Chiral bis(N-arylamino)phosphine-oxazolines: Synthesis and application in asymmetric catalysis

Article abstract of DOI:10.1002/adsc.200800221

N-Arylation or N-alkylation of chiral 1,2-diamines followed by ring closure with phosphorus trichloride (PCl₃) and subsequent coupling with an oxazoline alcohol resulted in a new class of N,P ligands. The corresponding iridium tetrakis[3,5-bis(trifluormethyl)phenyl]borate (BAr_F) complexes were found to be efficient catalysts for the enantioselective hydrogenation of unfunctionalized olefins and α,β-unsaturated carboxylic esters.

Full text of DOI:10.1002/adsc.200800221

FULL PAPERS
DOI: 10.1002/adsc.200800221
Chiral Bis(N-arylamino)phosphine-oxazolines: Synthesis and
Application in Asymmetric Catalysis
Marc Schçnleber,a Robert Hilgraf,a and Andreas Pfaltza,*
a
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
Fax : (+41)-61-267-1103; e-mail: Andreas.Pfaltz@unibas.ch
Received: April 11, 2008; Published online: August 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800221.
Abstract: N-Arylation or N-alkylation of chiral 1,2- found to be efficient catalysts for the enantioselec-
diamines followed by ring closure with phosphorus tive hydrogenation of unfunctionalized olefins and
trichloride (PCl3) and subsequent coupling with an a,b-unsaturated carboxylic esters.
oxazoline alcohol resulted in a new class of N,P li-
gands. The corresponding iridium tetrakis
ACHTRE[UNG 3,5-bis(tri- Keywords: alkenes; asymmetric hydrogenation; ho-
fluormethyl)phenyl]borate (BArF) complexes were mogeneous catalysis; iridium; N,P ligands
Introduction
complexes of ligands 5 gave high enantioselectivites
in the hydrogenation of unfunctionalized olefins, their
Iridium complexes with chiral N,P ligands have activity was low, and consequently, only low conver-
emerged as efficient catalysts for the asymmetric hy- sions were achieved. In view of the encouranging
drogenation of unfunctionalized alkenes and certain enantioselectivities, we decided to study additional
classes of functionalized olefins for which no suitable structural variants of these ligands with N-aryl instead
chiral catalysts were available before.[1–5] After initial of N-sulfonamido groups, to see if a more electron-
studies of iridium catalysts based on phosphinooxazo- rich P atom would have a positive effect on catalytic
lines 1 (PHOX ligands),[6] we and others developed activity. Herein, we describe the synthesis of a series
many further chiral oxazoline ligands, such as phos- of bis(N-arylamino)phosphine-oxazolines and their
phinites 2 and 3,[7] which have considerably extended use in the iridium-catalyzed hydrogenation of unfunc-
the application range of Ir-catalyzed asymmetric hy- tionalized olefins and a,b-unsaturated carboxylic
drogenation (for non-oxazoline-based ligands, see esters.
refs.[1–3,5]). As further variations of this structural
motif we reported phosphite-oxazolines 4 and bis(N-
sulfonylamino)phosphine-oxazolines 5.[8] Although Ir Results and Discussion
Synthesis of Bis(N-arylamino)phosphine-oxazolines
The synthesis of ligands 6a–g started from commer-
cially available, enantiomerically pure 1,2-diphenyl-
AHCTREeUNG thylenediamine and 1,2-diaminocyclohexane. Amina-
tion according to the procedure of Wagaw and Buch-
wald[9] led to the arylated diamines 6a–e,g in 31–99%
yield.
For the synthesis of the arylmethyl-substituted di-
AHCTREaUNG mine 6f an alternative route was chosen: 1-methyl-
naphthalene was converted to its brominated product
8 in 95% yield,[10] which was reacted with diamine 7
and sodium tert-butoxide to give the desired alkylated
diamine 6f (Scheme 1).
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Marc Schçnleber et al.
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one case (R2 =2-naphthyl) the yield was only 11%,
possibly due to steric hindrance. These ligands were
found to be air-stable solids, which can be stored at
room temperature for several months without decom-
position.
Attempts to prepare analogous ligands from other
secondary diamines such as (1R,2R)-diphenylethy-
lene-1,2-diamine substituted at the nitrogen atoms
with 2-methoxyphenyl or 1-naphthyl groups, or (R)-
2,2’-bis[(4-methoxyphenyl)amino]-1,1’-binaphthalene
failed. The corresponding ligands could not be isolat-
ed, although in the first two cases, NMR spectroscopy
indicated that the cyclic chlorophosphine had actually
formed.
