Gold-Catalyzed Vinyl Ether Hydroalkynylation: An Alternative Pathway for the Gold-Catalyzed Intermolecular Reaction of Alkenes and Alkynes

Article abstract of DOI:10.1021/acs.orglett.6b03228

In this report, the gold-catalyzed intermolecular reaction of vinyl ethers and terminal alkynes is investigated. Utilizing a triazole gold catalyst lessens gold decomposition in the presence of the vinyl ether and affords an alkynylation product instead of the [2 + 2] product. This protocol has been expanded to include glycal substrates, which undergo a one-pot alkynylation-Ferrier reaction to produce functionalized sugars in moderate to excellent yields with high diastereoselectivity.

Full text of DOI:10.1021/acs.orglett.6b03228

Letter
pubs.acs.org/OrgLett
Gold-Catalyzed Vinyl Ether Hydroalkynylation: An Alternative
Pathway for the Gold-Catalyzed Intermolecular Reaction of Alkenes
and Alkynes
Seyedmorteza Hosseyni, Courtney A. Smith, and Xiaodong Shi*
The Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, United States
S
* Supporting Information
ABSTRACT: In this report, the gold-catalyzed intermolecular
reaction of vinyl ethers and terminal alkynes is investigated.
Utilizing a triazole gold catalyst lessens gold decomposition in
the presence of the vinyl ether and affords an alkynylation
product instead of the [2 + 2] product. This protocol has been
expanded to include glycal substrates, which undergo a one-pot
alkynylation−Ferrier reaction to produce functionalized sugars in moderate to excellent yields with high diastereoselectivity.
old-catalyzed alkene and alkyne activation has greatly
enriched synthetic research by providing new strategies of
advantage of the unique stability of TA-Au and report the TA-
Au-catalyzed condensation of vinyl ethers and terminal alkynes.6
This represents a divergent pathway of the gold-catalyzed
intermolecular alkene−alkyne condensation reaction.
It is important to note that vinyl ethers have yet to be utilized
as the alkene source in previous gold-catalyzed intermolecular
alkene−alkyne condensations. Compared with regular alkenes,
vinyl ethers bear a more electron-rich CC double bond due to
the donation of oxygen’s electron density into the π system. This
additional π-electron density may lead to decomposition (e.g.,
polymerization) of the vinyl ether substrate upon treatment with
Lewis acids.7 Furthermore, the high electron density of vinyl
ethers may also accelerate [L-Au]+ decomposition.
Our recent study involving the TA-Au-catalyzed enyne
cycloisomerization of homopropargyl alcohols and terminal
alkynes provides preliminary evidence in support of vinyl ether-
promoted decomposition of Au. In this study, we demonstrated
that the presence of vinyl ether leads to decomposition of [L-
Au]+. However, employing a triazole ligand, and thus a more
stabilized gold complex, successfully prevented this catalyst
decomposition.8 Therefore, we postulated that the TA-Au
catalyst may also be viable in our proposed transformation
(Scheme 1B). To begin our investigation, we first evaluated the
stability of PPh3AuNTf2 and [PPh3Au(TA-H)]OTf (TA-Au) in
the presence of vinyl ether 1a and 2 equiv of terminal alkyne 2a
(Scheme 2).
G
bond formation.1 In general, Au(I) cations exhibit increased π-
acid reactivity toward alkynes in comparison with alkenes. One
illustration of this difference in reactivity is the gold-catalyzed
enyne cycloisomerization, which is proven to be a powerful
strategy for facile access to complex substrates.2 In particular, the
intermolecular enyne cycloisomerization of alkynes and alkenes,
as reported by Echavarren and co-workers, produces diversified
cyclobutenes.3 This reaction commences with alkene nucleo-
philic addition to a gold-activated terminal alkyne (Scheme 1A).
Scheme 1. Au(I) Catalysis: Intermolecular Reactions of
Alkenes and Alkynes
The final cyclobutene product is formed via ring expansion of a
cyclopropane intermediate and subsequent elimination. This
reaction is valuable because it enables straightforward access to
the cyclobutene skeleton, which is challenging to synthesize
using other methods. Moreover, this method has been
successfully employed for macrocycle ring formation,4 as well
as for the total synthesis of natural products.5 We wondered if
using vinyl ethers as the alkene source would afford the
analogous cyclobutene product.
As shown in Scheme 2, monitoring the reaction with 31P NMR
confirmed the quantitative decomposition of PPh3AuNTf2 to
form (PPh3)2Au+ (eq 1).9 Under identical conditions, less than
10% of [PPh3Au(TA-H)]OTf decomposed (eq 2), which
highlights the improved stability of the TA-Au catalyst.
