Gold-Catalyzed Hydrophenoxylation of Propargylic Alcohols and Amines: Synthesis of Phenyl Enol Ethers

Article abstract of DOI:10.1021/acs.orglett.9b01208

A practical method for the synthesis of phenyl enol ethers is reported. The combination of a gold(I) catalyst and potassium carbonate selectively mediates the addition of phenols to propargylic alcohols/amines in a chemo-, regio-, and stereoselective fash

Full text of DOI:10.1021/acs.orglett.9b01208

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Gold-Catalyzed Hydrophenoxylation of Propargylic Alcohols and
Amines: Synthesis of Phenyl Enol Ethers
Victor Laserna, Catherine Jeapes Rojas, and Tom D. Sheppard*
Department of Chemistry, University College of London, 20 Gordon Street, London, WC1H 0AJ, U.K.
S
* Supporting Information
ABSTRACT: A practical method for the synthesis of phenyl
enol ethers is reported. The combination of a gold(I) catalyst
and potassium carbonate selectively mediates the addition of
phenols to propargylic alcohols/amines in a chemo-, regio-,
and stereoselective fashion in high yield. The resulting enol
ethers are formed exclusively with a Z-configuration and can
be obtained from a wide array of phenols and propargylic alcohols or amines with the reaction showing excellent functional
group tolerance.
he discovery of new synthetic methods for the formation
of C−O bonds has drawn considerable attention in
Scheme 1. Alkoxylation of Alkynes
T
synthetic organic chemistry.1−4 A straightforward approach
consists of the addition of alcohols to unsaturated bonds,
particularly alkynes,5,6 in an intramolecular7−10 or intermo-
lecular11−14 fashion. Compared to the well-developed hydro-
amination of alkynes,15 hydroalkoxylation reactions are far less
developed, probably due to the lower nucleophilicity of
alcohols compared to amines. The addition of alcohols to
alkynes yields enol ethers, although acetal products have also
been observed16 as double addition of the alcohol is possible.
These additions usually have a high activation barrier which is
overcome by the use of harsh conditions or through metal
catalysis. Many methods have been reported over the years
with a wide array of metal catalysts including Cu,17 Rh,18 Ru,19
Ir,20 Pt,21 or Au.22
generally occur at the internal carbon to give ketals or enol
ether products, but internal alkynes usually give mixtures of
regioisomers, especially when the two substituents have similar
electronic properties. With intramolecular reactions the
regioselectivity of the reaction can usually be controlled, as
the kinetically faster ring formation will usually determine the
selectivity.
Propargylic alcohols are very versatile and readily available
starting materials, where the presence of the alcohol group
adjacent to the alkyne gives a distinctive reactivity which differs
significantly from simple internal alkynes. Numerous methods
to access useful building blocks from propargylic alcohols have
been described in the literature. Our group and others have
reported the gold-catalyzed transformation of propargylic
alcohols into enones,32 bromo/fluoro enones,33,34 hetero-
cycles,35,36 or geminal dihalides.37 Recently we described how
propargylic alcohols activated by the Gagosz catalyst38 could
undergo regioselective hydroamination with anilines under
mild conditions.39 We demonstrated that the OH played a key
role in significantly increasing the reactivity of the triple bond,
Over the past decade Au catalysis has become considered as
the most effective approach to achieve hydration,23 hydro-
amination,15 or hydroalkoxylation6 of alkynes efficiently with a
wide range of substrates. Gold-catalyzed intramolecular
hydroalkoxylation of alkynes has proved to be a very useful
method for the synthesis of various heterocycles24−26 and
natural products,27 and although more challenging, the
intermolecular version of this reaction has been described for
primary13 and secondary22 alcohols and, to a lesser extent, with
phenols.28−30 In these reactions, internal alkynes usually show
considerably lower reactivity in comparison to terminal
alkynes,6 but the nature of the alcohol species has a more
significant impact on the feasibility of the reaction, with the
addition of primary and secondary alcohols to alkynes being
relatively well-known, even at mild temperatures (Scheme 1).
