C?N Axial Chiral Hypervalent Iodine Reagents: Catalytic Stereoselective α-Oxytosylation of Ketones

Article abstract of DOI:10.1002/chem.202005253

A simple synthesis of a library of novel C?N axially chiral iodoarenes is achieved in a three-step synthesis from commercially available aniline derivatives. C?N axial chiral iodine reagents are rarely investigated in the hypervalent iodine arena. The potential of the novel chiral iodoarenes as organocatalysts for stereoselective oxidative transformations is assessed using the well explored, but challenging stereoselective α-oxytosylation of ketones. All investigated reagents catalyse the stereoselective oxidation of propiophenone to the corresponding chiral α-oxytosylated products with good stereochemical control. Using the optimised reaction conditions a wide range of products was obtained in generally good to excellent yields and with good enantioselectivities.

Full text of DOI:10.1002/chem.202005253

Communication
doi.org/10.1002/chem.202005253
Chemistry—A European Journal
&
Organocatalysis
CÀN Axial Chiral Hypervalent Iodine Reagents: Catalytic
Stereoselective a-Oxytosylation of Ketones
Haifa Alharbi,[a] Mohamed Elsherbini,[a, b] Jihan Qurban,[a, c] and Thomas Wirth*[a]
they are gaining an increased interest as redox-active media-
Abstract: A simple synthesis of a library of novel CÀN ax-
ially chiral iodoarenes is achieved in a three-step synthesis
from commercially available aniline derivatives. CÀN axial
chiral iodine reagents are rarely investigated in the hyper-
valent iodine arena. The potential of the novel chiral io-
doarenes as organocatalysts for stereoselective oxidative
transformations is assessed using the well explored, but
challenging stereoselective a-oxytosylation of ketones. All
investigated reagents catalyse the stereoselective oxida-
tion of propiophenone to the corresponding chiral a-oxy-
tosylated products with good stereochemical control.
Using the optimised reaction conditions a wide range of
products was obtained in generally good to excellent
yields and with good enantioselectivities.
tors in oxidative electrochemical transformations.[9]
Among the wide spectrum of chiral hypervalent iodine re-
agents and chiral iodoarene catalysts, axial chiral iodine-con-
taining scaffolds are very promising from structural and syn-
thetic perspectives. Numerous enantioselective oxidative trans-
formations have been achieved with high levels of stereocon-
trol using axial chiral hypervalent iodine reagents under stoi-
chiometric and catalytic reaction conditions.[3] The majority of
axial chiral hypervalent iodine reagents and their iodoarene
precursors contain a chiral CÀC axis such as biphenyls 1,[10] bi-
naphthyls 2[11] or spiroindanes 3[12] (Figure 1). On the other
hand, axial chiral iodoarenes containing a chiral CÀN axis such
as 4 are rarely investigated in the context of hypervalent
iodine chemistry. Hence, the synthesis of such compounds
with a chiral CÀN axis and the investigation of their potential
in stereoselective oxidative transformations is of great interest.
To the best of our knowledge, only one report on the synthesis
and reactivity of C-N axial chiral hypervalent iodine reagents
emerged during the final preparation of this work.[13]
Hypervalent iodine compounds are very attractive in modern
synthetic chemistry as they are environmentally and economi-
cally viable alternatives to transition metal reagents.[1] Al-
though the history of chiral hypervalent iodine reagents can
be traced back to the seminal work by Pribram published in
1907,[2] it took almost a century till they became active players
in stereoselective synthesis.[3] Nowadays, chiral hypervalent
iodine reagents are widely used in a wide range of stereoselec-
tive transformations, including, but not limited to, stereoselec-
tive difunctionalisation of alkenes,[4] a-functionalisation of car-
bonyl compounds,[5] oxidation of sulfur compounds,[6] phenol
dearomatisation,[7] and oxidative rearrangements.[8] In addition
Figure 1. Examples of axially chiral iodoarene scaffolds.
