Rhodium-Catalyzed Carbene Transfer Reactions for Sigmatropic Rearrangement Reactions of Selenium Ylides

Article abstract of DOI:10.1021/acs.orglett.9b01092

The rearrangement of selenium ylides is even today almost unexplored, although it would provide access to important organoselenium compounds with broad downstream applications. In this report, the first systematic study of sigmatropic rearrangement reactions of selenium ylides using a simple rhodium catalyst with catalyst loadings as low as 0.01 mol % is described. Selenium oxide pyrolysis of the rearrangement products gives access to important 1,1-disubstituted butadienes.

Full text of DOI:10.1021/acs.orglett.9b01092

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium-Catalyzed Carbene Transfer Reactions for Sigmatropic
Rearrangement Reactions of Selenium Ylides
Sripati Jana and Rene M. Koenigs*
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
S
* Supporting Information
ABSTRACT: The rearrangement of selenium ylides is even
today almost unexplored, although it would provide access to
important organoselenium compounds with broad downstream
applications. In this report, the first systematic study of
sigmatropic rearrangement reactions of selenium ylides using a
simple rhodium catalyst with catalyst loadings as low as 0.01
mol % is described. Selenium oxide pyrolysis of the rearrange-
ment products gives access to important 1,1-disubstituted
butadienes.
rganoselenium compounds are highly valuable reagents
or catalysts with widespread application in the synthesis
Scheme 1. Sigmatropic Rearrangement Reactions of
Selenium Ylides and Their Application in the Synthesis of
1,1-Butadienes
O
of complex natural products, drugs, or materials and are
regarded as an important tool for the efficient introduction of
new functional groups.1,2 The reaction of selenium compounds
with electrophilic oxygen or nitrogen species is well-
documented and furnishes selenoxides or selenimides that,
depending on the substitution pattern of the starting selenide,
undergo elimination or rearrangement reactions.3,4 On the
contrary, the reaction of organoselenium compounds with
electrophilic carbenes has been explored significantly less and
no systematic study of their reactivity has previously been
reported.5,6
In the presence of an electrophilic carbene, the lighter group
VI elements, oxygen and sulfur, react under ylide formation
and subsequent rearrangement.7−10 The heavier homologues
have been investigated much less, and selenium ylides find very
limited applications.5,6 Even today, there are only singular
examples that describe sigmatropic rearrangement reactions of
selenium ylides via deprotonation reactions of a C−H acidic,
preformed selenonium salt under basic reaction conditions.5 In
1995, Uemura reported a single example describing the
catalytic rearrangement of a selenium ylide using copper or
rhodium catalysts, though the efficiency and yield were poor.6
Against this background, we became interested in studying
the catalytic formation of selenium ylides and their application
in sigmatropic rearrangements, which would open up pathways
toward densely functionalized organoselenium compounds
with applications in organic synthesis. More specifically, the
development of a relay protocol consisting of [2,3]-sigmatropic
rearrangement of an allylic selenide followed by oxidation to
the selenium oxide and syn elimination should allow an
efficient, stereoselective entry into valuable 1,1-disubstituted
butadiene derivatives (Scheme 1).
provided the desired rearrangement product 3 in excellent
yield (72−99%) in a short reaction time (Table 1, entries 1
and 2). Chiral rhodium catalysts, however, provided the
desired reaction product only in diminished yield and with
only negligible enantioselectivity (Table 1, entry 4). All other
catalysts investigated, based on iron, cobalt, copper, or silver,
did not provide the desired reaction product (Table 1, entry
5).11 In all reactions, analysis of the crude reaction mixture
revealed only diminutive amounts of byproducts formed and
the diazoalkane and selenide remained almost untouched,
which might be attributed to catalyst poisoning via complex-
ation of the Lewis basic selenide. In further studies, the
influence of solvent was investigated. We observed a marked
effect of water and could isolate rearrangement product 3 in
almost quantitiative yield in a water/DCM mixture.11 Notably,
even with only 0.1 mol % Rh2(OAc)4 catalyst, we could
observe a complete conversion of diazoalkane 2 after reaction
for 120 min and the product was isolated in excellent yield
We thus began our investigations by studying the reaction of
allylic selenide 1 with phenyl diazoacetate 2 in the presence of
different typical carbene transfer catalysts. Rh(II) catalysts
Received: March 28, 2019
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.9b01092
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Organic Letters
Letter
Table 1. Optimization of the Catalytic [2,3]-Sigmatropic
Rearrangement of Selenium Ylides
Scheme 2. Substrate Scope of the Catalytic [2,3]-
Sigmatropic Rearrangement of Selenium Ylides
a
no.
