Enantioselective and regioselective ruthenium-catalyzed decarboxylative etherification of allyl aryl carbonates

Article abstract of DOI:10.1002/chem.200800619

A study was conducted to investigate enantioselective and regioselective ruthenium-catalyzed decarboxylative etherification of allyl aryl carbonates. It was observed that allyl aryl carbonates react in the presence of CpRu and pyridylmonooxazoline ligands, to produced branched allyl aryl ethers in high enantiomeric purity and significant regioselectivity. The transformation demonstrated an effective enantioselective decarboxylative etherification. Initial investigations were conducted, using conditions similar to that of the enantioselective rearrangement of allyl β-keto esters. Allyl carbonate was treated with catalytic amounts of CpRu in THF at room temperature. The scope of the asymmetric protocol was also studied with allyl carbonates, with improved conditions.

Full text of DOI:10.1002/chem.200800619

COMMUNICATION
DOI: 10.1002/chem.200800619
Enantioselective and Regioselective Ruthenium-Catalyzed Decarboxylative
Etherification of Allyl Aryl Carbonates
Martina Austeri, David Linder, and JØrôme Lacour*[a]
Among the large variety of synthetic methods yielding
non-racemic products, one of the most documented is the
attack of nucleophiles onto allyl–metal intermediates yield-
ing chiral allylic products in high enantiomeric purity.[1]
With unsymmetrical allyl–metal intermediates, the regiose-
lectivity of the reaction is of central importance and it can
be controlled by the metal at-play.[2] In this respect, several
ruthenium derivatives have proven to be largely effective
for the introduction of nucleophiles at the more substituted
position leading to branched (b) rather than linear (l) prod-
ucts [Eq. (1)].[3] Cp*Ru derivatives, and Cp’Ru moeties that
contain cyclopentadienyl rings with tethered ligands, are
generally preferred over CpRu compounds (Cp*=C5Me5,
Cp=C5H5).[4–9]
been reported so far. The first concerns the etherification of
allyl chlorides with phenols. Bruneau and Renaud and co-
workers have first shown that combinations of Cp*Ru 1a
and box-type ligands (e.g. 1, Figure 1) afford allyl aryl ethers
Figure 1. Ruthenium complexes a: R=Me, b: R=H and chiral diimine
ligands.
Typical substrates are primary or secondary allyl carbo-
nates and chlorides, and effective allylic alkylation, amina-
tion, and etherification reactions have been developed.[4–10]
If non-racemic secondary allyl carbonates are used, the reac-
tions proceed stereospecifically with, possibly, complete
transfer of chirality.[4] Recently, it was shown that allyl b-
keto esters[10] and alcohols[11] can also be employed as sub-
strates in related processes.
in good enantioselectivity and decent regioselectivity (up to
82% ee, b/l 1.6:1 to 6.5:1).[12] Very recently, Onitsuka et al.
have reported excellent results for this reaction using
planar-chiral Cp’Ru 2 as mediator (up to 95% ee and, b/l>
20:1) and shown that only allyl halides provide high selectiv-
ity in this reaction.[13] The second kind of transformation
concerns the decarboxylative rearrangement of allyl b-keto
esters. Recently, our group has shown that CpRu 1b and
pyridine–imine ligands (e.g. L2 and L3, Figure 1) afford
non-racemic g,d-unsaturated ketones through regio- and
Efficient Ru-catalyzed enantioselective allylic substitu-
tions of linear unsymmetrical substrates are nevertheless
rare. Only two types of successful transformations have
ꢀ
enantioselective C C bond forming reactions (up to 85%
ee and, b/l>20:1).[14] However, none of these transforma-
tions have used allyl carbonates as starting materials. The
lack of any examples of this process was striking as these
moieties are very common starting materials in asymmetric
metal-catalyzed processes.[1,15] A possible reason is the facile
reaction of these substrates with Cp*Ru 1a and CpRu 1b to
[a] M. Austeri, D. Linder, Prof. J. Lacour
DØpartement de Chimie Organique, UniversitØ de Gen›ve
Quai Ernest Ansermet 30, 1211 Gen›ve 4 (Switzerland)
Fax : (+41) 22-379-3215
E-mail: jerome.lacour@chiorg.unige.ch
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800619.
Chem. Eur. J. 2008, 14, 5737 – 5741
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5737

