Reactivity of Stabilized Vinyldiazo Compounds toward Alkenyl- And Alkynylsilanes under Gold Catalysis: Regio- And Stereoselective Synthesis of Skipped Dienes and Enynes

Article abstract of DOI:10.1021/acs.orglett.1c01381

We report the gold-catalyzed reaction of vinyldiazo compounds and alkenylsilanes to produce skipped dienes, which are common structural motifs in an array of bioactive compounds. This carbon-carbon bond-forming transformation proceeds with complete regio- and stereoselectivity with the silyl group serving as a regio- and stereocontrolling element. Likewise, the use of alkynylsilanes as reaction partners yielded skipped enynes resulting from a C(sp)-C(sp3) coupling. Mechanistic experiments and DFT studies have provided support for a stepwise mechanism.

Full text of DOI:10.1021/acs.orglett.1c01381

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Letter
Reactivity of Stabilized Vinyldiazo Compounds toward Alkenyl- and
Alkynylsilanes under Gold Catalysis: Regio- and Stereoselective
Synthesis of Skipped Dienes and Enynes
Olaya Bernardo, Kota Yamamoto, Israel Fernández,* and Luis A. López*
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ABSTRACT: We report the gold-catalyzed reaction of vinyldiazo compounds and
alkenylsilanes to produce skipped dienes, which are common structural motifs in an
array of bioactive compounds. This carbon−carbon bond-forming transformation
proceeds with complete regio- and stereoselectivity with the silyl group serving as a
regio- and stereocontrolling element. Likewise, the use of alkynylsilanes as reaction
partners yielded skipped enynes resulting from a C(sp)−C(sp3) coupling.
Mechanistic experiments and DFT studies have provided support for a stepwise
mechanism.
́
́
ince the seminal work by Nolan, Diaz-Requejo, Perez and
much less common than that involving the carbenic position
Sco-workers in 2005, the marriage of diazo reagents with (Scheme 1b).10
1
On the other hand, owing to their availability, nontoxic
gold catalysts has proven to be an extremely fertile field of
research leading to reactivity modes previously unattainable by
using more traditional catalysts.2 In this realm, initial
developments focused on the use of ethyl diazoacetate and
aryl substituted derivatives thereof.3 The implementation of
these gold-based methodologies to vinyldiazo compounds is
more recent. Liu and co-workers in 2011 reported the
synthesis of quinoline oxide derivatives through a gold-
catalyzed formal [3 + 3] cycloaddition of vinyldiazo
compounds with nitroso derivatives.4 Since this pioneering
study, the use of gold catalysts in transformations of vinyldiazo
compounds has gained increasing interest.5 In this regard, in
the past decade, our group and others have explored the
reactivity of a number of unsaturated substrates toward
vinyldiazo compounds under gold catalysis (Scheme 1a). In
2013, we reported the gold-catalyzed reaction of stabilized
vinyldiazo compounds and unbiased alkenes to yield 2,6-diene
derivatives.6 Despite a broad scope of alkenes, this method-
ology generally yields an inseparable mixture of positional
isomers and E and Z stereoisomers. Concurrently, Davies and
co-workers reported the highly enantioselective Au(I)-
catalyzed [3 + 2] cycloaddition between vinyldiazoacetates
and enol ethers.7,8 Our group also reported the regioselective
synthesis of functionalized cyclopentene derivatives through
the gold-catalyzed [3 + 2] cycloaddition of vinyldiazo
compounds and styrenes.9
nature, and right balance between reactivity and stability,
organosilicon compounds have reached an indisputable
position as valuable reagents in organic synthesis.11 In
particular, vinylsilanes undergo a facile electrophilic substitu-
tion as a result of the ability of the carbon−silicon σ bond to
stabilize an adjacent carbocation (the so-called β-silicon effect)
(Scheme 1c).12
We surmised that merging the electrophilic character of the
postulated gold carbene intermediate generated from vinyl-
diazo compounds with the innate ability of vinylsilanes to
undergo electrophilic-induced desilylation could enable new
synthetic opportunities. Herein, we report the gold-catalyzed
reaction of vinyldiazo reagents with vinylsilanes leading to
functionalized skipped dienes, structural motifs found in
numerous natural products and bioactive compounds.13,14
Extension of this carbon−carbon bond formation to
alkynylsilanes was also accomplished providing skipped enynes.
