Rhodium-catalyzed enantioselective hydrogenation of (E)-enol acetate acids

Article abstract of DOI:10.1002/adsc.201200934

The novel rhodium complex [Rh(S)-Phanephos(cod)]-catalyzed hydrogenation of disubstituted (E)-enol acetate carboxylic acids is reported. The catalytic cycle works under 30 bar of hydrogen under conventional heating giving different 3-acetoxy-2,3-disubstituted carboxylic acids with ee ≥90%. Hydrogenation occurred also under microwave dielectric heating without eroding the enantioselectivity but improving the overall efficiency of the process. With microwaves, hydrogen pressure and reaction time required for complete hydrogenation dropped to 5 bar and 30 min, respectively. The best performance of this catalyst under microwave irradiation was TON 100, TOF 196 h^-1 with ee 99% on a 6-g scale. Copyright

Full text of DOI:10.1002/adsc.201200934

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DOI: 10.1002/adsc.201200934
Rhodium-Catalyzed Enantioselective Hydrogenation of (E)-Enol
Acetate Acids
Giada Arena,a Giuseppe Barreca,b Luca Carcone,b Elena Cini,a Giovanni Marras,b
Hans G. Nedden,c Marcello Rasparini,b,* Stephen Roseblade,c Adele Russo,a
Maurizio Taddei,a,* and Antonio Zanotti-Gerosac
a
Dipartimento di Biotecnologie, Chimica e Farmacia, Universitꢀ di Siena, Via A. Moro 2, 53100 Siena, Italy
E-mail: maurizio.taddei@unisi.it
Chemessentia srl, Via Bovio 6, 28100 Novara, Italy
b
E-mail: marcello.rasparini@chemessentia.it
Johnson Matthey Catalysis and Chiral Technologies, 28 Cambridge Science Park, Milton Road, Cambridge CB4 0FP, U.K.
c
Received: October 22, 2012; Published online: February 22, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200934.
Abstract: The novel rhodium complex [Rh(S)-
Phanephos(cod)]-catalyzed hydrogenation of disub-
lyst,[2a] several transition metal-catalyzed or enzymatic
processes have been developed for the DKR/enantio-
selective hydrogenation of a-substituted-b-keto
esters.[2]
ACHTUNGTRENNUNG
stituted (E)-enol acetate carboxylic acids is report-
ed. The catalytic cycle works under 30 bar of hydro-
gen under conventional heating giving different 3-
acetoxy-2,3-disubstituted carboxylic acids with ee
ꢀ90%. Hydrogenation occurred also under micro-
wave dielectric heating without eroding the enantio-
selectivity but improving the overall efficiency of
the process. With microwaves, hydrogen pressure
and reaction time required for complete hydrogena-
tion dropped to 5 bar and 30 min, respectively. The
best performance of this catalyst under microwave
irradiation was TON 100, TOF 196 hÀ1 with ee 99%
on a 6-g scale.
Engaged in a total synthesis of Aliskiren (Rasilezꢁ,
1), the first direct renin inhibitor marketed by Novar-
tis for the treatment of hypertension,[3] we came
across the possibility to drive the synthesis through
the intermediate A, obtained in turn from b-keto
ester B (Scheme 1).
In order to verify this hypothesis, the preparation
of compound 4a was accomplished through alkylation
of b-keto ester 3 with iodide 2. Compound 3 was ob-
tained as recently described,[4] whereas iodide 2 was
prepared from the corresponding enantiomerically
pure acid, in turn obtained by catalytic asymmetric
hydrogenation following literature reports.[5] These
compounds were reacted using LiH as the base to
give product 4a in 71% yield (Scheme 1). When enan-
tioselective hydrogenation was attempted using
Keywords: asymmetric catalysis; carboxylic acids;
hydrogenation; microwave heating; phosphane li-
gands
[Ru(S)-BINAPACTHNUTRGNEUNG(C6H6)]Cl as the catalyst, lactone 5
was isolated with good yield (90%) and moderate ee
(85%). However, when lactone 5 was treated with
b-Keto esters are very versatile intermediates in or- amine 6 in order to obtain the final target (just a Cur-
ganic synthesis for their disposition to be alkylated at tius rearrangement on the COOEt position separated
the carbon located between the carbonyls. From these compound 7 from Aliskiren), the expected amide was
compounds, the corresponding a-alkyl-b-hydroxy only detected in the LC-MS of the crude reaction
esters can be obtained, setting the stereochemistry of mixture while all attempts to isolate it produced again
the motif through the stereoselective reduction of the the lactone 5 through an unusual displacement of an
carbonyl in the b-keto ester.[1] It has been demonstrat- amide NHR group induced by an alcohol as the nu-
ed that applications of asymmetric hydrogenation to cleophile.
