Synthesis, characterisation, electronic spectra and electrochemical investigation of ferrocenyl-terminated dendrimers

Article abstract of DOI:10.1016/j.tet.2013.03.047

Two new dodecaferrocenyl dendrimers have been prepared using a sixfold Sonogashira coupling reaction and sixfold Huisgen cycloaddition, respectively. In addition, a dendron containing four ferrocenyl groups has been synthesised by conventional synthetic methods. Electrochemical studies have shown that all the ferrocene units are electrochemically equivalent. Moreover, when treated with acid these compounds form multicharged methylium near-infrared-absorbing dendritic dyes with a high extinction coefficient.

Full text of DOI:10.1016/j.tet.2013.03.047

Tetrahedron 69 (2013) 3885e3895
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis, characterisation, electronic spectra and electrochemical
investigation of ferrocenyl-terminated dendrimers
,
a
a
Carolina Villalonga-Barber a , Kalliopi Vallianatou , Spyros Georgakopoulos ,
*
Barry R. Steele a, Maria Micha-Screttas a, Efrat Levin b, N. Gabriel Lemcoff b
a Institute of Biology, Medicinal Chemistry and Biotechnology, National Hellenic Research Foundation, Vas. Constantinou Ave. 48, 11635 Athens, Greece
b Department of Chemistry, Ben-Gurion University of Negev, Beer-Sheva 84105, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2012
Received in revised form 18 February 2013
Accepted 11 March 2013
Available online 15 March 2013
Two new dodecaferrocenyl dendrimers have been prepared using a sixfold Sonogashira coupling re-
action and sixfold Huisgen cycloaddition, respectively. In addition, a dendron containing four ferrocenyl
groups has been synthesised by conventional synthetic methods. Electrochemical studies have shown
that all the ferrocene units are electrochemically equivalent. Moreover, when treated with acid these
compounds form multicharged methylium near-infrared-absorbing dendritic dyes with a high extinction
coefficient.
Keywords:
Dendrimer
Ferrocenyl
Ó 2013 Elsevier Ltd. All rights reserved.
Near-infrared
Carbenium ion
1. Introduction
Ferrocene, an interesting compound widely used as standard in
electrochemistry due to its reversible one-electron redox behav-
Dendrimers are monodisperse hyperbranched macromolecules
and with characteristic structural features: the core of the mole-
cule, the branches spreading out from the core and the functional
groups on the periphery. The properties of dendrimers are domi-
nated by the tunable surface functional groups, which present the
opportunity to develop novel functional materials for medicine,
biology, chemistry, physics and engineering.
Dendrimers are excellent scaffolds for the attachment of chro-
mophores at their peripheral positions and useful optical proper-
ties of dyes located on dendrimer shells have been described.1
Near-infrared (NIR) dyes, with absorption and/or emission in
the spectral range 700e1400 nm, are of great interest because of
their promising applications in biosensing probes, optical com-
munication, photovoltaics, solar cells, etc. These dyes require
a narrow HOMOeLUMO gap, which in organic materials is com-
iour, has been used as the donor substituent in pushepull systems
to generate NIR dyes not only by us,3 but also by others,4 while
ferrocenyl-containing dendrimers have been widely described,5,6
and promising applications include artificial enzymes, molecular
batteries, sensors, photoswitchable hosts, etc. NIR dendrimers are
also known,7 but ferrocenyl NIR dendrimers are, to the best of our
knowledge, only known where the ferrocene substituent is present
in its oxidized ferrocenium state.7k
In this letter, we report the design and synthesis of two novel
dodecaferrocenyl dendrimers using a sixfold Sonogashira coupling
reaction and sixfold Huisgen cycloaddition that form multicharged
methylium near-infrared-absorbing dendritic dyes when treated
with acid.
2. Results and discussion
2.1. Synthesis
monly achieved through extensive
capped with strong electron donor and acceptor substituents.2
Many of the so-called pushepull or De
p-conjugated systems end-
peA systems (D¼donor,
A¼acceptor) share applications in the fields of nonlinear optics
Initially, we had planned to prepare a dendrimer containing
branches of type I, as an analogue of a ferrocene monomer3a pre-
viously synthesised by our group, which formed a stable crystalline
NIR dye when treated with acid. Our proposed route to branches of
this type was via addition of {2,6-dimethoxy-4-[2-(ferrocenyl)
ethenyl]phenyl}lithium (II) to a suitable ester (Scheme 1).
(NLOs) and organic electronics.
* Corresponding author. Tel.: þ30 2107273874; fax: þ30 210727831; e-mail ad-
dresses: carol@eie.gr, carol.villalonga@gmail.com (C. Villalonga-Barber).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.03.047

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C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
Scheme 1.
Because it was anticipated that carbocation formation could be
polymeric products. Ketone 15 was then converted to propynol 12b
by addition of lithium trimethylsilylacetylene with subsequent
desilylation (Scheme 5). Finally a sixfold Sonogashira reaction be-
tween iodine hypercore 8 and acetylene branch 12b gave den-
drimer 16b in 79% yield. Similarly, dendrimers 16c and 16d were
obtained in 68% and 84% yield, respectively (Scheme 6).
Unexpectedly, when the ferrocenyl dendrimer 16b was treated
with trifluoroacetic acid, the generated carbocation species was
found to be very unstable as the purple solution lost its colour
within seconds of its preparation (vide infra). It was assumed that
the instability was due to the presence of the triple bond despite
the fact that smaller triarylmethane dye ethynylogues had been
described to be fairly stable in aprotic solvents.8,13 Only Matsui
and co-workers have reported a remarkably unstable triaryl-
methane dye but its structure was not described.14 An alternative
dendrimer was therefore designed, which contained a triazole
linkage that would be constructed using click chemistry
methodology.15
Synthesis of dendrimer 20 (Scheme 8) required the
hexaacetylene-terminated core 10, which was prepared via the
three-fold esterification of 3,5-bis(2-propynyloxy)benzenemetha-
nol 916 with trimesoyl chloride in the presence of DMAP (see
Scheme 3). In order to prepare the azide-containing branch 18,
bromide 2a was transformed to the corresponding azide and the
product was treated with stilbenyllithium 193 at low temperature
(Scheme 7). In this last step it was important to keep the temper-
ature low in order to avoid side reactions at the azido functionality,
and for this reason the analogous reaction with {2,6-dimethoxy-4-
[2-(ferrocenyl)ethenyl]phenyl}lithium (II) failed.17 Finally, reaction
of core 10 with 8 equiv of azide 18 under Cu(I)-catalyzed Huisgen
‘click reaction’ conditions using the organosoluble copper catalyst
Cu(PPh3)3Br and microwave irradiation18 was found to give the
desired dendrimer 20 in 95% yield. Employing stoichiometric
amount15 (6.2 equiv) of azide 18 led to a dendrimer containing only
3 ferrocenyl branches as indicated by the integration of the signal
corresponding to the heterocyclic proton in the 1H NMR spectrum
and the mass spectral measurement.
a side reaction during branch manipulation, the attachment of the
carbinol-containing branches was planned as a late step in the
synthesis.
We initially synthesised a hypercore (5) lacking any function-
alities sensitive and/or reactive to lithiation (apart from the reactive
peripheral groups). This was prepared by Williamson etherification
between 1,1,1-tris(4-hydroxyphenyl)ethane and bromide 4, the
latter being prepared from ethyl 4-hydroxybenzoate in three steps
and 58% overall yield (Scheme 2). However, the addition of II to core
5 lead to a mixture containing the starting materials and un-
identified by-products and a convergent synthetic approach was
attempted instead. Thus alcohol 3a was first silylated, then reacted
with II and the product deprotected to afford 7b. Unfortunately,
benzyl alcohol 7b could not be transformed to the corresponding
benzyl bromide using mild reaction conditions, e.g., CBr4/PPh3. The
use of PBr3 was ruled out due to the acid sensitivity of the carbinol
functionalities. An alternative route via Mitsunobu reaction be-
tween 7b and trimesic acid was also attempted but the starting
materials were recovered and so an alternative dendrimer was
designed.
