New types of o-carborane-based chiral phosphinooxazoline (Cab-PHOX) ligand systems: Synthesis and characterization of chiral Cab-PHOX ligands and their application to asymmetric hydrogenation

Article abstract of DOI:10.1055/s-0028-1087934

o-Carborane-based chiral phosphinooxazoline (Cab-PHOX) ligands were synthesized for the first time and applied to the iridium- and rhodium-catalyzed hydrogenation of unfunctionalized and functionalized olefins with an enantioselectivity of up to 98% and 96%, respectively. The modularity of the Cab-PHOX ligands is highlighted by the facile preparation of a variety of sterically and electronically different ligands. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087934

LETTER
771
New Types of o-Carborane-Based Chiral Phosphinooxazoline (Cab-PHOX)
Ligand Systems: Synthesis and Characterization of Chiral Cab-PHOX
Ligands and Their Application to Asymmetric Hydrogenation
N
ew
T
ype
o
s
of
o
-Carbo
n
rane-Based
C
g
hiral Cab-PH
-
OX Lig
D
and
S
ystems ae Lee,a Thanh Thien Co,b Tae-Jeong Kim,*b Sang Ook Kang*c
a
Department of Chemistry, Chosun University, 375 Seosuk, Dong-gu, Gwangju 501-759, Korea
Department of Applied Chemistry, Kyungpook National University, Taegu, 702-701, Korea
Fax +82(41)8601334; E-mail: tjkim@knu.ac.kr
Department of Chemistry, Korea University, Sejong Campus, Chungnam 339-700, Korea
Fax +82(41)8675396; E-mail: sangok@korea.ac.kr
b
c
Received 8 November 2008
This paper reports a new class of o-carborane-based chiral
PHOX ligands that are highly enantioselective for a wide
range of substrates employing their iridium and rhodium
complexes. Previously, we reported a systematic study of
Abstract: o-Carborane-based chiral phosphinooxazoline (Cab-
PHOX) ligands were synthesized for the first time and applied to the
iridium- and rhodium-catalyzed hydrogenation of unfunctionalized
and functionalized olefins with an enantioselectivity of up to 98%
and 96%, respectively. The modularity of the Cab-PHOX ligands is o-carborane-based chelating bidentate ligand systems as
supporting ligands for metal complexes, such as C,N,13
highlighted by the facile preparation of a variety of sterically and
C,P,14 S,P,15 and Si,P.16 Carboranes contain ten boron at-
electronically different ligands.
Key words: chiral phosphinooxazoline ligands, o-carborane-based
bidentate ligands, asymmetric hydrogenation, unfunctionalized ole-
fins, functionalized olefins
oms in a small volume with a 3-dimensional peripheral
size similar to that of a benzene molecule. Carborane
(C2B10H12) has three isomers, known as ortho- (1,2-
C2B10H10), meta- (1,7-C2B10H10), and para-carborane
(1,12-C2B10H10), depending on the position of the two car-
bon atoms in the icosahedral cage framework; derivatives
of these carboranes can be produced by replacing the acid-
ic protons of one or both of the C–H units with other func-
tional groups.17 Therefore, it would be interesting to
examine the possibility of transition-metal-catalyzed
asymmetric reactions using o-carborane-based chiral bi-
dentate PHOX ligand systems (Cab-PHOX, Figure 1). To
the best of our knowledge, this is the first report of a P,N-
chelating chiral Cab-PHOX ligand being a versatile
ligand for metal-catalyzed asymmetric hydrogenation.
Herein we report our results on the design and synthesis
of a new class of o-carborane-based chiral PHOX ligands
(4 and 5) starting from commercially accessible o-carbo-
rane, chlorodiphenyl- or chlorodicyclohexylphosphine,
and L-valinol as a chiral amino alcohol, as well as their ap-
plication to the iridium- and rhodium-catalyzed enantio-
selective hydrogenation of unfunctionalized and
functionalized olefins.
