Synthesis of fluoranthenes by hydroarylation of alkynes catalyzed by gold(I) or gallium trichloride

Article abstract of DOI:10.3762/bjoc.7.178

Electrophilic gold(I) catalyst 6 competes with GaCl₃ as the catalyst of choice in the synthesis of fluoranthenes by intramolecular hydroarylation of alkynes. The potential of this catalyst for the preparation of polyarenes is illustrated by a s

Full text of DOI:10.3762/bjoc.7.178

Synthesis of fluoranthenes by hydroarylation of
alkynes catalyzed by gold(I) or gallium trichloride
Sergio Pascual1, Christophe Bour1, Paula de Mendoza1
and Antonio M. Echavarren*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 1520–1525.
1Institute of Chemical Research of Catalonia (ICIQ), Av. Països
Catalans 16, 43007 Tarragona, Spain and 2Additional affiliation:
Departament de Química Analítica i Química Orgànica, Universitat
Rovira i Virgili, C/ Marcel·li Domingo s/n, 43007 Tarragona, Spain
doi:10.3762/bjoc.7.178
Received: 08 September 2011
Accepted: 28 October 2011
Published: 14 November 2011
Email:
Antonio M. Echavarren* - aechavarren@iciq.es
This article is part of the Thematic Series "Gold catalysis for organic
synthesis".
* Corresponding author
Guest Editor: F. D. Toste
Keywords:
alkynes; gold(I) catalysis; hydroarylation; polyarenes
© 2011 Pascual et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Electrophilic gold(I) catalyst 6 competes with GaCl3 as the catalyst of choice in the synthesis of fluoranthenes by intramolecular
hydroarylation of alkynes. The potential of this catalyst for the preparation of polyarenes is illustrated by a synthesis of two func-
tionalized decacyclenes in a one-pot transformation in which three C–C bonds are formed with high efficiency.
Introduction
Electrophilic activation of alkynes in functionalized substrates reaction can also be carried out with GaCl3 [32] and Pt(II) [33]
by gold catalysts allows for the synthesis of complex molecules as catalysts. In contrast, alkynyl furans react with gold to give
under mild conditions [1-8]. Alkynes can react in gold- phenols by using Au(III), Au(I) [1,2,34-37], or Pt(II) as the
catalyzed Friedel–Crafts-type reactions with arenes to give catalyst [38,39].
products resulting from the intermolecular hydroarylation of the
alkynes (or alkenylation of the arenes) [9-21]. In addition to In our efforts towards the synthesis of large polyarenes [40-43],
gold, the intramolecular version of this reaction was also carried which are related to the fullerenes [44], we used the palladium-
out with Ru(II) [22], Pt(II) [12,22,23], Pt(IV) [24], Ga(III) catalyzed arylation reaction as the main tool [45-48]. We
[25,26], and Hg(II) [27,28] as catalysts.
decided to try the triple hydroarylation of substrates of type 1 to
give 3,9,15-triaryldiacenaphtho[1,2-j:1',2'-l]fluoranthenes 2
Electron-rich indoles also react with alkynes in the presence of with X and Y substitutes at strategic positions, which could be
gold catalysts to form 6–8-membered rings [29-31]. A similar activated by palladium in subsequent intramolecular arylations
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Beilstein J. Org. Chem. 2011, 7, 1520–1525.
Scheme 1: Proposed metal catalyzed annulation for the synthesis of triaryldiacenaphtho[1,2-j:1',2'-l]fluoranthenes 2.
(Scheme 1). Substituted fluoranthenes are of interest since some donating phosphite ligand, led almost quantitatively to 4'
derivatives have been shown to be useful in light-emitting (Table 1, entry 2). Under these conditions, AuCl3 was not effec-
devices [49-52]. Fluoranthene derivatives have already been tive as a catalyst (Table 1, entry 3). As previously reported
synthesized by palladium-catalyzed arylation reactions [53,54]. [25,26], GaCl3 is an excellent catalyst for the cyclization of 3 to
Strategically halogenated decacyclenes with a substitution give 4' (Table 1, entry 4). In all cases the reaction proceeds
pattern similar to that of 2 have been used for the synthesis of exclusively though the 6-exo-dig pathway.
circumtrindene by flash vacuum pyrolysis [55]. Here we report
the results on the synthesis of large polyarenes 2 and more
simple 3-arylfluoranthenes by using gold(I)- or gallium(III)-
catalyzed hydroarylation reactions.