We conceived a dendrimer containing acetylene functionalities
between the core and the carbinol-containing branches (16a),
which would increase the conjugation8 of the chromophore units
and present the possibility of dendrimer assembly through
a Sonogashira coupling reaction. To this end, hypercore compound
8, containing the iodo functionalities required for the carbon-
ecarbon bond formation, was prepared by the condensation of
trimesoyl chloride and benzyl alcohol 3b (Scheme 3), the latter
being prepared analogously to 3a (see Scheme 2).
The acetylene-terminated ferrocenyl branch 12a, required for
the palladium-catalysed cross coupling reaction, was synthesized
in 53% overall yield by addition of II onto ethyl 3-
trimethylsilylpropynoate9 followed by in situ base-promoted
desilylation (Scheme 4). However, the envisaged coupling be-
tween 8 and 12a was unsuccessful. This unreactivity might have
been due to the presence of too many electron donating sub-
stituents on the acetylenic branch10 as a Sonogashira coupling was
successfully achieved with 1,1-bis(4-((E)-2-ferrocenylethenyl)phe-
nyl)prop-2-yn-1-ol (12b), 1,1-diphenylprop-2-yn-1-ol (12c) and
1,1-bis(4-methoxyphenyl)prop-2-yn-1-ol (12d)11 (Scheme 6) vide
infra.
2.2. Optical properties
The UVevisible absorption spectra were recorded in chloroform,
both for the carbinols and for the corresponding triarylmethyl dyes
generated by adding solutions of trifluoroacetic acid to solutions of
the carbinols.
The absorptions at around 450 nm for the ferrocene containing
carbinols (7a, 7b, 16b and 20) are due to ferrocene ded bands.19
When this carbinols are treated with acid, carbocations are gen-
erated20 and their electronic spectra exhibit an additional low-
The modified acetylenic branch 12b was synthesized from 4,40-
bis(bromomethyl) benzophenone,12 which was transformed to the
corresponding bis-phosphonate (14) and then reacted with ferro-
cenecarboxaldehyde to generate ketone 15 in 49% yield as the only
geometric isomer (two steps, Scheme 5). In the latter Horner
Emmons reaction it was important to use an excess of the aldehyde
and run the reaction at high dilution to avoid the formation of
energy bands, one in the visible region (
l¼540e600 nm), which

C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
3887
Scheme 2. Reagents and conditions: i, 1,4-dibromobutane, K2CO3, acetone, 56 ꢀC (2a: 89%; 2b: 89%); ii, 3,5-dihydroxybenzylalcohol, K2CO3, 18-crown-6 (or TBAI for 3b), acetone,
56 ꢀC (3a: 75%; 3b: 93%); iii, CBr4, Ph3P, CH2Cl2, 25 ꢀC (87%); iv, (1,1,1-tris(4-hydroxyphenyl)ethane, K2CO3, 18-crown-6, acetone, 56 ꢀC (85%); v, TBSCl, imidazole, CH2Cl2 rt (quant.);
vi, II, Et2O, 35 ꢀC (72%); vii, TBAF, THF, rt, 3 h (93%).
is assigned as charge transfer (CT) and another in the NIR region
¼850e1025 nm) assigned as metal to ligand charge transfer
(MLCT) (dpe *), processes, which are in accordance with Barlow’s
theoretical model.21 These bands present both a bathochromic shift
in respect to -conjugated triphenylmethane dyes due to the
lowering of the energy of the MLCT transitions in the ferrocenyl
dyes.4c The broad MLCT band that appears in the NIR region shows
a pH dependency, similar as the one found for smaller molecules;3
as the acid concentration is increased, the NIR band gradually de-
creased (except for compound 20). The disappearance of the NIR
band at high acid concentration is attributed to the protonation of
the iron atom of the ferrocenyl groups, which removes the
possibility of MLCT.22 Similar pH dependency has been observed for
dialkylamino terminated -conjugated triphenylmethane dyes
(l
p
p
where N-protonation weakens and finally cancels the pushepull
character of these carbacationic dyes.7a,23
p
The acid solution of carbinol 16b was highly unstable and a rapid
colour change (green, purple and brown successively) could be
observed seconds after its preparation. This instability had not been
anticipated based on known stable ethynologs of triphenylmethane
dyes prepared by Akiyama9,13 although it is known that propargylic
alcohols rearrange in acidic conditions to
a,b-unsaturated ke-
tones.24 By contrast, compound 16c needed a couple of hours to
react with the acid and turn blue.

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C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
Scheme 3. Reagents and conditions: i, benzene-1,3,5-tricarbonyl chloride, 3b or 9, DMAP, CH2Cl2, 40 ꢀC, 18 h (8: 90%; 10: 78%).
Scheme 4. Reagents and conditions: i, ethyl 3-trimethylsilylpropynoate, Et2O, 40 ꢀC, 18 h; ii, KOH, MeOH, THF, 25 ꢀC, 2 h (53% over two steps).
Scheme 5. Reagents and conditions: i, P(OEt)3, 156 ꢀC, 3 h (65%); ii, FcCHO, NaH, THF, 66 ꢀC, 30 min (76%); iii, (a) TMS-acetylene, n-BuLi, THF, ꢁ78 ꢀC to rt, 18 h; (b) KOH, MeOH, rt,
1.5 h (85%).
Compound 20 behaved slightly differently and needed higher
concentrations of acid in order to reach the maximum absorption of
the NIR band. Once reached, this band also disappeared at lower
pH. Presumably protonation of the multiple triazole rings occurs
first and therefore a higher concentration of acid is required to
achieve quantitative carbocation formation.
most probably due to the steric crowding around the central carbon
atom that does not allow these dyes to achieve planarity.
2.3. Electrochemical properties
The electrochemical properties of compounds 7a, 16b and 20
were investigated by CV with dichloromethane as solvent in the
presence of tert-butylammonium perchlorate as the supporting
Carbocations obtained from dendrimers 16b and 20 present an
NIR band red-shifted with respect to branches 7a and 7b. This is

C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
3889
Scheme 6. Reagents and conditions: iv, Pd(PPh3)2Cl2, CuI, Et3N, DMF, acetylene 150 ꢀC, (16a: 0%, 16b: 79%, 16c: 68%, 16d: 84%).
Scheme 7. Reagents and conditions: i, NaN3, H2O/acetone,
D
, 18 h (75%); ii, FcC2H4C6H4Li (19), THF, ꢁ78 ꢀC, 30 min (65%).
electrolyte, using graphite rods as working and counter electrodes
and an Ag/Agþ reference electrode at ambient temperature. The
redox process was found to be very similar for the three com-
pounds, which display a quasi-reversible one-electron redox wave
attributed to the Fe(III)/Fe(II) redox couple (iox/ired¼w1), therefore
indicating that the four (7a) and twelve (16b and 20) ferrocene
units are electrochemically equivalent. The redox potentials are
summarized in Table 2.

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C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
Scheme 8. Reagents and conditions: i, Cu(PPh3)3Br, DIPEA, THF, MW, 140 ꢀC, 20 min (95%).
Table 2
Table 1
Redox potentials of ferrocenyl dendrimersa
Electronic spectra of carbinols and their carbenium ions
Entry
Compound
D
E (mV)
E0 (mV)
Compound
[Dye] M
[TFA] M
lVis (log
3 )
lNIR (log
3 )
1
2
3
7a
16d
20
104
126
765
þ61
þ58
þ5
7a
1.50E-4
1.50E-5
3.70E-5
1.8E-4
1.8E-5
1.8E-5
1.8E-5
2.1E-5
1.0E-5
4.2E-4
2.1E-5
1.0E-5
1.0E-5
5.0E-5
5.0E-6
5.0E-6
d
453 (3.60)
594 (4.75)
592 (4.68)
453 (371)
594 (4.79)
600 (4.85)
603 (4.77)
449 (4.25)
517 (3.63)
268 (3.80)
576 (4.96)a
260 (5.16)
576 (5.25)
453 (4.19)
544 (5.19)
540 (5.44)
2.0E-4
5.2E-2
d
858 (4.46)
d
a
7b
Experimental conditions: CH2Cl2; TBAP (0.1 M); concentration in electroactive
compound: 1ꢂ10ꢁ3 M; Ag/Agþ reference; 100 mV Sꢁ1 sweep rate.
