Selective oxidation of alcohol-d1to aldehyde-d1using MnO2

Article abstract of DOI:10.1039/d1ra05405h

The selective oxidation of alcohol-d₁to prepare aldehyde-d₁was newly developed by means of NaBD₄reduction/activated MnO₂oxidation. Various aldehyde-d₁derivatives including aromatic and unsaturated ald

Full text of DOI:10.1039/d1ra05405h

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Selective oxidation of alcohol-d1 to aldehyde-d1
using MnO2†
Cite this: RSC Adv., 2021, 11, 28530
Hironori Okamura, ‡ Yoko Yasuno, ‡ Atsushi Nakayama,
Katsushi Kumadaki,
Kohei Kitsuwa, Keita Ozawa, Yusaku Tamura, Yuki Yamamoto
*
and Tetsuro Shinada
The selective oxidation of alcohol-d1 to prepare aldehyde-d1 was newly developed by means of NaBD4
reduction/activated MnO2 oxidation. Various aldehyde-d1 derivatives including aromatic and unsaturated
aldehyde-d1 can be prepared with a high deuterium incorporation ratio (up to 98% D). Halogens
(chloride, bromide, and iodide), alkene, alkyne, ester, nitro, and cyano groups in the substrates are
tolerated under the mild conditions.
Received 14th July 2021
Accepted 16th August 2021
DOI: 10.1039/d1ra05405h
rsc.li/rsc-advances
expensive catalysts. Moreover, the synthetic examples of
1. Introduction
substituted acrolein and propynal-d1 are much less than those
12,13
Deuterium (2H, d) is a stable, non-radioactive, and safe isotope
of hydrogen (1H). Since its discovery,1 d has been widely utilized
in organic chemistry, biochemistry, analytical chemistry, phar-
maceutical science, and drug discovery.2,3 Because of the high
demand for d-labelled molecules in the scientic research
elds, many efforts have been devoted to developing a new
method for the synthesis of d-labelled molecules.
of alkyl and aryl aldehyde-d1,
though recently developed
NHC-catalysed H–D exchange reactions allowed access to
various substituted acrolein-d1 derivatives.9 In this context,
development of a new d-incorporation method which allows
exible synthesis of aromatic and unsaturated aldehyde-d1
2
remains to be a challenging synthetic task (Scheme 1).
Method A using Dꢀ as a deuterium source has been recog-
nized as a robust and conventional synthetic method to prepare
aldehyde-d1 2 (Scheme 2). The synthesis is typically performed
Aldehyde-d1
2 has received signicant attention as
a synthetic target due to the facts that aldehyde 1 is a useful
feedstock in organic synthesis. Various methods have been
performed in the synthesis of alkyl and aryl aldehyde-d1. For
example, more than 40 syntheses (25 different reaction condi-
tions) of benzaldehyde-d1 (PhCDO) were conducted even since
2018 in the studies to develop new d-incorporation method or
reaction mechanism using PhCDO.4–8
The previous synthetic approaches to access d-labelled
molecules are classied into 5 types; (A) addition of Dꢀ fol-
lowed by oxidation, (B) carbonyl Umpolung approach, (C)
radical reaction, (D) transition metal-catalysed reaction, and (E)
others. Recently, mild, one-step, and catalytic syntheses of
aldehyde-d1 2 have been achieved by deuteration of the Breslow
intermediates,9 deuteration of acyl radicals,6,10 and transition
metal-catalysed deuterium incorporation.11 However, the
previous synthetic methods including the modern direct
syntheses oen suffered from drawbacks such as over-
deuteration, requirements of harsh conditions (high and low
temperature, and strong base and acids), and the use of
Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi, Osaka,
558-8585 Japan. E-mail: shinada@sci.osaka-cu.ac.jp
Scheme 1 Synthesis of aldehyde-d1 derivatives. (A) reduction of the
formyl with Dꢀ and oxidation, (B) carbonyl Umpolung approach, (C)
radical H–D exchange, (D) transition metal-catalysed H–D exchange,
and (E) others.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra05405h
‡ HO and Y. Yasuno contributed equally to this study.
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Scheme 2 Oxidation of deuterated alcohols. Eqn (1): oxidation of
alcohol-d2, eqn (2): selective oxidation of alcohol-d1.
in two steps; (i) reduction of carboxylic acid derivatives using
LiAlD4 to provide alcohol-d2 3 and (ii) oxidation to aldehyde-d1 2
(eqn (1)).14 In this approach, the deuterium incorporation ratio
(%D) of the commercially available Dꢀ sources such as LiAlD4
(>98 atom) is reliably transferred into the product. On the other
hand, the use of highly reactive LiAlD4 oen limits the synthetic
scope. Under the conditions, various functional groups such as
nitro, nitrile, ester, and acid moieties, and alkene and alkyne
with electron-withdrawing group(s) are not tolerated. To over-
come the limitation, we emerged selective oxidation of alcohol-
d1 derivatives 4 (Scheme 2(eqn (2))). It is expected that various
alcohol-d1 4 can be prepared by the mild NaBD4 reduction. The
next selective oxidation of D (H/D selectivity) is the key to this
approach. Recently, oxidation of benzyl alcohol-d1 (PhCDHOH)
with PCC or PDC was conducted to prepare PhCDO with ꢁ85%
D.4a,4e,4q On the other hand, further efforts to improve the
selectivity (%D) in the selective oxidation have not been well-
examined. Herein, we would like to report that NaBD4 reduc-
tion followed by activated MnO2 oxidation (NaBD4/MnO2
system). The simple and mild protocol allows expansion of the
synthetic range of aldehyde-d1 2 including not only aromatic
aldehyde-d1 derivatives but also substituted acrolein-d1 and
propynal-d1 derivatives with high %D (up to 98%).
Scheme 3 Reduction of aldehyde 1 with NaBD4.
reagents (entry 1, Table 1). Treatment of 4a with 23 eq. of MnO2
in CH2Cl2 gave aldehyde-d1 2a with 92% D in 2 h. The use of
pyridinium dichlorochromate (PDC) gave 2a in good selectivity
(88% D).4e However, the isolate yield was moderate (entry 2).
Dess–Martin periodinane oxidation, 2,2,6,6-tetramethylpiper-
idine 1-oxyl (TEMPO) oxidation in the presence of PhI(OAc)2,
and Parikh–Doering oxidation (sulfur trioxide–pyridine
complex in dimethyl sulfoxide (DMSO)) resulted in lower
selectivities (74, 76 and 66%D, entries 3–5).
Activated MnO2 oxidation was successfully expanded to the
synthesis of various aldehyde-d1 2a–2aa with high %D (85–96%
D) (Scheme 4A–C). Chloride, bromide, iodide, methoxy, ethoxy,
or methylene acetal, nitrile, ester, nitro, and alkyne groups on
the aromatic ring of 4c–4q are preserved under the mild
oxidation conditions (Scheme 4A). Substituted acrolein 4r–4v
and propynal 4w–4aa smoothly underwent MnO2 oxidation to
provide 2r–2aa without loss of the alkene and alkyne moieties
(Scheme 4B and C). The synthetic utility was further demon-
strated by the synthesis of 2v with a bromo group at the a-
position of cinnamaldehyde. In addition, Bz, THP, and TBS
protecting groups of 4y, 4z, and 4aa were also maintained under
the conditions. These propargyl alcohols 4y, 4z, and 4aa were
smoothly converted to the corresponding propynal derivatives
2y, 2z, and 2aa with high %D, respectively.
2. Results and discussion
In a similar manner to the previous synthetic examples of
NaBH4 reduction of aldehyde 1, the reduction with NaBD4 gave
the corresponding alcohol-d1 derivatives 4 with excellent func-
tional group compatibility and yields (Scheme 3). Chloride,
bromide, iodide, methoxy, ethoxy, or methylene acetal, nitrile,
ester, nitro, and alkyne groups on the aromatic ring of 4c–4q
were tolerated under the conditions. Substituted acrolein and
propynal 1r–1aa also underwent smooth NaBD4 reduction to
provide 4r–4aa without loss of the alkyne and alkene moieties,
and tetrahydropyanyl (THP), benzoyl (Bz), and tert-butyldime-
thylsilyl (TBS) protecting groups.
In conjunction with our recent efforts toward elucidation of
biosynthetic reaction mechanisms of terpene synthases using d-
labelled prenols,15,16 we needed geranylgeraniol-d2 (6) as an
enzyme substrate. Previously, the synthesis of 6 (ref. 17) and
other acyclic prenol-d2 derivatives18 was performed in four steps
from 5 via reduction of ester 7 with LiAlD4. However, commer-
cially available LiAlD4 is almost out of stock in recently years. In
We next examined the key oxidation of alcohol-d1 4 using 4-
phenylbenzyl alcohol-d1 (4a) (Table 1). As a result, activated
MnO2 was found to be superior to other general oxidation
ꢂ
addition, low temperature conditions (ꢀ20 C) is required for
the LiAlD4 reduction to avoid the undesired 1,4-reduction. We
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Table 1 Oxidation of alcohol-d1 4aa
Entry
Conditions
Yieldb (%)
%Dc
1
2
3
4
5
MnO2 (23 eq.), 1 h
PDC (1.2 eq.), MS4A, 2 h
Dess–Martin periodinane (1.5 eq.), 5 min
TEMPO (0.01 eq.), Bu4NHSO4 (0.05 eq.), NaOCl (1.2 eq.), 1 h
DMSO (10 eq.), SO3–pyridine (4 eq.), iPr2NEt (5 eq.), 1.5 h
92
51
84
96
75
92
88
74
76
66
a
b
c
0.5 mmol scale. Isolated yield. %D for 2a is calculated based on the integration ratios of aldehyde and aromatic proton. MnO2 ¼ activated
manganese dioxide, PDC ¼ pyridinium dichlorochromate, MS4A ¼ molecular sieves 4A, TEMPO ¼ 2,2,6,6-tetramethylpiperidine 1-oxyl, DMSO
¼ dimethyl sulfoxide.
expected that NaBD4/MnO2 system would be an alternative to subsequently reduced by NaBD4 to provide geranylgeraniol-d2
the LiAlD4 procedure to prepare 6, conveniently. According to (6) in 70% yield over four steps with satisfactory deuterium
the literature,19 geranylgeraniol (5) was converted to aldehyde 8 incorporation ratio (94% D). Under the conditions, the unde-
by MnO2 oxidation (Scheme 5). Aldehyde 8 was subjected to sired 1,4-addition reaction was not observed. Thus, an
NaBD4/MnO2 to deliver d-enriched aldehyde 9 which was
Scheme 4 MnO2 oxidation of alcohol-d1 4. (A) synthetic examples of aromatic aldehyde-d1 2, (B) synthetic examples of substituted acrolein-d1
a
2, and (C) synthetic examples of substituted propynal-d1 2. 2 h, b6 h, c12 h.
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cycloaddition reaction, and transition metal catalysed trans-
formations. In this context, NaBD4/MnO2 system would offer
vital opportunity to the synthesis of highly functionalized d-
labelled molecules via facile preparation of aromatic and
unsaturated aldehyde-d1 2. Deuterium-labelled compounds are
oen needed for the investigation of the mechanisms or
determination of the rate-limiting step. The present synthetic
method supports the studies from the viewpoint of the facile
preparation of aldehyde-d1 2 and its derivatives. Further appli-
cation and mechanism studies are ongoing in our laboratory.
Author contributions
Scheme 5 Synthesis of geranylgeraniol-d2 (6) from geranylgeraniol
(5).
Y. Yasuno and HO are contributed equally. Y. Yasuno, HO, and
TS designed the synthetic route. TS wrote the manuscript. HO,
Y. Yasuno, and AN prepared ESI.† HO, Y. Yasuno, AN, K.
Kumadaki, K. Kitsuwa, KO, YT, and Y. Yamamoto performed
syntheses of 2.
operationally simple and mild deuteration of prenols-d2 was
achieved by application of NaBD4/MnO2 system.
Previously, Brecker et al. investigated 13C kinetic isotope
effects (KIEs) in the oxidation of cinnamyl alcohol using MnO2,
Dess–Martin periodinane, and Swern oxidation (DMSO/(COCl)2/
Et3N) to gain insight into the reaction mechanism.20 Compar-
ison of the kinetic isotope of effects revealed the following order
MnO2 > Dess–Martin oxidation z Swern oxidation. The higher
13C KIE using MnO2 displayed that the C–H bond breaking in
the intermediate is irreversible and rate-determining, and the
oxidation proceeded via energy rich transition state. On the
other hand, the lower 13C KIEs observed in Swern oxidation and
Dess–Martin oxidation indicated that the intramolecular C–H
bond cleavage in these oxidation reaction processes would not
be slower to be rate-limiting.
Experimental results in Table 1 clearly shows that the
degrees of %D are as follows MnO2 > PDC > TEMPO z Dess–
Martin > SO3–pyridine/DMSO. It is speculated that higher %D of
MnO2 oxidation and lower %D of SO3–pyridine/DMSO oxidation
would correlate to the 13C KIE data (MnO2 > Swern oxidation). It
is interesting to note that the %D value in Scheme 4 depended
on the substrates. The oxidation of propargyl alcohols 4w–4aa
resulted in higher %D than those of the other alcohols. The
oxidation of 4w–4aa needed a longer reaction period to
complete the reactions. As mentioned in the previous 13C KIE
studies, the rate limiting steps of the MnO2 oxidation relies on
the C–H cleavage step of the reaction intermediate. It is
considered that the slower C–H cleavage would provide the
higher %D.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported from JSPS Kakenhi
(JP19H04661 for TS, JP20K09396 for AN, and JP21K14629 for
YYasuno), the OCU ‘Think globally, act locally’ Research Grant
for Young Scientists 2021 through the hometown donation fund
of Osaka City for AN, the Uehara Memorial Foundation for AN,
and the Sasakawa Scientic Research Grant from The Japan
Science Society for HO.
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