Angewandte
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Chemie
A representative reaction profile obtained over 1 h at
208C is shown in Figure 2. This data could be fit to a simple
kinetic model for successive bimolecular reactions with H2O2,
from which rate constants of 0.033, 0.013 and 0.00050mÀ1 sÀ1
were derived for the reactions of thiol, sulfenic acid, and
sulfinic acid, respectively. These results suggest that the thiol
is most easily oxidized, followed by sulfenic acid and sulfinic
acid.
Scheme 2. Synthesis of fluorinated 9-triptycenethiol (1), sulfenic acid
(2), sulfinic acid (3), and sulfonic acid (4). i) BuLi, THF, À788C, then
I2. ii) tBuSH, CuI, tBuOK, 1,10-phen, 1108C, PhCH3. iii) BuLi, THF,
À788C, then NFSI. iv) Anthralinic acid, isoamylnitrite, TCA, THF.
v) NBS, DMF, 08C. vi) BuLi, PhH/MTBE, À208C, then (2-PyrS)2.
vii) RuCl3, NaIO4, H2O/EtOAc. THF=tetrahydrofuran, phen=1,10-phe-
nanthroline, TCA=trichloroacetic acid, NBS=N-bromosuccinimide,
DMF=N,N-dimethylformamide, TFA= trifluoroacetic acid,
Figure 2. Representative reaction profile for the oxidation of 9-fluoro-
10-triptycenethiol (5 mm) by H2O2 (50 mm) in buffered methanol at
sspH 11.5 (20 mm 2,2,6,6-tetramethylpiperidine) at 208C. Concentra-
tions were determined directly by 19F NMR relative to an internal
standard (PhCF3). Solid lines are from numerical fitting of the data to
a simple kinetic scheme for consecutive bimolecular reactions of the
thiol, sulfenic acid, and sulfinic acid with H2O2.
mCPBA=m-chloroperbenzoic acid, Pyr=pyridine, NFSI=N-fluoroben-
zenesulfonimide, MTBE=methyl tert-butyl ether.
Given these results, we independently synthesized the
fluorinated 9-triptycenesulfenic (2), sulfinic (3),[13] and sul-
fonic (4) acids (Scheme 2) and confirmed that they corre-
spond to the two intermediates and final product, respec-
tively, observed in the oxidations (see Figure 1).
With the authentic reaction intermediates in hand, we also
determined the rate constants for each reaction step inde-
pendently (Figure 3), obtaining essentially indistinguishable
results from those obtained by fitting the data in Figure 2, that
is, kobs = 0.029, 0.014, and 0.00058mÀ1 sÀ1 for the oxidation of
the thiol, sulfenic acid, and sulfinic acid, respectively.
The role of a thiol/thiolate pre-equilibrium in the oxida-
tion of thiols by H2O2 has been demonstrated.[6h] We sought to
both confirm that this was also the case for the current system,
and also investigate its role in the reactions of the sulfenic and
sulfinic acids with H2O2. Results of reactions carried out in
buffered methanol at sspH[14] 6.6 to 13.5 are shown in Figure 4.
The observed rate constants for thiol and sulfenic acid
&
Figure 3. Initial rates of reaction of 9-fluoro-10-triptycenethiol ( ), 9-
*
fluoro-10-triptycenesulfenic acid ( ), and 9-fluoro-10-triptycenesulfinic
oxidation were dependent on the pH of the medium, reaching
~
acid ( ) as a function of H2O2 concentration in buffered methanol at
s
s
a maximum at around pH ꢀ 12 and 13, respectively, which
sspH 11.5 (20 mm 2,2,6,6-tetramethylpiperidine) at 208C. Reaction
progress was determined directly by 19F NMR on 5 mm of substrate.
roughly correspond to the pKa values of the thiol and sulfenic
acids (determined independently to be 11.6 and 12.8 under
the same conditions, see the Supporting Information).[15] The
plateau at sspH values above the pKa indicate that the inherent
reactivity of the sulfenate anion is greater than of the thiolate.
The rate of sulfinic acid oxidation was independent of pH in
the range that was studied, which is expected since it exists as
2
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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