Triazole-gold promoted intermolecular propargyl alcohol addition to alkyne: the reaction cascade toward substituted allenes

Article abstract of DOI:10.1039/c5cc08233a

Gold-catalyzed intermolecular propargyl alcohol addition to alkyne was achieved. The reaction of propargyl alcohol with a typical gold catalyst gave the hydration product almost exclusively. The triazole gold catalyst (TA-Au) successfully prevented the hydration, giving the vinyl ether followed by a 3,3-rearrangement. Synthetically useful substituted allenes were formed with high efficiency, substrate tolerability and chemoselectivity. Also, more than 95% chirality transfer from propargyl alcohol to allene was obtained.

Full text of DOI:10.1039/c5cc08233a

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Triazole-Gold Promoted Intermolecular Propargyl Alcohol
Addition to Alkyne: Reaction Cascade Toward Substituted
Allenes
Cite this: DOI: 10.1039/x0xx00000x
Seyedmorteza Hosseyni,a Shengtao Ding,a Yijin Su,b Novruz G. Akhmedov,b Xiaodong Shia*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Gold-catalyzed intermolecular propargyl alcohol addition to versatility of gold catalysis. 5 Herein, we report successful
alkyne was achieved. Reaction of propargyl alcohol with typical intermolecular propargyl alcohol addition to alkyne using the
gold catalyst gave almost exclusively hydration product. Triazole triazole-gold catalyst (TA-Au) to form allene.
We sought to enlist propargyl alcohols as a precursor for allene
synthesis due to its general availability and ease of synthesis.
However, there were very few examples reported in the literature
using this approach due to A) derivatization or Lewis acid was
usually required to convert OH to a good leaving group and B)
formation of propargyl cation intermediates might cause various
gold catalyst (TA-Au) successfully prevented the hydration, giving
the vinyl ether followed by 3,3-rearrangement. Synthetically
useful substituted allenes were formed with high efficiency,
substrate tolerability and chemoselectivity. Also more than 95%
chirality transfer from propargyl alcohol to allene was obtained
.
In the last decade, there has been a rapid growth of
homogeneous gold catalysis.1 Triggered by the breakthrough
example of ambient alkyne hydration (Teles hydration),2 the
predominant reactivity of gold catalysts was identified as a
highly effective π-acid toward alkyne, alkene and allene
activation.3 In general, compared to intramolecular functional
group rearrangements, transformations that enable reactions
between multiple components (intermolecular) is of greater
interest because it allows rapid increase of structural
complexity through the accumulation of functionalities.
However, like other catalytic systems, gold catalysis
encountered unbalanced development between the
intramolecular transformations (far more advanced) and their
intermolecular counterparts (much less developed). For
example, while the gold catalyzed intramolecular Conia-ene
reaction was originally reported in 2004, the similar
intermolecular Nakamura reaction was not achieved until
2014. 4 Thus, developing new catalytic systems that can
facilitate the challenging intermolecular transformations will
provide new opportunity to further advance the synthetic
side reactions.
One recent successful example was the
stereospecific reduction of propargyl alcohol using 1.1 equivalent of
[Cp2Zr(H)Cl], reported by Ready and co-workers (Scheme 1A).6
A) With stoichiometric Schwartz reagent
OPMB
OH
Cp2Zr(H)Cl (1.1 equiv), rt
EtMgCl (1.0 equiv), Tol
70% yield, 98% ee
Ready et al. ref 6
•
H
H3C
98% ee
H3C
OPMB
B) With gold catalyst
D
[XPhosAu(NCPh)]+SbF6- (4%)
CDCl3, 60 o
Ph
D
75% yield
Ph
D
•
Gagosz et al. ref 7
C
O
Ph
Activation of allene with the same gold catalyst
[Au]
OBn
OH
[Au]
90% yield
no allene obtained
•
same condition
O
HO
C) Proposed reaction cascade: highly efficient but very challenging
R3
O
[Au]
R3
R2
R3
R3OCH2C
O
•
[Au]
[Au]
OH
R1 = Ph; ref 12b
2
+
R1
R2
known
R3
HO
?
