Anodic oxidation of dithiane carboxylic acids: A rapid and mild way to access functionalized orthoesters

Article abstract of DOI:10.1021/acs.orglett.0c01324

A new electrochemical methodology has been developed for the preparation of a wide variety of functionalized orthoesters under mild and green conditions from easily accessible dithiane derivatives. The new methodology also offers an unprecedented way to access tri(fluorinated) orthoesters, a class of compound that has never been studied before. This provides the community with a rapid and general method to prepare libraries of functionalized orthoesters from simple and readily available starting materials.

Full text of DOI:10.1021/acs.orglett.0c01324

pubs.acs.org/OrgLett
Letter
Anodic Oxidation of Dithiane Carboxylic Acids: A Rapid and Mild
Way to Access Functionalized Orthoesters
Anthony D. Garcia, Matthew C. Leech, Alessia Petti, Camille Denis, Iain C. A. Goodall, Adrian P. Dobbs,
and Kevin Lam*
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Supporting Information
ACCESS
ABSTRACT: A new electrochemical methodology has been developed for the
preparation of a wide variety of functionalized orthoesters under mild and green
conditions from easily accessible dithiane derivatives. The new methodology also offers
an unprecedented way to access tri(fluorinated) orthoesters, a class of compound that
has never been studied before. This provides the community with a rapid and general
method to prepare libraries of functionalized orthoesters from simple and readily
available starting materials.
rthoesters are highly reactive compounds that have found
many applications in synthesis such as diol protecting
reagents and lead to the formation of significant amounts of
inorganic salts as byproducts. The Pinner synthesis is the most
employed method for the preparation of orthoesters, even
though it is mostly restricted to aliphatic nitriles. It requires the
use of gaseous HCN or HCl to prepare an imidate salt, which is
then slowly decomposed in the presence of an alcohol, usually
over several days, to form the desired orthoesters in modest
yields.13,14 Unsurprisingly, common functional groups, such as
nitriles, amides, or even alkenes are incompatible with such
harsh reaction conditions.
O
groups,1 acylating agents,2 Claisen rearrangement reagents,3 or
coupling partners in heterocyclic chemistry (Scheme 1).4−6
Scheme 1. Common Reactions Using Orthoesters
Recently, electrosynthesis has gained significant attention
from the synthetic community.15−17 Electrosynthesis represents
a practical, green, and economically relevant way to activate
small organic molecules. Whereas several electrosyntheses of
orthoesters have been reported before (Scheme 2b),18−21 none
of them provide a general route to access functionalized
orthoesters, and previously reported electrosynthetic examples
are generally restricted to very specific compounds. Therefore,
in this Letter, we would like to disclose a new general, practical,
versatile, and scalable electrochemical methodology to access
both aromatic and aliphatic orthoesters in good yields.
Orthoesters have also found applications in medicinal chemistry.
For instance, the treatment of betamethasone, an anti-
inflammatory and immunosuppressive compound, with an
orthoester decreases its absorption through the skin and
increases its potency.7 Recently, orthoesters have also been
employed in supramolecular and material science.8 Never-
theless, the methods used to prepare them remain nontrivial and
lengthy and usually require harsh conditions that significantly
limit the tolerance of common functional groups.9 Therefore,
unsurprisingly, only a few orthoesters are commercially
available, which may have impeded the development of their
chemistry.
We would like to introduce dithiane carboxylic acid 1
(DTCOOH) as a nucleophilic orthoester/ester equivalent
(Figure 1).22 The anodic oxidation of the dithiane moiety
Received: April 16, 2020
Several chemical methods have been developed to access
orthoesters (Scheme 2a).10−12 Unfortunately, most of them rely
on unstable precursors in addition to dangerous and toxic
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Scheme 2. Common Methods to Prepare Orthoesters
Scheme 4. Preparation of Electrolysis Precursors 5
converted directly into 5 by treatment with 1,3-propandithiol
(Scheme 4C).
