Ruthenium-catalyzed redox isomerization of trifluoromethylated allylic alcohols: Mechanistic evidence for an enantiospecific pathway

Article abstract of DOI:10.1002/anie.201200827

Transfer news: A synthetic approach to chiral β-CF₃- substituted saturated carbonyl compounds has been developed in which ruthenium complexes efficiently catalyze the redox isomerization of CF₃-bearing allylic alcohols by an intramol

Full text of DOI:10.1002/anie.201200827

Angewandte
Chemie
DOI: 10.1002/anie.201200827
Enantiospecific Catalysis
Ruthenium-Catalyzed Redox Isomerization of Trifluoromethylated
Allylic Alcohols: Mechanistic Evidence for an Enantiospecific
Pathway**
Vincent Bizet, Xavier Pannecoucke, Jean-Luc Renaud,* and Dominique Cahard*
The challenge to generate enantiopure molecules featuring
a trifluoromethyl motif at a stereogenic carbon center
increasingly stimulates high interest in many laboratories.
The recent success of the direct introduction of a CF3 group by
nucleophilic, electrophilic, or radical processes[1] nicely com-
plements the equally challenging approach of exploiting
prochiral trifluoromethylated substrates.[2] The stereoselec-
tive construction of an all-C CF3-bearing stereogenic tertiary
carbon center without any heteroatom substituent undoubt-
edly belongs to the most fundamental topics in fluorine
chemistry and remains highly challenging. For this purpose,
a direct a-trifluoromethylation of carbonyl compounds can be
achieved by electrophilic addition to enolates or enol
derivatives; however, remote trifluoromethylation at the
b position of a carbonyl function is not possible and requires
alternative synthetic approaches from b-CF3-substituted a,b-
unsaturated carbonyl compounds, for example, by hydro-
genation,[3] conjugate reduction,[4] and conjugate addition.[5]
In addition, sigmatropic rearrangements of mixed ketene
acetals[6] or allyloxy acetates[7] have been achieved with high
efficiency and complete chirality transfer through well-
defined cyclic transition states.
literature, although recent efforts have enabled the incorpo-
ration of a fluorine atom into the isomerized product.[10,11]
Herein, we present a hitherto unknown ruthenium-catalyzed
redox isomerization of trifluoromethylated allylic alcohols as
a novel synthetic route to enantioenriched trifluoromethyl
carbonyl compounds. Several issues were addressed during
the search for the optimal reaction conditions including:
a) the choice of the catalyst, b) the impact of the highly
electron-withdrawing CF3 group: electronic, steric, and posi-
tion effects, c) the gain of mechanistic insights, and d) the
stereocontrol of the newly created stereogenic center.
A screening of the reaction conditions provided the
optimized results compiled in Table 1 (see the Supporting
Information for full details). The desired b-CF3-substituted
saturated ketones were obtained in quantitative yields by
using 1 mol% [RuCl2(PPh3)3]. The reactions are very clean
and the use of more than 1 mol% catalyst is detrimental to
the yield because of the persistence of the corresponding
enone, which results in an incomplete redox process. In this
case, the abstraction of the hydrogen atom a to the OH group
by the metal is not followed by a subsequent 1,4-hydride
addition.
The catalytic redox isomerization is an efficient, selective,
atom-economic, one-step process for the isomerization of
Interestingly, the use of wet toluene doesnꢀt impede the
reaction.[12] The amount and the nature of the base is another
crucial parameter, since in the absence of base no conversion
is observed, and inorganic bases, in particular Cs2CO3, give
better results than organic ones. Lowering the amount of
Cs2CO3 should theoretically allow the reaction to go to
=
a C C bond of O-allylic substrates into saturated carbonyl
compounds. In this method, a transition metal assists the
migration of the carbon–carbon double bond into an enol,
which tautomerizes to the carbonyl compound. It is a con-
ceptually attractive approach, which compares favorably with
the more conventional sequential two-step oxidation and
reduction reactions or vice versa.[8] Ruthenium, rhodium, and
iridium have been particularly successful in this transforma-
tion, with an emphasis placed on the asymmetric version.[9]
Surprisingly, the redox isomerization of an allylic alcohol
containing a CF3-olefin moiety has no precedence in the
Table 1: Redox isomerization of b-CF3-substituted allylic alcohols.