R1
R1
R2
R2
[Au]
O
1
4
3
L
R1
R2
+
O
Au
H
O
R1 = alk; ref 12a
[L-Au]+
H
O
H
ref 9
R1
R2
R1
a Department of Chemistry, University of South Florida, Tampa, FL 33620, USA,
E-mail: xmshi @usf.edu
5
R2
Challenges: 1) Intramolecular addition of propargyl alcohol 1 over fast hydration
2) Preventing further cyclization of allene.
Scheme 1. Synthesis of allenes from propargyl alcohols.
Although a stoichiometric amount of Schwartz reagent was
required, this reaction provided the first example of asymmetric
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 1

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DOI: 10.1039/C5CC08233A
allene synthesis from readily available enantiomericaly enriched enone 5a) with no desired intermolecular addition product
propargyl alcohols.
obtained. Solvent screening (CH2Cl2, CH3CN, dioxane, toluene, etc.)
revealed toluene as the optimal choice because it gave less
hydration of 1a. Notably, furan 6a was observed under this
condition with a very low yield (7%). Screening of primary ligand
revealed that XPhos bound gold cation produced more furan 6a
(27%), suggesting that the desired intermolecular addition
occurred.
[Rh(COD)Cl2]2, AgOTf, RuCl3, all gave no allene.
Another interesting example was the gold catalyzed propargyl
benzyl ether rearrangement reported by Gagosz and co-
workers.7 Using [XPhosAu(NCPh)]+ (Echavarren catalyst) at 60
oC, the desired allenes were prepared in modest to good yields
through a proposed 1,5-H shift. While this transformation was
very interesting, it suffered from the allene activation by the
same gold catalyst, especially when nucleophile was presented
in the same molecule. With the presence of nucleophilic
hydroxyl group, cyclization occurred through gold catalyzed
allene activation (formation of dihydro furan) with no allene
isolated (Scheme 1B).
Our proposed strategy was the intermolecular addition of
propargyl alcohol to terminal alkynes. Besides the clear kinetic
difference between inter- and intra-molecular reactions, the
existence of competitive reaction paths is the key challenge
that hindered the success of intermolecular transformations.
Table 1. Screening of gold catalysts.a
HO
O
N
n-Bu
Ph
Ph
R
N
Ph
Ph
N
O
n-Pr
1a
[Au]
rt
O
Ph
6a
+
•
4a
O
Ph
2a (1.2 equiv)
n-Pr
Ph
7a
TA-H: R = H
TA-Me: R = Me
n-Pr
Ph
5a
convn of
1a (%)
(%)
[Au] cat.
solv (wet)
time
4a
0
6a
0
5a
41
72
28
18
16
24
7a
39
48
65
65
85
60
PPh3AuCl/AgOTf (5%)
PPh3AuNTf2 (5%)
PPh3AuNTf2 (5%)
IPrAuNTf2 (5%)
dioxane
dioxane
toluene
toluene
toluene
toluene
6 h
6 h
50
83
66
67
76
77
0
0
0
7
12 h
12 h
12 h
12h
0
6
XPhosAuNTf2 (5%)
(ArO)3PAuNTf2 (5%)
0
27
6
0
Gold
catalyzed
propargyl
alcohol
derivative
[(Ph3PAu)3O]+SbF6- (5%)
[XPhosAu(MeCN)]+TfO- (5%) toluene
[XPhosAu(TA-H)]+TfO- (5%)
[XPhosAu(TA-Me)]+TfO- (5%)
[XPhosAu(TA-Me)]+TfO- (1%)
(2a, 1.8 equiv)
toluene
14 h
14 h
14 h
14 h
12 trace <5
<5
25
<5
25
25
70
rearrangements (intramolecular reactions) have been well
studied.8 However, the propargyl alcohol has not been used as
a nucleophile in gold catalysis due to the rapid hydration
(formation of enone) promoted by the same gold catalyst.9
Once the first step of the proposed mechanism takes place,
the second step, which is the gold-catalyzed propargyl vinyl
ether Saucy-Marbet rearrangment is well known to occur.10
Notably, the synthesis of propargyl vinyl ether is problematic,
which we propose to generate in situ. So far, the most
practical approach in preparing propargyl vinyl ether was the
vinyl ether exchange promoted by stoichiometric amount of
Hg(OAc)2.11 The other challenge of the proposed reaction was
the chemoselectivity, because it is well known that the
resulting allenone can easily undergo further cyclization.12 So,
the choice of the catalyst is very critical to be reactive for the
intermolecular addition of propargyl alcohol to alkyne while
remaining inert toward interamolecular alleneone cyclization.