(simple) racemic a-substituted b-keto esters proceed
The difficulties encountered in controlling the lac-
through dynamic kinetic resolution (DKR). The tonization of a b-hydroxy ester such as 7 prompted us
method allows for the creation of two contiguous ster- to explore an alternative reduction of a preformed
eocentres starting from racemic substrates. Since the enol acetate carboxylic acid derivative. Double bond
seminal reports on the use of a chiral ruthenium cata- reduction should give the alkyl acetate (no free OH),
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lated with a favourable orbital overlapping when the
acetate and the carboxylate are in a trans arrange-
ment. Compound 8a was at first employed to investi-
gate the double bond hydrogenation. A broad screen
of catalysts and reaction conditions led to the first en-
couraging results when the reaction was carried out in
1,2-dichloroethane (DCE) at 808C and 30 bar of H2
using different (diphosphine)-Rh and Ru complexes
(Table 1). While MeBoPhoz[10] or BINAP type ligands
were unsatisfactory (entries 1–3 in Table 1), achiral
[11]
[RhACTHNUGTERNU(GNN dppf)ACHUTNGTRNENUG(cod)]BF4 or achiral [RhACHUTNGERT(GNNNU dipf)ACHTNGURTEN(NUGN cod)]BF4
gave moderate selectivity and conversion (entries 4
and 5).
[12]
Catalysis
with
[Rh(S)-PhanephosACHTNGUTERNNU(G cod)]BF4
AHCTUNGTREG(NNUN entry 6) gave comparable conversion and diastereo-
meric ratio with some by-products as revealed by
HPLC analysis of the crude. Toluene, EtOH, THF
and EtOAc all gave scarce conversion (data not in-
cluded in the Table), whereas CH2Cl2 gave slightly im-
provements over DCE (entry 7) with less catalyst
(5%) at lower temperature (608C). Under these con-
ditions, product 10 was isolated in 90% yield as a 95:5
ratio of diastereomers.
In order to minimize the amount of by-products
and increase the stereoselectivity through a stronger
coordinative centre located on the substrate, the free
acid 9 was prepared from 8b and 4b, the latter ob-
tained with a synthetic sequence analogue to that de-
scribed in Scheme 1 for 4a.
Removal of the tert-butyl protection with TFA in
CH2Cl2 at 08C gave free acid 9 in high yields (Scheme
in Table 1). Hydrogenation of 9 under the conditions
developed for ester 8a, using both [Rh(S)-Phanephos-
Scheme 1. Retrosynthesis of Aliskiren and attempted prepa-
ration of the key intermediate 4a. R1 =MeOCH2CH2CH2-.
A
ACHTUNGTRENNUNG(dipf)-
AHCTUNGTRENNUNG
so lactonization could not occurr. Moreover, having 30 bar H2 over 18 h gave only moderate conversion to
a conformationally defined double bond, the setting product 11. A series of experiments performed with
of the stereochemistry of A should rely upon the syn [RhACTHNUTRGENN(GU dipf)ACHTUGNTREN(NUGN cod)]BF4 (5 mol%) showed that the use of
hydride addition rather than on a DKR process. MeOH gave improved reactivity (entry 10) whereas,
Asymmetric hydrogenation of enol ethers has been in the presence of Et3N, conversion increased even
successfully accomplished under chiral homogenous more (entry 11). While [Rh
catalysis and exploiting coordinating or non-coordi- duced some side-products, [Rh(S)-Phanephos-
nating functionalized substrates.[6] However, unlike
(cod)]BF4 in MeOH plus 0.7 equiv. of Et3N gave the
ACHUTGTNRENN(GU dipf)CAHTUGNTREN(NUGN cod)]BF4 still pro-
AHCTUNGTRENNUNG
the case of a-enol esters, the asymmetric hydrogena- highest conversion (99% conversion to product, 95%
tion of b-enol esters has not been so largely investi- isolated yields) and stereoselectivity (99:1) (entry 12).
