Gold-catalyzed cyclization/oxidative [3+2] cycloadditions of 1,5-enynes with nitrosobenzenes without additional oxidants

Article abstract of DOI:10.1002/anie.201209850

Golden control: The title reaction (see scheme) proceeds with high stereocontrol to generate the heterocyclic products in good yield. Experiments to probe the mechanism were performed. Copyright

Full text of DOI:10.1002/anie.201209850

Angewandte
Chemie
DOI: 10.1002/anie.201209850
Synthetic Methods
Gold-Catalyzed Cyclization/Oxidative [3+2] Cycloadditions of
1,5-Enynes with Nitrosobenzenes without Additional Oxidants**
Chun-Hao Chen, Yen-Ching Tsai, and Rai-Shung Liu*
Metal-catalyzed [2+2+m] cycloadditions (m = 1–3) of 1,n-
enynes (n = 5–7) with small molecules are powerful tools for
accessing complicated hetero- and carbocycles.[1,2] These
reactions are appealing because their mechanisms can be
altered by using different metal complexes. As shown in
Scheme 1, low-valent metal species react with these enynes to
This oxidative cycloaddition was realized with the treat-
ment of the 1,5-enyne 1a (1 equiv) with nitrosobenzene (2a;
1.5 equiv) and [P(tBu)2(o-biphenyl)AuCl]/AgNTf2 (5 mol%)
in 1,2-dichloroethane (DCE; 258C, 24 h) to give the oxidative
cycloadduct 3a in 21% yield, together with the cycloisome-
rization product 4a in 70% yield (Table 1, entry 1).[7] The
Table 1: Tests over various gold catalysts.
Entry[a] Au
n
Solvent t [h]
Yield [%][b]
3a 4a 5a
1
2
3
4
5
6
7
[LAuCl]/AgNTf2
[LAuCl]/AgSbF6
[LAuCl]/AgSbF6
1.5 DCE
1.5 DCE
4.0 DCE
24
14
4
4
5
21 70 18
41 53 35
82
77
6
8
57(75)[c]
(76)[c]
(73)[c]
(82)[c]
–
[PPh3AuCl]/AgSbF6 4.0 DCE
[IPrAuCl]/AgSbF6
[LAuCl]/AgSbF6
[LAuCl]/AgSbF6
4.0 DCE
4.0 CH2Cl2
4.0 CH3CN 24
72 16
3
85
–
6
87
Scheme 1. Metal-catalyzed cycloadditions on 1,n-enynes.
[a] [1a]=0.3m, 258C. [b] Yield of product isolated from silica gel column.
[c] Yield determined by NMR spectroscopy. L=P(tBu)2(o-biphenyl),
IPr=1,3-bis(diisopropylphenyl)imidazol-2-ylidene), Tf=trifluorometha-
nesulfonyl.
form metallocycloalkene intermediates (Ia) initially,[3]
whereas gold(I) catalysts generate the cyclopropyl gold
carbene Ib which has a resonance structure, that is, the
alkenylgold carbocation Ib’.[4] We sought a new catalytic
[2+2+m] cycloaddition of 1,n-enynes to generate an addi-
tional functionality on the resulting cycloadducts. Although
catalytic cycloadditions of nitrosobenzenes have been inten-
sively studied,[5,6] we are aware of no report on the cyclo-
addition of nitroso species with 1,n-enynes (n = 5–7).[6] We
report herein the gold-catalyzed cyclization/oxidative
[3+2] cycloadditions of 1,5-enynes with nitrosobenzenes.
Notably, nitrosobenzenes not only provide two-atom building
units, but also lead to the alkyne oxidation. Such a reaction
pattern is unprecedented in metal-catalyzed cycloadditions of
1,n-enynes.
yield of 3a was increased to 41% with [{P(tBu)2(o-biphenyl)}-
AuCl]/AgSbF6 (5 mol%, entry 2). For the two above-men-
tioned entries, we also isolated 1,2-diphenyldiazene oxide
(5a) which was identified upon comparison of its NMR
spectra and mass data with those of an authentic sample.[8]
The structure of 3a was confirmed by an X-ray diffraction
study to confirm a cis-fused ring system with the methyl group
and its neighboring hydrogen atom cis to each other.[9]
Accordingly, three equivalents 2a are required to achieve
the reaction. With an increased loading (4 equiv) of 2a, the
yield of 3a was significantly increased to 82% (entry 3).