catalyst
Rh2(OAc)4 (1 mol %)
Rh2(TPA)4 (1 mol %)
Rh2(esp)2 (1 mol %)
time
yield (%)
1
2
3
4
15 min
15 min
30 min
60 min
91
72
76
Rh2(S-DOSP)4 (1 mol %)
43
(3% ee)
5
Co(salen), FeTPPCl, TBA[Fe], AgOTf,
CuOTf1/2C6H6 (1 mol %)
Rh2(OAc)4 (1 mol %)
Rh2(OAc)4 (1 mol %)
Rh2(OAc)4 (0.1 mol %)
Rh2(OAc)4 (0.01 mol %)
24 h
no
reaction
b
6
45 min
45 min
120 min 86
24 h 67
84
99
c
7
cd
,
8
9
ce
,
a
For the reaction, 1 (0.2 mmol) and 2 (1.1 equiv) were dissolved in
1.5 mL of DCM, the catalyst was added, and the mixture was stirred
b
c
at room temperature. In water as the solvent. In a 1:4 DCM/water
mixture. On a 1 mmol scale. On a 5 mmol scale.
d
e
(86%). In additional experiments, we further reduced the
catalyst loading to parts per million levels of Rh2(OAc)4 (0.01
mol %, 100 ppm) and the rearrangement product was obtained
on a 5 mmol scale in good yield (1.2 g, 67%), which
corresponds to a turnover number of 6700 and demonstrates
the applicability of this reaction on a gram scale (Table 1, entry
9).
isolated under the reaction conditions presented here in
excellent isolated yields (Scheme 3).
Encouraged by these observations, we next investigated the
applicability and generality of this rearrangement reaction.
Different diazoesters, including styryl diazoacetate, smoothly
reacted with allyl selenide 1 to the desired homoallylic
selenides in excellent yields (Scheme 2). The substitution
pattern of the aryl ring of diazoester 2 had little influence on
the reaction yield, and different electron-donating groups and
halogens in all positions of the aromatic ring were compatible
with the reaction. Notably, ethyl diazoacetate, as a model
acceptor-only diazoalkane, also smoothly underwent the
rearrangement reaction yet with a slightly diminished yield of
reaction product 3q. We subsequently investigated different
allylic selenides and were delighted to observe that the seleno-
Doyle−Kirmse reaction smoothly furnishes homoallylic
selenides in good to high yields and a broad variety of
functional groups, such as nitro, nitrile, and trifluoromethyl, are
well-tolerated (Scheme 2). Similarly, an aliphatic selenide
reacted in this transformation to provide reaction product 3ae
in good isolated yield. Very intriguingly, thiophene and
pyridine heterocycles are compatible under these reaction
conditions, and no poisoning of the rhodium catalyst occurred.
The corresponding heterocyclic substituted homoallylic
selenides were obtained in very good yields (Scheme 2, entries
3ac and 3ad). Upon investigation of the cinnamyl-substituted
selenide, the rearrangement product was obtained in good
yield, with only little diastereoselectivity (Scheme 2, entry 3af).
It should be noted that the observed diastereoselectivity is in
the same range as the diastereoselectivity for classic Doyle−
Kirmse reactions.12
Scheme 3. Substrate Scope of the Reaction of Propargylic
Selenide with Different Diazoalkanes
To further investigate the generality of selenium-based
rearrangement reactions, we next turned our attention to
selenium compounds that should promote either [1,2]-
sigmatropic rearrangement reactions or [2,3]-sigmatropic
rearrangements. If ethyl 2-(phenylselanyl)acetate 6a is used,
a Sommelet−Hauser or [2,3]-sigmatropic rearrangement takes
place and allows access to α-seleno-substituted esters 10a. The
reaction product corresponds to a formal o-C−H functional-
ization of the aromatic substituent of 2 (Scheme 4). The
robustness of the Sommelet−Hauser reaction was also
investigated using 100 ppm Rh2(OAc)4, and ester 10a was
obtained in 34% yield, which corresponds to a turnover
We next studied the reaction of propargyl selenides 4 with
α-aryldiazoacetates, which provides efficient access to allenyl
selenides. To our delight, the desired allenyl selenides could be
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DOI: 10.1021/acs.orglett.9b01092
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Letter
homoallylic selenide by addition of hydrogen peroxide
(Scheme 6a). Indeed, this approach proved to be highly
Scheme 4. Substrate Scope of the Sommelet−Hauser
Rearrangement
Scheme 6. Applications in Olefin Synthesis
number of 3400. This transformation also readily takes place in
the presence of cyano-substituted selenide 6b as well as
different donor−acceptor substituted diazoalkanes and now
gives access to o-C−H functionalization products in good to
high isolated yields. Notably, meta-substituted diazoalkanes
reacted in a highly regioselective fashion to yield the
corresponding triply substituted phenyl acetic acid ester 10i
in high isolated yield.
