Practical solvent-free ruthenium trichloride-mediated benzannulation approach to fused functionalized arenes

Article abstract of DOI:10.1002/adsc.201500186

The solvent- and ligand-free [2+2+2]ruthenium-promoted cycloaddition of α,ω-diynes and alkynes provides a facile and efficient strategy for the synthesis of substituted benzene-derived systems. The search for the optimal reaction conditions revealed the unprecedented catalytic activity of ruthenium trichloride for benzannulation reactions and this atom-economical process allowed the synthesis of fused arenes including dihydrobenzofurans, isoindolines, indanes in good to high yields. This practical protocol also gave rise to the preparation of pentasubstituted aromatic derivatives and was applied to the one-gram scale synthesis of a functionalized heterocycle.

Full text of DOI:10.1002/adsc.201500186

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DOI: 10.1002/adsc.201500186
Practical Solvent-Free Ruthenium Trichloride-Mediated
Benzannulation Approach to Fused Functionalized Arenes
Jeremy Jacquet,a Anne-Laure Auvinet,a Anil Kumar Mandadapu,a
Mansour Haddad,a Virginie Ratovelomanana-Vidal,a,* and VØronique Micheleta,*
a
PSL Research University, Chimie ParisTech – CNRS, Institut de Recherche de Chimie Paris, 11 rue P. et M. Curie, 75005
Paris, France
Fax: (+33)-1-4407-1062; phone: (+33)-1-4427-6742; e-mail: virginie.vidal@chimie-paristech.fr or
veronique.michelet@chimie-paristech.fr
Received: February 23, 2015; Revised: March 30, 2015; Published online: April 29, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500186.
Abstract:
The
solvent-
and
ligand-free
[2+2+2]ruthenium-promoted cycloaddition of a,w-
diynes and alkynes provides a facile and efficient
strategy for the synthesis of substituted benzene-de-
rived systems. The search for the optimal reaction
conditions revealed the unprecedented catalytic ac-
tivity of ruthenium trichloride for benzannulation
reactions and this atom-economical process allowed
the synthesis of fused arenes including dihydroben-
zofurans, isoindolines, indanes in good to high
yields. This practical protocol also gave rise to the
preparation of pentasubstituted aromatic deriva-
tives and was applied to the one-gram scale synthe-
sis of a functionalized heterocycle.
Scheme 1. Selected [Ru] catalysts for [2+2+2]cycloaddition
reactions of a,w-diynes.
Keywords: alkynes; benzannulation; cycloaddition;
a,w-diynes; ruthenium; solvent-free conditions
processes greener, the development of atom-economi-
cal synthetic procedures involving, for instance, sol-
vent-free conditions has become crucial.[6]
To the best of our knowledge, solventless metal-cat-
alyzed [2+2+2]cycloadditions of diynes have been
scarcely described although the search for environ-
Benzannulation is an atom-economical and a straight- mentally friendly protocols is essential.[7] To complete
forward approach to access complex structures start- our recent interest in [2+2+2]cycloaddition reac-
ing from relatively simple materials. Transition metal- tions,[8] we were wondering if a more practical strat-
catalyzed [2+2+2]cycloadditions have proven to be egy could be envisaged to access fused arenes in the
a powerful tool in the construction of highly substitut- absence of solvent and in the absence of additional
ed arenes. Indeed, the [2+2+2]cycloaddition has been ligand by using the commercially available, cheap and
widely reported with several metals, the most easy to handle RuCl3·nH2O. Noteworthy, Mitsudo
common being cobalt, nickel, iridium, rhodium and and Tanaka reported the inefficiency of RuCl3 and
ruthenium.[1] Ruthenium-catalyzed [2+2+2]cycloaddi- the RuCl3/amine system toward [2+2+2]cycloaddi-
tions[2–5] have been reported using mainly elaborated tions of terminal alkynes with dimethyl acetylenedi-
and expensive Ru complexes such as Cp*Ru(cod)Cl, carboxylate and cyclotrimerization of ethyl phenyl-
[RuCp(MeCN)3]PF6,
[RuCl2(p-cymene)]2, propiolate.[9] We and others previously demonstrated
À
À
À
[Cp*RuCl2]2, Grubbs (I and II), Hoveyda–Grubbs cat- the efficiency of RuCl3·nH2O in C H, C C, C N and
alysts (Grubbs III), or Ru(ethene)(CO)4, Ru3(CO)12 C O bond forming processes.
(Scheme 1) combined with organic solvents, addition- report here the first efficient RuCl3-catalyzed
al ligands, supported systems or microwave assistance. [2+2+2]cycloaddition reactions of a,w-diynes and
Due to environmental concerns to make chemical alkynes under solvent-free conditions.