Furthermore, reactions with either 5% PPh3AuNTf2 (eq 1) or
5% [PPh3Au(TA-H)]OTf (TA-Au) (eq 2) resulted in 100%
conversion of the vinyl ether. However, no expected cyclobutene
Surprisingly, in sharp contrast to the previously reported [2 +
2] cycloaddition pathway, C-2-hydroalkynylation of the vinyl
ether was obtained as the dominant product. Herein, we take
Received: October 27, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03228
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
was added slowly to the reaction mixture over 3 h. Satisfyingly,
this slow addition resulted in an increase in the yield to 78%
(entry 7).
Scheme 2. Initial Evaluation of Propargyl Ether Synthesis
It is plausible that the formation of 3a arises from alkyne
addition to the oxocarbenium B that is formed in situ. Although
an analogous metal-catalyzed hydroalkynylation of iminiums,11
carbonyls,12 and oxoniums13 was previously reported, copper (I/
II) salts were not able to promote the transformation. Other
metal catalysts and acids also did not produce the desired product
3a (entry 8). These results highlight the exceptional reactivity of
the gold catalyst in promoting this challenging transformation.
With the optimal conditions in hand, we investigated the scope of
the reaction (Table 2).
product A was observed. After careful analysis of the reaction
mixture, the hydroalkynylation product 3a was identified in both
reactions as the only intermolecular condensation product
incorporating both substrates.
Encouraged by these preliminary results, we conducted a
detailed screening of the reaction conditions (Table 1).
a,b
Table 2. Substrate Scope of Propargyl Ether Synthesis
a
Table 1. Optimization of the Reaction Conditions
b
time conv yield
catalyst
(h)
(%)
(%)
1
2
3
4
5
6
7
3% PPh3AuNTf2
3% IPrAuNTf2
2
2
2
2
4
4
6
100
100
100
100
100
100
100
24
<10
16
3% XPhosAuNTf2
c
3% (ArO)3PAuNTf2
44
c
3% (ArO)3PAu(TA-H)OTf
54
c
c
3% (ArO)3PAu(TA-Ph)OTf
56
3% (ArO)3PAu(TA-Ph)OTf (1a was added
slowly over 3 h)
78
8
9
other tested catalysts (5%): CuI, Cu(OTf)2,
Ga(OTf)3, ZnBr2, HOTf etc.
6
6
100
100
trace
<50
a
other tested solvents: toluene, CH3CN, THF,
MeNO2
Reaction conditions: In a 2 mL vial, gold catalyst (0.03 mmol) was
added to a solution of 2a (2 mmol) in CHCl3 (0.70 mL). The
resulting reaction mixture was stirred, and 1a (1 mmol) in 0.63 mL of
CHCl3 was added dropwise via syringe pump over 3 h. Isolated yields
a
Reaction conditions: Catalyst was added to a solution of 1a (1 mmol)
and 2a (2 mmol) in CDCl3 (1.33 mL) and the mixture stirred at room
b
b
temperature. 1H NMR yields were calculated using 1,3,5-trimethox-
are reported. Ar = 2,4-di-tert-butylbenzene.
c
ybenzene as an internal standard. Ar = 2,4-di-tert-butylbenzene.
Fortunately, this reaction tolerates a wide range of vinyl ethers
and alkynes. In evaluating the substrate scope of the alkyne, both
electron-deficient (3b,c,l,o) and electron-rich (3d,e,m) aromatic
alkynes produced good yields. Using m-diethynylbenzene
yielded the monoalkynylation product 3i in 58% yield. The
conjugate enyne 3h is also a viable substrate for this reaction,
demonstrating the mildness of the conditions. Additionally,
aliphatic alkynes bearing cyclopropyl (3f) and cyclopentyl (3g)
substitutents can be used. A heteroaromatic-substituted alkyne
was also tolerated, as the thiophene alkyne yielded 73% (3j).
Concerning the reaction scope of vinyl ethers, both cyclic (3a−j;
3n,o) and linear (3k−m) vinyl ethers afforded moderate yields of
the desired product. However, monosubstituted vinyl ethers
yielded the product in less than 30% yield, presumably due to
their higher reactivity and relatively fast polymerization.
Trisubstituted methyl vinyl ethers (3p,q) afforded the highest
yield (91−93%) because of the aberrance of side products in
comparison with di- and monosubstituted vinyl ethers. However,
tetra-substituted vinyl ethers did not produce the desired product
due to their low reactivity presumably from steric hindrance. In
addition, using furan and benzofuran vinyl ether substrates
resulted in low conversion due to the necessary disruption of
aromaticity during the reaction. Overall, this reaction tolerated a
wide range of alkynes and vinyl ethers.
