Highly enantioselective Rh-catalysed hydrogenation of 1-alkyl vinyl esters using phosphine-phosphoramidite ligands

Article abstract of DOI:10.1002/chem.201303066

MatPhos, a good mate for hard tasks: The asymmetric hydrogenation of 1-alkyl vinyl esters, thwarted so far by mediocre ee values and low activities, can now be achieved with MatPhos/Rh catalysts with ee values of 96-99 % for a variety of substrates at low catalyst loadings (0.1-1 mol %) and under mild conditions (5-20 bar H₂, room temperature). After hydrolysis, the corresponding chiral secondary alkyl alcohols can be obtained in high enantiopurities providing a general and practical route to this important product class. Copyright

Full text of DOI:10.1002/chem.201303066

COMMUNICATION
DOI: 10.1002/chem.201303066
Highly Enantioselective Rh-Catalysed Hydrogenation of 1-Alkyl Vinyl Esters
Using Phosphine–Phosphoramidite Ligands
Tina Maria Konrad, Pascal Schmitz, Walter Leitner,* and Giancarlo Franciꢀ*[a]
Dedicated to Prof. Dr. Manfred T. Reetz on the occasion of his 70 birthday
Asymmetric hydrogenation is probably the most advanced
and useful transition-metal-catalysed transformation for the
synthesis of chiral building blocks both on laboratory and in-
dustrial scale.[1] Excellent levels of enantioselectivity have
been achieved for a broad variety of substrates and func-
tionalities at often high productivities. However, no effective
transition-metal catalyst for the asymmetric hydrogenation
of aliphatic ketones is known so far.[1,2] Whereas outstanding
enantioselectivity can be obtained for ketones bearing an
additional functionality such as (hetero)aryl,[3] olefin,[3,4] and
cycloalkyl[3,4a,5] groups, simple methyl alkyl ketones cannot
be hydrogenated with synthetically useful enantiomeric
excess. The two alkyl chains do not provide sufficient struc-
tural differentiation for high enantio-discrimination by typi-
cal transition-metal catalysts.
Currently, chiral secondary alkan-2-ols are accessible in
high enantiomeric purity (>95% ee) by enzymatic routes
only, either upon reduction of the corresponding ketones[6]
or through (dynamic) kinetic resolution of the alcohols.[7]
Therefore, this class of substrates represents a significant
gap in the scope of the otherwise extremely efficient and
well-developed catalytic hydrogenation methodology.
ic system for this class of substrates was reported by Reetz,
Gooßen and co-workers.[16] With Rh-monophosphite cata-
lysts, ees up to 90% could be achieved for furan-2-carbox
ACHTUNGTRENNUNGyl-
AHCTUNGTRENNUNG
lower enantioselectivities of 86%, 74%, and 42% ee were
obtained with the corresponding benzoate,[17] acetate and
pivalate derivatives, respectively. More recently, Gooßen
and co-workers showed that the same catalyst can success-
fully be applied in the hydrogenation of alk-2-en-2-yl esters
leading to good to high enantioselectivities (77–98% ee).[18]
Herein, we report that rhodium catalysts comprising che-
lating phosphine–phosphoramidite ligands are very efficient
systems for hydrogenation of 1-alkyl vinyl esters and other
enol esters, opening a general access to chiral secondary al-
cohols including for the first time simple alkan-2-ols in syn-
thetically useful enantiopurity.
Electronically unsymmetrical phosphine–phosphorami-
dites proved to be excellent ligands for asymmetric catalysis,
in particular for hydrogenation reactions.[19] The QuinaPhos
structure was reported as the first example of this class of li-
gands.[20] The 1-naphthyl-substituted QuinaPhos L1 and the
related MatPhos[21] L2 (Figure 1) have been successfully ap-
The asymmetric hydrogenation of 1-alkyl vinyl esters fol-
lowed by hydrolysis constitutes an alternative catalytic route
to the corresponding chiral alcohols.[8] In addition to acyla-
tion of methyl alkyl ketones,[9] other synthetic methods for
the generation of enol esters such as the Ru-catalysed addi-
tion of acids to alkynes[10] can also be envisaged in retrosyn-
thetic schemes towards alkan-2-ols by this route. While
useful enantioselectivities (95–99% ee) could be obtained
for vinyl esters substituted in the 1-position with cyclic,[11] ar-
omatic,[11,12] carboxylic,[13] trifluoromethyl[14] or cyano[12c]
groups, only very few examples with acceptable enantiose-
lectivities have been reported up to now for non-functional-
ised 1-alkyl vinyl derivatives.[15] The most prominent catalyt-
Figure 1. Bidentate phosphine–phosphoramidite ligands used in this
study.