Entry
R1
R2
T [8C]
t [h]
Yield[a] [%]
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Me
Bn
p-MeOC6H4
p-CF3C6H4
Ph
30
30
30
30
30
60
30
60
60
30
70
40
2
4
5
3
2
2
2
3
3
2
12
3
99
99
99
99
99
99
99
99
99
99
99
99
[*] V. Bizet, Prof. Dr. X. Pannecoucke, Dr. D. Cahard
UMR CNRS 6014 C.O.B.R.A., Universitꢀ et INSA de Rouen
1 rue Tesniꢁre, 76821 Mont Saint Aignan (France)
E-mail: dominique.cahard@univ-rouen.fr
Ph
Me
Me
Me
Me
Ph
Prof. Dr. J. L. Renaud
Bn
UMR CNRS 6507 LCMT, Universitꢀ de Caen—ENSICAEN
Avenue du Marꢀchal Juin, 14050 Caen (France)
E-mail: jean-luc.renaud@ensicaen.fr
o-MeOC6H4
p-MeOC6H4
p-BrC6H4
Ph
9
10
11
12
[**] This work was promoted by the interregional CRUNCh network. V.B.
thanks the Rꢀgion Haute-Normandie for a fellowship. Johnson–
Matthey is acknowledged for a generous loan of catalysts.
tBu
Bn
H
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200827.
[a] Yields were determined by 19F NMR spectroscopy using trifluoro-
toluene as an internal standard.
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completion (see Scheme 3), but in practice
one equivalent of base is actually needed,
probably because of the heterogeneity of the
system. These results clearly demonstrate
that the steric bulk of the CF3 group, whose
molar volume is similar to that of an
isopropyl group, is not detrimental to the
reactivity. It is noteworthy that temperatures
of 70–1008C are usually required for the
isomerization of b-disubstituted substrates
with Ru or Rh complexes.[8c,9e,13] Attempts
to isolate the isomerization product of
a primary allylic alcohol (R1 = H, R2 = Ph)
led to the corresponding saturated alcohol as
the main product together with aldol con-
densation products. Indeed, the expected
aldehyde is prone to reduction under the
Scheme 1. Isotope-labeling and crossover experiments.
reaction conditions, and its a-CH group is susceptible to
deprotonation, which generates aldol products.
1a and 1b resulted in no deuterated ketone [D1]-2b being
detectable, thus establishing that the redox isomerization is an
intramolecular process. Furthermore, to evaluate the impact
of a noncoordinated enone as a possible intermediate
resulting from a dissociation from the metal, we carried out
a reaction between 1a and 3. The enone 3 was consumed, but
the formation of ketone 2b was not observed, and 2a was
found to be the major product with significant amounts of
aldol condensation products. This result indicates that the
intermediate enone remains coordinated to the metal center
in the redox isomerization process.
Next we undertook a comparison of trifluoromethylated
allylic alcohols and nonfluorinated substrates (Table 2).
When the CF3 group was replaced by a methyl group
(either the E or Z isomer), the corresponding trisubstituted
allylic alcohols required a much higher reaction temperature
and a longer reaction time but the yields were very poor and
some by-products were observed (Table 2, entries 2 and 3
versus entry 1). Even, a disubstituted allylic alcohol (Table 2,
entry 4) failed to provide the isomerized product in good
yield. These observations indicate that the bulkiness of the
CF3 substituent doesnꢀt affect the reaction and that the
electron-withdrawing effect of the CF3 group significantly
enhances the rate of the migratory insertion step (see
mechanism in Scheme 3). This finding suggests that the
rate-determining step in the redox isomerization of CF3-
substituted substrates is the b-elimination.
