) related to the protons of the
steric group \OCH3 within 3–4 ppm (complexes 3) and of \OC(CH3)3

76
L. Canovese et al. / Inorganic Chemistry Communications 32 (2013) 74–77
Scheme 2. Synthetic protocol for the synthesis of type 3 complexes.
within 1– 2 ppm (complexes 4) and by the singlet ascribable to the
methyl substituent of the vinyl group in all the complexes at ca.
2.5 ppm. Notably, the diastereotopicity of the CH2-Py protons (two dou-
blets centered at ca. 5.5 ppm, J = 14–15 Hz) bears witness to the ligand
chelation. In the 13C NMR spectra of complexes 3 the two signals due to
the \OCH3 carbons resonate at ca. 50 ppm, whereas in the case of com-
plexes 4 the signals of the primary carbons \OC(CH3)3 are at ca. 30 ppm
and those of the tertiary ones \OC(CH3)3 at ca. 80 ppm. In all the com-
plexes studied the carbonyl carbons resonate as distinct signals within
165–175 ppm whereas the methyl substituent of vinyl fragment at ca.
20 ppm and the carbene carbon within 170–180 ppm. Quite interest-
ingly, the low temperature 1H NMR spectra of the vinyl derivatives with-
out the methyl substituent in the pyridine ring (3A–C; 4A–C) in CDCl3
and CD2Cl2 are characterized by the splitting of some signals, indicating
the presence of two isomers in solution. Since the concentration of the
isomers is always comparable and slightly dependent on the ligand
and solvent nature (see Table 1 in Supplementary), we suggest that
the small difference in energy between the species is due to the presence
in solution of two conformers rather than of a couple of N\Cl cis and
N\Cl trans geometric isomers (Scheme 3). As a matter of fact a
computational approach based on the DFT calculation indicates that
the difference of energy in the case of the conformers of complex 3A is
ΔE =0.07 kcal mol−1 whereas the same difference when the cis and
trans isomers are taken into consideration is ΔE = 6.8 kcal mol−1. DFT
calculations were performed with the DMol3 [14] program, and geome-
tries of the compounds were optimized at DFT PBE [15] level using
double numerical polarized basis set (DNP) [16]. Relativistic effects of
core electrons were computed using DFT Semi-core Pseudopotentials [17].
The former result fits nicely with the observed isomeric distribution.
In the case of complexes bearing the methyl substituted pyridines as
chelating ligands (3D and 4D), owing to the weakening of the Pd\N
bond due to the distortion imposed by the methyl group from the
main plane of the complex [4] the fluxionality of the ring is always op-
erative and therefore no isomers are detected at any accessible
temperature.
Furthermore, the 1H NMR spectra of the complexes bearing bulky R2
substituents (3B–C and 4B–C) at 233 K display the splitting of the sig-
nals of the ortho methyl or ortho i-propyl groups owing to the hindered
rotation of the substituted phenyl fragment around the N\C bond.
According to the reaction reported in Scheme 4(A), the Pd(II) vinyl
complexes in the presence of ethynyl-stannanes and deactivated
olefins yield Pd(0) olefin derivatives and the extensively conjugated
1-R″ -1,2-R-2-arylethynyl ethenes. In this respect the described cross-
coupling process can be identified with the final part of a Stille reaction
and reasonably proceeds through the formation of type I intermediates
which rapidly collapse via reductive elimination to the final products
[18]. We therefore decided to compare the influence of different specta-
tor ligands on the reactivity of palladium vinyl complexes by reacting
the compounds reported in Scheme 4B with tributyl-phenylethynyl-
stannane in the presence of fumaronitrile [19] in CDCl3 at 298 K. We
have chosen the complexes 1, 2 [6] and 3D since it is possible to distin-
guish the influence on their reactivity of the fragment of the bidentate
Scheme 3. Schematic representation of the conformation isomers.

L. Canovese et al. / Inorganic Chemistry Communications 32 (2013) 74–77
77
Scheme 4. (A) Transmetalation reaction between palladium vinyl complexes and stannane. (B) Studied Palladium vinyl complexes.
[4] (a) I. Shimuzu, J. Tsuji, J. Chem. Lett. (1984) 203;
ligand bearing phosphine, thioetheric sulfur or carbene carbon, respec-
(b) M. Ahmar, B. Cazes, J. Gore, Tetrahedron Lett, 25 (1984) 4505–4508;
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tively, all the other substituents to palladium being almost the same.
We have monitored the reaction progress by detecting the inten-
sity of the relevant 1H NMR peaks of the reaction product 2-methyl-
3-phenylethynyl-but-2-ene dioic acid dimethyl ester and measured
the reaction time. The reaction times are:
(e) P.W.N.M. van Leeuwen, G. van Koten, Catalysis: an integrated approach to
homogeneous, heterogeneous and industrial catalysis, Elsevier, 1993.
[5] J.H. Groen, C.J. Elsevier, K. Vrieze, W.J.J. Smeets, A.L. Spek, Organometallics 15 (1996)
3445–3455, (and refs. therein).
[6] L. Canovese, F. Visentin, G. Chessa, P. Uguagliati, C. Santo, A. Dolmella, Organome-
tallics 24 (2005) 3297–3308.
[7] K.J. Cavell, D.S. McGuinness, Coord. Chem. Rev. 248 (2004) 671–681.
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1386–1390.
2ð10′ Þb1ð7h Þ b 3D ðmore than 24hÞ:
e
e
The reduced reactivity of complex 3D is clearly due to the compar-
atively higher σ-donor capability of the carbene which, while enhanc-
ing the charge density on the metal, disfavors the substitution of the
chloride with the less electron withdrawing alkenyl group. It is also
possible to state that steric hindrance plays a minor role since the
less crowed derivative 3D displays the lowest reactivity.
[10] (a) A.A.D. Tulloch, A.A. Danopoulos, S. Winston, s. Kleinhenz, G. Eastham, J. Chem.
Soc. Dalton Trans. (2000) 499–4506;
(b) L. Canovese, F. Visentin, C. Levi, C. Santo, V. Bertolasi, Inorg. Chim. Acta 390
(2012) 105–118.
[11] R.H. Crabtree, Organometallics 23 (2004) 2467.
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Appendix A. Supplementary material
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.inoche.2013.03.026.
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