Thus, our proposed intermolecular propargyl alcohol addition
to alkyne will be very attractive as a practical new route to
allene synthesis from simple starting materials, if proper
conditions are revealed.
Our group have recently reported 1,2,3-triazole-gold (TA-
Au) complexes as effective catalysts to activate alkyne 3 over
allene 4, giving the desired allene ketone/aldehyde 4 without
further cyclization.13 This process was highly efficient (with 1%
loading providing, up to 95% yield at rt). To explore the
possibility of intermolecular propargyl alcohol addition to
alkyne, mixture of 1a and alkyne 2a were employed to react
with various gold catalysts. The summarized screening condi-
tions are shown in Table 1 (see details in ESI).
25
55
77
20
18
0
25
<5
0
toluene
toluene
toluene
(45 oC)
100
82
100
94
0
0
22
8 h
other tested metal catalysts (5%), including Pd(OAc)2, CuI, Cu(OTf)2, PtCl2,
[Rh(COD)Cl2]2, AgOTf, RuCl3, all gave no allene.
a
Reaction condition: 2a (0.6 mmol) and catalyst (0.025 mmol) in
toluene (0.7 mL) was added to a solution of 1a (0.5 mmol). See detailed
screening conditions in the Electronic Supplementary Information. 1H-NMR
yields by 1,3,5-trimethoxybenzene as internal standard are shown.
The optimal result was obtained using TA-Me modified TA-
Au catalyst, although hydration of alkyne 2a caused the
incomplete conversion of 1a. Increasing the amount of 2a to
1.8 equivalents led to the complete consumption of 1a.
o
Finally, with 1% gold loading at 45 C, the desired allene 4a
was obtained in 94% isolated yield (1.8 equiv of alkyne 4 was
generally needed).
Other metal catalysts have also been tested, such as Pd(II),
Ag(I), Cu(I/II), Rh(I) and Ru(III) as well as propionic acid. Allene
formation was not observed in those cases (see ESI), which
highlighted the superior reactivity of the gold cation toward
alkyne activation. With the optimal conditions revealed, we
explored the substrate scope. The results are summarized in
Table 2.
With TA-Au as the catalyst, this new transformation
tolerated a large group of substrates. First, the reaction
worked well with aromatic alkynes (4a and 4b), aliphatic
alkynes (4c and 4d), conjugated 1,3-enyne (4f and 4g) and
heteroaromatic alkynes (4l), giving the desired allenes in good
to excellent yields. Both electron rich (4r) and electron
deficient (4k) aromatic alkynes were suitable substrates. Linear
(4c) and branched (4d) aliphatic alkynes also worked well
under this catalytic system even with primary alkyl chloride (a
good leaving group, 4e) worked well. In addition, the reaction
tolerated conjugated enynes, giving the highly reactive allene-
enones, which are very challenging building blocks to be
synthetized using other methods. Both cases demonstrated
good functional group tolerability under mild reaction
First, rapid decomposition of the gold catalyst was observed
with the mixture of gold and silver salts and only 50% conversion of
1a was obtained with 5% catalyst loading (PPh3AuCl/AgOTf). Silver-
free gold catalyst PPh3AuNTf2 gave a slower decomposition rate,
reaching a higher conversion of 1a. However, as discussed above,
significant gold-catalyzed 1a hydration was observed (formation of
2 | J. Name., 2012, 00, 1-3
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condition of this new approach. However, internal alkynes did received with very low yield of allene observed (<15%). This is
not give the desired reactions due to the poor reactivity of the likely due to the formation of benzylic carbocation, which led
alkyne. Applications of harsher conditions (such as higher to various side reactions. Overall, as successful example of
temperature) lead to the formation of a complex reaction intermolecular propargyl alcohol addition for the one-pot
mixture likely caused by the similar reactivity of the two synthesis of allenones, this new method tolerates a good
alkynes (hard to differentiate by catalyst).