Structural information about the bis(N-arylamino)-
phosphine-oxazolines ligands geometry could be ob-
tained from a crystal structure of an Rh complex de-
rived from a phenyloxazoline analogue of ligand 10g
(Figure 1).[11] Addition of the ligand to a solution of
(COD)Cl]2 followed by anion exchange from ClÀ
[RhACHTREUNG
À
to PF6 afforded the complex in good yield. Slow re-
crystallization from ethyl acetate/cyclohexane at
À188C yielded orange needles suitable for X-ray
analysis.
Scheme 1. Synthesis of diamine 6f.
The diazaphospholidine ring of the ligand adopts a
nearly planar conformation. The tolyl groups shield
the coordination sphere next to the phosphorus atom.
The secondary amines 6a–g were treated with phos- These structural features are similar to those of the
phorus trichloride and triethylamine at À788C in tolu- bis(N-sulfonylamino)phosphine-oxazolines 5, implying
ene and slowly warmed to room temperature over-
night. Slow warming was essential to suppress the for-
mation of by-products. After filtration of the suspen-
sion and removal of the solvent, yellow solids were
1
obtained and characterized by H and 31P NMR spec-
troscopy. These air- and moisture-sensitive P-chloro-
diazaphospholidines were directly converted into the
corresponding N,P ligands 10a–g by stirring with tri-
ACHTREUNGethylamine, DMAP and oxazoline alcohol 9 overnight
in moderate to good yields of 38–64% (Scheme 2). In
Scheme 2. Synthesis of bis(N-arylamino)phosphine-oxazo-
line ligands 10a–g.
Figure 1. Crystal structure of Rh complex 11 derived from a
phenyloxazoline analogue of ligand 10g.
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Chiral Bis(N-arylamino)phosphine-oxazolines
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that substitution of the electron-withdrawing arylsul-
In the hydrogenation of (E)-1,2-diphenylpropene
fonamide groups of ligand 5 with the more electron- 13, catalysts 12b and 12g derived from diphenylethy-
donating arylamino groups of ligand 10 alters mainly lenediamine and cyclohexanediamine, respectively,
the electronic rather than the steric properties of the showed the highest enantioselectivity (>99% ee) (see
AHCTREUNG
ligand.
Table 1, entries 1 and 2). Catalysts 12c–e also exerted
Table 1. Enantioselective hydrogenations of olefins with cat-
alysts 12a–g.
Iridium-Catalyzed Hydrogenation
Entry Substrate Catalyst Conversion [%][a] ee [%][b]
Iridium complexes of bis(N-arylamino)phosphine-ox-
azoline ligands 10a–g were synthesized using our stan-
dard protocol: a solution of [IrACHTRE(UNG COD)Cl]2 and the N,P
ligand in dichloromethane was heated under reflux
followed by ion exchange by treatment with NaBArF
(sodium tetrakisACHTREUNG[3,5-bis(trifluormethyl)phenyl]borate)
in a two-phase dichloromethane-water system. Subse-
quent chromatography on a silica gel column afforded
the desired iridium complexes 12a–g in good yields.
These complexes were then tested as catalysts in the
hydrogenation of unfunctionalized olefins 13–18,
using 1 mol% of catalyst in dichloromethane at room
temperature for 2 h under 50 bar hydrogen pressure
(Scheme 3). Lower catalyst loading or reduction of
hydrogen pressure resulted in incomplete conversions
and lower enantioselectivities.
1
2
3
4
5
6
7
8
13
13
14
14
15
15
16
17
18
18
18
18
12b
12g
12b
12g
12c
12d
12f
12f
12a
12b
12e
12f
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99 (R)
>99 (R)
>99 (R)
>99 (R)
93 (R)
94 (R)
88 (S)
70 (S)
82 (S)
82 (S)
83 (S)
9
10
11
12
82 (S)
[a]
[b]
Determined by GC.
Determined by GC or HPLC using chiral columns.
high enantiocontrol of>98% ee (see Supporting In-
formation). The 3-methoxyphenyl-substituted com-
plex 12a and the 1-naphthylmethyl-substituted com-
plex 12f led to inferior enantiomeric excesses of 95%
and 88%. In comparison to the analogous tosylated
ligand 5a[8] [R1 =-(CH2)4-, R2 =4-tolyl, R3 =tert-butyl,
15% conversion, 94% ee], these new N,P ligands
showed higher enantiocontrol and activity. Similar
trends were also observed with the coresponding 4-
methoxy-substituted substrate 14 (see entries 3, 4 and
Supporting Information).