We began our investigation by studying the gold-catalyzed
reaction of ethyl 2-diazobut-3-enoate (1a) and (E)-trimethyl-
(styryl)silane (2a). Initial experiments were conducted in
dichloromethane as the solvent at room temperature in the
presence of 5 mol % of the corresponding gold complex. In
order to overcome potential competitive side reactions,15 slow
addition of a solution of vinyldiazo reagent 1a to a solution of
Received: April 22, 2021
Published: May 13, 2021
Although a detailed mechanistic study of these trans-
formations has not yet been undertaken, most of them have
been rationalized on the basis of the initial generation of a
highly electrophilic gold carbene complex, which can undergo
attack of the unsaturated reagent to the vinylogous position.
This vinylogous reactivity of vinyldiazo compounds had been
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Scheme 1. Background of the Present Study
Scheme 2. Scope of the Gold-Catalyzed Reaction of
a b
,
Vinyldiazo Compounds 1 and Vinylsilanes 2
a
Reaction conditions: 1 (0.15 mmol), 2 (0.30 mmol, 2 equiv),
JohnPhosAuCl (5 mol %), NaBArF4 (5 mol %), CH2Cl2 (1.8 mL), rt.
b
c
Yield of isolated products. Isolated as a 4:1 mixture of (2E,5E) and
d
(2Z,5E) isomers. Four equivalents of the vinylsilane were used.
e
vinylsilane 2a (2 equiv) and the catalytic system was
performed. Under these controlled addition conditions, a
Isolated as a 10:1 mixture of (3E,6E) and (3Z,6E) isomers.
catalytic system composed of JohnPhosAuCl (5 mol %) and
F−
NaBArF (5 mol %, BAr4
= 3,5-bis(trifluoromethyl)-
by product 3ag (51% yield). However, the reaction with a
substrate featuring a nitro group (2h; R3 = p-NO2−C6H4, R4 =
H) proceeded sluggishly to produce diene 3ah with a
significantly lower yield (19%). On the other hand, ortho-
and meta-substitution on the aryl group of the vinylsilane were
well accommodated, as exemplified by products 3ai (60%
yield) and 3aj (55% yield). A 2,2-disubstituted vinylsilane,
4
phenylborate) as a halide scavenger outperformed other
gold(I) catalysts tested delivering (2E,5E)-6-phenylhexa-2,5-
dienoate (3aa) in 85% yield after chromatographic purification
(eq 1; see the Supporting Information for details on the
namely (2,2-diphenylvinyl)trimethylsilane (2k; R3 = R4
=
C6H5), was also transformed into the corresponding skipped
diene 3ak in good yield (75%).
We next evaluated Z-alkenyl silanes under our reaction
conditions. Pleasingly, treatment of ethyl 2-diazobut-3-enoate
(1a) with (Z)-trimethyl(styryl)silane (2l, R3 = H, R4 = C6H5)
led to ethyl (2E,5Z)-6-phenylhexa-2,5-dienoate (3al) in
moderate yield (55%), albeit with complete regio- and
stereoselectivity.
Our studies indicate that an aryl group on the β-position of
the vinylsilane is paramount to the success of the present
reaction. Indeed, the parent vinyltrimethylsilane 2m (R3 = R4 =
H) proved unreactive under our optimized conditions, as did
β-alkyl-substituted alkenylsilanes such as 2n (R3 = cyclohexyl,
R4 = H). Furyl-substituted alkenylsilane 2o (R3 = 2-furyl, R4 =
H) also failed to undergo the present transformation. Instead
of the desired product, a mixture of products was observed.16
Thereupon, the structural variation of the vinyldiazo
component was addressed. Benzyl 2-diazobut-3-enoate (1b;
EWG = COOBn, R1 = R2 = H) reacted well with vinylsilanes
2a (R3 = C6H5, R4 = H), 2b (R3 = p-Me-C6H4, R4 = H), and 2f
(R3 = p-Br−C6H4, R4 = H), thereby providing the desired
products 3ba (73%), 3bb (81%), and 3bf (82%), respectively.
screening study). Under these conditions, the reaction could
be scaled up to 1 mmol with minimal erosion of yield (75%
yield, 162.2 mg).