In the case of benzylic selenide 11, the desired [1,2]-
sigmatropic or Stevens rearrangement readily took place,
though the reaction product was obtained only as a complex
mixture. When the solvent was changed to only water, the
rearrangement product was obtained in good isolated yield.
Similarly, a variety of different donor−acceptor diazoesters
smoothly reacted to selectively provide the Stevens rearrange-
ment product (Scheme 5). In an experiment with 100 ppm
Rh2(OAc)4 as a catalyst, Stevens product 12a was obtained in
29% yield on a 5 mmol scale reaction.
versatile in the stereoselective synthesis of the Z-configured,
1,1-disubstituted butadiene 13 in 56% yield over two steps; a
consecutive reaction protocol with purification of the
intermediate homoallylic selenide produced 13 in similar
yield (60%).13 The stereochemical outcome of this syn
elimination can be rationalized by the transition state in
Scheme 6c, in which the vinyl group and the phenyl ring are
arranged, for steric reasons, preferentially in a trans
conformation that results in the Z configuration of the
butadiene product. Most importantly, the stereochemistry is
complementary to protocols relying on the reaction of
diazoalkanes with electrophilic palladium−allyl complexes.14
A similar protocol could also be applied to the product of the
Stevens rearrangement to provide trisubstituted olefine 14, yet
only as a 1:1 mixture of diastereoisomers (Scheme 6b), which
may be a result of the similar steric hindrance of both
transition states (Scheme 6c).
Finally, we decided to study the mechanism of this
rearrangement reaction. More specifically, it would be highly
important to understand if rearrangement reactions of
selenium ylides undergo rearrangement via metal-bound or
free ylide intermediates. We therefore conducted a set of
control experiments using different Rh(II) catalysts and
aryldiazoacetates in the diastereoselective rearrangement
reaction of selenide 15, yet the catalyst had an only minor
effect on the selectivity of the rearrangement reaction, which is
indicative of a rearrangement process without participation of
the catalyst and, thus, a free ylide reaction mechanism (Scheme
7a). This is also in line with the observation mentioned above
when using chiral Rh(II) catalysts, which gave an only
negligible enanantiomeric excess in the rearrangement reaction
of 1 (Table 1, entry 4). We also probed our recently developed
photochemical protocol in this transformation;15 however,
only the dimerization reaction of the diazoalkane was observed.
In further control experiments, we investigated the influence of
the aryldiazoacetate and studied different esters. Although a
Organoselenium compounds play an important role in the
construction of new functional groups, and the selenium oxide
pyrolysis reaction is one of the textbook examples of
applications of selenium compounds.1 We thus tested a one-
pot protocol consisting of an initial rhodium-catalyzed
rearrangement reaction and a subsequent oxidation of the
Scheme 5. Substrate Scope of the Stevens Rearrangement
C
DOI: 10.1021/acs.orglett.9b01092
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Letter
Detailed experimental procedures and spectral data for
Scheme 7. (a) Control Experiment with Different Rhodium
Catalysts, (b) Control Experiments with Different
Aryldiazoacetates, and (c) Proposed Reaction Mechanism
of the Rearrangement Reaction of Selenium Ylides
1
all compounds, including copies of H, 13C, and 19F
NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rene.koenigs@rwth-aachen.de.
ORCID
Rene M. Koenigs: 0000-0003-0247-4384
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.M.K. gratefully acknowledges the Dean’s Seed Fund of
RWTH Aachen University. Funded by the Deutsche
Forschungsgemeinschaft (DFG, German Research Founda-
tion), via Project 408033718.
ABBREVIATIONS
TPP, meso-tetraphenylporphyrin; TBA[Fe], [Bu4N][Fe-
(CO)3(NO)]
■
REFERENCES
■
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On the basis of the literature of rearrangement reactions of
sulfur ylides and the data presented here, we propose the
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01092.
D
DOI: 10.1021/acs.orglett.9b01092
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Letter
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