Efficient two-step synthesis of 11,11′-dithiobis[1-(2-bromo-2- methylpropionyloxy)undecane], a conventional initiator for grafting polymer brushes from gold surfaces via ATRP

Article abstract of DOI:10.1080/00397910903350008

11,11′-Dithiobis[1-(2-bromo-2-methylpropionyloxy)undecane], a conventional initiator for grafting polymers from gold surfaces, was synthesized in two steps from 11-mercapto-1-undecanol in 88-92% overall yield. Oxidative dimerization of 11-mercapto-1-undecanol with a catalytic amount of sodium iodide and 30% hydrogen peroxide in ethyl acetate proceeded in 96% yield. Esterification with 2-bromoisobutyryl bromide in dichloromethane was clean and almost quantitative (92% yield) with pyridine used as base, whereas triethylamine gave a messy reaction (64% yield). Alternatively, esterification with 2-bromo-2-methyl-propanoic acid in dichloromethane occurred readily under Steglich′s conditions with N,N′-dicyclohexylcarbodiimide (DCC) and a catalytic amount of dimethylaminopyridine (DMAP; 88% yield). Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903350008

Synthetic Communications1, 40: 3000–3007, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903350008
EFFICIENT TWO-STEP SYNTHESIS OF
11,11’-DITHIOBIS[1-(2-BROMO-
2-METHYLPROPIONYLOXY)UNDECANE], A
CONVENTIONAL INITIATOR FOR GRAFTING POLYMER
BRUSHES FROM GOLD SURFACES VIA ATRP
Serena Belegrinou, Violeta Malinova, Raffaello Masciadri, and
Wolfgang Meier
Department of Chemistry, University of Basel, Basel, Switzerland
11,110-Dithiobis[1-(2-bromo-2-methylpropionyloxy)undecane], a conventional initiator for
grafting polymers from gold surfaces, was synthesized in two steps from 11-mercapto-1-
undecanol in 88–92% overall yield. Oxidative dimerization of 11-mercapto-1-undecanol with
a catalytic amount of sodium iodide and 30% hydrogen peroxide in ethyl acetate proceeded
in 96% yield. Esterification with 2-bromoisobutyryl bromide in dichloromethane was clean
and almost quantitative (92% yield) with pyridine used as base, whereas triethylamine gave a
messy reaction (64% yield). Alternatively, esterification with 2-bromo-2-methyl-propanoic acid
in dichloromethane occurred readily under Steglich’s conditions with N,N0-dicyclohexylcarbo-
diimide (DCC) and a catalytic amount of dimethylaminopyridine (DMAP; 88% yield).
Keywords: Atom transfer radical polymerization (ATRP); gold surface; self-assembled initiator mono-
layer; Steglich esterification
INTRODUCTION
The seminal discovery by Nuzzo and Allara[1] at the Bell Laboratories that
gold surfaces can be conveniently functionalized with self-assembled monolayers
of alkanethiols gave a tremendous boost to the field of surface engineering.[2] In
the course of our ongoing investigations[3,4] on the synthesis of triblock copolymers
by ‘‘grafting from’’ gold surfaces, we were in need of an efficient synthesis of the atom
transfer radical polymerization initiator 11,110-dithiobis[1-(2-bromo-2-methylpro-
pionyloxy)undecane] (3) (Scheme 1). A previously reported pioneering synthesis[5]
(synthesis I; Scheme 1) relied on the dimerization of 11-mercapto-1-undecanol 1 with
toxic bromine as a first step, followed by acylation of the resulting diol 2 with
2-bromoisobutyryl bromide and triethylamine as base. 2-Bromoisobutyryl bromide
is a very strong acylating agent which may react with the excess of triethylamine
and=or the disulfide bridge present in diol 2. The moderate reported yield of 64%
led us to presume that uncontrolled collateral reactions had taken place. In fact, this
Received August 11, 2009.
Address correspondence to Wolfgang Meier, Department of Chemistry, University of Basel,
Klingelbergstrasse 80, CH-4056 Basel, Switzerland. E-mail: wolfgang.meier@unibas.ch
3000

SYNTHESIS OF DISULFIDE-BASED ATRP INITIATOR
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Scheme 1. Syntheses of 11,110-dithiobis[1-(2-bromo-2-methylpropionyloxy)undecane] 3: Synthesis I (Ia)
Br2=DCM=10% KHCO3=25 ꢁC; (Ib) TEA=DCM=0–25 ꢁC=24 h; synthesis II (IIa) cat. NaI=30% H2O2=
AcOEt=0–5 ꢁC=1 h; (IIb) method A cat. DMAP=DCC=DCM=24 h; method B Py=DCM=0–25 ꢁC=24 h.
was the case when we repeated Shah’s procedure[5] (synthesis I). Lowering the initial
reaction temperature to ꢀ70 ꢁC did not improve the yield. We therefore conducted a
more thorough investigation, which resulted in a satisfactory improvement of the
yield. Details of the strategy are discussed in the following article.
RESULTS AND DISCUSSION
The synthesis of the diol 2 from 11-mercapto-1-undecanol 1 has been pre-
viously reported using sodium borate (92% yield),[6] stochiometric iodine in meth-
anol (90% yield),[7] N-phenyltriazolinedione in toluene (61% yield),[8] or starting
from 11-bromoundecanol using sodium thiosulfate and iodine in water=ethanol.[9]
We went a step further and applied a recently described mild and general
method for dimerization of thiols reported by Kirihara et al.,[10] who used a