The dithiane n-OctDTCOOH 5a (5, R= n-Oct) was chosen
as a model compound for the initial mechanistic and
optimization studies due to the low volatility of the
corresponding orthoester. The anodic oxidation of dithianes
in aqueous media has been reported before. One common
general feature among the previous reports is that an important
fouling of the electrodes occurs during the electrolysis.26 To
solve this issue, Steckhan et al. reported the use of a
triarylamminium radical cation as an electrooxidant mediator.27
The use of a mediator allows the redox reaction to take place in
the bulk of the solution rather than at the electrode surface, thus
avoiding fouling. Unfortunately, in our case, the use of a
mediator would only allow for an electron transfer with the easily
oxidizable sulfur atoms and would leave the carboxylic acid
untouched due to its high oxidation potential.28
Cyclic voltammetric (CV) experiments performed on 5a
show that the compound is oxidized at Epa = 1.38 V vs Fc+/Fc, a
potential similar to the one reported for the oxidation of
dithianes.27 The CV studies also confirmed the anodic fouling of
the electrode, as can be seen from the current crossings at 0.8
and 1.15 V vs Fc+/Fc (see Figure CV1 in the Supporting
Information).
followed by a Hofer−Moest decarboxylation should lead to the
desired orthoester.17,23,24
The addition of an equivalent of collidine leads to the
formation of an anion with a lower oxidation potential (Epa = 1.0
V vs Fc+/Fc) and also completely suppresses the fouling of the
electrode (see Figure CV2 in the Supporting Information). The
addition of a second equivalent shows that only traces of acid
remain, whereas adding 5 equiv of base converts the carboxylic
acid entirely into its carboxylate.
On the basis of our previous experience with the anodic
decarboxylation of carboxylic acids, we decided to investigate
the use of “Kolbe-type” anodic decarboxylation conditions.29−31
The dithiane carboxylic derivative 5 was treated with a base
to generate the corresponding carboxylate 5−. The neutraliza-
tion of the carboxylic acid serves three purposes: It makes the
medium conductive, it favors the adsorption of the negatively
charged carboxylate onto the positively charged anode, and it
avoids fouling of the electrodes.
Figure 1. DTCOOH, a new orthoester and ester synthon.
Dithiane carboxylic acid 1, a bench-stable crystalline solid, is
readily prepared, on a large scale, by treating glyoxylic acid 4
with 1,3-propanedithiol in the presence of a catalytic amount of
PTSA (Scheme 3).25
Scheme 3. Preparation of DTCOOH
We then explored the preparative anodic oxidation of 5a. A
full optimization table can be found in the Supporting
Information, whereas salient results are summarized in Table
1. At the end of the electrolysis, the orthoester can be easily
isolated by a rapid extraction of the electrolysis solution using
hexane and either a NaOH or a NaHCO3 aqueous solution,
followed by the evaporation of the hexane. As shown by GC−
MS analysis, 1,2-dithiolane 9 is the main byproduct produced
during the electrolysis. The disulfide is volatile enough to be
removed under reduced pressure, leaving the orthoester
The synthesis of the alkylated electrolysis precursors 5 is
simple and rapid, with high yields obtained. This can be achieved
in several ways. For instance, treating DTCOOH 1 with two
equivalents of n-BuLi leads to the formation of [Li+]2[12−]. The
dianion can then be alkylated with a suitable electrophile
(Scheme 4A). Alternatively, aldehydes 6 can be converted into
their corresponding dithianes, deprotonated, and then carboxy-
lated with CO2 (Scheme 4B). Finally, α-keto acids can be
B
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Table 1. Optimization Parameters
entry
conc. 5a (mM)
base (equiv)
current density (mA·cm−2
)
Faradays (F·mol−1
)
electrodes (+/-)
yield 8a (%)
a
1
36
36
36
18
18
9
1
1
1
1
1
2
2
36
18
36
36
36
36
36
15
15
15
15
30
30
30
Pt/Ni
Pt/Ni
C/C
C/C
C/C
C/C
C/C
21
20
28
42
58
99
76
a
2
b
3
b
4
b
5
b
6
b
7
181
a
b
No workup. Solvent removed under reduced pressure after electrolysis. Reaction mixture quenched with aqueous NaHCO3 and subsequently
extracted with hexanes.
sufficiently pure to be used without further purification in the
next reaction step.
Scheme 5. Possible Mechanism
Because of their inherent instability, NMR yields have been
measured for the orthoesters using dibromomethane as an
internal standard.
The concentration of 5a was shown to be a crucial parameter.
Salient results are summarized in Table 1. On a small scale (∼0.1
mmol) (entries 1−7), diluted conditions (9 mM) gave the best
results (entry 7). Surprisingly, when 1.81 mmol of 5a was
electrolyzed in 10 mL of methanol (181 mM), only a minor drop
in yield was observed (entry 8).