Reports of the addition of tertiary alcohols and phenols are
much rarer and usually involve harsher conditions. Further-
more, simple gold phenolates which could potentially be
formed in the reactions have been reported to be unstable,
potentially leading to catalyst decomposition.31 An added
problem when dealing with internal alkynes is the regiose-
lectivity of the reaction. Alcohol additions to terminal alkynes
Received: April 5, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01208
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and directing the attack of the aniline regioselectively to form
the 3-hydroxyimine.
a range of gold catalysts to explore the effect of ligands and
counteranions41 on the reaction. Comparable results were
obtained with different noncoordinating counterions, and
remarkably the reaction even proceeded effectively with a
chloride counterion although the isolated yield of product was
lower. A range of phosphine ligands were similarly effective in
the reaction, while a gold complex bearing an NHC ligand was
slightly less reactive (see Supporting Information). As
PPh3AuNTf2 is available commercially and gave the highest
isolated yield of product, this was used in further reactions.
The phenyl enol ethers were formed exclusively as the Z-
isomer (NOE data for compound 2c; see Supporting
Information), confirming that the gold catalyst promotes
trans addition of the phenol across the alkyne.
Inspired by these results, we decided to explore the
hydrophenoxylation of propargylic alcohols, with a view to
improving the regioselectivity of previously reported alcohol
additions to internal alkynes. Hydrophenoxylation of alkynes
has seldom been reported,28−30 and although gold catalysts can
mediate the reaction, harsh conditions are usually required and
the reactions typically lack selectivity. We hereby report a
method for the hydrophenoxylation of propargylic alcohols to
yield phenyl enol ethers with high chemo- and regioselectivity
under mild conditions. The robustness of the reaction is
demonstrated by a broad substrate scope in terms of both
phenols and propargylic alcohols. The reaction can also be
extended to Boc-protected propargylic amine substrates.
To investigate this reaction we chose the propargylic alcohol
1, derived from the addition of 1-hexyne to valeraldehyde. In
our previous work, we have observed that addition of 4-
nitrophenol to the reaction of a propargylic alcohol with a gold
catalyst promotes the formation of 3-hydroxyketone over
Meyer−Schuster rearrangement to the enone.40 However, little
or no addition of the phenol to the alkyne was observed in
these reactions. Our first attempts at phenol addition in this
study were not successful, and mixing phenol, 1, and gold
catalyst in chloroform at room temperature gave only trace
amounts of the addition product 2a, together with the
corresponding enone 3 and 3-hydroxyketone 4 after 18 h.
Previously reported phenoxylation reactions of alkynes have
employed a stoichiometric amount of base28,29 to partially
deprotonate the phenol and increase its nucleophilicity. To our
delight, upon the addition of sodium acetate the selectivity of
the reaction for the addition product 2a increased significantly
(Table 1, entry 3). Sodium acetate proved to lead to
nonselective reactions, producing significant amounts of
enone side-product 3. Optimization of the base and reaction
parameters led us to identify K2CO3 and 50 °C as the ideal
conditions. The optimization was carried out in CHCl3, but
alternative nonchlorinated solvents can also be used including
toluene, fluorobenzene, or trifluorotoluene. We also examined
Once optimal conditions were identified, we set out to
determine the scope of the reaction. First we focused on
screening phenols using substrate 1 as the propargylic alcohol
(Scheme 2). We examined the compatibility of the reaction
Scheme 2. Enol Ether Scope with Phenols
Table 1. Optimization of the Reaction Conditions
with functionalized phenols and found that it tolerated a wide
range of substituents on the phenol including electron-
donating (Me, NH2, OMe) or electron-withdrawing (NO2,
CF3, F, Cl, Br, and Ph) groups. The reaction does not seem to
be significantly affected by sterics, as it proceeded smoothly
with hindered phenols containing Cl or Me at the ortho
positions (2f, 2g, 2h, and 2k), and even when both ortho
positions are substituted (2l). To our surprise, under the
reaction conditions a phenol reacts preferentially to an
adjacent aniline (2n) which we have shown can attack
propargylic alcohols under similar reaction conditions in the
absence of a base.39 This result illustrates the higher reactivity
of deprotonated phenols and supports previous proposals that
the gold catalyst can have a dual role in alkyne phenoxylations,
activating both the alkyne and the phenol.29,30 Gold phenolates
are reported in the literature31 and have been proposed as the
nucleophilic species which attacks the electrophilic gold−
alkyne complex.29 Hexafluoroisopropanol, which has a
comparable acidity to phenol, can also undergo the reaction
(2p), but typical alcohols (MeOH, etc.) were unreactive, even
U1
entry
base
solvent
temp (°C) conv selectivity (2a:3:4)
1
2
3
4
5
6
7
8
9
−
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
PhMe
rt
rt
rt
rt
40
0
5:70:25
−
Cs2CO3
NaOAc
Na2CO3
K2CO3
K2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
58
24
27
99
82
72
85
78
67:32:1
91:8:1
98:1:1
98:1:1
98:1:1
98:1:1
98:1:1
98:1:1
rt
50
50
50
50
50
PhCF3
C6H5F
10
B
DOI: 10.1021/acs.orglett.9b01208
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
with the addition of stronger bases such as tert-butoxides or
hydrides.
