DOI: 10.1002/chem.201602347

Full Paper

& Au-Catalyzed Reactions

a,b-Unsaturated Gold(I) Carbenes by Tandem Cyclization and 1,5Alkoxy Migration of 1,6-Enynes: Mechanisms and Applications
Pilar Calleja,[a] Óscar Pablo,[a] Beatrice Ranieri,[a] Morgane Gaydou,[a] Anthony Pitaval,[a]
María Moreno,[a] Mihai Raducan,[a] and Antonio M. Echavarren*[a, b]
Abstract: 1,6-Enynes bearing OR groups at the propargyl
position generate a,b-unsaturated gold(I)-carbenes/ gold(I)
stabilized allyl cations that can be trapped by alkenes to
form cyclopropanes or 1,3-diketones to give products of aalkylation. The best migrating group is p-nitrophenyl ether,

which leads to the corresponding products without racemization. Thus, an improved formal synthesis of (+)-schisanwil+
sonene A has been accomplished. The different competitive
reaction pathways have been delineated computationally.

Introduction
The study of gold(I)-catalyzed reactions of 1,n-enynes has led
to the discovery of a wealth of cyclization modes, including
mechanistically intriguing skeletal rearrangements and nucleophilic addition reactions to cycloaddition processes.[1] In this
context, we recently found that 1,6-enynes such as 1 bearing
propargyl alcohols, ethers, or silyl ethers react with gold(I) catalysts through the usual type of highly delocalized cyclopropyl
gold(I) intermediates 2,[2] which then undergo a new type of
1,5-migration of the OR groups to generate species 3[3] that we
postulated as intermediate between a,b-unsaturated gold(I)
carbenes and gold(I)-stabilized allyl cations.[2, 4] In the presence
of carbon nucleophiles such as indole or furans, products 4[3]
or trienes such as 5[5] were obtained by Friedel–Crafts-type reactions (Scheme 1). Intermediates 3 can also react with electron-rich alkenes to form the corresponding cyclopropanes,[3]
a reaction which is also characteristic of gold(I) carbenes 2.[6–8]
We demonstrated the potential of this tandem cyclization/1,5OR migration/cyclopropanation to form products 6 that were

[a] P. Calleja, Dr. Ó. Pablo, Dr. B. Ranieri, M. Gaydou, A. Pitaval, Dr. M. Moreno,
M. Raducan, Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Barcelona Institut of Science and Technology (BIST)
¨
Av. Paısos Catalans 16, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
[b] Prof. A. M. Echavarren
Departament de Química Analítica i Química Orgµnica
Universitat Rovira i Virgili, C/ Marcel·li Domingo s/n
43007 Tarragona (Spain)
Supporting information for this article and ORCID for one of the authors
can be found under:
http://dx.doi.org/10.1002/chem.201602347.
 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
Chem. Eur. J. 2016, 22, 13613 – 13618

Scheme 1. Intermolecular trapping of the intermediates of the gold-catalyzed cyclization/1,5-OR migration of enynes 1.

key intermediates in the first total synthesis of the natural sesquiterpene (+)-schisanwilsonene A (7).[9]
+
This type of gold(I)-catalyzed 1,5-migration has been found
to compete[10] with 1,2- and 1,3-migrations of propargylic carboxylate groups.[10] Related processes have been found in the
gold(I)-catalyzed reactions of dienynes 8, which undergo cyclization/1,5-OR
migration/intramolecular
cyclopropanation
through intermediates 9 to form stereoselectively hexahydroazulenes 10,[3] which were the key intermediates in our total
synthesis of the sesquiterpernes (À)-epiglobulol (11) and (À)4b,7a-aromadendranediol (12) (Scheme 2).[11] Alternatively,
when the gold(I)-catalyzed reaction was performed in the presence of allyl alcohol, this external nucleophile reacted to give
9’, which underwent intramolecular cyclopropanation to give
rise to the sesquiterpene (À)-4a,7a-aromadendranediol (13).[11]

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Full Paper
ether 1 c, and acetate 1 d, and benzyl ethers 1 f, g failed to give
any of the expected products 14 f, g (Table 1, entries 2–7). Poor
results were obtained with catalysts B and C bearing tBuXphos
as the ligand (Table 1, entries 8 and 9). The best results were
obtained using the less electrophilic catalysts D–G with more
donating NHC ligands (Table 1, entries 10–13), which have
been proposed to enhance the carbene-like character of the
intermediates in gold(I)-catalyzed reactions.[1g,h]

Table 1. Gold(I)-catalyzed reaction of enynes 1 a–g with 1,3-diphenyl-1,3propandione.
Scheme 2. Intramolecular trapping of the intermediates of the gold-catalyzed cyclization/1,5-OR migration of dienynes 8.

