Solvent-free direct cyclopalladation of sulfides on silica gel

Article abstract of DOI:10.1016/j.inoche.2012.09.029

Direct cyclopalladation of benzyl methyl sulfide, benzyl phenyl sulfide and 1,3-bis(methylthiomethyl)benzene using Pd(OAc)₂ can be accomplished using silica gel instead of a solvent. The yields of the corresponding cyclopalladated complexes in

Full text of DOI:10.1016/j.inoche.2012.09.029

Inorganic Chemistry Communications 26 (2012) 64–65
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Inorganic Chemistry Communications
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Solvent-free direct cyclopalladation of sulfides on silica gel
1
1
⁎
Jonathan E. Kukowski , Jessica R. Lamb , Valeria A. Stepanova, Irina P. Smoliakova
Chemistry Department, University of North Dakota, 151 Cornell Street, Stop 9024, Grand Forks, ND 58202-9024, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Direct cyclopalladation of benzyl methyl sulfide, benzyl phenyl sulfide and 1,3-bis(methylthiomethyl)ben-
zene using Pd(OAc)2 can be accomplished using silica gel instead of a solvent. The yields of the corresponding
cyclopalladated complexes in these green transformations are very similar to those obtained using conven-
tional solvent conditions. The X-ray crystallographic data for trans-Pd(HL)2Cl2 (HL=benzyl methyl sulfide)
are reported.
Received 28 June 2012
Accepted 29 September 2012
Available online 5 October 2012
Keywords:
C,S-palladacycle
© 2012 Elsevier B.V. All rights reserved.
S,C,S-palladacycle
Reactions on silica gel
Cyclopalladation of sulfides
Solvent-free synthesis of organic compounds and coordination
complexes is a well-known area of research [1–4] that is becoming
more important due to environmental concerns. When at least one
of the reagents is liquid, sometimes it is possible to use that chemical
as both a reactant and a solvent, e.g. in preparation of acid halides
from the corresponding carboxylic acids using excess SOCl2. However,
this approach is limited to certain reactions and cannot be considered
environmentally friendly. When both reagents are solid, their grind-
ing can result in the formation of the corresponding products [4].
However, this so-called mechanochemical approach has been applied
to a limited scope of reactions usually occurring at room temperature
[1,4]. In another solvent-free methodology, a solid absorbent, such as
silica gel or aluminum oxide, is used as a reaction medium [2,3,5–7].
While organic transformations on silica gel with or without solvents
are well documented, solvent-free preparations of organometallic
compounds are very rare [8,9]. It is noteworthy that there have
been sporadic reports on the preparation of organometallic com-
plexes on silica gel or aluminum oxide in the presence of a solvent
[10–12]. Recently, our group described preparation of C,N- and
C,P-cyclopalladated complexes (CPCs) by direct cyclopalladation
using either Pd(OAc)2 or Na2PdCl4 on silica gel applying solvents on
the purification step only [13–15]. In this communication, we report
the extension of this methodology to the synthesis of C,S- and S,C,
S-types of palladacycles from sulfides.
When Pd(OAc)2 in acetic acid was used, cyclopalladation of sulfide 1a
occurred at 90 °C in 30 min [19]. After treatment with LiCl, the corre-
sponding dimeric CPC 2a was isolated in 72% yield [19]. Solvent-free
reactions of sulfide 1 with Na2PdCl4 on silica gel were performed in a
1:1 ratio of the reagents using room temperature, 50, 80 and 110 °C.
The reaction time was varied from 1 to 72 h. In all experiments with
Na2PdCl4, only the coordination complex Pd(HL)2Cl2 (3a, HL=1a)
was isolated. The highest quantitative yield of 3a was obtained in
the room temperature reaction on SiO2 after 18 h of stirring. When
Na2PdCl4 was replaced by Pd(OAc)2, cyclopalladation of sulfide 1a
occurred at 50 °C to afford CPC 2a in 52% (Scheme 1 and Table 1,
entry 1). The yield of complex 2a was improved by adding the weak
base NaOAc [20] and increasing the time of the reactions (Table 1,
entries 2–5) [21]. Elevating the temperature to 80 °C led to a decreased
yield of the CPC (entry 6).
In all reactions with Pd(OAc)2 after treatment with LiCl, the corre-
sponding coordination complex 3a was isolated as well. The struc-
tures of complexes 2a and 3a were confirmed by 1H and 13C{1H}
NMR spectroscopy as well as by comparing spectroscopic data and
melting points of these compounds with those reported earlier
[17,19]. The structure and trans geometry of the coordination com-
plex in its solid state were proven by X-ray crystallographic analysis
(Fig. 1) [22].
Next, cyclopalladation of benzyl phenyl sulfide (1b) on SiO2 was
studied (Scheme 1 and Table 1, entries 7–9). The best yield (50%) of
the corresponding CPC (2b) was obtained in the reaction with
Pd(OAc)2 at 65 °C for 24 h (entry 8). For comparison, the same
yield of 2b was achieved in the only reported successful palladation
of this sulfide [Pd(OAc)2, acetic acid, 90 °C, 1 h] [23]. Similar to sulfide
1a, in all reactions with 1b, the coordination complex 3b [24] was iso-
lated as well (entries 7–9). Structures of the cyclopalladated and co-
ordination complexes 2b and 3b were confirmed by 1H and 13C{1H}
NMR spectra.
Benzyl methyl sulfide 1a was used in our study for determina-
tion of the best conditions for cyclopalladation of sulfur-containing
preligands on silica gel. Previously, several groups have unsuccessfully
attempted preparation of the corresponding palladacycle from this
preligand using Li2PdCl4 or Na2PdCl4 in refluxing MeOH [16–18].
⁎
Corresponding author. Tel.: +1 701 777 3942; fax: +1 701 777 2331.
E-mail address: ismoliakova@chem.und.edu (I.P. Smoliakova).
Undergraduate researcher.
1
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.09.029