A small amount of the desired product was formed when PPh3
was employed as the primary ligand (24%; entry 1). However,
when an electron-rich, N-heterocyclic carbene ligand (10%;
entry 2), or the bulky, electron-rich XPhos (2-dicyclohexylphos-
phino-2′,4′,6′-triisopropylbiphenyl) ligand was used (16%; entry
3), the yield was diminished. Gratifyingly, switching to an
electron-deficient phosphite ligand increased the yield to 44%
(entry 4). This is likely due to the increased electrophilicity of its
gold cation, which may favor the formation of gold acetylide C.
The use of triazole−gold further increased the yields, reasonably
attributed to the improved stability of the catalyst (entries 5 and
1
6). Monitoring the reaction via H NMR indicated that the
dihydropyran underwent self-condensation in the presence of
gold. Theoretically, 1a can attack intermediate B to produce
undesired products. In support of this binding mode, Jones and
co-workers have demonstrated that vinyl ethers can undergo
complexation with gold to form η1 or η2 complexes.10
To provide evidence supporting the self-condensation of
dihydropyran 1a in the presence of gold, a control experiment
was performed in which 1a was reacted with TA-Au in the
absence of 2a. 1H NMR monitoring indicated that vinyl ether 1a
converts to an uncharacterized dimerization product in the
presence of gold. To avoid this self-condensation pathway, 1a
B
DOI: 10.1021/acs.orglett.6b03228
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Letter
a c
Table 3. Substrate Scope of Glycosidation Reaction −
After evaluating our new findings, we were keen to expand this
chemistry to carbohydrate research. In recent years, gold catalysis
has played a pivotal role in the development of glycosidation
reactions.14 We identified glycals, which contain a vinyl ether
functional group, as an excellent substrate to expand our
research.
It has been recently reported that a combination of ascorbic
acid and Cu(OTf)2 catalyzes the alkynylation and subsequent
Ferrier-type elimination of glycal 4a to produce 5a.15 We
wondered whether this condition could be applied to
unfunctionalized vinyl ethers. To demonstrate the difference in
reactivity of 1a and 5a, we subjected 1a and 2a to this previously
reported literature condition (Scheme 3). Surprisingly, no
alkynylation product 3a was observed under these conditions,
and rapid decomposition of 1a was detected via 1H NMR.
Scheme 3. Copper-Catalyzed Reactions
a
Reaction conditions: In a 2 mL vial, gold catalyst (0.03 mmol) was
added to a solution of 2a (2 mmol) in 1,2-dichloroethane (1.33 mL),
followed by addition of 4a (1 mmol). The resulting mixture allowed
stirring at 70 °C for 4 h. Isolated yields are reported unless otherwise
b
c
stated. Ar = 2,4-di-tert-butylbenzene. Diastereomeric ratio deter-
d
mined via 1H NMR. 1H NMR yield was calculated using 1,3,5-
trimethoxybenzene as an internal standard.
Scheme 4. Mechanistic Investigation
These results clearly highlight the enhanced reactivity of the
gold catalytic method we have reported. Thus, to expand our
reaction scope to include the Ferrier-type elimination of glycals,
we subjected various glycal substrates to the optimized catalytic
gold conditions. To ensure full conversion of the glycal, we
increased the temperature to 70 °C. Rewardingly, moderate to
excellent yields of the desired product 5a were observed with
only one diastereomer isolated for a majority of the substrates
1
(Table 3). The diastereomeric ratio was determined via H
NMR, and the relative stereochemistry was assigned on the basis
of previous literature reports.16
no conversion of the vinyl ether (eq 2). Adding 10%
PPh3AuNTf2 and stoichiometric gold acetylide E to 1a resulted
in decomposition of both the gold catalyst and vinyl ether with
no formation of the desired product (eq 3). Using the triazole
gold catalyst (ArO)3PAu(TA-Ph)OTf instead of PPh3AuNTf2
decreased the yield of the reaction due to the reduced reactivity
of TA-Au (eq 4). Finally, treating 1a with a combination of 2%
HOTf and stoichiometric gold acetylide E afforded the desired
product 3a in 65% yield (eq 5).17
These results suggest that gold acetylide is the nucleophile for
the observed hydroalkynylation reaction and that residual acid
may promote the transformation. There are few instances of this
reactivity mode of gold(I) found in the literature.18 Furthermore,
the main challenge of this class of reactions is avoiding vinyl ether
promoted gold decomposition. This signifies the use of a
triazole−gold catalyst, which is inherently more stable, in
promoting this challenging transformation.