(E)- and (Z)-2-aryl-2-butenes are more demanding
substrates which usually react with lower enantiocon-
trol than the corresponding methylstilbenes 13 and
14.[1–8] With (E)-2-(4-methoxyphenyl)-2-butene 15 the
best results were achieved with ligands 12c and 12d
with up to 94% ee (Table 1, entries 5 and 6), which
exceed the enantioselectivities achieved with PHOX
ligand 1a (>99% conversion, 81% ee) and the tosylat-
ed ligand 5a (65% conversion, 84% ee).[8]
Scheme 3. Iridium-catalyzed hydrogenations of unfunction-
alized olefins.
For the corresponding (Z)-isomer 16, complex 12f
with 1-naphthylmethyl substituents proved to be the
best catalyst affording 88% ee, whereas all other com-
plexes gave inferior results (52 and 79% ee) (see
Table 1, entry 7 and Supporting Information).
Complex 12f was also the best catalyst for the re-
duction of 2-(4-methoxyphenyl)butene 17 (entry 8)
with an enantiocontrol of 70% ee. A clear decrease in
enantioselectivity to 14% ee was observed by using
ligand 12g with a 1,2-diaminocyclohexane backbone
instead of the (1R, 2R)-diphenylethylene-1,2-diamine
backbone (see Supporting Information).
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Using 6-methoxy-1-methyl-3,4-dihydronaphthalene
18 as substrate a similar trend could be observed. In-
dependently of the substitution pattern of the N-aryl
substituents, all catalysts (12a, 12b, 12e and 12f)
based on the 1,2-diphenylethylene-1,2-diamine frame-
work showed enantioselectivites between 82–83% ee.
Only complex 12g, with a cyclohexane-1,2-diamine
backbone, gave a lower ee of 72% (see Table 1, en-
tries 9–12 and Supporting Information). In compari-
son, the related tosylated ligand 5a gave very poor re-
sults for this substrate (>99% conversion, 18% ee).
Hydrogenation of N-(1-phenylethylidene)aniline,
(E)-2-methyl-3-phenylprop-2-enol and 3-methylcyclo-
hexenone resulted in low enantioselectivities (see
Supporting Information).
Table 3. Enantioselective Ir-catalyzed hydrogenation of a,b-
unsaturated esters 21–24.[a]
Entry Substrate Catalyst Conversion [%][b] ee [%][c]
a,b-Unsaturated carboxylic acids can be reduced
with high enantioselectivity using Ru catalysts.[12,13]
For analogous esters only a few suitable catalytic sys-
tems have been reported so far. The best results have
been achieved by semicorrin cobalt complexes[14]
using sodium borohydride as reducing agent, and with
certain iridium complexes.[4,5b,7,16] Complexes 12a–g
proved to be very efficient catalysts for this class of
substrate. In the hydrogenation of ethyl b-methylcin-
namate 19 excellent enantioselectivities of up to 98%
ee were obtained (Table 2).
1
2
3
4
5
6
7
8
21
21
22
22
22
22
23
24
12a
12b
12a
12c
12e
12g
12b
12e
71
95
88 (S)
88 (S)
>99
>99
>99
>99
>99
>99
88 (S)[17]
88 (S)
88 (S)
88 (S)
90 (R)
72 (S)[18]
[a]
All reactions were carried out with 1 mol% of catalyst in
CH2Cl2 at 50 bar H2 at 238C for 2 h.
Determined by GC.
[b]
[c]
Determined by GC or HPLC using chiral columns.
Table 2. Enantioselective Ir-catalyzed hydrogenation of
ethyl b-methylcinnamate 19.[a]
For (E)- and (Z)-ethyl 3-methyl-5-phenylpent-2-
enoates 22 and 23 the highest enantioselectivities so
far have been achieved with a cobalt complex derived
from the semicorrin ligand 25 (94% ee for 22 and
23).[14]
Entry
Catalyst
Conversion [%][b]
ee [%][c]
1
2
3
4
5
6
7
12a
12b
12c
12d
12e
12f
12g
94
97 (R)
97 (R)
98 (R)
96 (R)
97 (R)
90 (R)
96 (R)
>99
>99
>99
>99
97
>99
[a]
Iridium catalysts 12a, 12c, 12e, and 12g all gave
88% ee in the hydrogenation of (E)-ester 22 (Table 3,
entries 3 to 6). (Z)-Ester 23 reacted with somewhat
higher enantioselectivity of 90% ee with catalyst 12b,
whereas complexes 12d and 12g gave 86% and 83%
ee. In comparison, PHOX ligand 1a led to 72% and
All reactions were carried out with 1 mol% of catalyst in
CH2Cl2 at 50 bar H2 at 238C for 2 h.
