Enantioselective iridium-catalyzed hydrogenation of β,γ- unsaturated carboxylic acids: An efficient approach to chiral 4-alkyl-4-aryl butanoic acids

Article abstract of DOI:10.1002/anie.201107802

Chiral acids: A highly enantioselective iridium-catalyzed hydrogenation of β,γ-unsaturated carboxylic acids is developed for the preparation of chiral 4-alkyl-4-aryl butanoic acids (see scheme). Copyright

Full text of DOI:10.1002/anie.201107802

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Angewandte
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DOI: 10.1002/anie.201107802
Iridium-Catalyzed Hydrogenation
Enantioselective Iridium-Catalyzed Hydrogenation of b,g-Unsaturated
Carboxylic Acids: An Efficient Approach to Chiral 4-Alkyl-4-aryl
Butanoic Acids**
Song Song, Shou-Fei Zhu, Shuang Yang, Shen Li, and Qi-Lin Zhou*
Chiral acids are important intermediates for the construction
of biologically active compounds. In particular, optically
active 4-alkyl-4-aryl butanoic acids 1 and their derivatives
have been widely used in the synthesis of various natural
products that have stereogenic centers in the benzylic position
(Scheme 1).[1] Although many catalytic enantioselective
approaches have been developed for the synthesis of diverse
chiral compounds, there is, to the best of our knowledge, no
direct method for asymmetric preparation of 4-alkyl-4-aryl
butanoic acids 1.
Optically active 4-alkyl-4-aryl butanoic acids 1 are gen-
erally prepared from chiral materials by means of multistep
transformations.[1b–e,k] The transition-metal-catalyzed asym-
metric hydrovinylation of alkenes,[1a] hydrogenation of a,b-
unsaturated carboxylic acids,[1f] isomerization of allylic alco-
hols,[1j] and some enzymatic catalytic methods[1h,i] have all
been used to construct a chiral benzyl center, but a subsequent
Scheme 1. Applications of optically active 4-alkyl-4-aryl butanoic acids.
multistep transformation is required to obtain the target 4-
alkyl-4-aryl butanoic acids 1. Thus, a reliable and efficient
catalytic asymmetric method for the preparation of these
chiral acids is highly desirable. The transition-metal-catalyzed
asymmetric hydrogenation of 4-alkyl-4-aryl-3-butenoic acids
2, which can be prepared from easily obtained materials,[2]
potentially provides an atom-efficient and reliable approach
Scheme 2. Convenient synthesis of optically active 4-alkyl-4-aryl butanoic
acids.
to chiral 4-alkyl-4-aryl butanoic acids 1 (Scheme 2). Various
chiral ruthenium, rhodium, and iridium catalysts have been
developed for the hydrogenation of a wide range of a,b-
unsaturated carboxylic acids.[3] However, only a few examples
of catalytic asymmetric hydrogenation of b,g-unsaturated
acids have been reported.[4] The enantioselective hydrogena-
tion of 4-alkyl-4-aryl-3-butenoic acids 2 has not been ach-
ieved.[5] Herein we describe a highly enantioselective iridium-
catalyzed hydrogenation of 2. By using a newly developed
chiral spiro phosphine–oxazoline ligand bearing an a-naph-
thylmethyl group on the oxazoline ring, we have prepared
various chiral 4-alkyl-4-aryl butanoic acids through the
iridium-catalyzed asymmetric hydrogenation reaction with
high enantioselectivities (up to 97% ee). The concise total
syntheses of natural products (R)-aristelegone-A, (R)-curcu-
mene, and (R)-xanthorrhizol were also accomplished with this
asymmetric hydrogenation reaction as the key step.
