SilverCatalyzed Cascade Carboxylation and Cyclization of Trimethyl(2-methylenebut-3-yn-1-yl)silane Derivatives

Article abstract of DOI:10.1021/acs.orglett.5b03023

C-C bond-forming carboxylation and cyclization of trimethyl(2-methylenebut-3-yn-1-yl)silane derivatives and carbon dioxide was developed. Silver catalysts and CsF promoted the reaction to afford the corresponding 2-furanone and 2-pyrone derivatives in goo

Full text of DOI:10.1021/acs.orglett.5b03023

Letter
pubs.acs.org/OrgLett
SilverCatalyzed Cascade Carboxylation and Cyclization of
Trimethyl(2-methylenebut-3-yn-1-yl)silane Derivatives
Kohei Sekine, Yuta Sadamitsu, and Tohru Yamada*
Department of Chemistry, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
S
* Supporting Information
ABSTRACT: C−C bond-forming carboxylation and cyclization of trimethyl(2-methylenebut-3-yn-1-yl)silane derivatives and
carbon dioxide was developed. Silver catalysts and CsF promoted the reaction to afford the corresponding 2-furanone and 2-
pyrone derivatives in good-to-high yields. The structure of the 2-furanone was confirmed by single-crystal X-ray crystallography,
which revealed that the geometry of the exo-olefin was that of a Z-isomer. When an aromatic ring-substituted alkyne was used, 2-
furanone derivatives were selectively obtained via 5-exo-dig cyclization, whereas the reaction of alkyl-substituted alkynes produced
2-pyrone derivatives with high selectivity.
Recently, carbon dioxide incorporation into propargyl
alcohols,8a,b,e propargylamines,8c−e and o-alkynylanilines8f,g
using silver catalysts and base was reported. In these reactions,
silver catalysts, acting as a π-Lewis acid, activate C−C triple
bonds to promote cyclization of the intermediates, such as
carbonates and carbamates. Activation of C−C triple bonds by
silver salts was a crucial step; otherwise, the intermediates
would readily revert back to starting materials. The silver
catalytic system was applied to ketone-containing alkynes to
give the corresponding lactones9a or furans having a carboxyl
group9b in good-to-high yields via C−C bond formation
between the substrates and carbon dioxide. Next, organosilane
compounds were utilized as a nucleophile for carbon dioxide,
which led to C−C bond-forming carboxylation and cyclization.
On the basis of previous studies, 5-membered ring lactones
and 6-membered ring lactones were expected from the reaction
of carbon dioxide and an allylsilane containing a C−C triple
bond (Scheme 1). The present report describes a silver-
catalyzed cyclization following carboxylation of trimethyl(2-
methylenebut-3-yn-1-yl)silane derivatives with carbon dioxide.
arbon dioxide has been utilized as a carbon source for the
Csynthesis of fine chemicals due to its abundant supply and
easy handling, despite its thermodynamic stability. Methods to
transform carbon dioxide into high-value chemicals have been
developed,1 and carbon dioxide has been incorporated into
alkyne derivatives during the synthesis of heterocyclic
compounds.2 Propargyl alcohols3 and propargyl amines4 were
converted to the corresponding cyclic carbonates and
oxazolidinones in the presence of a transition metal, organic
base, phosphine, or carbene catalyst. Though cyclic carbonates
and oxazolidinones are important structures in materials and
pharmaceutical sciences, their hydrolysis would readily promote
decarboxylation. To incorporate carbon dioxide into organic
molecules, formation of C−C bonds between the substrate and
carbon dioxide is important. Cyclization following C−C bond-
forming carboxylation can afford the corresponding lactones
that are not easily decarboxylated. However, few examples of
cyclization following carboxylation of alkyne derivatives with
C−C bond formation have been reported.5,7g
Organosilane compounds, such as allylsilane, arylsilane, and
vinylsilane, are useful reagents for new C−C bond formation.
For example, Hosomi−Sakurai allylation6 has been used to
provide homoallyl alcohols, which are an important framework
for the total synthesis of natural products and medicinal
compounds. Studies related to the carboxylation of organo-
silane compounds using carbon dioxide have been reported;
fluoride-mediated or Lewis acid mediated carboxylation
afforded the corresponding carboxylic acids or esters.7
However, for allylsilane compounds, only one system involving
Lewis acid mediated carboxylation has been reported.6c
Therefore, allylsilane compounds have additional potential for
carbon dioxide incorporation.