Next, we investigated the substrate scope of this gold-
catalyzed C(sp2)−C(sp3) bond formation (Scheme 2). Keep-
ing diazo compound 1a as the reaction partner, we first studied
the variation of the vinylsilane component 2. In this regard, we
found that para-substituted substrates bearing electron-
donating groups at the aryl moiety, such as methyl (2b; R3
= p-Me-C6H4, R4 = H) and methoxy (2c; R3 = p-MeO-C6H4,
R4 = H), engaged in this gold-catalyzed coupling reaction
providing the corresponding skipped dienes 3ab and 3ac in
74% and 45% yield, respectively. F-, Cl-, and Br-substituted 2-
arylvinylsilanes (2d−f; R3 = p-X-C6H4, R4 = H) also performed
well to provide the corresponding dienes in good yields (3ad,
92%; 3ae, 61%; 3af, 70%). Even a strong electron-withdrawing
trifluoromethyl substituent at the para position of the aryl ring
(2g; R3 = p-CF3−C6H4, R4 = H) was tolerated as exemplified
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Likewise, reaction of tert-butyl 2-diazobut-3-enoate (1c; EWG
= COOtBu, R1 = R2 = H) with 2a furnished the expected diene
3ca in 68% yield. Next, variation of the vinyl moiety of the
diazo component was studied. Substitution at the C3 atom was
tolerated as illustrated by the formation of diene 3da in 64%
yield when ethyl 2-diazo-3-metylbut-3-enoate (1d; EWG =
COOEt; R1 = Me, R2 = H) was reacted with (E)-
trimethyl(styryl)silane (2a). The reaction of diazoacetate 1d
with vinylsilane 2d (R3 = p-F−C6H4, R4 = H) proceeded
similarly delivering the expected product 3dd in 50% yield.
Noteworthy, substitution at the C3 atom of the vinyl moiety
had a noticeable effect on the stereochemical outcome, as
dienes 3da and 3dd were isolated as 4:1 mixtures of (2E,5E)
and (2Z,5E) isomers. Unfortunately, probably because of steric
hindrance, C4-substituted vinyldiazo compounds did not
perform well in this transformation.
thus demonstrating that the extra hydrogen incorporated in the
final product does not come from the solvent. Next, we
conducted the model reaction in CH2Cl2 as solvent in the
presence of D2O (2 equiv). Following conventional workup,
diene 3aa-D with the deuterium label incorporated exclusively
in the 2-position was isolated (eq 4). Although compound 3aa-
D was isolated in low yield (20%) because of partial
decomposition of the starting diazo compound, this result
would confirm that external water participates in the present
transformation. Finally, reaction of deuterated vinyldiazo
compound 1c-D with (E)-trimethyl(styryl)silane (2a) led to
skipped diene 3ca-D without positional scrambling of the
deuterium label (eq 5).
Based on these control experiments and previous gold-
catalyzed transformations of vinyldiazo compounds, a likely
mechanism is proposed in Scheme 3. The process is suggested
To further explore the scope of our transformation we next
evaluated the reactivity of vinyldiazo ketones toward (E)-
trimethyl(styryl)silane (2a).17 In this regard, we found that 2-
diazo-1-phenylbut-3-en-1-one (1e; EWG = COPh, R1 = R2 =
H) was also amenable to the present transformation delivering
(2E,5E)-1,6-diphenylhexa-2,5-dien-1-one (3ea) in 55% yield
with complete regio- and stereoselectivity. On the other hand,
3-diazo-4-methylpent-4-en-2-one (1f; EWG = COMe, R1 =
Me, R2 = H) reacted with vinylsilane 2a to give the expected
product 3fa in 40% yield as a 10:1 mixture of (3E,6E) and
(3Z,6E) isomers.
Scheme 3. Proposed Mechanism for the Formation of
Skipped Dienes 3
Next, we briefly investigated the reactivity of silyl group
protected enoldiazoacetates.18 Gratifyingly, reaction of TMS-
protected enoldiazoacetate 1g with silanes 2a (Ar = Ph) and
2d (Ar = p-F−C6H4) resulted in the formation of a reaction
mixture from which compounds 4ga and 4gd were obtained in
55% and 44% yield, after column chromatography (eq 2). It
should be mentioned here that, along with compounds 4,
diethyl 2-diazo-3,6-dioxooctanedioate (5) was also produced
as a minor byproduct (8−10%).19
to begin with the decomposition of the diazo derivative 1
leading to the corresponding gold carbene intermediate I,
which would undergo attack of the vinylsilane 2 to the
vinylogous position with generation of a carbocationic species
II, stabilized by π-conjugation from the adjacent phenyl group
and additionally by hyperconjugation from the TMS group
placed in the β-position. Intermediate II would evolve through
an intramolecular 1,4-migration of the TMS group delivering
diene III,20 which in the presence of trace amounts of water
present in the reaction medium would lead to the final diene 3.
