Gold-catalyzed cyclization of 1,6-diyne-4-en-3-ols: Stannyl transfer from 2-tributylstannylfuran through Au/Sn transmetalation

Article abstract of DOI:10.1002/anie.201201523

Gold-tinted: A gold catalyzed cyclization reaction of 1,6-diyne-4-en-3-ols, incorporating an in situ stannyl transfer reaction involving 2-tributylstannylfuran, gives synthetically valuable 2-stannylnaphthalenes (see scheme; DCE=dichloroethane). A gem-diaurated furan complex, was isolated and fully characterized by X-ray crystallographic analysis, and provides strong evidence for a tin to gold transmetalation step. Copyright

Full text of DOI:10.1002/anie.201201523

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DOI: 10.1002/anie.201201523
Gold Catalysis
Gold-Catalyzed Cyclization of 1,6-Diyne-4-en-3-ols: Stannyl Transfer
from 2-Tributylstannylfuran Through Au/Sn Transmetalation**
Yifeng Chen, Ming Chen, and Yuanhong Liu*
In recent years, gold complexes and salts have emerged as
powerful homogeneous catalysts for a wide variety of
synthetic transformations owing to their superior chemo-
selectivity and activity.[1] For example, gold species can act as
efficient alkynophilic Lewis acids, which can activate
p systems towards nucleophilic attack. Despite significant
diversity in the types of exisiting gold catalyzed reactions, the
development of reactions that involve the functionalization of
organogold intermediates containing a gold–carbon s bond
still remains a major challenge in this area. Such gold-
containing intermediates are captured most frequently by
a proton,[1] and much less frequently with alternative electro-
philes such as carbocation,[2] sulfonyl,[3] silicon,[4] and halo-
gen[5] electrophiles. Recent research has indicated that trans-
metalation of the in situ formed gold intermediates could
into alkyne substrates (Scheme 1). Herein, we describe our
discovery of a highly efficient gold-catalyzed cycloisomeriza-
tion/stannylation cascade reaction of 1,6-diyne-4-en-3-ols,
[15]
Scheme 1. Transmetalation reactions of organogold intermediates.
extend the scope of the gold-catalyzed reactions to include,
[6]
À
for example, C C cross-coupling reactions. Organogold
thus leading to synthetically useful stannyl naphthalenes;
2-tributylstannylfurane is used as the source of stannane.
Investigations into the mechanism of the reaction reveal that
tin-to-gold as well as gold-to-tin transmetalation steps are
involved in the process.
Recently, we developed a highly efficient gold-catalyzed
cascade reaction between 1,6-diyne-4-en-3-ols and furans to
give phenanthryl ketones; the reaction involved a Friedel—
Crafts alkylation, furan-yne cyclization, and a heteroenyne
metathesis reaction.[14a] Interestingly, when 2-tributylstannyl-
furan was used as the furan component, and the reaction was
conducted in dichloroethane for a period of 1.5 hours, in the
presence of 5 mol% of [(PPh3)AuCl] and 5 mol% of AgSbF6,
diyne 1a was not converted into the expected phenanthrene.