Phosphinooxazolines (PHOX, Figure 1)1 are efficient,
non C2-symmetric, chiral P,N-chelating classes of ligand
that were first described by Pfaltz,2 Helmchen,3 and
Williams.4 Their development was inspired by Crabtree’s
catalyst [(COD)Ir(PCy3)(py)][PF6],5 which features a
phosphine and pyridine ligand in its coordination sphere.
Crabtree’s catalyst is highly active in catalytic hydrogena-
tion reactions, particularly in highly substituted olefins.
The PHOX ligands were used successfully in transition-
metal-catalyzed asymmetric reactions such as the hydro-
genations of olefins,6 allylic alkylations,7 Heck reactions,8
and other processes.9 The efficiency of the PHOX ligands
has been in part attributed to their ability to create distin-
guishable coordination sites trans to the phosphorus and
nitrogen donors, thereby enhancing the selectivity.10
Modification of the PHOX ligands is mainly undertaken
at the a-carbon to the oxazoline nitrogen,11 the two aryl
groups on phosphorus,11 and the arene backbone.12
New o-carborane-based chiral PHOX (Cab-PHOX, 4 and
5) ligands were synthesized according to the mechanism
shown in Scheme 1. The starting materials, phosphino-o-
carboranes CabPR2 (1 R = Ph; 2 R = Cy)18 and 2-bromo-4-
isopropyl-oxazoline 3,19 can be prepared easily using the
procedure reported in the literature. CabPPh2 (1) or CabPCy2
(2) was treated with 1.1 equiv of BunLi followed by the
addition of 1.1 equiv of 2-bromo-4-isopropyl-oxazoline
(3) in THF at –78 °C (Scheme 1). The target Cab-PHOX
ligands (4 and 5) were obtained in high yields (88–93%),
which were subsequently transformed into their corre-
sponding transition-metal complexes according to a mod-
O
O
Ph2P
N
N
R2P
= BH
= C
i-Pr
R
Cab-PHOX
PHOX
4 R = Ph
5 R = Cy
Figure 1 Chiral bidentate ligands PHOX and Cab-PHOX (4, 5)
SYNLETT 2009, No. 5, pp 0771–0774
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x.
x
x.
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Advanced online publication: 24.02.2009
DOI: 10.1055/s-0028-1087934; Art ID: U11808ST
© Georg Thieme Verlag Stuttgart · New York

772
J.-D. Lee et al.
LETTER
O
N
Table 1 Rh-Catalyzed Asymmetric Hydrogenation of a,b-Unsatur-
i-Pr
N
ated Functionalized Olefinsa
Br
PR2
H
Li
PR2
i-Pr
R1
R1
n-BuLi
3
[Rh(NBD)2]BF4 (2.0 mol%)
O
PR2
ligand (2.1 mol%)
THF, 0 °C
THF, –78 °C
R3O2C
9–11
R2
R3O2C
R2
H2 (10 bar), THF, 24 h
= BH
= C
1 R = Ph
2 R = Cy
6–8
4 R = Ph
5 R = Cy
Entry Substrate R1
R2
R3
Ligand Yield ee
(%)b (%)c
Scheme 1 Synthesis of new chiral Cab-PHOX ligands 4 and 5
1
2
6
7
8
H
NHAc Me
NHAc Me
4
5
22
50
65 (S)
84 (S)
ification of the standard procedure (see Supporting
Information).
