Gold-catalyzed cyclization of propargylic diynes: Ethers vs acetates - Related products but different pathways Dedicated to Prof. Hubert Schmidbaur on the occasion of his 80th birthday.

Article abstract of DOI:10.1016/j.jorganchem.2015.04.053

Diyne substrates bearing one propargylic ether instead of the previously published propargylic acetates were subjected to a gold catalyst. α-Naphthol derivatives were obtained as products of the cycloisomerization. The close relationship of the products t

Full text of DOI:10.1016/j.jorganchem.2015.04.053

Journal of Organometallic Chemistry xxx (2015) 1e7
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Gold-catalyzed cyclization of propargylic diynes: Ethers vs acetates e
Related products but different pathways
a
a
a
a
€
Johannes Schadlich , Tobias Lauterbach , Matthias Rudolph , Frank Rominger ,
A. Stephen K. Hashmi a
Organisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
b Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
, b, *
a
€
a r t i c l e i n f o
a b s t r a c t
Article history:
Diyne substrates bearing one propargylic ether instead of the previously published propargylic acetates
Received 5 March 2015
Received in revised form
27 April 2015
Accepted 29 April 2015
Available online xxx
were subjected to a gold catalyst.
a
ꢀNaphthol derivatives were obtained as products of the cyclo-
isomerization. The close relationship of the products to the corresponding cyclizations implicated a
related mechanistic scenary at first, but further studies favour a mechanistic pathway that is completely
different.
© 2015 Elsevier B.V. All rights reserved.
Dedicated to Prof. Hubert Schmidbaur on
the occasion of his 80th birthday.
Keywords:
Cycloisomerization
Diynes
Gold catalysis
Naphthols
Vinyl cations
1. Introduction
corresponding acetate derivatives [4c,d,g]. In this case an initial 1,2-
migration of the acetal and a subsequent transfer of the so formed
In the highly active field of homogeneous gold catalysis [1],
diynes are gaining importance as starting materials for gold-
catalyzed transformations. The common reaction principles are
dual gold-catalyzed transformations [2], reactions in which one of
the alkynes is transferred into a nucleophilic part which then un-
dergoes a subsequent cyclization with the remaining alkyne [3] and
carbene transfer reactions in which an alkyne-derived gold carbe-
noid is transferred over a pendant alknye followed by classical
carbenoid transformations as terminating step [4].
During our studies on the gold-catalyzed transformation of 1,6-
diynes 1 and 3, we observed a completely different reactivity which
depended on the oxygen functionalities (Scheme 1). While in the
case of an unprotected propargylic alcohol a complex cascade, that
is most probably initiated by an attack of the nucleophilic oxygen
gold carbenoid delivered b-naphtole derivates 4 as final products of
the reaction cascade. Inspired by this strong influence of the oxygen
functionality in the precursors, we considered the incorporation of
ether moieties in the starting materials 5 as well. As these pre-
cursors are not known to deliver gold carbenoids and in addition
due to the increased sterical bulkiness of the methoxy group a
nucleophilic addition pathway should be less favoured, we were
curious if an alternative reaction pathway might be opened for this
type of reactants. The results of our studies will be discussed in this
contribution.
2. Results and discussion
Appropriate starting materials for our investigations were pre-
pared via Sonogashira coupling of the corresponding bromoalde-
hydes, addition of an ethynyl Grignard reagent to the aldehyde and
alkylation of the obtained propargylic alcohols (see SI for further
details). Initial tests were conducted with model diyne 5a. The re-
sults with different catalyst are summarized in Table 1. With the
well established IPrAuNTf2 catalyst, complete conversion was
monitored and 88% yield of a single product could be obtained
onto the triple bond, delivered b-keto-naphtalines 2 as the prod-
ucts [5], a completely different picture was obtained for the
* Corresponding author. Organisch-Chemisches Institut, Ruprecht-Karls-Uni-
€
versitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany.
E-mail address: hashmi@hashmi.de (A.S.K. Hashmi).
http://dx.doi.org/10.1016/j.jorganchem.2015.04.053
0022-328X/© 2015 Elsevier B.V. All rights reserved.