5.2E-3
5.2E-2
1.0E-1
d
855 (4.50)
865 (4.12)
d
16b
16c
16d
1.3E-2
d
959 (4.5)
3. Conclusions
2.6E-2
d
In conclusion, dendrimers with ferrocenyl units at the surface
were synthesised in good yields. The cyclic voltammetry studies
revealed that they all undergo a reversible single electron transfer
process. When treated with acid these dendrimers absorbed with
a high extinction coefficient in the near-infrared area, although the
dye derived from 16b is very unstable.
3.9E-1
d
20
20
5.2E-2
2.6E-1
999 (4.85)
1024 (5.11)
a
This band reached its maximum intensity in about 2 h after addition of the acid.

C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
3891
4. Experimental section
4.1. General details
filtered, evaporated under reduced pressure and purified by col-
umn chromatography (100% CH2Cl2) to give benzyl alcohol 3a
(2.67 g, 75%) as a white solid, mp 91e93 ꢀC; Rf 0.28 (50% Et2O in
hexane); nmax 3526, 2963, 2987, 1712, 1604, 1245, 1164 cmꢁ1
; dH
All reactions requiring dry or inert conditions were conducted in
flame-dried equipment under an atmosphere of argon. Ethers were
distilled from sodium-benzophenone; (chlorinated) hydrocarbons,
amines, MeOH and DMF from CaH2. Reactions were monitored by
TLC using commercially available Merck silica gel 60 F254 glass-
backed plates. Visualisation of reaction components was achieved
with a UV lamp (254 nm) and with anisaldehyde or KMnO4 stains.
Silica gel column chromatography was carried out on silica gel 60
(300 MHz, CDCl3) 7.99 (4H, d, J 8.9, 4ꢂ CH), 6.91 (4H, d, J 8.9, 4ꢂ CH),
6.52 (2H, d, J 2.1, 2ꢂ CH), 6.38 (1H, t, J 2.1, CH), 4.63 (2H, d, J 5.9,
CH2OH), 4.53 (4H, q, J 7.1, 2ꢂ OCH2CH3), 4.10e4.00 (8H, m, 4ꢂ
CH2O), 2.10e2.00 (8H, m, 4ꢂ CH2) and 1.39 (6H, t, J 7.1, 2ꢂ CH3); dC
(75 MHz, CDCl3) 166.4 (C, quat.), 162.6 (C, quat.), 160.3 (C, quat.),
143.4 (C, quat.), 131.5 (CH), 122.8 (C, quat.), 113.9 (CH), 105.1 (CH),
100.5 (CH), 67.6 (CH2), 67.4 (CH2), 65.3 (CH2), 60.6 (CH2), 25.9 (CH2,
two overlapping carbons) and 14.4 (CH3). MS (EI) m/z¼603.29
([MþNa]þ, 100%). HRMS (ESI): [MþH]þ, found 581.27582. C33H41O9
requires 581.27451.
(particle size 40e63
mm) as supplied by Merck. Size exclusion
column chromatography was performed with Bio BeadsÒ S-X1. 1H
and 13C NMR spectra were recorded with a Varian 300 (300 MHz
and 75 MHz for 1H and 13C, respectively) or a Varian 600 (600 MHz
and 150 MHz for 1H and 13C, respectively). Chemical shifts are re-
ported relative to the internal solvent [e.g., dH 7.27, dC (central line of
t) 77.0 for CDCl3]. Coupling constants (J) are given in Hertz. Mass
spectra were obtained using a Thermo Scientific LCQ Fleet (ESI)
spectrometer. HRMS were determined by using a Thermo Scientific
LTQ Orbitrap Velos (ESI) spectrometer. MALDI-TOF mass spec-
trometry was performed on a BRUKER DALTONICS REFLEX IV
Matrix-Assisted Laser Desorption Ionization Time-Of-Flight spec-
trometer operating in linear mode using dithranol or 2,5-
dihydroxybenzoic acid as matrix. IR spectra were obtained on
a Brucker ATR-IR spectrometer Model Tensor 27. UVevisible spec-
tra were recorded on a Hitachi Model U-2001. Melting points (ꢀC)
are uncorrected.
4.2.3. Dendron 4. A mixture of alcohol 3a (0.58 g,1.0 mmol), carbon
tetrabromide (0.46 g, 1.40 mmol) and triphenylphosphine (0.36 g,
1.4 mmol) in CH2Cl2 (20 mL) was stirred at 25 ꢀC for 3 h. Then H2O
(15 mL) was added and the product extracted into CH2Cl2
(3ꢂ15 mL), washed with brine (15 mL), dried and evaporated under
reduced pressure. The crude product was purified by column
chromatography (gradient elution: 100% CH2Cl2 to 20% Et2O in
CH2Cl2) to give benzyl bromide 4 (0.56 g, 87%) as a white solid, mp
88e90 ꢀC; Rf 0.71 (80% Et2O in hexane); nmax 2961, 1704, 1604, 1249,
1162, 766 cmꢁ1
;
dH (300 MHz, CDCl3) 7.98 (4H, d, J 8.9, 4ꢂ CH), 6.91
(4H, d, J 8.9, 4ꢂ CH), 6.53 (2H, d, J 2.2, 2ꢂ CH), 6.38 (1H, t, J 2.2, CH),
4.40 (2H, s, CH2Br), 4.35 (4H, q, J 7.1, 2ꢂ OCH2CH3), 4.09 (4H, t, J 5.5,
2ꢂ CH2O), 4.02 (4H, t, J 5.5, 2ꢂ CH2O), 2.00e1.96 (8H, m, 4ꢂ CH2)
and 1.38 (6H, t, J 7.1, 2ꢂ CH3); dC (75 MHz, CDCl3) 166.2 (C, quat.),
162.5 (C, quat.), 160.1 (C, quat.), 139.6 (C, quat.), 131.4 (CH), 122.7 (C,
quat.), 113.9 (CH), 107.4 (CH), 101.3 (CH), 67.5 (CH2), 67.4 (CH2), 60.5
(CH2), 33.5 (CH2), 25.8 (CH2), 25.7 (CH2) and 14.3 (CH3). ESI mass
spectrum: MS (EI) m/z¼642.79/644.77 (1:1) ([M]þ, 100%), 550.82
(85), 246.84 (95). HRMS (ESI): [MþH]þ, found 643.18836.
C33H40Br79O8 requires 643.19011.
Cyclic voltammograms (CV) were carried out using a PalmSense
potentiostat (Palm Instruments, Houten, The Netherlands). The
cyclic voltammetry was performed in a three-electrode standard
cell, using graphite rod as working and counter electrodes and an
Ag/Agþ reference electrode. Measurements were conducted in
CH2Cl2 containing 100 mM Bu4NClO4. Solutions of the solutions of
the ferrocenyl derivatives were 1 mM in concentration. The refer-
ence electrode was separated from the bulk solution by a fritted-
glass bridge filled with 100 mM AgNO3 solution.
Microwave reactions were carried out with a CEM Discover
300 W monomode microwave instrument. The closed vessels used
were special glass tubes with self-sealing septa that controlled
pressure with appropriate sensors on the top (outside the vial). The
temperature was monitored through a non-contact infrared sensor
centrally located beneath the cavity floor.
