Gold(I) - Catalyzed domino reaction of aziridinyl alkynes

Article abstract of DOI:10.1002/chem.201000628

Chemical Equation Presented Aziridinyl alkynes strike gold: A novel gold(I)-catalyzed domino transformation of aziridinyl alkynes with arenes to construct 1,2,3,4-tetrahydroisoquinoline and 3,4-dihydroisoquinoline structural motifs (see scheme), especially sterically congested Syn-3,4-disubsti contzonetuted 1,2,3,4-tetrahydroisoquinolines, is described. A plausible mechanism proceeding through a benzylic cation is given based upon deuterium-labeling and control experiments as well as the observed diastereoselectivities.

Full text of DOI:10.1002/chem.201000628

COMMUNICATION
DOI: 10.1002/chem.201000628
Gold(I)-Catalyzed Domino Reaction of Aziridinyl Alkynes
Zhen Zhang[a] and Min Shi*[a, b]
A novel gold(I)-catalyzed domino transformation of aziri-
arylamines in good yields.[4] Herein, we report a novel gold-
catalyzed tandem reaction of aziridinyl alkynes that leads to
1,2,3,4-tetrahydroisoquinoline and 3,4-dihydroisoquinoline
structural motifs under mild conditions.[5,6]
dinyl alkynes with arenes to construct 1,2,3,4-tetrahydroiso-
quinoline and 3,4-dihydroisoquinoline structural motifs, es-
pecially sterically congested syn-3,4-disubstituted 1,2,3,4-tet-
rahydroisoquinolines, is described herein. A plausible mech-
anism that proceeds through a benzylic cation is presented
on the basis of deuterium-labeling and control experiments
as well as the observed diastereoselectivities.
Preliminary experiments directed towards optimizing the
conditions of the transformation were carried out with aziri-
dinyl alkyne 1a in the presence of silver or gold salts, as
well as combinations of Au and Ag, in toluene (2a), at 208C
(room temperature). We found that, by using a silver salt,
AuCl3, AuCl3/AgOTf, NaAuCl4·2H2O, or HAuCl4·2H2O as
the catalyst, the reaction afforded the ring-opened product
3a in moderate to good yields, but none of the desired prod-
uct 4a (Table 1, entries 1–7). Compound 3a was obtained as
the sole product, in 82% yield, in the presence of
HAuCl4·2H2O (5 mol%) after 30 min and in 88% yield in
the presence of 20 mol% of this catalyst within 2 min
(Table 1, entries 6 and 7, respectively; see the Supporting In-
formation for some other examples). However, the com-
Gold-catalyzed domino reactions are powerful tools in or-
ganic synthesis utilized to access complex molecular frame-
works that have drawn increasing attention.[1] One of the
most interesting aspects of this field is the introduction of
strained small-ring systems as molecular building blocks to
initiate such domino reactions, because the relief of ring
strain provides a potent thermodynamic driving force. Since
the first example of a gold-catalyzed transformation of cy-
clopropane was reported by Thomas in 1976,[2] the number
of gold-catalyzed reactions that use strained small rings, in-
cluding cyclopropane, cyclopropene, oxirane, aziridine, and
so on, has increased rapidly. It is known that aziridine is a
versatile building block for the synthesis of many nitrogen-
containing biologically active molecules. Among the proce-
dures for the ring opening of aziridines, nucleophilic ring-
opening reactions are one of the major routes to form
highly functionalized compounds.[3] Moreover, a highly re-
giospecific AuCl3/AgOTf-catalyzed ring-opening reaction of
aziridines by arenes has recently been reported, affording b-
bined catalyst [AuACTHNUTRGNEUNG(PPh3)Cl]/AgOTf (5 mol%) produced 3a
in 35% yield, along with the domino-reaction product 4a in
44% yield, at 08C (Table 1, entry 8). Further optimization
of the reaction conditions revealed that the use of [Au-
AHCTUNTGRENGUN(N PPh3)Cl]/AgSbF6 (10 mol%) as the catalyst produced 4a in
79% yield, as the sole product, at room temperature (208C;
Table 1, entry 15) and that increasing the reaction tempera-
ture to 40 or 608C did not further improve the reaction out-
come (Table 1, entries 18 and 19).
