Tandem Insertion/[3,3]-Sigmatropic Rearrangement Involving the Formation of Silyl Ketene Acetals by Insertion of Rhodium Carbenes into S-Si Bonds

Article abstract of DOI:10.1021/acs.orglett.1c00229

Allyl 2-diazo-2-phenylacetates are shown to react with trimethylsilyl thioethers in the presence of rhodium(II) catalysts to generate α-allyl-α-thio silyl esters. The transformation involves a tandem process involving formal rhodium-catalyzed insertion of the carbene group into the S-Si bond to generate a silyl ketene acetal, followed by a spontaneous Ireland-Claisen rearrangement. The silyl ester products were isolated as the corresponding carboxylic acids after aqueous workup. Intramolecular cyclopropanation of the allyl fragment generally competes with addition of the heteroatom to the carbene center. The reaction occurs under mild conditions and in high yield, allowing for rapid entry into rearrangement tetrasubstituted products. Propargyl esters were shown to generate the corresponding α-allenyl products.

Full text of DOI:10.1021/acs.orglett.1c00229

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Letter
Tandem Insertion/[3,3]-Sigmatropic Rearrangement Involving the
Formation of Silyl Ketene Acetals by Insertion of Rhodium Carbenes
into S−Si Bonds
Jason R. Combs, Yin-Chu Lai, and David L. Van Vranken*
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ABSTRACT: Allyl 2-diazo-2-phenylacetates are shown to react
with trimethylsilyl thioethers in the presence of rhodium(II)
catalysts to generate α-allyl-α-thio silyl esters. The transformation
involves a tandem process involving formal rhodium-catalyzed
insertion of the carbene group into the S−Si bond to generate a
silyl ketene acetal, followed by a spontaneous Ireland−Claisen rearrangement. The silyl ester products were isolated as the
corresponding carboxylic acids after aqueous workup. Intramolecular cyclopropanation of the allyl fragment generally competes with
addition of the heteroatom to the carbene center. The reaction occurs under mild conditions and in high yield, allowing for rapid
entry into rearrangement tetrasubstituted products. Propargyl esters were shown to generate the corresponding α-allenyl products.
of oxygen and nitrogen nucleophiles to metal carbene
intermediates have been used for a number of powerful
processes involving Mannich5 and aldol reactions (Figure 1B)6
and 1,4-additions (Figure 1C).7 Protonation of enolate
intermediates is probably important in a variety of rhodium-
catalyzed asymmetric X−H insertion reactions of α-diazo-
esters.8−10
We reasoned that enolates derived from addition of
heteroatoms to metal-carbene intermediates might be trapped
as silylketene acetals that could engage in further tandem
processes such as [3,3]-sigmatropic rearrangements (Figure
1D). In this work, we report the first tandem process involving
formal insertion of a metal carbene into a S−Si bond to
generate a silyl ketene acetal followed by a spontaneous
Ireland−Claisen rearrangement.
any rhodium-catalyzed reactions of α-diazocarbonyl
Mcompounds involve addition of heteroatom nucleo-
philes, giving rise to enolate intermediates.1 In the earliest
related work, Ando and co-workers showed that allyl thioethers
react with α-diazoesters to generate allyl sulfonium inter-
mediates that undergo [2,3]-rearrangement (Figure 1A),2 but
[2,3]-rearrangement of sulfonium ylides does not rely on the
presence of a carbonyl group.3 A number of variants of this α-
allylation have been reported, including asymmetric variants of
the reaction.4 The enolate intermediates derived from addition
Tetrasubstituted α-thiocarboxylic acids are present in a
variety of biologically active molecules. Agents that reverse
multidrug resistance in mammalian tumors11 and inhibitors of
HIV-1 reverse transcriptase12 such as those in Figure 2 contain
the tetrasubstituted α-thiocarbonyls. Hence, there is an interest
in catalytic methods resulting in α-thiocarboxylic acids.