4.2.4. Hypercore 5. A mixture of bromide 4 (1.8 g, 2.79 mmol),
1,1,1-tris(4-hydroxyphenyl)ethane (0.24 g, 0.79 mmol), K2CO3
(0.87 g, 6.3 mmol) and 18-crown-6 (0.02 g, 0.07 mmol) in acetone
(50 mL) was heated at 56 ꢀC. After 20 h the reaction mixture was
cooled, filtered, evaporated under reduced pressure and purified
by size exclusion column chromatography (100% CH2Cl2) to give
a glassy solid, core 5 (1.34 g, 85%); Rf 0.41 (40% EtOAc in hexane);
nmax 2953, 1706, 1604, 1508, 1249, 1165, 770 cmꢁ1
; dH (300 MHz,
CDCl3) 7.99 (12H, d, J 8.8, 12ꢂ CH), 7.01 (6H, d, J 8.5, 6ꢂ CH), 6.91
(12H, d, J 8.8, 12ꢂ CH), 6.86 (6H, d, J 8.5, 6ꢂ CH), 6.58 (6H, s, 6ꢂ
CH), 6.40 (3H, s, 3ꢂ CH), 4.95 (6H, s, CH2), 4. 35 (12H, q, J 7.1, 6ꢂ
OCH2CH3), 4.15e3.95 (24H, m, 12ꢂ CH2O), 2.11 (3H, s, CH3),
2.10e1.95 (24H, m, 12ꢂ CH2) and 1.38 (18H, t, J 7.1, 6ꢂ CH3); dC
(75 MHz, CDCl3) 166.3 (C, quat.), 162.6 (C, quat.), 160.3 (C, quat.),
156.7 (C, quat.), 142.0 (C, quat.), 139.5 (C, quat.), 131.5 (CH), 129.6
(CH), 122.8 (C, quat.), 113.9 (CH, two overlapping carbons), 105.8
(CH), 100.7 (CH), 69.9 (OCH2), 67.6 (OCH2), 67.4 (OCH2), 60.6 (CH2),
50.6 (C, quat.), 30.7 (CH3), 25.9 (CH2), 25.8 (CH2) and 14.3 (CH3).
MALDI mass spectrum: calcd for C119H132NaO27 [MþNa]þ: 2015.88.
Found: 2015.61.
4.2. Synthesis
4.2.1. Ethyl 4-(4-bromobutoxy)benzoate (2a).25 Ethyl 4-hydroxy-
benzoate (2.5 g, 15.0 mmol) was added over a period of 15 min to
a solution of 1,4-dibromobutane (13.0 g, 30.0 mmol) and K2CO3
(4.2 g, 30.0 mmol) in acetone (50 mL) at 56 ꢀC. The resulting mixture
was heated at 56 ꢀC for 4 h, then cooled, filtered, evaporated under
reduced pressure and purified by column chromatography (20%
CH2Cl2 in hexane) to give bromo-ether 2a (4.0 g,13.28 mmol, 89%) as
a white solid, Rf 0.65 (50% Et2O in hexane); nmax 2955, 1706, 1605,
1509,1273,1101, 769 cmꢁ1
;
dH (300 MHz, CDCl3) 8.00 (2H, d, J 8.9, 2ꢂ
CH), 6.91 (2H, d, J 8.9, 2ꢂ CH), 4.36 (2H, q, J 7.1, OCH2CH3), 4.06 (2H, t,
J 6.2, CH2O), 3.50 (2H, t, J 6.2, CH2Br), 2.07e1.97 (4H, m, CH2) and 1.37
(3H, t, J 7.1, CH3); dC (75 MHz, CDCl3) 166.3 (C, quat.), 162.5 (C, quat.),
131.5 (CH), 122.9 (C, quat.), 113.9 (CH), 66.9 (CH2), 60.6 (CH2), 33.3
(CH2), 29.3 (CH2), 27.7 (CH2) and 14.4 (CH3).
4.2.5. Dendron 6. A mixture of benzyl alcohol 3a (4.4 g, 7.5 mmol),
tert-butyldimethylsilyl chloride (2.3 g, 9.1 mmol) and imidazole
(1.5 g, 22.5 mmol) in CH2Cl2 (40 mL) was stirred at 25 ꢀC. After 2 h
the reaction mixture was filtered and the solution was washed
with brine, dried and evaporated under reduced pressure to give
silyl ether 6 (5.2 g, quant.) as a white solid, mp 85e86 ꢀC; Rf 0.6
(50% Et2O in hexane); nmax 2953, 2857, 1728, 1604, 1162, 1057,
4.2.2. Dendron 3a. A mixture of bromide 2a (4.0 g, 13.3 mmol), 3,5-
dihydroxybenzyl alcohol26 (0.84 g, 6.0 mmol), K2CO3 (2.9 g,
21.0 mmol) and 18-crown-6 (0.2 g, 0.7 mmol) in acetone (50 mL)
was heated at 56 ꢀC. After 48 h the reaction mixture was cooled,
832 cmꢁ1
d, J 8.7, 4ꢂ CH), 6.48 (2H, d, J 2.1, 2ꢂ CH), 6.32 (1H, t, J 2.1, CH),
;
dH (300 MHz, CDCl3) 7.99 (4H, d, J 8.7, 4ꢂ CH), 6.90 (4H,

3892
C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
4.66 (2H, s, CH2), 4.34 (4H, q, J 7.1, 2ꢂ OCH2CH3), 4.08 (4H, t, J 5.5,
2ꢂ CH2), 4.01 (4H, t, J 5.5, 2ꢂ CH2), 2.00e1.95 (8H, m, 4ꢂ CH2),
1.39 (6H, t, J 7.1, 2ꢂ CH3), 0.97 (9H, s, SiC(CH3)3) and 0.1 (6H, s,
Si(CH3)2); dC (75 MHz, CDCl3) 166.3 (C, quat.), 162.6 (C, quat.),
160.0 (C, quat.), 143.9 (C, quat.), 131.4 (CH), 122.7 (C, quat.), 113.9
(CH), 104.2 (CH), 99.9 (CH), 67.6 (OCH2), 67.3 (OCH2), 64.8 (OCH2),
60.6 (CH2), 25.9 (CMe3), 25.9 (CH2), 25.8 (CH2), 18.3 (C, quat.), 14.3
(CH3) and ꢁ5.3 (SiMe2). MS (EI) m/z¼717.38 ([MþNa]þ, 100%).
HRMS (ESI): [MþH]þ, found 695.35995. C39H55O9Si requires
695.36099.
evaporated under reduced pressure and purified by column chro-
matography (100% hexane) to give bromo-ether 2b (10.0 g,
28.17 mmol, 89%) as a white solid, mp 28e29 ꢀC (lit.27 26e27 ꢀC); Rf
0.2 (100% hexane); dH (300 MHz, CDCl3) 7.55 (2H, d, J 8.9, 2ꢂ CH),
6.67 (2H, d, J 8.9, 2ꢂ CH), 3.95 (2H, t, J 6.0, CH2O), 3.48 (2H, t, J 6.0,
CH2Br), 2.10e2.01 (2H, m, CH2) and 1.97e1.88 (2H, m, CH2); dC
(75 MHz, CDCl3) 158.7 (C, quat.), 138.2 (CH), 116.8 (CH), 82.7 (C,
quat.), 66.9 (CH2), 33.3 (CH2), 29.3 (CH2) and 27.7 (CH2). MS (EI) m/
z: 353.82 (100%), 355.82 (85).