Results and Discussion
First, we examined the cyclization of 3 to give 4 or 4'
[22,24,26] (Table 1) with cationic gold(I) catalysts 5 [56] and 6
Figure 1: Cationic gold complexes 5 and 6.
[57] (Figure 1), which have been demonstrated to be amongst
the best catalysts in many gold(I)-catalyzed cyclizations [6,58].
No reaction was observed with complex 5 after heating for The cyclization of 9-(3-phenylprop-2-ynyl)-9H-fluorene (7a) to
5 min at 70 °C in CH2Cl2 under microwave irradiation (Table 1, form 3-phenylfluoranthene (8a) [59] was also examined by
entry 1), whereas the more electrophilic 6, bearing a less using catalysts 5, 6, and GaCl3 (Table 2). Since the initial
Table 1: Hydroarylation of 3 to give dihydronaphthalene 4'.a
entry
MXn
4' (yield, %)
1
2
3
4
5
—b
99
6
AuCl3
GaCl3
—c
99
a2 mol % catalyst, microwave irradiation, 5 min. b100% 3 was recovered. c87% 3 was recovered.
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Beilstein J. Org. Chem. 2011, 7, 1520–1525.
led to decomposition of 7a under these conditions (Table 2,
entry 6), GaCl3 led to 8a, although satisfactory results were
only obtained in toluene at 70 °C (Table 1, entry 10). Interest-
ingly, FeCl3 was also catalytically active, although fluoran-
thene 8a was only obtained in moderate yields (Table 2, entries
11 and 12). The reaction of 3a with Pd(OAc)2 as catalyst
proceeded differently to give known (E)-9-(3-phenylallylidene)-
9H-fluorene (9) [60], presumably via the formation of the
corresponding allene as an intermediate (Scheme 2).
Table 2: Hydroarylation of 9-(3-phenylprop-2-ynyl)-9H-fluorene (7a) to
give 3-phenylfluoranthene (8a).a
entry MXn (mol %)
solvent
CH2Cl2
T (ºC) t (h) yield (%)
1
5 (2)
70b
0.7
1
—c
—d
64
2
5 (5)
toluene 110
3
6 (5)
CH2Cl2
CH2Cl2
CH2Cl2
r.t.
r.t.
r.t.
17
16
16
17
17
17
26
4
6 (2)
70
5
6 (1)
70
6
PtCl2 (5)
AuCl3 (5)
InCl3 (5)
GaCl3 (2)
GaCl3 (2)
toluene 110
toluene 110
toluene 110
—c
—c
—d
16e
7
8
9
CH2Cl2
toluene
r.t.
Scheme 2: Pd(OAc)2-catalyzed isomerization of 7a to form (E)-9-(3-
phenylallylidene)-9H-fluorene (9).
10
11
12
70b
r.t.
0.2 57
FeCl3·6H2O (10) DCEf
FeCl3·6H2O (5) DCEf
40
36e
34e
70b
0.2
Substrates 7b–j, prepared by alkylation of fluorenyl lithium
with the corresponding propargyl bromide or by Sonogashira
couplings of 9-(prop-2-ynyl)-9H-fluorene [61], were cyclized
by using gold(I) complex 6 or GaCl3 as the catalyst (Table 3).
Although both catalysts could be used for the synthesis of
3-arylfluoranthenes 8b–h, better yields were obtained with
aCrude reaction mixtures were aromatized by heating in toluene with
DDQ (3 equiv) for 12 h. bMicrowave irradiation. cNo reaction. dProduct
decomposition. eYield determined by 1H NMR. fDCE = 1,2-dichloro-
ethane.
gold(I)-catalyzed reaction provided a mixture of 3-phenyl- GaCl3 in toluene at 100 °C. However, in the case of 9-(3-
1,10b-dihydrofluoranthene, 3-phenyl-1,2,3,10b-tetrahydrofluo- bromoprop-2-yn-1-yl)-9H-fluorene (7i), gold(I) complex 6 gave
ranthene, and 8a, the crude mixtures were treated with excess more satisfactory results (Table 3, compare entries 10 and 11).