form RuIV allyl carbonate complexes of type 3a and 3b (Fig-
ure 1).[8e,g] The derived complexes are more catalytically-
active than 1a and 1b and lead to racemic products only.
The development of an efficient enantioselective allyla-
tion reaction using allyl carbonates as substrates and ruthe-
nium catalysts was then worth studying. Herein we report
that allyl aryl carbonates can be used. They react in the
presence of CpRu 1b and easy-to-prepare pyridylmonooxa-
zoline (pymox) ligands to afford branched allyl aryl ethers
in high enantiomeric purity and excellent regioselectivity
(up to 87% ee and, b/l>20:1).[16] This transformation is, to
our knowledge, the first example of an effective enantiose-
lective decarboxylative etherification.
In view of the successful results with aryloxides as nucleo-
philes in intermolecular Ru-catalyzed displacement reac-
tions,[13,14] we reasoned that allyl aryl carbonates of type 4
(Table 1) ought to be ideal substrates as, after carbon diox-
ide extrusion, the resulting aryloxides could react directly
with allyl fragments.[17] Surprisingly, only few reports have
been devoted to the formation of allyl aryl ethers by metal-
catalyzed, intramolecular decarboxylation of allyl aryl car-
bonates.[18,19] To our knowledge a single attempt has been re-
ported at developing an enantioselective version of this
transformation (up to 24% ee).[20] This lack of examples was
making the study even more interesting.
and L3 (10 mol%),[15] the reaction proceeded but modest re-
sults were obtained (conversion up to 24% after 2 h, up to
73% ee). A screening of chiral ligands was then performed,
and of pymox derivatives in particular.[22] A selection is pre-
sented in Figure 1. Ligands L4 to L6 were synthesized fol-
lowing the procedure of Bolm et al. by condensing commer-
cially available enantiopure 1,2-aminoalcohols onto 2-cyano-
pyridine with a catalytic amount of ZnCl2.[23,24] Whereas
ligand L4 allowed the reaction to proceed with decent con-
version (2 h, 74%) and moderate enantioselectivity (5a:
56% ee), essentially no reaction was observed with more
sterically hindered, tert-butyl substituted L5. With ligand L6,
derived from (1R,2S)-cis-1-amino-2-indanol, the reaction
was the fastest (2 h, 100% conversion) and the desired
branched adduct 5a afforded with good overall selectivity
(84% ee, b/l>95:5).[25] The results are summarized in
Table 1. Interestingly, longer reaction times (12 h) leads to a
small but definite decrease in both, ee and, b/l values. This
result will be explained later in the course of the study.
With the improved conditions at hand (THF, 258C, L6 10
mol%), the scope of the asymmetric protocol was studied
with allyl carbonates 4b–f (Table 2). At first substituents
were introduced on the aromatic nucleus of the cinnamyl
fragment (4b: R=p-Cl, 4c: R=p-NO2). From 4a–c, a grad-
