.
Angewandte
Communications
13C NMR (176 MHz, [D6]DMSO): d = 145.6 (s, triazole), 123.9 (s, C-
1), 122.1 (s, triazole), 78.5 (br, PG), 75.7 (m, C-2), 70.1 (br, PG), 60.2
(br, PG), 50.3 (br, PG), 30.3 (s, C-3), 18.6 ppm (s, C-4). 19F NMR
As an example of the oxidation of thioethers, we subjected
thioanisol to the reaction conditions (not shown in Table 2).
We were delighted to see that the very high sulfoxide
selectivity typical for sulfoxidations in fluoroalcohol solvents
was maintained by the dendritic catalysts 6a,b, as only the
sulfoxide PhS(O)Me was formed in quantitative yield, and no
sulfone.
An additional advantage of the dendritic catalysts is their
potential recovery for multiple uses. In the current case, the
catalysts 6a,b were successfully recovered by ultrafiltration.
The catalysts were re-used twice without noticeable losses in
the yield of the product epoxide when cyclooctene was used
as the test substrate.
In conclusion, we could show that immobilization of
fluoroalcohol monomers on a soluble dendritic support is
a suitable method for the generation of organocatalysts that
promote transformations by multiple hydrogen-bond net-
works. In the current case, the high local concentration of
fluoroalcohol groups on the polymeric surface was exploited
for the electrophilic activation of hydrogen peroxide. Epox-
idations with hydrogen peroxide, hitherto attainable in
fluoroalcohol solvents, were achieved for the first time with
catalytic amounts of fluoroalcohol units. This positive den-
dritic effect not only validates the multifunctional catalyst
design concept, but also supports the transition-state model
with multiple HFIP molecules for the catalytic epoxidation of
olefins. Similarly, the selective sulfoxidation of thioethers with
H2O2 could be achieved with our catalytic dendritic polymers.
We are convinced that this novel catalytic principle will find
further use, for example, in further electrophilic oxidations
using peroxide as a terminal O donor, or in other trans-
formations requiring substrate activation/transition-state sta-
bilization by multiple hydrogen bonding.[21]
~
(376 MHz, [D4]MeOH): d = ꢀ77.01 ppm (s). IR (neat): n ¼3145,
3079, 2956, 2882, 2736, 1732, 1704, 1556, 1454, 1283, 1199, 1137, 1035,
967, 930 cmꢀ1
.
In the same fashion, the polymeric catalyst 6b was prepared from
the TMS-protected alkyne 4b and polyglycerol azide 5 in 81% yield
(1.4 g), with a loading of 2.9 mmol fluoroalcohol head groups per
gram.
1H NMR (700 MHz, [D6]DMSO): d = 7.88–7.30 (m, 1H, tri-
azole), 5.30–4.63 (functionalized primary/secondary PG groups),
4.07–3.03 (PG), 2.68–2.43 (m, 2H, H5), 1.97–1.83 (m, 2H, H3), 1.83–
1.65 ppm (m, 2H, H4). 13C NMR (176 MHz, [D6]DMSO): d = 146.4 (s,
triazole), 123.9 (s, C-1), 122.3 (s, triazole), 78.4 (br, PG), 75.9 (m, C-2),
70.2 (br, PG), 60.2 (br, PG), 50.2 (br, PG), 30.3 (s, C-3), 25.3 (s, C-5),
22.1 ppm (s, C-4). 19F NMR (376 MHz, [D4]MeOH): d = ꢀ76.92 ppm
~
(s). IR (neat): n ¼3148, 3089, 2952, 2875, 1728, 1704, 1552, 1462, 1444,
1375, 1286, 1273, 1206, 1178, 1137, 1053, 989, 930, 871, 808 cmꢀ1
.