As expected, carbon graphite electrodes (entry 3) have been
shown to be superior to platinum electrodes at achieving a
multielectron transfer (entry 2). Presumably, this is because the
substrate is better adsorbed on carbon than on a metallic surface.
The current density was shown to have very little impact on the
reaction as long as it was ≥18 mA·cm−2 (entries 1, 2, 5, and 6).
No significant difference in yield was observed upon the
exchange of NaOMe for ammonia as the base; however, a
notable decrease was observed upon the use of t-BuOK,
presumably due to its limited solubility in methanol (see
Supplementary Table 1 in the Supporting Information).
Although, in theory, a catalytic amount of base should be
sufficient because methoxide ions are continuously produced at
the electrode by the reduction of the methanol at the cathode,
we found that using two equivalents helps to reduce the
impedance of the system and minimize the Joule effect.
Similar to what we have observed during our previous
investigations, oxygen and moisture have little to no effect on the
electrolysis.32,33
by undergoing an intramolecular electron transfer from the
carboxylate to the sulfur radical cation (see Figure M1 in the
Supporting Information for an alternative mechanism.)24
With these conditions in hand, we explored the scope and
limitations of the anodic oxidation of dithiane carboxylic acid
derivatives.
Scheme 5 shows a possible mechanism. The reaction starts
with the oxidation of one of the two sulfur centers of 5− because
their oxidation potential is much lower than that of a
carboxylate. The dithiane then opens to form the corresponding
thionium B. This intermediate is then attacked by the solvent to
form the mixed S,O-acetal C, which quickly decomposes into the
oxonium D by releasing a disulfide, which is oxidized into the
corresponding dithiolane 9. Alternatively, C’s sulfur moiety can
be oxidized and release the oxonium D as well as a 1,3-
propandithiyl radical that cyclizes into 9. Finally, another
molecule of solvent adds onto the oxonium D to form a
carboxylic acetal that decarboxylates to form the desired
orthoester 8 following a classical Kolbe/Hofer−Moest decar-
boxylation. Alternatively, a reviewer made the interesting
suggestion that the intermediate A could decarboxylate rapidly
A large number of functional groups, such as alkenes, alkynes,
silyl ethers, Boc carbamates, and nitriles, are not tolerated by the
classic Pinner reaction due to the highly acidic conditions used.
In addition, the Pinner reaction is known to be unsuccessful for
the preparation of aromatic orthoesters.14 As can be seen from
Figure 2, our methodology is compatible with both aliphatic and
aromatic substituents. The anodic oxidation of 5 tolerates a wide
variety of functional groups such as alkenes (8b), alkynes (8h),
silyl groups (8e), halides (8c,d,k,l,r,s,t,z7), fluorinated groups
(8m,q), nitriles (8j,p), ethers (8x,y,v), and amides (8n). For the
cyclic ether 8y and amide 8n, the electrolyses were only
undertaken for 15 F·mol−1 because prolonged electrolysis of
these compounds led to the formation of the Shono-type
oxidation product (α-methoxylation of the ether or amide).34
Replacing the dithiane with the less expensive dibenzyl
C
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Figure 2. Scope of the electrochemical orthoesterification. aTypical procedure: In a 10 mL ElectraSyn cell equipped with two graphite electrodes, 0.1
mmol (or 1.81 mmol) of 5 was dissolved in 10 mL of MeOH and 2 equiv of NaOMe (for OMe orthoesters) or t-BuOK (for OEt and OCH2CF3
orthoesters). The solution was electrolyzed for 30 or 15 F·mol−1 with a current density of 36 mA·cm−2. bBecause of the instability of the orthoesters, the
yields were measured by NMR using dibromomethane as a reference. cFrom the corresponding dibenzyl thioacetal.
thioacetal led to a slight decrease in the yield of the orthoester
(8z), and the side product, dibenzyldisulfide, was difficult to
remove completely due to its low volatility.
demonstrate their synthetic utility (Scheme 6). The anodically
prepared orthoesters were successfully used for the synthesis of
heterocyclic compounds using metal and Lewis-acid-free
Using ethanol or trifluoroethanol as solvents for the
electrolysis allowed us to easily access a series of triethyl and
trifluoroethyl orthoesters. Reports of tri(ethyl)orthoesters are
even scarcer in the literature than those about their trimethyl
analogues. Moreover, there are only two known tri-
(trifluoroethyl) orthoesters reported as byproducts of a reaction
and with no further study on them. In our hands, we found that
tri(trifluoroethyl) orthoesters were highly reactive and hydro-
lyze extremely rapidly compared with their nonfluorinated
analogues.