J.E. Kukowski et al. / Inorganic Chemistry Communications 26 (2012) 64–65
65
SR
Pd
Cl
Pd
Cl
1. Pd(OAc)2,SiO2
2. LiCl, acetone
Bn
R
SR
R
+
S
S
Pd(OAc)2
LiCl
Bn
SiO2, 50 oC, 120 h
2
Cl
acetone, rt
SMe
MeS Pd
1a,b
MeS
SMe
a: R = Me
b: R = Ph
Cl
5
2a,b
3a,b
4
Scheme 1. Cyclopalladation of sulfides 1a,b on SiO2.
Scheme 2. Preparation of complex 5 on SiO2.
Young and the University of Minnesota X-Ray Crystallographic Labo-
ratory for the data obtained for complex 3a.
Table 1
Selected reactions of sulfides 1a and 1b with Pd(OAc)2 on SiO2.
Appendix A. Supplementary material
Entry Ligand Base
Temp. (°C) Time, h Yield of 2a,b, % Yield of 3a,b (%)
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1b
1b
1b
None
NaOAc 50
None 50
NaOAc 50
None 50
NaOAc 80
NaOAc 50
NaOAc 65
NaOAc 80
50
18
18
120
120
168
48
120
24
18
52
61
63
67
81
45
31
50
42
18
15
14
16
12
12
26
40
16
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.inoche.2012.09.029.
References
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It is important to note that mixing sulfides 1a and 1b with
Pd(OAc)2 in a 1:1 molar ratio and heating the resulting mixtures
without solvent nor silica gel at 50 °C for 18 h resulted in the forma-
tion of a very dark viscous matter, which did not contain the corre-
sponding cyclopalladated complexes according to analytical TLC.
We also expanded our solvent-free methodology to the metalation
of 1,3-bis(methylthiomethyl)benzene 4, which can give a pincer-type
CPC. In the only report of the successful direct cyclopalladation of this
preligand, the reaction with Pd(OAc)2 followed by treatment with LiCl
furnished the S,C,S-pincer complex 5 “in less than 10% yield” [19]. Sim-
ilarly, in the reaction on SiO2, disulfide 4 reacted with Pd(OAc)2
followed by ligand metathesis with LiCl to afford 8% yield of CPC 5
(50 °C, 120 h; Scheme 2). Increasing the temperature to 80 °C did not
improve the yield of the pincer complex. The structure of compound 5
was confirmed by comparing 1H and 13C{1H} NMR spectra with those
reported earlier [19]. No other complexes were identified in these
transformations.
In summary, it was demonstrated that direct cyclopalladation of
sulfides can occur on silica gel using Pd(OAc)2 without using solvents.
Isolated yields of the sulfide-derived CPCs synthesized in the reac-
tions on silica gel were very similar to those obtained in solutions.
Compared to the corresponding reactions using acetic acid as the sol-