Determined by GC.
[b]
[c]
Determined by GC or HPLC using chiral columns.
Ethyl a-methylcinnamate 21 reacted with lower but 78% ee for (E)- and (Z)-ester 22 and 23, respectively
still respectabele enantioselectivity (Table 3). In this (see Supporting Information).
case, catalysts 12a and 12b gave the best results with
The benzyl ester of (E)-tiglic acid 24 afforded only
up to 88% ee. These values are comparable to the moderate enantioselectivities of up to 72% ee with
enantioselectivities reported for the hydrogenation of catalyst 12e (Table 3, Entry 8), whereas up to 90% ee
the corresponding free acid with a ruthenium
BINAP) catalyst.[13b] In comparison, PHOX ligand 1a sponding free acid with a BINAP-like ligand derived
afforded only 3% ee for this substrate.
from equilenin.[15]
ACHTRE(UNG H8- was reported for the hydrogenation of the corre-
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Chiral Bis(N-arylamino)phosphine-oxazolines
FULL PAPERS
ethyl acetate (10:1)] to give the product as a pale yellow
solid; yield: 39%.
Conclusions
Bis(N-arylamino)phosphine-oxazolines have proved
to be efficient ligands for the iridium-catalyzed asym-
metric hydrogenation of unfunctionalized olefins and,
particularly, a,b-unsaturated carboxylic esters. Our re-
sults show that these ligands form more reactive iridi-
um catalysts than the previously reported N-sulfonyl
derivatives. Crystal structure data indicate that the
two ligand classes possess sterically similar coordina-
tion spheres, implying that the higher reactivity pri-
marily results from an electronic influence of the
more electron-rich phosphorus atom of the N-aryl-
substituted ligands rather than from steric effects. In
view of the modular nature and easy synthesis from
readily available chiral precursors, these ligands
should also prove useful for other applications in
asymmetric catalysis.
General Procedure for the Synthesis of Ligands 10a–g
AHCTREUNG
AHCTREUNG
phospholidine (10a): Triethylamine (4.2 equiv.) was dis-
solved in toluene (1.8 mLmmolÀ1) and cooled to À788C.
Freshly distilled phosphorus trichoride (2.1 equiv.) and a sus-
pension of diamine 6a (1 equiv.) in toluene (10 mLmmolÀ1)
were added and the reaction mixture was allowed to slowly
warm to room temperature overnight. After filtration under
argon and removal of the solvent under high vacuum, the
residue was dissolved in toluene (7.4 mLmmolÀ1) and
cooled to À788C. A solution of NEt3 (10 equiv.) and DMAP
(1.02 equiv.) in toluene (1 mLmmolÀ1) was added very
slowly dropwise followed by a solution of the oxazoline al-
cohol 9 (1 equiv.) in toluene (3 mLmmolÀ1). The reaction
mixture was allowed to slowly warm to room temperature
overnight. After column chromatography (4”10 cm,
hexane/ethyl acetate, 5/1) the desired product was obtained
as white solid; yield: 61%.
Experimental Section
General Procedure for the Synthesis of Iridium
Complexes of Ligands 12a–g
Typical Procedures for the Bis-Arylation of Chiral
Diamines
Ligand (1 equiv.) and [IrACHTREU(NG COD)Cl]2 (0.5 equiv.) were dis-
ACHTREUNG
solved in dichloromethane (36 mLmmolÀ1) and heated
under reflux for 1 h. After cooling to room temperature
NaBArF (1.5 equiv.) and water (11 mLmmolÀ1) were added
and the reaction mixture was stirred vigorously for 20 min.
The phases were separated and the aqueous phase was ex-
tracted twice with dichloromethane. The combined organic
phases were washed with water and the solvent was evapo-
rated. The residue was purified by chromatography (di-
chloromethane, 4”10 cm) to give the product.
