Enantioselective synthesis of optically active alkanephosphonates via rhodium-catalyzed asymmetric hydrogenation of β-substituted α,β-unsaturated phosphonates with ferrocene-based monophosphoramidite ligands

Article abstract of DOI:10.1002/adsc.200800388

A series of chiral β-substituted alkane-phosphonates was synthesized in high enantioselectivities via the first rhodium-catalyzed asymmetric hydrogenation of the corresponding β-substituted-α,β-unsaturated phosphonates using a ferrocene-derived monophosphoramidite ligand, with which up to 99.5% ee have been achieved for the hydrogenation of (E)-substrates and 98.0% ee for (Z)-substrates.

Full text of DOI:10.1002/adsc.200800388

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DOI: 10.1002/adsc.200800388
Enantioselective Synthesis of Optically Active Alkanephos-
phonates via Rhodium-Catalyzed Asymmetric Hydrogenation
of b-Substituted a,b-Unsaturated Phosphonates with Ferrocene-
Based Monophosphoramidite Ligands
Zheng-Chao Duan,a,b Xiang-Ping Hu,a,* Dao-Yong Wang,a,b Jia-Di Huang,a,b
Sai-Bo Yu,a,b Jun Deng,a,b and Zhuo Zhenga,*
a
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, Peopleꢀs
Republic of China
Fax : (+86)-411-8468-4746; e-mail: xiangping@dicp.ac.cn or zhengz@dicp.ac.cn
The Graduate School of Chinese Academyof Sciences, Beijing 100039, Peopleꢀs Republic of China
b
Received: March 4, 2008; Revised: June 24, 2008; Published online: August 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800388.
Abstract: A series of chiral b-substituted alkane-
phosphonates was synthesized in high enantioselec-
tivities via the first rhodium-catalyzed asymmetric
hydrogenation of the corresponding b-substituted-
a,b-unsaturated phosphonates using a ferrocene-de-
rived monophosphoramidite ligand, with which up
to 99.5% ee have been achieved for the hydrogena-
tion of (E)-substrates and 98.0% ee for (Z)-sub-
strates.
methods for the enantioselective synthesis of these
compounds is still highlydesirable.
In our ongoing efforts toward the search for new
efficient approaches for the asymmetric synthesis of
chiral phosphonate derivatives, we have been interest-
ed in the application of catalytic asymmetric hydroge-
nation because of its inherent efficiencyand atom
[5]
economy. Catalytic asymmetric hydrogenation is a
powerful tool for the enantioselective synthesis of 3-
arylbutanoic acid derivatives,[6] which are structurally
similar to the b-substituted alkanephosphonates of in-
terest. We therefore envisioned that it should be pos-
sible to prepare a phosphono analogue of an a,b-un-
saturated carboxylic acid ester as a substrate for cata-
lytic asymmetric hydrogenation. Indeed, such a strat-
egyhas been extensivelyapplied in the synthesis of a
varietyof chiral phosphono analogues of a-amino
acid esters, b-amino acid esters, a- and b-hydroxy acid
derivatives, by catalytic asymmetric hydrogenation of
Keywords: alkanephosphonates; asymmetric cataly-
sis; hydrogenation; monophosphoramidite ligands;
rhodium
Opticallyactive alkanephosphonic acid derivatives
have received significant attention recentlybecause of
their interesting biological properties as phosphorus the corresponding a-amidoalkenephosphonates, b-
analogues of carboxylic acids,[1] as well as their syn- amidoalkenephosphonates, a-acyloxyalkenephospho-
[2]
thetic utilityas chiral building blocks.
Although nates, and b-ketoalkanephosphonates.[7] However,
some stoichiometric or catalytic asymmetric syntheses asymmetric synthesis of b-substituted alkanephospho-
have been developed for constructing chiral 1-aryl- nates via catalytic hydrogenation of a,b-unsaturated
ACHTREUNG
ethanephosphonates in the past decades,[3] the enan- phosphonates is still unexplored. Herein we report for
tioselective synthesis of chiral b-substituted alkane- the first time the highlyenantioselective rhodium-cat-
phosphonates bya catalytic method is still rarelyex-
plored. To the best of our knowledge, onlyone publi-
cation byHayashi et al. has described the enantiose-
alyzed asymmetric hydrogenation of b-substituted al-
kenephosphonates with a 1-ferrocenylethylamine-de-
rived monodentate phosphoramidite ligand, with
lective
alkaneACHTREUNG
synthesis
phosphonates by a catalytic asymmetric 1,4-ad- nates were prepared in up to 99.5% ee.
