Synthesis of poly(pyridylthioether) dendrimers incorporating a Fe 2(CO)6 cluster core

Article abstract of DOI:10.1016/j.tet.2004.12.020

New pyridylthioether-based dendrons bearing a thiol moiety at their focal point have been prepared by a convergent synthetic approach. These dendrons were readily attached to a Fe₂(CO)₆ core to generate two-directional dendritic molecules incorporating an iron-carbonyl cluster.

Full text of DOI:10.1016/j.tet.2004.12.020

Tetrahedron 61 (2005) 1755–1763
Synthesis of poly(pyridylthioether) dendrimers incorporating
a Fe2(CO)6 cluster core
a
a
a
b
Gavino Chessa,a, Luciano Canovese, Fabiano Visentin, Claudio Santo and Roberta Seraglia
*
a
`
Dipartimento di Chimica, Universita di Venezia, Calle Larga S. Marta 2137, I-31123 Venezia, Italy
b
`
`
CNR, Centro di Studio sulla Stabilita e Reattivita dei Composti di Coordinazione, Via Marzolo 1, I-35100 Padova, Italy
Received 7 October 2004; revised 17 November 2004; accepted 9 December 2004
Available online 25 December 2004
Abstract—New pyridylthioether-based dendrons bearing a thiol moiety at their focal point have been prepared by a convergent synthetic
approach. These dendrons were readily attached to a Fe2(CO)6 core to generate two-directional dendritic molecules incorporating an iron-
carbonyl cluster.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
co-workers who prepared metallodendrimers containing a
[Fe4S4]2C cluster core unit via replacement of bulky
aliphatic thiols at the four vertices of this cluster by
dendron-functionalized aromatic thiols.12 The main object
of Gorman’s research group, however, was to understand
the relationship between dendritic structure and electro-
chemical properties of encapsulated iron–sulfur clusters.13
Organic–inorganic composites deriving from the encapsula-
tion of metal clusters within dendrimer molecules are
emerging as a new class of nanosized materials that are
expected to have applications in many areas including
catalysis,1 molecular recognition,2 and photoactive device
engineering.3 In these materials, the chemical interaction
between an organic array and the inorganic counterpart can
be either covalent or noncovalent. The noncovalent method
of incorporation, which uses dendrimers as both nanoreactor
that sequesters metal ions and stabilizer, leads to dendrimer-
encapsulated metal nanoclusters. Moreover, the size of such
metallic nanoparticles can be controlled by the size of the
dendrimer template.4 Typically, the synthetic strategy to
achieve encapsulation relies on complexation of metal ions
into the dendrimer interior and their subsequent chemical
reduction to zero-valent form. Most syntheses of these
nanocomposites concern noble metals1,4–7 but dendrimers
Because dendritic wedges bearing a thiol functionality at
their focal point play a key role in the chemistry of both
noncovalent and covalent dendrimer-encapsulated metal
clusters we decided to explore the synthesis of a new series
of thiol-functionalized pyridylthioether-based dendrons.
There are two features of our dendrons that make them
appealing as precursors of dendron-functionalized inorganic
clusters. First, the S–N–S terdentate ligands constituting the
dendritic framework are potential binding sites of transition
metals.14 This, therefore, may enable the preparation of
heterometallic dendritic assemblies. Second, the pyridyl-
thioether-based repeating units are directly connected
through phenyl groups affording a scaffold that could
facilitate the electron transfer between dendron units and a
focal point.
incorporating copper8 have also been reported. Recently,
9
´
Frechet-type dendritic wedges focally functionalized
with a metal-coordinating group, such as a thiol10 or
4-pyridone11 moiety, were assembled around gold clusters
affording dendron-stabilized gold nanoparticles in which the
average size of the metallic nanoparticles seems to be
correlated to the generation number of the dendritic wedges.
A prerequisite to successfully achieve the covalent encap-
sulation of transition-metal clusters using these types of
focally-functionalized dendrons, is an efficient synthetic
methodology that provides easy access to products of high
purity.
The covalent encapsulation of a metal cluster inside
dendritic architectures was first reported by Gorman and
Keywords: Dendrimers; Metallodendrimers; Dendrons; Pyridylthioethers;
Iron-carbonyl cluster.
* Corresponding author. Tel.: C39 0412348570; fax: C39 0412348517;
e-mail: chessa@unive.it
To accomplish such a task we have chosen to investigate the
chemistry of the dinuclear hexacarbonyls of formula
[Fe2(CO)6(SR)2]. It has been previously shown that the
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.12.020

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G. Chessa et al. / Tetrahedron 61 (2005) 1755–1763
reactions between Fe3(CO)12 and bulky aromatic thiols
afford compounds of the type [Fe2(CO)6(m-SR)2] in good
yields.15
Since the order of reactivity of the aryl halides towards
oxidative addition to a Pd(0) complex suggests that
iodopyridines are much more effective substrates than the
corresponding chloropyridines, the chloro-substituted pyri-
dine 2 was converted to the corresponding iodo derivative 6
in 93% yield by sonochemical reaction with acetyl chloride,
and NaI in dry CH3CN.19 Unfortunately, the coupling
reaction of 6 with boronic acid 4 under anhydrous
conditions in the presence of CsF as base20 only afforded
7 in moderate yield (40–60%). However, the yields compare
well with those reported by Lohse,21 who investigated the
reactivity of methyl 4-chloropyridine-2-carboxylate in a
Suzuki reaction. These relatively low yields, presumably
due to the sensitivity to base of the esters functions,
prompted us to pursue the preparation of 5 from diol 3 by a
four-step procedure that proved to give superior results
regarding achievable yield of compound 5.
We focused our efforts on the preparation of analogous
systems with dendritic topologies. The results of the
synthetic studies which allowed us to assembly novel
pyridyl thioether dendrons around an iron-carbonyl core are
presented here.