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J. Schadlich et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
2
Scheme 2. Applied catalysts.
besides the one IPr-substituent (Entry 4). A simpler NAC complex
(Cat B) [7] only delivered moderate yield (Entry 5) and isonitrile
complex Cat C showed no full conversion (Entry 6). In a series of
phosphane ligands very good results were obtained with sterically
shielded Buchwald type ligands SPhos and XPhos (Entries 7 and 8),
while simple Ph3PAuNTf2 turned out to be inefficient (Entry 9). The
phosphite complex Cat D also catalyzed the transformation but
yields were only moderate (Entry 10). As due to the terminal alkyne
in the substrate a dual activation pathway cannot be ruled out, a
test reaction was conducted in the presence of dual activation
catalyst Cat E (Entry 11) [8]. The poor result for this catalyst class
gives a clear hint that a dual activation pathway is unlikely for this
transformation. A control experiment with only AgNTf2 showed
only low conversion and only a trace of the desired product was
obtained (Entry 12). A tenfold increase of the substrate concen-
tration led to a significant drop in yield (Entry 13) and a screening of
different counter ions in combination with the IPr complex was
unsuccessful (Entries 14e17).
With the optimized conditions in hand we turned our focus on
the evaluation of the substrate scope of the cycloisomerization
(Table 2). A preparative scale of the reaction with diyne 5a deliv-
ered the corresponding naphthol derivate 6a in 78% yield (Entry 1).
The installation of two electron-donating methoxy groups in the
aromatic backbone of starting material 5b led to a significant drop
in yield and the corresponding product was only obtained in 47%
yield (Entry 2). Fortunately, we were able to obtain crystals suitable
for X-ray crystal structure analysis. The obtained solid state mo-
lecular structure unambiguously verifies the assignment of the
Scheme 1. Known reactivity patterns for oxygen-substituted diynes 1 and 3 and our
planned investigation.
(Entry 1). NMR analysis revealed that no migration of the methoxy
group had taken place and an
a-naphthol 6a was obtained as
product. The structure of the cycloisomerized product shows a
strong relationship to that of carbene migration products 4. Like in
the case of substrates 3, a methyl group migration also takes place
in this case which finally delivers a tetra-substituted double bond in
the products. Further experiments revealed that simple AuCl and
NaAuCl4 cannot efficiently catalyze the transformation (Entries 2
and 3). Much better results were obtained with Cat A [6]
(Scheme 2), a NHC complex with a large 15-membered ring
Table 1
Catalyst screening.
Entrya
[Au]
AgX
Conversion [%]
Yield [%]
1
2
3
4
5
6
7
8
IPrAuNTf2
AuCl
NaAuCl4
Cat A
Cat B
Cat C
SPhosAuCl
XPhosAuCl
PPh3AuNTf2
Cat D
100
49
29
100
95
70
100
100
37
100
37
88
18
0
80
59
27
88
80
0
obtained a-naphthol structure bearing a tetra-substituted double
bond in peri-position (Fig. 1). Substrates 5c and 5d with only one
donating methoxy or methyl group in the backbone delivered good
to excellent results again (Entry 3 and 4), while only a low yield was
obtained with acetal protected starting material 5e (Entry 5).
Electron-withdrawing groups were tolerated well, no matter if
nitro or fluoro groups were installed in the backbone (Entries 6 and
7). As a next step we tested heteroaromatic and nonaromatic
backbones. Probably due to the strong coordinating properties of
the pyridine no reaction took place for starting material 5h (Entry
8). The conversion of thiophene substrate 5i only led to an insep-
arable mixture of products. In the case of substrates 5j and 5k with
cyclic aliphatic backbones, the 5-membered ring turned out to be
completely unreactive even at 80 ꢁC (Entry 10), while in the case of
the 6-membered homologue, a slow conversion delivered the
desired product 6k in low yield (Entry 11). Next we probed the
possibility to vary the non-terminal alkyne substituent but none of
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
9
10
11
12
13
14
15
16
17
AgNTf2
AgNTf2
74
0
Cat E
43
6
IPrAuNTf2
IPrAuCl
IPrAuCl
IPrAuCl
IPrAuCl
100
100
19
53
16
38b
15
1
AgPF6
AgSbF6
AgBF4
AgOTf
15
2
a
80 mmol 5a, 5 mol% gold catalyst and 5 mol% silver salt were stirred in CH2Cl2
(4 ml) at rt; after 16 h conversions and yield were determined by GC using n-
dodecan as internal standard.
b
tenfold concentration (0.2 M).
