Gold(I)-Catalyzed Allene–Diene–Alkyne Coupling Reaction to Polycycles

Article abstract of DOI:10.1002/ejoc.202001190

In general, the design of complex polycycles from simple building blocks is challenging and requires many reaction steps. Herein, we present that complex polycycles can be synthesized starting from two alkyne building blocks in a one-pot reaction using go

Full text of DOI:10.1002/ejoc.202001190

Communication
doi.org/10.1002/ejoc.202001190
EurJOC
European Journal of Organic Chemistry
Cascade Reaction
Gold(I)-Catalyzed Allene–Diene–Alkyne Coupling Reaction to
Polycycles
[
a]
[a]
[a]
Nina Semleit, Mathis Kreuzahler, and Gebhard Haberhauer*
Abstract: In general, the design of complex polycycles from by allene–diene–alkyne coupling. Quantum chemical investiga-
simple building blocks is challenging and requires many reac- tions confirm the assumed mechanism. In sum, five new C–C
tion steps. Herein, we present that complex polycycles can be bonds are built in one sweep, which is extraordinary since non-
synthesized starting from two alkyne building blocks in a one- identical building blocks are combined in this reaction. The in-
pot reaction using gold(I) as catalyst, in yields up to 77 %. troduced new allene–diene–alkyne coupling reaction paves the
Mechanistically, the process can be described as cascade reac- way for several new syntheses of cycles and polycycles consid-
tion in two steps: first, the allene is formed by gold(I)-catalyzed ering potential intermolecular reactions.
dearomatization, second, the desired product is then obtained
In organic chemistry the development of efficient and new
C–C coupling reactions is highly important for the synthesis of
complex molecules. Especially, catalysis by transition metals has
[
1]
become an important tool to design such molecules. One of
these transition metal-catalyzed C–C coupling reactions is the
[
2–6]
catalytic asymmetric dearomatization (CADA).
By using
cheap, functionalizable and ubiquitous aromatic systems for
this C–C coupling reaction it is possible to synthesize more
[
2–6]
complex polycycles.
catalysts to build up new C–C bonds, homogeneous gold catal-
In search of efficient transition metal
[
7–11]
ysis has come to the fore.
up to four new C–C bonds
Currently, it is possible to form
during a reaction cascade by
[
12]
means of homogeneous gold catalysis if the combination of
identical building blocks (e.g. oligomerization) is not taken into
[
13]
account.
tions compared to other transition metals.
gold-catalyzed dearomatization involving one alkyne unit was
However, gold has been rarely used in CADA reac-
[
14–17]
An example of
[
18]
shown by Bandini et al.
They converted naphthyl propargyl
ethers 1 to naphthalene-2-one derivatives 2 (Scheme 1a). This
reaction can be considered as a Claisen rearrangement with
subsequent intramolecular hydroxy group addition to the
newly formed allene.
Scheme 1. Gold(I)-catalyzed dearomatization (a), haloalkynylation (b), cycliza-
tion (c) of alkynes and allene–diene–alkyne coupling reaction (d).
Our previous work in the field of gold catalysis was focused
on the conversion of haloacetylenes (Scheme 1b). The advan-
few years we and other research groups presented haloalkynyl-
[23–25]
[26–29]
ation reactions of different alkynes
and alkenes.
[
19]
tage of these compounds is their easy accessibility and their
Mechanistic investigations proved that the formation of prod-
ucts obtained by addition has to take place via rearrange-
[
20–22]
thermal stability at standard conditions.
During the last
[
30]
ment.
However, the restriction of previous systems is that
[
a] N. Semleit, M. Kreuzahler, Prof. Dr. G. Haberhauer
only conjugated alkynes can be used. While attempting to
transfer the concept of haloalkynylation to non-conjugated
haloacetylenes, we discovered a gold(I)-catalyzed allene–diene–
alkyne coupling reaction to polycycles. Herein, we describe the
application range as well as mechanistical investigations of this
novel type of cascade reaction.
Institut für Organische Chemie, Universität Duisburg-Essen
Universitätsstraße 7, 45117 Essen Germany
E-mail: gebhard.haberhauer@uni-due.de
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.202001190.