CTHREUNG
ACHTREUNG
(1.8 equiv.) and sodium tert-butoxide (2.8 equiv.) were
added and the solution stirred for further 30 min. After
adding
(1 equiv.) the reaction mixture was refluxed for 4 h and after
cooling to room temperature hydrolyzed with water. The so-
lution was filtered and dried over Na2SO4. The solvent was
evaporated and the residue was purified by chromatography
[5”10 cm, hexane/ethyl acetate (10:1 to 5:1)] and recrystalli-
zation (CH2Cl2/hexane) to give the product as a white solid;
yield: 80%.
(1R,2R)-(+)-1,2-diphenyl-1,2-ethanediamine
Rhodium Complex 11
Ligand 10g (150 mg, 0.240 mmol) and [RhCATHRE(UNG COD)Cl]2
(61 mg, 124 mmol) were dissolved in dichloromethane
(1.3 mL) and stirred for two hours at room temperature.
Ammonium hexafluorophosphate (57 mg, 0.350 mmol) was
added and the reaction mixture was stirred overnight. The
suspension was washed with water (4 mL) and the solvent
evaporated. Crystals suitable for X-ray analysis were ob-
tained by recrystallization from ethyl acetate/cyclohexane at
À188C.
1-(Bromomethyl)naphthalene (8): 1-Methylnaphthalene
(1.4 g, 10 mmol) was dissolved in dry benzene (100 mL) and
NBS (1.79 g, 10 mmol) and catalytic amounts of AIBN were
added. The reaction mixture was heated at 808C for 2.5 h.
After cooling to room temperature the reaction mixture was
filtered through Celite and the solvent evaporated. The resi-
due was purified by chromatography [hexane/ethyl acetate
(5:1)] and subsequent recrystallization (pentane/dichlorome-
thane) to give the product as a colorless solid; yield: 2.11 g
(95%).
General Procedure for Ir-Catalyzed Hydrogenation
ACHTREUNG
The alkene (1 equiv.) and Ir complex (1 mol%) were dis-
solved in CH2Cl2 (10 mLmmolÀ1 substrate) in a high-pres-
sure autoclave (without exclusion of oxygen), pressurized to
50 bar with hydrogen and stirred for 2 h at room tempera-
ture. The solvent was removed and the residue was suspend-
ed in heptane, filtered through a syringe filter (CHROMA-
FIL O-20/15 MS PTFE 0.2 mm, Macherey–Nagel) and the
filtrate was directly used for GC and chiral HPLC analysis
to determine the conversion and enantiomeric excess (for
analytical procedures and data, see refs.[7b,c]).
A
7
(50 mL) and NaO-t-Bu (978 mg, 9.8 mmol) was added. After
stirring for 1.5 h at room temperature, 1-(bromomethyl)-
naphthalene 8 (1.40 g, 6.3 mmol) was added and the reaction
mixture heated under reflux for 2 h. After cooling to room
temperature the mixture was quenched with water (5 mL),
the phases were separated and the combined organic phases
dried over MgSO4. The residue obtained after evaporation
of the solvent was purified by chromatography [hexane/
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Analytical characterization data for all compounds are
available in the Supporting Information.
[10] S. Futamura, Z. M. Zong, Bull. Chem. Soc. Jpn. 1992,
65, 345–348.
[11] CCDC 238429 contains the supplementary crystallo-
graphic data for the Rh complex described in this
paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[12] H. Takaya, T. Ohta, N. Sayo, H. Kumobayashi, S. Aku-
tagawa, S. Inoue, I. Kasahara, R. Noyori, J. Am. Chem.
Soc. 1987, 109, 1596–1597; R. Noyori, Angew. Chem.
2002, 114, 2108–2123; Angew. Chem. Int. Ed. 2002, 41,
2008–2022; reviews: J. M. Brown, in: Comprehensive
Asymmetric Catalysis, Vol. I, Chapter 5.1, (Eds.: E. N.
Jacobsen, A. Pfaltz, H. Yamamoto), Springer, Berlin,
1999, pp. 121–182; T. Ohkuma, M. Kitamura, R.
Noyori, in: Catalytic Asymmetric Synthesis, 2nd edn.,
(Ed.: I. Ojima), Wiley-VCH, New York, 2000, pp 1–
110.
Acknowledgements
Financial support by the Swiss National Science Foundation
and the Federal Commission for Technology and Innovation
(KTI Project No 5189.2 KTS) is gratefully acknowledged.
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