Intramolecular cyclopropanation of α-diazo allyl esters is
well precedented with copper(I) and rhodium(II) catalysts13
and has previously been shown to outpace the addition of
sulfonamides to rhodium carbenes derived from diazoester 1
leading to cycopropanolactone 2 (Scheme 1).14 We set out to
Received: January 20, 2021
Published: April 1, 2021
Figure 1. Related reactions of metal enolate intermediates derived
from carbene insertion: past and present work.
https://doi.org/10.1021/acs.orglett.1c00229
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2841−2845
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intermolecular attack by Nu−SiMe3 relative to intramolecular
cyclopropanation, nitrogen nucleophiles were explored (trime-
thylsilyl azide, O,N-bis-trimethylsilyl acetamide, and N-
trimethylsilylpiperidine), but those reactions also resulted in
cyclopropanation and/or complex mixtures. Finally, when the
nucleophilicity was increased further by employing PhS−
SiMe3, the reaction was found to generate the product
expected for formal S−Si insertion followed by Ireland−
Claisen rearrangement in 56% yield. The initial silyl ester was
hydrolyzed to the carboxylic acid 3 in the aqueous workup. We
set out to optimize the reaction conditions to improve the
yield of the addition/rearrangement product 3.
Figure 2. Bioactive molecules containing tetrasubstituted α-
We employed conditions similar to those used by Zhou and
co-workers for S−H insertion (2 mol % Rh2(OAc)4, 2 equiv
nucleophile, 60 °C) over 1 h, followed immediately by workup
(Table 2);10 under those conditions, the product of addition/
thiocarbonyl motifs.
Scheme 1. Cyclopropanation Competes with Addition of
the Nucleophile
Table 2. Optimization of Conditions for the Insertion/
Rearrangement Reaction
equiv
entry diazo 1
mol %
Rh2(OAc)4
equiv
PhSSiMe3
time
(h)
yield
(%)
T (°C)
1
2
3
4
5
6
1.0
1.0
1.0
1.3
1.3
1.0
1
2
2
2
1
2
2.0
2.0
2.0
1.0
1.0
1.2
1 + 1
1
1 + 1
1 + 1
1 + 1
1 + 1
40
60
60
60
60
60
56
61
79
91
70
92
explore the addition of a variety of O−Si, N−Si, and S−Si
nucleophiles in order to assess the competition between
addition of the Nu−Si nucleophile relative to cyclopropana-
tion.
Initially, we explored the reaction of PhO−SiMe3 with allyl-
α-phenyl-α-diazoacetate 1 using typical conditions for O−H
insertion (1 mol % Rh2(OAc)4, 2 equiv PhO−SiMe3, CH2Cl2,
40 °C, slow addition over 1 h; Table 1).15 Ireland−Claisen
rearrangement 3 was isolated in 61% yield. When the reaction
was allowed to proceed for an additional 1 h after addition of
the diazo compound was complete, the yield improved to 79%
(Table 2, entry 3). At high concentrations, strong nucleophiles
are expected to complex with the rhodium and reduce the
concentration of active rhodium catalyst.20 The presence of
unreacted diazo compound suggested that the catalyst was
being deactivated, possibly by reversible or irreversible
coordination of the thiol(ate)21 and/or silylation of the acetate
ligands22 to generate AcOSiMe3 and unreactive rhodium-
sulfide complexes. When the stoichiometry of silyl ether was
reduced from 2 equiv to 1 equiv, with a slight excess of the
diazo compound, the yield of addition product 3 was further
improved to 91% (entry 4). The yield was lower when the
catalyst loading was reduced to 1 mol % (entry 5). The yield
was similar when a slight excess (1.2 equiv) of the S−Si
nucleophile was used (entry 6), leading to a 92% yield of the
desired addition/rearrangement product 3. The scope of the
addition/rearrangement process was explored using the
conditions in Table 1, entry 6.
The scope of the reaction was initially explored with respect
to the diazo compound on a 0.5 to 0.8 mmol scale (Table 3).
Para-, meta-, and even ortho-substituted aryl groups all gave
good yields of the addition/rearrangement product (2b−2d).