4.2.9. Dendron 3b. A mixture of bromide 2b (8.6 g, 24.22 mmol),
3,5-dihydroxybenzyl alcohol26 (1.2 g, 8.56 mmol), K2CO3 (4.7 g,
34.01 mmol) and TBAI (0.1 g, 0.27 mmol) in acetone (100 mL) was
heated at 56 ꢀC. After 48 h the reaction mixture was cooled, filtered,
evaporated under reduced pressure and purified by column chro-
matography (30% EtOAc in hexane) to give benzyl alcohol 3b (5.49 g,
93%) as a white solid, mp 73e74 ꢀC; Rf 0.22 (30% EtOAc in hexane);
4.2.6. Dendron 7a. To a solution of II3a (20.0 mmol) in Et2O
(100 mL) silyl ether 6 (1.75 g, 2.5 mmol) was added as a solid in
portions. The mixture was then warmed to reflux. After 18 h NH4Cl
(satd aq solution) (30 mL) was added and the solution was
concentrated CH2Cl2 (100 mL) was added, the layers were sepa-
rated, the aqueous layer extracted with CH2Cl2 (2ꢂ50 mL) and the
combined organic layer was dried, filtered and evaporated to give
an orange solid. The crude was stirred in Et2O and the product was
filtered and purified by size exclusion column chromatography
(100% toluene). Silyl ether 7a was obtained as an orange solid
(3.64 g, 72%), Rf 0.42 (2% Et3N in CH2Cl2); dH (600 MHz, acetone-d6)
7.33 (4H, d, J 8.8, 4ꢂ CH), 7.00 (4H, d, J 16.1, 4ꢂ CH]), 6.76 (4H, d, J
8.8, 4ꢂ CH), 6.75 (8H, s, 8ꢂ CH), 6.73 (4H, d, J 16.1, 4ꢂ CH]), 6.53
(2H, d, J 2.3, 2ꢂ CH), 6.41 (1H, t, J 2.3, CH), 6.22 (2H, s, 2ꢂ OH), 4.70
(2H, s, CH2O), 4.51 (8H, J 2.0, 8ꢂ FceCH), 4.27 (8H, J 2.0, 8ꢂ
FceCH), 4.13 (20H, s, FceCH), 4.10e4.00 (8H, m, 4ꢂ CH2), 3.47
(24H, s, 8ꢂ OCH3), 1.95e1.93 (8H, m, 4ꢂ CH2), 0.95 (9H, s,
SiC(CH3)3) and 0.11 (6H, s, Si(CH3)2); dC (75 MHz, acetone-d6) 162.2
(C, quat.), 160.3 (C, quat.), 158.8 (C, quat.), 145.9 (C, quat.), 143.7 (C,
quat.), 138.9 (C, quat.), 129.8 (CH), 128.4 (CH]), 128.0 (CH), 127.9
(CH]), 114.1 (C, quat.), 106.4 (CH), 106.2 (CH), 101.5 (CH), 85.5 (C,
quat.), 81.0 (C, quat.), 70.9 (CH), 70.7 (CH), 69.3 (OCH2), 69.1
(OCH2), 68.6 (CH), 66.4 (OCH2), 57.8 (OMe), 27.9 (CH2), 27.8 (CH2),
27.3 (SiCMe3), 19.9 (SiCMe3) and ꢁ4.0 (SiMe2). MALDI mass spec-
trum: calcd for C115H122Fe4O14Si [MꢁOH]þ: 1978.60. Found:
1978.415.
nmax 3320, 2925, 2872, 1584, 1241, 1164, 815 cmꢁ1
; dH (600 MHz,
CDCl3) 7.54 (4H, d, J 6.2, 4ꢂ CH), 6.67 (4H, d, J 6.2, 4ꢂ CH), 6.50 (2H,
d, J 2.1, 2ꢂ CH), 6.35 (1H, t, J 2.1, CH), 4.61 (2H, s, CH2OH), 4.02e3.95
(8H, m, 4ꢂ CH2O) and 1.97e1.93 (8H, m, 4ꢂ CH2); dC (150 MHz,
CDCl3) 160.3 (C, quat.), 158.8 (C, quat.), 143.3 (C, quat.), 138.1 (CH),
116.9 (CH), 105.1 (CH), 100.5 (CH), 82.6 (CI), 67.50 (CH2), 67.4 (CH2),
65.3 (CH2), 25.9 (CH2) and 25.8 (CH2). MS (EI) m/z¼505.20 (100%),
711 ([MþNa]þ, 40). HRMS (ESI): [MþNa]þ, found 711.00643.
C27H30I2O5Na requires 711.00748.
4.2.10. Hypercore 8. A mixture of benzyl alcohol 3b (2.4 g,
3.47 mmol), benzene-1,3,5-tricarbonyl chloride28 (265 mg,
1.00 mmol) and DMAP (423 mg, 3.47 mmol) in CH2Cl2 (50 mL) was
heated at 40 ꢀC. After 18 h the reaction mixture was cooled, filtered
and evaporated under reduced pressure to give hypercore 8 (1.99 g,
90%) as a white solid, mp 93e94 ꢀC; Rf 0.41 (50% Et2O in hexane);
nmax 2926, 2872, 1724, 1233, 1165, 814 cmꢁ1
; dH (600 MHz, CDCl3)
8.90 (3H, s, 3ꢂ CH), 7.51 (12H, d, J 8.5, 12ꢂ CH), 6.65 (12H, d, J 8.5,
12ꢂ CH), 6.56 (6H, s, 6ꢂ CH), 6.40 (3H, s, 3ꢂ CH), 5.30 (6H, s, 3ꢂ
CH2), 4.05e3.90 (24H, m, 12ꢂ OCH2) and 1.95e1.90 (24H, m, 12ꢂ
CH2); dC (75 MHz, CDCl3) 164.7 (C, quat.), 160.3 (C, quat.), 158.8 (C,
quat.), 138.1 (CH), 137.5 (C, quat.), 134.9 (CH), 131.2 (C, quat.), 116.8
(CH), 106.7 (CH), 101.2 (CH), 82.6 (C, quat.), 67.5 (OCH2, two over-
lapping carbons), 67.3 (OCH2), 25.9 (CH2) and 25.8 (CH2). MALDI
mass spectrum: calcd [MþNa]þ for C90H90I6NaO18: 2243.029.
Found: 2243.150.
4.2.7. Dendron 7b. TBAF (0.73 mL, 0.73 mmol; 1 M in THF) was
added to a solution of silyl ether 7a (970 mg, 0.48 mmol) in THF
(50 mL) and the resulting mixture was stirred at 25 ꢀC for 3 h. Then
the solution was concentrated and the product extracted into
CH2Cl2 (50 mL), washed with brine (20 mL), dried and evaporated.
The crude was purified by size exclusion chromatography (100%
CH2Cl2) and dendron 7b was obtained as an orange solid (840 mg,
93%), mp 135e138 ꢀC; Rf 0.35 (2% Et3N in CH2Cl2); nmax 3474, 2931,
4.2.11. 1,1-Bis(4-((E)-ferrocen-1-enyl)-2,6-dimethoxyphenyl)prop-2-
yn-1-ol (12a). To a solution of stilbenyllithium II3a (10.0 mmol) in
Et2O (100 mL) was added silyl ether 119 (440 mg, 2.57 mmol) in THF
(20 mL). The mixture was then warmed to reflux. After 18 h NH4Cl
(satd aq solution) (30 mL) was added and the solution was con-
centrated. CH2Cl2 (100 mL) was added, the layers were separated,
the aqueous layer extracted with CH2Cl2 (2ꢂ50 mL) and the com-
bined organic layers were dried, filtered and evaporated to give an
orange solid. The crude was dissolved in THF (20 mL), a solution of
KOH (190 mg, 3.3 mmol) in MeOH (4 mL) was added and the re-
action mixture was then stirred at 25 ꢀC. After 2 h the solvent was
evaporated and the product was extracted into CH2Cl2 (2ꢂ50 mL),
washed with brine, dried, filtered and purified by size exclusion
column chromatography (100% CH2Cl2). Acetylene 12a was ob-
tained as an orange solid (1.02 g, 53%), mp 135e138 ꢀC; Rf 0.17 (100%
1596, 1231, 1104, 815 cmꢁ1
; dH (600 MHz, acetone-d6) 7.32 (4H, d, J
8.9, 4ꢂ CH), 7.00 (4H, d, J 16.1, 4ꢂ CH]), 6.76e6.73 (12H, m, 12ꢂ
CH), 6.73 (4H, d, J 16.1, 4ꢂ CH]), 6.54 (2H, d, J 2.1, 2ꢂ CH), 6.41 (1H,
t, J 2.1, CH), 6.22 (2H, s, 2ꢂ OH), 4.55 (2H, d, J 6.0, CH2O), 4.51 (8H, J
1.7, 8ꢂ FceCH), 4.28 (8H, J 1.7, 8ꢂ FceCH), 4.13 (20H, s, FceCH),
4.10e4.00 (8H, m, 4ꢂ CH2), 3.47 (24H, s, 4ꢂ OCH3) and 1.95e1.93
(8H, m, 4ꢂ CH2); dC (75 MHz, acetone-d6) 162.2 (C, quat.), 160.3 (C,
quat.), 158.7 (C, quat.), 146.9 (C, quat.), 143.7 (C, quat.), 138.9 (C,
quat.), 129.8 (C, quat.), 128.4 (CH]), 127.9 (CH]), 127.9 (C, quat.),
114.1 (CH), 106.6 (CH), 106.4 (CH), 101.5 (CH), 85.5 (C, quat.), 81.0 (C,
quat.), 70.9 (CH), 70.7 (CH), 69.2 (OCH2), 69.0 (OCH2), 68.6 (CH),
65.6 (OCH2), 57.8 (OMe), 27.9 (CH2) and 22.4 (CH2). MALDI mass
spectrum: calcd for C109H108Fe4O14 [MꢁOH]þ: 1864.51. Found:
1864.49. HRMS (ESI): [MþH]þ, found 1864.51349. C109H108Fe4O14
requires 1864.51313.