Other gold catalysts such as [PPh3 AuCl]/AgSbF6 and
[IPrAuCl]/AgSbF6 also produced the desired cycloadduct 3a
(entries 4 and 5). [{P(tBu)2(o-biphenyl)}AuCl]/AgSbF6 in
CH2Cl2 gave 3a with a yield of up to 85% (entry 6), but
afforded 4a exclusively (87%) with CH3CN as the solvent
(entry 7). The yield of 5a was almost the same as that for 3a
according to the NMR analysis of the crude reaction mixture
(entries 3–6).
[*] C. H. Chen, Y. C. Tsai, Prof. Dr. R.-S. Liu
Department of Chemistry, National Tsing Hua University
Hsinchu (30013) Taiwan (China)
E-mail: rsliu@mx.nthu.edu.tw
[**] We thank National Science Council, Taiwan, for financial support of
this work.
The oxidative cycloadditions of 2a (4 equiv) with various
1,5-enynes (1b–1i) bearing a variety of alkynyl and phenyl
substituents (R, X, and Y) is shown (Table 2). The reactions
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209850.
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Table 2: Oxidative [3+2] cycloadditions using various enynes.
Table 3: Oxidative cycloadditions on 1,5-enynes.
Entry[a] Enyne
t [h]
Yield [%][b]
Entry[a]
Enyne
t [h]
Yield [%][b]
1
2
1j (n=1)
1k (n=2)
11
2.5
77 (3j)
72 (3k)
9 (4j)
11 (4k)
X=Y=H
R=nBu (1b)
R=iPr (1c)
R=cyclopropyl (1d)
R=Ph (1e)
1
2
3
4
10
7
8
73 (3b)
6 (4b)
5 (4c)
11 (4d)
10 (4e)
77 (3c)
77 (3d)
81 (3e)
36
R=Me
3
1l
5.5
82 (3l)
8 (4l)[c]
5
6
7
8
X=Cl, Y=H (1 f)
X=H, Y=Cl (1g)
X=OMe, Y=H (1h)
X=H, Y=OMe (1i)
3
5
5
4
80 (3 f)
83 (3g)
– (3h)
78 (3i)
9 (4 f)
9 (4g)
80 (4h)
8 (4i)
[a] [1]=0.30m, L=P(tBu)2(o-biphenyl). [b] Yield of product isolated from
silica gel column.
4
5
(E)-1m (R1 =Ph,R2 =Me) 14
(Z)-1m (R1 =Me,R2 =Ph) 15
75 (3m)
77 (3m)
10 (4m)
9 (4m)
were
run
with
[{P(tBu)2(o-biphenyl)}AuCl]/AgSbF6
(5 mol%) in CH2Cl2 (258C, 0.3m). We made no attempts to
isolate 5a for these reactions. Various internal alkynes (1b–e)
were used in this reaction to give the desired compounds 3b–
e together with the indene derivatives 4b–e in small quantities
(entries 1–4). The reactions are sensitive to the substituents
on the phenyl ring (1 f–i); we obtained the desired cyclo-
adducts 3 f,g and 3i (entries 5, 6, and 8) in satisfactory yields,
whereas 1h gave only the cycloisomerization product 4h in
80% yield (entry 7). This substituent effect provides insight
into the mechanism of the reaction.[10]
6
7
(E)-1n
24
20
63 (3n)
67 (3n)
–
–
(R1=4-MeOC6H4,R2 =H)
(Z)-1n
(R1=H, R2=4-MeOC6H4)
Table 3 shows our results on additional 1,5-enynes (1j–p).
For the substrates 1j–l bearing various trisubstituted alkenes
including cyclopentylidene, cyclohexylidene, and 3-pentyli-
dene groups (entries 1–3), the corresponding products 3j–
l were obtained in reasonable yields (72–82%). We also
prepared the trisubstituted alkene 1m with both E and
Z configurations (entries 4 and 5), and their gold-catalyzed
reactions resulted in the same cycloadduct 3m with the
methyl group cis to the neighboring hydrogen atom. Notably,
this cis-configured product is 0.75 kcalmolÀ1 higher in energy
than the trans-configured isomer.[11] Similarly, both trans- and
cis-styrene derivatives of 1n (entries 6 and 7) gave the same
cycloadduct 3n, wherein the two adjacent hydrogen atoms are
8
9
1o (R=Me)
1p (R=nBu)
14
15
85 (3o)
82 (3p)
–
–
[a] [1]=0.3m, 5 mol% [{P(tBu)2(o-biphenyl)}Au]SbF6, CH2Cl2, 2a
(4 equiv), 258C. [b] Yield of product isolated from silica gel column.
