Gold(I)-catalyzed synthesis of dihydrodibenzoquinolizinium salts

Article abstract of DOI:10.1002/adsc.201700126

A gold-catalyzed cyclization of 1-alkynyl-2-aryl tetrahydroisoquinolines is described for the synthesis of novel dihydrodibenzoquinolizinium salts. The reaction mechanism is likely to involve a 6-endo-dig cyclization and subsequent oxidation by air to give a relatively stable arylgold intermediate. This gold species undergoes protodeauration under acidic conditions to afford the title compounds. An NMR study was performed to gain further evidence and insight on the presence of the arylgold intermediate and the reaction mechanism. (Figure presented.).

Full text of DOI:10.1002/adsc.201700126

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DOI: 10.1002/adsc.201700126
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Gold(I)-catalyzed synthesis of dihydrodibenzoquinolizinium salts
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Niels Mariena and Guido Verniesta,*
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Research Group of Organic Chemistry, Department of Chemistry and Department of Bio-engineering Sciences, Faculty of
Science and Bio-engineering Sciences, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium
E-mail: guido.verniest@vub.be
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Received: February 2, 2017; Revised: April 19, 2017; Published online: && &&, 0000
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14 Dedicated to Professor Dirk Tourwe on the occasion of his 70 birthday.
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Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201700126
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relevant polyheterocycles,[7] we envisaged these 1-
alkynyl-2-aryl tetrahydroisoquinolines 1 as promising
starting materials for the synthesis of tetrahydrodiben-
zoquinolizines or their oxidized analogues.
Abstract: A gold-catalyzed cyclization of 1-alkyn-
yl-2-aryl tetrahydroisoquinolines is described for
the synthesis of novel dihydrodibenzoquinolizinium
salts. The reaction mechanism is likely to involve a
6-endo-dig cyclization and subsequent oxidation by
air to give a relatively stable arylgold intermediate.
This gold species undergoes protodeauration under
acidic conditions to afford the title compounds. An
NMR study was performed to gain further evidence
and insight on the presence of the arylgold inter-
mediate and the reaction mechanism.
Keywords: Cyclization; Gold; Homogeneous catal-
ysis; Nitrogen heterocycles; Oxidation
Scheme 1. Proposed cyclization of 1 to polycyles 2.
Results and Discussion
The aforementioned starting materials 1 were synthe-
Introduction
sized from commercially available substrates in a
39 Homogeneous gold catalysis has proven to be an simple two-step process. A Cu(I)-catalyzed cross-
40 increasingly valuable addition to the organic chemist’s coupling of tetrahydroisoquinoline (THIQ) with an
41 toolbox.[1] In particular, the activation of triple bonds aryliodide afforded N-arylated THIQs.[8] A subse-
42 towards a nucleophilic attack has opened synthetic quent cross dehydrogenative coupling with a terminal
43 pathways to a large number of biologically active alkyne, afforded the desired substrates 1.[9]
44 motifs.[2] A number of the pioneering reactions in this
One of the available substrates 1a was chosen as a
45 field started from a-heteroatom substituted alkynes.[3] model substrate and subjected to various catalytic
46 The recent surge in “cross-dehydrogenative coupling” systems, as summarized in Table 1
47 (CDC) reactions will undoubtedly boost this research
Initial attempts (entries 1–4) provided a proof-of-
48 further as it offers facile entries to these particular concept as a transformation of 1a towards 3a was
49 starting materials.[4] For example, substituted N-phenyl observed, although in relatively low conversions.
50 propargylamines are readily available via this method- When AuPPh3 type catalysts were evaluated, the
51 ology and can be transferred chemoselectively to formation of an intermediate arylgold species 4 was
52 quinolines[5] and indoles[6] via a 6-endo-dig or 5-exo- detected in the reaction mixture pointing to a slow
53 dig cyclization respectively. To our knowledge, related protodeauration as a final step towards 3a. This
54 1-alkynyl-2-aryl tetrahydroisoquinolines 1 have never unexpected compound 4 could be isolated in trace
55 been employed as starting materials in a cyclization amounts via preparative HPLC and was analyzed by
56 strategy (Scheme 1). Due to our group’s ongoing LC-MS and NMR. In subsequent attempts, a number
57 interest in employing gold-catalyzed reactions towards of variables were tested to get an insight in the
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present in air. An oxygen atmosphere or the addition
of external oxidants was also found to be unsuitable to
drive the conversion to completion (entry 6–7; both
under dry conditions). This further strengthened our
hypothesis that protodeauration of 4 to 3a was the
bottleneck in this reaction.