CH2Cl2); nmax 3447, 3293, 2930, 2162, 1634, 1598, 1118, 812 cmꢁ1
; dH
(600 MHz, CDCl3) 7.00 (1H, s, OH), 6.80 (2H, d, J 16.1), 6.63 (4H, s, 4ꢂ
CH), 6.60 (2H, d, J 16.1), 4.44 (4H, t, J 1.8, 4ꢂ FceCH), 4.28 (4H, t, J 1.8,
4ꢂ FceCH), 4.14 (10H, s, 10ꢂ FceCH), 3.76 (12H, s, 4ꢂ OMe) and
2.68 (1H, s, CH); dC (75 MHz, CDCl3) 157.7 (C, quat.), 137.5 (C, quat.),
126.9 (CH]), 125.8 (CH]), 122.6 (C, quat.), 104.5 (CH), 88.2 (C,
quat.), 83.1(C, quat.), 70.9 (C, quat.), 70.7 (CH), 69.2 (FceCH), 69.0
4.2.8. 1-(4-Bromobutoxy)-4-iodobenzene
(2b).27 4-Iodophenol
(7.0 g, 31.82 mmol), 1,4-dibromobutane (27.5 g, 127.36 mmol) and
K2CO3 (8.8 g, 63.67 mmol) were stirred in acetone (200 mL) at
56 ꢀC. After 12 h the reaction mixture was cooled, filtered,

C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
3893
(FceCH), 66.8 (FceCH), 56.7 (OMe). MS (EI) m/z¼419.44 (100%),
748.24 ([M]þ, 75). HRMS (ESI): [M]þ, found 748.15879. C43H40O5Fe2
requires 748.15691.
(4H, d, J 8.2, 4ꢂ CH), 6.99 (2H, d, J 16.2, 2ꢂ CH), 6.78 (2H, d, J 16.2 2ꢂ
CH), 4.53 (4H, t, J 2.0, 4ꢂ FceCH), 4.28 (4H, t, J 2.0, 4ꢂ FceCH), 4.12
(10H, s, 10ꢂ FceCH) and 3.36 (1H, s, 1CH); dC (75 MHz, CDCl3) 145.4
(C, quat.), 139.0 (C, quat.), 128.2 (CH]), 127.2 (CH), 126.3 (CH), 126.2
(CH]), 88.0 (C, quat.), 84.4 (C, quat.), 76.35 (CH), 74.17 (C, quat.),
70.0 (FceCH), 69.9 (FceCH) and 67.8 (FceCH); MS (EI) m/z¼628.25
([MþNa]þ, 100%). HRMS (ESI): [M]þ, found 628.11315. C39H32Fe2O
requires 628.12400.
4.2.12. 4,40-Bis[(diethylphosphono)methyl]benzophenone
(14). A
mixture of 4,40-dimethylbenzophenone (5 g, 23.8 mmol), NBS
(9.3 g, 52.3 mmol) and AIBN (cat.) in CCl4 (10 mL) was irradiated
with a 275 W lamp for 1 h and then heated at 76 ꢀC overnight. The
reaction mixture was then cooled, filtered and evaporated under
reduced pressure to give 4,40-bis (bromomethyl) benzophenone12
as a white solid; dH (300 MHz, CDCl3) 7.79 (4H, d, J 8, 4ꢂ CH),
7.52 (4H d, J 8 4ꢂ CH) and 4.55 (4H, s, 2ꢂ OCH2Br). The crude
dibromide was dissolved in triethyl phosphate (25 mL) and the
mixture was heated at 150 ꢀC for 3 h. The reaction mixture was then
cooled and the unreacted phosphate and diethyl ethylphosphonate
(formed during the reaction) were removed by vacuum distillation.
Purification of the residue by column chromatography (gradient
elution: 100% CH2Cl2 to 6% MeOH in CH2Cl2) afforded phosphonate
14 as a yellow oil (7.45 g, 65%); nmax 2982, 1713, 1654, 1243,
4.2.15. General procedure for the Sonogashira cross coupling re-
action. Dendrimer 16b. An oven-dried vessel was charged with the
aryl iodide (80 mg, 0.036 mmol), the terminal alkyne (157 mg,
0.25 mmol), Pd(PPh3)2Cl2 (9 mg, 0.0125, 5 mol %), and CuI (5 mg,
0.025 mmol, 10 mol %). The vessel was evacuated and backfilled
with argon three times. To the vessel, degassed DMF (5 mL) and
degassed triethylamine (1 mL) were added.
The mixture was stirred at 80 ꢀC overnight. After filtration
through CeliteÒ, the solvent was evaporated and the resulting
residue was taken into CH2Cl2 (30 mL) and washed with brine
(15 mL). The organic layer was separated, dried and evaporated and
the crude purified by size exclusion column chromatography (100%
CH2Cl2) to give dendrimer 16b as an orange solid (150 mg, 79%), mp
1017 cmꢁ1
;
dH (300 MHz, CDCl3) 7.74 (4H, d, J 8.0, 4ꢂ CH), 7.40 (4H,
dd, J 8.0 and 2.5, 4ꢂ CH), 4.10e3.98 (8H, m, 4ꢂ OCH2CH3), 3.21 (4H,
d, J 22.2, 2ꢂ CH2) and 1.25 (12H, t, J 7.1, 4ꢂ OCH2CH3); dC (75 MHz,
CDCl3) 195.6 (C, quat.), 136.5 (C, quat., d, J 6.5), 135.9 (C, quat., d, J
3.5), 130.1 (CH, d, J 3), 129.5 (CH, d, J 6.5), 62.1 (OCH2, d, J 7), 33.7
(PCH2, d, J 137.5) and 16.2 (CH3, d, J 6). MS (EI) m/z¼505.31
([MþNa]þ, 100%). HRMS (ESI): [M]þ, found 482.1625. C23H32O7P2
requires 482.1623.
>250 ꢀC; nmax 1725, 1595, 1241, 808 cmꢁ1
; dH (600 MHz, CDCl3) 8.89
(3H, s, 3ꢂ CH), 7.63 (24H, d, J 7.5, 24ꢂ CH), 7.40 (36H, m, 36ꢂ CH),
6.86 (12H, d, J 16, 12ꢂ CH]), 6.81 (12H, d, J 8, 12ꢂ CH), 6.68 (12H, d,
J 16, 12ꢂ CH]), 6.54 (6H, s, 6ꢂ CH), 6.40 (3H, s, 3ꢂ CH), 5.28 (6H, s,
3ꢂ CH2), 4.45 (24H, s, 24ꢂ FceCH), 4.27 (24H, s, 24ꢂ FceCH), 4.12
(60H, s, 60ꢂ FceCH), 4.00e3.96 (24H, s, 12ꢂ CH2) and 1.90e1.97
(24H, s, 12ꢂ CH2); dC (75 MHz, CDCl3) 164.7 (C, quat.), 160.2 (C,
quat.), 159.2 (C, quat.), 143.6 (C, quat.), 137.2 (C, quat.), 134.9 (CH),
133.2 (CH), 131.1 (C, quat.), 128.6 (C, quat.), 127.2 (CH]), 126.3 (CH),
125.6 (CH), 125.3 (CH]), 114.4 (CH), 114.3 (C, quat.), 106.7 (CH),
101.1 (CH), 90.3 (C, quat.), 87.1 (C, quat.), 83.1 (C, quat.), 74.5 (C,
quat.), 69.1 (FceCH), 69.1 (FceCH), 67.4 (3ꢂ OCH2), 66.9 (FceCH),
25.8 (CH2) and 25.8 (CH2). MALDI mass spectrum: calcd for
C324H276Fe12O24: 5221.25. Found: 5220.76.
