Ruthenium-catalyzed C-H bond activation of Michael acceptors: An unusual reactivity leading to allylsilanes

Article abstract of DOI:10.1002/adsc.200800600

We report here an improved catalyst for the functionalization of Michael acceptors, involving C-C bond formation via C-H bond activation, using an in situ generated ruthenium active species. Moreover, on some particular substrates, the C-H functionalizati

Full text of DOI:10.1002/adsc.200800600

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DOI: 10.1002/adsc.200800600
À
Ruthenium-Catalyzed C H Bond Activation of Michael
Acceptors: An Unusual Reactivity Leading to Allylsilanes
Marc-Olivier Simon,a Rꢀmi Martinez,a Jean-Pierre GenÞt,a and Sylvain Darsesa,*
a
Laboratoire de Synthꢁse Sꢀlective Organique (UMR 7573, CNRS), Ecole Nationale Supꢀrieure de Chimie de Paris, 11
rue P&M Curie, 75231 Paris cedex 05, France
Fax : (+33)-1-4407-1062; e-mail: sylvain-darses@enscp.fr
Received: September 29, 2008; Revised: December 2, 2008; Published online: January 12, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800600.
Abstract: We report here an improved catalyst for
the functionalization of Michael acceptors, involv-
lation of styrenes.[7] The catalytically active ruthenium
species was generated in situ from the reaction of in-
expensive [RuCl2ACTHNGUTERNNU(G p-cym)]2 (p-cym=p-cymene) and
sodium formate, in association with a phosphane
ligand.
À
À
ing C C bond formation via C H bond activation,
using an in situ generated ruthenium active species.
À
Moreover, on some particular substrates, the C H
functionalization resulted unexpectedly in the for-
mation of allylsilanes rather than in the expected
conjugated adducts, affording a new straightforward
methodology to access useful stereodefined trisub-
Even if intermolecular alkylation processes involv-
À
ing chelation-assisted aromatic C H bond activation
have been largely explored,[6] the activation of vinylic
À
C H bond of alkenes, and particularly Michael ac-
stituted allylsilanes via C H bond activation. Pre-
ceptors, has been scarcely explored.[8] Indeed, Murai
and Trost have shown that some cyclic a,b-unsaturat-
ed esters,[8b] and a limited number of particular a,b-
unsaturated ketones[8b,c] and aldehydes[8d] could be al-
kylated in the b-position in the presence of vinylsi-
À
liminary results have shown that they were reactive
in the allylation of aldehydes, providing an access
to alcohols bearing a quaternary carbon center.
À
À
Keywords: allylsilanes; C C bond formation; C H
activation; ruthenium; silanes
lanes and a catalytic amounts of RuH2(CO)ACTHUNGETRNNU(G PPh3)3.
In all the reactions examined, conjugated alkylation
products were obtained in moderate yields.
We want to report here an improved catalyst for
the functionalization of Michael acceptors, using an in
situ generated ruthenium active species (Scheme 1).
[1]
À
À
Catalytic processes involving C H bond activation
Moreover, on some particular substrates, the C H
are highly desirable, not only because they allow the bond functionalization resulted unexpectedly in the
functionalization of more easily available starting ma- formation of allylsilanes rather than in the expected
terials (atom economy concept[2]) but also because conjugated adducts, affording a straightforward meth-
they produce clean reactions (reduced salts amounts). odology to access useful trisubstituted allylsilanes[9]
À
À
À
In the field of C C bond formation via C H activa- via C H bond activation. Preliminary results have
tion, several catalytic reactions have been developed shown that they were reactive in the allylation of al-
recently involving mainly low valent transition metals dehydes, providing an access to alcohols bearing a
like palladium, rhodium and particularly ruthenium quaternary carbon center.
complexes which have been extensively developed in
The feasibility of the activation of a,b-unsaturated
substrates was evaluated in the reaction of methyl cy-
that field.[1,3]
À
Among the catalytic processes for C C bond for-
mation via C H bond activation, hydroarylation pro-
À
cesses, catalyzed by transition metal, allow the func-
tionalization of alkenes with a total atom economy.[1,4]
We recently reported a highly efficient catalytic
system, generated in situ from easily available Ru(II)
sources, allowing either the hydroarylation of vinylsi-
Scheme 1. Ruthenium-catalyzed functionalization of Mi-
lane (Murai reaction)[5,6] or the anti-Markovnikov ary- chael acceptors.
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Marc-Olivier Simon et al.