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J. Schadlich et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
3
Table 2
Substrate scope.
Entry
1
Starting material
Product
Yield [%]a
78
1H NMR data for 6 in CDCl3 (ppm values)
1.40 (s, 3H), 1.92 (s, 3H), 2.00 (s, 3H), 4.00 (s, 3H), 6.79 (d, 7.8 Hz, 1H), 7.09 (d,
7.8 Hz, 1H), 7.43e7.48 (m, 2H), 7.72e7.75 (m, 1H), 8.27e8.29 (m, 1H).
5a
6a
2
47
1.42 (s, 3H), 1.93 (s, 3H), 2.00 (s, 3H), 3.94 (s, 3H), 3.99 (s, 3H), 4.02 (s, 3H), 6.70
(7.8 Hz, 1H), 6.96 (d, 7.8 Hz, 1H), 7.02 (s, 1H), 7.56 (s, 1H).
6b
6c
5b
3
4
74
90
1.31 (s, 3H), 1.82 (s, 3H), 1.90 (s, 3H), 3.87 (s, 3H), 3.92 (s, 3H), 6.70 (d, 7.9 Hz,
1H), 6.78 (d, 7.9 Hz, 1H), 7.04 (dd, 9.0 Hz, 2.4 Hz, 1H), 7.49 (d, 2.4 Hz, 1H), 7.57 (d,
1H).
5c
1.42 (s, 3H), 1.94 (s, 3H), 2.00 (s, 3H), 2.50 (s, 3H), 3.99 (s, 3H), 6.72 (d, 7.7 Hz,
1H), 7.05 (d, 7.7 Hz, 1H), 7.29 (dd, 8.5 Hz, 1.6 Hz, 1H), 7.48 (d, 1.6 Hz, 1H), 8.18 (d,
8.5 Hz, 1H).
5d
6d
5
40
1.40 (s, 3H), 1.89 (s, 3H), 1.95 (s, 3H), 3.96 (s, 3H), 6.01 (s, 2H), 6.70 (d, 7.8 Hz,
1H), 6.94 (d, 7.8 Hz, 1H), 7.04 (s, 1H), 7.57 (s, 1H).
5e
5f
6e
6f
6
7
87
83
1.37 (s, 3H), 1.93 (s, 3H), 1.98 (s, 3H), 4.04 (s, 3H), 6.90 (d, 7.9 Hz, 1H), 7.29 (d,
7.9 Hz, 1H), 7.82 (d, 9.2 Hz, 1H), 8.18 (dd, 9.2 Hz, 2.4 Hz, 1H), 9.23 (d, 2.4 Hz, 1H).
1.38 (s, 3H), 1.91 (s, 3H), 1.98 (s, 3H), 3.99 (s, 3H), 6.81 (d, 7.8 Hz, 1H), 7.04 (d,
7.8 Hz, 1H), 7.21 (ddd, 10.8 Hz, 9.2 Hz, 2.7 Hz, 1H), 7.72 (dd, 9.2 Hz, 5.7 Hz, 1H),
7.87 (dd, 10.7 Hz, 2.7 Hz, 1H).
5g
6g
8
9
-
-
no reaction
unselective
e
e
5h
5i
10
11
12
-
no reaction
e
5j
32b
1.41 (s, 3H), 1.68e1.78 (m, 4H), 1.79 (s, 3H), 1.84 (s, 3H), 2.50 (t, 2H), 2.68 (t,
6.4 Hz, 2H), 3.81 (s, 3H), 6.64 (d, 8.2 Hz, 1H), 6.77 (d, 8.2 Hz, 1H).
5k
6k
-
unselective
e
5l
(continued on next page)
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J. Schadlich et al. / Journal of Organometallic Chemistry xxx (2015) 1e7
4
Table 2 (continued )
Entry
13
Starting material
Product
-
Yield [%]a
1H NMR data for 6 in CDCl3 (ppm values)
unselective
e
5m
14
-
-
unselective
e
5n
5o
15
16
unselective
e
e
20c
7
5p
17
18
-
unselective
e
5q
15
1.43 (s, 3H), 1.92 (s, 3H), 2.00 (s, 3H), 3.94 (s, 3H), 4.00 (s, 3H), 5.25 (s, 2H), 6.77
(d, 7.8 Hz, 1H), 6.95 (d, 7.8 Hz, 1H), 7.04 (s, 1H), 7.35 (t, 7.2 Hz, 1H), 7.42 (t, 7.2 Hz,
2H), 7.53 (d, 7.2 Hz, 2H), 7.64 (s, 1H).