scope of propargyl alcohols and alkynes.
The reaction scope of propargyl alcohol was also
investigated and functional group tolerability was observed.
We have previously demonstrated that TA-Au could serve
as chemoselective catalyst in promoting propargyl vinyl ether
chirality transfer.14 To validate whether chirality transfer could
be achieved under this more challenging intermolecular
condition, we prepared (+)-1a (96% ee) and submitted it to the
reaction condition. To our excitement, more than 95%
chirality transfer was achieved (scheme 2), which highlighted
the enhanced selectivity of TA-Au in this challenging
intermolecular reaction cascade.
Table 2. Allene synthesis substrate scope.a
R4
OH
O
•
[XPhosAu(TA-Me)]+TfO- (1%)
toluene, 45 o
R2
R3
R2
R3
1
+
R4
C
R1
2
R1
Cy
4
Ph
C6H4-3-Cl
n-Bu
Ph
Scheme 2. Effective chirality transfer with TA-Au.
O
•
O
•
O
O
•
OH
[XPhosAu(TA-Me)]+TfO- (1%)
Ph
•
O
+
Ph
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
Ph
Ph
Ph
toluene, 45 o
C
•
Ph
Ph
4a: 94%
4c: 96%
n-Pr
(-)-4a
4b: 90%
4d: 88%
2a
(+)-1a
96% ee
94% yield
91% ee
(CH2)3Cl
O
•
O
•
O
•
O
In conclusion, we report herein the first successful example of
intermolecular propargyl alcohol addition to alkyne as a general
approach for the synthesis of substituted allenones. This method
used simple starting materials and was carried out under mild
conditions (1% gold loading) with more than 95% chirality transfer.
In contrast, other tested catalysts (gold or other metals) gave
almost no desired allene products due to the lack of
chemoselectivity and inevitable gold-catalyzed propargyl alcohol
hydration side reaction
n-Pr
•
n-Pr
n-Pr
Ph
Ph
n-Pr
Ph
Ph
4e: 85%
4g: 72%
4f: 74%
4h: 65%
S
Ph
C6H4-3-Cl
C6H4-3-F
O
•
O
•
O
•
O
n-Pr
n-Pr
n-Pr
•
n-Bu
Ph
n-Bu
n-Bu
n-Pr
n-Bu
Cy
4i: 91%
4j: 85%
4k: 86%
4l: 72%
n-Bu
Ph
C6H4-3-F
We thank the NSF (CHE-1362057) and NSFC (21228204) for
financial support.
O
•
O
•
O
•
O
•
Ph
Ph
Ph
Ph
Bn
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4n: 91%
4m: 88%
4o: 85%
4p: 86%
C6H4-4-t-Bu
Ph
O
•
O
•
O
•
O
•
n-Bu
n-Bu
n-Bu
n-Bu
4t: 70%
4s: 73%
4q: 88%
4r: 79%
n-Bu
C6H4-3-F
Ph
O
•
O
•
O
•
Ph
n-Bu
H
O2N
4u: 70%
4w: 74%
4v: 44%
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secondary (4i) and tertiary (4m) propargyl alcohols worked
well under the standard condition. At the R2 position, both
aliphatic alkyne and aromatic alkyne were suitable. Terminal
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This journal is © The Royal Society of Chemistry 2012
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