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S. BELEGRINOU ET AL.
catalytic amount of sodium iodide and hydrogen peroxide in ethyl acetate. When
we subjected the precursor 11-mercapto-1-undecanol (1) (Scheme 1; synthesis II)
to Kirihara’s conditions, the reaction was strongly exothermic and terminated in a
few minutes. When we repeated the reaction at 0–5 ꢁC, there was no exothermic
effect, the product precipitated gradually from the reaction mixture, and the reaction
was completed after 1 h. To ensure high purity and yield, we ultimately crystallized
the product from methanol. With these modifications, the desired diol 2 was
obtained in 96% yield.
In the second step, we observed a messy reaction when using the strong acylat-
ing agent 2-bromoisobutyryl bromide and triethylamine as base according to the
seminal procedure by Shah et al.[5] (synthesis I), which was cited and utilized for
the synthesis of diester 3 in numerous subsequent publications[3,4,11–26] without
substantial modification.
To improve the yield of this step, we envisaged two strategies: changing the
acylating agent and changing the base (synthesis II, methods A and B). We therefore
applied Steglich’s esterification conditions[27] using 2-bromo-2-methyl-propionic
acid, a catalytic amount of DMAP, and a slight excess of DCC (synthesis II,
method A) in analogy to a previously reported synthesis[28] of structurally related
polymerization initiator 2,20-dithiobis[1-(2-bromo-2-methylpropionyloxy)ethane]
(5)[29–41] (Scheme 2).
In contrast to diester 3, which has been synthesized always by the same pro-
cedure, diester 5 was synthesized according to three different protocols, namely
Shah,[29,30,34,38] Tsarevsky,[28,31,35,39,41] and Matsuura.[32,33,36,40]
2-Bromo-2-methyl-propanoic acid reacted with DCC exothermically and
afforded the corresponding isourea derivative in situ. A catalytic amount of DMAP
smoothly catalyzed the esterification at 5 ꢁC but because it was difficult to assess the
endpoint of the heterogeneous reaction, stirring at room temperature was continued
overnight to assure complete conversion. The reaction mixture was directly poured
on a column of silica gel, which had previously been conditioned with dichloro-
methane, and the product was eluted in high purity and yield. Other by-products
in the reaction mixture spotted by thin-layer chromatography (TLC) were not eluted
and did not contaminate the desired diester 3, which was gratifyingly obtained in
88% yield. The diester 3 was sensitive to oxydative degradation upon sustained
exposure to air and should thus be stored under nitrogen.
Triethylamine is often replaced by the more expensive but sterically more hin-
dered N-ethyldiisopropylamine (Hunig’s base),[42] which has proven to be effective in
¨
alkylations and acylations. Pyridine is less basic and more nucleophilic because of
the sp2 character of the lone electron pair on the nitrogen. In research, pyridine
is widely used for acylations especially in combination with acid anhydrides.
Matsuura[32] obtained 82% yield when using pyridine as base for the synthesis of
the shorter chain analogous polymerization initiator 5 (Scheme 2, Matsuura’s
method). By applying Matsuura’s conditions to the diol 2, we obtained the desired
diester 3 in 92% yield (synthesis II, method B). Thus, most likely the real culprit
for the diminished yield in Shah’s procedure[5] is triethylamine and not 2-bromoiso-
butyryl bromide. In fact, these two reagents, when mixed in the absence of the diol 2,
gave rise to a brown tar, which was found to be a multicomponent mixture by
1H NMR.

SYNTHESIS OF DISULFIDE-BASED ATRP INITIATOR
3003
Scheme 2. Syntheses of shorter chain polymerization initiator 5: Matsuura’s method Py=CHCl3=0–25 ꢁC=
24 h; Tsarevsky’s method cat. DMAP=DCC=THF.
CONCLUSIONS
In summary, we have developed a practical and efficient two-step synthesis
for the gold surface polymerization initiator 2-bromo-2-methyl-propanoic acid
11-[[110-(2-bromo-2-methyl-1-oxopropoxy)undecyl]dithio]undecyl ester (3) by
exploiting procedures established for shorter chain anolog 5. To our knowledge,
these procedures have not been applied productively to the synthesis of the longer
chain analog 3. We did not see an obvious reason why researchers had continued
to use the procedure described by Shah et al.[5] despite its nonoptimized yield. This
triggered us to search for improvements, and here we suggest a solution.
EXPERIMENTAL
11,11’-Dithiobis-1-undecanol (2)
11-Mercapto-1-undecanol (1) (10.2 g, 48.4 mmol) and a catalytic amount of
sodium iodide (73 mg, 0.484 mmol) were dissolved in AcOEt (100 mL) under nitro-
gen and cooled in ice. Then 30% hydrogen peroxide (5 ml, 48.4 mmol) was added

