Self-buffering hybrid gold-polyoxometalate catalysts for the catalytic cyclization of acid-sensitive substrates

Article abstract of DOI:10.1002/chem.201201007

Grafting of a gold complex to an organo-polyoxometalate delivers catalytically active bitopic hybrids. The gold end activates allenes, while the metal-oxide surface can capture protons (see scheme). The scope of the gold-catalyzed oxacyclization of allenols is expanded to highly sensitive tertiary benzylic alcohols. Copyright

Full text of DOI:10.1002/chem.201201007

COMMUNICATION
DOI: 10.1002/chem.201201007
Self-Buffering Hybrid Gold-Polyoxometalate Catalysts for the Catalytic
Cyclization of Acid-Sensitive Substrates
Nathalie Duprꢀ,[a] Christian Brazel,[a] Louis Fensterbank,[a] Max Malacria,[a]
Serge Thorimbert,*[a] Bernold Hasenknopf,*[a] and Emmanuel Lacꢁte*[a, b]
The recent development of functionalized polyoxometa-
lates (POMs)[1] opens broad new perspectives for catalysis
involving these clusters.[2] Merging of the properties of
POMs and organic or organometallic fragments either
through functional cations[3] or through covalent attachment
can lead to new reactivity,[4] or better turnovers.[5,6]
Herein, we report that the weak basicity of the oxo li-
gands in organometallic Au-polyoxotungstate covalent hy-
brids significantly improves the scope of the gold-mediated
oxacyclization to acid-sensitive allenes.
This work started from the observation that the usually
efficient Au-catalyzed oxacyclization of allenes[7] failed for
the cyclization of bis-arylic substrates, such as 1a,[8] deliver-
ing a low yield of vinylallene 2a (Scheme 1). The same elim-
ination was observed in all the different conditions we tried
and with varied diaryl substrates (see Table S1 in the Sup-
porting Information). We attributed this to the adventitious
acidic traces present in the gold catalysts, which dehydrated
the very sensitive bis-benzylic tertiary alcohols more rapidly
than their gold-catalyzed cyclizations.
known that heteropoly acids are compatible with gold catal-
ysis.[10] We also thought that tethering to the POM might fa-
cilitate recycling of the gold catalysts, owing to the specific
solubilities of POMs.[11]
Organometallic hybrids 3–5 were chosen as representative
potentially self-buffering catalysts (Scheme 2). Compound 3
is a derivative of an alkyl-diaryl phosphine, whereas 4 and 5
are derivatives of a triarylphosphine with different geome-
tries. Understanding the interaction of organic and organo-
metallic units with the POM surface is still an unsolved
problem. We felt that the different substitutions in the
spacer would lead to different intramolecular interactions of
the gold atom with the oxide surface,[12] and provide a
handle to control reactivity as well as progress toward map-
ping of the POM/organics recognition.
Scheme 1. Dehydration of acid-sensitive bis-benzylic b-allenyl alcohol 1a.
POMs are the conjugated bases of the strong heteropoly
acids.[9] Thus, we hypothesized that POMs might be benefi-
cial as buffers in the reactions considered, since it is already
Scheme 2. Preparation of the hybrid catalysts (TBA=tetrabutyl ammoni-
um; TBABr=tetrabutyl ammonium bromide).
[a] N. Duprꢀ, Dr. C. Brazel, Prof. L. Fensterbank, Prof. M. Malacria,
Prof. S. Thorimbert, Prof. B. Hasenknopf, Dr. E. Lacꢁte
UPMC Sorbonne universitꢀs
The desired precatalysts were prepared in good yields
(89–98%) from coupling the corresponding w-amino gold
phosphine complexes (6–8) to the activated Dawson a1-or-
Institut Parisien de Chimie Molꢀculaire (UMR CNRS 7201)
4 place Jussieu, C. 229, 75005 Paris (France)
Fax : (+33)1-44-27-73-60
ganotin-substituted
polyoxotungstate
[P2W17O61{Sn-
(CH2)2CO}]6À (9) in acetonitrile.[13] The crude products were
E-mail: serge.thorimbert@upmc.fr
stirred with TBA-loaded Amberlyst cation-exchange resin
to remove all traces of triethylammonium formed during the
coupling around the POM. Introduction of the aminophos-
phines before complexation of AuCl is also possible, albeit
in lower yields. Variable amounts of phosphine oxide were
observed after the isolation of the POM–phosphine ligands.
bernold.hasenknopf@upmc.fr
[b] Dr. E. Lacꢁte
ICSN CNRS
Av. de la Terrasse, 91198 Gif s/Yvette Cedex (France)
E-mail: emmanuel.lacote@univ-lyon1.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201007.
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POMs 3–5 were characterized by NMR and IR spectro-
scopy, mass spectrometry, and elemental analysis. All techni-
ques were in agreement with the proposed structures (see