[c] E/Z=2.5.
Table 4: Oxidative Cycloadditions with Nitrosobenzenes.
1
cis configured according to the H NMR NOE spectra (see
the Supporting Information). To our delight, the reactions
also proceeded with the nonbenzoid substrates 1o and 1p
(entries 8 and 9), thus delivering the products 3o and 3p,
respectively.
Table 4 shows the reactions using different nitrosoben-
zenes (2b–f). The cycloadducts 6b–f and 1,2-diphenyldiazene
oxides 5b–f were obtained in nearly the same proportions.
The reactions worked efficiently not only for the electron-
deficient nitrosobenzenes 2b–d (entries 1–3), but also for the
electron-rich analogues 2e,f (entries 4 and 5). The reaction
time of 4–8 hours for the nitroso species 2e,f is attributed to
their ability to coordinate to the gold catalyst. In all cases 4a
was produced in small proportions (0–11%).
Entry
Nitroso
t [h]
Yield [%][b]
compound[a]
6
4a
5
1
2
3
4
5
X=NO2 (2b)
X=Cl (2c)
X=Br (2d)
X=tBu (2e)
X=OMe (2 f)
0.5
1
0.5
4
71 (6b)
77 (6c)
79 (6d)
80 (6e)
91 (6 f)
11
6
5
7
–
67 (5b)
71 (5c)
75 (5d)
76 (5e)
84 (5 f)
8
[a] [1a]=0.3m, nitroso (4 equiv), [{P(tBu)2(o-biphenyl)}Au]SbF6, CH2Cl2,
258C. [b] Yield of product isolated from silica gel column.
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Equation (1) shows relevant data to support the interme-
diacy of the gold carbenes A. We prepared the acyclic 1,5-
enyne 7 which reacted with 2a (4 equiv) and the gold catalyst
(5 mol%) to deliver the tricyclic species 8 in 78% yield.
Formation of this compound arose from a 1,2-cyclopropane
migration to the neighboring gold carbene center in species
A. Equation (1) represents the first formal [2+2+1] cyclo-
addition for the enyne/nitroso functionalities.
Scheme 3. Deuterium-labeling experiment.
a very small proton content (< 10%). The 13C NMR spectrum
resembled that of the fully deuterated sample [D8]-5d (see the
Supporting Information). Its FAB mass spectra showed
primarily the signal for [D8]-5d. Again, this deuterium-
labeling experiment confirms that the monomeric form 2d
is actually the oxygen source for 6d.
A plausible mechanism (Scheme 4) to account for the
stereoselectivity of 3m from either (E)-1m or (Z)-1m is
shown. For (Z)-1m, the initially formed cyclopropyl carbene
The nitrosobenzenes 2a–e are in equilibrium with
a dimeric species in solution.[12] Their respective reactivities
toward the oxidation of gold carbenes should be addressed.
We tested the gold-catalyzed oxidation of the diazocarbonyl
species 9 with a nitroso species to compete with its catalytic
diazo decomposition to the olefin 12 (Scheme 2). The treat-
Scheme 4. A plausible mechanism for gold-catalyzed oxidative cycloaddition.
Scheme 2. Oxidation of gold carbenes by mono- or dimeric nitroso
species. EWG=CO2Et, Ar=4-BrC6H4
B has a an alkenylgold carbocation resonance structure,
which enables a [3+2] cycloaddition with nitrosobenzene to
form the cyclic transition-state C by either a concerted or
stepwise process.[13] This process is feasible because the
alkenyl phenyl group lies at the less hindered equatorial
position. This cycloaddition generates the gold carbene D
with the observed stereochemistry. For (E)-1m, the nitroso
cycloaddition might encounter difficulty because the corre-
sponding cyclic transition-state C’ has the phenyl group at the
axial position. This proposed stereochemistry is analogous to
that for the Sn(OTf)2-catalyzed [3+2] cycloaddition of sub-
stituted cyclopropane derivatives with aldehydes.[14,15] We
postulate an equilibrium between B and B’ because of their
significant alkenylgold carbocation resonance structures.
ment of this diazo species with [{P(tBu)2(o-biphenyl)}AuCl]/
AgSbF6 (5 mol%) and monomeric the nitroso species 2d
(1 equiv) gave the dicarbonyl compound 10 and 1,2-diphe-
nyldiazene oxide 5d in 42 and 40% yield, respectively. With
2d’ (1 equiv),[13] the desired 10 and 11 were obtained in 11 and
2% yields, respectively, together with the tetrasubstituted
olefin 12 in large 72%. Accordingly, we postulate that 2d’ is
a poor oxygen donor and the resulting 10 and 11 presumably
arose from the slow release of the monomeric form 2d.