Scheme 1. Cyclization Following Carboxylation of
Allylsilanes Containing Alkynes
Received: October 19, 2015
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.5b03023
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Organic Letters
Letter
Enyne compound 1a containing an allylsilane moiety was
employed as a model substrate in the presence of CsF and
MeOH in DMF under 1 MPa CO2 atmosphere. Methanol was
added as a proton source for complete turnover of the catalytic
cycle. Initially, various metal salts were examined (Table 1).
Various alkynylaromatics were next explored under the
optimized conditions (Table 2, entries 1−12). Electron-
Table 2. Substrate Scope Using the Silver Catalytic System
Table 1. Examination of Metal Salts
a
b
c
entry
catalyst
none
yield 2 + 3 (%)
2:3
d
1
2
3
4
5
6
trace
e
CuI
trace
(PPh3)AuCl
Pd(OAc)2
AgF
17
49
70
79
28
73
73
73
80
80
78
95:5
89:11
77:23
88:12
87:13
83:19
89:11
91:9
AgOTf
f
7
AgOTf
g
8
AgOTf
9
AgOAc
10
11
12
AgBF4
(IMes)AgCl
(IPr)AgCl
(IPr)AgCl
93:7
94:6
h
13
> 95:5
a
a
The reaction was carried out with 0.15 mmol of substrate in 1.5 mL
The reaction was carried out with 0.15 mmol of substrate in 1.5 mL
b
c
b
c
d
of solvent. Isolated yield. Determined by 1H NMR. 4a was obtained
in 54% yield after methylation. 4a was produced in 63% yield after
methylation. KF was used intsead of CsF. TBAT (tetrabutylammo-
nium difluorotriphenylsilicate) was used intsead of CsF. 40 °C for 24
h.
of DMF. Isolated yield. The ratio was calculated based on isolated
yields. 50 °C. 20 mol % (IPr)AgCl was employed. 48 h. Not
detected. Using AgOTf instead of (IPr)AgCl.
d
e
f
e
g
f
g
h
donating groups induced production of the pyrone 3. For
substrates 1b (p-Me), 1c (m-Me), and 1e (p-OMe), the
corresponding furanones 2b, 2c, and 2e, and pyrones 3b, 3c,
and 3e were obtained with ratios of 94:6, 95:5, and 88:12,
respectively (Table 2, entries 2, 3, and 5). The reaction of
substrate 1d (o-Me) required higher temperatures, compared to
the optimized conditions, to afford the corresponding furanone
2d and pyrone 3d in 43% and 29% yields, respectively. A
methyl group at the ortho-position should hinder the approach
of silver catalysts, therefore requiring a greater amount of
catalyst, higher temperatures, and a longer reaction time (Table
2, entry 4). In contrast, substrates containing electron-
withdrawing groups such as p-COOEt (1f), p-Ac (1g), p-CN
(1h), and p-CF3 (1i) gave the corresponding furanones 2f−i
selectively in high yields, while pyrone derivatives 3f−i were
not observed (Table 2, entries 6−9). 1-Naphthyl and 2-
naphthyl isomers affected yield and selectivity; substrate 1j (1-
naphthyl) could be converted to furanone 2j and pyrone 3j in
18% and 15% yields, respectively (Table 2, entry 10). In
contrast, 2-naphthyl-substituted furanone 2k (2-naphthyl) was
produced selectively in 80% yield (Table 2, entry 11). The
regioselectivity for substrates 1b (p-Me), 1c (m-Me), 1d (o-
Me), 1j (1-naphthyl), and 1k (2-naphthyl) was assumed to be
due to steric effects from the substituent at the ortho-position;
silver catalysts were hindered from occupying the appropriate
position for 5-exo cyclization to promote 6-endo cyclization. For
2-pyridyl-substituted substrate 1l, the reaction afforded only the
Without any catalysts or with CuI, a trace amount of 2-
furanone 2a was detected, although methyl ester 4a was
obtained after methylation by trimethylsilyldiazomethane
(Table 1, entries 1 and 2). The results indicated that
carboxylation of 1a proceeded; in contrast, cyclization was
minimal. The metal salts (PPh3)AuCl and Pd(OAc)2 promoted
the reaction to give 2-furanone 2a and 2-pyrone 3a in 17% and
49% yields, respectively (Table 1, entries 3 and 4). In the
presence of AgF, the reaction proceeded smoothly to afford a
70% yield of a mixture of 77:23 furanone 2a/pyrone 3a (Table
1, entry 5). In this case, (3-methylbut-3-en-1-yn-1-yl)benzene
5a was obtained in 8% yield, which was produced by
protonation of a carbanion equivalent generated from substrate
and CsF. For fluoride sources, CsF was the most suitable for
this reaction (Table 1, entries 6−8).10 Various silver salts were
screened to improve the selectivity for 2a (Table 1, entries 5, 6,
and 9−12). The results indicated that an N-heterocyclic
carbene ligand slightly improved selectivity for the furanone
2a; (IMes)AgCl and (IPr)AgCl produced 2a and 3a in ratios of
93:7 and 94:6, respectively (Table 1, entries 11 and 12).