Density Functional Theory (DFT) calculations provided
further support for the proposed mechanism (see the
Supporting Information for details). In particular, our
calculations suggest that (a) the generation of the carbene
intermediate I occurs stepwise involving the migration of the
transition metal fragment from the initially formed gold(I)-
alkene complex followed by N2 release; (b) the formation of
intermediate II can be considered as a barrierless reaction; (c)
the intramolecular 1,4-migration of the TMS group is a highly
exergonic concerted process which occurs with a rather low
barrier of 10.3 kcal/mol; and (d) the desilylation of
intermediate III to provide the final product 3 is promoted
by the gold catalyst in the presence of a water molecule.
According to our calculations, the high exergonicity of the
process (ΔG = −27.5 kcal/mol) would be the driving force for
the key intramolecular 1,4-migration of the TMS group
(transformation of intermediate II into III).
Several control experiments were performed to gain insight
into this coupling reaction between vinyldiazo compounds and
alkenylsilanes. First, we found that conducting the model
reaction in deuterated dichloromethane gave the skipped diene
3aa without incorporation of deuterium in its structure (eq 3),
The proposed mechanism would account well for the
observed stereochemical outcome. Thus, the E-configuration
of the fragment arising from the vinyldiazo compound would
be ascribed to the preferred conformation of the carbene
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intermediate.21 On the other hand, the retention in the
configuration of the carbon−carbon double bond of the
vinylsilane is well-established and has been interpreted as a
consequence of the β-silicon effect.22
Encouraged by the results obtained with vinylsilanes, we
next wondered whether alkynylsilanes could serve as suitable
coupling partners, thus providing a convenient access to
skipped enynes. These motifs are highly attractive because they
are valuable building blocks and also structural motifs in
natural products and biologically active compounds and new
synthetic procedures for their synthesis would be highly
desirable.23
We first studied the reaction of ethyl 2-diazo-3-butenoate
(1a) with 1-phenyl-2-trimethylsilylacetylene (6a). Gratifyingly,
after a slight reoptimization of the reaction conditions, we
found that using [JohnPhosAu(MeCN)][SbF6] (5 mol %) as
the catalyst provided ethyl (E)-6-phenylhex-2-en-5-ynoate
(7aa) in 52% yield (Scheme 4).
alkyl-substituted alkynylsilanes also engaged in this trans-
formation providing the corresponding skipped enynes 7aj,
7ak, and 7al in moderate yields.
According to DFT calculations, this C(sp)−C(sp3) bond-
forming transformation is suggested to occur through the
generation of a vinylic carbocation followed by a concerted
intramolecular 1,4-silyl migration similar to that commented
above (see Supporting Information).
In conclusion, we have devised a simple and efficient route
to skipped dienes based on the gold-catalyzed reaction of
vinyldiazo compounds and alkenylsilanes. In most cases this
transformation proceeds with complete regio- and stereo-
selectivity with the silyl group serving as a regio- and
stereocontrolling group. Alkynylsilanes are also well suited
for the current transformation providing differently substituted
skipped enynes in moderate yields. The results reported herein
expand the range of carbon−carbon bond-forming reactions
available from vinyldiazo compounds under gold catalysis.
ASSOCIATED CONTENT
* Supporting Information
Scheme 4. Scope of the Gold-Catalyzed Reaction of
Vinyldiazoacetates 1 and Alkynylsilanes 6
■
a b
,
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01381.
Experimental procedures, characterization data, compu-
tational details, and copies of NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Luis A. López − Departamento de Química Orgánica e
Inorgánica, Instituto Universitario de Química
Organometálica “Enrique Moles and Centro de Innovación en
Química Avanzada (ORFEO−CINQA), Universidad de
Oviedo, 33006 Oviedo, Spain; orcid.org/0000-0002-
0893-6298; Email: lalg@uniovi.es
Israel Fernández − Departamento de Química Orgánica I and
Centro de Innovación en Química Avanzada
(ORFEO−CINQA), Facultad de Ciencias Químicas,
Universidad Complutense de Madrid, 28040 Madrid, Spain;
orcid.org/0000-0002-0186-9774; Email: israel@
quim.ucm.es
a
Reaction conditions: 1 (0.15 mmol), 6 (0.60 mmol, 4 equiv),
[JohnPhosAu(MeCN)][SbF6] (5 mol %), CH2Cl2 (1.8 mL), rt.
Yield of isolated product.