Instead, 2-stannyl naphthalene 2a was obtained in 53% yield,
together with dihydroisobenzofuran 3a in 19% yield, the
complexes are good precursors for transmetalation reactions
involving nickel,[7] palladium,[6e,7a,8] rhodium,[9] tin,[10]
iron,[7a,11] ruthenium,[11] and other metal species,[7a,12] as
demonstrated by the research groups of van Koten, Blum,
Hashmi, and others (Scheme 1). However, whereas most of
these studies involved the use of stoichiometric amounts of
pre-formed organogold reagents, the transmetalation of
a gold intermediate generated in situ within a gold-catalyzed
reaction is quite rare.[8c,10,11,13] Blum and co-workers showed
that the use of a mixture of gold and palladium catalysts was
effective for the carbostannylation of electron-deficient
alkynes and that it proceeds through successive palladium-
to-gold and gold-to-tin transmetalation steps.[10] However,
reactions that are catalyzed by a gold catalyst, in the absense
of other metal catalysts, thus featuring a direct transmetala-
tion from gold to tin and allowing for a direct access to
organostannanes, are not known. During our ongoing
research program on gold-catalyzed cyclization reactions of
alkynes for the synthesis of polycyclic aromatic compounds,[14]
we found that gold(I) complexes can be used as catalysts for
the regioselective incorporation of a stannyl functional group
À
latter resulting from an intramolecular O H bond addition to
the alkyne moiety (Table 1, entry 1). The presence of the
stannyl group in 2a reveals that the stannyl group was
transferred from 2-tributylstannylfuran to the diyne substrate
during the reaction. The use of a OTfÀ-containing gold
complex, which was prepared in situ by treating [(PPh3)AuCl]
with AgOTf, improved the yield of 2a to 71%; however,
significant amounts of 3a and an isomeric isochromene
[*] Y.-F. Chen, M. Chen, Prof. Y.-H. Liu
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
derivative,
3-phenyl-1-(phenylethynyl)-1H-isochromene
(4a), were also obtained (Table 1, entry 2). Further experi-
ments revealed that the use of Echavarrenꢀs catalyst (A),
which contains a bulky biarylphosphine ligand, gave much
improved yields of 2a. When 5 mol% of A was used, a clean
conversion of diyne 1a into the stannane was achieved (88%
yield) and only trace amounts of 3a was detected (Table 1,
entry 3). The use of a lower loading of catalyst A (2 mol%)
afforded 2a in a comparable yield of 87% upon isolation
(Table 1, entry 4). These results indicate that the nature of the
345 Lingling Lu, Shanghai 200032 (P. R. China)
E-mail: yhliu@mail.sioc.ac.cn
[**] We thank the National Natural Science Foundation of China (Grant
Nos. 20872163, 21121062, and 21125210), the Chinese Academy of
Science, and the Major State Basic Research Development Program
(Grant No. 2011CB808700) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201523.
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Table 1: Optimization studies for gold-catalyzed cyclization/stannyl
transfer reactions.
Entry Catalyst (mol%)
RSnBu3
Yield [%] 2a[a] Yield [%] 3a[a]
1
[(PPh3)AuSbF6] (5)
[(PPh3)AuOTf] (5)
A (5)
53
71
88
87
59
76
45
45
19
2
26 (3.6:1)[b]
3
4
4
A (2)
6
5[c]
6[d]
7
A (2)
23 (2.9:1)[b]
10
A (2)
AuCl3 (5)
A (2)
38 (1:2.3)[b]
[e]
8
–
[e]
9
A (2)
A (2)
24
22
–
10
12
[a] Yield of isolated product. [b] 3-phenyl-1-(phenylethynyl)-1H-isochro-
mene 4a was also formed. The ratio 3a/4a is shown in parentheses.
[c] 508C, 16 h. [d] 1.2 equiv of 2-tributylstannylfuran was used. [e] Not
determined.
Scheme 2. Yields of isolated products. [a] The yield in parentheses was
obtained by using 5 mol% of [(PPh3)AuCl]/AgOTf and 1.2 equiv of 2-
tributylstannylfuran. DCE=dichloroethane.
ligand on the gold(I) complex influences the chemoselectivity
of the reaction. Decreasing the amount of 2-stannylfuran
from 2 equivalents to 1.2 equivalents resulted in a lower yield
of 2a (76% yield, Table 1, entry 6). The use of other tin
reagents, such as 2-thienyl-, phenyl-, and vinylstannane
resulted in lower product yields (22–45%, Table 1,
entries 8–10). When the reaction was conducted in the
absence of 2-tributylstannylfuran at room temperature for
a period of 15 minutes using 2 mol% of catalyst A, phenyl(2-
phenylnaphthalen-1-yl)methanone (5)[15a] was isolated in
68% yield, albeit containing a very small amount of an
impurity. The reaction of 1a using 1 equivalent of Bu3SnOTf,
in the absense of a gold catalyst, resulted in rapid decom-
position of the starting material.