The proposed mechanism for these inter- and intramolecular
reactions was based on the isolation of diverse products but
not on a rigorous study of this intriguing process and its several possible competitive cycloisomerization pathways. Furthermore, although the chirality transfer in the intramolecular processes was satisfactory, partial racemization was observed in
the formation of intermediates 2 when RO = AcO, which led to
(+)-schisanwilsonene A (7) with 9:1 e.r. Here we report that
+
1,3-dicarbonyl compounds can also be used as the C-nucleophiles to trap the putative a,b-unsaturated gold(I) carbenes 3
leading to products of formal alkylation. This and additional
studies on the intermolecular trapping of intermediates 3 with
alkenes have allowed selecting p-nitrophenyl ether as the protecting group of choice in these reactions. This led us to develop an improved formal synthesis of (+)-schisanwilsonene A (7)
+
in which the key step proceeds with total retention of configuration. We have also found cases in which a skeletal rearrangement takes place preferentially to form six-membered ring
compounds. A detailed computational study has been performed to understand the mechanisms of these complex transformations.

Entry

[AuLL’]X

1 a–g

14 a–g
(yield) [%]

1
2
3
4
5
6
7
8[a]
9[a]
10
11
12
13

A
A
A
A
A
A
A
B
C
D
E
F
G

1a
1b
1c
1d
1e
1f
1g
1a
1a
1a
1a
1a
1a

14 a (58)
14 b (14)
14 c (18)
14 d (14)
14 e (44)
14 f (À)
14 g (À)
14 a (8)
14 a (8)
14 a (67)
14 a (67)
14 a (62)
14 a (71)

[a] = 70–75 min

Results and Discussion
Selection of the best OR migration group
1,3-Dicarbonyl compounds react as C-nucleophiles with 1,6enynes in the presence of gold(I) catalysts by formal attack at
the alkene via opening of the cyclopropane of intermediates
of type 2.[1h] We therefore decided to examine whether enynes
1 would react with this type of nucleophiles at the alkene,
through intermediates 2, or at the terminal alkyne carbon
through intermediates 3. In the event, reaction of enynes
1 with 1,3-diphenyl-1,3-propandione in the presence of gold(I)
complexes gave products of a-alkylation of the dicarbonyl
compounds 14 by trapping of intermediates 3 at C-1 (Table 1).
Using catalyst [(JohnPhos)Au(NCMe)]SbF6 (A), the best migrating group proved to be p-nitrophenyl (PNP) ether in substrate
1 a, which led to adduct 14 a in 58 % yield after 30 min at 24 8C
(Table 1, entry 1). p-Anisyl ether derivative 1 e gave 14 e in
a moderate 44 % yield (Table 1, entry 5), whereas lower yields
(14–18 %) were obtained with the free alcohol 1 b, methyl
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1,6-Enyne 1 a reacted smoothly with other 1,3-dicarbonyl
compounds and b-ketoesters as the C-nucleophiles using the
optimal IPr gold(I) complex G to form adducts 14 h–n in 59–
82 % yield in 30 min at 24 8C (Table 2).
p-Nitrophenyl ether 1 a was also the substrate of choice for
the cyclization/1,5-OR migration/intermolecular cyclopropanation with cyclohexene and norbornene to form cyclopropanes
15 a, b in satisfactory yields as mixtures of exo and endo diastereomers by using catalyst A under very mild conditions
(Scheme 3). Dihydropyrane reacted similarly to give 14 c, although in this case catalyst D gave better results. Indene, benzofuran, 5-bromobenzofuran, and benzothiophene gave the
corresponding adducts 15 d–g more stereoselectively. The configuration of the mayor product 15 g in the reaction with benzothiophene was assigned by X-ray diffraction.[12] This product

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Table 2. Products of the reaction of 1,6-enyne 1 a with 1,3-dicarbonyl
compounds and b-ketoesters in the presence of catalyst G (24 8C,
30 min).