Authors
b
Olaya Bernardo − Departamento de Química Orgánica e
Inorgánica, Instituto Universitario de Química
Organometálica “Enrique Moles and Centro de Innovación en
Química Avanzada (ORFEO−CINQA), Universidad de
Oviedo, 33006 Oviedo, Spain
Kota Yamamoto − Departamento de Química Orgánica e
Inorgánica, Instituto Universitario de Química
Organometálica “Enrique Moles and Centro de Innovación en
Química Avanzada (ORFEO−CINQA), Universidad de
Oviedo, 33006 Oviedo, Spain
Concerning the reaction scope of this C(sp)−C(sp3)
coupling, a variety of aryl-substituted trimethylsilylacetylenes
6 were subjected to the previously developed reaction
conditions providing the corresponding skipped enynes 7 in
moderate yields and complete regio- and stereoselectivity. As
seen in Scheme 4, both alkyl groups and halogens could be
accommodated in the aryl moiety. In line with the results
displayed by vinylsilanes, substrates bearing strong electron-
withdrawing groups performed poorly. In fact, the use of a p-
trifluoromethylphenyl substituted alkynylsilane led to the
corresponding skipped enyne 7af with a low yield (38%),
while only traces of product 7ag were observed in the reaction
of 1a with a substrate featuring a p-nitrophenyl group. In
contrast, a substrate having a 3-methylphenyl group was
suitable for the current reaction as illustrated by the formation
of the corresponding product 7ah in moderate yield. On the
other hand, a 1-naphthyl-substituted alkynylsilane could also
provide the desired skipped enyne 7bi in 20% yield. Notably,
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01381
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from Ministerio de Ciencia e Innovación,
Agencia Estatal de Investigación (AEI) and Fondo Europeo de
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Palladium-Catalyzed 1,4-Difunctionalization of Butadiene To Form
Skipped Polyenes. J. Am. Chem. Soc. 2013, 135, 4167−4170.
(14) For synthetic approaches to skipped dienes, see: (a) Mailig, M.;
Hazra, A.; Armstrong, M. K.; Lalic, G. Catalytic Anti-Markovnikov
Hydroallylation of Terminal and Functionalized Internal Alkynes:
Synthesis of Skipped Dienes and Trisubstituted Alkenes. J. Am. Chem.
Soc. 2017, 139, 6969−6977 and references cited therein. Recent
relevant contributions on the synthesis of skipped dienes: (b) Wang,
C.-G.; Zhang, Y.; Wang, S.; Chen, B.; Li, Y.; Ni, H.-L.; Gao, Y.; Hu,
P.; Wang, B.-Q.; Cao, P. Nickel-Catalyzed Carboalkenylation of 1,3-
Dienes with Aldehydes and Alkenylzirconium Reagents: Access to
Skipped Dienes. Org. Lett. 2021, 23, 535−541.
(15) The gold-catalyzed dimerization of vinyldiazo compounds has
been reported: Zhang, D.; Xu, G.; Ding, D.; Zhu, C.; Li, J.; Sun, J.
Gold(I)-Catalyzed Diazo Coupling: Strategy towards Alkene
Formation and Tandem Benzannulation. Angew. Chem., Int. Ed.
2014, 53, 11070−11074.
(16) Electron-rich heteroarenes are known to be alkylated by
vinyldiazo compounds under gold catalysis (see ref 6). This
competing pathway might account for the failure of the reaction
with furyl-substituted alkenylsilane 2o.
(17) For a recent gold-catalyzed transformation of vinyldiazo
ketones, see: Kardile, R. D.; Liu, R. S. Gold(I)-Catalyzed Reactions
between 2-(1-Alkynyl)-2-Alken-1-Ones and Vinyldiazo Ketones for
Divergent Synthesis of Nonsymmetric Heteroaryl-Substituted Triaryl-
methanes: N- versus C-Attack Paths. Org. Lett. 2020, 22, 8229−8233.
(18) Recent contribution: Dong, K.; Zheng, H.; Su, Y.; Humeidi, H.;
Arman, H.; Xu, X.; Doyle, M. P. Catalyst-Directed Divergent Catalytic
Approaches to Expand Structural and Functional Scaffold Diversity
via Metallo-Enolcarbene Intermediates. ACS Catal. 2021, 11, 4712−
4721.
Desarrollo Regional (FEDER) (Grants PID2019-107469RB-
I00, PID2019-106184GB-I00, and RED2018-102387-T) is
gratefully acknowledged. K.Y. was supported by a Japan Public-
Private Partnership Student Study Abroad Program TOBI-
TATE Young Ambassador Program. We also acknowledge the
technical support provided by Servicios Científico-Técnicos de
la Universidad de Oviedo. We also thank Prof. J. M. González
(Universidad de Oviedo) for helpful discussions.
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