Examination of the reaction temperature10 revealed that the
ratio of furanone 2a to pyrone 3a was >95:5 if the reaction was
conducted at 40 °C, which afforded a mixture of 2a and 3a in
78% yield (Table 1, entry 13).
B
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Letter
furanone 2l in 73% yield. The reaction of the alkyl-substituted
alkynes substrates 1m and 1n also proceeded (Table 2, entries
13−15). Interestingly, the corresponding pyrones 3m and 3n
were obtained exclusively in 77% and 65% yields, respectively
(Table 2, entries 13 and 14). For tert-butyl (1o), pyrone 3o was
produced in a greater yield than that of the furanone 2o (Table
2, entry 15). Selectivity for pyrone 3 when using alkyl-
substituted substrates 1m, 1n, and 1o was due to the electron-
donating properties of the alkyl groups. A similar tendency was
noticed upon introduction of electron-donating groups into
alkynylaromatics (Table 2, entries 2−5).
exo-olefin of 2d was that of a Z isomer. The structure based on
heteronuclear multiple bond correlation (HMBC) (see the
Supporting Information for details) also agreed with that
suggested by the X-ray analysis.
In conclusion, a C−C bond-forming cyclization following
carboxylation of trimethyl(2-methylenebut-3-yn-1-yl)silane de-
rivatives and carbon dioxide was promoted by silver catalysts
and CsF. It afforded the corresponding 2-furanone and 2-
pyrone derivatives in good-to-high yields. The structure of the
2-furanone was confirmed by single-crystal X-ray crystallog-
raphy and HMBC, which revealed that the geometry of the exo-
olefin was that of a Z-isomer. Results also showed that, when
using an aromatic ring-substituted alkyne, 2-furanone deriva-
tives were selectively obtained via 5-exo-dig cyclization, whereas
the reaction of alkyl-substituted alkynes produced 2-pyrone
derivatives with high selectivity.
Alkyl-substituted allylsilane 1p also was investigated (Scheme
2). The reaction required a higher temperature to provide an
Scheme 2. Reactions of Alkyl-Substituted Allylsilanes
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03023.
Experimental procedures and analytical data for new
compounds (PDF)
X-ray crystallographic data for 2d (CIF)
AUTHOR INFORMATION
Corresponding Author
■
adequate yield of furanone 2p and pyrone 3p. Only trace
amounts of furanone 2p and pyrone 3p were observed at 40
°C. However, raising the temperature to 60 °C increased the
yields of furanone 2p and pyrone 3p to 52% and 11%,
respectively.
*E-mail: yamada@chem.keio.ac.jp.
Notes
The authors declare no competing financial interest.
Next, we investigated the nucleophilic addition to carbon
dioxide of substrate 1q, which contains a trisubstituted alkene
moiety (Scheme 2). Previous studies on fluoride-activated
allylation6c indicated that a carbanion (ion pair or equivalent)
was generated via Si−F bond formation. This was supported by
the result that nucleophilic addition of multisubstituted
allylsilanes occurred preferably from the less substituted carbon
atom, which gave homoallylalcohols with loss of regioselectiv-
ity. In this case, 1q was transformed to three isomers, 2q-γ
produced by attack from the γ-carbon, and 2q-α and 3q-α were
produced by the addition from the α-carbon. These
observations suggest that nucleophilic addition of a carbanion
(ion pair or equivalent) to carbon dioxide occurred to produce
the carboxylate. The structure of 2d was confirmed by single-
crystal X-ray crystallography (Figure 1). The geometry of the
ACKNOWLEDGMENTS
This work was financially supported by JSPS Research
Fellowships for Young Scientists.
■
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