2. Results and discussion
The synthetic route we have adopted to prepare a
dendrimer-encapsulated Fe2(CO)6 core unit consists of
treating commercially available Fe3(CO)12 with a den-
dron-functionalized aromatic thiol which was synthesized
via convergent methodology.9 The building block designed
for the synthesis of all dendron generations, namely
compound 12, is depicted in Scheme 2, along with the
synthetic approach we have developed for its preparation.
Thus, as outlined in Scheme 2, the hydroxyl functions of 3
were acylated with Ac2O and trietylamine to afford the
corresponding acetate 8 in essentially quantitative yield. To
increase the reactivity of 8 in cross-coupling reactions the
chloro group of this compound was replaced by iodo
employing trans-halogenation conditions (AcCl/NaI,
CH3CN, room temperature, ultrasonic irradiation) to
produce compound 9 in 95% yield. As expected, halopyri-
dine 9 furnished the arylpyridine 10 in excellent yield (96%)
by a Suzuki reaction using boronic acid 4 as coupling
partner of 9, Pd(PPh3)4 as catalyst, NaHCO3 as base, and
toluene/H2O as solvent system. Due to the acetyl groups
protecting the hydroxyl moieties, the solubility of 10 in
organic solvents was significantly enhanced relative to the
unprotected derivative allowing easy separation and puri-
fication by silica gel chromatography of this material after
reaction. Furthermore, the acetate moiety can be selectively
and cleanly removed under mild basic conditions. Accord-
ingly, deacetylation of compound 10 by alkaline metha-
nolysis generated the diol 5, as formed by Suzuki coupling
of 3 with 4, in 98% yield (88% overall yield from 3).
The synthesis of 12 was initiated from the commercial
diacid 1, (Scheme 1) which was converted to dimethyl
4-chloropyridine-2,6-dicarboxylate 2 in 71% yield accord-
ing to suitably modified literature procedures.16 Subsequent
reduction of the ester functionalities to alcohols, using
NaBH4 in methanol,17 afforded diol 3 in 96% yield.
Halopyridine 3 was then treated with boronic acid 4,
prepared18 in high yield and purity from 4-bromobenzene-
thiol, employing the Suzuki cross-coupling conditions
(toluene/H2O, Na2CO3, Pd(PPh3)4) to give the aryl-
substituted pyridine derivative 5 in 61% yield after
chromatographic purification. Although we have obtained
the desired compound 5 in reasonable yield, the purification
step was arduous because of the difficulty in separating 5
from the starting materials. To overcome this problem and
to improve the yield of cross-coupling product an alternative
approach to 5 was explored.
Scheme 1. Synthesis of the intermediates 5 and 7.

G. Chessa et al. / Tetrahedron 61 (2005) 1755–1763
1757
Scheme 2. Synthesis of the building block 12.
Reaction of 5 with methanesulfonyl chloride and triethyl-
amine furnished the corresponding chloride 11 in 96%
yield, which was subsequently converted into its iodo
analogue 12 in 99% yield by reaction with NaI in acetone
because iodides easily undergo a nucleophilic substitution.
On the contrary, direct conversion of diol 5 to diiodide 12 by
The reactivity of the pyridine-based building block 12
evidently emerges in the synthesis of the first-generation
dendron. As a matter of fact, coupling diiodide 12 with
slightly over 2 equiv of thiophenol under standard ether-
ification conditions (DMF/K2CO3) proceeded quickly (2 h,
room temperature) to give the protected first-generation
dendron 13 in 97% yield (Scheme 3).
22
I2/imidazole/PPh3 gave poor yields.
Scheme 3. Synthesis of dendrons: (i) DMF/K2CO3, rt; (ii) thioanisole, TFA, CF3SO3H, rt.

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G. Chessa et al. / Tetrahedron 61 (2005) 1755–1763
It is well-known that some S-thiophenol protecting groups,
such as thioethers, are particularly stable and their selective
removal may be difficult to accomplish in high yield.23
Nevertheless, the tert-butyl moiety in compound 13 could
be easily removed using the thioanisole-trifluoromethane-
sulfonic acid (TFMSA)-trifluoroacetic acid system24 to
produce the desired thiol-funtionalized dendron 14 in 95%
yield. In accordance with a convergent growth strategy thiol
14 was then reacted with diiodide 12, under conditions
similar to those used for 13, to give the second-generation
dendron 15 (79%), which was deprotected to the corre-
sponding thiol 16 in 97% yield by reacting with thioanisole-
trifluoromethanesulfonic acid (TFMSA)-TFA at room
temperature for 4 h (Scheme 3). While the deprotection
proceeds smoothly, the purification of the crude product of
this reaction by silica gel chromatography proved to be
unsuitable affording the thione form of 16 (Chart 1), as
confirmed by NMR and mass spectrometry analysis. Thus to
circumvent this problem, the thiol 16 was purified by
repeated precipitation from dichloromethane–hexane
mixtures.
The third-generation dendritic wedge 17 was prepared in a
similar way to its earlier generation analogue. However, the
coupling of 12 with thiol 16 gives 17 in moderate yields
(61%), presumably due both to steric problems and the
inherent low stability of the thiol moiety. Following the
same procedure that gave compounds 14 and 16, dendron 17
was deprotected to furnish the corresponding thiol deriva-
tive 18 in 92% yield.
All dendrons display NMR spectra consistent with their
structures. The aliphatic regions in the H NMR spectra
1
provide the most important information since the aliphatic
protons give well-resolved signals. The number of singlets
at around 4.25–4.36 ppm due to CH2S protons clearly
corresponds with the generation of the dendron involved.
Moreover, the relative integrations for these signals and the
aromatic proton signals match perfectly with the proposed
structures.
