Access to Vinyl Ethers and Ketones with Hypervalent Iodine Reagents as Oxy-Allyl Cation Synthetic Equivalents

Article abstract of DOI:10.1002/ange.202006707

We report an Umpolung strategy of enol ethers to generate oxy-allyl cation equivalents based on the use of hypervalent iodine reagents. Under mild basic conditions, the addition of nucleophiles to aryloxy-substituted vinylbenziodoxolone (VBX) reagents, easily available in two steps from silyl alkynes, resulted in the stereoselective formation of substituted aryl enol ethers. The reaction was most efficient with phenols as nucleophiles, but preliminary results were also achieved for C- and N- nucleophiles. In absence of external nucleophiles, the 2-iodobenzoate group of the reagent was transferred. The obtained aryl enol ethers could then be transformed into α-difunctionalized ketones by oxidation. The described “allyl cation”-like reactivity contrast with the well-established “vinyl-cation” behavior of alkenyl iodonium salts.

Full text of DOI:10.1002/ange.202006707

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Titel: Access to Vinyl Ethers and Ketones with Hypervalent Iodine
Reagents as Oxy-Allyl Cation Synthetic Equivalents
Autoren: Nina Declas and Jerome Waser
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Access to Vinyl Ethers and Ketones with Hypervalent Iodine
Reagents as Oxy-Allyl Cation Synthetic Equivalents
Nina Declas[a] and Jerome Waser*[a]
In the memory of Prof. Dr. Kilian Muñiz
Abstract: We report an Umpolung strategy of enol ethers to generate
oxy-allyl cation equivalents based on the use of hypervalent iodine
reagents. Under mild basic conditions, the addition of nucleophiles to
aryloxy-substituted vinylbenziodoxolone (VBX) reagents, easily
available in two steps from silyl alkynes, resulted in the
stereoselective formation of substituted aryl enol ethers. The reaction
was most efficient with phenols as nucleophiles, but preliminary
results were also achieved for C- and N- nucleophiles. In absence of
external nucleophiles, the 2-iodobenzoate group of the reagent was
transferred. The obtained aryl enol ethers could then be transformed
into -difunctionalized ketones by oxidation. The described "allyl
cation"-like reactivity contrast with the well-established "vinyl-cation"
behavior of alkenyl iodonium salts.
acting as vinyl cation equivalents,[9a] a well established polarityfor
alkenyl iodonium salts.[11]
In standard organic reactions, new bonds are formed between
atoms of opposite polarity.[1] First introduced by Seebach,[2] the
Umpolung approach -inverting the reactivity of one of the
partners- allowed chemical transformations impossible based on
the inherent polarity of the reactants. In this context, hypervalent
iodine compounds are broadly used in organic synthesis as
efficient group transfer reagents via Umpolung of nucleophiles.[3]
Enolates, enol ethers and enamines are among the most
important nucleophilic synthons in synthetic chemistry.[4] The
Umpolung of enolates with hypervalent iodine reagents is well
established,[5] but it is only recently that the involved enolonium
species could be characterized by Szpilman and co-workers.[6]
Nevertheless, controlling transformations involving highly reactive
intermediates formed in situ is challenging, and an access to
stable reagents would be highly desirable.
Scheme 1. O-Vinylbenziodoxolones (VBXs) as vinyl and oxy-allyl cation
equivalents (A) and application to the synthesis of functionalized ketones (B).
Herein, we report a new mode of reactivity of O-VBX reagents,
allowing the Umpolung of enol ethers to give formal oxy-allyl
cations instead of vinyl cations (Scheme 1A, right). Oxy-allyl
cations have been used as transient electrophilic specie for the
reaction with nucleophiles[12] or with dienes in [4
+ 3]
cycloadditions.[13] They are also formed in the versatile Nazarov
cyclization.[14] Enantioselective processes have been recently
developed.[15] Access to this type of reactive intermediates from
hypervalent iodine reagents has not been reported to the best of
our knowledge. Hypervalent iodoallyl intermediates have been
proposed in sigmatropic [2,3] and [3,3] rearrangements.[16]
Reactive allylic cation intermediates were accessed recently
using diazo-substituted hypervalent iodine reagents by Suero.[17]
In our work, we now disclose a different approach based on the
treatment of O-VBX reagents with base and nucleophiles for the
formation of C-O, C-N and C-C bonds in allylic position. The
obtained aryl enol ethers were transformed into -difunctionalized
ketones under oxidative conditions (Scheme 1B), resulting in a 3-
4 steps synthesis from the corresponding silylated alkynes.