plied subsequently in Rh-catalysed hydrogenations of func-
tionalised alkenes,[20–22] Ru-catalysed hydrogenations of
functionalised ketones,[21,23] and Ir-catalysed hydrogenation
of 2-substituted-quinolines.[21]
Considering the structural similarity to enamides, which
could be hydrogenated with high enantioselectivities with
Rh/phosphine–phosphoramidites catalysts,[21,22b] we decided
to examine (Ra,Sc)-1-Naph-QuinaPhos L1 and (Sa,Sc)-Mat-
Phos L2 in the Rh-catalysed asymmetric hydrogenation of
[a] Dr. T. M. Konrad, P. Schmitz, Prof. Dr. W. Leitner, Dr. G. Franciꢀ
RWTH Aachen University
Institut fꢁr Technische und Makromolekulare Chemie
Worringerweg 1, 52074 Aachen (Germany)
Fax : (+49)241-80-22177
E-mail: leitner@itmc.rwth-aachen.de
francio@itmc.rwt-aachen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303066.
Chem. Eur. J. 2013, 19, 13299 – 13303
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13299

Table 1. Asymmetric hydrogenation of oct-1-en-2-yl acetate 1 and pent-1-en-2-yl benzoate 7 under various
1-alkyl vinyl esters. The sub-
strates were prepared either
from the corresponding ketones
by acid-catalysed acylation with
isopropenyl acetate[9] or by the
Ru-mediated addition of car-
boxylic acids to alkynes devel-
oped by Gooßen and co-work-
ers. The latter procedure leads
to the Markovnikov adducts
with good to high selectivities
for acetates (88.9–97.8%) and
conditions.[a]
Entry
Ligand
Sub
Solvent
pH2
[bar]
T
[8C]
Conv.[b]
[%]
ee[c]
[%]
1
2
3
4
U
1
1
1
1
1
1
7
7
7
7
7
7
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
MeOH
EtOAc
2-Me-THF
2-Me-THF
CH2Cl2
CH2Cl2
20
20
20
20
20
20
20
20
20
5
30
0
30
>99
>99
>99
>99
>99
>99
>99
>99
77
84 (S)
91 (S)
96 (R)
98 (R)
99 (R)
99 (R)
99 (R)
99 (R)
91 (R)
95 (R)
91 (R)
99 (R)
0
5[d]
6[e]
7[e,f]
8[e,f]
9[e,f]
10[g]
À20
RT
RT
RT
RT
RT
0
>99
69
>99
furan-2-carboxylate
(96.6%), 11
20
5
12[h]
RT
whereas with benzoic acid
almost exclusively the corre-
sponding 1-alkyl vinyl ben-
zoates were isolated (99%)
(see the Supporting Informa-
tion for details).[10] For screen-
[a] Reaction conditions: substrate 1 or 7=0.78 mmol; catalyst [Rh
1 h. [b] Determined by 1H NMR spectroscopy. [c] Determined by chiral GC. [d] t=4 h. [e] Catalyst formed in
situ from [Rh(nbd)2]BF4/L. [f] t=3 h. [g] t=17 h. [h] t=5 h, p(H2)=5 bar, catalyst loading=1 mol%.
ACHTUNGTNER(NUNG cod)L]BF4 =0.5 mol%; solvent=1 mL; t=
AHCTUNGTRENNUNG
ing purposes, the mixtures of the two regioisomers were sub-
jected to hydrogenation and full conversion of the linear
isomer to the achiral ester was observed without interfer-
ence with the asymmetric transformation.[24]
Oct-1-en-2-ol acetate 1 was selected as the benchmark
substrate for the initial investigations on the asymmetric in-
duction (Scheme 1). Rhodium catalysts were generated in
initial TOF5min of 5600 hÀ1. This value is 5 to 900 times
higher than that for previously reported catalytic sys-
tems.[8,11, 12,16,26]
Variation of the solvent revealed that also more benign
solvents such as MeOH, EtOAc, and 2-Me-THF are excel-
lent reaction media for this transformation. Their use result-
ed in 99% ee (entries 7 and 8), and 91% ee (entry 9) in the
hydrogenation of pent-1-en-2-yl benzoate 7 at 20 bar H2 and
room temperature.
situ from [Rh
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG(nbd)2]BF4 (nbd=norbornadiene) or [Rh-
gands, or by using a preformed complex [Rh
(cod)L]BF4 (L1
Due to the high catalyst activity, lower H2 pressures can
be used as demonstrated by conducting the hydrogenation
of 7 in a glass autoclave at 5 bar H2 pressure and 1 mol%
catalyst loading in CH2Cl2. Full conversion to 7a was ach-
ieved within 5 h at room temperature with a remarkable ee
of 99% (entry 12). Thus, the pressure reduction led to a sig-
nificantly higher ee as compared to the enantioselectivity of
91% ee obtained at 20 bar and 08C (entry 11). The same
trend was observed using 2-Me-THF as the solvent, increas-
ing the ee from 91% at 20 bar to 95% ee at 5 bar H2 pres-
sure (entries 9 and 10).
or L2).[21]
Scheme 1. Asymmetric hydrogenation of oct-1-en-2-ol acetate 1.