the Supporting Information for details). For example, the
31P NMR spectrum of 3 showed three peaks. Two (d=À12.1
and À6.6 ppm) were those of the Dawson phosphates, with
the P1 (d=À6.6 ppm) peak showing coupling satellites from
the tin coupling. The third peak (d=28.3 ppm) was charac-
teristic of an aurated phosphine. IR spectra display the typi-
cal strong bands for the Dawson phosphotungstates (in the
700–1100 cmÀ1 range). The bands at higher wavenumber are
those of the organic ligands and counterions. Finally, the
mass spectra show intense signals of the intact anions of 3–5
associated with TBA and H+ cations, confirming their struc-
ture. Minor fragmentations correspond to the loss of AuCl
or ClÀ.
In a typical catalytic cyclization, allene 1a was treated
with catalyst 3 (5 mol%) in the presence of AgSbF6
(5 mol%) in dichloromethane at room temperature. To our
pleasure, compound 10a was isolated in nearly quantitative
yield (Table 1, entry 1) after two days. Diversely substituted
b,b-diaryl b-hydroxy allenes possessing electron-donating
and electron-withdrawing substituents also gave the expect-
ed cyclized products in excellent yields (Table 1, entries 2–
4). Triarylphosphine derivatives (4 and 5) reacted similarly,
albeit the reactions were slower taking 5 days to reach com-
pletion (Table 1, entries 5–8).
catalytic system. This drives the reaction toward cyclization
by allene activation without byproduct formation. Further
support for this hypothesis can be derived from the lack of
reaction in the absence of a cationic gold species.
We designed several control experiments to probe our
tethering approach. First, omission of the POM–Au hybrid
resulted in formation of vinylallene 2a, showing that
AgSbF6 alone also leads to protolysis (Table S1 in the Sup-
porting Information). Second, the reactions were tested with
a POM additive that was not covalently bound to the gold
(Table 1, entries 9–12). Dawson derivative TBA6ACHTUNRGTENUNG[P2W18O62]
was chosen because of its structural similarity to catalysts 3–
5. In all cases, cyclized products 10a–d were isolated, albeit
after prolonged reaction times. In the case of the bis(p-me-
thoxyaryl) substituent, however, a significant amount of
elimination was observed (37%; Table 1, entry 10). When
pyridine was used as a base for the reaction of 1b (Table 1,
entry 13), the reaction was really slow (more than ten days
to reach 90% completion) and some vinylallene 2b was
again observed (6%) together with the expected 10b, isolat-
ed in 76% yield.[7a]
Overall, the controls showed that the hybrid POM–Au
catalysts are the best compromise between yield and reac-
tion rates, for the cyclization of very sensitive allenols. Silver
hexafluoroantimonate is able to generate protons also, but
not to promote cyclization by itself, thus the cyclized prod-
ucts do come from gold activation of the allenes.
The less sensitive allenyl alcohol 1e also led to the cycli-
zation, delivering 80% of 2e after three days (Scheme 3).
This substrate provides a key comparison with the literatur-
e.[7a] In the absence of an amine buffer, the yield for the cyc-
lization of 1e was in the sixties, and slightly improved with
our catalysts. Amine buffering was a way to improve yields,
but as before this impacted even more on overall reactivity,
since the cyclization took five days to reach a yield similar
to the one obtained with 3.
Table 1. Cyclization of acid-sensitive b,b-diaryl b-hydroxy allenes cata-
lyzed by the POM–Au hybrids.
Entry
Substrate
Ar
Cat.
Product, yield [%]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1a
1b
1a
1b
1a
1b
1c
1d
1b
Ph
3
3
3
3
4
4
5
5
–
–
–
–
10a, 95
10b, 96
10c, 97
10d, 88
4-MeO-C6H4
3,5-Me2-C6H3
4-F-C6H4
Ph
4-MeO-C6H4
Ph
4-MeO-C6H4
Ph
4-MeO-C6H4
3,5-Me2-C6H3
4-F-C6H4
4-MeO-C6H4
10a, 91[a]
10b, 96[a]
10a, 99[a]
10b, 80[a]
10a, 75[c]
10b, 55[d]
10c, 58[e]
10d, 85[f]
10b, 76[h]
[b]
9
[b]
10
11
12
13
[b]
Scheme 3. Cyclization of allenol 1e.
[b]
[g]
–
The POM–Au conjugates are more efficient than running
the reaction in the mere presence of a POM. It is known
that POMs can be ligands for transition metals.[6,14] Thus, the
gold atom (especially in its cationic form) may interact with
the polyoxometallic surface, as in A (Scheme 4a), resulting
in catalyst deactivation. It might also be possible that the
POM is basic enough to also capture the proton of the oxo-
nium vinylgold intermediate B (Scheme 4b). The proto-
[a] Reaction took 5 days. [b] Conditions: AuClPPh3 (5 mol%), AgSbF6