After having successfully used O-VBX reagents as vinyl
cation equivalents with thiols,[9a] we attempted to extend this
reactivity to phenols as nucleophiles. However, the reaction of O-
VBX (1a) and para-cresol (2a) under basic conditions led to the
unexpected formation of allyl ether 3a as main product (Table
1).[18] In 1,2-dimethoxyethane (DME) as solvent with 1.2
equivalents of potassium tert-butoxide as base, 3a could be
obtained in 80% yield (entry 1). Other solvents gave a lower yield
(See Supporting Information). When various bases were tested,
cesium carbonate gave a similar NMR yield as KOtBu, but with
higher reproducibility (entry 2),[19] whereas organic bases, such
Cyclic benziodoxol(on)es (BX) reagents are more stable and
especially useful for group transfer reactions.[7] Recently, Miyake
and co-workers[8] and our group[9a] reported the first synthesis of
enol ethers and enamides-based vinyl benziodoxol(on)es (VBX)
reagents by the reaction of nucleophiles with ethynyl
benziodoxol(on)es (EBX)[10] (Scheme 1A, left). The enhanced
reactivity of the hypervalent bond allowed the use of VBX as
electrophiles in palladium-catalyzed Stille cross-couplings at
room temperature for the formation of aryl, vinyl, alkynyl, and
alkyl-substituted Z-enamides and enol ethers.[9a] Moreover, they
reacted directly with thiol nucleophiles to form thio-enamides,
[a]
N. Declas, Prof. Dr. J. Waser
Laboratory of Catalysis and Organic Synthesis, Ecole Polytechnique
Fédérale de Lausanne, EPFL, SB ISIC LCSO, BCH 4306
1015 Lausanne (Switzerland)
Email: jerome.waser@epfl.ch
Homepage : http://lcso.epfl.ch/
Supporting information for this article is given via a link at the end of
the document. Raw data for NMR, IR and MS is available at
zenodo.org, DOI: 10.5281/zenodo.3894256
1
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10.1002/ange.202006707
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as pyridine and triethylamine gave no reactivity (entries 3-4). O-
VBX 1a was also recovered when performing the reaction without
base (entry 5). Incomplete conversion of 1a was observed when
using a catalytic amount or one equivalent of base (entries 6 and
7). In the case of a larger excess of base (entry 8), formation of
the allylic ester 4a resulting from addition of 2-iodo benzoate was
increased. In fact, small amounts of 4a were always observed in
this transformation. Allyl ester such as 4a are valuable building
blocks in synthetic chemistry. Furthermore, all the parts of reagent
1a are incorporated in the product, resulting in high atom
economy.[20] Therefore, we optimized the formation of ester 4a in
absence of external nucleophiles (see Supporting Information for
details). In presence of 20 mol% anisole as a non-participating
nucleophilic additive, 4a could be obtained as the only product in
65% yield (entry 9).
3a was obtained in 83% yield. When conditions B where
employed, only 63% yield was observed. The difference came
from the larger amount of allyl ester 4a formed.[21] 4-Bromo- and
4-iodo-phenols afforded the corresponding products 3b and 3c in
respectively 75% and 73% yields. 4-Methoxyphenol ether 3d was
obtained in a reduced yield of 45%, due to increased formation of
ester 4a. Boronic acid pinacol ester 3e was formed in 71% yield.
A meta-methoxy group was also well tolerated (product 3f). When
an unprotected amine-containing substrate was employed, 35%
of product 3g was still obtained, but significant decomposition was
observed. Aryl enol ether 3h bearing an ortho ester group was
obtained in 89% yield.
A
sterically highly hindered
hydroxyacetophenone still afforded compound 3i in 43% yield. 2-
Naphthol was successfully converted into the corresponding aryl
enol ether 3j in 72% yield.
The scope of the reaction could be extended to more complex
phenols (Scheme 2B). Protected tyrosine gave the desired
product 3k in 96% yield. Two natural products containing phenols
-capsaicin and estradiol- could be used subsequently in the one-
pot protocol B to give highly functionalized product 3l in 41% yield.