gated,[7] probably due to difficulties in controlling the Finally the hydrogenation with the enantiomeric cata-
E/Z stereochemistry and in processing the reduced lysts [Rh(R)-PhanephosACHTNUTRGNEUNG(cod)]BF4 (5 mol%) was at-
ethers into the more synthetically valuable alcohols.[8] tempted (entry 14). A reversed (although lower) ste-
Moreover, most of the examples reported concern reoselectivity (dr=25:75, entry 14) was obtained, sug-
more simple substrates when compared to our case.
gesting that a degree of stereocontrol is exerted by
Thus, a general investigation programme was start- the chiral catalyst.[13] Although coordination of Rh to
ed to find the best conditions for the catalytic trans- the acetoxy group on the double bond is likely to
formation. Treating 4a (R=Et) with acetic anhydride occur while working on enol acetate ester such as 8a,
in the presence of TEA and DMAP gave 8a in high the increased activity of [Rh(S)-PhanephosACTHNURTGNENG(U cod)]BF4
yield as a single (E) isomer (scheme in Table 1).[9] shown on the enol acetate carboxylate 9 (Table 1,
The high (E)-stereoselectivity of the process, unex- entry 12) must be explained by a tighter coordination
À
plainable with a simple steric hindrance, might be re- to the CO2 group. The high ee value obtained makes
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Rhodium-Catalyzed Enantioselective Hydrogenation of (E)-Enol Acetate Acids
Table 1. Hydrogenation of enol acetate esters or acids.[a]
Entry
Substrate
Catalyst (%), Solvent, Temperature, Time[c]
[Rh(S)-MeBoPhoz(cod)]BF4 (10), DCE, 808C, 24 h
Product [%][b]
dr[c]
1
2
3
4
5
6
7
8
8a
8a
8a
8a
8a
8a
8a
9
9
9
9
9
G
10 (5)
10 (1)
10 (5)
50:50
–
–
[Rh(S)-BINAP(cod)]BF4 (10), DCE, 808C, 24 h
[(S)-BINAP RuCl2A(dmf)] (10), DCE, 808C, 24 h
(cod)]BF4 (10), DCE,808C, 24 h
(cod)]BF4 (10), DCE, 808C, 24 h
R
R
[Rh
[Rh
N
E
10 (89)
10 (73)
10 (89)
10 (90)
11 (62)
11 (70)
11 (82)
11 (93)
11 (95)
11 (83)
11 (92)
11 (95)[d]
85:15
90:10
85:15
95:5
95:5
95:5
95:5
96:4
99:1
97:3
25:75
99:1
ACHTUNGTRENNUNG
[Rh(S)-Phanephos
[Rh(S)-Phanephos
[Rh(S)-PhanephosACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
9
[Rh
[Rh
[Rh
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
10
11
12
13
14
15
[Rh(S)-PhanephosACHTUNGTRENNUNG
9
9
9
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a]
R1 =MeOCH2CH2CH2-.
Experiments done on a 30-mg scale of substrate in a Biotage Endeavour under 30 bar of H2.
Determined by HPLC.
[b]
[c]
[d]
Reaction done in a microwave oven equipped with a gas addition accessory at 5 bar of H2.
likely that both coordination of Rh to the acetoxy or (entry 15 in Table 1). When scaled to 6 grams of 9, the
to the carboxylate give rise to the same sense of MW-assisted hydrogenation gave 11 in high yield with
asymmetric induction.
In order to increase the efficiency of the process, of the catalytic process.