[10,11]
À
In this context, we
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Table 1. Optimization of the reaction conditions.[a]
when the catalyst loading was decreased to 2 mol%
and the quantity of n-hexyne to 6 equivalents
(Table 1, entry 21).[13]
With these promising results, we decided to investi-
gate the scope of the solvent-free RuCl3·nH2O-pro-
moted [2+2+2]cycloaddition of a,w-diynes and al-
kynes (Table 2). Thus, the next step was the screening
of different class of diynes 3–5 with n-hexyne. To our
delight, the reaction proceeded with N-tethered 3 and
4 (Table 2, entries 1 and 2) and O-tethered 5 diynes
Entry Catalyst [mol%]
Conv.[b] [%] Yield[c] [%]
1
FeCl3 (8)
–
–
2
3
4
Fe(acac)3 (8)
FeSO4·7H2O (8)
AuCl3 (8)
–
–
–
–
–
–
Table 2. [Ru]-catalyzed cycloaddition reactions of carbon-,
5
AuCl (8)
–
–
nitrogen- and oxygen-tethered diynes.[a]
6
7
8
9
HAuCl4 (8)
PPh3AuCl/AgOTf (8)
AgOTf (8)
AgSbF6 (8)
InBr3 (8)
In(OTf)3 (8)
InCl (8)
Cu(OTf)2 (8)
CuI (8)
–
–
–
–
–
–
–
–
10
11
12
13
14
15
16
17
18
19
20
21[e]
–
–
–
–
–
–
–
–
–
–
CuSO4·5H2O (8)
–
–
[Ir][d] (4)
100
80
–
80
65
–
IrCl3 (8)
PdCl2 (8)
PtCl2 (8)
RuCl3·nH2O (8)
RuCl3·nH2O (2)
20
100
100
15
85
79
[a]
Reaction conditions: 0.4 mmol of diyne 1 and 10 equiva-
lents of n-hexyne, screw-capped tube.
Determined by 1H NMR.
Isolated yields.
[Ir]=[{Ir(H)[rac-binap]}2(m-I)3]I.
6 equivalents of n-hexyne
[b]
[c]
[d]
[e]
The first set of experiments was carried out using
several transition metal complexes. The optimization
showed that RuCl3·nH2O was the best catalytic
system to access indane 2 (Table 1).
Indeed, iron (Table 1, entries 1–3), gold(I) and
gold(III) (Table 1, entries 4–7), silver (Table 1, en-
tries 8 and 9), indium (Table 1, entries 10–12), and
copper complexes (Table 1, entries 13–15) were totally
ineffective toward cycloaddition. In our previous
report, we showed the efficiency of an Ir(III) complex
in i-PrOH.[8] Employing 4 mol% of the Ir(III) com-
plex[12] allowed the same transformation to provide
indane derivative 2 in 80% yield (Table 1, entry 16).
The use of the IrCl3 complex afforded the corre-
sponding indane 2, in a lower 65% yield (Table 1,
entry 17). The catalytic activity of other transition
metal complexes such as PdCl2 and PtCl2 was evaluat-
ed (Table 1, entries 18 and 19), only the latter allowed
the isolation of a small amount of 2. The same reac-
tion was carried out with 8 mol% of RuCl3·nH2O and
furnished 2 in 85% yield (Table 1, entry 20). Pleasing-
ly, indane 2 was obtained with a similar yield of 79%
[a]
Conditions: 1 or 2 mol% [Ru], 1 equivalent of diyne and
5 or 6 equivalents of alkyne (see the Supporting Informa-
tion for details).
[b]
Isolated yields.
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Practical Solvent-Free Ruthenium Trichloride-Mediated Benzannulation
(Table 2, entry 3) with satisfactory yields up to 75%.
Then, we turned our attention to cycloalkylalkynes.
The reaction proved to be successful with
cyclopropylacetylene (Table 2, entries 4–7, 65–80%
yields) and cyclohexylacetylene (Table 2, entry 8). Re-
wardingly, the isolated yield of isoindoline 9 was sig-
nificantly improved under solvent-free conditions
using RuCl3·nH2O compared to that obtained in our
previous report[8] using an Ir(III) complex (entry 4,
80% yield vs. 61% yield).
Alkynyl alcohols could also be involved in the cy-
cloaddition with different functionalized diynes pro-
viding the desired fused arenes in respectable yields
(Table 2, entries 9 and 10, up to 71% yield). Ether-
linked (Table 2, entry 11) and halogen-containing al-
kynes (Table 2, entries 12 and 13, 72% and 77%
yield) could be envisaged, the cycloaddition furnished
the desired compounds 16, 17 and 18 in good yields
up to 77%. The reaction of phenylacetylene with
oxygen-tethered diyne 5 was also successful as the
corresponding adduct 19 was isolated in 77% yield
(Table 2, entry 14).
We then decided to evaluate the activity of
RuCl3·nH2O catalyst toward cycloaddition reactions
of functionalized carbon-, nitrogen-tethered and di-
substituted diynes (Scheme 2). We were pleased to
find that the reaction conditions were compatible
with such protective groups as the corresponding
functionalized indanes 27, 28 and 29 were respectively
obtained in 84%, 69% and 82% yields (Scheme 2).
The scope of the solvent-free reaction was further ex-
tended to the synthesis of challenging polysubstituted
arenes.[1,2] The cycloadditions were conducted in the
presence of 2 mol% of ruthenium catalyst.