Addition of an equivalent of trifluoroacetic acid
(entry 8) to the reaction mixture facilitated the
protodeauration step,[12] significantly boosting the
observed conversion to 75%. Further increasing the
amount of acid gave an adverse effect on the
conversion (entry 9). Control experiments using only
AgNTf2 and/or TFA resulted in low or no conversion
(entries 10–11), proving the necessity for the gold
catalyst to enable a clean conversion at room temper-
ature.
In an attempt to increase conversions further, TFA
was substituted by the stronger methanesulfonic acid.
Further optimization of the amount of equivalents of
MsOH used (entry 15) resulted in excellent conver-
sions towards 3a. Comparing entries 2 and 15, the
addition of MsOH proved to be essential for increas-
ing conversion. Remarkably, this increase was not
observed when a gold (III) catalytical system was
used, as shown in entries 3 and 17. Taking into account
that MsOH is a much stronger acid than TFA, two
additional control experiments (entries 12–13) were
performed but no conversion towards 3a was de-
tected.
Considering that water as a proton source was
sufficient to get low conversion, it was also tested if a
biphasic approach (i.e. a large excess of water) could
also afford the desired polycyclic 3a. While conver-
sions were lower as compared to the addition of acids,
we observed that 3a showed good solubility in
aqueous media. This observation was used to facilitate
workup: after quenching the slight excess MsOH with
a solid-supported base, 3a was extracted into the
aqueous phase. Removal of water via evaporation or
freeze-drying afforded the desired 3a deemed pure via
HPLC/NMR, effectively removing the need for tedi-
ous chromatographic purification.
Table 1. Evaluation of catalytical systems for conversion of
1a to 3a.
Entry Conditions
Conversion (%)
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2
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AuPPh3Cl/AgOTf
AuPPh3Cl/AgNTf2
Au(pic)Cl2
AuCl3
AuPPh3Cl/AgNTf2, Ar atm.
AuPPh3Cl/AgNTf2, O2 atm.
AuPPh3Cl/AgNTf2, 1eq DDQ, Ar
atm.
AuPPh3Cl/AgNTf2, 1eq TFA
AuPPh3Cl/AgNTf2, 2eq TFA
<20
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<20
Trace
Trace
Complex mix-
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ture
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0
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10 AgNTf2, 1eq TFA
11 1eq TFA
12 1eq. MsOH
0
13 Excess MsOH
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14 AuPPh3Cl/AgNTf2, 1eq MsOH
15 AuPPh3Cl/AgNTf2, 1.1eq MsOH
16 AuPPh3Cl/AgNTf2, 1.5eq MsOH
17 Au(pic)Cl2, 1eq MsOH
18 AuPPh3Cl/AgNTf2, H2O
85
94
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[a] Test reactions were typically performed on a 75 mmol scale
in CH2Cl2 at room temperature for 16 h under air
atmosphere, using 10 mol% of (coÀ)catalyst.
[b] Conversion as observed via HPLC analysis.
Subsequently, a number of 1-alkynyl-2-aryl tetrahy-
droisoquinolines 1 were subjected to the optimized
reaction conditions to investigate the scope of the
reaction (Scheme 3, Table 2). Structures similar to the
one used for catalyst screening (3a–c, Table 2) were
Scheme 2. Evaluation of catalytic systems.
48 reaction mechanism and optimize conversion. The
49 obtained 3a can be related to annulated quaternary
50 quinolinium compounds (e.g. berberine), which repre-
51 sent an important class of natural products with a
52 wide array of bioactivities.[10] To our surprise, there are
53 only a few reports available towards these polyhetero-
54 cycles 3.[11]
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Performing the reaction under inert atmosphere
56 (entry 5) resulted in a lack of conversion towards 3a,
57 suggesting the need for either oxygen or moisture
Scheme 3. Scope evaluation using optimized conditions.
