Gold(I)/(III)-catalyzed synthesis of cyclic ethers; Valency-controlled cyclization modes

Article abstract of DOI:10.1021/acs.orglett.5b01046

Strategic use of oxophilic (hard) gold(III) and π-philic (soft) gold(I) catalysts provides access to two types of cyclic ethers from propargylic alcohols. Thus, heating propargylic alcohols with an oxophilic gold(III) catalyst (AuBr₃) results i

Full text of DOI:10.1021/acs.orglett.5b01046

Letter
pubs.acs.org/OrgLett
Gold(I)/(III)-Catalyzed Synthesis of Cyclic Ethers; Valency-Controlled
Cyclization Modes
Nobuyoshi Morita,* Arisa Yasuda, Motohiro Shibata, Shintaro Ban, Yoshimitsu Hashimoto,
Iwao Okamoto, and Osamu Tamura*
Showa Pharmaceutical University, Machida, Tokyo, 194-8543, Japan
S
* Supporting Information
ABSTRACT: Strategic use of oxophilic (hard) gold(III) and π-philic (soft) gold(I)
catalysts provides access to two types of cyclic ethers from propargylic alcohols. Thus,
heating propargylic alcohols with an oxophilic gold(III) catalyst (AuBr3) results in
cyclization to afford cyclic ethers bearing an acetylenic moiety, due to coordination of
gold(III) to the oxygen of the propargylic hydroxyl group. On the other hand, propargylic
alcohols with a π-philic gold(I) catalyst (Ph3PAuNTf2) induces Meyer−Schuster
rearrangement to afford α,β-unsaturated ketones, which undergo gold(III)-catalyzed
intramolecular oxa-Michael addition to afford cyclic ethers bearing a carbonyl group, due
to coordination of gold(III) to the oxygen of the carbonyl group.
oxophilic (hard) gold(III)-catalyzed oxa-Michael addition11,12
old catalysts were initially recognized as π-acidic catalysts
(route B, Figure 1). Thus, strategic use of the two gold catalysts
Gthat activate unsaturated bonds, such as alkynes, allenes,
enables diversity-oriented synthesis of two types of cyclic ethers.
We first examined cyclization of the propargylic alcohol (route
A, Figure 1) by using alcohol 1a as a model substrate with small
amounts of a gold(III) catalyst (Table 1). Thus, treatment of 1a
and alkenes, for nucleophilic attack to form C−C, C−O, C−N,
and C−S bonds.1 Later, groups led by Gevorgyan2b and
Campagne3c reported the oxophilic character4 of gold(III)
catalysts, which efficiently activate oxygen functionalities even in
the presence of an unsaturated bond (allene/alkyne). Since then,
nucleophilic substitution reactions of benzyl alcohols5 or their
acetates6 via oxophilic gold(III)-catalyzed activation of the
hydroxyl or acetoxy group have been reported. We rationalized
these observations in terms of the hard and soft acids and bases
(HSAB) principle, which states that metal ions in low valence
states exhibit soft character, whereas metal ions in high positive
oxidation states show hard character.7,8 Thus, gold(I) catalysts
may behave as soft acids and gold(III) catalysts as hard acids. On
the basis of this working hypothesis, we designed synthetic
methods to obtain two types of cyclic ethers from the same
propargyl alcohols by means of valency-controlled gold-catalyzed
regiodivergent activation (Figure 1). Here, we report an
oxophilic (hard) gold(III)-catalyzed cyclization of propargylic
alcohols (route A, Figure 1) and a π-philic (soft) gold(I)-
catalyzed Meyer−Schuster rearrangement9,10 followed by
Table 1. Optimization of Reaction Conditions with Gold(III)
Catalyst
entry
catalyst (mol %)
AuBr3 (5)
solvent
temp
time
yield
1
2
3
4
CH2Cl2
CH2Cl2
DCE
rt
4 h
63%
64%
80%
80%
73%
83%
40%
HAuCl4·3H2O (5)
AuBr3 (5)
rt
4 h
reflux
reflux
reflux
reflux
reflux
5 min
5 min
5 min
5 min
5 min
HAuCl4·3H2O (5)
AuBr3 (5)
DCE
a
5
DCE
6
7
AuBr3 (5)/AgNTf2 (15)
AgNTf2 (5)
DCE
DCE
a
The reaction was carried out at high concentration (0.73 M). DCE =
ClCH2CH2Cl.