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Table 1. Ruthenium-catalyzed b-alkylation of Michael ac-
clopent-1-enecarboxylate (1a) with triethoxyvinylsi-
lane (2a), using an in situ generated ruthenium cata-
lyst.[6,7] Several conditions were examined for the re-
action of these model substrates and the previously
reported conditions were found to be suitable, i.e.,
conducting the reaction at lower temperature: 1208C
ceptors.[a]
À
compared to 1408C for aromatic C H bond activation
(Table 1).[5] We also found that the use of (4-
CF3C6H4)3P as ligand of the ruthenium afforded
higher yields, even allowing us to run the reaction in
dioxane at 1008C.[7] Under these conditions, good
yields were achieved for the functionalization of
cyclic substrates with triethoxyvinylsilane (Table 1,
entries 1–4). Indeed, a,b-unsaturated esters (C5 or C6
rings), ketones or amides were alkylated in the b-posi-
tion with yields ranging from 63 to 99%. Trisubstitut-
ed or phenyl-substituted linear a,b-unsaturated amide
1e or 1f (entries 5 and 6), and the useful Weinreb[10]
amide 1h (entry 8) reacted equally well, even if, in the
later case, the yield was moderate. In the same way, a
phenyl-substituted a,b-unsaturated ester was also al-
kylated with good yield (entry 10). However, linear
alkyl-substituted a,b-unsaturated substrates reacted
more sluggishly, and the formation of the expected
adduct was hampered by the formation of isomers
(entries 7 and 9). Among those isomers, b,g-unsaturat-
ed species (allylsilanes) were the major by-products
observed. The observation of the formation of such
isomers is in contrast with Trostꢃs results[8b] where,
under quite similar conditions but using PPh3 as
ligand, it was found that, on some linear substrates,
the use of dioxane as solvent inhibited the isomeriza-
tion.
Entry
1
Product
Yield[b] [%]
94 (87)
3aa
3ba
3ca
2
3
>99 (>99)
(63)[c]
4
5
3da
3ea
83 (50)
99[f] (76)
6
7
8
9
3fa
3ga
3ha
3ia
64 (30)
50[d]
32 (30)
45[e] (19)
80[f] (50)
All tentative optimization studies (solvent, temper-
ature, ligand), in order to achieve the exclusive for-
mation of only one isomer at the expense of the
other, failed. However, with these substrates (en-
tries 7 and 9), the conjugated adducts 3ga and 3ia
could be isolated in moderate yields (45 and 50%, re-
spectively).
10
3ja
We were pleased to find that the reaction conduct-
ed on crotyl derivatives, under otherwise identical
conditions, afforded nearly exclusively trisubstituted
allylsilanes at the expense of the conjugated adducts
(Table 2). We previously reported an example of the
reaction of crotylamide affording allylsilane.[5] How-
ever, even if a good yield of allylsilane was achieved,
the isomeric E/Z ratio was only 85:15, and we also
faced reproducibility problems. During the course of
our studies, we found that higher isomeric ratios were
observed using (4-CF3C6H4)3P as ligand and conduct-
ing the reaction in dioxane at lower temperature.[11]
[a]
Reactions conducted with 1 mmol of 1, 2 equiv. of 2, 2.5
mol% of [RuCl2A(p-cym)]2, 30 mol% NaHCO2 and 15
mol% of (4-CF3C6H4)3P, at 1008C in 1 mL 1,4-dioxane
for 20 h.
Isolated yields. Between parenthesis yields obtained
using PPh3 as ligand, conducting the reaction in toluene
at 1208C.
Reaction conducted in toluene at 1408C.
Formation of 4% of allylsilane.
CTHUNGTRENNUNG
[b]
[c]
[d]
[e]
[f]
Formation of 18% of allylsilane.
Reaction time of 7 h.
Indeed, the reaction of tert-butylcrotylamide (1k) with E/Z ratio >97:3 (entry 1). Other allylsilanes, derived
vinylsilane 1a, in the presence of 2.5 mol% [RuCl2A(p- from 1k, could be prepared by reacting amide 1k with
CTHUNGTRENNUNG
cym)]2, 30 mol% of sodium formate and 15 mol% of different vinylsilanes (entries 1–4), still with high ste-
(4-CF3C6H4)3P as ligand, in dioxane at 1008C, allowed reoselectivities. In the same way, other crotylamides
the formation of allylsilane 4ka in 69% yield and an reacted equally well allowing the formation of allylsi-
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Ruthenium-Catalyzed C H Bond Activation of Michael Acceptors
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Table 2. Ruthenium-catalyzed formation of allylsilanes via
[a]
À
C H activation.