DDQ in toluene under reflux to provide pure 8a. No reaction or The reaction proceeded satisfactorily with aryl-substituted
decomposition was observed when the reaction was carried out substrates bearing either electron-donating (p-Me, o-OMe) or
with gold(I) complex 5 (Table 2, entries 1 and 2). In contrast, electron-withdrawing (p-Cl, p-Br, p-CN, p-NO2) groups.
the more electrophilic gold(I) complex 6 with phosphite as the However, no reaction was observed for n-butyl derivative 7j
ligand led to 8a in 64–70% yield by stirring at room tempera- with 6 or with GaCl3 (Table 3, entries 12 and 13).
ture in CH2Cl2 (Table 2, entries 3–5). Satisfactory results were
obtained by simply using 1 mol % of 6 (Table 2, entry 5). No Cyclization of substrate 7k, having an electron-rich aryl group
reaction was observed with PtCl2 or AuCl3 even after heating at the alkyne, with catalyst 6 gave 1,10b-dihydrofluoranthene 9
in toluene under reflux (Table 2, entries 3–5). Whereas InCl3 cleanly in quantitative yield (Scheme 3).
Scheme 3: Gold(I)-catalyzed hydroarylation of 7k to give 1,10b-dihydrofluoranthene 9.
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Beilstein J. Org. Chem. 2011, 7, 1520–1525.
Table 3: Hydroarylation of 7b–j to give 3-substituted fluoranthenes 8b–i.a
entry
fluorene
R
MXn (mol %)
solvent
T (ºC)
t (h)
yield (%)
1
7b
7b
7c
7d
7e
7f
p-Tol
GaCl3 (5)
6 (5)
toluene
CH2Cl2
toluene
toluene
toluene
CH2Cl2
toluene
toluene
toluene
CH2Cl2
toluene
CH2Cl2
toluene
100b
r.t.
0.2
17
0.2
0.2
0.2
17
0.2
0.2
2
45
28
71
88
92
17
57
44
74
44
21
—c
—c
2
p-Tol
3
p-ClC6H4
p-NCC6H4
p-O2NC6H4
o-MeOC6H4
o-MeOC6H4
o-BrC6H4
C6F5
GaCl3 (5)
GaCl3 (2)
GaCl3 (2)
6 (5)
100b
100b
70b
4
5
6
r.t.
7
7f
GaCl3 (5)
GaCl3 (5)
GaCl3 (5)
6 (5)
100b
100b
100b
r.t.
8
7g
7h
7i
9
10
11
12
13
Br
20
0.2
7
7i
Br
GaCl3 (5)
6 (5)
100b
r.t.
7j
n-Bu
7j
n-Bu
GaCl3 (2)
70b
0.2
aCrude reaction mixtures were aromatized by heating in toluene with DDQ (3 equiv) for 12 h. bMicrowave irradiation. cNo reaction.
Derivatives 1a and 1b were readily prepared by the triple alkyl- with an average yield per C–C bond formation that is greater
ation of the lithium anion of 4,9,14-trimethoxytruxene than 90%.
(Scheme 4) [41,62]. The cyclization reaction was carried out
efficiently with gold(I) catalyst 6 (15 mol %) at room tempera- Conclusion
ture in CH2Cl2 to give triaryl substituted diacenaphtho[1,2- Highly electrophilic gold(I) catalyst 6 with a bulky phosphite
j:1',2'-l]fluoranthenes (decacyclenes) 2a and 2b in very good ligand competes with GaCl3 as the catalyst of choice for the
overall yields after aromatization of the crude products with hydroarylation of alkynes. The synthetic potential of this cata-
DDQ. Remarkably, this triple hydroarylation occurs efficiently lyst is illustrated by the synthesis of functionalized triarylated
Scheme 4: Gold(I)-catalyzed triple hydroarylation of 1a,b to give 2a,b.
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