©
2020 The Authors. European Journal of Organic Chemistry published by
Wiley-VCH GmbH. · 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.
In the course of studies relating to gold(I)-catalyzed reactions
of non-conjugated haloacetylenes with alkynes, compound
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classes 12 and 13 got into our focus of interest (Scheme 1d).
We wanted to examine if the yield obtained in the cascade
These molecules can be prepared in a few steps from readily reaction can be increased. Therefore, we tested different gold
accessible compounds. The use of two methyl groups in ortho catalysts in the first step (entries 1–8 in Table 1). Nevertheless,
position is necessary to avoid gold(I)-catalyzed cyclization to we were not able to improve the yield (47 %) that was obtained
the corresponding 2H-chromene; this type of reaction has by using [JohnPhosAu(NCMe)]SbF . The usage of sterically de-
6
[
31–34]
already been observed for terminal alkynes (Scheme 1c).
similar concept was used for propargylic ethers
manding ligands like XPhos led to distinctly lower yields for
A
1
polycycle 14a (30 %; entry 4). If another type of phosphine li-
[
18]
(
Scheme 1a). The undesired cyclization could actually be pre- gand like trimethylphosphine was employed, no conversion
vented and 2,5-dihydrofurans 2 emerged from gold(I)-catalyzed was observed (entry 6). If JohnPhosAuCl was used with
asymmetric dearomatization. Regarding haloacetylenes 12 and NaBArF24 as counterion, the main product was no longer poly-
1
3 adequate cyclization reactions are not expected as halogen cycle 14a but the multiple unsaturated system 15a (entry 3).
atoms are monovalent. This system was identified as allene by NMR spectroscopy. The
As test reaction the conversion of chloroacetylene 12 with use of gold(III) complex dichloro(2-picolinato)gold(III) led ex-
[
36]
[
35]
[37]
alkyne 5a and [JohnPhosAu(NCMe)]SbF6 as gold catalyst was clusively to allene 15a (entry 8). The use of an NHC ligand
investigated via NMR experiments in deuterated chloroform. ended up in a product mixture in which polycycle 14a and
Within only two hours at room temperature the starting mate- allene 15a were formed in a 2:1 ratio (entry 7). Thus, the initially
rial was completely consumed. Fortunately, one single main applied catalyst [JohnPhosAu(NCMe)]SbF turned out to be the
6
product (yield: 47 %) was obtained (see Figure S8 and Table 1). best choice. In the next step, the equivalents of both starting
The yields of all side products amounted to less than 5 %. How- materials and catalyst as well as temperatures and solvents
ever, the NMR spectra showed that not the expected enyne were varied (entries 9–17). The highest yield of polycycle 14a
dimer but a polycycle was formed. Structural investigations via (52 %) was obtained, when chloroacetylene 12 was converted
2
D NMR spectroscopy revealed that the main product can be with 2.5 mol-% of the catalyst and two equivalents of alkyne
ascribed to tetracycle 14a (Figures S9–S11). Analysis of the con- 5a at room temperature in chloroform.
nectivity in tetracycle 14a shows that five C–C bonds were
After optimization of the reaction conditions, we evaluated
formed in the course of the reaction. Additionally, one C–O the substrate scope of this reaction on a preparative scale.
bond was broken, and one hydrogen atom was rearranged. Therefore, we used different arylacetylenes (3–7; see Scheme 2)
Until now, we are not aware of any case in which so many in which both triple bond substituents were varied. At first, we
C–C bonds were formed by gold catalysis in one reaction cas- had a closer look at alkyl arylacetylenes 5. The length of the
cade, if the combination of two identical building blocks (e.g. alkyl chain plays no important role, if the second substituent is
[
13]
oligomerization) is not taken into account.
a phenyl group (14a–c). In all cases the yields amount to ca.