Scheme 4. First-generation synthesis of (+)-schisanwilsonene A (7) via cycli+
zation/1,5-OR migration/cyclopropanation.[9]

Scheme 3. Gold(I)-catalyzed cyclization/1,5-OR migration/intermolecular cyclopropanation of 1 a.

was formed in lower yield most likely as a consequence of the
inhibition of the catalytic reaction by coordination of gold(I) to
the thioether 15 g. Interestingly, in the last three cases, the
electron-rich heterocycles undergo cyclopropanation, which is
in contrast to what we observed previously for indole, which
gave product 4 after formal alkylation at C-3 (Scheme 1).[3]
A second-generation formal synthesis of (+)-schisanwilso+
nene A
The gold(I)-catalyzed reaction of racemic 1 a with the bis-TBS
ether of 2-methylenepropane-1,3-diol gave 6 a in good yield as
Chem. Eur. J. 2016, 22, 13613 – 13618

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a single diasteromer (Scheme 4).[9] Enantioenriched acetate 1 d
(96:4 e.r.) reacted in slightly lower yield to afford 6 b in 91:9 e.r.
This partial racemization was explained as a result of a 1,2-acyloxy rearrangement competing as a minor pathway.[9]
Although the formation of enantioenriched 6 b allowed us
to complete the first total synthesis of (+)-schisanwilsonene A
+
(7) via hexahydroazulene diol 16 a (90:10 e.r.) (Scheme 4),
a better solution has now been found by using enantioenriched 1 a as the substrate for the cyclization/1,5-OR migration/
cyclopropanation cascade. Thus, alcohol 1 b was protected as
the PNP ether by nucleophilic aromatic substitution of its potassium salt with p-fluoronitrobenzene and 18-crown-6 to give
1 a with full retention of the configuration (94.5:5.5 e.r.)
(Scheme 5). The key gold(I)-catalyzed reaction of 1 a with the
bis-TBS ether of 2-methylenepropane-1,3-diol, followed by desilylation with TBAF, provided diol 6 c in 68 % yield without detectable racemization, within experimental error (94:6 e.r.). The
configuration of 6 c was determined by X-ray diffraction.[12] This
result demonstrates that the enyne cyclization/1,5-OR migration/cyclopropanation cascade takes place without any racemization, which excludes the involvement of a propargyl carbocation as an intermediate in the process. In the racemic series,
6 c has been converted into hexahydroazulene diol 16 a according to the original sequence followed by a two-setp deprotection of the PNP group (76 % yield).[13]

Exceptions to the rule: no migration of the propargyl group
We also tried the reaction of 1,6-enynes 17 a, b with a propargylic amine (Scheme 6). In the case of 17 a, the electron-rich pmethoxyphenyl amine undergoes a gold(I)-catalyzed intramolecular hydroarylation with the terminal alkyne[14] to give dihydroquinoline 18, whereas PNP derivative 17 b afforded 7-methylene-2-azabicyclo[2.2.1]heptane 19 as the major product,
whose structure was determined by X-ray diffraction.[12]

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Scheme 5. Enantio-retentive synthesis of key intermediate 6 c in the synthesis of (+)-schisanwilsonene A (7).
+

Scheme 7. Cyclizations of enynes 20 a, b and 22 with gold(I) catalyst A without OR migration and the proposed mechanism for the formation of 23.

rangement products 27 under mild conditions (Table 3). The
reaction proceeds satisfactorily with substrates bearing phenyl
or aryl groups with electron-withdrawing substituents (Table 3,
entries 1–5 and 8; Figure 1), whereas enynes 26 f, g with
a more electron-rich anisyl group gave complex reaction mix-

Table 3. Gold(I)-catalyzed reaction of 1,6-enynes 26 a–h to give endotype single-cleavage rearrangement products 27 a–h.

Entry

We had reported that 1,6-enyne 20 a reacts with methanol
in the presence of catalyst A to give stereoselectively adduct
21 in which the migration of the benzyloxy group has not
taken place[15] (Scheme 7). Interestingly, alcohol 20 b also reacted in the presence of catalyst A without migration of the OH
group to give 22, the product of an endo-type single cleavage
rearrangement.[16] This reaction was better performed in the
presence of 4 Š molecular sieves. Surprisingly, in the absence
of molecular sieves, diene 23 was obtained as the major product of the reaction, whose structure was determined by X-ray
diffraction. A speculative mechanism for the formation of this
unexpected product could involve a reaction of 22 with allyl
cation 24 to form a new allyl cation 25, followed by aromatization by proton loss and dehydration (Scheme 7).
1,6-Enynes 26 a–h with tertiary propargyl hydroxyl or trimethylsilyloxy groups and different aryl groups at the alkene also
react with catalyst A to give endo-type single cleavage rearChem. Eur. J. 2016, 22, 13613 – 13618

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R1

R2

Product
(yield) [%]

1
2
3
4
5
6
7
8

Scheme 6. Reaction of propargyl amines 17 a, b with gold(I) catalyst A.