Under the optimized conditions, the reaction worked best with
phenols. Nevertheless, promising results were obtained with
several C- and N-nucleophiles (Scheme 2B). Diketone and
ketoester-derived products 3m and 3n were obtained in 31 and
65% yield respectively, whereas tosyl amide 3o was isolated in
33% yield.
Table 1. Optimisation of the addition of p-cresol (2a) to O-VBX 1a.
Base
Yield of 3a
Yield of 4a
Entry
Base
(%)[a]
(%)[a]
Having explored the reactivity of various nucleophiles, we turned
to the scope of O-VBX reagents with para-cresol (2a) as
nucleophile (Scheme 2C). Propyl-substituted O-VBX 1b led to
formation of allylic ether 3p in 65% yield with a complete E-
stereoselectivity.[22] The formation of 3p also confirmed that the
new C-O bond was formed at the position of the iodine atom. A
longer alkyl chain was also well tolerated and product 3q was
obtained in 57% yield. A hypervalent iodine reagent bearing a free
alcohol could be converted into allylic ether 3r. Ethers 3s and 3t
containing a protected alkyne and a phthalimide group could also
be accessed. We could also perform this transformation with O-
VBX bearing phenol with an iodide in ortho position to give
products 3u-w in 44-61% yield. An electron-donating group in
para position was also well tolerated (products 3x and 3y). When
the reaction was examined for N-VBX reagent 1l bearing a
sulfonamide group, the product 3z could be observed by NMR in
about 30% yield. However, isolation of the product always led to
partial decomposition and isomerization to the more stable
internal alkene.
We then examined the synthesis of allyl esters using conditions B
(Scheme 2D). Primary alkyl chains were well tolerated on the EBX
reagents, and products 4b-d were obtained in good yield
(Scheme 6A). EBXs bearing functional groups such as a
protected alkyne, a phthalimide, an alkene or a chloride could be
converted to the corresponding products 4e-h in 60% to 82% yield.
Halogen substituents were also tolerated. The ortho-iodo, meta-
bromo and pentafluoro, derivates 4i-m were obtained in 40% to
70% yield. In addition, phenols bearing an electron-donating
methoxy group in para position or an electron-withdrawing
aldehyde in ortho position could also be used to give products 4n-
p in 58-64% yield.
equivalents
1
2
KOtBu
Cs2CO3
Pyridine
Et3N
1.20
1.20
1.20
1.20
-
80
79
NR
NR
NR
35
60
62
-
15%
15%
NR
3
4
NR
5
none
NR
6
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
0.20
1.00
2.00
1.20
6%
7
8%
8
32%
65%
9[b]
Reactions conditions: Substrate 1a (0.100 mmol), para-cresol (2a) (0.100
mmol), base (0.120 mmol) and DME (0.1 M) at 25 °C. NR = No reaction. [a]NMR
yield determined by addition of 0.1 mmol of CH2Br2 as an internal standard after
the reaction. When determined, the yield of 4a is given in parenthesis.
[b]Reaction performed without 2a in presence of 0.20 equiv. anisole as additive.
To explore the scope of formation of aryl enol ethers and
esters, two conditions were used: conditions A, starting from
isolated O-VBX 1 and conditions B, using an one-pot two-step
procedure from the corresponding EBX reagent 5 without
isolation of the intermediate O-VBX 1 (Scheme 2). For the
addition of external nucleophiles to give allyl ethers 3, method A
usually gave better yields. For allylic esters 4, the yields were
nearly identical for both methods, and the more practical method
B was therefore preferred.
The scope of phenol nucleophiles was first investigated
(Scheme 2A). On a 0.3 mmol scale, using conditions A, substrate
2
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Scheme 2. Scope of the reaction. For A. B. and C. the yield of ester product 4 is given in parenthesis. [a]Only partial conversion of 1a was observed. [b]Reaction
time was 24 h. [c]The reaction was performed on gram scale.
In some cases, formation of the O-VBX reagent 1 was
successful, but the reaction stopped at this stage. In particular,
this was the case for secondary alkyl groups, in absence of an
allylic C-H bond, for sterically hindered O-VBX or for simple VBX
lacking the ether group[23] (see Scheme S1 in Supporting
Information for more details). To highlight the efficiency of the
transformation, gram scale syntheses were performed (Scheme
2C and 2D). Allylic ether 3x was isolated in 63% yield using
method A, while one-pot procedure B gave 66% of allylic ester 4e.