The hydrogenation of 1 in the presence of QuinaPhos
(Ra,Sc)-L1 in CH2Cl2 at 308C, 20 bar H2 pressure, and a cata-
lyst loading of 0.5 mol% led to full conversion within one
hour and an enantioselectivity of 84% ee (Table 1, entry 1).
This already represents a higher value as compared to that
obtained by using DuPhos[15,25] or monophosphite ligands[16]
under optimized conditions. By lowering the temperature to
08C, the enantioselectivity could be increased up to 91% ee
(entry 2). Even higher enantioselectivities were obtained
with MatPhos (Sa,Sc)-L2 under the same conditions: 96% ee
at 308C, 98% ee at 08C and 99% ee at À208C (entries 3–5).
Using THF as the reaction medium, an excellent enantiose-
lectivity of 99% ee could be achieved even at room temper-
ature (entry 6).
To exemplify the incorporation of the hydrogenation step
in a synthetic sequence for the generation of chiral sec
ACHTUNGTRENNUNGond-
AHCTUNGTREGaNNNU ry alcohols, the hydrolysis of 1a (99% ee) was carried out
using a mixture of aqueous NaOH and MeOH (Scheme 2).
Scheme 2. Hydrolysis of 1a to octan-2-ol 1b.
Octan-2-ol 1b could be obtained with full retention of the
enantiomeric purity of 99% ee confirming that no racemisa-
tion occurred under the conditions of standard hydrolytic
procedures.
The substrate scope of the Rh/MatPhos (Sa,Sc)-L2 catalyst
was evaluated by systematic variation of the substitution
To estimate the catalyst activity of the (Sa,Sc)-L2/Rh
system, the pressure drop during the hydrogenation of 1 in
THF (initial H2 pressure 20 bar) at a substrate-to-catalyst
ratio of 1000 was recorded (see the Supporting Informa-
tion). Full conversion was obtained within 30 min, with an
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Asymmetric Hydrogenation of 1-Alkyl Vinyl Esters
COMMUNICATION
pattern (Figure 2).[27] Full conversion with excellent enantio-
selectivities of 99% ee could be obtained within 1 h at room
temperature at low catalyst loading (0.5 mol%) in the hy-
drogenation of linear 1-alkyl vinyl acetates 1–4 irrespective
of the chain length. Particularly, noteworthy are the results
achieved with but-1-en-2-ol acetate 4, the simplest and most
challenging representative of this class of substrates. For the
first time, the corresponding saturated ester could be ob-
tained with >99% ee by hydrogenation opening an efficient
chemo-catalytic route to enantiomerically pure butan-2-ol.
The hydrogenation of the 1-alkyl vinyl benzoates 5–7 re-
sulted also in excellent enantioselectivities of 98–99% ee.
Again, the asymmetric induction largely surpasses the best
values reported in the literature for this ester class (90% ee
for substrate 6[12b]). As the addition of benzoic acid to al-
kynes occurs with 99% regioselectivity (vide supra), this
class of substrates provides a particularly attractive synthetic
pathway avoiding a separation step along the reaction se-
quence. Hydrogenation of hex-1-en-2-yl furan-2-carboxylate
8 led within 1 h at 08C to full conversion and an ee of 98%
(best ee reported 94%, at À208C, t=20 h).[16] Thus, high ac-
tivities and excellent enantioselectivities could be achieved
with Rh/ACTHNUGTREN(UNG Sa,Sc)-L2 independently of the ester auxiliary.
Next, the hydrogenation of bulkier 1-alkyl vinyl acetates
with cyclic substituents or branched alkyl chains was investi-
gated. The hydrogenation of 9 bearing a cyclohexyl substitu-
ent at the b-position of the olefinic carbon proceeded with
similar enantiocontrol and activity as for the previous sub-
strates. Lower enantioselectivities of 80% and 86% ee were
obtained in the hydrogenation of 10 and 11 comprising a cy-
clohexyl and a cyclopropyl ring directly adjacent to the ole-
finic carbon, respectively.[4a,5] In contrast, the hydrogenation
of substrate 12 with a tert-butyl group in the same position
resulted again in high enantioselectivity of 96% ee. Higher
catalyst loading (2 mol%) and prolonged reaction time
(18 h) were required for quantitative conversion because of
the steric encumbrance of this substrate.