[a] H. Alharbi, Dr. M. Elsherbini, Dr. J. Qurban, Prof. Dr. T. Wirth
School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff, CF10 3AT (UK)
E-mail: wirth@cf.ac.uk
Herein, we report a simple synthesis of a small library of
novel CÀN axial chiral iodoarenes, starting from commercially
available aniline derivatives and investigate their potential as
chiral organocatalysts using the extensively studied— yet chal-
lenging— hypervalent iodine mediated stereoselective a-oxyto-
sylation of ketones as a model reaction.[5c–d,14]
[b] Dr. M. Elsherbini
current address: Department of Chemistry
University of Huddersfield
Queensgate, Huddersfield HD1 3DH (UK)
[c] Dr. J. Qurban
current address: Department of Chemistry
Faculty of Applied Science
Umm Al-Qura University, Makkah (Saudi Arabia)
In contrast to axial chiral biaryl systems, the methods avail-
able for the stereoselective construction of CÀN axial chiral
compounds are limited. As we are interested to develop a
facile and rapid access to the target molecules, we want to
avoid the use of specialised and/or complex or expensive cata-
lysts and reagents. Therefore, our synthesis relies on chiral res-
olution to keep the synthetic route simple and to access opti-
cally active target molecules from simple and cheap commer-
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202005253.
ꢀ 2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
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Chemistry—A European Journal
cially available chemicals. In addition, chiral resolution would
enable access to diastereomers of each compound enabling a
rapid construction of the target library of chiral iodoarenes.
The synthesis commences with the electrophilic iodination
of anilines using molecular iodine[8f] to give the corresponding
iodoanilines 5a and 5b in 88% and 40% yield, respectively.
Racemic iodosulfonamides 6a–d were obtained in good yields
by treatment of iodoanilines 5 with the sulfonyl chloride deriv-
atives, namely, p-tosyl chloride (TsCl), p-nosyl chloride (NsCl)
and p-anisylsulfonyl chloride (AnCl). Reaction of the racemic
mixtures 6a–d with (S)-lactate esters under Mitsunobu reaction
conditions[8f] led to the formation of the corresponding diaste-
reomeric mixtures of 7 that were easily separated by crystalli-
sation or column chromatography. The reaction led to the for-
mation of the SC-N diastereomer as major isomer and the RC-N
diastereomer as the minor isomer in all cases with the de rang-
ing from 10% to 30%. As a result, ten novel optically active CÀ
N axial chiral iodoarenes (7a–j) were synthesised in satisfactory
yields over three simple chemical steps (Scheme 1).
Figure 2. 3D structure and absolute configuration of diastereomers 7a and
7b.
Table 1. Screening of pre-catalysts 7 in the enantioselective a-tosyloxyla-
tion of acetophenone 8a.[a]
Entry
ArÀI
9a Yield [%]
9a ee [%][b]
9a abs. config.
reagent
1
2
3
4
5
6
7
8
9
10
7a
7b
7c
7d
7e
7 f
7g
7h
7i
40
85
93
96
81
78
45
62
53
20
47
40
43
67
31
59
21
64
9
R
S
R
S
R
S
R
S
R
S
7j
55
[a] Reactions were carried out with 0.027 mmol of 7, 0.27 mmol of 8a,
0.81 mmol of mCPBA and 0.81 mmol of TsOH in acetonitrile (5 mL) at
room temperature for 72 h. [b] Enantiomeric excesses were determined
by chiral-phase HPLC analysis.
Scheme 1. Synthesis of novel C-N axial chiral iodoarenes 7a–j. Ts: 4-toluene-
sulfonyl; Ns: 3-nitrobenzenesulfonyl; An: 4-methoxybenzenesulfonyl.
The absolute configuration of the iodoarenes 7 were as-
signed through analysis of the X-ray crystallographic struc-
tures.[15] The 3D structures of the diastereomers 7a and 7b are
shown in Figure 2 while other X-ray structures (7d, 7e, 7h) are
found in the supporting information.
entry 7). The best results were obtained using pre-catalyst 7d
(entry 4) where (S)-9a was formed in excellent yield (96%) and
with good enantioselectivity (67% ee). On the other hand, pre-
catalyst 7c gave the best results for the opposite enantiomer
(R)-9a. Importantly, the configuration of the stereogenic centre
of 9a seems solely depending on the configuration of the
chiral CÀN axis where the RC-N configuration always forms 9a
with (S)-configuration. This proves that the stereochemical in-
duction is mainly controlled by the chiral axis and not by the
stereocentre in the lactate moiety.