4.2.13. 4,40-Bis[(E)-2-(ferrocenylethenyl)]benzophenone (15). A so-
lution of ferrocenecarboxaldehyde (1.78 g, 8.32 mmol) in THF
(25 mL) was added to a suspension of sodium hydride (210 mg,
5.2 mmol; 60% in mineral oil) in THF (5 mL) and the mixture was
heated to 70 ꢀC. Then, a solution of phosphonate 14 (500 mg,
1.04 mmol) in THF (25 mL) was slowly added over a period of 1 h.
Upon completion of addition the reaction mixture was stirred at
70 ꢀC for another 30 min and then MeOH (10 mL) was added and
the solvent evaporated. The residue was redissolved into CH2Cl2
(50 mL), washed with brine (20 mL), dried and evaporated. The
crude mixture, containing the product and excess ferrocene-
carboxaldehyde, was taken into Et2O (50 mL) and the insoluble
portion was filtered. Ferrocenyl derivative 15 was obtained as an
orange solid (480 mg, 76%), which was used without further puri-
4.2.16. Dendrimer 16c. This compound was prepared from 12c
(190 mg, 0.91 mmol) and 7 (290 mg, 0.13 mmol) according to the
general procedure for Sonogashira cross coupling reaction. The
crude product was purified by size exclusion column chromatog-
raphy (100% CH2Cl2) to give dendrimer 16c as a light yellow solid
(240 mg, 68%), mp 88e90 ꢀC; nmax 1724, 1601, 1507, 1242, 1167,
fication in the next step; nmax 1633, 1592, 1176, 813, cmꢁ1
; dH
(300 MHz, CDCl3) 7.80 (4H, d, J 8.0, 4ꢂ CH), 7.53 (4H, d, J 8.0, 4ꢂ CH),
7.06 (2H, d, J 16.3, 2ꢂ CH), 6.76 (2H, d, J 16.3, 2ꢂ CH), 4.51 (4H, s, 4ꢂ
FceCH), 4.34 (4H, s, 4ꢂ FceCH) and 4.17 (10H, s, 10ꢂ FceCH); dC
(75 MHz, CD2Cl2) 195.6 (C, quat.), 142.5 (C, quat.), 136.3 (C, quat.),
131.1 (CH), 130.9 (CH]), 125.8 (CH), 125.1 (CH), 62.1 (OCH2, d, J 7),
83.1 (C, quat.), 70.1 (FceCH), 69.9 (FceCH) and 67.8 (FceCH). MS
(EI) m/z¼602.25 (Mþ, 100%). HRMS (ESI): [M]þ, found 602.09814.
C37H30Fe2O 602.10 requires 602.0995.
696 cmꢁ1
;
dH (600 MHz, acetone-d6)
d
8.85 (3H, s, 3ꢂ CH), 7.70
(24H, d, J 7.5, 24ꢂ CH), 7.41 (12H, d, J 8.8, 12ꢂ CH), 7.31 (24H, t, J 7.5,
24ꢂ CH), 7.22 (12H, t, J 7.5, 12ꢂ CH), 6.91 (12H, d, J 8.8,12ꢂ CH), 6.65
(6H, d, J 2.0, 6ꢂ CH), 6.46 (3H, t, J 2.0, 3ꢂ CH), 5.64 (6H, s, 6ꢂ OH),
5.34 (6H, s, 3ꢂ OCH2), 4.05e4.02 (24H, s, 12ꢂ CH2) and 1.92e1.89
(24H, s, 12ꢂ CH2); dC (75 MHz, acetone-d6) 165.0 (C, quat.), 161.3 (C,
quat.), 160.3 (C, quat.), 147.5 (C, quat.), 138.9 (C, quat.), 135.0 (CH),
133.8 (CH), 132.3 (C, quat.), 128.8 (CH), 0, 127.9 (CH), 5, 126.8 (CH),
115.5 (CH), 107.3 (CH), 101.7 (CH), 92.2 (C, quat.), 86.8 (C, quat.), 74.7
(C, quat.), 68.3 (OCH2), 68.2 (OCH2), 67.2 (C(O)OCH2), 26.58 (CH2)
and 26.5(CH2). MALDI mass spectrum: calcd for C180H162O27
[MþH2O]þ: 2755.13. Found: 2755.88.
4.2.14. 1,1-Bis(4-((E)-ferrocen-1-enyl)phenyl)prop-2-yn-1-ol (12b).
n-BuLi (0.85 M in methylcyclohexane, 8.3 mL, 7.9 mmol) was added
dropwise to
a solution of (trimethylsilyl)acetylene (830 mg,
8.5 mmol) in anhydrous THF (50 mL) at ꢁ78 ꢀC. After 30 min at
ꢁ78 ꢀC a solution of ketone 15 (1.7 g, 2.8 mmol) in dry THF (50 mL)
was added at ꢁ78 ꢀC and the reaction mixture was then allowed to
warm up to rt and stirred at that temperature overnight. Then,
a solution of KOH (240 mg, 4.3 mmol) in MeOH (3 mL) was added
and stirring was continued until TLC indicated that deprotection
was complete (1.5 h). The solvent was then evaporated, the crude
was taken into CH2Cl2 (50 mL) and the solution was washed with
brine (20 mL), dried and evaporated to yield prop-2-yn-1-ol 12b
(1.50 g, 85%) as an orange solid, mp 209e211 ꢀC; nmax 3538, 3286,
4.2.17. Dendrimer 16d. This compound was prepared from 12d
(268 mg, 1 mmol) and 7 (317 mg, 0.14 mmol) according to the
general procedure for Sonogashira cross coupling reaction. The
crude product was purified by size exclusion column chroma-
tography (100% CH2Cl2) to give dendrimer 16d as a green solid
(360 mg, 84%), mp 139e141 ꢀC; nmax 3467, 1595, 1561, 1232, 1104,
815 cmꢁ1
;
dH (300 MHz, acetone-d6) 8.87 (3H, s, 3ꢂ CH),
7.55e7.52 (24H, br, 24ꢂ CH), 7.38e7.35 (12H, br, 12ꢂ CH),
6.85e6.50 (36H, m, 12ꢂ CHþ24ꢂ CH), 6.53 (6H, s, 6ꢂ CH), 6.39
(3H, s, 3ꢂ CH), 5.27 (6H, s, 3ꢂ OCH2), 4.05e3.85 (24H, s, 12ꢂ CH2),
1631, 804 cmꢁ1
;
dH (600 MHz, CDCl3) 7.59 (4H, d, J 8.2, 4ꢂ CH), 7.45

3894
C. Villalonga-Barber et al. / Tetrahedron 69 (2013) 3885e3895
3.57 (36H, s, 12ꢂ OCH3) and 1.95e1.85 (24H, s, 12ꢂ CH2); dC
(75 MHz, CDCl3) 164.7 (C, quat.), 160.2 (C, quat.), 159.1 (C, quat.),
158.8 (C, quat.), 137.8 (C, quat.), 137.5 (C, quat.), 134.8 (CH), 133.1
(CH), 131.1 (C, quat.), 129.3 (CH), 127.3 (CH), 5, 114.4 (CH), 106.7
(CH), 101.2 (CH), 90.8 (C, quat.), 87.7 (C, quat.), 74.1 (C, quat.), 67.5
(OCH2), 67.4 (OCH2), 67.3 (C(O)OCH2), 55.2 (OMe), 25.9 (CH2) and
25.8 (CH2); MALDI mass spectrum: calcd for C192H180O36: 3062.22.