Table 1. Optimization of the reaction conditions for the gold(I)-catalyzed coupling of 12 with alkyne 5a.[a]
Entry
Catalyst
Conditions
Yield [%]
cycle
Conversion
[%]
allene
1
2
3
4
5
6
7
8
9
[JohnPhosAu(NCMe)]SbF
JohnPhosAuCl (5 mol-%)/AgNTf
JohnPhosAuCl (5 mol-%)/NaBArF24 (10 mol-%)
6
(5 mol-%)[35]
(10 mol-%)[37]
RT, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
3
3
3
3
3
3
3
3
3
3
47
37
9
30
38
–
24
–
52
–
43
40
26
37
44
49
34
–
–
42
–
–
100
97
100
92
100
0
97
100
100
21
100
100
100
100
100
100
100
2
[
37]
tBuXPhosAuNTf
2
(5 mol-%)
CyJohnPhosAuCl (5 mol-%)/AgSbF
6
(10 mol-%)
Me
3
PAuCl (5 mol-%)/AgSbF
6
(10 mol-%)
–
[
37]
IPrAuCl (5 mol-%) /AgNTf
2
(10 mol-%)
13
98
–
19
–
–
–
23
9
–
[
36]
Dichloro(2-picolinato)gold(III) (5 mol-%)
[JohnPhosAu(NCMe)]SbF
[JohnPhosAu(NCMe)]SbF
[JohnPhosAu(NCMe)]SbF
[JohnPhosAu(NCMe)]SbF
[JohnPhosAu(NCMe)]SbF
[JohnPhosAu(NCMe)]SbF
[JohnPhosAu(NCMe)]SbF
[JohnPhosAu(NCMe)]SbF
[JohnPhosAu(NCMe)]SbF
6
6
6
6
6
6
6
6
6
(2.5 mol-%)
(1 mol-%)
10
11
12
13
14
15
16
17
(2.5 mol-%)
(2.5 mol-%)
(2.5 mol-%)
(2.5 mol-%)
(2.5 mol-%)
(2.5 mol-%)
(2.5 mol-%)
RT, 2 h, C
6
D
6
RT, 2 h, DCM
RT, 2 h, DCE
–10 °C, 2 h, CDCl
0 °C, 2 h, CDCl
RT, 2 h, CDCl
RT, 2 h, CDCl
3
3
[
[
b]
c]
3
3
–
[
5
a] Yields were determined by 1H NMR using cyclooctane as internal standard. If not stated otherwise, the concentration of alkyne 13 was 0.15
a was added in 2 molar equivalents. [b] 3 molar equivalents of alkyne 5a were added. [c] 1 molar equivalent of alkyne 5a was added.
M and alkyne
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40 %. The yield was increased by a methoxy group in para posi- version to polycycle 14q was not complete. Even after one
tion (57 %, 14d), if a propyl group is attached to the triple bond week at room temperature or after a few days at 40 °C, the
as second substituent. A lower yield was found if the second allene and only small amounts of polycycle 14q (11 %) were
substituent is a methyl group (26 %, 14e).
found.
Next, we wanted to elucidate the mechanism of this remark-
able coupling reaction. In the first step, the role of the allene
was investigated; we wanted to find out whether this allene
embodies an intermediate or a side product. Therefore, we con-
verted chloroacetylene 12 without alkyne 5a using gold(III) as
catalyst. The allene 15a was isolated in 57 % yield (Scheme 3a).
Afterwards, we converted 15a under the optimized reaction
conditions with [JohnPhosAu(NCMe)]SbF as catalyst and 5a by
6
NMR experiments. Polycycle 14a was obtained in 55 % yield.
Thus, the formation of 15a via gold-catalyzed Claisen re-
arrangement can be described as the first sequence of the cas-
cade reaction.
Scheme 3. Gold(I)-catalyzed reaction of chloroacetylenes 12 and 16 in pres-
ence and absence of alkyne 5a.
Interestingly, the NMR yield of 14a after conversion of 15a
with alkyne 5a (55 %) was not much higher than the NMR yield
of 14a obtained in the reaction of 12 and 5a (52 %, Scheme 3a).