3
4
Ph
Ph
4
5
90
85
96 (S)
83 (S)
Hydrogenation of a,b-unsaturated carboxylic acids and a-
dehydroamino acid derivatives has been a typical reaction
to test the efficiency of new chiral ligands. Indeed, a num-
5
6
H
H
4
5
24
20
94 (S)
87 (S)
ber of chiral phosphine ligands with great structural diver- a Absolute configurations of the products are given in parentheses
after the ee values. Conditions: H2 pressure = 10 bar in all entries; all
reactions performed at r.t. for 24 h; catalyst loading 2.0 mol% in all
sity are found to be effective for Rh-catalyzed
hydrogenation of these acid derivatives.20 Thus, in the
entries. All reactions were performed in freshly distilled THF.
first set of experiments, we performed the Rh-catalyzed
asymmetric hydrogenation of a-dehydroamino acid deriv-
atives to benchmark the potential of 4 and 5 for asymmet-
ric catalysis. Here, it is worth noting that there is virtually
no precedence for the PHOX-type ligand to be employed
in asymmetric hydrogenation of functionalized olefins
such as those mentioned above. Hydrogenation was con-
b GC yield.
c Determined by chiral capillary GC on a Chiralsil-Val column (25 m)
and the product configuration by a comparison with the literature val-
ues.
[BArF = tetrakis-3,5-bis(trifluoromethyl)phenylborate] and
2.1 mol% of 4 and 5. The results are summarized in
ducted under a H2 pressure of 10 bar in the presence of 2 Table 2. Indeed, our systems demonstrate excellent enan-
mol% of the catalysts prepared in situ from tioselectivity (up to 98% ee) and catalytic activity (up to
[Rh(NBD)2]BF4 and 2.1 mol% of chiral ligands (4 and 5), 97%) regardless of the types of ligand or substrate. These
and the results are summarized in Table 1. The results are results may well be compared with those obtained with
remarkable in that extremely high enantioselectivity (up Pfaltz’s or Kim’s Ir-systems for the hydrogenation of re-
to 96% ee) and catalytic activity are achieved. It should be lated trisubstituted nonfunctionalized olefins (see entry 9,
pointed out that few systems have been reported on Rh- Table 2 for comparison).23,24 The steric bulkiness of sub-
catalyzed hydrogenation of a,b-unsaturated carboxylic strate seems to play a minor role on chemical yields and
acids such as (E)-2-methylcinnamic acid (or ester), al- ee (%) as deduced from Table 2. For instance, a dramatic
though some successful results have been obtained with decrease in both yield and ee (%) is observed with the sub-
Ru-based catalysts.21 Enantioselectivity is a function of
strate 12 that carries a bulky phenyl group. In addition, the
both substrates and ligands. Namely, of two substrates,
size of the phosphine substituents, occupying the semi-
(E)-2-methylcinnamate and (Z)-2-acetamidoacrylate, the hindered quadrant, is important for substrates, such as
latter gives the lower ee (%) and the lower chemical yield compounds 12 and 15 (entries 1, 2, 7, and 8, Table 2),
(entry 1 and 2). When the comparison is made between where a change from phenyl to cyclohexyl leads to a
phosphine moieties (PPh2 and PCy2) the former gives slightly increase in enantioselectivity in both cases. Allyl-
higher ee (%) in the hydrogenation of (E)-2-methylcin- ic alcohol 14 (entries 5 and 6, Table 2) worked exception-
namate (entries 3–6).
ally well for all Ir complexes.
Encouraged by the results shown in Table 1, we further In conclusion, we have developed a new class of readily
pursued the Ir-catalyzed asymmetric hydrogenation of un- available, o-carborane-based chiral PHOX ligands 425 and
functionalized olefins, 12–15, which belong to a class of
5
26 for use as ligand for cationic Rh- and Ir-complexes in
substrates that resist high enantioselectivity by conven- catalytic asymmetric hydrogenation. They provide a new
tional Ru- and Rh-phosphine-type catalysts.22 Recently, entry into powerful catalysts for asymmetric hydrogena-
however, a breakthrough has been made independently tion a series of a,b-unsaturated carboxylic acids (or esters)
by Pfaltz23 and Kim24 who have demonstrated that Ir- and unfunctionalized olefins. In some cases they may
complexes of PHOX or (iminophosphoranyl)ferrocenes even serve as practical catalysts.
