Efficient ruthenium-catalyzed synthesis of [3]dendralenes from 1,3-dienic allylic carbonates

Article abstract of DOI:10.1039/b913595b

Selective elimination from 1,3-dienic allylic carbonates occurs in the presence of a catalytic amount of [Ru(C₅Me₅)(4,4′- di-Bu^t-2,2′-bipyridine)(CH₃CN)]PF₆ and provides efficient access to a variety

Full text of DOI:10.1039/b913595b

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Efficient ruthenium-catalyzed synthesis of [3]dendralenes from 1,3-dienic
allylic carbonatesw
Kassem Beydoun,a Hui-Jun Zhang,ab Basker Sundararaju,a Bernard Demerseman,a
Mathieu Achard,a Zhenfeng Xib and Christian Bruneau*a
Received (in Cambridge, UK) 8th July 2009, Accepted 3rd September 2009
First published as an Advance Article on the web 17th September 2009
DOI: 10.1039/b913595b
Selective elimination from 1,3-dienic allylic carbonates occurs in
the presence of a catalytic amount of [Ru(C5Me5)(4,40-di-But-
2,20-bipyridine)(CH3CN)]PF6 and provides efficient access to a
variety of [3]dendralenes.
Acyclic cross-conjugated polyolefins, known as [n]dendralenes,
have received new interest due to their applications in domino
[4 + 2] cycloadditions where they act as multidienes to
synthesize polycyclic frameworks.1 Metal-catalyzed coupling
reactions,1e–g,2 pyrolysis,3 thermolytic cracking of vinyl
sulfolene,4 siloxane elimination,5 Mitsunobu dehydration1a,b
Scheme 2 Formation of (diene)ruthenium(II) complexes.
and Hofmann elimination6 have provided specific access to
[Ru(Cp*)(CH3CN)3]+ cation (Cp*
= C5Me5) at room
distinct [n]dendralenes.
temperature to afford the same, but unexpected, (diene)-
ruthenium(II) derivative (Scheme 2). Further evidence for this
reactivity was obtained by conducting a similar reaction
with crotyl ethyl carbonate that resulted in the symmetrical
unsubstituted butadiene ligand.w By contrast, tert-butyl allylic
carbonates have recently been reported to form stable
[Ru(Cp*)(Z3-CH2CHCHR)(Z2-O2COBut)][PF6] (R = Prn, Ph)
allyl ruthenium(IV) complexes.8 Moreover, b-H elimination
reactions from allyl ruthenium complexes are scarce and occur
under stoichiometric conditions.9
On the other hand, allylic carbonates are typical substrates
for allylation reactions and we have recently reported a
ruthenium-catalyzed synthesis of functionalized 1,3-dienes
starting from 1,3-dienic allylic carbonates (Scheme 1).7
We now report a novel synthesis of 2-substituted and
2,5-disubstituted [3]dendralenes via a ruthenium(IV)-catalyzed
elimination reaction starting from appropriate 1,3-dienic
allylic carbonates (Scheme 1).
Indeed, the formation of an (allyl)ruthenium(IV) intermediate
will be involved as the preliminary step for both the allylation
and elimination pathways (Scheme 1). During the course of
our studies, we observed that the PrnCH(OCO2Et)CHQCH2
and PrnCHQCHCH2(OCO2Et) carbonates, as branched and
linear isomers, respectively, reacted with the ruthenium(II)
In our hands, the formation of diene complexes was slow
enough at room temperature to allow detection of the (allyl)-
ruthenium(IV) intermediate that can be trapped with acetic
acid to form the more stable (allyl)(acetato)ruthenium(IV)
derivative10 (Scheme 2).w Thus, the basic properties of the
(EtOCO2)2ꢀ ligand in the (allyl)ruthenium(IV) intermediate
will account for the subsequent deprotonation of the allylic
ligand to generate the coordinated diene besides release of
ethanol and carbon dioxide.
We next sought for a catalytic system starting from 1,3-
dienic allylic substrates that were available via enyne cross
metathesis.7,11 An elimination reaction from 1a in the presence
of ruthenium precatalysts I–III (Fig. 1) was investigated under
various conditions and the results are given in Table 1.
In the presence of 5 mol% of precatalyst II bearing a
disubstituted 4,40-di-But-2,20-bipyridine ligand, along with
Scheme 1 Allylation and elimination pathways.
a
´
UMR 6226 CNRS – Universite de Rennes 1, Sciences chimiques de
Rennes, Catalyse et Organometalliques, Campus de Beaulieu,
´
35042 Rennes Cedex, France. E-mail: christian.bruneau@univ-rennes1.fr;
Fax: +33 223236939; Tel: +33 223236283
b Beijing National Laboratory for Molecular Science (BNLMS),
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry, Peking University,
Beijing 100871, China
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for complexes and compounds
2a–j. See DOI: 10.1039/b913595b
Fig. 1 Ruthenium precatalysts.
ꢁc
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Table 1 Elimination reaction from dienic carbonate 1aa
Table 2 Synthesis of [3]dendralenes from dienic allylic carbonates 1a–ja
Entry Substrate R1
R2 Product
Yield (%)b
2a 75
Entry
Base
Catalyst
T/1C
Yield (%)b
1
2
3
4
5
6
K2CO3
K2CO3
K3PO4
Cs2CO3
None
None
None
None
None
II
II
II
II
II
II
II
I
25
60
60
60
60
90
90
90
90
13
35
35
28
38
75
97
8
1
2
1a
1b
Ph
H
H
p-Tolyl
2b 63
2c 72
3
1c
Ph
Me
7c
8c
9c
III
5
4
5
6
7
8
1d
1e
1f
Ph
Et
H
2d 66
2e 65
2f 75
2g 78
2h 73
a
All reactions were carried out in a closed Schlenk tube under an inert
atmosphere in acetonitrile (3 mL) as solvent for a reaction time of 12 h
n-C6H13
b
with a 1/0.05/1.2 1a/[Ru]/base molar ratio. Yield based on GC
c
analysis using tetradecane as internal standard. Additional presence
n-C6H13 Me
of powdered 4 A MS (500 mg per mmol of 1a).
1g
1h
n-C4H9
n-C4H9
H
one equivalent of potassium carbonate at room temperature,
1a afforded the [3]dendralene 2a in a modest 13% yield, as