This means that the one-pot cascade reaction is almost as effi-
cient as the stepwise process. Please note, the reaction of 12
with [JohnPhosAu(NCMe)]SbF as catalyst without alkyne 5a led
6
to a complete conversion (Scheme 3a), but only a variety of
oligomeric compounds was formed, which could not be sepa-
rated by column chromatography. Therefore, we assume that –
Scheme 2. Evaluation of the substrate scope of the gold(I)-catalyzed coupling
of haloacetylenes 12 and 13 with alkynes 3–7.
+
utilizing JohnPhosAu as catalytic species – allene 15a (which
A methyl group in ortho position lowers the yield to 23 % is formed in the first step) reacts with the diene unit of the
14f). If the alkyl group in 5 is substituted by chlorine (3a), no cycle. This leads to the formation of oligomeric structures due
(
significant change was observed (40 %, 14h). The correspond- to the absence of intercepting alkynes. Another preparative
ing bromo derivative 14g was isolated in 13 % yield. Terminal mechanistic indication was achieved via conversion of 16 under
alkynes (6) are also tolerated; the corresponding products were the optimized reaction conditions. Bicycle 17 was obtained in
obtained in yields of ca. 25 % (14i, 14j and 14l) and 10 % (14k), 67 % yield whether alkyne 5a was present or not. The formation
respectively. Diarylacetylenes (7) form as well polycycles with of the product underlines the assumption that the allene unit
1
2. The resulting products were isolated in yields up to 77 %. is cyclized with the diene unit building a five-membered ring.
In these products the more electron-rich aromatic ring is bound The formed cation can be stabilized by deprotonation. Thus,
to the position which is closer to the carbonyl group. If bromo the second sequence of the cascade reaction probably starts
derivative 13 was used instead of chloroacetylene 12, the con- with the formation of a five-membered ring.
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The above-mentioned experimental indications were used to plex is accompanied by free-energy reduction amounting to
calculate the cascade reaction mechanism via quantum chemi- 15.4 kcal/mol (Figure 1). Please note that the coordination of
+
cal methods. Arylacetylene 12 and alkyne 6a were utilized JohnPhosAu to acetylene 6a (ΔG = –16.2 kcal/mol) is energeti-
as model substances. In order to optimize the geometrical pa- cally favored compared to the complexation of 12. However,
rameters of all stationary points, the density functional the reactions of the thusly formed complex with acetylenes 6a
[
38–40]
B3LYP
Johnson damping
-31G(d) was applied for the elements C, H, O, P, and Cl, 18a, which is formed by coordination of JohnPhosAu to acetyl-
together with the dispersion correction via Becke– and 12, respectively, require high activation energies, making
[
41]
(D3BJ) was employed. As basis sets the complexation of 6a a dead end (Figures S8–9). Complex
+
6
whereas def2-TZVP was used for Au. Furthermore, single-point ene 12, was used as reference for all subsequent reaction steps.
calculations on the optimized structures were performed using The rate-determining step of the first cascade reaction se-
B3LYP-D3BJ with basis sets 6-311++G(d,p) (for C, H, O, P, and Cl) quence (allene formation) is the cyclization to oxonium ion 20a
and def2-TZVP (for Au). To take solvent effects into account, (10.0 kcal/mol). This ion converts into gold(I) allene complex
chloroform was considered as reaction solvent by using the 22a in the second step. Therefore, a barrier of 7.4 kcal/mol
[
42]
SMD model.
needs to be overcome.
The first sequence of the cascade reaction is the formation
of the allene starting with chloroacetylene 12 and JohnPhosAu
After that, two reaction paths are possible: on the one hand,
decomplexation can take place leading to the catalytic species
+
as catalytic-active species. The coordination of the gold(I) com- and allene 15a (this allene formation corresponds to the end
Figure 1. Reaction pathway of the gold(I)-catalyzed dearomatization of chloroacetylene 12. The indicated free-energy values (ΔG in kcal/mol) were calculated
using B3LYP-D3BJ(SMD) and are relative to gold complex 18a. [Au]+ = JohnPhosAu .
+
Figure 2. Reaction pathway of the addition of acetylene 6a to cation 24a. The indicated free-energy values (ΔG in kcal/mol) were calculated using B3LYP-
+
+
D3BJ(SMD) and are relative to acetylene 6a and cation 24a. [Au] = JohnPhosAu .