The success of this hydrogenation method largely
of
b-aryl-substituted which a varietyof chiral b-substituted alkanephospho-
dition to 1-alkenephosphonates using a chiral phos-
phine-Rh catalyst and arylboroxines as arylating re- depended on the development of a method for the ef-
agents.[4] Therefore, the development of new catalytic ficient synthesis of single isomers of b-substituted-a,b-
Adv. Synth. Catal. 2008, 350, 1979 – 1983
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1979

Zheng-Chao Duan et al.
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Table 1. The asymmetric hydrogenation of diethyl (E)-(2-
phenyl-1-propene)phosphonate (1a).[a]
unsaturated phosphonates 1 since a mixture of Z- and
E-isomers will be normallyformed in the snythesis
and sometimes theyare not easyto be separated. For
the hydrogenation of a substrate containing two iso-
mers, it is generallydifficult to achieve high enantio-
selectivity.[8] Fortunately, we found that the target sub-
strates could be easilyprepared through a simple and
versatile synthetic method as shown in Scheme 1. By
the reaction of tetraethyl methylenediphosphonate
with various aryl methyl ketones, the E-isomer is
formed predominantlyand a small amount of the Z-
isomer formed can be removed bycolumn chroma-
[b]
EntryLigand
Solvent Conversion [%]
CH2Cl2
ee [%][c]
[d]
1
DuPHOS
–
–
[9]
2
3
4
5
6
7
8
9
10
11
12
13
BINAP
BoPhoz
ACHTREU(GN Rc,Sa)-3b
CH2Cl2 84
CH2Cl2 100
CH2Cl2 100
30.3
75.0
95.6
55.5
0
tography.
MonoPhos CH2Cl2 35
(Sa)-4b CH2Cl2 21
(Rc,Sa)-3b
(Rc,Sa)-3b
(Rc,Sa)-3b
(Rc,Sa)-3b
(Rc,Sa)-3a
(Rc,Sa)-3c
(Rc,Sa)-3d
[d]
A
A
U
E
AHCTREUNG
A
AHCTREUNG
MeOH
THF
PhMe
–
–
–
–
–
–
[d]
[d]
i-PrOH 70
CH2Cl2 47
CH2Cl2 100
CH2Cl2 65
36.1
7.0
98.3
93.4
Scheme 1. Synthesis of b-substituted a,b-unsaturated phos-
phonates (1).
[a]
All reactions were carried out with 0.25 mmol of sub-
strate at room temperature under an H2 pressure of 40
atm in 2 mL of the indicated solvent for 24 h.
Degrees of conversion were determined byGC.
The ee values were determined byHPLC on a chiral
column.
An exploratoryligand screening experiment was
performed at room temperature under 40 atm of H2
pressure in CH2Cl2, and employed a diverse array of
chiral monodentate and bidentate phosphorus ligands.
Some representative ligands screened are listed in
Figure 1. As shown in Table 1, most of the bidentate
phosphorus ligands tested, which have proven to be
successful for the highlyenantioselective hydrogena-
tion of various olefins, gave unsatisfactoryresults in
[b]
[c]
[d]
Not determined because of low conversion.
while BINAP onlygave low enantioselectivityand in-
complete conversion (entry2). A BoPhoz-type ligand
this transformation. For example, DuPHOS showed was effective for this hydrogenation, however, the
no activityunder the reaction conditions (entry1),
enantioselectivitywas moderate even with use of the
finelymodified ligand (entry3).
To our delight, we found that (Rc,Sa)-FAPhos 3b, a
monodentate phosphoramidite ligand recentlydevel-
oped by us for the highly efficient Rh-catalyzed asym-
metric hydrogenation of a variety of functionalized
C=C double bonds,[13] unexpectedlyafforded an ee
value of up to 95.6% and full conversion (entry4).
More interestingly, subsequent investigations of the
effect of the monophosphoramidite structure on this
hydrogenation showed that the ferrocene fragment in
these monodentate ligands has a crucial role in the re-
activityand enantioselectivit.y Thus, both MonoPhos
(4a) and the phenyl analogue of (Rc,Sa)-3b, (Rc,Sa)-4b,
gave the poor results in this transformation (entries 5
and 6). Further experiments in an effort to attain
higher enantioselectivitybyoptimizing the reaction
conditions failed. In a solvent screening experiment, a
dramatic solvent effect was observed; however, no re-
sults surpassed that obtained in CH2Cl2. For example,
the reaction in solvents such as MeOH, THF, or tolu-
ene led to verylow reactivity(entries 7–9), while low
enantioselectivityand incomplete conversion were
Figure 1. Structures of some representative ligands for asym-
metric hydrogenation.
