azide or with two thiols. Thiol addition is found to be faster
than the triazole formation with the continuous UV irradiation
for the former being the penalty to pay. On the other hand,
9
different thiol–X reactions have been compared on the thiol
beads. The thiol–isocyanate reaction is the fastest, being
closely followed by the thiol–norbornene reaction. The
full comparison is as follows: isocyanate > norbornene >
acrylate E isothiocyanate > maleimide E isolated ene >
a-bromo ester > epoxide E aziridine. The kinetics of the
reaction is found to be more dependent on the reactivity of a
compound than on the class it belongs to or on the mechanism
of the addition. This comparison may be the first step to guide
researchers, especially in the field of SPS, to choose the most
suitable ligation reaction according to their needs since a vast
number of conjugations are recently being claimed to be part
of the click family.
Fig. 3 Kinetic comparison of 9 thiol–X reactions on the high loading
thiol bead. Solid (red) triangles for isocyanate (14), open circles for
norbornene (8), open triangles (solid line) for acrylate (9), solid (blue)
diamonds (dashed-dotted line) for isothiocyanate (15), open squares
(
(
dotted line, shared with ene) for maleimide (10), solid (green) circles
dotted line) for ene (7) and solid (black) squares for methyl bromo-
Marie Curie Early Stage Research Training Fellowship
(
contract number 020643, Sendichem project, MTG), the
propionate (11). Curves are drawn arbitrarily to guide the eye.
Erasmus Belgica program (JB), the Belgian Program on
Interuniversity Attraction Poles initiated by the Belgian State,
Prime Minister’s office (Program P6/27) and Dr B. De Geest
(confocal analysis) are acknowledged.
compared to the reaction with isocyanate. Following these two
reactions, thiol–acrylate (phosphine catalyzed) and thiol–
isothiocyanate (NEt3 catalyzed) reactions exhibited quite
comparable performances, reaching 75% thiol peak height
depletion in 8 hours. Thus, n-hexyl isothiocyanate (15) had a
lower but still acceptable reactivity towards thiols in comparison
to n-hexyl isocyanate (14). Moreover, it should be noted that
the actual conversions, which are proportional to area rather
than to height ratios, should be even higher in all cases as the
decrease in the peak area is higher than the decrease of the
height of the same peak.
Notes and references
1 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int.
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C. Barner-Kowollik, F. Du Prez, P. Espeel, C. Hawker, T. Junkers,
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6
0–62.
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1
Further interpretation of Fig.
performances for N-methyl maleimide (10) and isolated ene
7) although they work via anionic and radical mechanisms,
3 shows comparable
4
5
Y. R. Hua and A. H. Flood, Chem. Soc. Rev., 2010, 39, 1262–1271.
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(
respectively (dotted line represents both). Their maximum
reaches B60% thiol peak height depletion after 8 hours. We
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efficient catalysts such as 1,8-diazabicyclo[5.4.0]undec-7-ene
6 H.-A. Klok, Macromolecules, 2009, 42, 7990–8000.
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B. A. Laurent and S. M. Grayson, J. Am. Chem. Soc., 2006, 128,
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1
(
DBU) and stronger UV sources, respectively.
7
928–7933.
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found to be the slowest working conjugation. It has a poor
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2
12 M. Amblard, J. A. Fehrentz, J. Martinez and G. Subra, Mol.
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1
1
1
1
1
1
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NEt
ZnCl
3
was used as the catalyst. Nevertheless, whereas DBU and
can catalyze the addition of N-tosyl aziridine (13) to a
2
maximum of B30% in 8 hours, the use of BF3 and tri-
butylphosphine totally failed such as in the case of NEt3.
Similarly, 62% thiol peak depletion was observed when DBU
was used for 1,2-epoxybutane (12). Other conjugations can
also be further improved by extended reaction times, by using
other catalysts and by shaking the reaction mixture. For
instance thiol–acrylate and thiol–maleimide reactions reached
quantitative conversion within the error margins of the FTIR
analysis in 15 and 24 h, respectively.
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2
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In conclusion, it was demonstrated in this communication
that novel, high loading monodisperse alkyne or thiol
functionalized beads can be easily manufactured by a tubing-
needle based microfluidic setup via thiol–yne chemistry. Their
ability to serve as ligation platforms has been demonstrated in
a comparative manner. The yne beads react either with an
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2
2
3 B. M. Rosen, G. Lligadas, C. Hahn and V. Percec, J. Polym. Sci.,
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4
654 Chem. Commun., 2011, 47, 4652–4654
This journal is c The Royal Society of Chemistry 2011