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isolated in decent yields. An ester and alkyl group where oxidation occurs first, followed by cyclization
were also tolerated (Entries 3d–e), where it is note- (e.g. by electrocyclization) is not probable given the
worthy that the electron withdrawing ester moiety did negative results obtained by performing the reaction
not shift the reactivity towards attack at the b-carbon. under oxygen atmosphere and the fact that no
Removal of the electron donating group at the N-aryl arylgold species 4 would be obtained in this case. Also
group (R2) still resulted in the formation of 3f a [3,3]-sigmatropic rearrangement as reported by
although a lower conversion and isolated yield was Blechert for N-arylpropargylamines is unlikely be-
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observed.
cause the reaction proceeds at room temperature and
a strained cyclic allene would be formed as intermedi-
ate.[14] The formation of vinyl- and arylgold species as
catalytic resting states has been described and can give
rise to isolable monoaurated, diaurated or Au/Ag-
dimetallated species.[13,15] In our case no dimetalated
compounds were observed via MS or 31P NMR
analysis of the reaction mixture in CD2Cl2. This
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Table 2. Evaluation of scope towards 3.
Product
R1
R2
Isolated yield (%)
3a
3b
3c
3d
3e
3f
Ph
OMe
OMe
OMe
OMe
OMe
H
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4-MePh
3-FPh
COOEt
Butyl
Ph
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corroborates with the findings of Gagne and Ham-
mond that monometallated species are generally
observed with electron deficient aryl or vinyl species.
The arylgold species 4 is then protodeaurated
under acidic conditions to afford the desired end
A plausible mechanism for the gold-catalyzed product 3a. This proposed mechanism is consistent
22 cyclization is shown in Scheme 4. Activation of the with the observed necessities for air and acid to be
23 triple bond in 5 by the cationic gold catalyst, via the present in the reaction.
24 free base 1a present in the mixture through acid-base
An NMR study was performed in an attempt to
25 equilibrium could facilitate a nucleophilic attack by gain further evidence for the proposed mechanism.
26 the aromatic system, resulting in a 6-endo-dig cycliza- An equimolar amount of a preformed and filtered
27 tion. The presence of acid also helps in preventing gold catalyst in CD2Cl2 was added to a solution of 1a
28 deactivation of the catalyst by formation of complexes in CD2Cl2 and stirred at room temperature. The
1
29 such as 6. Considering the observation of arylgold reaction was followed via H, 13C and 31P NMR on
30 species 4 in several reaction mixtures, a swift oxidation regular intervals. After one hour, near full consump-
31 of the proposed intermediate 8 to 4 gives rise to tion (>95%) of 1a was observed. The reaction
32 aromatic stabilization.[13] A reverse order of events mixture appeared to consist of 4 and 3a in a 1:2.5
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ratio. Several NMR spectra were indeed in agreement
with the proposed arylgold intermediate 4: significant
deshielding of nearby aromatic protons, a doublet in
3
13C NMR with a JC-P coupling constant similar to that
of other CÀAu-P systems and a singlet at 40 ppm in
31P NMR (Figure 1, Scheme 5).[15a,b] The significant
Scheme 4. Proposed mechanism for gold-catalyzed cycliza-
tion of 1a to 3a.
Scheme 5. NMR study to validate the formation of arylgold
species 4.
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amount of protodeauration already observed here
In the optimized reaction conditions, MsOH and
could be explained by the formation of HNTf2 in situ. most probably also traces of water are present from
the very start of the reaction. Therefore, the NTf2
counterion of the used catalysts is likely to be
interchanged with present coordinating groups. It is
known that PPh3AuNTf2 reacts with water to give less
reactive PPh3AuOH which can further lead to dimeric
hydroxonium or trimeric (Ph3PAu)3O+ species with a
characteristic peak at 23.9 ppm in 31P NMR.[18] In our
case, we did not observe such species because of the
presence of the Bronsted acid which is known to
reactivate formed PPh3AuOH.[19] As such the addition
of MsOH to the mixture could perform a triple role in
this reaction: reactivation of the catalyst, enhancing
the protodeauration rate and neutralizing the basic
tetrahydroisoquinoline nitrogen which has been linked
to deactivation of the gold catalyst in related sys-
tems.[20]
Figure 1. 31P NMR spectra in CD2Cl2 for (a) Ph3PAuNTf2, (b)
PPh3AuNTf2 after several days in CD2Cl2, (c) Reaction of
20 1a, and 1 eq. PPh3AuNTf2 after 1 h, (d) Reaction of 1a, and
Conclusion
In conclusion, a gold-catalyzed cyclization of 1-
alkynyl-2-aryl tetrahydroisoquinolines towards novel
dihydrodibenzoquinolizinium salts was developed.
1 eq. PPh3AuNTf2 after 1 h, followed by quenching with
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TFA.