with 5 mol % of AuBr3 or HAuCl4·3H2O induced cyclization to
furnish cyclic ether 2a bearing an acetylenic moiety in good yield
(entries 1 and 2). Optimization of the reaction conditions was
carried out with propargylic alcohol 1a using gold(III) catalysts.
Higher temperature dramatically accelerated the reaction. On
heating propargylic alcohol 1a with 5 mol % of the gold(III)
catalyst, AuBr3 or HAuCl4·3H2O, in refluxing ClCH2CH2Cl
Received: April 11, 2015
Figure 1. Our strategy for using gold(I)/(III) catalysts.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01046
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(DCE), the reaction was completed within 5 min to give cyclic
ether 2a in 80% yield (entries 3 and 4). To generate a cationic
gold catalyst, a silver catalyst (AgNTf2) was used as cocatalyst in
the reaction (entry 6). The system of AuBr3 (5 mol %) and
AgNTf2 (15 mol %) was found to slightly increase the yield of the
desired product 2a. On the other hand, we found that AgNTf2 (5
mol %) alone as well as AuBr3 (5 mol %) was capable of
catalyzing the cyclization, but with a significantly low yield (entry
3 vs 7). It is noteworthy that treatment of 1a at high
concentration (0.73 M solution) with 5 mol % of AuBr3
provided cyclic ether 2a in 73% yield without dimer formation
(entry 5), even though a gold(III)-catalyzed intermolecular ether
formation of phenyl acetylenic carbinol with ethanol at even
lower concentration (0.2 M) has been reported.3b In contrast,
other transition metal catalysts or Lewis acids, such as Sc(OTf)3,
CuI, and BF3·OEt2, afforded no cyclized product.
catalyzed cyclization of propargyl alcohols that is available for
five- to seven-membered rings. The transformation of 1 to 2 is
reminiscent of the Nicholas reaction.13 However, the overall
process using the Nicholas reaction would require at least three
steps: complexation of 1 with a stoichiometric amount of
Co2(CO)8, treatment of the resulting complex with a Lewis acid
for cyclization, and decomplexation of the alkyne−dicobalt
complex with a stoichiometric amount of oxidizing reagent. In
contrast, the present cyclization offers several advantages: (1)
one-step transformation of 1 to 2, (2) rapid cyclization within 5
min, (3) only a catalytic amount of AuBr3 is required, and (4)
H2O is the only byproduct.
During optimization of reaction conditions for gold(III)-
catalyzed cyclization with propargylic alcohol 1a, we surprisingly
found that use of 30 mol % of NaAuCl4·3H2O in CH2Cl2
afforded cyclic ether 2a in 38% yield along with another cyclic
ether 3a having a carbonyl group in 37% yield (Scheme 1).
Formation of 3a may be rationalized by postulating intra-
molecular oxa-Michael addition of α,β-unsaturated ketone 4a
produced by Meyer−Schuster rearrangement of 1a.