ual decrease in the reactivity of the allylic substitution was
noticed. In terms of regioselectivity, whereas no change was
observed with 4b, a sharp decrease proceeded with 4c (b/l
4b:>95:5 and 4c: 75:25). These variations resulting from
the presence of an electron-withdrawing atom or group are
in line with a previous result in this field.[8a] Interestingly, in
terms of enantioselectivity, little difference is observed with
these three substrates which would tend to indicate that the
enantio- and the regiodetermining steps of this reaction are
distinct and independent. A series of allyl carbonates with
substituents on the aryloxy moiety (4d–f) was also prepared.
Their structures are detailed in Tables 1and 2. Not surpris-
ingly, slower reactions resulted from the introduction of
electron-donating, p-Me on the “leaving-group” (4d and 4e)
with, however, little effect on the regioselectivity (b/l 90:10)
and enantiomeric purity of the branched products (84–
85% ee). With 4 f, a clear activation resulted from the pres-
ence of the electron-withdrawing, p-NO2 group, however, at
the expense of the enantiomeric purity of 5 f (34% ee).
Table 1. Ligand screening for allyl carbonate 4a.[a]
Allyl Ligand t [h] Conversion [%] ee[b] Configuration[c] b/l ratio[d]
4a
4a
4a
4a
4a
4a
4a
4a
–
24
2
2
2
2
2
2
12
34
37
24
14
74
<10
100
100
–
–
–
–
83:17
93:7
93:7
93:7
91:9
–
bpy
L2
L3
L4
L5
L6
L6
60
73
56
–
84
78
(ꢀ)
(ꢀ)
(ꢀ)
–
(+)
(+)
>95:5
91:9
[a] 1b (10 mol%), ligand (10 mol%), THF, 258C, c = 0.5m; the results
being the average of at least two runs. [b]Determined by CSP-HPLC.
[c]Sign of the optical rotation. [d]Ratios of branched (5a) to linear (6a)
Table 2. Ru-catalyzed etherification of allyl carbonates 4a to 4 f.[a]
1
Allyl
R
R’
t
Conversion
ee[b] Configuration[c] b/l
ratio[d]
products determined by H NMR spectroscopy (400 MHz).
[h] [%]
4a
4b
4c
4d
4e
4 f
H
H
H
H
2.0 100
84
87
85
84
85
34
(+)
(+)
(ꢀ)
(ꢀ)
(ꢀ)
(ꢀ)
>95:5
>95:5
75:25
90:10
90:10
87:13
Initial experiments were conducted using conditions simi-
lar to that of the enantioselective rearrangement of allyl b-
keto esters.[15] Allyl carbonate 4a (Table 1, R, R’=H) was
treated with catalytic amounts of 1b (10 mol%) in THF at
room temperature (Table 1).[21] Importantly, without ligand,
little reactivity was observed (34% conversion after 24 h);
the presence of 2,2’-bipyridine (bpy)[5] accelerating the reac-
tion and improving the regioselectivity. In presence of L2
Cl
NO2
H
Cl
H
2.5
7.0
92
87
90
90
94
Me 2.5
Me 3.5
NO2 0.5
[a] 1b (10 mol%), L6 (10 mol%), THF, 258C, c = 0.5m; the results
being the average of at least two runs. [b] Determined by CSP-HPLC.
[c]Sign of the optical rotation. [d]Ratios of branched (5) to linear (6)
1
products determined by H NMR spectroscopy (400 MHz).
5738
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Chem. Eur. J. 2008, 14, 5737 – 5741