1980
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Enantioselective Synthesis of Optically Active Alkanephosphonates
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observed when the reaction was carried out in i- sponding 2-arylpropanephosphonates with high enan-
PrOH (entry10). Since the synthetic method towards
the FAPhos ligand is highlymodular, the structural
tioselectivity(97.9–99.3% ee). The best enantioselec-
tivityof 99.3% ee was obtained in the hydrogenation
optimization of FAPhos was therefore performed. of the substrate 1d with a 4-methyl group in the
The results indicated that the substituent effect of the phenyl ring (entry 4). These results indicated that the
ligands was significant, affecting not onlythe reactivi-
present catalytic system has a high tolerance to the
tybut also the enantioselectivity. When ligand 3a with substituted pattern and electronic properties of the
À
an N H proton was used, both reactivityand enantio-
substituent on the phenyl ring in terms of both reac-
tivityand enantioselectivit.y Diethly ( E)-[2-(2-naph-
selectivitywere decreased dramatically(entry11). By
À
use of ligand 3c with an N Et group, the enantiose- thyl)-1-propene)phosphonate (1j) was also hydrogen-
lectivitywere further improved to 98.3% ee with com- ated to afford 2-(2-naphthyl)propanephosphonate (2j)
plete conversion (entry12). Ligand 3d with a benzyl in full conversion and 96.7% ee (entry10). In the hy-
group in the amino moietyalso gave good enantiose-
lectivity, however, lower reactivity was observed ic thiophenyl group, chiral phosphonate 2k was ob-
(entry13). taind in full conversion and 99.5% ee (entry11).
drogenation of the substrate 1k, with a heteroaromat-
With these encouraging results in the hydrogena- These results demonstrated the efficiencyof the pres-
tion of (E)-1a, we proceeded to investigate the scope ent catalytic system in the synthesis of chiral 2-aryl-
of this new transformation on various b-aryl-substitut- propanephosphonates.
ed a,b-unsaturated phosphonates (E)-1a–k, using
To broaden the synthetic interest of this highly
(Rc,Sa)-3c as ligand and CH2Cl2 as the standard sol- enantioselective procedure, a set of structurallydi-
vent. As shown in Table 2, entries 1–9, a wide range verse a,b-unsaturated phosphonates, including (E)-b-
of substituted b-phenyl-a,b-unsaturated phosphonates alkyl-b-methyl-substituted substrate [(E)-1l], (Z)-b-
(E)-1a–i were hydrogenated to provide the corre- alkyl-b-methyl-substituted substrate [(Z)-1l], and (E)-
b-ethyl-b-phenyl-a,b-unsaturated phosphonate [(E)-
1m], was prepared and submitted to the optimized
Table 2. The asymmetric hydrogenation of diethyl (E)-(2-
aryl-1-propene)phosphonate (1).[a]
Rh-catalyzed hydrogenation conditions (Scheme 2).
The result indicated that the Rh/(Rc,Sa)-3c complex is
also highlyefficient for the hydrogenation of b,b-di-
alkyl-substituted substrate (E)-1l, providing the hy-
AHCTREUNG
AHCTREUNG
drogenation product in full conversion with an ee
value of 98.2%. For the hydrogenation of (Z)-sub-
strate [(Z)-1l] with the present catalytic system, excel-
lent enantioselectivity(98.0% ee) and full conversion
were also achieved, however, the absolute configura-
tion of the hydrogenation product is opposite to that
obtained from the hydrogenation of the (E)-substrate.
The hydrogenation of (E)-b-ethyl-b-phenyl-a,b-unsa-
turated phosphonate [(E)-1m] proved to be as effi-
cient as that of its b-methyl analogues, giving the hy-
drogenation product in full conversion with an ee
value of 97.7%.
As reported byHayashi et al., opticallyactive alka-
nephosphonates, containing the stereogenic carbon
center at the b-position, can be used as chiral building
blocks for the synthesis of optically active alkenes by
the Horner–Emmons-type reaction without loss of en-
antiomeric purity(Scheme 3).