The diene 13 with a conjugated double bond could be
chemo- and enantioselectively
hydrogenated. When the reac-
tion time was limited to 5 h,
conversion of 94% of 13 occur-
red, leading to the correspond-
ing allyl alcohol derivative 13a
with 99% ee.[15,25] By prolong-
ing the reaction time to 48 h,
the remaining C=C double
bond could be also hydrogenat-
ed to give the fully saturated
nonan-2-yl acetate 13aꢁ while
retaining the enantiopurity
almost completely.
A high enantioselectivity of
97% ee was achieved also in
the hydrogenation of the func-
tionalised cyanovinyl acetate
14. The conjugated strong elec-
tron-withdrawing group de-
creases the reactivity of the ole-
finic bond towards the hydroge-
nation, but full conversion
could still be achieved in 8 h by
increasing the catalyst loading
to 1 mol%.[28] As expected, the
hydrogenation of phenyl vinyl
acetate 15 proceeded smoothly
with an almost perfect enantio-
control of 99% ee.
In conclusion, we have dis-
closed here an efficient and
general catalytic system based
on rhodium complexes of phos-
phine–phosphoramidite ligands
for the asymmetric hydrogena-
tion of vinyl enol esters, open-
ing an attractive chemical route
Figure 2. Products exemplifying the broad scope of the hydrogenation method developed in this study. Full
conversion (>99%) was achieved in all cases, except for 6/6a (97%), 13/13a (94%) and 14/14a (99%). See
the Supporting Information for details.
Chem. Eur. J. 2013, 19, 13299 – 13303
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13301

W. Leitner, G. Franciꢀ et al.
1999, pp. 199–246; c) T. Ohkuma, R. Noyori, in Handbook of Ho-
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to chiral secondary alkan-2-ols with excellent enantioselec-
tivities. Using the Rh/(Sa,Sc)-(MatPhos) (L2) system, linear
AHCTUNGTRENNUNG
unfunctionalised alk-1-en-2-ol esters even with short chain
lengths could be hydrogenated for the first time with enan-
tioselectivities of 99% ee. Variation of the ester group or
substitution at the 1-position of the enol ester is tolerated
widely, allowing a remarkable broad range of structural di-
versity in the substrates. This provides a large degree of flex-
ibility to choose the most favourable synthetic pathway con-
sidering the generation of the vinyl esters as well as the iso-
lation procedures. In favourable cases, the reaction can be
carried out in “green” solvents and at low hydrogen pres-
sures, allowing even the use of standard glass equipment.
Thus, the procedure appears very attractive as a practical
chemo-catalytic alternative to enzymatic methods for the
synthesis of highly enantiomerically enriched alkan-2-ols.
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Typical hydrogenation procedure: A high-pressure autoclave (10 mL)
equipped with a glass inlet and stirring bar was charged under an argon
atmosphere with the desired substrate (0.78 mmol) and a stock solution
of the catalyst (1 mL, 3.9 mm) in the desired solvent. The autoclave was
then pressurised with H2 (20 bar) and stirred at 1000 rpm for 1 h, unless
otherwise stated. After releasing the pressure, a small sample of the mix-
ture was immediately analysed by 1H NMR spectroscopy to determine
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silica SiO2 to remove the catalyst, and the enantiomeric excess was then
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Acknowledgements
We thank H. Eschmann and J. Wurlitzer for GC and HPLC measure-
ments, K. Thenert and J. Nießen for valuable experimental assistance,
and Johnson Matthey and Heraeus for the donation of metal precursors.
The research leading to these results has received funding from the Euro-
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Keywords: asymmetric hydrogenation · chiral alcohols ·
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Asymmetric Hydrogenation of 1-Alkyl Vinyl Esters
COMMUNICATION
[25] N. W. Boaz (Eastman Chemical Co., Kingsport, Tenneseee),
EP872467 A1.
[28] The first example of the asymmetric hydrogenation of this latter
compound was reported recently from Vidal-Ferran and co-workers
using a phosphine–phosphite/Rh catalyst (ee=97%; t=18 h, cata-
lyst=2 mol%).[12c]
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[27] Additional hydrogenation reactions carried out with the QuinaPhos
ligand can be found in the Supporting Information.
Received: June 24, 2013
Revised: August 2, 2013
Published online: September 11, 2013
Chem. Eur. J. 2013, 19, 13299 – 13303
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