These yields are nearly identical to the one obtained on small
scale, showing the robustness of the procedures.
be used in a reported palladium oxidative cyclization[24] to
generate benzofuran 6 in high yield (Eq. 1). Under palladium-
catalyzed hydrogenation conditions, ketone 7 resulting from
cleavage of the aryl-O bond and reduction of the aryl iodide was
obtained in 96% yield (Eq. 2). Finally, the electron-rich nature of
the enol ether make it well-suited for oxidative modification, with
the potential for accessing more highly functionalized ketones.
Indeed, when using 1.2 equivalents of meta-chloroperbenzoic
acid (m-CPBA) buffered with sodium bicarbonate, the epoxides
8a and 8b could be obtained in quantitative yield starting from the
corresponding enol ethers (Eq. 3).[25] Interestingly, the addition of
the in situ generated meta-chlorobenzoic acid was observed in
absence of buffer to give -benzoylated ketones such as 9a and
We then examined further functionalization of the obtained
aryl enol ethers (Scheme 3). The ortho-iodine substrate 3u could
3
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9b in moderate to good yields (Eq. 4).[26] Treatment of the isolated
epoxide 8a with acetic acid lead to the same regioselective
formation of -acetoxy ketone 10 in 72% yield (Eq. 5). This
straightforward oxidation method allows to make use of the
formed enol ether resulting in an overall C-H functionalization of
the initial propargylic C-H bond.
Keywords: Hypervalent Iodine Reagents • Umpolung • Allyl
Cation • Enol Ethers • Vinylbenziodoxolones
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the corresponding allyl iodane under basic conditions. The high
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the formation of an allyl cation. Indeed, ring-opening of an
adjacent cyclopropane and a [4+3] reaction with furan in low yield
were observed, supporting such an intermediate (See Scheme S5
and S6 in SI). However, the evidence does not allow to exclude a
direct SN2 pathway and further experiments will be needed to
understand the observed transformations.
In conclusion, hypervalent iodine reagents have been used as
oxy-allyl cation surrogates for the stereoselective synthesis of aryl
enol ethers by reaction with phenols. In absence of external
nucleophiles, the in-situ generated benzoate group reacted,
resulting in the formation of allylic esters. The reaction most
probably proceeds via an electrophilic allylic intermediate and
both SN1 or SN2 pathways appear feasible at this stage. The
obtained enol ethers could be transformed into -difunctionalized
ketones under oxidative conditions, demonstrating the synthetic
utility of the transformation.
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Acknowledgements
We thank the Swiss National Science Foundation (SNSF, grant
nos. 200020_182798) and EPFL for financial support.
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[18] The structure of 3a was confirmed by X-ray analysis. The data are
available free of charge from the Cambridge Crystallographic Data
Center (CCDC number 1989749).
[19] The reaction mixture was heterogenous, but no dependence on the batch
of cesium carbonate was observed. Using finely grinded salt also did not
change the result. See Supporting Information for details.
[20] Selected examples of atom-economical reactions with hypervalent iodine
reagents: a) S. G. Modha, M. F. Greaney, J. Am. Chem. Soc. 2015, 137,
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publication, DOI: 10.1021/acs.orglett.0c01150.
[21] The reaction profile is usually very clean and lower yields were generally
due to increase amount of ester 4a. It is difficult to recognize clear trends
in the formation of the ester products, although generally electron-rich
phenols gave larger amounts of 4a, in-line with the accelerating effect of
nucleophilic additives. See Supporting Information for exact yields of
isolated esters 4.
[22] The structure of 3p was confirmed by X-ray analysis. The data are
available free of charge from the Cambridge Crystallographic Data
Center (CCDC number 1989757). The structure and geometry of the
other products is assumed to be the same based on the similar NMR
spectra. The olefin geometry was further confirmed by NOE experiments
on products 4c and 4l.
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complex product mixture.
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5
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Entry for the Table of Contents
Access to oxy-allyl cation equivalents from alkynes via hypervalent iodine reagents is described. The stereoselective transformation
of Vinylbenziodoxolones (VBXs) gives aryl enol ethers bearing an allylic ether or ester group and corresponds to a Umpolung of the
nucleophilic reactivity of enol ethers. The obtained products are easily transformed into -difunctionalized ketones under oxidative
conditions.
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