Found: 3062.146.
spectrum: calcd for C48H36NaO12 [MþNa]þ: 827.21. Found: 827.19
(100%).
4.2.21. General procedure for the azideealkyne click reaction cata-
lyzed by Cu(PPh3)3Br. Dendimer 20. A 10 mL reaction vessel was
charged in air with the acetylene-terminated core 10 (81 mg,
0.1 mmol), the azide 18 (640 mg, 0.80 mmol; 8 equiv), N,N-diiso-
propylethylamine (60
m
l, 0.35 mmol), Cu(PPh3)3Br29 (23 mg,
0.025 mmol) and THF (3 mL). The vessel was sealed and submitted
to microwave irradiation for 20 min at 140 ꢀC, using an initial power
of 70 W. The mixture was then allowed to cool to room tempera-
ture, the solvent was evaporated and the product was purified by
size exclusion chromatography (100% CH2Cl2) to give dendrimer 20
as an orange solid (530 mg, 95%); dH (600 MHz, CDCl3) 8.88 (3H, s,
3ꢂ CH), 7.59 (6H, s, 6ꢂ HeteCH), 7.36 (24H, d, J 8.2, 24ꢂ AreCH),
7.25 (24H, d, J 8.2, 24ꢂ AreCH), 7.20 (12H, d, J 8.5, 12ꢂ AreCH), 6.86
(12H, d, J 16.1, 12ꢂ CH]), 6.78 (12H, d, J 8.5, 12ꢂ AreCH), 6.68 (12H,
d, J 16.1, 12ꢂ CH]), 6.66 (6H, s, 6ꢂ AreCH), 6.61 (3H, s, 3ꢂ AreCH),
5.28 (6H, s, 3ꢂ CH2), 5.11 (12H, s, 6ꢂ CH2), 4.45 (24H, s, 24ꢂ FceCH),
4.36 (12H, t, J7.0, 6ꢂ CH2), 4.28 (24H, s, 24ꢂ FceCH), 4.13 (60H, s,
60ꢂ FceCH), 3.91 (12H, t, J 5.9, 6ꢂ CH2), 3.01 (6H, s, 6ꢂ OH),
2.13e1.98 (12H, m, 6ꢂ CH2) and 1.83e1.66 (12H, m, 6ꢂ CH2); dC
(75 MHz, CDCl3) 164.6 (C, quat.), 159.5 (C, quat.), 157.7 (C, quat.),
145.5 (C, quat.),143.6 (HeteC, quat.),139.3 (C, quat.),137.9 (C, quat.),
136.6 (C, quat.), 134.9 (AreCH), 131.1 (C, quat.), 129.2 (AreCH), 128.1
(AreCH), 127.1 (CH]), 125.5 (CH]), 125.0 (AreCH), 122.9
(HeteCH), 113.62 (AreCH), 107.5 (AreCH), 101.7 (AreCH), 83.2
(FceC, quat.), 81.4 (C, quat.), 69.2 (FceCH), 69.1 (FceCH), 66.8
(FceCH), 66.7 (ArCH2OC(O)þCH2OAr), 61.96 (OCH2eHet), 50.01
(NCH2), 27.1 (CH2) and 26.1 (CH2). MALDI mass spectrum: calcd for
C330H294Fe12N18O24 [M]þ: 5566.45. Found: 5566.87.
4.2.18. Ethyl 4-(4-azidobutoxy)benzoate (17). A solution of bromide
2a (1.6 g, 5.3 mmol) and sodium azide (1.7 g, 26,6 mmol) in acetone
(20 mL) and H2O (5 mL) was heated to 56 ꢀC. After 18 h the mixture
was concentrated, diluted with CH2Cl2 (30 mL) and washed with
water (15 mL). The combined organic layer was dried, filtered and
evaporated to give azide 17 as a clear oil (1.0 g, 75%); Rf 0.51 (50%
Et2O in hexane); nmax 2979, 2093, 1707, 1605, 1247, 1166, 1021,
769 cmꢁ1
;
dH (300 MHz, CDCl3) 8.00 (2H, d, J 8.4, 2ꢂ CH), 6.91 (2H,
d, J 8.4, 2ꢂ CH), 4.35 (2H, q, J 7.2, CH2O), 4.05 (2H, t, J 6.1, CH2O), 3.38
(2H, t, J 6.1, CH2N3), 1.96e1.75 (4H, m, 2ꢂ CH2) and 1.39 (3H, t, J 7.2,
CH3); dC (75 MHz, CDCl3) 166.1 (C, quat.), 162.4 (C, quat.), 131.5 (CH),
122.8 (C, quat.), 113.8 (CH), 67.2 (OCH2), 60.4 (OCH2), 50.9 (N3CH2),
26.2 (CH2), 25.5 (CH2) and 14.2 (CH3); MS (EI) m/z¼263.97 ([M]þ,
100%). HRMS (ESI): [MþH]þ, found 264.13473. C13H18N3O3 requires
264.13427.
4.2.19. (4-(4-Azidobutoxy)phenyl)bis(4-((E)-ferrocen-1-enyl)phenyl)
methanol (18). To a solution of 193 (0.8 g, 2.18 mmol) in THF (20 mL)
at ꢁ78 ꢀC was added a solution of ester 17 (217 mg, 0.87 mmol) in
THF (5 mL) and the resulting mixture was stirred at ꢁ78 ꢀC for
30 min NH4Cl (satd aq solution) (15 mL) was then added and the
solution was concentrated. CH2Cl2 (20 mL) was added, the layers
were separated, the aqueous layer was extracted with CH2Cl2
(2ꢂ20 mL) and the combined organic layers were dried, filtered
and evaporated to give an orange solid. The crude was then purified
by column chromatography (gradient elution: 10% ethyl acetate in
hexane to 20% ethyl acetate in hexane) to give carbinol 18 as an
orange solid (450 mg, 65%), mp 82e83 ꢀC; Rf 0.26 (10% ethyl acetate
in hexane); dH (300 MHz, CDCl3) 7.39 (4H, d, J 8.5, 4ꢂ CH), 7.27 (4H,
d, J 8.5, 4ꢂ CH), 7.23 (2H, d, J 9.0, 2ꢂ CH), 6.88 (2H, d, J 16.1, 2ꢂ
CH]), 6.85 (2H, d, J 9.0, 2ꢂ CH), 6.70 (2H, d, J 16.1, 2ꢂ CH]), 4.47
(4H, t, J 1.8, 4ꢂ FceCH), 4.29 (4H, t, J 1.8, 4ꢂ FceCH), 4.14 (10H, s,
10ꢂ FceCH), 4.01 (2H, t, J 6.0, CH2O), 3.38 (2H, t, J 6.0, CH2N3) and
1.95e1.75 (4H, m, 4ꢂ CH2); dC (75 MHz, CDCl3) 158.0 (C, quat.),
145.5 (C, quat.), 139.2 (C, quat.), 136.8 (C, quat.), 129.2 (CH), 128.1
(CH), 127.2 (CH]), 125.4 (CH]), 125.3 (CH), 113.8 (CH), 83.2 (C,
quat.), 81.6 (C, quat.), 69.2 (FceCH), 69.1 (FceCH), 67.1 (OCH2), 66.9
(FceCH), 51.2 (N3CH2), 26.5 (CH2) and 25.8 (CH2). MS (EI) m/
z¼793.21 ([M]þ, 100%). HRMS (ESI): [M]þ, found 793.20642.
C47H43Fe2N3O2 requires 793.20486.
4.3. Sample preparation for the pH-dependent absorption
measurements
Stock solutions for the dyes (1.0E-5 M to 2.0E-4 M see Table 1 in
main text) and TFA in CHCl3 were prepared. The dye solutions were
prepared by mixing 1 or 2 mL of the dye base solution with varying
amounts of the TFA solutions and the final volumes adjusted to 5 or
10 mL with CHCl3.
Acknowledgements
This work was supported by funding provided by the European
Commission for the FP7-REGPOT-2009-1 Project ’ARCADE’ (Grant
Agreement No. 245866). The COST Action TD0802 is also thanked
for promoting the collaboration among the authors.
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