5r
6r
19
51
1.03 (t, J ¼ 7.4 Hz, 3H), 1.41 (s, 3H), 1.52e1.61 (m, 2H), 1.88e1.93 (m, 5H), 1.99
(s, 3H), 3.93 (s, 3H), 4.01 (s, 3H), 4.13 (t, 6.3 Hz, 2H), 6.69 (d, 7.8 Hz, 1H), 6.92 (d,
7.8 Hz, 1H), 7.01 (s, 1H), 7.58 (s, 1H).
5s
6s
a
Isolated yield.
Reaction in DCE at 80 ꢁC.
Product mixture with allene 7 as main product.
b
c
the substrates showed a selective reaction course (Entries 12e14).
Unfortunately this was also the case for substrates bearing non-
terminal alkynes at the propargylic moiety (Entries 15e17). In
this case the only product that could be identified was allene 7 that
can be explained by a 1,5-hydride shift with subsequent elimina-
tion of formaldehyde, a sequence that is in analogy to an allene
synthesis reported by the Gagosz group [9]. Finally the role of the
ether substituent was investigated. A lower yield was obtained
with benzylic ether 5r which might be explained by competing
hydride shift pathways that are favoured in the case of a benzylic
position (Entry 18). Substrate 5s with a butyl ether moiety deliv-
ered comparable yields to its methyl ether derivative 5b again
(Entry 19).
As the obtained product of the transformation strongly resem-
bled the structure of the carbene transfer products 4 derived from
propargylic acetates (compare Scheme 1), we considered that a
related pathway might have taken place for methoxy substituted
diynes as well (Scheme 3, upper part). Coordination to the terminal
triple bond by the gold catalyst and a subsequent hydride shift [10]
might deliver acceptor-substituted vinyl gold species IIa which in
another mesomeric form can be regarded as carbenoid IIb [11]. The
subsequent steps (carbene transfer and 1,2-methylshift) would be
in complete accordance to our previous results with the acetate
precursors [4c]. Even if this mechanistic picture cannot be
completely ruled out, it is not explainable that starting materials
like substrate 5m, which formed the corresponding products in the
case of a propargylic acetates [4g], turned out to be unsuitable for
this reaction. Besides the limitation to a tert-butyl substituent at the
alkyne, it is not explainable that substrates bearing two non-
Fig. 1. Solid state molecular structure of compound 6b.
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5
Scheme 3. Possible mechanistic scenarios.
Scheme 4. Control experiment with propargylic ethers 8 and 10.
terminal alkyne substituents were unsuitable substrates as well. To
evaluate if a carbenoid species can be generated from propargylic
ethers, we converted methyl-propargylicether 8 under the reaction
conditions in the presence of externally offered alkenes (Scheme 4,
upper part). No cyclopropanation products 9 could be obtained
which makes it unrealistic that carbenoids can be formed by this
precursors [12]. Even the intramoleculary offered alkene in com-
pound 10 did not deliver any cyclopropanation product under the
standard conditions (Scheme 4, upper part). Based on this finding,
we postulate a mechanism in which the tert-butyl alkyne reacts as a
nucleophile under generation of vinyl cation III that can be stabi-
lized by the attached tert-butyl group by hyperconjugation. A
subsequent 1,2-methyl shift followed by rearomatization of inter-
mediate IV would then deliverer the product after proto-
demetalation of the gold fragment. Besides the stepwise formation
of intermediate V/V′ a concerted pathway including 1,2-methyl
shift and cyclization is also a reasonable alternative.
strong relationship of the products to the corresponding acetate
starting materials, the scope for this transformation is not that
broad. This might be explained by a completely different reaction
pathway that is based on the nucleophilic addition of the second
alkyne moiety instead of a carbene transfer pathway. This might
open up new possibilities for the design of new gold-catalyzed
reactions.