Finally, the conversion from tert-butyl protected dendron to
the corresponding thiol derivative was easily confirmed by
the complete absence of the signal associated with the tert-
butyl moiety coupled with the presence of a new resonance
at 3.52–3.55 ppm for the SH group.
Chart 1. Thione form of pyridylthioether-based dendrons focally
functionalized with a thiol unit.
The last step of our approach involved the assembly of the
Scheme 4. Synthesis of dendrimers with Fe2(CO)6 as core.

G. Chessa et al. / Tetrahedron 61 (2005) 1755–1763
1759
synthesized dendrons around a transition-metal cluster. To
this end we initially reacted Fe3(CO)12 with thiol 14 in THF,
using triethylamine as a base, followed by treatment with
HgCl2 in a one-pot procedure (Scheme 4), which has been
suggested from the communication describing the reactions
of alkyl- and arylmercuric halides with [Et3NH][((-CO)-
((-RS)Fe2(CO)6] complexes.25 According to the above
procedure, the desired cluster compound 19 was obtained
in 74% yield after chromatographic purification. The
analogous reaction of Fe3(CO)12 with dendron-functional-
ized thiols 16 and 18 afforded the iron cluster dendrimers 20
and 21 in 69 and 49% yield, respectively (Scheme 4).
Preliminary experiments confirm that these metal cluster
core dendrimers are capable of binding the same number of
palladium atoms as the tridentate SNS units affording a
metallodendrimer that incorporates two kinds of transition
metal centres in its structure. The preparation and
investigation of these complexes are currently underway.
We are also investigating the electrochemical properties of
these dendritic systems to gain insight into structure-
property relationships.
4. Experimental
The formation of metallodendrimers 19–21 was confirmed
by disappearance of the thiol signal in their 1H NMR
spectra. The 1H NMR spectrum of the third generation
dendrimer 21 in CDCl3 was broad and structureless,
probably as a consequence of restricted movement of the
attached dendrons, but consistent with the proposed
structure in DMSO-d6 at 50 8C. However, the structure of
21 could be established unambiguously by 13C NMR that
showed the expected resonances for the aliphatic and
aromatic carbons of the dendritic building block as well as
the signal for the carbonyl groups of the cluster core. Further
evidence for the formation of 19–21 was provided by their
IR spectra that showed in the carbonyl region the expected
absorbances for complexes of this type.15 To confirm the
molecular masses of the synthesized metallodendrimers,
the MALDI-TOF technique was applied. Unfortunately,
MALDI-TOF mass spectra of molecules 19–21 did not
show the expected molecular ion peaks, but instead signals
corresponding to the mass of the starting dendron were
observed. Therefore these results indicate that the structure
of the iron-carbonyl core dendrimers is too weak to
withstand the conditions of the MALDI-TOF analysis.
4.1. General comments
Tetrahydrofuran (THF) was distilled from sodium benzo-
phenone ketyl immediately before use. Methanol and
˚
acetone were dried and stored over 3 and 4 A molecular
sieves, respectively. N,N-Dimethylformamide (DMF),
dichloromethane, acetonitrile and triethylamine were dis-
tilled from calcium hydride. Other solvents and reagents
were used as received. Flash chromatography was per-
1
formed on 230–400 mesh silica gel (Macherey–Nagel). H
NMR and 13C NMR spectra were obtained in CDCl3
solutions, unless otherwise indicated, on a Bruker Avance
300 spectrometer using the solvent signal as internal
standard. IR spectra were recorded on a Perkin–Elmer
FT-IR spectrometer. Melting points were taken in capillary
tubes with a Buchi 535 apparatus and are uncorrected.
Mass spectra of dendrimers were obtained by Matrix
Assisted Laser Desorption Ionisation Mass Spectrometry
(MALDI-MS), using a Voyager-DE PRO instrument
(Applied Biosystems, Foster City, CA, USA), operating in
positive linear mode. The instrumental conditions were:
acceleration voltage: 20 keV; grid voltageZ93%; guide
wireZ0.3%; delay timeZ200 ns. The matrix used was
(2-(p-hydroxy-phenylazo)benzoic acid (HABA), at a con-
centration of 10 mg/mL in choloroform. 0.5 mg of den-
drimer was dissolved in 1 mL of CHCl3 and 5 mL of this
solution were added to the same volume of the matrix
solution. About 1 mL of the resulting solution was deposited
on the stainless steel sample holder and allowed to dry
before introduction into the mass spectrometer.
Electrospray ionization technique was also used as an
alternative mass analysis in an attempt to provide
characterization of the dendritic complexes. However,
these attempts were unsuccessful.
Though the mass spectrometric studies did not provide
straightforward evidence for the formation of 19–21, the
elemental analysis data coupled with the NMR and IR
spectra offered clear indication that the desired compounds
had been successfully obtained.
ESI mass spectra were obtained using an LCQ (Finnigan,
Palo Alto, CA, USA), operating in positive ion modes. The
entrance capillary temperature was 270 8C and the capillary
voltage was kept at C3 kV. Sample solutions (at a
concentration of about 5!10K6 M in CHCl3) were
introduced by direct infusion at a flow rate of 8 mL/min.
The He pressure inside the trap was kept constant. The
pressure directly read by ion gauge (in the absence of the N2
stream) was 2.8!10K5 Torr.
3. Conclusions
Following a convergent strategy a series of novel pyridyl-
thioether dendrimers that incorporate a Fe2(CO)6 unit in the
core have been assembled by coupling commercially
available triiron dodecacarbonyl with synthetic mono-
dendrons. These dendrons have been constructed by
employing the activated and protected building block
4-[4-tert-butylthio(phenyl)]-2,6-bis(iodomethyl)pyridine
(12) which has been efficiently generated by a multistep
sequence. The key dendron growth steps involved coupling
via Williamson reaction conditions followed by deprotec-
tion of the thiol function. The formation of the novel
metallodendrimers is supported by 1H, 13C, and FT-IR
spectroscopy, and elemental analyses.
4.1.1. Dimethyl 4-chloropyridine-2,6-dicarboxylate (2).
A mixture of anhydrous 4-hydroxypyridine-2,6-dicarb-
oxylic acid (64.45 g, 0.32 mol) and PCl5 (200.2 g,
0.96 mol) in CCl4 (300 mL) was heated under reflux for
4 h. Then dry methanol (200 mL) was added dropwise over
a period of 40 min, and the resulting mixture was heated
under reflux for 1 h. The solvent was evaporated to afford a
tan yellow solid which was dissolved in water (500 mL) and

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G. Chessa et al. / Tetrahedron 61 (2005) 1755–1763
neutralized with Na2CO3. The resulting solid was collected
by filtration, washed with water and dissolved in CHCl3
(400 mL). The organic solution was washed with saturated
aqueous Na2CO3 (3!200 mL), brine (200 mL), and dried
over MgSO4. Removal of the solvent under reduced
pressure and recrystallization of the residue from methanol
gave 2 as white crystals (52.6 g, 71%). Mp 139–140 8C. IR
(KBr), n: 1722, 1710, 1578 cmK1. 1H NMR: d 4.02 (s, 6H,
CH3), 8.29 (s, 2H, PyH). 13C NMR: d 53.9 (CH3), 128.7
(3,5-PyC), 147.2 (4-PyC), 149.8 (2,6-PyC), 164.5 (CO).
Anal. calcd for C9H8ClNO4 (229.6): C, 47.08; H, 3.51;Cl,
15.44; N, 6.10. Found: C, 46.85; H, 3.44;Cl, 15.70; N, 6.00.
residue from methanol gave 6 as a white solid (6.51 g, 93%).
Mp 174–175 8C. IR (KBr) n: 1751, 1558 cmK1. 1H NMR: d
4.03 (s, 6H, CH3), 8.67 (s, 2H, PyH). 13C NMR: d 53.3
(CH3), 106.9 (4-PyC), 137.0 (3,5-PyC), 148.2 (2,6-PyC),
163.8 (CO). Anal. calcd for C9H8INO4 (321.1): C, 33.67; H,
2.51; N, 4.36. Found: C, 33.72; H, 2.66; N, 4.38.
4.1.5. Dimethyl 4-[4-tert-butylthio(phenyl)]pyridine-2,6-
dicarboxylate (7). A solution of 6 (500 mg, 1.56 mmol),
4-(tert-butylthio)phenylboronic acid
4
(393 mg,
1.87 mmol), CsF (473 mg, 3.12 mmol) and Pd(PPh3)4
(90 mg, 0.078 mmol) in dry DME (10 mL) was stirred
under argon at 60 8C for 18 h. After cooling to room
temperature, the reaction mixture was extracted with ethyl
acetate (2!25 mL). The combined organic extracts were
washed with water (2!50 mL) and dried over MgSO4. The
solvent was removed under reduced pressure and the residue
subjected to flash chromatography on silica gel eluting with
CH2Cl2/ethyl acetate 4:1. Removal of the solvent followed
by precipitation of the residue from CH2Cl2/hexane gave 7
as a yellowish solid (251 mg, 45%). Mp 139–140 8C. IR, n:
1741, 1716, 1603 cmK1. 1H NMR: d, 1.34 (s, 9H, t-Bu) 4.06
(s, 6H, CH3), 7.67–7.74 (m, 4H, PhH), 8.55 (s, 2H. PyH).
13C NMR: d 31.0 (C(CH3)3), 46.7 (C(CH3)3), 53.3 (CH3),
125.6 (3,5-PyC), 127.1 (2-PhC), 135.8 (1-PhC), 136.3
(4-PyC), 138.1 (3-PhC), 148.9 (4-PhC), 150.4 (2,6-PyC),
165.2 (CO). Anal. calcd for C19H21NO4S (359.44): C,
63.49; H, 5.98; N, 3.90. Found: C, 63.66; H, 5.91; N, 4.05.
4.1.2. 4-Chloro-2,6-bis(hydroxymethyl)pyridine (3). This
compound was prepared from 2 following a literature
procedure17 in 96% yield.
4.1.3. 4-[4-tert-Butylthio(phenyl)]-2,6-bis(hydroxy-
methyl)pyridine (5). Method A. A stirred mixture of diol
3 (1.00 g, 5.76 mmol), 4-(tert-butylthio)phenylboronic acid
4 (2.42 g, 11.52 mmol), Pd(PPh3)4 (1.33 g, 1.52 mmol) in
toluene (100 mL) and saturated aqueous Na2CO3 (50 mL)
was heated under reflux under argon for 64 h. After cooling
to room temperature the reaction mixture was extracted with
ethyl acetate (3!50 mL) and the combined organic extracts
were dried over MgSO4. The solvent was removed under
reduced pressure and the residue was subjected to flash
chromatography on silica gel eluting with 5–10% methanol
in CHCl3. Removal of the solvent followed by precipitation
of the residue from CH2Cl2/hexane gave 5 as a white solid
(1.07 g, 61%).
4.1.6. 4-Chloro-2,6-bis(acetoxymethyl)pyridine (8). To a
stirred solution of 3 (5.00 g, 28.8 mmol) in dry CH2Cl2
(150 mL) and dry triethylamine (13.11 g, 129.6 mmol) was
added acetic anhydride (11.76 g, 115.21 mmol). The
reaction mixture was heated under reflux under argon for
2 h and then cooled to room temperature. CH2Cl2 (100 mL)
was added, and the resulting solution was washed with
saturated aqueous NaHCO3 (50 mL), water (2!50 mL),
and dried over MgSO4. After filtration, the solvent was
removed from the filtrate and the pure product (7.24 g, 98%)
was obtained as a white solid. Mp 50–51 8C. IR (KBr), n:
Method B. A stirred solution of 10 (4.00 g, 10.32 mmol) and
CH3ONa (4.13 mmol, from 94 mg of Na) in dry methanol
(50 mL) was heated under reflux under argon for 4 h. Then
the reaction mixture was evaporated to dryness and the
residue taken up in CH2Cl2 (150 mL). The resulting organic
solution was washed with water (2!50 mL) and dried over
MgSO4. The solvent was removed under reduced pressure
and the residue was precipitated from CH2Cl2/hexane to
afford 5 as a white solid (3.08 g, 98%). Mp 119–120 8C. IR
(KBr), n: 1610 cmK1. 1H NMR (DMSO-d6): d 1.29 (s, 9H,
t-Bu) 4.59 (d, 4H, JZ5.9 Hz, CH2), 5.44 (d, 2H, JZ5.9 Hz,
OH), 7.62 (s, 2H, PyH), 7.65 (d, 2H, JZ8.5 Hz, 3PhH), 7.78
(d, 2H, JZ8.5 Hz, 2-PhH). 13C NMR (DMSO-d6): d 31.1
(C(CH3)3), 46.5 (C(CH3)3), 64.5 (CH2), 116.0 (3,5-PyC),
127.4 (2-PhC) 133.6 (4-PhC), 137.9 (3-PhC), 138.7
(1-PhC), 147.6 (4-PyC), 162.3 (2,6-PyC). Anal. calcd for
C17H21NO2S (303.4): C, 67.29; H, 6.98; N, 4.62. Found: C,
67.32; H, 6.81; N, 4.49. ESI/MS m/z: 304 [MCH]C (Rel.
Int.Z100%).
1
1732, 1578 cmK1. H NMR: d 2.19 (s, 6H, CH3), 5.20 (s,
4H, CH2), 7.30 (s, 2H, PyH). 13C NMR: d, 20.7 (CH3), 65.9
(CH2), 120.7 (3-PyC), 145.5 (4-PyC), 157.3 (2-PyC), 170.2
(CO). Anal. calcd for C11H12ClNO4 (257.7): C, 51.27; H,
4.69; N, 5.44. Found: C, 51.46; H, 4.84; N, 5.42.
4.1.7. 4-Iodo-2,6-bis(acetoxymethyl)pyridine (9). This
compound was prepared analogously to 6 by reacting 8
(4.45 g, 17.29 mmol), acetyl chloride (4.07 g, 51.87 mmol),
and NaI (18.14 g, 121.03 mmol) in dry CH3CN (90 mL),
except the reaction mixture was sonicated for 18 h instead of
5 h while the bath temperature was allowed to warm to
65 8C. After workup, the crude product was purified by
precipitation from CH2Cl2/hexane to give 9 as a pale yellow
solid (5.73 g, 95%). Mp 116–117 8C. IR (KBr), n: 1751,
4.1.4. Dimethyl 4-iodopyridine-2,6-dicarboxylate (6).
Acetyl chloride (5.15 g, 65.61 mmol) was added to a
mixture of chloropyridine 2 (5.00 g, 21.77 mmol) and NaI
(65.28 g, 435.5 mmol) in dry CH3CN (150 mL) at 0 8C. The
reaction mixture was sonicated for 5 h under an argon
atmosphere maintaining the bath temperature below 50 8C.
After cooling to 0 8C saturated aqueous Na2CO3 (75 mL)
and CH2Cl2 (150 mL) were added. The organic layer was
washed with saturated aqueous Na2S2O3 (100 mL), water
(2!100 mL), and dried over MgSO4. Removal of the
solvent under reduced pressure and recrystallization of the
1
1558 cmK1. H NMR: d 2.18 (s, 6H, CH3), 5.15 (s, 4H,
CH2), 7.66 (s, 2H, PyH). 13C NMR: d 20.8 (CH3), 65.7
(CH2), 106.7 (4-PyC), 129.7 (3,5-PyC), 156.4 (2,6-PyC),
170.3 (CO). Anal. calcd for C11H12INO4 (349.12): C, 37.84;
H, 3.46; N, 4.01. Found: C, 37.95; H, 3.49; N, 4.19.
4.1.8. 4-[4-tert-Butylthio(phenyl)]-2,6-bis(acetoxy-
methyl)pyridine (10). A mixture of boronic-acid 4

G. Chessa et al. / Tetrahedron 61 (2005) 1755–1763
1761
(2.65 g, 12.6 mmol), 9 (4.00 g, 11.46 mmol), Pd(PPh3)4
(264 mg, 0.229 mmol) in toluene (90 mL) and saturated
aqueous NaHCO3 (75 mL) was stirred under argon at 50 8C
for 72 h. After cooling to room temperature, the reaction
mixture was extracted with ethyl acetate (3!50 mL) and
the combined organic extracts were dried over MgSO4. The
solvent was removed under reduced pressure and the residue
was purified by flash chromatography on silica gel eluting
with CH2Cl2/ethyl acetate 4:1. Removal of the solvent
followed by recrystallization of the residue from methanol
gave 10 as a white solid (4.25 g, 96%). Mp 157–158 8C. IR
(KBr), n: 1745, 1736, 1610 cmK1. 1H NMR: d, 1.29 (s, 9H
t-Bu) 4.59 (s, 4H, CH2), 7.62 (s, 2H, PyH), 7.65 (d, JZ
8.5 Hz, 2H, 3-PhH), 7.78 (d, JZ8.5 Hz, 2H, 2-PhH). 13C
NMR: d, 20.9 (CH3), 31.0 (C(CH3)3), 46.4 (C(CH3)3), 66.8
(CH2), 119.0 (3,5-PyC), 127.1 (3-PhC), 134.5 (1-PhC),
137.9 (2-PhC), 138.1 (4-PyC), 149.5 (4-PhC), 156.3 (2,6-
PyC), 170.6 (CO). Anal. calcd for C21H25NO4S (387.49): C,
65.09; H, 6.50; N, 3.61. Found: C, 65.11; H, 6.48; N, 3.42.
anhydrous K2CO3 (2.19 g, 15.86 mmol) and benzenethiol
(1.33 ml, 13.01 mmol) in dry DMF (30 mL) was added 12
(3.32 g, 6.34 mmol) portionwise over 10 min. The resulting
mixture was stirred at room temperature under an argon
atmosphere for 2 h and then evaporated to dryness. The
residue was extracted with CH2Cl2 (100 mL) and the
organic extract was washed with saturated aqueous
Na2CO3 (50 mL), water (2!50 mL), and dried over
MgSO4. After removal of the solvent under reduced
pressure, the resulting crude product was recrystallized
from methanol to give 13 as a white solid (2.98 g, 97%). Mp
54–55 8C. IR (KBr), n: 1599 cmK1. 1H NMR: d, 1.33 (s, 9H,
t-Bu), 4.31 (s, 4H, CH2), 7.22 (tt, 2H, JZ7.1, 1.4 Hz, CH2S-
PhH), 7.27 (tt, 4H, JZ7.3, 1.4 Hz, CH2S-PhH), 7.37 (dd,
4H, JZ8.0, 1.5 Hz, CH2S-PhH), 7.37 (s, 2H, PyH), 7.41 (d,
2H, JZ8.3 Hz, 2-PhH), 7.59 (d, 2H, JZ8.3 Hz, 3-PhH). 13C
NMR: d, 31.4 (C(CH3)3), 41.0 (CH2), 46.8 (C(CH3)3), 119.7
(3,5-PyC), 126.8, 127.4, 129.3, 130.3, 134.5, 136.2, 138.2,
and 138.6 (PhC), 149.2 (4-PyC), 158.4 (2,6-PyC). MALDI-
TOF MS m/z: 488 [MCH]C; calcd for C29H30NS3: 488.7.
4.1.9. 4-[4-tert-Butylthio(phenyl)]-2,6-bis(cloromethyl)-
pyridine (11). Methanesulfonyl chloride (8.49 g,
74.15 mmol) was slowly added over 10 min to a cooled
solution (0 8C) of 5 (7.50 g, 24.72 mmol) in dry CH2Cl2
(100 mL) containing triethylamine (8.0 g, 79.1 mmol)
under argon. The mixture was stirred at room temperature
for 2 h, then was heated to reflux for 14 h and, after cooling
to room temperature, CH2Cl2 (100 mL) and saturated
aqueous NaHCO3 (100 mL) were added. The organic
layer was washed with water, dried over MgSO4 and
filtered. The filtrate was passed through a silica plug eluting
with CH2Cl2. Removal of the solvent under reduced
pressure gave 11 as a pale brown solid (8.10 g, 96%). Mp
82–83 8C. IR (KBr), n 1605 cmK1. 1H NMR: d, 1.35 (s, 9H
t-Bu) 4.74 (s, 4H, CH2), 7.64 (d, JZ8.6 Hz, 2H, 2-PhH),
7.67 (s, 2H, PyH), 7.68 (d, JZ8.6 Hz, 2H, 2-PhC). 13C
NMR: d 31.4 (C(CH3)3), 46.9 (C(CH3)3), 46.9 (CH2), 120.4
(3,5-PyC), 127.5 (2-PhC), 135.2 (4-PhC), 138.1 (1-PhC),
138.4 (3-PhC), 150.5 (4-PyC), 157.4 (2,6-PyC). Anal. calcd
for C17H19Cl2NS (340.31): C, 60.00; H, 5.63; N, 4.12.
Found: C, 59.81; H, 5.58; N, 4.26.
4.1.12. HS-G1 (14). To a stirred solution of 13 (4.22 g,
8.64 mmol) in thioanisole (20 mL) was slowly added
trifluoroacetic acid (2.9 mL, 38.9 mmol) and triflic acid
(2.7 mL, 30.25 mmol). The resulting mixture was kept
stirring at room temperature under an argon atmosphere for
4 h and then CH2Cl2 (100 mL) was added. The organic layer
was washed with saturated aqueous NaHCO3 (50 mL),
water (2!50 mL), and dried over MgSO4. After filtration,
the organic phase was evaporated to dryness and the residue
was purified by repeated precipitation from CH2Cl2/hexane
to give 14 as a yellow solid (354 g, 95%). Mp 110–110.5 8C.
IR (KBr) n: 1605 cmK1. 1H NMR: d, 3.54 (s, 1H, SH), 4.30
(s, 4H, CH2), 7.19 (tt, 2H, JZ7.0, 1.5 Hz, CH2S-PhH), 7.26
(btt, 4H, JZ7.5, 1.5 Hz, CH2S-PhH), 7.33-7.31 (m, 4H,
CH2S-PhH), 7.32 (s, 2H, PyH), 7.34–7.83 (m, 4H, PhH). 13
C
NMR: d, 40.4 (CH2), 118.9 (3,5-PyC), 126.4, 127.5, 128.8,
129.4, 129.9, 132.7, 135.1, and 135.6 (PhC), 148.7 (4-PyC),
157.8 (2,6-PyC). MALDI-TOF MS m/z: 432 [MCH]C,
calcd for C25H22NS3: 432.6.
4.1.13. tert-BuS-G2 (15). This compound was prepared
analogously to 13 by reacting 14 (1.00 g, 2.32 mmol),
K2CO3 (396 mg, 2.87 mmol) and 12 (577 mg, 1.10 mmol)
in dry DMF (15 mL), except the reaction mixture was
stirred at room temperature over a period of 4 h instead of
2 h. After workup, the crude product was purified by flash
chromatography on silica gel eluting with 2.5–5% ethyl
acetate in CH2Cl2 to give 15 as a pale yellow glassy solid
4.1.10. 4-[4-tert-Butylthio(phenyl)]-2,6-bis(iodomethyl)-
pyridine (12). A stirred solution of 11 (3.02 g,
8.86 mmol) and NaI (6.64 g, 44.31 mmol) in dry acetone
(50 mL) was heated under reflux under argon for 2 h. The
solvent was removed under reduced pressure and the residue
was dissolved in CH2Cl2 (100 mL). The organic solution
was washed with saturated aqueous Na2S2O3 (100 mL),
water and dried over MgSO4. The mixture was filtered and
the solvent was removed from the filtrate affording 12 as a
pale yellow solid (4.58 g, 99%). Mp 155 8C Dec. IR (KBr),
1
(987 mg, 79%). IR (KBr) n: 1601 cmK1. H NMR: d, 1.32
(s, 9H, t-Bu), 4.27 (s, 8H, CH2), 4.36 (s, 4H, CH2), 7.17 (tt,
4H, JZ7.2, 1.3 Hz, CH2S-PhH), 7.24 (tt, 8H, JZ7.5,
1.4 Hz, CH2S-PhH), 7.33–7.48 (m, 18H, 2,6-PhH and
CH2S-PhH), 7.29 (s, 4H, Py0H), 7.46 (s, 2H, PyH), 7.59
(d, 2H, JZ8.3 Hz, 3,5-PhH). 13C NMR: d, 30.9 (C(CH3)3),
39.9 (CH2), 40.4 (CH2), 46.3 (C(CH3)3), 118.9 (3,5-Py0C),
119.4 (3,5-PyC), 126.3, 126.9, 127.3, 128.8, 129.2, 129.7,
134.4, 135.6, 135.7, 137.6, 137.8, and 137.9 (PhC), 148.6
(4-Py0C), 149.1 (4-PyC), 157.6 (2,6-PyC), 157.8 (2,6-Py0C).
MALDI-TOF MS m/z: 1132. [MCH]C, calcd for
C67H60N3S7: 1131.7.
1
n: 1603 cmK1. H NMR: d, 1.34 (s, 9H, t-Bu) 4.56 (s, 4H,
CH2), 7.49 (s, 2H, PyH) 7.59 (d, JZ8.5 Hz, 2H, 2-PhH),
7.66 (d, JZ8.5 Hz, 2H, 3-PhH). 13C NMR: d 5.7 (CH2),
30.9 (C(CH3)3), 46.4 (C(CH3)3), 119.7 (3,5-PyC), 126.9
(2-PhC), 134.6 (4-PhC), 137.5 (1-PhC), 137.9 (3-PhC),
149.7 (4-PyC), 158.8 (2,6-PyC). Anal. calcd for C17H19I2NS
(523.21): C, 39.02; H, 3.66; N, 2.68. Found: C, 39.11; H,
3.61; N, 2.83. ESI/MS m/z: 524 [MCH]C (Rel. Int.Z
100%).
4.1.11. tert-BuS-G1 (13). To a stirred suspension of
4.1.14. HS-G2 (16). This compound was prepared

1762
G. Chessa et al. / Tetrahedron 61 (2005) 1755–1763
analogously to 14 starting from 15 (2.00 g, 1.77 mmol) and
purified by repeated precipitation from CH2Cl2/hexane to
give 16 as a yellow glassy solid (1.85 g, 97%). IR (KBr) n:
1607 cmK1. 1H NMR: d 3.55 (s, 1H, SH), 4.28 (s, 8H, CH2),
4.34 (s, 4H, CH2), 7.15–7.45 (m, 38H, PyH, Py0H, PhH and
CH2S-PhH). 13C NMR: d, 39.9 (CH2), 40.4 (CH2), 118.9
(3,5-Py0C), 119.0 (3,5-PyC), 126.3, 127.3, 127.4, 128.8,
129.4, 129.5, 135.5,0 135.6, 135.7, 136.6, 137.6, and 138.1
(PhC), 148.7 (4-Py C), 148.9 (4-PyC), 157.5 (2,6-PyC),
157.7 (2,6-Py0C). MALDI-TOF MS m/z: 1075 [MCH]C,
calcd for C63H52N3S7: 1075.6.
(CH2), 119.1 (3,5-PyC), 126.4, 127.2, 129.2, 129.5, 129.6,
133.1, 136.7, and 137.8 (PhC), 148.2 (4-PyC), 158.8 (2,6-
PyC), 208.7 (CO). Anal. calcd for C56H40Fe2N2O6S6
(1141.01): C, 58.95; H, 3.53; N, 2.46. Found: C, 58.30; H,
3.58; N, 2.50.
4.1.18. [Fe2(CO)6]-[S-G2]2 (20). This compound was
prepared analogously to 19 starting from 16 (1.00 g,
0.931 mmol) and purified by flash chromatography on silica
gel eluted with 7.5–10% ethyl acetate in CH2Cl2 to give 20
as a red glassy solid (778 mg, 69%). IR (KBr) n: 2073, 2037,
1
1996, 1601 cmK1. H NMR: d, 4.25 (s, 16H, CH2), 4.32
(s, 8H, CH2), 7.12–7.41 (m, 76H, PyH, Py0H, PhH and
CH2S-PhH). 13C NMR: d, 39.8 (CH2), 40.5 (CH2), 118.9
(3,5-Py0C), 119.1 (3,5-PyC), 126.3, 126.6, 126.8, 127.3,
128.8, 129.0, 129.7,0 132.5, 134.5, 135.5, 135.7, and 137.6
(PhC), 148.40 (4-Py C), 148.6 (4-PyC), 157.7 (2,6-PyC),
157.8 (2,6-Py C), 207.9 (CO). Anal. calcd for C132H100Fe2-
N6O6S14 (2426.86): C, 65.33; H, 4.15; N, 3.46. Found: C,
64.86; H, 3.94; N, 3.57.
4.1.15. tert-BuS-G3 (17). This compound was prepared
analogously to 13 by reacting 16 (2.00 g, 1.86 mmol),
K2CO3 (311 mg, 2.25 mmol) and 12 (453 mg, 0.866 mmol)
in dry DMF (25 mL), except the reaction mixture was
stirred at room temperature for 18 h instead of 2 h. After
workup, the crude product was purified by flash chroma-
tography on silica gel eluting with CH2Cl2/EtOAc/NEt3
9:1:0.5 to give 17 as a pale yellow glassy solid (1.27 g,
1
61%). IR (KBr) n: 1602 cmK1. H NMR: d, 1.31 (s, 9H,
t-Bu), 4.26 (s, 16H, CH2), 4.30 (s, 8H, CH2), 4.33 (s, 4H,
CH2) 7.15 (tt, 8H, JZ7.0, 1.4 Hz, CH2S-PhH), 7.22 (tt,
16H, JZ7.6, 1.3 Hz, CH2S-PhH), 7.27–7.60 (m, 58H, PyH,
Py0H, Py00H, PhH and CH2S-PhH). 13C NMR: d, 31.0
(C(CH3)3), 39.8 (CH2), 39.9 (CH2), 40.5 (CH2), 46.4
(C(CH3)3), 118.9 (3,5-Py00C), 119.1 (3,5-Py0C), 119.4
(3,5-PyC), 126.4, 126.9, 127.3, 127.4, 128.9, 129.1, 129.2,
129.3, 129.8, 134.5, 135.3, 1300 5.6, 135.8, 137.7,0137.8, and
138.1 (PhC), 148.6 (4-Py C), 149.0 (4-Py C), 149.2
(4-PyC), 157.5 (2,6-PyC), 157.6 (2,6-Py0C), 157.8 (2,6-
Py00C). MALDI-TOF MS m/z: 2418 [MCH]C, calcd for
C143H120N7S15: 2417.5.
4.1.19. [Fe2(CO)6]-[S-G3]2 (21). This compound was
prepared analogously to 19 starting from 18 (1.00 g,
0.423 mmol) and purified by flash chromatography on silica
gel eluted with 5% ethyl acetate in CHCl3 to give 21 as a
pale red glassy solid (520 mg, 49%). IR (KBr) n: 2073,
2036, 1995, 1599 cmK1. 1H NMR (DMSO-d6, 50 8C): 4.25
(bs, 32H, CH2), 4.26 (bs, 16H, CH2), 4.32 (bs, 8H, CH2),
7.10 (bt, 16H, JZ7.2, CH2S-PhH), 7.20 (bt, 32H, JZ7.2,
CH2S-PhH), 7.29–7.47 (m, 116H, PyH, Py0H, Py00H, PhH
and CH2S-PhH). 13C NMR: d, 39.7 (CH2), 39.9 (CH2), 40.5
(CH2), 118.9 (3,5-Py00C), 119.0 (3,5-Py0C), 119.2 (3,5-
PyC), 126.3, 126.7, 126.9, 127.3, 128.8, 129.2, 129.7, 134.5,
135.1, 135.5, 135.7, 1037.6, 138.0, and 138.1 (PhC), 148.6
(4-Py00C), 148.8 (4-Py C), 149.0 (4-PyC), 157.3 (2,6-PyC),
157.5 (2,6-Py0C), 157.8 (2,6-Py00C) 207.8 (CO). Anal. calcd
for C284H220Fe2N14O6S30 (4998.55): C, 68.24; H, 4.44; N,
3.92. Found: C, 68.18; H, 4.25; N, 3.71.
4.1.16. HS-G3 (18). This compound was prepared analo-
gously to 14 starting from 17 (1.27 g, 0.525 mmol) and
purified by repeated precipitation from CH2Cl2/hexane to
give 18 as a yellow glassy solid (1.18 g, 92%).
IR (KBr) n: 1603 cmK1. 1H NMR: d 3.52 (s, 1H, SH), 4.25
(s, 16H, CH2), 4.30 (s, 8H, CH2), 4.34 (s, 4H, CH2) 7.12–
7.38 (m, 82H, PyH, Py0H, Py00H, PhH and CH2S-PhH). 13C
NMR: d 39.7 (CH2), 39.8 (CH2), 40.4 (CH2), 118.9 (3,5-
Py00C), 119.0 (3,5-Py0C), 119.1 (3,5-PyC), 126.3, 127.3,
127.5, 127.6, 128.8, 128.9, 129.1, 129.2, 129.7, 134.9,
135.1, 135.5, 135.7, 1037.6, 137.9, and 138.0 (PhC), 148.6
(4-Py00C), 148.8 (4-Py C), 149.0 (4-PyC), 157.4 (2,6-PyC),
157.5 (2,6-Py0C), 157.8 (2,6-Py00C). MALDI-TOF MS m/z:
2362 [MCH]C, calcd for C139H112N7S15: 2361.4.
Acknowledgements
`
We thank the MURST (Ministero per l’ Universita e la
Ricerca Scientifica e Tecnologica) for financial support and
Mrs. L. Gemelli for technical assistance.
References and notes
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The resulting mixture was stirred under argon for 14 h,
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After the solvent was removed, the residue was purified by
flash chromatography on silica gel eluted with CHCl3 to
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