At the outset of this study, (E)-4-phenylpent-3-enoic acid
was hydrogenated in the presence of 1 mol% chiral spiro
iridium complexes 3 (Scheme 3) under 6 atm H2 pressure at
658C. When catalyst (Sa,S)-3a was used, complete conversion
with 60% ee was obtained (Table 1, entry 1). A diastereoiso-
mer of (Sa,S)-3a, complex (Ra,S)-3a, gave lower conversion
[*] S. Song, Prof. S.-F. Zhu, S. Yang, S. Li, Prof. Q.-L. Zhou
State Key Laboratory and Institute of Elemento-organic Chemistry
Nankai University, Tianjin 300071 (China)
E-mail: qlzhou@nankai.edu.cn
[**] We thank the National Natural Science Foundation of China, the
National Basic Research Program of China (2010CB833300 and
2011CB808600), and the Ministry of Education of China (B06005)
for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107802.
Scheme 3. Chiral spiro phosphine–oxazoline/iridium complexes.
BArFÀ =tetrakis[3,5-bis(trifluoromethyl)phenyl]borate.
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Table 1: Optimization of the reaction conditions.[a]
was obtained under the standard reaction conditions
(entry 15).
With the optimal reaction conditions determined, various
4-alkyl-4-aryl-3-butenoic acids 2 were hydrogenated. The
substituent on the phenyl ring of the substrate had a negligible
influence on the enantioselectivity, and all of the tested
substrates afforded essentially the same enantioselectivity
(92–96% ee; Table 2, entries 2–10). The reactivity of (E)-4-
(naphthalen-2-yl)pent-3-enoic acid (2k) was similar to that of
2a, and an enantioselectivity of 97% ee was obtained
Entry
Catalyst
Additive
pH [atm]
Conv. [%][b]
ee [%][c]
2
1
2
3
4
5
6
7
8
(Sa,S)-3a
(Ra,S)-3a
(Sa,S)-3b
(Sa)-3c
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
iPr2NEt
Et2NH
Cs2CO3
6
6
6
6
6
6
6
6
30
3
100
45
80
85
100
94
100
100
100
100
100
98
100
100
21
60
rac.
20
15
92
85
95
94
90
96
95
95
95
95
–
(Sa,S)-3d
(Sa,S)-3e
(Sa,S)-3 f
(Sa,S)-3g
(Sa,S)-3 f
(Sa,S)-3 f
(Sa,S)-3 f
(Sa,S)-3 f
(Sa,S)-3 f
(Sa,S)-3 f
(Sa,S)-3 f
Table 2: Asymmetric hydrogenation of 4-alkyl-4-aryl-3-butenoic acids.[a]
9
10
11[d]
12[d,e]
13[d]
14[d]
15[d]
3
3
3
3
Entry
Ar: R (2)
Product
Yield [%]
ee [%]
1
C6H5: Me (2a)
1a
1b
1c
1d
1e
1 f
1g
1h
1i
1j
1k
1l
1m
1n
97
97 (98)
96
96
98
96
97
98
97
98
98
96
96
95
94 (93)
94
96
95
92
95
95
95
95
97
90
92
2[b]
3
4-MeC6H4: Me (2b)
4-MeOC6H4: Me (2c)
4-FC6H4: Me (2d)
3
[a] Reaction conditions: 0.5 mmol scale, [substrate]=0.25 molLÀ1
1.0 equiv additive, 12 h. [b] Determined by NMR spectroscopy.
[c] Determined by chiral-phase HPLC or supercritical fluid chromatog-
raphy (SFC) analysis of the corresponding anilide. [d] Using 0.5 mol%
catalyst. [e] Using 0.5 equiv NEt3.