Substitution on the allyl fragment was less forgiving. Reaction
of an ester with a dimethylallyl group (1f), which is more bulky
but more nucleophilic than an allyl group, again resulted
exclusively in cyclopropanation. The E-cinnamate ester 1g also
resulted mainly in cyclopropanation but afforded a low yield of
addition/rearrangement product 2g (17%) as an inseparable
mixture of diastereomers in a 2.7:1 ratio (1H NMR).4b,e,23 A 2-
Table 1. Only Highly Nucleophilic Nu−SiMe3 Compounds
Outpace Cyclopropanation
Nu-SiMe3
PhO−SiMe3
N3−SiMe3
cyclopropane 2
adduct 3
44%
78%
CH3C(OSiMe3)N−SiMe3
C6H10N−SiMe3
PhS−SiMe3
77%
<15%
56%
rearrangements of silylketene acetals generally proceed at or
below room temperature,16 but after 1 h syringe pump
addition of the diazo compound, the reaction was allowed to
run for an additional 1 h to ensure completion of the
rearrangement. Addition of the diazo compound by syringe
pump reduces unwanted carbene dimer formation.17
Intramolecular addition of the alkene (cyclopropanation)
dominated over intermolecular addition of PhO−SiMe3 (Table
1). The crude reaction mixture showed a small amount of
another product consistent with the addition of phenol and
protonation of the enolate.18 Other rhodium catalysts,
Rh2(hfb)4 and Rh2(cap)4, generated more complex mixtures
dominated by cyclopropane 2.19 Reasoning that increasing the
strength of the nucleophile might increase the rate of
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Table 3. Scope of the Diazoester Substrate in the Addition/
Rearrangement Reaction
Table 4. Scope of the Diazoester Substrate in the Addition/
Rearrangement Reaction
afforded good yields of the addition/rearrangement products
(2j and 2k). Phenyl trimethylsilyl ethers with reduced
reactivity gave lower yields of products due to competing
cyclopropanation. For example, the bulky 2,5-dimethylphenyl
thioether 3l generated the addition rearrangement product 2l
in 43% yield. Similarly, the p-chlorophenyl thioether 3m led
also generated a lower yield of product 2m (59%) due to
competing cyclopropanation. Aliphatic sulfur nucleophiles
afforded good yields.
Silyl ketene acetals are valuable synthetic intermediates for a
number of transformations.27 In order to establish the
intermediacy of a silyl ketene acetal, the reaction was carried
out in CDCl3 using methyl 2-diazo-2-phenylacetate 6, which
lacks the allyl group needed for the Ireland−Claisen rearrange-
ment. 1H NMR and 13C NMR of the reaction mixture showed
the presence of silyl ketene acetals corresponding to the E and
Z isomers (Scheme 3).
bromoallyl moiety led to a modest 57% yield (2h). It is
possible the bromine could be interacting competitively with
the carbene center,24 which could slow the rate of nucleophilic
attack by sulfur and allow time for catalyst decomposition.
Ireland−Claisen rearrangements of propargyl esters are not
very common,25 but the addition/rearrangement of propargyl
ester 4 (Scheme 2) proceeded smoothly to afford a satisfying
82% yield of the allene product 5 on a 1 mmol scale.
The scope of the reaction was then explored with respect to
the sulfur nucleophile (Table 4). Trimethylsilyl thioethers were
prepared by the method of Baba and co-workers through
silylation of the lithium thiolates and purification through
vacuum distillation.26 Substituted phenyl thiothers generally
Hypothetically, the silyl ketene acetal (Z)-7 could arise
through intramolecular silyl transfer from sulfur to oxygen. If
the silyl ketene acetal (E)-7 is formed stereospecifically from
Scheme 3. Confirmation of Silyl Ketene Acetal
Intermediates
Scheme 2. Addition/Rearrangement of Propargyl Esters Is
Successful
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the corresponding Z-ylide, then transfer of trimethylsilyl must
occur intermolecularly.
In summary, the reaction of trimethylsilyl thioethers with α-
phenyl-α-diazoesters has been shown to generate silyl ketene
acetals. Silyl ketene acetals derived from allyl diazoesters
undergo further [3,3]-sigmatropic rearrangements to generate
tetrasubstituted carboxylic acid derivatives. This tandem
process offers an alternative to [2,3]-rearrangement of
sulfonium ylides and is robust in terms of aromatic substituent
scope. This type of addition-rearrangement process could
prove useful in the synthesis of bioactive molecules containing
a tetrasubsituted α-thiocarboxylic acid motif.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00229.
Experimental procedures and spectroscopic data (PDF)
AUTHOR INFORMATION
Corresponding Author
■
David L. Van Vranken − Department of Chemistry, University
of California, Irvine, Irvine, California 92617, United States;
orcid.org/0000-0001-5964-7042; Email: david.vv@
uci.edu
Authors
Jason R. Combs − Department of Chemistry, University of
California, Irvine, Irvine, California 92617, United States
Yin-Chu Lai − Department of Chemistry, University of
California, Irvine, Irvine, California 92617, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00229
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported with funds from the U.C. Irvine
School of Physical Sciences.
■
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