3004
S. BELEGRINOU ET AL.
slowly from a dropping funnel. The product started to precipitate partially a few
minutes after the addition of hydrogen peroxide. The reaction temperature did not
rise. Cooling and stirring in ice was continued for 1 h. AcOEt, which partially dis-
solved the product, was evaporated from the suspension. Then MeOH (100 mL)
was added, and the suspension was stirred in ice for 15 min. The solid was filtered
off, washed twice with cold MeOH (2 ꢂ 20 mL), and thoroughly dried first at
20 mbar=25 ꢁC for 1 h and then at 0.01 mbar=40 ꢁC overnight to afford 9.5 g (96%)
1
of white crystals. Mp 83 ꢁC. H NMR (400 MHz, CDCl3) d ¼ 1.22–1.42 (m, 30 H,
CH2), 1.56 (quin, 4 H, CH2CH2S), 1.66 (quin, 4 H, CH2CH2O), 2.67 (t, J ¼ 8 Hz,
4 H, SCH2), 3.63 (t, J ¼ 8 Hz, 4 H, OCH2). MS (ESIþ): m=z ¼ 429 (M þ Na)þ.
MS (ESIꢀ): m=z ¼ 441 (M þ Cl)–. TLC on silica gel in hexane–AcOEt 2:1, Rf ¼ 0.23.
11,11’-Dithiobis[1-(2-bromo-2-methylpropionyloxy)undecane] (3)
Method A. 11,110-Dithiobis-1-undecanol (2) (5 g, 12.3 mmol) was suspended
under nitrogen in DCM (100 mL). 2-Bromo-2-methylpropionic acid (4.3 g,
25.8 mmol) and N,N0-dicyclohexylcarbodiimide (DCC) (5.33 g, 25.8 mmol) were
added, the latter in portions. These two reagents reacted exothermically, leading
to a rise in temperature of 10 ꢁC. The reaction mixture was cooled in ice and treated
with a catalytic amount of 4-dimethylaminopyridine (DMAP) (315 mg, 2.58 mmol).
Stirring in ice was continued for 5 h, and then the mixture was allowed to warm to
room temperature overnight. The reaction mixture was directly purified by chroma-
tography on silica gel (column 12 cm high, 8 cm in diameter) in DCM to afford 7.6 g
(88%) of colorless oil.
Method B. 2-Bromoisobutyryl bromide (1.36 mL, 10.8 mmol) was added drop-
wise to a stirred suspension of 11,110-dithiobis-1-undecanol (2) (2 g, 4.9 mol) and dry
pyridine (0.975 mL, 10.8 mmol) in dry dichloromethane (40 mL) at 0–5 ꢁC under a
nitrogen atmosphere. The reaction mixture was stirred at 0–5 ꢁC for 2 h and then at
room temperature for 24 h. Afterward, the mixture was diluted with DCM (60 mL)
and extracted with cold 1 N HCl (2 ꢂ 100 mL), saturated NaHCO3 (2 ꢂ 100 mL),
and saturated NaCl (1 ꢂ 100 mL). All aqueous phases were back-extracted with
DCM (1 ꢂ 100 mL). The crude product (3.7 g) was purified by chromatography on sil-
ica gel in hexane=DCM 1:1 to afford 3.2 g (92%) of colorless oil.
1H NMR (400 MHz, CDCl3) d ¼ 1.28 (s, 20 H, CH2), 1.37 (m, 8 H, CH2
CH2CH2O=S), 1.64 (m, 8 H, CH2CH2O=S), 1.93 (s, 12 H, CH3), 2.67 (t, J ¼ 8,
4 H, SCH2), 4.16 (t, J ¼ 8, 4 H, OCH2). 13C NMR (100 MHz, CDCl3) d ¼ 26.21
(CH2), 28.77 (CH2), 28.95 (CH2), 29.59 (CH2), 29.64 (CH2), 29.65 (CH2), 29.89
=
(CH2), 31.22 (CH2), 39.59 (CH2S), 56.44 (C), 66.58 (CH2O), 172.16 (C O). MS
(ESIþ): m=z ¼ 727 (M þ Na)þ. MS (ESIꢀ): m=z ¼ 783 (M þ Br)–. TLC on silica
gel in hexane=DCM 1:1, Rf ¼ 0.33.
ACKNOWLEDGMENTS
The authors thank the Swiss BNF fund for financial support and Carsten Kroll
for measuring the mass spectra.

SYNTHESIS OF DISULFIDE-BASED ATRP INITIATOR
3005
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