,
4
5
6
7
8
9
3-BrC6H4: Me (2e)
3-MeC6H4: Me (2 f)
3-MeOC6H4: Me (2g)
3,4-Me2C6H3: Me (2h)
3-MeO-4-MeC6H3: Me (2i)
3,4-(OCH2O)C6H3: Me (2j)
2-naphthyl: Me (2k)
2-thiophenyl: Me (2l)
4-MeOC6H4: Et (2m)
4-MeOC6H4: iPr (2n)
10[c]
11
12[c]
13
14[c]
and racemic product (entry 2). This result demonstrated that
the chiralities in (Ra,S)-3a were unmatched in their ability to
induce enantioselectivity in the hydrogenation reaction. The
effect of the oxazoline ring substituent of the ligand on the
enantioselectivity of the reaction was studied. Changing the
substituent from benzyl to isopropyl [(Sa,S)-3b] or H [(Sa)-3c]
decreased both conversion and enantioselectivity (entries 3
and 4). The substituents on the phosphorous of the ligand
strongly affected the enantioselectivity (entries 1, 5, and 6):
catalyst (Sa,S)-3d, with 3,5-dimethylphenyl substituents on
the phosphorous, exhibited the best chiral induction (92% ee,
entry 5). To further improve the enantioselectivity, we
synthesized two new ligands with an a-naphthylmethyl or
a b-naphthylmethyl group on the oxazoline ring, and the
corresponding catalysts (Sa,S)-3 f and (Sa,S)-3g were prepared
by means of a previously reported method.[6] Both (Sa,S)-3 f
and (Sa,S)-3g showed higher enantioselectivities in the
hydrogenation reaction than did (Sa,S)-3d (entries 7 and 8).
The reaction conditions were optimized with catalyst
(Sa,S)-3 f. Experiments on hydrogen pressure showed that the
reaction gave higher enantioselectivity under lower hydrogen
pressure (Table 1, entry 10 vs. entries 7 and 9), which is similar
to our previous observations in the hydrogenation of a,b-
unsaturated carboxylic acids.[3j] The catalyst loading can be
reduced to 0.5 mol% without diminishing the reactivity and
enantioselectivity (entry 11). Reducing the basic additive
NEt3 to 0.5 equiv slightly decreased the conversion under
standard reaction conditions (entry 12). Aside from NEt3,
other organic bases, such as iPr2NEt and NHEt2, were also
suitable additives for the hydrogenation reaction, affording
identical results to NEt3 (entries 13 and 14). On the other
hand, the use of the inorganic base Cs2CO3 drastically
decreased the reactivity of the catalyst; only 21% conversion
97
88
[a] The reaction conditions and analysis were the same as those in
Table 1, entry 11. Full conversions were obtained in all cases. [b] The data
in parentheses were obtained at 1.52 g scale. [c] Using 1 mol% catalyst.
(entry 11). The Ar group of 2 can also be a heterocycle. For
example, the hydrogenation of (E)-4-(thiophen-2-yl)pent-3-
enoic acid (2l) ran smoothly and afforded the desired product
with 90% ee in the presence of 1 mol% catalyst (entry 12).
Changing the R group of the acid 2 from methyl to ethyl (2m)
slightly reduced the enantioselectivity (entry 13). However,
when R was a bulky isopropyl (2n), 1 mol% of the catalyst
was required for complete conversion, and the ee decreased to
88% (entry 14). This asymmetric hydrogenation reaction
could easily be carried out on a gram scale, which is a benefit
for practical applications. For instance, (E)-4-p-tolylpent-3-
enoic acid (2b) was hydrogenated smoothly on a 1.52 g scale
under the standard hydrogenation conditions to produce the
desired product without compromise of either yield or
enantioselectivity (entry 2).
To further demonstrate the utility of the hydrogenation
reaction, three natural sesquiterpenes, (R)-aristelegone-A,
(R)-curcumene, and (R)-xanthorrhizol were conveniently
synthesized from the products of the asymmetric hydro-
genation (Scheme 4). With the hydrogenation product 1i as
starting material, (R)-aristelegone-A,[7] was obtained through
a Brønsted acid catalyzed Friedel–Crafts acylation and
a subsequent demethylation with Et2NCH2CH2SNa in 68%
overall yield (steps a and b). Note that the present procedure
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Scheme 4. Synthetic applications of hydrogenation products. Reaction
conditions: a) trifluoroacetic acid, trifluoroacetic anhydride, 08C;
b) Et2NCH2CH2SNa, DMF, reflux; c) CH3I, K2CO3, DMF, RT; d) diiso-
=
Scheme 5. Substrate and mechanistic studies.
butylaluminum hydride, CH2Cl2, À788C; e) (CH3)2CH PPh3, THF,
À158C.