We next investigated the effect of substituents at the alkyne
terminus of propargylic alcohols 1b−f on the cyclization; we
chose AuBr3 as the gold(III) catalyst because it is less
hygroscopic than HAuCl4·3H2O (Table 2). Alkyne-terminal
Table 2. AuBr3-Catalyzed Cyclization of Various Propargylic
Alcohols 1
Scheme 1. Gold-Catalyzed Synthesis of Two Types of Cyclic
Ethers 2a and 3a
additive
entry
1
R1
R2
n
(mol %)
2
yield
60%
1
2
3
4
5
6
7
8
9
1b
1b
1c
1c
1d
1e
1f
t-Bu
t-Bu
n-Hex
n-Hex
c-Hex
H
H
H
H
H
H
H
Me
H
H
2
2
2
2
2
2
2
1
3
none
2b
2b
2c
2c
2d
2e
2f
AgNTf2 (15)
none
22%
The formation of 3a from 1a11,12 prompted us to examine the
reaction in the presence of π-philic (soft) gold(I) catalysts (Table
3). The desired product 3a was not formed at all from 1a in the
18%
AgNTf2 (15)
AgNTf2 (15)
none
60%
50%
not detected
52%
Ph
none
Table 3. Optimization of Reaction Conditions in Gold(I)-
Catalyzed Synthesis of Cyclic Ethers 3a Having a Carbonyl
Group
1g
1h
Ph
none
2g
2h
80%
Ph
none
46%
substituents influenced cyclization of propargylic alcohols 1b−f.
Propargylic alcohol 1b bearing a tert-butyl group with AuBr3 (5
mol %) was smoothly transformed to the corresponding product
2b in good yield (entry 1), but the cationic gold catalyst
generated by AuBr3 (5 mol %) and AgNTf2 (15 mol %) with
alcohol 1b afforded product 2b in low yield along with many
unidentified products (entry 2). On the other hand, treatment of
propargylic alcohol 1c having an n-hexyl group with AuBr3 (5
mol %) furnished the corresponding product 2c in low yield
(entry 3), while the system of AuBr3 (5 mol %) and AgNTf2 (15
mol %) smoothly catalyzed cyclization of alcohol 1c to give the
product 2c in good yield (entry 4). Propargylic alcohol 1d also
underwent cyclization, affording the corresponding product 2d
in moderate yield (entry 5). The reaction of alcohol 1e with a
terminal alkyne gave no cyclized product 2e (entry 6). Tertiary
propargyl alcohol 1f was efficiently converted into the
corresponding cyclic ether 2f in good yield (entry 7). To our
delight, this cyclization was found to be effective for construction
of five- and seven-membered cyclic ethers, as well as six-
membered ones (entries 8 and 9). Thus, propargylic alcohols 1g
and 1h reacted under similar conditions to those used for 1a−f to
afford five- and seven-membered ring products 2g and 2h in 80%
and 46% yields, respectively. This is the first noble metal-
entry
catalyst (mol %)
solvent
time
yield
1
2
3
4
5
AuCl (10)
CH2Cl2
CH2Cl2
toluene
toluene
toluene
3 days
3 days
3 days
18 h
−
−
−
36%
Ph3PAuCl (10)
Ph3PAuOCOCF3 (5)
[Ph3PAuNTf2]2PhMe (1)
[Ph3PAuNTf2]2PhMe (1), 1 equiv of
MeOH
18 h
78%
presence of 10 mol % of AuCl, Ph3PAuCl, or 5 mol % of
Ph3PAuOCOCF3 (entries 1−3), but the reaction using a
14
cationic gold(I) catalyst, [Ph3PAuNTf2]2PhMe (1 mol %), gave
cyclic ether 3a bearing a carbonyl group in 36% yield (entry 4).
Further optimization of the reaction conditions using
[Ph3PAuNTf2]2PhMe was carried out. As a result, MeOH was
found to be efficient as an additive. Thus, treatment of
propargylic alcohol 1a with 1 mol % of [Ph3PAuNTf2]2PhMe
and 1 equiv of MeOH9b,c smoothly gave cyclic ether 3a in 78%
yield (entry 5).
Next, we examined the scope of the π-philic gold(I)-catalyzed
reaction of 1 leading to carbonyl-containing cyclic ethers 3
B
DOI: 10.1021/acs.orglett.5b01046
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Organic Letters
Letter
(Table 4). Treatment of alcohols 1 having various substituents at
the alkyne terminal with 1 mol % of [Ph3PAuNTf2]2PhMe and 1
and gold(III) catalyst. After confirming consumption of the
starting alcohol 1h and production of α,β-unsaturated ketone 4h
through gold(I)-catalyzed Meyer−Schuster rearrangement, a
gold(III) catalyst (5 mol % of AuBr3) was added to induce oxa-
Michael addition, and the desired product 3h was obtained in
good yield (Scheme 4).
Table 4. Gold(I)-Catalyzed Meyer−Schuster Rearrangement
Followed by Oxa-Michael Addition
Scheme 4. One-Pot Synthesis of Seven-Membered Ring 3h
entry
1
1
R1
R2
H
X
time
18 h
yield (cis/trans)
3i 94%
1i
p-MeO-
C6H4
CH2
2
3
4
5
1c
1d
1b
1j
(10:1)
1k
1l
n-Hex
c-Hex
t-Bu
H
CH2
CH2
CH2
CH2
4 h
3c 89%
3d 82%
3b 55%
H
1 day
2 days
1 day
H
Ph
Me
3j
84%
(98:2)
6
7
Ph
Ph
H
H
O
20 h
3k 89%
3l 67%
Next, we examined Meyer−Schuster rearrangement and oxa-
Michael addition of other propargylic alcohols 1g,m,n (Table 5).
NBoc
1 day
equiv of MeOH in toluene at room temperature afforded the
corresponding carbonyl-containing cyclic ethers 3 in excellent
yield (entries 1−3), except in the case of propargylic alcohol 1b
with a t-Bu group, which required a prolonged reaction time and
gave only a moderate yield of 3b (entry 4). A 10:1 diastereomeric
mixture of secondary alcohol 1j also underwent cyclization cis-
selectively, giving rise to 2,6-tetrahydropyran 3j in 84% yield
(entry 5). The incorporation of oxygen (1k) or nitrogen (1l) into
the ether provided 1,4-dioxane 3k or morpholine 3l in 89% and
67% yields, respectively (entries 6 and 7).
Table 5. One-Pot Synthesis of Five- and Seven-Membered
Ring 3 by Gold(I)/(III) Catalyst
Next, we attempted to construct seven-membered ring 3h
under similar reaction conditions, but the reaction with
propargylic alcohol 1h mainly afforded α,β-unsaturated ketone
4h (86%) with only a trace amount of the desired product 3h
(Scheme 2). To accelerate the oxa-Michael addition of α,β-
entry
1
R
n
additive (mol %)
time
3 yield
28%
1
2
3
4
5
6
1g
Ph
1
1
1
1
3
3
none
1 day
1 day
1 day
5 h
3g
1g
Ph
AuBr3 (5)
none
3g
83%
34%
71%
trace
40%
1m
1m
1n
1n
n-Hex
n-Hex
n-Hex
n-Hex
3m
3m
3n
3n
AuBr3 (5)
none
1 day
1 day
Scheme 2. Attempt to Construct Seven-Membered Ring 3h by
Gold(I) Catalyst
AuBr3 (5)
Compounds 1g,m,n were transformed into cyclic ethers 3g,m,n
in low yields (entries 1 and 3) or in a trace amount (entry 5) in
the absence of gold(III) catalyst, whereas the addition of the
gold(III) catalyst greatly improved the yields of the products
(entries 2, 4, 6). Although a few similar ether formation reactions
using noble metal catalysts have been reported,11,12 their scopes
are quite limited. It should be noted that the present method is
the first system that provides access to five- to seven-membered
cyclic ethers from propargylic alcohols.
A plausible mechanistic model for gold-catalyzed formation of
the two types of cyclic ether is shown in Scheme 5. In both cases,
the complex A3 would be formed as a common reaction
intermediate, whose character would play a pivotal role in
determining the reaction pathway. Oxophilic gold(III) in
complex A strongly activates the hydroxyl group (activation a)
to induce cyclization by intramolecular nucleophilic substitution,
furnishing cyclic ether 2 bearing an acetylenic moiety. On the
other hand, a π-philic gold(I) catalyst strongly activates the triple
bond of propargylic alcohols 1 (activation b). Thus, activation b
by π-philic gold(I) promotes addition of methanol9c (A → B) to
generate an allenyl ether (B → C → D), which undergoes
hydrolysis (D → E) to afford α,β-unsaturated ketone E. In the
case of six-membered ring formation, ketone E (n = 2) cyclizes
smoothly to give 3 (n = 2) because it has the lowest ring strain. In
the case of five- or seven-membered ring formation (n = 1 or 3),
unsaturated ketone 4h produced by Meyer−Schuster rearrange-
ment, we chose an oxophilic (hard) gold(III) catalyst which
could activate the carbonyl group by coordination to oxygen.2,8
Thus, use of the oxophilic gold(III) catalyst AuBr3 (5 mol %)
resulted in smooth oxa-Michael addition from α,β-unsaturated
ketone 4h, affording the desired cyclic ether 3h in good yield
(Scheme 3).15
With these results in hand, we tried one-pot synthesis of seven-
membered ring 3h from propargylic alcohol 1h using a gold(I)
Scheme 3. Acceleration of Oxa-Michael Addition by
Oxophilic Gold(III) Catalyst to Construct Seven-Membered
Ring 3h
C
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Organic Letters
Letter
(p) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45,
7896.
Scheme 5. Mechanistic Proposal for Gold(III)-Catalyzed
Cyclization and Gold(I)-Catalyzed Meyer−Schuster
Rearrangement Followed by Gold(III)-Catalyzed Oxa-
Michael Addition
(2) (a) Dudnik, A. S.; Sromek, A. W.; Rubina, M.; Kim, J. T.; Kel’in, A.
V.; Gevorgyan, V. J. Am. Chem. Soc. 2008, 130, 1440. (b) Sromek, A. W.;
Rubina, M.; Gevorgyan, V. J. Am. Chem. Soc. 2005, 127, 10500.
(3) For coordination and activation by gold catalysis of propargylic
alcohols, see: (a) Debleds, O.; Gayon, E.; Vrancken, E.; Campagne, J.-M.
Beilstein J. Org. Chem. 2011, 7, 866. (b) Georgy, M.; Boucard, V.;
Debleds, O.; Zotto, C. D.; Campagne, J.-M. Tetrahedron 2009, 65, 1758.
(c) Georgy, M.; Boucard, V.; Campagne, J.-M. J. Am. Chem. Soc. 2005,
127, 14180.
(4) (a) Aponick, A.; Li, C.-Y.; Biannic, B. Org. Lett. 2008, 10, 669.
(b) Reich, N. W.; Yang, C.-G.; Shi, Z.; He, C. Synlett 2006, 1278.
(5) (a) Hikawa, H.; Suzuki, H.; Azumaya, I. J. Org. Chem. 2013, 78,
12128. (b) Hikawa, H.; Suzuki, H.; Yokoyama, Y.; Azumaya, I. J. Org.
Chem. 2013, 78, 6714. (c) Biswas, S.; Samec, J. S. M. Chem.Asian J.
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2013, 8, 974. (d) Cuenca, A. B.; Mancha, G.; Asensio, G.; Medio-Simon,
M. Chem.Eur. J. 2008, 14, 1518. (e) Terrasson, V.; Marque, S.;
Georgy, M.; Campagne, J.-M.; Prim, D. Adv. Synth. Catal. 2006, 348,
2063.
(6) Mertins, K.; Iovel, I.; Kischel, J.; Zapf, A.; Beller, M. Adv. Synth.
Catal. 2006, 348, 691.
oxophilic (hard) gold(III) activates the carbonyl group of E (n =
1 or 3) efficiently8 to furnish cyclic ethers 3 (n = 1 or 3) having a
carbonyl group.
In summary, we present gold(I)/(III)-catalyzed regiodiver-
gent syntheses of two types of cyclic ethers from propargylic
alcohols, by making use of the hard−soft principle. We are
currently applying the method to the synthesis of biologically
active cyclic ether derivatives. Experimental and theoretical
investigations on the reaction mechanism are also in progress.
(7) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533.
(8) For computational studies on Lewis acid catalyzed reactions
including gold catalysts, see: Yamamoto, Y. J. Org. Chem. 2007, 72, 7817.
Heats of formation of the complexes of cyclohexylacetylene (C6H11−
CCH) with AuCl and AuCl3 were calculated to be 36.2 and 30.9 kcal/
mol, respectively, whereas those of cyclohexylcarbaldehyde (C6H11−
CHO) with AuCl and AuCl3 were estimated to be 32.7 and 35.1 kcal/
mol, respectively. These computational data are consistent with the
softer (π-philic) nature of the gold(I) catalyst and harder (oxo-philic)
character of the gold(III) catalyst.
(9) For recent examples of gold-catalyzed Meyer−Schuster rearrange-
ment of propargylic alcohols, see: (a) Hansmann, M. M.; Hashmi, A. S.
K.; Lautens, M. Org. Lett. 2013, 15, 3226. (b) Pennell, M. N.; Turner, P.
G.; Sheppard, T. D. Chem.Eur. J. 2012, 18, 4748. (c) Pennell, M. N.;
Unthank, M. G.; Turner, P.; Sheppard, T. D. J. Org. Chem. 2011, 76,
1479. (d) Rieder, C. J.; Winberg, K. J.; West, F. G. J. Org. Chem. 2011, 76,
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and characterization data, 1H and 13
C
NMR spectra, and HRMS for all novel compounds. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.orglett.5b01046.
50. (e) Ramon
́
, R. S.; Gaillard, S.; Slawin, A. M. Z.; Porta, A.; D’Alfonso,
AUTHOR INFORMATION
Corresponding Authors
■
A.; Zanoni, G.; Nolan, S. P. Organometallics 2010, 29, 3665. (f) Ramon
́
,
R. S.; Marion, N.; Nolan, S. P. Tetrahedron 2009, 65, 1767. (g) Egi, M.;
Yamaguchi, Y.; Fujiwara, N.; Akai, S. Org. Lett. 2008, 10, 1867.
(h) Lopez, S. S.; Engel, D. A.; Dudley, G. B. Synlett 2007, 949. (i) Lee, S.
I.; Baek, J. Y.; Sim, S. H.; Chung, Y. K. Synthesis 2007, 2107. (j) Engel, D.
A.; Dudley, G. B. Org. Lett. 2006, 8, 4027.
* E-mail: morita@ac.shoyaku.ac.jp.
* E-mail: tamura@ac.shoyaku.ac.jp.
Notes
The authors declare no competing financial interest.
(10) For reviews on Meyer−Schuster rearrangement, see: (a) Cadier-
no, V.; Crochet, P.; García-Garrido, S. E.; Gimeno, J. Dalton Trans. 2010,
39, 4015. (b) Engle, D. A.; Dudley, G. B. Org. Biomol. Chem. 2009, 7,
4149. (c) Meyer, K. H.; Schuster, K. Chem. Ber. 1922, 55, 819.
(11) Reactions of this type involving gold catalysts have only limited
scope; see: (a) Wohland, M.; Maier, M. E. Synlett 2011, 1523.
(b) Schwehm, C.; Wohland, M.; Maier, M. E. Synlett 2010, 1789.
(12) Reactions of this type in the presence of platinum catalysts have
been reported; see: Liang, Q.; Qian, M.; Razzak, M.; De Brabander, J. K.
Chem.Asian J. 2011, 6, 1958.
́
(13) For reviews on the Nicholas reaction, see: (a) Martín, T.; Padron,
ACKNOWLEDGMENTS
■
This work was supported by Platform for Drug Discovery,
Informatics, and Structural Life Science from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
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DOI: 10.1021/acs.orglett.5b01046
Org. Lett. XXXX, XXX, XXX−XXX