Enyne

26 a
26 b
26 c
26 d
26 e
26 f
26 g
26 h

H
TMS
H
TMS
H
H
TMS
H

H
H
p-NO2
p-NO2
o-NO2
p-OMe
p-OMe
o-Br

27 a (52)
27 b (95)
27 c (88)
27 d (72)
27 e (98)
27 f (À)
27 g (À)
27 h (84)

Figure 1. X-ray diffraction structure of product 27 h (Table 3, entry 8).

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Full Paper
tures (Table 3, entries 6 and 7). Somewhat surprisingly, the reaction of the corresponding methyl ethers led only to decomposition.
Mechanistic discussion
We examined computationally the evolution of enynes Ia–c coordinated with AuL + (L = PMe3). In all cases, in agreement with
calculations, Ia–c reacted by 5-exo-dig pathways to form preferentially IIa–c (Schemes 8 and 9), which correspond to intermediates of type 2 in Scheme 1.[17] The alternative 6-endo-dig
pathway leading to IIIa–c was less favorable in all cases.
Whereas in the case of PNP-protected intermediate IIa, the migration to form VIa proceeds in a direct manner through TS3a
(Scheme 8), the migration of the OR group in complexes IIb
and IIc proceeds via bicyclic intermediates IVb and IVc, which
then lead to products of 1,5-migration VIb and VIc through
very low barrier transition states TS5b and TS5c, respectively
(Scheme 9). The alternative evolution of IIa–c to products of
single-cleavage rearrangement Va–c was found to be thermodynamically the most favorable pathway,[18] although kinetically
the 1,5-migration is more favorable as TS4a–c have higher energies than TS3a–c (Schemes 8 and 9).

Scheme 9. Alternative reaction pathways for the evolution of gold(I) complexes Ib and Ic into the products of 1,5-OR migration VIb and VIc. DFT calculations (B3LYP, 6-31G(d,p) (C, H, P, O) and SDD (Au), CH2Cl2). Values for free
energies in kcal molÀ1.

Figure 2. Computed structure for minimum VIb (Scheme 9). Bond lengths
are in Š.

Scheme 8. Alternative reaction pathways for the evolution of gold(I) complexes Ia into the product of 1,5-OR migration VIa (L = PMe3). DFT calculations (B3LYP, 6-31G(d,p) (C, H, P, O, N) and SDD (Au), CH2Cl2). Values for free
energies in kcal molÀ1.

Table 4. Evaluation on the ligand effect for the computed trends of
gold(I) complex Ib.[a]

PMe3

The calculated structure for minimum VIb shows very similar
CÀC bond lengths of 1.41 and 1.39 Š (Figure 2). The AuÀC
bond length is 2.04 Š, which might correspond to a single
metal–carbon bond, and is similar to that found in well-characterized heteroatom-stabilized gold(I) carbenes.[3, 4] Overall, the
calculated structure fits better with a gold(I)-stabilized allylic
cation.
The observed reactivity trends when L = PMe3 are reproduced in cases where the ligand on gold(I) is changed to bulkier PPh3 phosphine or the model NHC ligand 1,3-dimethylimidazol-2-ylidene (Table 4).
Chem. Eur. J. 2016, 22, 13613 – 13618

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13617

Ib
TS1b
IIb
TS2b
IIIb
TS3b
IVb
TS5b
VIb
TS4b
Vb

PPh3

NHC[b]

0.0
3.6
À1.1
7.4
À3.1
2.1
À6.6
À6.6
À15.7
6.7
À37.5

0.0
5.8
À1.5
9.3
À2.5
4.4
À4.3
À5.1
À13.9
7.0
À35.4

0.0
9.5
1.6
11.3
0.1
7.9
À2.3
À0.7
À13.3
11.0
À33.1

[a] DG energies are given in kcal molÀ1. [b] 1,3-Dimethylimidazol-2-ylidene.
 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
MEC (FPU fellowship to M.R. and P.C.), the European Research
Council (Advanced Grant No. 321066), the AGAUR (2014 SGR
818), the Marie Curie program (postdoctoral fellowship
658256-MSCA-IF-EF-ST to B.R.), and the ICIQ Foundation. We
also thank the ICIQ X-ray diffraction and chromatograpy units.
Keywords: cycloisomerization · cyclopropanation · density
functional calculations · gold · rearrangement