To conclude, herein we report the efficient reaction of vinyl
ethers with terminal alkynes, producing a hydroalkynylation
product. The previously reported [2 + 2] product was not
observed in any of the studied cases, suggesting a new
mechanistic route of this transformation. This reaction was
performed under mild conditions and exhibits high functional
group tolerance. A mechanistic investigation revealed the in situ
generation of HOTf and gold acetylide as key components of the
As with the reaction of simple vinyl ethers, a wide range of
glycals could be used in the reaction. Glycals with −OAc (5a−f),
−OBn (5i), or −OBz (5l) at the C-3 position were well tolerated.
In addition, utilizing glycals bearing an OTBS substituent at the
C-3 position of glycal could also afford the product in good yields
(5j−k). Substrates with a methyl group at the C-6 position were
used to investigate whether the presence of an OAc substituent at
the C-6 position influenced the reaction yield or stereochemistry
due to its proximity to the reaction center. The yield of these
substrates bearing a C-6-methyl group did not vary considerably
(5g,h). Notably, the cholesterol derivative 5m was formed in
moderate yield (64%), further illustrating the tolerance of the
reaction. However, aliphatic alkynes were not compatible with
the glycal transformation under the optimized reaction
conditions.
With the substrate scope of the transformation explored, we
turned our attention to investigating the mechanism of the
transformation. We synthesized gold acetylide E to test whether
the intermediate could be used to form the desired product. Vinyl
ether 1a and gold acetylide E were subjected to a variety of
conditions (Scheme 4). When 1a and 2a were treated with a
catalytic amount of HOTf, 1a underwent rapid decomposition
and 3a was not observed (eq 1). Treating 1a with a
stoichiometric amount (1 equiv) of gold acetylide E resulted in
C
DOI: 10.1021/acs.orglett.6b03228
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(4) Obradors, C.; Leboeuf, D.; Aydin, J.; Echavarren, A. M. Org. Lett.
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reaction. An alkynylation−Ferrier reaction employing glycals was
also realized, producing functionalized glycal derivatives with
excellent diastereoselectivity.
(6) Recent publications on TA-Au catalysis: (a) Motika, E. S.; Wang,
Q.; Akhmedov, N. G.; Wojtas, L.; Shi, X. Angew. Chem., Int. Ed. 2016, 55,
11582−11586. (b) Yang, Y.; Qin, A.; Zhao, K.; Wang, D.; Shi, X. Adv.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b03228.
■
S
(7) (a) Aoshima, S.; Yoshida, T.; Kanazawa, A.; Kanaoka, S. J. Polym.
Sci., Part A: Polym. Chem. 2007, 45, 1801−1813. (b) Hashmi, A. S. K.;
Materials and methods, compound characterization, and
NMR spectra (PDF)
Schafer, S.; Goker, G.; Eisenbach, C. D.; Dirnberger, K.; Zhao-Karger,
̈
̈
Z.; Crewdson, P. Aust. J. Chem. 2014, 67, 500−506. (c) Nzulu, F.;
Telitel, S.; Stoffelbach, F.; Graff, B.; Morlet-Savary, F.; Lalevee, J.;
́
AUTHOR INFORMATION
■
Fensterbank, L.; Goddard, J. P.; Ollivier, C. Polym. Chem. 2015, 6,
Corresponding Author
*E-mail: xmshi@usf.edu.
ORCID
4605−4611.
(8) Hosseyni, S.; Wojtas, L.; Li, M.; Shi, X. J. Am. Chem. Soc. 2016, 138,
3994−3997.
(9) Zhdanko, A.; Strobele, M.; Maier, M. E. Chem. - Eur. J. 2012, 18,
̈
Xiaodong Shi: 0000-0002-3189-1315
Notes
14732−14744.
(10) (a) Sriram, M.; Zhu, Y.; Camp, A. M.; Day, C. S.; Jones, A. C.
Organometallics 2014, 33, 4157−4164. (b) Zhu, Y.; Day, C. S.; Jones, A.
C. Organometallics 2012, 31, 7332−7335. (c) Mazzone, G.; Russo, N.;
The authors declare no competing financial interest.
Sicilia, E. Organometallics 2012, 31, 3074−3080. (d) Roithova,
Jankova, S.; Jasíkova, L.; Vana, J.; Hybelbauerova,
Ed. 2012, 51, 8378−8382.
́
J.;
ACKNOWLEDGMENTS
We thank the NIH (1R01GM120240-01), NSF (CHE-
1362057), and NSFC (21629201) for financial support.
́
̌
́
́
̌
́
S. Angew. Chem., Int.
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Hammond, G. B. J. Am. Chem. Soc. 2010, 132, 916−917.
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