Under the optimized conditions, it was found that, for a
variety of arenes, such as benzene (2b), 1,4- and 1,2-xylene
(2c and d), mesitylene (2e), 1,2- and 1,4-dimethoxybenzene
[a] Z. Zhang, Prof. M. Shi
(2 f and g), and benzoACTHNUTRGNE[NUG 1,3]dioxole (2h), the reaction pro-
Laboratory for Advanced Materials & Institute of Fine Chemicals
East China University of Science and Technology
130 Mei Long Road, Shanghai 200237 (China)
ceeded smoothly to give the desired 1,2,3,4-tetrahydroiso-
quinolines 4b–h in moderate to high yields, as a sole prod-
ucts (Table 2, entries 1–7). By using furan (2i) as the electro-
[b] Prof. M. Shi
phile, the reaction afforded 4i in 44% yield if [AuACTHNUTRGNEUNG(PPh3)Cl]/
AgOTf (10 mol%) was used as the catalyst (Table 2,
entry 8), because 4i was formed in low yield in the presence
of [AuACHTNUTGRENUNG(PPh3)Cl]/AgSbF6. The examination of naphthalene
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (China)
Fax : (+86)21-64166128
(2j) and anisole (2k) in this domino reaction revealed that
the corresponding two regioisomers 4j (a- and b-4j) and 4k
(ortho- and para-4k) were formed in good total yields
E-mail: Mshi@mail.sioc.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000628.
Chem. Eur. J. 2010, 16, 7725 – 7729
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1. Optimization of the reaction conditions.[a]
Scheme 1. The removal of the para-nitrobenzenesulfonyl group from
compounds 4; Mes=mesityl.
line derivative, presumably due to the steric and electronic
properties of 2 f (Scheme 2). The ring-opened intermediate
of the reaction of electron-rich arene 2 f with 1 f may more
easily undergo nucleophilic attack of the aromatic carbon to
the gold(I)-activated alkyne than attack by the nitrogen
Entry
Catalyst [(mol%)]
T [8C]
t [min]
Yield [%][b]
3a[c]
4a[c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
AgOTf (5)
AgSbF6 (5)
AuCl3 (5)
AuCl3/AgOTf (5)
NaAuCl4·2H2O (5)
HAuCl4·4H2O (5)
HAuCl4·4H2O (20)
RT
RT
RT
RT
RT
RT
RT
0
RT
RT
RT
0
120
50
60
30
150
30
2
15
40
120
120
60
70
30
50
1 day
30
80
30
33
75
67
41
37
82
88
35
16
54
23
14
55
–
–
–
–
–
–
–
–
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
[Au
N
44
57
–
17
47
8
65
79
23
66
76
78
0
RT
RT
RT
60
40
60
–
23
–
–
–
Scheme 2. The gold(I)-catalyzed reaction of aziridine 1 f with arene 2 f.
[a] Unless otherwise stated, all reactions were conducted with 1a
(0.200 mmol) with the catalyst in toluene (1.0 mL). [b] Isolated yield
based upon aziridine 1a. [c] The product includes traces of a regioisomer.
[d] IPr=1,3-diisopropylimidazolylidene. [e] The reaction was run in
CH3NO2 (1.0 mL) with toluene (1.2 equiv).
atom in the amine moiety, since this nucleophilic attack is
sterically favored and the aromatic ring is very electron-rich
in this particular case. Alternatively, an aryl–gold complex
of the electron-rich arene with gold(I) may be formed,
which may be responsible for this intramolecular hydroary-
lation reaction, although the formation of an aryl–gold com-
plex is not proven at the present stage.[9]
To verify the reaction pathway, a control experiment was
performed using 3a as the starting material and it was found
that 4a could be obtained in 98% yield by using [Au-
(Table 2, entries 9 and 10). However, electron-deficient
arenes, such as bromobenzene (2l) and nitrobenzene (2m),
are not suitable substrates for this domino reaction, since
complex product mixtures were formed under the standard
conditions (Table 2, entries 11 and 12).
Under the optimized conditions, it was also found that,
for a variety of aziridinyl alkynes 1, the reactions proceeded
smoothly with arenes 2e, a, f, or h to give the corresponding
tetrahydroisoquinoline derivatives in moderate to good
yields. For the substituted aziridines 1d and h–k, the prod-
ucts 4n, s–v, and x were obtained as syn- and anti-isomeric
mixtures (Table 3, entries 3, 8–11, and 13).[7] By using aziri-
dinyl alkyne 1l [R1 =trimethylsilyl (TMS)] as the substrate
the reaction afforded 4w in 79% yield, in which the elimina-
tion of the TMS group occurred during the reaction with 2e
(Table 3, entry 12).
ACHTUNGTRENUN(NG PPh3)Cl]/AgSbF6 as the catalyst, indicating that compounds
3 are intermediates in this domino reaction. Moreover, deu-
terium labeling experiments were carried out utilizing 1a as
the substrate in [D6]benzene ([D6]-2b), under the standard
conditions. The corresponding product [D]-4b was formed
in 50% yield with 20% of this product containing D as the
olefinic proton ([D6]-4b), supporting the internal hydrogen
transfer from the arene to the final product as shown in
Scheme 3 (see the Supporting Information for more de-
tails).[10]
The para-nitrobenzenesulfonyl group (Ns) can easily be
removed from products containing it by treatment with thio-
phenol in the presence of K2CO3. This gives access to the
corresponding 3,4-dihydroisoquinolines 5, in good yields
(Scheme 1).[8]
It should be noted that the reaction of 1 f with 2 f gave
product 6 rather than the corresponding tetrahydroisoquino-
Scheme 3. A deuterium-labeling experiment.
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Aziridinyl Alkynes
COMMUNICATION
Table 2. Gold(I)-catalyzed reaction of aziridine 1a with arenes 2b–m.[a]
ionic structures (R)-7 and (S)-7
(Scheme 4).[11a] The preferred
direction of attack from ArH
should correlate to the pre-
ferred conformation and occurs
from the bottom face of the
plane due to steric hindrance.
For the zwitterionic intermedi-
ates (R)-7 and (S)-7, syn-3 is
formed preferentially, which is
then smoothly transformed into
the corresponding final product
syn-4.
On the basis of the afore-
mentioned experiments, a plau-
sible reaction mechanism is out-
lined in Scheme 5. The initial
Entry
1
Arene
Product
Yield [%][b]
52
2b
2c
4b, R1 =R2 =R3 =R4 =H
2
3
4c, R1 =R3 =Me, R2 =R4 =H
4d, R1 =R2 =H, R3 =R4 =Me
79
2d
2e
2 f
97[c]
À
step is the breaking of the C N
4
5
4e, R1 =R2 =R4 =Me, R3 =H
98
bond in the presence of a Lewis
acid, such as AgSbF6, to give
the corresponding zwitterionic
intermediate 7.[12] Then, the
4 f, R1 =R2 =H, R3 =R4 =OMe
4g, R1 =R3 =OMe, R1 =R2 =H
78, 63[d]
Friedel–Crafts reaction of
a
6
7
2g
2h
71, 55[d]
69[d]
benzylic cation with arene 2
takes place to afford intermedi-
ate 8, which gives intermediate
4h, R3 =R4 =OCH2, R2 =R4 =H
4i
3
through an intramolecular
proton transfer.[13] The gold(I)-
catalyzed intramolecular nucle-
ophilic addition of the nitrogen
atom to the alkyne takes place
via intermediate 9 to produce
intermediate 10, which affords
the corresponding final product
4 and eliminates the gold cata-
lyst.
8
2i
44[e]
a-4j
b-4j
9
2j
60[d] (1.5:1)
97 (1.7:1)
ortho-4k, R1 =OMe, R2 =R3 =R4 =H
para-4k, R4 =OMe, R1 =R2 =R3 =H
In summary, we have devel-
10
11
12
2k
2l
oped
a novel gold-catalyzed
domino reaction of aziridinyl
alkynes with various arenes to
give the corresponding 1,2,3,4-
tetrahydroisoquinoline and 3,4-
dihydroisoquinoline derivatives
in moderate to good yields,
under mild conditions, with syn-
stereoselectivities. The reaction
mechanism has been discussed
and is based upon deuterium-
labeling and control experi-
ments, as well as the observed
diastereoselectivities. The mod-
[f]
–
–
–
[f]
2m
–
[a] All reactions were conducted with aziridine 1a (0.200 mmol), [AuACTHUNTRGNE(UNG PPh3)Cl]/AgSbF6 (10 mol%) in 2 as the
solvent (1.0 mL) at 608C for 30 min unless otherwise specified. [b] Isolated yield based upon aziridine 1a.
[c] The product includes traces of a regioisomer. [d] The reaction was run with 2 (2.0 equiv) in dichloroethane
(DCE) (1.0 mL) at 608C for 30 min. [e] [AuACTHNUTRGNEUNG(PPh3)Cl]/AgOTf (10 mol%) was used as the catalyst for 30 min at
RT. [f] Complex mixture of products.
According to the stereochemistry of the products 4n, s–v,
and x (shown in Table 3), syn-stereoselectivity is favored in
this reaction. This stereochemical outcome can be rational-
ized by using the Friedel–Crafts reaction mechanism of a
benzylic cation, according to the literature.[11] The preferred
reaction conformations should, therefore, resemble zwitter-
erate to high diastereoselectivities can be rationalized by
conformational restriction of the corresponding benzylic
cation. Further investigations on this kind of domino reac-
tion, staring from chiral aziridines or other highly strained
small rings, are ongoing in our laboratory.