To gain mechanistic insights into this redox isomerization
reaction we carried out the reactions illustrated in Sche-
me 1.[8e] Isomerization of deuterium-labeled 1a ([D1]-1a)
provided valuable information on the mechanism. Although
some H-D scrambling occurred during the process (from [D1]-
1a > 95% D to [D1]-2a 81% D), the deuterium was exclu-
sively incorporated at the b-carbon atom, clearly demonstrat-
ing a 1,3-migration pathway. The coexistence of catalytic
species [Ru(D)Cl(PPh3)3], [RuDH(PPh3)3], and [RuD2-
(PPh3)3] has been previously discussed by Bꢁckvall and co-
workers, and the mixed hydride [RuDH(PPh3)3] could cause
the H-D scrambling that provides a fraction of the non-
deuterated ketone 2a.[14] A crossover experiment with [D1]-
We next examined the stereochemistry of the 1,3-hydride
shift. Optically enriched starting allylic alcohols were success-
fully prepared from the corresponding enones by Noyoriꢀs
ruthenium(II)-catalyzed transfer hydrogenation using [RuCl-
(p-cym){(R,R)-Tsdpen}] (p-cym = p-cymene, Tsdpen = N-(p-
toluenesulfonyl)-1,2-diphenylethylenediamine) and the azeo-
tropic formic acid/triethylamine mixture.[15] This reaction
raised the problem of the selective reduction of the keto
group in the a,b-unsaturated enones, and the present result
constitutes a rare example of the chemoselective transfer
hydrogenation of a,b-unsaturated enones[16] and a unique
case with CF3-substituted enones. Furthermore, the enantio-
selectivity of the transfer hydrogenation was very high for
enones having an aryl ketone moiety, but only moderate in
the case of enones having an alkyl ketone moiety and
preparative HPLC was required to reach high ee values
(Table 3, alcohols in entries 4 and 5). However, we studied the
particular case of a poor ee value (Table 3, entry 2). When
[RuCl2(PPh3)3] was used as the catalyst, we were delighted to
observe that the initial alcohols and the saturated ketones had
very similar ee values„ with a very high enantiospecificity
(es)[17] ranging from 97 to 100% [%es = 100(product ee)/
(reactant ee)].[18] These results indicate an excellent transfer
of chirality during the redox isomerization, but this reaction is
not as easy as it first seems because other ruthenium catalysts
do not behave similarly. For example, the transfer of chirality
is not complete with [RuCl2(C6H6)]2 (82% es), [RuCl2(p-
cym)]2 (79% es), and [RuCp*(MeCN)3]PF6 (42% es; Cp* =
C5Me5).
Table 2: Fluorinated versus nonfluorinated substrates.
Entry
R1
R2
T [8C]
t [h]
Yield[a] [%]
1
2
3
4
Ph
Ph
Me
H
CF3
Me
Ph
30
2
98
10
21
33
100
100
30
15
15
9
Ph
Of special interest is the experiment described in Table 3,
entry 3, for which we chose a b-CF3-substituted secondary
[a] Yields of pure isolated products.
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Table 3: Enantiospecific redox isomerization.
Entry R1
R2
Alcohols
e.r.
Ketones
yield[a] [%] e.r.
es [%]
98.5(R):1.5 100
1
2
3
4
5
Ph
Me Ph
Ph
Me Bn
tBu Ph
Ph
98.5(R):1.5 97
62:38 93
p-BrC6H4 98.5(R):1.5 93
62:38
100
97
98
97(R):3
98.5:1.5
99.5:0.5
99.5:0.5
99.5:0.5
67
75
100
[a] Yields of pure isolated products.
allylic alcohol bearing a bromoaryl moiety because its
prochiral trigonal b carbon center can serve as a convenient
probe for the detection of the 1,3-chirality transfer and the
bromine atom can facilitate the determination of the absolute
configuration of the newly created stereogenic center by X-
ray analysis (Scheme 2).[19]
Scheme 3. Mechanism rationale for enantiospecific redox isomerization.
of ruthenium-catalyzed isomerization of nonfluorinated sec-
ondary allylic alcohols described by Ikariya et al. which gives
an intermediate with 74% ee during the synthesis of musco-
ne.[9a]
To illustrate the usefulness of our approach, we prepared
the (S)-CF3 analogue of citronellol, which has olfactory
properties dissimilar to those of the natural product. This
synthetic fragrance compound was previously synthesized by
Seebach and co-workers by a multistep process involving
a resolution.[22] We herein report a shorter and enantioselec-
tive route to (S)-CF3-citronellol in which the key step is an
enantiospecific redox isomerization (Scheme 4).