can catalyze a variety of unfunctionalized olefins with
very high enantioselectivities (up to 99% ee). The reaction
was carried out in CH2Cl2 at room temperature for 24
hours under a H2 pressure of 10 bar in the presence of
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
2 mol% of catalysts prepared in situ from [Ir(cod)2]BArF
Synlett 2009, No. 5, 771–774 © Thieme Stuttgart · New York

LETTER
New Types of o-Carborane-Based Cab-PHOX Ligand Systems
773
Table 2 Ir-Catalyzed Hydrogenation of Unfunctionalized Olefinsa
(8) (a) Loiseleur, O.; Meier, P.; Pfaltz, A. Angew. Chem., Int.
Ed. Engl. 1996, 35, 200. (b) Loiseleur, O.; Hayashi, M.;
Keenan, M.; Schmees, N.; Pfaltz, A. J. Organomet. Chem.
1999, 576, 16.
R4
R4
[Ir(COD)2]BArF (2.0 mol%)
R5
ligand (2.1 mol%)
R5
(9) (a) Cho, Y.-H.; Kina, A.; Shimada, T.; Hayashi, T. J. Org.
Chem. 2004, 69, 3811. (b) Frölander, A.; Moberg, C. Org.
Lett. 2007, 9, 1371. (c) Lu, Z.-L.; Neumann, E.; Pfaltz, A.
Eur. J. Org. Chem. 2007, 4189.
H2 (10 bar), CH2Cl2, 24 h
R6
R6
R7
R7
12–15
16–19
(10) Grützmacher, H. Angew. Chem. Int. Ed. 2008, 47, 1814.
(11) (a) Lightfoot, A.; Schnider, P.; Pfaltz, A. Angew. Chem. Int.
Ed. 1998, 37, 2897. (b) Tani, K.; Behenna, D. C.;
McFadden, R. M.; Stoltz, B. M. Org. Lett. 2007, 9, 2529.
(c) Sedinkin, S. L.; Rath, N. P.; Bauer, E. B. J. Organomet.
Chem. 2008, 693, 3081.
Entry Substrate R4 R5
R6
H
R7
H
H
H
Ligand Yield ee
(%)b (%)c
1
2
12
13
14
15
12
Me Ph
4
5
40 73 (S)
30 82 (S)
(12) (a) Braunstein, P.; Graiff, C.; Naud, F.; Pfaltz, A.;
Tiripicchio, A. Inorg. Chem. 2000, 39, 4468.
3
4
Me CO2Et
H
4
5
96 93 (S)
89 98 (S)
(b) Gilbertson, S. R.; Fu, Z. Org. Lett. 2001, 3, 161.
(c) Gilbertson, S. R.; Fu, Z.; Xie, D. Tetrahedron Lett. 2001,
42, 365. (d) End, N.; Stoessel, C.; Berens, U.; di Pietroa, P.;
Cozzi, P. G. Tetrahedron: Asymmetry 2004, 15, 2235.
(13) Lee, J.-D.; Kim, S.-J.; Yoo, D.; Ko, J.; Cho, S.; Kang, S. O.
Organometallics 2000, 19, 1695.
5
6
H
CH2OH CH3
4
5
90 95 (S)
97 96 (S)
7
8
Me
H
(CH2)2 OMe 4
85 88 (S)
95 90 (S)
5
(14) Lee, J.-D.; Ko, J.; Cheong, M.; Kang, S. O. Organometallics
9
Me Ph
H
H
PHOX >99 99 (S)d
2005, 24, 5845.
a Absolute configurations of the products are given in parentheses
after the ee values. Conditions: H2 pressure = 10 bar in all entries; all
reactions performed at r.t. for 24 h; catalyst loading 2.0 mol% in all
entries. All reaction performed in freshly distilled (CaH2, ethanol
free) CH2Cl2.