TON 100 and TOF 196 hÀ1, confirming the robustness
hydrogenation was attempted under microwave
The protocol developed for substrate 9 was extend-
(MW) dielectric heating.[14] Notwistanding the wide ed to the enantioselective hydrogenation of other
diffusion of microwave dielectric heating in organic (E)-enol acetate acids (Scheme 2). This reaction is
synthesis, not many examples on the application of not precedented in the literature although it is poten-
this technique to asymmetric synthesis and asymmet- tially very useful for the preparation of enantiomeri-
ric catalysis have been described until now.[15] We cally pure acetates of syn-b-hydroxy acids, structures
were pleased to verify that the high stereoselectivity present in many natural products and biologically rel-
observed in the autoclave was maintained under MW evant compounds.[16] Thus, starting from a-substituted
heating. Working at higher temperature (808C) but b-keto esters 12a–i, the corresponding (E)-enol ace-
for a shorter time (30 min) and under a lower H2 tates were selectively obtained by reaction with Ac2O
pressure (5 bar), the MW-assisted process gave prod- and TEA in the presence of DMAP. In any case,
uct 11 in 95% yields and 99:1 stereoselectivity using these reaction conditions, exclusively the (E)
Adv. Synth. Catal. 2013, 355, 1449 – 1454
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Giada Arena et al.
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under standard heating, compound 13a was submitted
to hydrogenation in an autoclave under the same re-
action conditions described above. After 60 min of
stirring at 808C (pre-heated oil bath) under 5 bar of
H2, no trace of product 14a was observed in the reac-
tion mixture. Even stirring for additional 24 h gave no
better results. Under these conditions (808 and 5 bar
of H2), compound 14a could be obtained in 45% (and
high ee) when the catalyst loading was increased to
15%. Alternatively, a result comparable with that ob-
tained with the microwaves, was achieved by heating
13a at 908C for 24 h under 30 bar of H2. After this
longer reaction conditions, compound 14a was isolat-
ed in 78% yield and 94% enanatiomeric purity.
In conclusion, a new highly stereoselective (and
enantioselective) hydrogenation of enol acetate car-
boxylates was carried out under MW dielectric heat-
ing, using [Rh(S)-PhanephosACHTUNRGTNEUNG(cod)]BF4 as catalyst.
This reaction can be considered as a valuable alterna-
tive to classic Ru-catalyzed DKR, especially for struc-
turally more complex molecules. Acces to syn-b-ace-
toxycarboxylic acids (and in principle to the corre-
sponding syn-b-hydroxy acids) was possible in good
yields and high enantiomeric excess. The use of mi-
crowaves as heating source allowed a drastic reduc-
tion of time, H2 pressure and catalyst loading required
for reduction, confirming that the tecnique is highly
suitable also for asymmetric catalytic reactions. How-
ever, the high enantioselection was maintained also
under conventional heating, making this reaction
a valuable tool for the preparation of chiral molecules
on small and medium to large scales.
Scheme 2. Microwave-assisted Rh-catalyzed enantioselective
synthesis of syn-b-hydroxy-a-alkyl carboxylic acids. R1 =
MeOCH2CH2CH2-.
Experimental Section
Hydrogenation under Microwave Dielectric Heating:
Typical Procedure
isomer was obtained in acceptable yields. After re-
moval of the tert-butyl protection, acids 13a–i were
submitted to MW-assisted hydrogenation with
Keto ester 4b (8.60 g, 13.7 mmol) was dissolved in acetic an-
hydride (25 mL), and 4-dimethylaminopyridine (0.75 g,
6.9 mmol) and Et3N (8.0 mL, 57.4 mmol) were added. The
reaction mixture was stirred for 3 h at room temperature.
The solvent was evaporated and the residue was taken up in
toluene and washed with a saturated solution of NaHCO3,
water and brine. The organic phase was dried over Na2SO4
and concentrated under reduced pressure. The residue was
[Rh(S)-PhanephosACTHNUTRGNE(UGN cod)]BF4 and TEA. The reduced
products 14a–i were obtained in good yields and high
enantiomeric purity just after 30 min of MW dielectric
heating at 908C under 5 bar of H2. The reaction
worked well on achiral (13a–g) or chiral (13h, i) enol
acetates. It was also compatible with the presence of
aromatic rings in different positions with respect to purified by flash column chromatography (hexane/ethyl ace-
tate 7:3) to give compound 8b as a yellow oil; yield: 6.59 g
(9.96 mmol, 72%).
the enol acetate.