In summary, we have disclosed a new approach to prepare
enantioenriched carbonyl compounds featuring a b-trifluoro-
methylated stereogenic carbon center that is notable for its
simplicity of execution and enantiospecificity. We have also
described a unique example of chemo- and enantioselective
transfer hydrogenation of b-CF3-a,b-unsaturated enones.
With regard to the issue of the mechanism, we have
conducted experiments that afford the first experimental
evidence in support of an intramolecular suprafacial enantio-
The (R)-allylic alcohol (97% ee) was fully converted into
the (R)-saturated ketone (94% ee) by a process that was 97%
enantiospecific, with the ruthenium center acting almost
exclusively on the Si face of the alkene moiety of the (R)-
allylic alcohol. The overall reaction can occur within the
coordination sphere of the ruthenium center through a supra-
facial enantiospecific 1,3-hydrogen atom transfer. This is
a unique demonstration of the stereochemical course of
a redox isomerization of an acyclic substrate.[20] In light of all
these observations and literature precedence,[21] we propose
the mechanism depicted in Scheme 3. Cesium carbonate
facilitates the formation of a 16-electron ruthenium alkoxide
complex that is further coordinated to the double bond to
account for the reactivity observed. Subsequent b-hydride
elimination produces the enone-hydride complex, in which
the enone remains coordinated until the 1,3-migratory
insertion of the hydride takes place from a single face of the
trigonal carbon center. The resulting ruthenium enolate is
then protonated by an incoming allylic alcohol and tauto-
merizes into the final saturated ketone concomitantly with the
release of the catalyst.
Our approach allows a CF3-bearing stereogenic carbon
center at a remote position of a carbonyl function to be
controlled with excellent enantiospecificity (up to 99% ee
and 100% es). It compares favorably with the unique example
Scheme 4. Reaction conditions for the asymmetric synthesis of (S)-
CF3-citronellol: a) [RuCl(p-cym){(R,R)-Tsdpen}], Et3N/HCO2H;
b) [RuCl2(PPh3)3], toluene, Cs2CO3; c) Beckmann rearrangement
through the corresponding oxime, then hydrolysis and reduction.
Scheme 2. Suprafacial enantiospecific 1,3-chirality transfer.
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specific 1,3-hydrogen transfer. This new synthetic approach
was applicable to the asymmetric synthesis of a CF3 analogue
of citronellol.
Experimental Section
General procedure: b-Trifluoromethylated allylic alcohol (1 mmol),
degassed toluene (2 mL), cesium carbonate (325.8 mg, 1 mmol), and
[RuCl2(PPh3)3] (9.6 mg, 10À2 mmol) were added to a Schlenk tube
under an inert atmosphere. The mixture was heated at a temperature
of 30 to 708C for 1 to 12 h (see Table 1), until 19F NMR spectroscopic
analysis showed full conversion. The reaction mixture was then
filtered through celite, concentrated under reduced pressure, and
purified by column chromatography on silica gel (petroleum ether/
ethyl acetate: 99:1) to give the corresponding b-trifluoromethylated
ketone.
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Received: January 31, 2012
Published online: May 15, 2012
[11] Similar substrates rearranged into b-CF3-substituted aryl
ketones by a triethylamine-mediated conjugation mechanism,
see: F. L. Qing, W. Z. Gao, Chin. J. Org. Chem. 2000, 20, 764 –
768. We thank one of the reviewers for drawing our attention to
this.
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Keywords: asymmetric catalysis · fluorine · isomerization ·
redox chemistry · ruthenium
.
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