b) General procedure for the catalytic epoxidation of alkenes:
The alkene (50 mmol, 1 equiv), bromobenzene (50 mmol, internal
standard), and the catalyst (0.2 equiv) were suspended in CH2Cl2
(0.4 mL, c = 0.125molLꢀ1) in a GC vial (1.5 mL). Hydrogen peroxide
(1 mmol, 50 wt% in H2O, 20 equiv) was added and the reaction
mixture was stirred at 408C for 15–72 h. 20 mL samples were
frequently taken, eluted over Al2O3/MnO2 with CH2Cl2 to quench
any remaining hydrogen peroxide, and analyzed by gas chromatog-
raphy. GC Method: Chiraldex g-TA column; flow 0.9 mLminꢀ1, 408C
for 5 min, then 48Cminꢀ1 up to 1208C, 1208C for 15 min, then
58Cminꢀ1 up to 1408C.
Received: July 26, 2012
Published online: November 21, 2012
Keywords: alcohols · epoxidation · hyperbranched polymer ·
.
organocatalysis · polyglycerol
[1] A. Berkessel in Modern Oxidation Methods, 2nd ed. (Ed.: J. E.
Bꢁckvall), Wiley-VCH, Weinheim, 2010, pp. 117 – 145.
[2] J.-P. Bꢂguꢂ, D. Bonnet-Delpon, B. Crousse, Synlett 2004, 18 – 29.
[3] a) A. Berkessel, J. A. Adrio, D. Hꢃttenhain, J.-M. Neudçrfl, J.
[4] a) A. Berkessel, M. R. M. Andreae, H. Schmickler, J. Lex,
[5] a) J. Legros, B. Crousse, J. Bourdon, D. Bonnet-Delpon, J.-P.
3993 – 3998; c) K. S. Ravikumar, Y. M. Zhang, J. P. Bꢂguꢂ, D.
[8] C. Hajji, S. Roller, M. Beigi, A. Liese, R. Haag, Adv. Synth.
[10] a) A. Sunder, R. Hanselmann, H. Frey, R. Mꢃlhaupt, Macro-
Experimental Section
a) “Click reaction” and characterization of polymeric catalysts:
1. In situ deprotection of the TMS-protected alkyne: Tetra-n-
butylammonium fluoride trihydrate (1.8 g, 5.72 mmol, 1.1 equiv) and
4a (1.52 g, 5.2 mmol) were stirred in THF until TLC showed complete
deprotection (ca. 30 min).
2. Click coupling: Diisopropylethylamine (88 mL, 0.52 mmol,
0.1 equiv) and polyglycerol azide
5 (515 mg, 5.2 mmol azide,
1 equiv) in THF were added to the deprotected fluorinated alcohol.
After the mixture had been stirred for 5 min, sodium ascorbate
(103 mg, 0.52 mmol, 0.1 equiv) in 1.5 mL Millipore water was added,
followed by copper(II) sulfate pentahydrate (130 mg, 0.52 mmol,
0.1 equiv) in 1.5 mL Millipore water. The reaction mixture was stirred
overnight at RT. TLC analysis indicated complete consumption of the
fluorinated alcohol. The solution was concentrated and the residue
was diluted in water and extracted with ethyl acetate. The combined
organic layers were washed several times with small portions of
saturated EDTA solution until the blue color of the aqueous phase
had disappeared. The crude product was further purified by ultra-
filtration (Millipore solvent-resistant stirred cell (XFUF07601);
solvent: methanol; membrane material: regenerated cellulose, molec-
ular-weight cut-off (MWCO) of the membrane: 5 kDa). The poly-
meric catalyst 6a was obtained in 84% yield (1.4 g) with a loading of
3.0 mmol fluoroalcohol head groups per gram.
1H NMR (700 MHz, [D6]DMSO): d = 8.16–7.42 (m, 1H, tri-
azole), 5.30–4.60 (functionalized primary/secondary PG groups),
4.09–3.01 (PG), 2.89–2.66 (m, 2H, H4), 2.26–2.03 ppm (m, 2H, H3).
742
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 739 –743