(15) Lee, H.-S.; Bae, J.-Y.; Kim, D.-H.; Kim, H. S.; Kim, S.-J.;
Cho, S.; Ko, J.; Kang, S. O. Organometallics 2002, 21, 210.
(16) Lee, Y.-J.; Lee, J.-D.; Kim, S.-J.; Keum, S.; Ko, J.; Suh, I.-
H.; Cheong, M.; Kang, S. O. Organometallics 2004, 23, 203.
(17) (a) Grimes, R. N. Carboranes; Academic Press: New York,
1970. (b) Nakamura, H.; Aoyagi, K.; Yamamoto, Y. J. Am.
Chem. Soc. 1998, 120, 1167. (c) Ohta, K.; Goto, T.;
Yamazaki, H.; Pichierri, F.; Endo, Y. Inorg. Chem. 2007, 46,
3966.
b GC yield.
c Determined by chiral capillary GC on a Chiralsil-Val column (25 m).
d The highest ee (%) obtainable with PHOX.23f
(18) Hill, W. E.; Silva-Trivino, L. M. Inorg. Chem. 1979, 18, 361.
(19) Leonard, W. R.; Romine, J. L.; Meyers, A. I. J. Org. Chem.
1991, 56, 1961.
(20) (a) Tang, W.; Zhang, X. Chem Rev. 2003, 103, 3029.
(b) Zhang X., Ed.Tetrahedron: Asymmetry 2004, 15, 2099.
(c) Brown, J. M. In Comprehensive Asymmetric Catalysis;
Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.; Springer:
Berlin, 1999, 121–182. (d) Ohkuma, T.; Kitamaru, M.;
Noyori, R. In Catalytic Asymmetric Synthesis; Ojima, I.,
Ed.; Wiley: New York, 2000, 1.
Acknowledgment
This study was supported by the Korea Research Foundation Grant
funded by the Korean Government (MOEHRD, Basic Research
Promotion Fund) (KRF-2007-521-C00185).
References and Notes
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109, 7876.
(22) Halterman, R. L. In Comprehensive Asymmetric Catalysis;
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Berlin, 1999, 183.
(23) (a) Lightfoot, A.; Schnider, P.; Pfaltz, A. Angew. Chem. Int.
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Catal. 2002, 344, 40. (c) Blackmond, D. G.; Lightfoot, A.;
Pfaltz, A.; Rosner, T.; Schnider, P.; Zimmermann, N.
Chirality 2000, 12, 442. (d) McIntyre, S.; Hörmann, E.;
Menges, F.; Smidt, S. P.; Pfaltz, A. Adv. Synth. Catal. 2005,
347, 282. (e) Bell, S.; Wüstenberg, B.; Kaiser, S.; Menges,
F.; Netscher, T.; Pfaltz, A. Science 2006, 311, 642.
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(3) Sprinz, J.; Helmchen, G. Tetrahedron Lett. 1993, 34, 1769.
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Tetrahedron Lett. 1993, 34, 3149.
(5) (a) Crabtree, R. H.; Felkin, H.; Morris, G. E. J. Chem. Soc.,
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(25) Synthesis of Cab-PHOX 4
To a stirred solution of CabPPh2 1 (0.99 g, 3.0 mmol) in 30
mL of THF, which was cooled to –10 °C, was added 2.5 M
n-BuLi (1.2 mL, 3.0 mmol) via a syringe. The resulting
solution was stirred at –10 °C for 1 h and then added 2-
bromooxazoline 3 (0.63 g, 3.3 mmol) through a cannula. The
reaction temperature was maintained at –10 °C for 1 h.
Subsequently the reaction mixture was warmed slowly to r.t.
After stirring for an additional 12 h, the solvent was removed
Synlett 2009, No. 5, 771–774 © Thieme Stuttgart · New York

774
J.-D. Lee et al.