4. Experimental Section (see SI for further informations)
All reagents and solvents were obtained from Acros, ABCR, Alfa
Aesar, SigmaeAldrich or VWR and were used without further pu-
rification unless otherwise noted. Deuterated solvents were pur-
chased from Euriso-Top. Absolute solvents were dried by a MB SPS-
800 with the aid of drying columns. Preparation of air- and
moisture-sensitive materials was carried out in flame dried flasks
under an atmosphere of nitrogen using Schlenk-techniques. Thin
layer chromatography (TLC) was performed using Polygram pre-
coated plastic sheets SIL G/UV254 (SiO2, 0.20 mm thickness) from
MachereyeNagel. Column chromatography was performed using
silica gel (40.0e63.0 nm particle size) from MachereyeNagel. NMR
spectra were recorded on Bruker Avance 500, Bruker Avance 300
3. Conclusion
In conclusion we could demonstrate that propargylic ethers can
be applied as substrates for diyne cylizations, too. Despite the
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6
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and Bruker ARX-250 spectrometers. Chemical shifts (in ppm) were
referenced to residual solvent protons [13]. Signal multiplicity was
determined as s (singlet), d (doublet), t (triplet), q (quartet) or m
(multiplet). 13C assignment was achieved via DEPT90 and DEPT135
or HSQC-me spectra. MS spectra were recorded on a Vakuum
Generators ZAB-2F, Finnigan MAT TSQ 700 or JEOL JMS-700 spec-
trometer. GC spectra were recorded on HP Agilent 5890 Series II
Plus with FID analyser. IR spectra (in cmꢀ1) were recorded on a
Bruker Vector 22 FT-IR. Crystal structure analysis was accomplished
on a Bruker APEX II Quazar diffractometer [14]. Elemental analysis
was performed on an Elementar Vario EL.
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Representative Synthesis of 6a; other products were obtained in
a similar fashion (see SI and Table 2).
300 mg (1.32
mmol) 5a were dissolved in 70 ml DCM. To this
solution were added IPrAuNTf2 (57.0 mg, 66.0
mmol, 5 mol%). The
reaction mixture was stirred at room temperature for 16 h. The
solvent was removed under reduced pressure. After purification by
flash column chromatography (SiO2, PE), 232 mg (1.03 mmol, 78%)
of 6a were obtained as an yellow oil.
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Rf (PE) ¼ 0.37; IR (reflection): ~n ¼ 3064, 2985, 2911, 2855, 1586,
1461, 1421, 1389, 1372, 1323, 1293, 1255, 1236, 1219, 1164, 1081,
1025, 817, 765 cm-1
J ¼ 1.3 Hz, 3H), 1.92 (s, 3H), 2.00 (s, 3H), 4.00 (s, 3H), 6.79 (d,
J ¼ 7.8 Hz, 1H), 7.09 (d, J ¼ 7.8 Hz, 1H), 7.43e7.48 (m, 2H), 7.72e7.75
(m, 1H), 8.27e8.29 (m, 1H) ppm; 13C NMR (CDCl3, 100 MHz):
;
1H NMR (CDCl3, 400 MHz):
d
¼ 1.40 (d,
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d
¼ 20.0 (q), 21.2 (q), 22.0 (q), 55.4 (q), 103.5 (d), 122.2 (d), 124.8 (d),
124.9 (d), 125.5 (d), 125.7 (s), 126.1 (d), 128.0 (s), 128.9 (s), 132.1 (s),
135.1 (s), 153.8 (s) ppm; MS (EI (þ), 70 eV): m/z (%) ¼ 226 (100)
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We thank Umicore AG & Co. KG for the generous donation of
gold salts.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.04.053.
(q) K. Hirano, Y. Inaba, N. Takahashi, M. Shimano, S. Oishi, N. Fujii, H. Ohno,
J. Org. Chem. 76 (2011) 1212e1227;
€
(r) A.S.K. Hashmi, T. Haffner, M. Rudolph, F. Rominger, Chem. Eur. J. 17 (2011)
8195e8201;
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