determined by GC analysis (entry 1). Better conversions
(30–40%) were obtained at 60 1C in the presence of various
bases (entries 2–4). Furthermore, a similar result (entry 5) was
obtained without the addition of base, which was expected to
facilitate deprotonation of the allylic ligand from a ruthenium(IV)
intermediate.
Me
9
1i
1j
C6H11
C3H5
H
2i 82
2j 68
10
Me
a
Complete consumption of 1a to generate 2a in 75% yield
occurred at 90 1C. However, the formation of alcohols resulting
from the hydrolysis of the carbonate group in 1a was also
detected, and this observation suggested that anhydrous
conditions should be more suitable. Thus, in the additional
presence of 4 A molecular sieves, triene 2a was selectively
produce in 97% yield (entry 7). Note that in the absence of
precatalyst II no reaction took place and complete recovery of
1a was obtained. Under the optimized conditions, precatalysts
I, bearing three labile acetonitrile ligands, and III, bearing a
strongly bonded N,O chelate, led to very modest conversions
of 8 and 5%, respectively (entries 8 and 9). The presence of a
bipyridine ligand in II has been shown to favour the formation
of dicationic (allyl)ruthenium(IV) species,12 and it prevented
coordination of the diene moiety,7 as required for the
establishment of a catalytic cycle.
All reactions were carried out in a closed Schlenk tube under an inert
atmosphere in acetonitrile (3 mL) as solvent for a reaction time of
12 h in the presence of II (5 mol%) along with powdered 4 A MS
b
(500 mg per mmol of 1). Isolated yield.
produced trienes 2c,d exclusively as 2,5-disubstituted
(E)-[3]dendralenes (entries 3 and 4). Similar good results were
obtained starting from dienic carbonates 1e,g containing an
n-butyl or n-hexyl aliphatic side chain to obtain 2-substituted
dendralenes 2e,g (entries 5, 7).
For compounds 2a–i, no Diels–Alder product resulting
from further thermal reaction was detected. Only the
cyclopropane derivative 2j dimerizes and oligomerizes during
concentration of the reaction mixture.
(E)-Selectivity was also observed during the formation of
2,5-disubstituted [3]dendralenes 2f,h starting from dienes 1f,h
(entries 6 and 8). Substrates featuring aliphatic cyclohexyl (1i)
or cyclopropyl (1j) moieties also afforded the expected trienes
2i,j in 82% and 68% isolated yield, respectively (entries 9
and 10).
Precatalyst II was selected to synthesize [3]dendralenes 2a–j
starting from carbonates 1a–j (Table 2). Thus, under our
optimal conditions, a wide range of dienic substrates afforded
the corresponding [3]dendralenes, which were isolated in very
good yields. Trienes 2a–j were isolated in a pure form after
pentane extraction of the acetonitrile layer followed by direct
chromatography without preliminary concentration. It should
be also mentioned that careful room temperature concentration
of triene solutions is required to prevent dimerization or
polymerization.1g Thus, 2-aryl-[3]dendralenes 2a (R1 = Ph)
and 2b (R1 = p-tolyl) were isolated in 75% and 63% yield,
respectively, starting from aryl dienic carbonates 1a (entry 1)
and 1b (entry 2). Interestingly, the reaction of dienic carbonates
1c,d featuring an a-substituted side chain (R2 = Me, Et)
These results show that a variety of [3]dendralenes can
be obtained as soon as the starting carbonate contains an
eliminable hydrogen in the b-position.
Using the aforementioned protocol but in the presence of a
nucleophile and a base, we finally undertook a concurrent
tandem allylation/elimination (ABIIC)(CIID) sequence.13 The
reaction was performed starting from the dienic dicarbonate
1k (Scheme 3) and dimethyl malonate in the presence of
K2CO3 at 100 1C. The functionalized [3]dendralene 2k was
isolated in 55% yield as a mixture of branched and linear
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Scheme 3 Tandem allylation/elimination sequence from dicarbonate
substrates.
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isomers, with an excellent regioselectivity (83 : 17) in favour of
the isomer containing a stereogenic center.2b In the same
manner, reaction of diene 1l with diethyl malonate afforded
the expected trienes 2l with good regioselectivity (75 : 25).
These results show that catalyst II catalyzes both the mono-
substitution and the elimination reactions. They reveal that the
nucleophilic substitution (allylation) is faster than the elimination,
and that catalyst II is not very efficient for double allylation.
Starting from a variety of N-, O- and C-nucleophiles, this
allylation/elimination sequence will provide a straightforward
access to functionalized dendralenes.
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´
ndez, F. Breher, P. S. Pregosin,
In conclusion, we have developed a new general and efficient
route based on a ruthenium-catalyzed elimination reaction to
prepare substituted [3]dendralenes starting from 1,3-dienic
allylic carbonates. Association of allylation and elimination
steps offers an access to a variety of functional trienes containing
a stereogenic center.
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F. Breher, Organometallics, 2006, 25, 1440.
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This work was supported by the Natural Science Foundation
of China and the international joint supervision of Ph.D. grant
(H.-J. Z.) from the Ambassade de France in China
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6582 | Chem. Commun., 2009, 6580–6582