The presence of a liphophilic chain (C10) (com-
pounds 14c–14i) was not detrimental for the yields
and the ee obtained, a remarkable result as many nat-
ural b-hydroxy acids are characterized by long carbon
chains.[17] The configuration of the products was deter-
mined by comparison with literature data (for 14d)[18]
and through the final transfomation of 11 into
a known Aliskiren advanced intermediate.[4] In order
This product was dissolved in TFA (25 mL) and the mix-
ture stirred at room temperature for 1 hour. The solvent
was evaporated under reduced pressure and the product ob-
tained as a yellow oil; yield: 6.09 g (9.95 mmol, 98%). Full
characterization of compounds 8b and 9 are reported in the
Supporting Information.
In the 80-mL vial of a CEM microwave oven Discovery
equipped with the gas addition apparatus,[19] the starting
to compare these results with the hydrogenation alkene 9 (6.0 g, 9.7 mmol) was dissolved in absolute MeOH
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Rhodium-Catalyzed Enantioselective Hydrogenation of (E)-Enol Acetate Acids
(15 mL) and the mixture was carefully degassed with argon.
TEA (1 mL, 10 mmol) and [(S)-Rh-Phanephos(cod)]BF4
[3] C. Jensen, P. Herold, H. R. Brunner, Nature Rev. Drug.
N
Disc. 2008, 7, 399–410.
(0.01 mmol%) were added under an argon atmosphere.
After 3 cycles of vacuum/nitrogen and 3 cycles of vacuum/
hydrogen, the yellow solution obtained was submitted to
pressurized H2 (5 bar) and heated at 808C (30 min) by mi-
crowave irradiation at 200 W. The flask was cooled and the
internal gas released. The reaction mixture was diluted with
toluene, and washed with a 10% AcOH solution until pH 5.
The organic phase was washed with water, azeotropically
dried and concentrated to a brown oily residue that resulted
to be practically pure on 1H NMR analysis; yield: 5.7 g
(95%). Chiral HPLC analysis after derivatization with di-
azomethane showed a diasteromeric ratio 99:1. An analyti-
cal sample was obtained after column chromatography with
EtOAc/hexane 1/1. 1H NMR (300 MHz, CDCl3): d=7.36–
7.26 (m, 5H), 6.75–6.63 (m, 3H), 5.18–5.02 (AB system, J=
12.2 Hz, 2H), 5.18–5.03 (m, 1H), 4.15–4.08 (m, 2H), 3.81 (s,
3H), 3.61–3.51 (m, 2H), 3.36 (s, 3H), 2.72–2.59 (m, 1H),
2.55–2.33 (m, 2H), 2.27–2.15 (m, 1H), 2.14–1.76 (m, 4H),
1.91 (s, 3H), 1.68–1.58 (m, 5H), 1.30–1.18 (m, 1H), 0.87–
0.79 (m, 12H); 13C NMR (75 MHz, CDCl3): d=177.0, 174.6,
170.2, 148.3, 147.8, 136.1, 134.1, 128.6 (2C), 128.5, 128.2
(2C), 121.3, 114.2, 111.7, 72.7, 69.7, 66.5, 66.2, 60.6, 58.7,
56.2, 48.3, 47.8, 44.1, 36.9, 31.1, 29.4, 29.0, 28.7, 21.2, 20.9,
20.4, 19.9, 19.8, 17.5; HR-MS (EI): m/z=637.3351, calcd. for
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dron Lett. 1994, 35, 4963–4966.
Mr. Roberto Rossi, Mrs. Alessandra Amorosi and Mrs. Mi-
caela Graglia (Chemessentia, Novara) are acknowledged for
providing analytical support. This work was financially sup-
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COMMUNICATIONS
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Chem. Soc. Chem. Commun. 1993, 1367–1370.
[19] Warning: we have developed the apparatus to use gas
under microwave dielectric heating and used it since 5
years in hydrogenation, hydroformylation and carbony-
lation without experiencing any trouble, explosion or
dangerous situation. However, as hydrogenation may
be intrinsecally exothermic, runaway reactions may be
possible and the microwave oven must be used in the
presence of suitable protection.
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Adv. Synth. Catal. 2013, 355, 1449 – 1454