In a separate experiment, the treatment of 1a with [D4]-
2d (1 equiv), 2d’ (1 equiv), and [{P(tBu)2(o-biphenyl)}-
Au]SbF6 (5 mol%) in CH2Cl2 (258C) gave the desired [D4]-
1
6d as a major component (D4/D0 = 9:1; Scheme 3). H NMR
analysis of 5d, using CDCl3 as an internal standard, indicated
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According to our control experiments (Schemes 2 and 3),
the subsequent oxidations of the resulting gold-carbenes D
rely on the monomeric nitroso species 2a.[16,17] This process is
expected to generate 3m and the gold nitrenes E simulta-
neously. We hypothesize that this nitrene species reacts
further with nitrosobenzene to regenerate the gold catalyst
and 5a. Notably, 2a is inactive toward the oxidation of species
B (or B’), which is not a pure gold carbene because of their
additional resonance structures.
Before this work, transition-metal catalyzed cycloaddi-
tions of 1,n-enynes (n = 5–7) were strictly restricted to the
[2+2+m] cycloaddition (m = 1–3) modes. In this work, we
achieved a cyclization/oxidative [3+2] cycloaddition of 1,5-
enynes and nitrosobenzene with high stereocontrol. These
oxidative cycloadditions are applicable a variety of 1,5-enynes
and nitrosobenzenes. According to our control experiments,
monomeric nitrosobenzenes not only react with the cyclo-
propyl gold-carbene intermediates, but also lead to the
oxidation of the key gold carbene intermediates.[18] This
novel enyne oxidative cycloaddition highlights the use of gold
catalysis.
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Received: December 10, 2012
Revised: February 20, 2013
Published online: March 19, 2013
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[9] CCDC 908146 (3a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Keywords: cycloadditions · enynes · gold · heterocycles ·
synthetic methods
.
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[10] For the 1,5-enyne 1h (Table 2, entry 7), the methoxy substituent
stabilizes the resonance structure of the cyclopropyl gold
carbene B’’ which tends to lose one proton to give the cyclo-
isomerization product 4h.
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[11] The energy differences between 3m and its diastereomer was
calculated using the B3LYP/6-311 ++ G** level of theory.
1H NOE spectra of compounds 3m and 3n are provided in the
Supporting Information.
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[13] We do not exclude the possibility that cyclic the transition-state
C is derived from the benzylic cation B’’ through a prior
coordination of the nitroso species to its cationic center to form
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F. A subsequent addition of the alkenylgold bond of F to the
coordinated nitrosobenzene also forms C. This process ration-
alizes well the nonstereospecificity of the cycloaddition.
[16] Nitrosobenzene was reported to undergo a nitrogen attack on
the less hindered gold carbenes (see Ref. [6b]), but this process is
likely impeded by steric hindrance for D in
our system. Attack of the oxygen atom of
nitrosobenzene on the gold carbene is also
documented in a nonoxidation process (see
Ref. [6a]).
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Johnson, J. Am. Chem. Soc. 2008, 130, 8642 – 8650; b) P. D.
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[17] We preclude the possibility that water and
oxygen are the oxygen sources for the oxida-
tion of D since we failed to obtain positive
evidence using either H2O18 or O2
.
18
[18] We have performed additional experiments
using a mixture of 2d’ (m equiv) and m2d (n equiv), thus
amounting to three ArNO molecules (2m+n = 3). Oxidative
cycloadduct 6d was obtained in decreased yields with increasing
proportions of 2d’ (m = 1.5, n = 0; m = 1, n = 1; m = o, n = 3). See
Table S1 in the Supporting Information.
[15] An alternative cyclic transition state C’’ is unlikely to occur
because the axial methyl group exerts steric interaction with the
bulky indene ring.
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