Acknowledgements

[1] Recent reviews: a) C. Obradors, A. M. Echavarren, Acc. Chem. Res. 2014,
47, 902 – 912; b) A. S. K. Hashmi, Acc. Chem. Res. 2014, 47, 864 – 876; c) L.
Zhang, Acc. Chem. Res. 2014, 47, 877 – 888; d) Y.-M. Wang, A. D. Lackner,
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7332.
[2] For a recent discussion on the nature of the carbene-like intermediates
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[3] E. JimØnez-NfflÇez, M. Raducan, T. Lauterbach, K. Molawi, C. R. Solorio,
A. M. Echavarren, Angew. Chem. Int. Ed. 2009, 48, 6152 – 6155; Angew.
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[4] Y. Wang, M. E. Muratore, A. M. Echavarren, Chem. Eur. J. 2015, 21, 7332 –
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[5] D. Leboeuf, M. Gaydou, Y. Wang, A. M. Echavarren, Org. Chem. Front.
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[6] a) C. Nieto-Oberhuber, S. López, M. P. MuÇoz, E. JimØnez-NfflÇez, E.
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[7] A. Prieto, M. R. Fructos, M. M. Díaz-Requejo, P. J. PØrez, P. PØrez-Galµn, N.
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[8] C. R. Solorio-Alvarado, Y. Wang, A. M. Echavarren, J. Am. Chem. Soc.
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[9] M. Gaydou, R. E. Miller, N. Delpont, J. Ceccon, A. M. Echavarren, Angew.
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[12] CCDC1480036 (6 c), CCDC 1480037 (19), CCDC 1480038 (27 b), CCDC
1480039 (23), CCDC 1480040 (15 g) contain the supplementary crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
[13] See the Supporting Information for details on the transformation of 6 c
into hexahydroazulene diol 16 a.
[14] a) C. Nevado, A. M. Echavarren, Chem. Eur. J. 2005, 11, 3155 – 3164; b) P.
de Mendoza, A. M. Echavarren, Pure Appl. Chem. 2010, 82, 801 – 820.
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[17] See the Supporting Information for details on the calculations.
[18] For an important mechanistic study on isomerization of 1,6-enynes catalyzed by InCl3 that also leads to this type of products, see: L.-G. Zhuo,
J.-J. Zhang, Z.-X. Yu, J. Org. Chem. 2012, 77, 8527 – 8540.

We thank MINECO (Severo Ochoa Excellence Accreditation
2014–2018 (SEV-2013-0319), project CTQ2013-42106-P), the

Received: May 17, 2016
Published online on August 16, 2016

Scheme 10. Alternative reaction pathways for the evolution of gold(I) complex Id into the product of 1,5-OR migration VId. Numbers in red correspond to the formation and evolution of IId“, the diastereomer of IId. DFT
calculations (B3LYP, 6-31G(d,p) (C, H, P, O) and SDD (Au), CH2Cl2). Values for
free energies in kcal molÀ1.

DFT calculations for the reaction of gold(I) complex Id led to
much less clear-cut results (Scheme 10). Although the 5-exo-dig
pathway leading to IId was again more favorable than the 6endo-dig cyclization to form IIId, the 1,5-migration of IId
through IVd to form VId was found to be the kinetically most
favorable pathway, which is not what was observed experimentally for 20 b and 26 a–h.

Conclusions
In general, 1,6-enynes bearing OR groups at the propargyl position react through intermediates that can be formulated in
a simplified manner as a,b-unsaturated gold(I) carbenes, which
react in general with alkenes to form cyclopropanes or 1,3-diketones to form products of a-alkylation. We have found that
among the various migrating OR groups, p-nitrophenyl ether
gives the best results. In addition, we have established that
the gold(I)-catalyzed enyne cyclization/1,5-OR migration/cyclopropanation cascade takes place without racemization, which
demonstrates that propargyl carbocations are not formed
under the reaction conditions. This has been applied for the
preparation of key intermediates for the synthesis of (+)-schi+
sanwilsonene A with higher enantiomeric purity. DFT calculations suggest that after the initial cyclization, the 1,5-OR migration proceeds stepwise through a cyclic intermediate although
the cleavage occurs through a very low barrier. However, additional mechanistic work is still required to understand why in
cases in which propargyl group does not migrate an endo-type
single-cleavage rearrangement is the most favorable reaction
pathway.

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 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim