JMS letters
Received: 30 September 2013
Revised: 4 December 2013
Accepted: 11 December 2013
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/jms.3325
Catalytic oxidation of alkanethiols and
dialkyldisulfides to alkanesulfonic acids by
H2O2/CH3ReO3 examined by electrospray
ionization mass spectrometry
Dear Sir,
Moreover, MTO has become one of the most widely used cata-
lysts for oxidation reactions involving H2O2, as it exhibits broad
versatility for facilitating the oxidation of a broad range of
functional groups.[6,7] It is now understood that this occurs via
the reversible formation of the mono- and diperoxorhenium
complexes (CH3Re(O2)O2 and CH3Re(O2)2O, respectively), which
allows efficient O atom transfer to a variety of nucleophilic
targets.[6–11]
Work in this lab has recently focused on developing a method by
which mixed self-assembled monolayers (SAMs) of alkanethiols
on gold surfaces may be quantitatively analyzed by electrospray
ionization mass spectrometry (ESI-MS). The approach that is cur-
rently being developed involves the use of hydrogen peroxide
to oxidatively remove these films for direct infusion into the
electrospray source. The rationale for these investigations, as well
as preliminary studies of the experimental conditions necessary
to produce various alkanesulfonates from their alkanethiol
precursors, has been detailed recently.[1] In short, the goal in
the preliminary studies has been the development of an oxida-
tion process that yields the same sulfur oxidation product for
each structurally distinct alkanethiol present. Such an approach
should facilitate the quantification of complex SAMs that consist
of multiple components.
An approach using MTO has therefore appeared to be a prom-
ising route for eliminating disulfide formation in our own studies
and has the added advantage that it involves only a minor
change to our current methodology (the addition of the catalyst).
Since disulfides are intermediate oxidation products between
thiols and sulfonic acids, we decided to examine whether excess
H2O2 in combination with the catalyst MTO would allow us to
quantitatively convert alkanethiol precursors (both short- and
long-chain species) directly to sulfonic acids without getting
trapped in-between with the semi-stable disulfide intermediate.
The effect of alkyl chain length on the efficacy of this process is
critical given that the focus of our methodology will be studies
of mixed alkanethiol SAMs whose components may differ signif-
icantly in chain length.[1] Thus, the research reported here
involves the application of MTO to the oxidation of both short
and long chain ω-mercaptoalkanoic acids and an examination
of the resulting solutions by ESI-MS to determine the sulfur oxida-
tion products. As mentioned above, we have chosen to employ
ω-mercaptoalkanoic acids as the means by which the general
conversion efficiency is assessed since the terminal carboxylic
acid moiety allows the observation of these products by MS. To
the authors’ knowledge, only one other publication has appeared
involving ESI-MS analyses of MTO-containing solutions, the focus
of which centered primarily on the base-catalyzed hydrolysis of
MTO and the resulting decomposition products.[12]
Recent studies in our lab of the oxidation of ω-
mercaptoalkanoic acids by H2O2 have indicated that not only
is the desired alkanesulfonic acid (1) produced by this process,
but that a disulfide-containing product (2) occurs as well:
HO2CðCH2Þ SH þ 3H2O2 → HO CðCH Þ
þ
3H2O
SO3H
2
2
ð Þ 10
10
1
HO2CðCH2Þ SꢀSðCH2Þ10CO2H
10 ð Þ
2 HO2CðCH2Þ10SH þ H2O2 →
2
þ
2H2O
where (1) was observed by negative-ion ESI-MS as the sulfo-
nate, and (2) was observed as the carboxylate monoanion.[2]
The addition of excess H2O2 did not appear to accelerate ox-
idation of the disulfide product to the sulfonate, as the former
seemed to be semi-stable. For the methyl-terminated
alkanethiols employed in our previous work, formation of
the corresponding disulfide product would not have been
detected by MS due to their being electrically neutral.[1] Thus,
the presence of the carboxylic acid moiety at the chain termi-
nus allows one to probe mass spectrally the creation of various
sulfur oxidation products resulting from this procedure. Since
All reagents for these experiments were used as received:
3,3′-dithiodipropanoic acid (3,3′-DTDPA: Acros Organics, 99%),
3-mercaptopropanoic acid (3-MPA: Aldrich, >99%), 11-
mercaptoundecanoic acid (11-MUDA: Aldrich, 95%), pentane-
1-thiol (C5SH: Aldrich, 98%), octane-1-thiol (C8SH: Aldrich,
98.5%), methanesulfonic acid (C1SO3H: Aldrich, 99.5%),
ethanesulfonic acid (C2SO3H: Aldrich, 95%), hydrogen peroxide
(Sigma-Aldrich, 30% w/w) and methyltrioxorhenium(VII)
(CH3ReO3, MTO: Aldrich, 71–76%). Solutions were freshly
our goal is to produce
(alkanesulfonates) upon exposure to H2O2, the presence of a
disulfide product is clearly undesirable.
a
single oxidation product
In recent years, several groups have shown that the room tem-
perature oxidation of various alkyl- and aryl-substituted thiols
and disulfides to the corresponding sulfonic acids can be accom-
plished using H2O2 if the oxidation is performed in combination
with the catalyst methyltrioxorhenium(VII) (CH3ReO3, MTO).[3–5]
*
Correspondence to: Brian W. Gregory, Department of Chemistry and Bio-
chemistry, Samford University, Birmingham, AL, 35229-2236, USA. E-mail:
J. Mass Spectrom. 2014, 49, 241–247
Copyright © 2014 John Wiley & Sons, Ltd.