Scheme 6. Synthetic Applications of Anodically Prepared
Orthoesters
Finally, we engaged the electrochemically synthesized
orthoesters in some of the possible subsequent reactions to
D
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conditions (see the Supporting Information for examples)35 as
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In conclusion, we have demonstrated that dithiane carboxylic
acids are versatile precursors of orthoesters. The starting
materials are easily prepared in high yields in a few steps from
readily available materials. The new electrosynthetic method-
ology is practical, mild, green, and economically relevant and
tolerates a wide variety of functional groups that are not usually
compatible with the classic methods to prepare orthoesters.
Finally, tri(trifluoroethyl) orthoesters are easily prepared using
this method, which would ease the study of this class of
compounds.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
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0003-1481-9212; Email: k.lam@greenwich.ac.uk
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Authors
Anthony D. Garcia − School of Science, University of Greenwich,
Chatham Maritime ME4 4TB, United Kingdom
Matthew C. Leech − School of Science, University of Greenwich,
Chatham Maritime ME4 4TB, United Kingdom; orcid.org/
0000-0003-3524-2897
Alessia Petti − School of Science, University of Greenwich,
Chatham Maritime ME4 4TB, United Kingdom
Camille Denis − School of Science, University of Greenwich,
Chatham Maritime ME4 4TB, United Kingdom
Iain C. A. Goodall − School of Science, University of Greenwich,
Chatham Maritime ME4 4TB, United Kingdom; orcid.org/
0000-0003-2212-2052
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Orthoesters: The Model Case of Trimethylorthobenzoate. Green Chem.
2013, 15 (8), 2252−2260.
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Electrochemical Methods since 2000: On the Verge of a Renaissance.
Chem. Rev. 2017, 117, 13230−13319.
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(16) Mohle, S.; Zirbes, M.; Rodrigo, E.; Gieshoff, T.; Wiebe, A.;
Waldvogel, S. R. Modern Electrochemical Aspects for the Synthesis of
Value-Added Organic Products. Angew. Chem., Int. Ed. 2018, 57, 6018−
6041.
Adrian P. Dobbs − School of Science, University of Greenwich,
Chatham Maritime ME4 4TB, United Kingdom; orcid.org/
0000-0002-7241-7118
(17) Leech, M. C.; Lam, K. Electrosynthesis Using Carboxylic Acid
Derivatives: New Tricks for Old Reactions. Acc. Chem. Res. 2020, 53,
121.
(18) Brinkhaus, K. H. G.; Steckhan, E.; Degner, D. Indirect
Electrochemical Side-Chain Oxidation of Alkyl Aromatic Compounds
- Selective Synthesis of Methyl Benzoates or Ortho-Benzoic Acid
Trimethylesters. Tetrahedron 1986, 42 (2), 553−560.
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Acid Trialkyl Esters. European Patent EP1362022A1, September 5,
2007.
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
(20) Lebreux, F.; Buzzo, F.; Marko, I. Studies in the Oxidation of
We are grateful to the Engineering and Physical Sciences
Research Council (Grant EP/S017097/1 to K.L. and M.C.L.)
for their financial support, the University of Greenwich (Vice
Chancellor’s Ph.D. Scholarship to A.P. and Fellowship to C.D.),
the University Alliance “Applied Biosciences for Health” Marie
Skłodowska-Curie Grant 801604 (Ph.D. scholarship to A.D.G.),
and IKA for their material support (to K.L.).
Carboxylic Acids: New Twists for an Old Reaction. Synthesis of Various
Cyclic Systems and Substituted Orthoesters. ECS Transactions 2008,
13, 1−10.
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(21) Rockl, J. L.; Hauck, A. V.; Schollmeyer, D.; Waldvogel, S. R.
Electrochemical Synthesis of Fluorinated Orthoesters from 1,3-
Benzodioxoles. ChemistryOpen 2019, 8 (9), 1167−1171.
(22) Garcia, A.; Leech, M.; Petti, A.; Denis, C.; Goodall, I. C. A.;
Dobbs, A.; Lam, K. Anodic Oxidation of Dithiane Carboxylic Acids: A
Rapid, Mild and Practical Way to Access Functionalised Orthoesters.
ChemRxiv 2020.
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