Scheme 2. Allylation of aldehydes with tri-substituted allylsi-
lanes.
Entry
1
Product
Yield[b] [%]
69 (66)
4ka
4kb
4kc
4kd
4la
Indeed, these preliminary results show the possibili-
À
ty to generate directly, from the C H bond activation
of readily available substrates, functionalized trisubsti-
tuted allylsilanes, whose synthesis is not generally
straightforward.[9]
2
3
4
5
61
The usefulness of such trisubstituted allylsilanes,
was evaluated in the allylation of aldehydes. Indeed,
reaction of the previously prepared allylsilane 4ka
with benzaldehyde using Yamamotoꢃs conditions,[12]
i.e., under Ag/Difluorphos catalysis, afforded alcohol
5, bearing a quaternary carbon center, in a 55% unop-
timized yield (Scheme 2). The latter was obtained as a
70/30 mixture of diastereoisomers.
70
58 (66)
58 (68)
To the best of our knowledge this example consti-
tutes one of the rare examples of catalytic allylation
of aldehydes with
6
7
4ma
60
a
trisubstituted allylsilane.[13]
Indeed, starting from readily available substrates, a,b-
unsaturated substrates, vinylsilanes and aldehydes, it
was possible to access highly functionalized structures
via two catalytic sequences involving intermediate
ruthenium-catalyzed generation of allylsilane and its
subsequent silver-catalyzed reaction with aldehyde.
Although the reaction mechanism is still under
study, the reaction course of this ruthenium-catalyzed
formation of either conjugated adduct or allylsilane
can be rationalized (Scheme 3) on the basis of previ-
ously reported studies on the proposed reaction
mechanism of the ruthenium-catalyzed functionaliza-
tion of acetophenones, which is occurring under simi-
lar conditions.[6b,14] Intermediate A, resulting from oxi-
dative addition of low-valent ruthenium(0) species
4na
4oa
50 (35)
50
8
[a]
Reactions conducted with 1 mmol of 1, 2 equiv. of 2, 2.5
mol% of [RuCl2A(p-cym)]2, 30 mol% NaHCO2 and 15
mol% of (4-CF3C6H4)3P, at 1008C in 1 mL 1,4-dioxane
for 20 h.
Isolated yields. E/Z ratio for isolated allylsilanes were
above 97:3. Between parenthesis yields obtained using
PPh3 as ligand, conducting the reaction in toluene at
1208C, isomeric ratio E/Z being lower than 90:10.
CTHUNGTRENNUNG
[b]
À
into the b C H bond of the Michael acceptor, reacts
with vinylsilane to produce the alkenyl-alkyl-rutheni-
lanes 4la and 4ma bearing benzyl- and piperidine-cro- um species B. At this stage, reductive elimination can
tylamide substituents in 58 and 60% yields, respec- occur, affording the intermediate D. However, it has
tively (entries 5 and 6). It also appeared that crotonic been demonstrated that, in this reaction, a particular
esters were also amenable to this reaction (entries 7 two-step reductive elimination process is operating,
and 8) allowing a straightforward access to trisubsti- involving the intermediate formation of a five-coordi-
tuted allylsilanes with an ester substituent. All the al- nated metallacycle with agostic interactions C.[14]
lylsilanes thus obtained have the (E) stereochemistry From this intermediate, reductive elimination can
(E/Z>97:3), which was confirmed by NOE NMR ex- occur, affording conjugated adducts 3.
periments. In all of the reactions conducted with
However, starting from metallacycle C, and depend-
those crotyl substrates, formation of the normal con- ing on the steric and electronic nature of the activated
jugated adduct was also observed, in yields ranging substrate, a b-hydride elimination can occur giving the
from 5 to 20%. The later were produced as a mixture intermediate F. Then, reductive elimination allows the
1
of Z/E isomers, as judged by GC/MS and H NMR regeneration of the active ruthenium(0) species and re-
analysis.
sults in the formation of the allylsilane 4. Further stud-
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Marc-Olivier Simon et al.
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Scheme 3. Proposed mechanistic interpretation for the allylsilanes formation.
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À
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Acknowledgements
This work was support by the Centre National de la Recher-
che Scientifique (CNRS). M.-O. Simon thanks the Ministꢀre
de l’Education et de la Recherche for a grant.
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