After the library synthesis of CÀN axial chiral iodoarenes,
their potential as organocatalysts for the stereoselective a-oxy-
tosylation of ketones was initially probed using propiophenone
8a as a model substrate, m-chloroperbenzoic acid (mCPBA,
3 equiv) as the terminal oxidant and p-toluenesulfonic acid
(TSA, 3 equiv) as nucleophile, according to a literature proce-
dure.[14b] The result of the catalyst screening (Table 1) shows
that catalytic amounts (10 mol%) of all iodoarenes 7 led to the
formation of the desired product 9a in good to excellent
yields and only with pre-catalyst 7j (entry 10) a low yield was
obtained. The enantiomeric excess of the resulted a-tosyloxy
ketone 9a was moderate to good (31–67% ee) in all cases
except for pre-catalysts 7i (9% ee, entry 9) and 7g (21% ee,
Using pre-catalyst 7d, the influence of other reaction param-
eters, such as solvent, stoichiometries of the terminal oxidant
and p-toluenesulfonic acid, and temperature were studied
(Table 2). Performing the reaction at 08C (entry 2) lead to a
slight increase in enantioselectivity (70% ee) but was accompa-
nied by a significant reduction in yield (26%). On the other
hand, increasing the reaction temperature to 508C (entry 3)
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Table 2. Screening of reaction parameters.[a]
Entry Solvent
x [equiv] Temp. [8C] 9a Yield [%] 9a ee [%][a]
1
2
3
4
5
6
7
8
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
Et2O
EtOAc
MeCN-CH2Cl2 (1:1)
MeCN-CH2Cl2 (1:2)
MeCN-CH2Cl2 (2:1)
EtOAc-CH2Cl2 (1:1)
3
3
3
2
5
3
3
3
3
3
3
3
20
0
96
26
94
92
96
70
40
64
94
90
93
72
67
70
66
52
64
73
76
56
75
73
72
75[b]
50
20
20
20
20
20
20
20
20
20
9
10
11
12
[a] Enantiomeric excesses were determined by chiral-phase HPLC analysis.
[b] Synthesis of (R)-9a using catalyst 7c (10 mol%).
did not show a significant change in the reaction outcome. De-
creasing the amount of mCPBA and p-toluenesulfonic acid
from 3 to 2 equivalents (entry 4) did not affect the yield, but
led to a reduced ee of 52% while using 5 equivalents (entry 5)
had no effect. Subsequent results show that the most effective
reaction parameter is the solvent. The reaction performed in
ethyl acetate did not perform well (entry 8), but with dichloro-
methane and diethyl ether an improved enantioselectivity of
73% and 76% ee was observed albeit with much smaller yields
(entries 6, 7). Using mixtures of acetonitrile and dichlorome-
thane (entries 9–11) was more efficient compared to using any
of the two solvents on their own. The best result, 94% yield
and 75% ee of 9a was obtained using a 1:1 mixture. On the
other hand, the best results for using the diastereomeric pre-
catalyst 7c [(R)-9a: 72% yield, 75% ee, entry 12] were obtained
with a 1:1 mixture of EtOAc and CH2Cl2 (see supporting infor-
mation, Table S1).
Scheme 2. Reaction scope of the stereoselective a-oxytosylation protocol.
electron-withdrawing group attached to the aromatic ring (Cl,
CF3, NO2) gave the products 9b, 9c, 9d and 9e in very good
yields and with good enantioselectivities under both condi-
tions, while the derivatives with electron-donating groups (Me,
OMe, tBu) gave the corresponding products 9 f, 9g and 9h in
lower yields, but without big influence on the enantioselectivi-
ty. Also, the naphthyl derivative 9i was obtained in good enan-
tioselectivity but with moderate yield. Changing the aliphatic
a-carbon from methyl to ethyl lead to product 9j in excellent
yields (>90%) and good stereoselectivities (77% and 80% ee)
under both conditions A and B, respectively, while introducing
a phenyl group at the aliphatic a-carbon led to complete loss
of stereoselectivity, giving 9k in high yields but as a racemate.
Heterocyclic ketones containing furan and thiophene moieties
provided the corresponding products 9l and 9m in good
(57%) to high (80%) enantiomeric excess and high yields for
the furan derivative 9l. The reactivity and selectivity of the re-
action showed dependence on the ring size of cyclic ketones.
1-Indanone afforded the corresponding product 9n in high
yields and in moderate ee, a-tetralone gave 9o in low conver-
sion and yield and poor ee while the seven-membered ring
ketone 1-benzosuberone was not reactive at all. Changing the
With the optimised reaction conditions for synthesising both
enantiomers of 9a, the scope of the substrates was investigat-
ed (Scheme 2). The reaction provided satisfactory to excellent
yields (31–96%) under both conditions A and B for most of
the products. Only the thiophene derivative 9m is formed in
poor yields and 1-benzosuberone was not reactive under the
reaction conditions forming only trace amounts of the pro-
duct 9p. The enantioselectivity was moderate to very good
under both reaction conditions ranging from 48–80% under
condition A and from 60% to 80% under condition B, except
the a-tetralone derivative 9o was formed in only 32 and 20%
ee, respectively. A comparison of the enantioselectivity of the
reaction using the conditions A and B with already reported
protocols[5c,14] using various classes of chiral iodoarenes/ hyper-
valent iodine reagents shows a good improvement of the ste-
reochemical induction of the reaction using the newly synthes-
ised C-N axial chiral iodoarenes 7, especially 7d and 7c and
demonstrates their potential as organocatalysts in stereoselec-
tive oxidative transformations. Propiophenone derivatives with
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sulfonic acid from p-toluenesulfonic acid to benzenesulfonic
acid and methanesulfonic acid was successful under both reac-
tion conditions and led to the formation of the propiophenone
derivatives 9q and 9r in excellent yields and with reasonable
enantioselectivity.
Acknowledgements
We thank the government of Saudi Arabia and the Northern
Borders University, KSA for the financial support and scholar-
ship to HA. We thank the Mass Spectrometry Facility, School of
Chemistry, Cardiff University, for mass spectrometric data. The
authors are also grateful to Dr. B. Kariuki, School of Chemistry,
Cardiff University, for X-ray crystallographic measurements.
The reaction mechanism was a subject of various stud-
ies.[10a,14c,d] The reaction has two possible mechanistic pathways
A and B (Scheme 3). Equilibration of Int-I and Int-II could lead
to racemisation and account for the lower stereoselectivity of
the reaction. Although, Beaulieu and Legault[14c] excluded path-
way B and demonstrated computationally that the reaction
proceeds via pathway A, the formation of the product from
Int-I through a SN2’ mechanism could also suffer from low ste-
reoselectivity.
Conflict of interest
The authors declare no conflict of interest.
Keywords: catalysis · hypervalent iodine · ketones
stereochemistry · a-oxytosylation
·
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In conclusion, ten novel C-N axial chiral iodoarenes have
been synthesised from simple aniline derivatives. Easily separa-
ble diastereomers were obtained via chiral resolution using lac-
tate esters, enabling rapid access to the target library of chiral
iodoarene reagents avoiding the use of complex and/or expen-
sive reagents and catalysts. Their application in the stereoselec-
tive a-oxytosylation of ketones demonstrated their potential as
efficient chiral organocatalysts as a wide range of ketones
could be transformed into the corresponding a-oxygenated
products in generally good to excellent yields up to 96% and
with good to high enantioselectivity up to 80% ee. Cyclic and
aliphatic ketones remain challenging substrates giving unsatis-
factory yields and/or enantioselectivities. The stereochemical
reaction is controlled mainly by the axial chirality.
Experimental Section
Chiral iodine pre-catalyst 7c or 7d (0.027 mmol, 0.1 equiv), mCPBA
(0.81 mmol, 3 equiv), and RSO3H (0.81 mmol, 3 equiv) were dis-
solved in a mixture of MeCN and dichloromethane (1:1) or a mix-
ture of EtOAc and dichloromethane (1:1) followed by the addition
of the appropriate ketone 8 (0.27 mmol, 1 equiv). The reaction mix-
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the reaction, the mixture was washed with sat. aq. NaHCO3 solu-
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Manuscript received: December 8, 2020
Revised manuscript received: January 7, 2021
Accepted manuscript online: January 11, 2021
Version of record online: February 8, 2021
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