The reaction mixture was subsequently quenched These compounds could be isolated from the reaction
26 via addition of 1 equivalent of TFA. Full protodeaura- mixture via a simple extraction protocol, avoiding the
27 tion of 4 to 3a occurred in ~30 min. As can be seen need for chromatographic purification. The reaction
28 from Figure 1c and 1d, significant catalyst decomposi- mechanism is likely to involve a 6-endo-dig cyclization
29 tion is observed during the reaction shown by the and subsequent oxidation by air to a relatively stable
30 presence of a signal at 45 ppm in 31P NMR, which is arylgold intermediate that undergoes protodeauration.
+
31 most probably a Au(PPh3)2 species.[16] This hypoth- The presence of this arylgold intermediate could be
32 esis was strengthened further by acquisition of a direct demonstrated via MS and NMR.
33 ESI-MS spectrum from the NMR sample, which
34 cleanly showed an m/z of 338 (3a) and 721 (Au
35 (PPh3)2 ). This is in correspondence with the known
Experimental Section
+
The gold-catalyzed cyclization of 1a to 3a is given as a
representative example.
36 decomposition of PPh3AuOTf where it has been
37 shown that the presence of p-substrates and water
38 significantly accelerates decomposition to amongst
(Ph3P)AuCl (14.6 mg, 0.029 mmol) and AgNTf2 (11.4 mg,
0.029 mmol) were dissolved in dichloromethane (2.9 mL)
and allowed to stir at room temperature for 15 minutes,
resulting in the precipitation of AgCl. To this suspension was
+
39 others Au(PPh3)2 species.[16]
40
When a similar reaction was performed under basic
41 conditions (1 equiv. of NEt3), the protodeauration step
42 slowed down significantly, and the gold intermediate 4 added 1a (100.0 mg, 0.29 mmol). After an additional 5
minutes of stirring, methanesulfonic acid (21 mL, 0.32 mmol)
was added and the resulting reaction mixture was allowed to
stir overnight. The acidic excess was quenched via addition
of a solid supported base (ISOLUTE Si-trisamine, loading
1.18 mmol/g; 127 mg, 0.15 mmol) and this mixture was
filtered through a small cotton plug. The organic phase was
then extracted with distilled water (53 3 mL). The combined
water phases were freeze-dried to afford 3a as an orange
solid in 55% yield (64.0 mg, 0.15 mmol). IR (neat) 1615,
1596, 1560, 1383, 1277, 1238, 1221, 1209, 1039, 1020, 821, 784,
43 could be isolated via chromatography from the
44 reaction mixture.[17]
45
In an attempt to detect intermediate vinyl gold
46 species 8, a reaction in an NMR-tube under Ar
47 atmosphere was performed in the presence of Et3N
48 (1 equiv.) to prevent protodeauration. After reaction
49 for 4 h, the mixture contained next to starting material
50 and the arylgold 4 also an unidentifiable reaction
51 intermediate (ratio 1:2:1, resp.). The latter disap-
52 peared after prolonged standing of the sample result-
53 ing in only 4 and 3a (leaving a gold mirror on the
54 NMR tube after 2 days). Due to the complexity of the
55 obtained NMR spectrum and lability of this inter-
56 mediate, the intermediacy of vinylgold species 8 could
57 not be proven.
1
755 cmÀ1. H NMR (250 MHz, CDCl3) d 9.23 (d, J=9.9 Hz,
1H), 8.09 (s, 1H), 8.02–7.94 (m, 2H), 7.68–7.59 (m, 5H), 7.56–
7.48 (m, 2H), 7.36 (d, J=2.9 Hz, 1H), 5.51 (t,=6.9 Hz, 2H),
3.88 (s, 3H), 3.60 (t, 6.8 Hz, 2H), 2.80 (s, 3H). 13C NMR (125
MHz, CDCl3) d 160.2, 156.5, 148.3, 137.0, 135.8, 135.5, 134.5,
130.9, 129.6, 129.3, 129.0, 128.8, 127.9, 127.2, 122.9, 119.9,
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107.6, 56.3, 49.1, 39.7, 27.0. HRMS M+ calculated 338.1539
found 338.1539.
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[11] a) R. Salsmans, G. Van Binst, Tetrahedron, 1974, 30,
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1976, 85, 11; c) M. Cꢄzkovꢁ, V. Kolivoska, I. Cꢄsarovꢁ,
ˇ
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The authors acknowledge the Vrije Universiteit Brussel
(starting grant for GV and AAP-PhD Fellowship for NM)
and the Research Fund-Flanders (FWO-KN249) for financial
support.
D. Saman, L. Pospꢄsil, Filip Teply, Org. Biomol. Chem.
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