Ruthenium-Catalyzed Decarboxylative Etherification
COMMUNICATION
To gain some insight on the nature of the asymmetric
transformation, a series of experiments was performed.
First, a 1:1 mixture of allyl carbonates 4b and 4d was treat-
To further characterize the 5!6 transformation, experi-
ments were performed under the following conditions:
608C, THF, 1b and L6 10 mol% each. A mixture of rac-5a
and 6a (b/l 92:08) was heated for 4 h. The, b/l ratio gradual-
ly changed in favor of the linear product to reach a 80:20
value. Importantly, a small enantiomeric excess could be de-
tected for the remaining branched substrate in favor of (ꢀ)-
5a (9% ee; see the Supporting Information for details). This
means that, out of the racemic mixture the (+)-5a enantio-
mer has reacted to some extent faster than (ꢀ)-5a during
this branched to linear transformation. As the dextrorotato-
ry enantiomer is the one preferentially formed during the
enantioselective etherification with L6 as ligand, this ex-
plains why longer reaction times are detrimental for the en-
antiomeric excess.
1
ed under the standard conditions (Scheme 1) and H NMR
analysis (500 MHz) of the resulting mixture indicated the
presence of products 5b and 5d, and cross-over products 5a
and 5e in a 1.4:2.0:1.0:1.4 ratio, respectively. This result in-
dicates that the etherification reaction proceeds through a
state where in situ generated nucleophilic and electrophilic
fragments are separated in solution resulting in a complete
cross-over.
A 1:1 mixture of allyl ethers 5b and 5d was also heated
(608C) in the presence of the catalytic combination and the
1
reaction was monitored by H NMR (400 MHz). After 7 h,
all possible branched and linear products compatible with
intermolecular processes were observed (5b, 5d, 6b, 6d and
cross-over 5a, 5e, 6a, 6e). A prolonged reaction time of
24 h leads to the quasi exclusive formation of the linear
products (6a, 6b, 6d, and 6e) in approximately equal
amounts. These results indicate that aryloxides can act as
leaving groups[26] under the reactions conditions if care is
not taken to avoid a displacement (lower temperature, short
reaction time).[27] Equilibration between branched and
linear products occurs, via a dissociative, intermolecular
mechanism leading to a loss of enantiomeric purity of the
initial branched allyl aryl ethers and, finally, to the exclusive
formation of the linear thermodynamically more-stable iso-
mers. The enantioselectivity observed during the reaction
probably results from the initial oxidative addition step; the
regioselectivity being then dictated by the structure of the
p-allyl complex.[5g,8g]
In conclusion, we have just described the first effective
decarboxylative etherification of allyl aryl carbonates using
a readily-prepared (one step from commercial sources)
pymox ligand L6 and readily available CpRu complex 1b.
Reaction conditions are most simple as only these two re-
agents are necessary. We have also characterized an equili-
bration reaction that must be taken into consideration for
any asymmetric development. Further studies are performed
to understand the intricate details of this transformation and
extend the results to other useful processes.
Scheme 1. -Reactions of 4b and 4d.
At that stage, we considered the result of the decarboxy-
lative etherification of 4a for which, after 12 h of reaction
time at 258C, lower levels of enantio- and regioselectivity
were obtained (78% ee, b/l 91:9). We wondered if an equili-
bration between products 5a and 6a did not occur under
the reaction conditions at the expense of the selectivity. Pre-
liminary experiments performed at 258C seemed to validate
this hypothesis, but these studies were hampered by slow ki-
netics. The decarboxylative etherification of 4a was thus
performed at 608C. The results are detailed in Table 3.
Clearly, full conversion was achieved rapidly at this higher
temperature. A result similar to that of the reaction at 258C
after 12 h was obtained after only 45 min at 608C (Table 3,
entry 1). Longer reaction times (Table 3, entries 2–6) result-
ed, as hypothesized, in a progressive loss of enantiomeric
purity and in an increase of the b/l ratio. Branched adduct
5a disappeared in favor of linear compound 6a which
became the exclusive product after reaction at 608C for
22 h.
Table 3. Etherification of allyl carbonate 4a.[a]
Entry
t [h]
Conversion [%]
ee[b]
Configuration[c]
b/l ratio[d]
Experimental Section
1
2
3
4
5
6
0.75
100
100
100
100
100
100
80
74
64
50
2
(+)
(+)
(+)
(+)
(+)
–
92:8
1.5
2.5
3.5
8.0
89:11
80:20
69:31
33:66
<5:95
General procedure for catalytic etherification of the allyl carbonates: I n
a 1 mL vial under dinitrogen atmosphere, [RuCpACTHERNGU(MeCN)3]ACHTREUNG[PF6] (1b;
6.3 mg, 14.4 mmol, 10 mol%) and pymox L6 ligand (3.4 mg, 10 mol%)
were dissolved in distilled anhydrous THF (300 mL). The resulting deep
red solution was stirred for 5 min at room temperature before the addi-
tion of the allyl aryl carbonates 4 (0.144 mmol). The reaction was stirred
under N2 at 258C until no trace of the starting material could be seen on
TLC (silica gel, Et2O/pentane 8:2). The reaction mixture was diluted
with Et2O/pentane 8:2 (1.5 mL). The precipitated metal salts were fil-
22
–
[a] 1b (10 mol%), L6 (10 mol%), THF, 608C, c = 0.5m. [b]Determined
by CSP-HPLC. [c]Sign of the optical rotation. [d]Ratios of branched
(5a) to linear (6a) determined by NMR spectroscopy (1H NMR,
400 MHz).
Chem. Eur. J. 2008, 14, 5737 – 5741
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www.chemeurj.org
5739

J. Lacour et al.
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Acknowledgements
We are grateful for financial support of this work by the Swiss National
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ꢀ
Keywords: allylic compounds · C O bond formation ·
enantioselectivity · N ligands · ruthenium
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5740
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Chem. Eur. J. 2008, 14, 5737 – 5741

Ruthenium-Catalyzed Decarboxylative Etherification
COMMUNICATION
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Received: April 2, 2008
Published online: May 21, 2008
Chem. Eur. J. 2008, 14, 5737 – 5741
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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