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Figure 3. Possible reaction mechanism for the gold(I)-catalyzed dearomatization (black and green arrows) leading to allene 15a and the allene–diene–alkyne
coupling (blue arrows) leading to polycycle 14i.
of the first cascade reaction sequence). On the other hand, the
In sum, we were able to show that polycycles can be de-
formation of bicyclic cation 24a can occur initiating the second signed from haloacetylenes and alkynes by gold(I) catalysis. In
cascade reaction sequence (allene–diene–alkyne coupling). Ac- our case, the haloacetylene was an o,o-dimethyl-substituted
cording to calculations, the second sequence (intramolecular propargyl ether. The reaction tolerates a wide range of alkyne
cyclization) is more favored (12.5 kcal/mol) than the formation substrates like alkylaryl, diaryl, halo- and terminal alkynes. The
of free allenes (15.3 kcal/mol). This agrees well with the results achieved preparative yields amount up to 77 %. Experimental
showing a polymerization in case of converting 12 in absence and quantum chemical investigations showed that the mecha-
+
of 5a with JohnPhosAu as catalytic species.
nism can be described as cascade reaction with two sequences.
Let us now consider the second cascade reaction sequence The first sequence is a gold(I)-catalyzed dearomatization in
allene–diene–alkyne coupling): the first step is again rate- which the corresponding allene is built from propargyl ether by
(
determining and corresponds to the addition of alkyne 6a to formal Claisen rearrangement (isolation of the allene is possi-
cation 24a with formation of vinyl cation 26a (Figure 2). The ble). The second sequence is a new reaction type: four new
activation energy for this step amounts to 9.1 kcal/mol. The C–C bonds are formed by an allene–diene–alkyne coupling
vinyl cation is stabilized by the phenyl ring, which also explains leading to the corresponding polycycle. Altogether, even five
the regioselectivity of the attack of 6a to 24a. A possible com- C–C bonds are newly formed during the whole cascade. The
peting reaction to the addition of alkyne 6a to cation 24a is described allene–diene–alkyne coupling reaction has the po-
the addition of alkyne 12 to cation 24a. However, the required tential to exploit new ways of polycycle formation starting from
activation energy for the latter reaction path is distinctly higher simple building blocks. Especially, the option of expanding the
(
15.5 kcal/mol; Figure S10). In the next step, two intramolecular reaction to obtain intermolecular products is promising and de-
C–C bonds are simultaneously formed leading to gold(I)-stabi- sires further exploration.
lized cation 28a. Proceeding from this σ complex, π complex
Supporting Information (see footnote on the first page of this
0a is formed by a 1,2-hydride shift. Decomplexation leads to article): Figures and Tables syntheses of the new compounds, NMR
3
the final product 14i.
In Figure 3 the most important intermediates are illustrated
in a catalyst cycle. The whole cycle consists of two sequences;
experiments, computational details, cartesian coordinates and abso-
lute energies for all calculated compounds, H NMR and C NMR
spectra of the new compounds.
1
13
in both gold(I) complexed allene 22a is passed. Allene forma- Conflict of Interest
tion takes place via two-step gold-catalyzed Claisen-like rear-
rangement (black in Figure 3). Allene 22a functions as origin of
two branches: on the one hand, the formation of allene 15a is
enabled by decomplexation (green in Figure 3), on the other
There is no conflict of interest to declare.
Acknowledgments
hand, the intramolecular 5-endo-trig-cyclization to 24a is possi- This work was supported by the Deutsche Forschungsgemein-
ble (blue in Figure 3). Alkyne 6a can be added to intermediate schaft (DFG; HA 2973/17-1). The authors would like to thank
2
3
4a ending up in the formation of gold complexed polycycle Adrian Schützmann, Petra Schneider and Katrin Steiger for syn-
0a via two further steps. Decomplexation leads to final prod- thetic support. Open access funding enabled and organized by
uct 14i and finishes the catalyst cycle.
Projekt DEAL.
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Keywords: C–C bond formation · Cyclization ·
2
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published by Wiley-VCH GmbH