With conditions for an efficient reaction established, we
next investigated the scope of the reaction using a variety of
substituted diynes (Scheme 2). The reaction proved to be
quite general with respect to variation of the R1 and R2 groups
on the alkyne moieties; the presence of aryl-, heteroaryl-, and
alkyl groups at these positions were tolerated, thus facilitating
access to a diverse series of products. When R1 was an aryl
group containing electron-withdrawing or electron-donating
groups, the desired stannyl naphthalenes 2b–2d were
obtained in good yields ranging from 76% to 87%. As the
aryl substituent R1 became more electron deficient, the
reaction efficiency decreased (product 2d, Scheme 2). The
presence of heteroaryl substituents such as 2-pyridyl and 2-
thienyl groups were also tolerated under the reaction
conditions, thus giving 2e, 2 f, and 2m in 45–74% yields.
Notably, when R1 is a thienyl group (product 2 f, Scheme 2),
the use of [(PPh3)AuOTf] afforded better results than the use
of catalyst A. The presence of alkenyl and cyclopropyl groups
was also tolerated in the reaction: 2g and 2h were obtained in
80% and 74% yields, respectively. The use of substrates
containing alkyl substituents also resulted in good yields of
the desired products (2i and 2o, Scheme 2). The substrate
containing a terminal alkyne (R1 = H) underwent the reaction
smoothly to give 2j in 42% yield. When R2 was a bulky tert-
butyl group, the efficiency of this reaction was not compro-
mised, and the corresponding product 2n was obtained in
83% yield. Substrates containing fluoro, methoxy, or meth-
ylenedioxy groups on the parent phenyl ring were also
2
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suitable for this reaction, and good to high yields of the
expected products were obtained in these cases (2p–2r).
Organostannanes are versatile building blocks for organic
synthesis. The utility of the stannyl-containing products 2 was
demonstrated in further transformations. For example,
destannylation of 2a with TsOH·H2O in CH2Cl2 afforded
naphthyl ketone 5 in 93% yield (see the Supporting
Information). Treatment of 2a with iodine gave the C2-
iodinated product 6 in 84% yield. The structure of 6 was
unambiguously confirmed by X-ray crystallography (see the
Supporting Information).[16] In general, sterically encumbered
stannanes, such as 2, are not good substrates for Stille
coupling reactions; however, after much investigation, we
found that the cross coupling of 2a with ethyl-4-iodobenzoate
proceeded smoothly in the presence of Pd0/CuCl/LiCl[17] to
give 7 in high yield (85%, Scheme 3).
yield. The above results suggest that the electrophilic Bu3Sn+
ion is formed in situ and then reacts with the gold intermedi-
ate. We wondered whether the Bu3Sn+ ion is generated
through a gold-catalyzed destannylation of 2-stannylfuran. In
a control experiment, 2-tributylstannylfuran in CDCl3 at
808C, in the presence of 5 equivalents of H2O and 5 mol% of
A, underwent destannylation (Scheme 5);[19] in the absense of
the gold catalyst, no reaction was observed. 2-tributylstan-
Scheme 5. Gold-catalyzed destannylation reaction.
nylfuran was treated with a stoichiometric amount of catalyst
A in dichloroethane at 808C for 2 hours to determine whether
a transmetalation from tin to gold was possible. Interestingly,
an air-stable gem-diaurated complex 11 was obtained in 51%
yield, a value based on the initial amount of gold complex A
(Scheme 6). The structure of 11 was confirmed by X-ray
Scheme 3. Transformation of 2a. DMF=N,N-dimethylformamide.
crystallographic analysis (Figure 1).[16] The Au1 C41 and
À
With regards to the mechanism, it occurred to us that the
stannylation might proceed by the trapping of an aryl gold
intermediate with a highly electrophilic tin species. To gain
a better understanding of the transmetalation of an aryl gold
species into an aryl tin species, we prepared the known
complex [2-naphthylAu(PPh3)] (8).[18] Treatment of 8 with
Bu3SnOTf[5e] afforded the product arising from a homocou-
pling reaction, 2,2’-binaphthyl (9, Scheme 4). We conducted
the reaction in the presence of 2 equivalents of PPh3 so that
the released gold triflate would be trapped as [AuL(PPh3)]+,
thus mimicking the conditions of our cyclization/stannylation
reaction more closely; in the event, the desired tributyl(naph-
thalene-2-yl)stannane (10) was obtained in 89% yield
(Scheme 4).[8d] Compound 8 could also react directly with
2-tributylstannylfuran at 808C to generate 10, albeit in a low
yield of 22%. However, the addition of 10 mol% of catalyst
A to the reaction mixture improved the yield of 10 to 51%;
the addition of 1 equivalent of catalyst A further improved
the yield of 10 to 65%, and also gave the product arising from
a cross-coupling reaction, 2-(naphthalen-2-yl)furan, in 20%
Scheme 6. Transmetalation of gold complex A with 2-tributylstannyl-
furan.