EntrySubstrate Ar
Conversion
[%][b]
ee [%] (configu-
ration)[c]
1
2
(E)-1a
(E)-1b
Ph
4-
100
100
98.3 (R)
98.8 (+)
MeOC6H4
3
4
5
6
7
8
9
10
11
(E)-1c
(E)-1d
(E)-1e
(E)-1f
(E)-1g
(E)-1h
(E)-1i
(E)-1j
(E)-1k
3-MeC6H4 100
4-MeC6H4 100
3-CF3C6H4 100
4-CF3C6H4 100
98.1 (+)[d]
99.3 (+)
98.3 (+)[d]
99.0 (+)
97.9 (+)
98.7 (+)
98.2 (+)[d]
96.7 (+)
99.5 (+)
4-FC6H4
100
4-ClC6H4 100
4-BrC6H4 100
2-naphthyl 100
2-thio-
phenyl
100
In conclusion, a series of chiral b-substituted alka-
nephosphonates was synthesized in high enantioselec-
tivities in the first Rh-catalyzed asymmetric hydroge-
nation of the corresponding b-substituted a,b-unsatu-
rated phosphonates using a 1-ferrocenylethylamine-
derived monophosphoramidite ligand, in which up to
99.5% ee has been achieved. Since these hydrogena-
tion substrates are easilyprepared, this method is po-
tentiallypractical for the synthesis of such chiral com-
pounds. The development of new catalytic methods to
[a]
All reactions were carried out with 0.25 mmol of sub-
strate at room temperature under an H2 pressure of 40
atm in 2 mL of CH2Cl2 for 24 h unless otherwise speci-
fied. Substrate/RhACHTREU(GN COD)2BF4/ligand=1/0.01/0.022.
[b]
[c]
Degrees of conversion were determined byGC.
The ee values were determined byHPLC on a chiral
column (Chiralpak AD-H or OJ-H). The absolute config-
uration was determined bycomparing the optical rota-
tion with the reported data.
[d]
The reactions were performed at 508C.
Adv. Synth. Catal. 2008, 350, 1979 – 1983
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asc.wiley-vch.de
1981

Zheng-Chao Duan et al.
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Scheme 2. Rh-catalyzed asymmetric hydrogenation of (E)-1l, (Z)-1l and (E)-1m.
Scheme 3. Example of a synthetic application of chiral diethyl 1-(2-phenyl)propanephosphonate 2a.
room temperature until the ketone disappeared as moni-
tored byTLC. The mixture was then diluted with ether and
synthesize chiral alkanephosphonates is still in prog-
ress.
washed with
a aqueous solution of saturated NH4Cl
(25 mL”1) and brine (25 mL”1). The organic layer was
separated, and dried over anhydrous NaSO4. After removal
of the volatiles, the residue was purified bycolumn chroma-
tography(silica gel, AcOEt/hexane, 1:1).
Experimental Section
General Methods
All reactions and manipulations were performed in a nitro-
gen-filled glove-box or under nitrogen using Schlenk tech-
niques unless otherwise noted. All solvents were distilled
under argon in the presence of the following desiccants:
sodium-benzophenone ketyl for diethyl ether (Et2O), tetra-
hydrofuran (THF), CaH2 for dichloromethane (CH2Cl2).
General Procedure for Asymmetric Hydrogenation of
b-Substituted a,b-Unsaturated Phosphonates
In a nitrogen-filled glove-box, [RhACHTRE(UGN COD)2]BF4 (1.0 mg,
0.0025 mmol) and (Rc,Sa)-3c (3.1 mg, 0.0055 mmol) were
dissolved in degassed CH2Cl2 (1 mL) in a 5-mL vial. After
stirring at room temperature for 15 min, a solution of a,b-
unsaturated phosphonate (E)-1a (64 mg, 0.25 mmol, S/C
100:1) in 1 mL of degassed CH2Cl2 was added. The resulting
mixture was transferred to an autoclave, which was then
charged with H2 (40 atm). The hydrogenation was per-
formed at room temperature for 24 hours. After carefully
releasing the hydrogen gas, the reaction mixture was puri-
fied through a plug of silica gel (eluting with a mixture of
hexanes/EtOAc, 2:1) to afford 2a. The enantiomeric excess
was determined byHPLC on a chiral stationaryphase.
General Procedure for the Preparation of b-
Substituted a,b-Unsaturated Phosphonates
A solution of (EtO)2P(O)CH2P(O)ACHTRE(UNG OEt)2 (1.44 g, 5 mmol)
in THF (2 mL) was slowlyadded at 0 8C to a slurryof NaH
(0.19 g, 5.5 mmol, 70% in oil) in THF (10 mL). After the ad-
dition, the mixture was stirred at room temperature for
0.5 h. To the mixture, a solution of ketone (4.25 mmol) in
THF (3 mL) was added. The reaction mixture was stirred at
1982
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Enantioselective Synthesis of Optically Active Alkanephosphonates
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Acknowledgements
We are grateful for financial support fromthe National Natu-
ral Science Foundation of China (20472083).
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