Chem. Eur. J. 2010, 16, 7725 – 7729
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www.chemeurj.org
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M. Shi and Z. Zhang
Table 3. The gold(I)-catalyzed reaction of aziridines 1b–l with arenes 2e, a, f, or h.[a]
the solvent evaporated under reduced
pressure and the residue purified by
flash column chromatography (silica
gel; eluent: EtOAc/petroleum ether
1:20). Compound 4a (80.0 mg) was
isolated as
yield.
a colorless oil in 78%
General procedure for the removal of
the para-nitrobenzenesulfonyl group
from 4: K2CO3 (0.570 mmol, 78.7 mg)
and PhSH (0.380 mmol, 39 mL) were
Entry
1 (R1/R2/R3) (trans/cis)[b]
Arene
t
Yield
[%][c]
d.r[b]
ACHTUNGTRENUN(NG syn/anti)
added to
a solution of 4p (0.19
[min]
0 mmol, 100 mg) in DMF (2.0 mL).
The reaction mixture was stirred at
room temperature for 4 h and moni-
tored by TLC. When the reaction was
complete, the mixture was quenched
with saturated NaHCO3 (10 mL) and
extracted with EtOAc (3ꢁ10 mL).
The organic phase was dried over an-
hydrous Na2SO4, the solvent evaporat-
ed under reduced pressure, and the
residue purified by flash column chro-
matography (silica gel; EtOAc/petro-
leum ether 1:4). Compound 5a
(32.4 mg) was isolated as a colorless
oil in 50% yield.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1b (nBu/H/Ts)
1c (nBu/H/Ns)
1d (nBu/Me/Ns) (4.0:1)
1e (Ph/H/Ts)
1 f (Ph/H/Ns)
2e
2e
2e
2e
2e
2e
2a
2e
2e
2e
2e
2e
2 f
2h
15
15
30
60
30
30
15
60
120
120
150
240
60
4l, 91
4m, 97
4n, 88
4o, 71
4p, 88
–
–
3.6:1
–
–
–
1g (Ph/H/Bs)
1g (Ph/H/Bs)
4q, 81
4r, 81[d]
4s, 71
–
1h (Ph/Me/Bs) (9.0:1)
1i (Ph/nPr/Bs) (4.8:1)
1j (Ph/nBu/Bs) (6.5:1)
1k (Me/Me/Ns) (4.6:1)
1l (TMS/H/Bs)
4.2:1
24:1
32:1
5.6:1
–
4t, 77
4u, 82
4v, 78
4w,[e] 79, R1 =H
4x, 42, 35[f]
4y, 85[d,f]
1k (Me/Me/Ns) (4.6:1)
1 f (Ph/H/Ns)
5.1:1
–
30
[a] All reactions were conducted with aziridine 1 (0.200 mmol), [AuACTHUNRTGNEUNG(PPh3)Cl]/AgSbF6 (10 mol%) in mesitylene
(1.0 mL) at 608C unless otherwise specified. [b] Determined by using the 1H NMR spectroscopic data. [c] Iso-
lated yield based on aziridine 1. [d] The product includes traces of a regioisomer. [e] H2O (1.0 equiv) was
added to the reaction system and product 4w was 2-(4-bromophenylsulfonyl)-4-mesityl-1-methylene-1,2,3,4-
tetrahydroisoquinoline. [f] The reaction was run using 2 (2.0 equiv) in DCE (1.0 mL) at 608C.
Scheme 4. The diastereoselective attack of an arene (ArH) on the zwit-
terionic intermediate 7.
Experimental Section
Further experimental procedures and compound characterization data
are available in the Supporting Information. CCDC-756210 (4r) and
CCDC-763709 (4t) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
General procedure for the gold-catalyzed domino reaction of aziridinyl
alkynes: A solution of [AuACHTNUTRGNEUNG(PPh3)Cl] (20.0 mmol, 10.0 mg) and AgSbF6
Scheme 5. A plausible reaction mechanism.
(20.0 mmol, 7.00 mg) in anhydrous toluene (2a; 0.50 mL) was stirred at
room temperature for 10 min. Then aziridinyl alkyne 1a (0.200 mmol,
83.7 mg) in toluene 2a (0.50 mL) was added into the reaction mixture
and stirred at 608C for 30 min. The reaction was monitored by TLC.
When the reaction was complete, the mixture was diluted with EtOAc,
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Aziridinyl Alkynes
COMMUNICATION
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Acknowledgements
We thank the Shanghai Municipal Committee of Science and Technology
(08dj1400100-2), the National Basic Research Program of China ((973)-
2009CB825300), and the National Natural Science Foundation of China
(20872162, 20672127, 20821002 and 20732008) for financial support and
Jie Sun for performing the X-ray diffraction experiments.
AHCTUNGTRENNUNG
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the basis of the NMR spectra and the X-ray crystal structure of 4t
(see the Supporting Information).
Keywords: aziridinyl alkynes
·
benzylic cations
·
·
dihydroisoquinolines
·
domino reactions gold
·
tetrahydroisoquinolines
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Received: March 11, 2010
Published online: June 8, 2010
Chem. Eur. J. 2010, 16, 7725 – 7729
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7729