À
Au2 C41 bond lengths, 2.098(7) and 2.144(6) ꢁ, respectively,
are longer than those of the normal 2 center/2 electron bonds
that are formed between gold and aryl groups (2.04–
2.08 ꢁ).[20] The Au1-C41-Au2 angle of 85.7(2)8 is much
smaller than those associated with normal tetrahedral
carbon atoms, and the distance between the two gold atoms
(2.8844(4) ꢁ) is close to that observed in metallic gold
(2.88 ꢁ), thus indicating a strong aurophilic interaction. In
addition, the large deviation of the P-Au-C ipso angle from
the linear configuration (1808) also supports the existence of
À
a
Au···Au interaction. The C42 C43 bond length of
1.376(13) ꢁ is significantly shorter than that of the corre-
À
sponding C C bond in 2-[(triphenyl)phosphine]gold]furan
(1.431(5) ꢁ),[21] thus indicating that the positive charge is
delocalized over the ring and that the aromaticity of the furan
moiety is reduced. Such gold moieties have been found in
various aryl gold(I) complexes,[21,22] including a structurally
characterized 2,2-diaurated furan complex [C4H3O-
(AuPPh3)2][BF4].[21] These results indicate that transmetala-
tion has indeed occurred, and we believe that the diaurated
furan complex 11 is formed through the addition of a second
Scheme 4. Trapping of organogold complex 8 with Bu3SnOTf.
Tf =trifluoromethanesulfonyl.
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Scheme 7. Proposed reaction mechanism.
Figure 1. Molecular structure of complex 11 (H atoms and counter
anion, SbF6À, are omitted for clarity). Thermal ellipsoids are shown at
30% probability. Selected bond lengths [ꢀ] and angles [deg]: Au1–C41
2.098(7), Au1–P1 2.298(2), Au1···Au2 2.8844(4), Au2–C41 2.144(6),
Au2–P2 2.2793(18); Au1-C41-Au2 85.7(2), C41-Au1-P1 167.5(3), C41-
Au2-P2 158.1(2).
Bu3Sn+ ion would then react with 19 to yield stannylnaph-
thalene 2 and regenerate the gold catalyst. However, a direct
transmetalation of 19 with 2-tributylstannylfuran to give 2
cannot be ruled out. Although 1a decomposed in the
presence of stoichiometric amounts of Bu3SnOTf, we did
not observe such decomposition in the reaction, presumably
because Bu3SnOTf is generated catalytically and only reacts
with the naphthyl gold intermediate preferentially.
To elucidate the reaction mechanism of the cyclization
process, 18O isotope labeling experiments were carried out.[26]
Treatment of 1a with 1 equivalent of H218O, 2-tributylstan-
nylfuran (2 equivalents), and 5 mol% of catalyst A resulted in
the formation of 2a without external 18O atom incorporation
at the carbonyl oxygen as indicated by EI-MS data. We also
prepared 18O-labelled 1a ([18O]-1a) wherein 49% of the
molecules contained the 18O atom. The treatment of [18O]-1a
with 2 equivalents of 2-tributylstannylfuran and 1 equivalent
of H2O (natural isotopic abundance) in the presence of
5 mol% of catalyst A afforded [18O]-2a and [18O]-3a without
any loss of 18O content as indicated by ESI-MS analysis. These
results strongly support our proposed mechanism in which
oxygen transfers from the benzylic postion to the triple
bond.[27,28]
In conclusion, we have described the first example of
a gold catalyzed cyclization and stannyl-transfer reaction of
1,6-diyne-4-en-3-ols using 2-tributylstannylfuran as the source
of stannyl group and without the use of additional metals. This
regioselective transformation provides an attractive new
route to a diverse range of stannyl substituted naphthylstan-
nanes and, when coupled with palladium-catalyzed Stille
cross-coupling reactions, leads to highly functionalized naph-
thalenes. Investigations toward understanding the mechanism
revealed that a gold to tin transmetalation reaction and
a gold-catalyzed destannylation reaction might be involved in
the domino process. We have also provided the first direct
crystallographic evidence that supports a tin to gold trans-
metalation reaction involving cationic gold complexes. The
method may find further application in the functionalization
equivalent of catalyst A to the monoaurated furan complex
12.[21] To the best of our knowledge, there are no crystallo-
graphic data associated with a transmetalation reaction