LETTER
under vacuum, and the resulting residue was taken up by
fresh column chromatography (RF = 0.6; hexane–benzene,
1:1). Chiral Cab-PHOX 4 was isolated from the reaction
(26) Synthesis of Compound 5
A procedure analogous to the preparation of compound 4
was used but instead starting from CabPCy2 2 (1.02 g, 3.0
mmol). Compound 5 was obtained as pale yellow oil (RF =
solution in 93% yield (1.23 g, 2.8 mmol).
12
HRMS: m/z calcd for [11B10
C
14N1H3016O31P]+: 439.5421;
0.5; hexane–benzene, 1:1; 1.19 g, 2.64 mmol, 88%).
20
12
found: 439.5432. Anal. Calcd: C, 54.65; H, 6.88; N, 3.19.
Found: C, 54.85; H, 7.02; N, 3.12. IR spectrum (KBr pellet):
n = 2604 (B–H), 1700 (C=N), 2982 (C–H), 2990, 3014
cm–1. 1H NMR (300 MHz, CDCl3): d = 0.92 [d, 3 H,
CH(CH3)2, 3JCH–CH3 = 6.6 Hz], 0.99 [d, 3 H, CH(CH3)2,
3JCH–CH3 = 6.9 Hz], 1.92 [m, 1 H, CH(CH3)2], 4.16 (m, 1 H,
CHN), 4.19 (t, 1 H, CH2O, 2JC–H = 8.4 Hz), 4.45 (t, 1 H,
CH2O, 2JC–H = 8.1 Hz), 7.43–7.82 (m, 10 H, PPh2). 11B NMR
(96.3 MHz, CDCl3): d = –3.12 (1 H), –5.74 (1 H), –8.93 (2
H), –12.49 (2 H), –14.02 (4 H). 13C NMR (75.4 MHz,
CDCl3): d = 14.2, 18.6, 30.5, 67.3, 70.5, 73.8, 81.7, 126.3,
126.5, 127.1, 127.7, 128.3, 128.6, 129.2, 129.7, 130.7,
131.2, 131.6, 132.8, 168.5. 31P NMR (121.5 MHz, CDCl3):
d = 12.7 (PPh2).
HRMS: m/z calcd for [11B10
C
20
14N1H4216O31P]+: 451.6373;
found: 451.6387. Anal. Calcd: C, 53.19; H, 9.37; N, 3.10.
Found: C, 53.33; H, 9.34; N, 3.11. IR spectrum (KBr pellet):
n = 2600 (B–H), 1698 (C=N), 2985 (C–H), 2996 cm–1.
1H NMR (300 MHz, CDCl3): d = 0.87 [d, 3 H, CH(CH3)2,
3JCH–CH3 = 6.3 Hz], 0.97 [d, 3 H, CH(CH3)2, 3JCH–CH3 = 6.9
Hz], 1.25 (m, 1 H, P-cyclo-CH), 1.35 (m, 2 H, P-cyclo-CH2),
1.79 (m, 2 H, P-cyclo-CH2), 1.86 (m, 1 H, CHN), 1.99 (m, 2
H, P-cyclo-CH2), 3.97 (m, 1 H, CHN), 4.08 (t, 1 H, CH2O,
3JCH–CH2 = 8.7 Hz), 4.34 (t, 1 H, CH2O, 3JCH–CH2 = 9.3 Hz).
11B NMR (96.3 MHz, CDCl3): d = –4.24 (1 B), –6.19 (1 B),
–9.48 (2 B), –10.51 (2 B), –14.90 (4 B). 13C NMR (75.4
MHz, CDCl3): d = 13.7, 16.9, 23.1, 23.4, 25.4, 25.8, 27.4,
27.7, 29.9, 30.3, 30.8, 64.2, 70.4, 73.5, 84.3, 168.2. 31P NMR
(121.5 MHz, CDCl3): d = 32.3 (PCy2).
Synlett 2009, No. 5, 771–774 © Thieme Stuttgart · New York

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