After determining the tolerance of the reaction conditions to
a broad range of phenols, a variety of propargylic alcohols were
examined. As can be observed in Scheme 3 a broad range of
steric hindrance with substrates bearing cyclic alkylic chains
(6a, 16a, and 17a), isopropyl (18a), or tert butyl groups (26a)
all giving good yields. Other functional groups such as diethyl
acetals (12a, 14a), furan rings (20a), trifluoromethyl groups
(23a), or alkenes (17a, 24a, and 25a) are also well tolerated.
Double addition reactions are also possible using this
methodology. Molecules with two phenols, such as phenolph-
thalein (Scheme 2, 2q), react with 2 equiv of propargylic
alcohol forming the double addition product in moderate yield.
Double addition reactions can also be achieved in molecules
with two alkyne moieties, including two different alkynes
linked to the same alcohol group (Scheme 3, 29a), and a
molecule containing two identical propargylic alcohol moieties
(30a). These systems open up the possibility of forming
molecules containing multiple enol ethers which could be of
interest in coatings applications, where polyvinyl ethers are
important starting materials.
Scheme 3. Scope with Respect to the Propargylic Alcohols
Following the successful phenoxylation of propargylic
alcohols, we decided to explore the reaction with analogous
propargylic amines, which could also have the potential to
undergo regioselective additions guided by hydrogen bond-
ing.42 As alkylic amines are known to bond strongly to gold
centers and prevent alkyne activation, we examined propargylic
amines bearing electron-withdrawing groups to reduce nitro-
gen nucleophilicity. Sulfonamides proved to be unreactive and
underwent deprotonation in the presence of K2CO3 and did
not react further. Gratifyingly we found that a range of Boc-
protected propargylic amines43 underwent the phenol addition
reaction with complete selectivity and in moderate to good
yields (Scheme 4).
Scheme 4. Hydrophenoxylation of Propargylic Amines
In conclusion we report a chemo-, stereo-, and regioselective
method for the hydrophenoxylation of propargylic alcohols/
amines to yield phenyl enol ethers in high yields. The method
is very robust, tolerating a significant range of electronically/
sterically diverse phenols, as well as propargylic alcohols and
propargylic amines with a wide array of substitution patterns
and functionalities. This is the first report describing the
hydrophenoxylation of propargylic alcohols and propargylic
amines, and further supports the role of adjacent hydrogen
bonding groups in increasing the reactivity of alkynes toward
gold-catalyzed addition of nucleophiles in a stereoselective and
regioselective fashion.
propargylic alcohols can be transformed into the correspond-
ing phenyl enol ethers. Primary (8a), secondary (5a, 14a, 18a,
19a, 21a, and 22a), and tertiary (6a, 7a, 12a, 16a, and 23a)
propargylic alcohols can be employed, including benzylic (9a,
10a, 11a, 13a, 15a, 17a, and 20a) alcohols, and all smoothly
undergo the addition reaction in high yield and with excellent
selectivity. Aromatic rings on the alcohol can include electron-
withdrawing groups (10a, 15a, 17a, 27a, or 28a) or electron-
donating groups (11a). Again the reaction shows tolerance of
C
DOI: 10.1021/acs.orglett.9b01208
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Catalysed by Cationic Gold(I) Complexes. Adv. Synth. Catal. 2010,
352, 1701−1710.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01208.
■
S
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Experimental procedures for the preparation of prop-
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1
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tom.sheppard@ucl.ac.uk.
ORCID
Tom D. Sheppard: 0000-0002-3455-1164
Notes
́
́
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We would like to thank the Leverhulme Trust for providing a
grant (RPG-2017-221) to support this work.
■
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