between an organostannane and a cationic gold complex.[23]
Interestingly, the catalytic activity of complex 11 was similar
to that of the catalyst A under the standard conditions
(2.0 equivalents of 2-(tributylstannyl)furan, dichloroethane,
808C, 2 hours), thus affording 2a in 87% yield; the use of
a gem-diaurated species as a catalyst has recently been
reported.[16d] The transmetalation from tin to gold may
proceed through a mechanism involving an interaction of
the furanyl p system with gold followed by elimination of the
Bu3Sn+ ion; this mechanism is analogous to that previously
reported for the transmetalation reaction between 2-stannyl-
furan and a palladium complex.[24]
Although a detailed mechanism has yet to be deter-
mined,[25] we propose the following mechanism for this
cascade reaction, based on the above observations
(Scheme 7). The reaction is probably initiated by the nucle-
ophilic attack of the hydroxy group onto the gold-coordinated
alkyne to give oxacyclic species 14, which could then undergo
À
C O bond cleavage, with the assistance of one molecule of
water, to give allenol 15. The allene moiety in 15 could be
further activated by gold to form complex 16. Subsequent
cyclization of 16, or alternatively its involvement in an aldol-
condensation via intermediate 17, would afford 2-naphthyl
gold intermediate 19. A trace amount of water should be
sufficient to induce the cyclization reaction because although
it would be consumed in the formation of allenol 15, it is
released during the aromatization process. In an adjoining
catalytic cycle, the gold complex could catalyze the destan-
nylation of 2-tributylstannylfuran through transmetalation
and protodemetalation; furan was indeed detected in the
crude reaction mixture. The resulting highly electrophilic
4
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Organomet. Chem. 2003, 670, 170; c) M. Ferrer, L. Rodrꢆguez, O.
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Received: February 24, 2012
Revised: March 26, 2012
Published online: && &&, &&&&
Keywords: alkynes · domino reactions · gold · stannanes ·
[13] It has been recently shown that one example from Ref. [8c]
needs only palladium to give the desired product in 37% yield,
see: A. S. K. Hashmi, C. Lothschꢂtz, R. Dçpp, M. Ackermann,
J. D. B. Becker, M. Rudolph, C. Scholz, F. Rominger, Adv. Synth.
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[26] See the Supporting Information for details.
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Angew. Chem. Int. Ed. 2012, 51, 1 – 7
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.
Angewandte
Communications
aromatization. Our results suggest that this mechanism is
unlikely because if it was operating, incorporation of 18O at the
carbonyl oxygen in the product 6a would be observed in the
gold-catalyzed reaction of 1a (natural isotopic abundance) and
2-tributylstannylfuran in the presence of 1 equivalent of H218O.
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A. Laguna, M. C. Blanco, M. C. Gimeno, Angew. Chem. 2007,
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Hashmi, M. Bꢂhrle, R. Salathꢃ, J. W. Bats, Adv. Synth. Catal.
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2008, 73, 8479.
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www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
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Angewandte
Chemie
Communications
Gold Catalysis
Y.-F. Chen, M. Chen,
Y.-H. Liu*
&&&& — &&&&
Gold-Catalyzed Cyclization of 1,6-Diyne-4-
en-3-ols: Stannyl Transfer from 2-
Tributylstannylfuran Through Au/Sn
Transmetalation
Gold-tinted: A gold catalyzed cyclization
reaction of 1,6-diyne-4-en-3-ols, incorpo-
rating an in situ stannyl transfer reaction
involving 2-tributylstannylfuran, gives
synthetically valuable 2-stannylnaphtha-
lenes (see scheme; DCE=dichloro-
ethane). A gem-diaurated furan complex,
was isolated and fully characterized by X-
ray crystallographic analysis, and pro-
vides strong evidence for a tin to gold
transmetalation step.
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!