vent, metalation of the sulfides on silica gel occurred at lower temper-
atures, but required a much longer reaction time. To get reproducible
results in reactions on silica gel, efficient stirring was essential.
[21] Typical procedure and data complex 2a: Preligand 1a (0.0214 mg, 0.155 mmol),
Pd(OAc)2 (0.0328 mg, 0.146 mmol), SiO2 (0.0544 mg; 375 mg per 1 mmol of
preligand; 100–230 mesh) and
a stirring bar were added to a small
round-bottom flask and the components were vigorously mixed using spatula
for ca. 1 min. The flask was fitted with a rubber septum and a 1-mL syringe
packed with CaCl2 and then was immersed in a preheated oil bath. After stirring
at 50 °C for 168 h, the mixture was transferred onto a glass filter and washed with
acetone (3×5 mL). LiCl (0.025 g, 0.60 mmol) was added to the filtrate, and the
resulting mixture was stirred overnight. The crude product was purified using
preparative TLC (silica gel, benzene). Complex 2a was isolated as a yellow solid
in 81% yield along with 12% of complex 3a, which appeared as an orange solid.
Mp 140 °C (decomp.); Rf 0.62 (10:1 benzene–acetone). 1H NMR (CDCl3, δ,
ppm): 2.21 (s, 3H,), 3.92 (d, J=14.6 Hz, 1H, CHA), 4.28 (d, J=14.6 Hz, 1H,
CHB), 6.91 (m, 1H, H3 arom), 6.98 (d, J=5.0 Hz, 2H, H4 and H5), 7.41 (d,
J=5.0 Hz, 2H, H6). 13C NMR (CDCl3, δ, ppm): 23.4 (SCH3), 47.3 (CH2S), 123.6
(C4 arom), 125.3 (C3 arom), 126.2 (C5 arom), 135.6 (C6 arom), 146.9 (C2
arom), 148.1 (C–Pd).
Acknowledgment
The authors acknowledge financial support from ND EPSCoR
through NSF grant No. EPS-0-0184442. The Doctoral Dissertation As-
sistantship to V.A.S. was provided by ND EPSCoR through NSF grant
No. EPS-0184442. The authors would like to thank Dr. Victor G.
[22] Crystallographic data for complex trans-3a have been deposited with the Cam-
bridge Crystallographic Data Centre as Supplementary Publication No. CCDC
888235. Copies of the data can be obtained free of charge on application to the
CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax: +44(1223)-336-033;
e-mail: deposit@ccdc.cam.ac.uk). Selected bond lengths (Å): Pd–Cl 2.2982(4),
Pd–S 2.3205(4), S–C(8) 1.8050(18), S–C(7) 1.8237(17), C(1)–C(2) 1.386(2). Se-
lected bond angles [°]: Cl′–Pd–Cl and S–Pd–S′ 180.000(17), Cl′–Pd–S and Cl–
Pd–S′ 85.819(16), Cl–Pd–S and Cl′–Pd–S′ 94.181(16).
[23] R. Bhawmick, P. Bandyopadhayay, Polyhedron 15 (1996) 2923–2926.
[24] Y. Takahashi, A. Tokuda, S. Sakai, Y. Ishii, J. Organomet. Chem. 35 (1972) 415–421.
Fig. 1. Molecular structure of trans-3a.