(5 mol%), TBA6A[P2W18O62] (5 mol%), CH2Cl2, RT. [c] Isolated yield. Re-
CHTUNGTRENNUNG
action was stopped after 8 days (88% completion). [d] Isolated yield. Re-
action took 10 days, 37% of vinylallene 2b was also isolated. [e] Isolated
yield. Reaction was stopped after 10 days (69% completion). [f] Isolated
yield. Reaction took one week. [g] Conditions: AuClPPh3 (2 mol%),
AgSbF6 (2 mol%), pyridine (2 mol%), CH2Cl2, RT. [h] Isolated yield.
Reaction was stopped after 10 days (90% completion). 6% of 2b was
also formed.
À
deauration of the vinyl gold bond in B would thus become
a slow rate-limiting step because it requires the deprotona-
tion of the POM surface of C.[15] This would also explain
why stronger bases reduce the reactivity even more.
This shows that the polyoxometalate is indeed able to
capture the adventitious protons from the gold/silver mixed
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Hybrid Gold-Polyoxometalate Catalysts
COMMUNICATION
Table 2. Recycling of the hybrid catalyst.
Entry
Substrate
Ar
Run
Product, yield [%]
1
2
3
4
5
6
1a
1a
1a
1b
1b
1b
Ph
Ph
Ph
1
2
3
1
2
3
10a, 88[a]
10b, 96
10c, 86[b]
10b, >99
10b, 96
p-MeO-C6H4
p-MeO-C6H4
p-MeO-C6H4
10b, 90[b]
[a] Reaction was stopped at 90% completion [b] Reaction took over 6
days to reach completion.
Scheme 4. Mechanistic hypotheses for the observed reactivity; a) deacti-
vation through Au–oxo interaction; b) deactivation through proton cap-
ture.
In conclusion, we have introduced a new catalytically
active organometallic hybrid of a polyoxometalate. The
unique covalent tethering of the gold complex to the POM
framework buffers the reaction mixture, which extends the
scope of the cycloisomerization of allenols to substrates ex-
tremely sensitive to dehydration. It also stabilizes the cation-
ic gold active site,[17] allowing the isolation and recovery of
the whole catalyst. Further work will focus on the accurate
mapping of the Au/POM interaction to design catalysts with
improved selectivity.
Attempts to fully characterize A were hampered by the
fact that the different ligands in solution can all interact
with the Au ion, preventing its clean precipitation. Nonethe-
31
À
less in the P NMR spectrum, the RPh2P AuCl signal at d=
28.3 ppm in 3 disappeared when stoichiometric AgSbF6 was
added. Two new signals at d=18.6 and 27.1 ppm were ob-
served in acetonitrile, but only one signal at d=22.1 ppm in
acetonitrile/water (19:1). This suggests that the coordination
sphere of Au is modified.
Acknowledgements
In contrast to the intermolecular complexation between
[AuPPh3]+ and [P2W18O62]6À, the intramolecular complexa-
tion[12,16] in hybrids 3–5 must differ in a way that hampers a
strong deactivating Au/POM association, either because it is
a more rigid system that cannot interact with the more elec-
tron-rich oxo ligands of the surface, or because of steric
shielding. Thus, the hybrids retain a significant catalytic ac-
tivity.
We thank the CNRS, UPMC, the German DFG (C. B.) and the French
ANR (grant 08-PCVI-0005 to E.L., S.T. and B.H.) for funding. Carlos
Afonso and Jean-Claude Tabet (UPMC) are acknowledged for their help
with mass spectrometry. FR2769 is acknowledged for technical assistance.
We also thank Prof. Alois Fꢃrstner (MPI Mꢃlheim) for helpful mechanis-
tic suggestions.
Keywords: allenes
composites · polyoxometalates · protonation
·
gold
·
organic–inorganic hybrid
We sought to take further advantage of the tether. We
surmised that the likely interaction between the gold atom
and the oxo ligands in A might stabilize the cationic gold
atom enough to lead to its recycling even if it is not involved
in the reactivity change.
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Received: March 24, 2012
Published online: && &&, 0000
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These are not the final page numbers!

Hybrid Gold-Polyoxometalate Catalysts
COMMUNICATION
Grafting of a gold complex to an
organo-polyoxometalate delivers cata-
lytically active bitopic hybrids. The
gold end activates allenes, while the
metal-oxide surface can capture pro-
tons (see scheme). The scope of the
gold-catalyzed oxacyclization of alle-
nols is expanded to highly sensitive ter-
tiary benzylic alcohols.
Hybrid Polyoxometalates
N. Duprꢀ, C. Brazel, L. Fensterbank,
M. Malacria, S. Thorimbert,*
B. Hasenknopf,* E. Lacꢁte* &&&& — &&&&
Self-Buffering Hybrid Gold-Polyoxo-
metalate Catalysts for the Catalytic
Cyclization of Acid-Sensitive
Substrates
Chem. Eur. J. 2012, 00, 0 – 0
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