The synthesis of various dimethylisoindolines 30–32
was successful in the presence of alkyl, ether or
chloro functionalized alkynyl partners (71–84%
yields). In the case of isoindoline 30, a notable in-
crease of the isolated yield was observed in the
presence of RuCl3·nH2O compared to the previous
Ir(III) complex[8] (84% vs. 43%). Diynes 24–26 bear-
Scheme 2. [Ru]-catalyzed [2+2++2]cycloaddition reactions of
functionalized diynes.
ing electron-withdrawing or electron-donating groups and gave rise, as anticipated,[3d] to a mixture of meta-
on the aryl moieties also reacted very nicely. Their cy- and ortho-substituted indanes 41 (72:28 ratio) in 72%
cloaddition reactions allowed an easy and effective yield [Scheme 3, Eq. (3)]. The reaction of 1,7-diynes
access to pentasubstituted aromatic rings, regardless was also tested and led to either no conversion or
of the alkynyl partner. Indanes 33–36 were isolated in degradation of diynes. No desired products were ob-
good to excellent yields (81–97%). The presence of served in all cases (see the Supporting Information).
a bromine atom such as in 35 was fully tolerated and
gives opportunities for further cross-coupling reac- cloaddition under the optimized conditions in the
tions.[14]
presence of 2 mol% of ruthenium catalyst
We finally performed a one gram-scale [2+2+2]cy-
Preliminary results were obtained in the presence (Scheme 4). The catalytic activity of the complex was
of internal alkynes such as but-2-yn-1-ol [Scheme 3, still excellent and the corresponding isoindoline 42
Eq. (1) and Eq. (2)]. The reaction of diynes 1 and 5 was isolated in 88% yield.
afforded the desired alcohols 37 and 38 in moderate
While the mechanism of the metal-catalyzed
yields (48% and 45%, respectively). In the case of [2+2+2]cycloaddition has been thoroughly studied,[1,2]
diyne 5, dimer 39 was also isolated in 9% yield. The it is not so clear how the active ruthenium(II) species
reactivity of unsymmetrical diyne 40 was attempted is generated in the case of some ruthenium cata-
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Jeremy Jacquet et al.
Scheme 3. Reactivity of internal alkynes and unsymmetrical diynes.
Experimental Section
General Procedure for [2+2+2]Cycloaddition
Reactions
To a sealed tube was added RuCl3·nH2O (1 or 2 mol%), fol-
lowed by addition of the diyne (1 equiv.) and the alkyne (5–
8 equiv.). The tube was sealed and the reaction mixture was
stirred at 1108C from 20 min to 18 h. The crude reaction
mixture was purified by flash chromatography over silica
gel. Experimental details are provided in the Supporting In-
formation.
Scheme 4. One gram-scale synthesis of 42.
lysts.[15,16] According to the recent report by PØrez-
Castells and co-workers,[4f] we may anticipate that the
RuCl3·nH2O catalyst has a similar behavior under
thermal conditions as the Grubbs II catalyst. The in
situ generation of an active Ru(II) species may also
occur via the reduction of Ru(III) by the alkyne.[10b,e]
Further studies are currently under investigation to
characterize organometallic intermediates and there-
fore shed the light on the mechanism.
In conclusion, we have demonstrated that fused
arenes bearing a range of substitution patterns can be
efficiently accessed via RuCl3-promoted [2+2+2]cy-
cloaddition of diynes and a,w-alkynes. This unprece-
dented protocol is more cost effective than conven-
tional methods using elaborated ruthenium com-
plexes. No additional ligands are used for the transfor-
mation. Moreover, RuCl3·nH2O is cheap, easy to
handle and not sensitive to water or air. The reaction
proceeds efficiently with low catalyst loading under
solvent-free conditions. This practicality renders this
process an attractive approach to access challenging
pentasubstituted aromatic compounds.
Acknowledgements
This work was supported by the Minist›re de l’Education et
de la Recherche and the Centre National de la Recherche Sci-
entifique (CNRS), A.-L. A. and A.K. M. are grateful to the
Fondation Pierre-Gilles de Gennes pour la Recherche for
grants. Johnson Matthey Inc. is acknowledged for generous
loans of HAuCl4, PdCl2 and PtCl2.
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Johansson Seechurn, M. O. Kitching, T. J. Colacot, V.
Snieckus, Angew. Chem. 2012, 124, 5150; Angew.
Chem. Int. Ed. 2012, 51, 5062; d) Palladium-Catalyzed
[15] For the use of Ru(III) complex ([Cp*RuCl2]2 and
Ru3(CO)12) as catalyst for [2+2+2]cycloadditions, see
refs.[3i,5b]
[16] For the use of Ru(IV) pre-catalyst in [2+2++2]reactions,
see: V. Cadierno, S. E. Garcia-Garrido, J. Gimeno, J.
Am. Chem. Soc. 2006, 128, 15094.
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Adv. Synth. Catal. 2015, 357, 1387 – 1392