acids. The concise syntheses of (R)-aristelegone-A, (R)-
curcumene, and (R)-xanthorrhizol, starting from chiral 4-
alkyl-4-aryl butanoic acids, show a high potential for the wide
application of this new catalytic asymmetric reaction in
organic synthesis.
is a novel and general route for the construction of tetralones
with a chiral benzyl center, a core structure for diverse natural
products.[1a,d,f,g,k] The (R)-curcumene[8] and (R)-xanthorri-
zol,[9,10] were synthesized from the hydrogenation products
1b and 1i respectively. The chiral acids were first transformed
into the corresponding aldehydes 5 by means of esterification
and a subsequent reduction (steps c and d). A Wittig reaction Experimental Section
General hydrogenation procedure: Substrate 2 (0.5 mmol), catalyst
was then performed to introduce the alkene moiety, giving
(R)-curcumene in 76% yield (step e). Finally, (R)-xanthorri-
zol was obtained by cleavage of the methyl group of 6. Thus,
the concise total syntheses of (R)-curcumene and (R)-
xanthorrizol were accomplished in three steps (from 1b,
68% overall yield) and four steps (from 1i, 70% overall
yield), respectively.
(Sa,S)-3 f (4.6 mg, 0.0025 mmol), NEt3 (51 mg, 0.5 mmol), and MeOH
(2 mL) were added to a hydrogenation tube containing a stir bar. The
hydrogenation tube was then put into an autoclave. The autoclave was
sealed and the atmosphere was purged five times with hydrogen. The
autoclave was then charged with hydrogen to 3 atm, and the reaction
mixture was subsequently stirred at 658C for 12 h. After venting the
autoclave, the reaction solution was treated with 5% NaOH (10 mL)
and extracted with Et2O (10 mL). The aqueous layer was then treated
with 3m HCl (pH 1) before being again extracted with Et2O (3 ꢀ
10 mL). The combined organic extracts were washed with a saturated
solution of NaCl, dried over MgSO4, and evaporated in vacuo to
produce the hydrogenation product. The product (0.5 mmol) was
reacted with aniline (50 mL, 0.55 mmol) in the presence of N,N-
To improve our understanding of the mechanism of the
hydrogenation reaction, we performed additional experi-
ments. As iridium complexes of phosphine–oxazoline ligands
are also well-known catalysts for the hydrogenation of
unfunctionalized olefins, work pioneered by the Pfaltz
group,[11] it would be interesting to know whether the carboxy
group in b,g-unsaturated acids 2 is necessary for the hydro-
genation reaction. The fact that the methyl ester of acid 2a,
(E)-methyl 4-phenylpent-3-enoate (7), is inert in the hydro-
genation under the standard reaction conditions indicates that
the functional carboxy group of the substrates 2 is crucial for
the reaction; it may act as an anchor to coordinate with
iridium and make the hydrogenation possible [Scheme 5,
Eq. (1)]. A deuterium-labeling study was also performed
[Scheme 5, Eq. (2)]. The essentially identical deuterium
distribution (1.1:1) at the b- and g-positions of the hydro-
genation product demonstrated that olefin migration did not
take place during the hydrogenation. A significant a-deute-
rium substitution was observed in both product and recovered
starting material, which can be attributed to H/D exchange
under basic conditions.
dimethylaminopyridine
(DMAP,
4 mg,
0.033 mmol)
and
dicyclohexylcarbodiimide (DCC, 110 mg, 0.53 mmol) in 1 mL THF
for 2 h. The reaction mixture was filtered through celite and the
filtrate was diluted with Et2O (10 mL), washed with 3m HCl (10 mL)
and saturated NaHCO3 (10 mL), and then dried over MgSO4. After
flash chromatography on silica gel with petroleum ether/ethyl acetate
(4:1), the desired amide was obtained and analyzed by supercritical
fluid chromatography (SFC) or HPLC to determined ee value.
Received: November 6, 2011
Revised: December 9, 2011
Published online: February 3, 2012
Keywords: asymmetric hydrogenation · iridium · P,N ligands ·
.
spiro compounds · unsaturated acids
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