A polymer-supported chiral fluorinated dirhodium(II) complex for asymmetric amination of silyl enol ethers

Article abstract of DOI:10.1002/adsc.201200219

The immobilization of dirhodium(II) tetrakis[N-tetrafluorophthaloyl-(S)- tert-leucinate], [Rh₂(S-TFPTTL)₄], has been accomplished by copolymerization of a dirhodium(II) complex-containing monomer with styrene and 1,6-bis(4-vinylbenzyloxy)hexane as a flexible cross-linker. The polymer-supported chiral fluorinated dirhodium(II) complex catalyzed the amination of silyl enol ethers with [N-(2-nitrophenylsulfonyl)imino] phenyliodinane (NsN=IPh) to provide α-amino ketones in high yields with high levels of enantioselectivity and could be used up to 20 times as the catalyst readily withstood stirring in the presence of the solid reactant. Copyright

Full text of DOI:10.1002/adsc.201200219

UPDATES
DOI: 10.1002/adsc.201200219
A Polymer-Supported Chiral Fluorinated Dirhodium(II) Complex
for Asymmetric Amination of Silyl Enol Ethers
Tadashi Oohara,a Hisanori Nambu,a Masahiro Anada,a Koji Takeda,a
and Shunichi Hashimotoa,*
a
Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
Fax : (+81)-11-706-4981; e-mail: hsmt@pharm.hokudai.ac.jp
Received: March 16, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200219.
Abstract: The immobilization of dirhodium(II) tet-
rakis[N-tetrafluorophthaloyl-(S)-tert-leucinate],
our knowledge, there is no immobilized dirhodium(II)
catalyst available for nitrene reactions.[17,18]
[Rh
(S-TFPTTL)4], has been accomplished by co-
Recently, we reported an effective immobilization
polymerization of a dirhodium(II) complex-contain-
ing monomer with styrene and 1,6-bis(4-vinylbenzy-
loxy)hexane as a flexible cross-linker. The polymer-
supported chiral fluorinated dirhodium(II) complex
catalyzed the amination of silyl enol ethers with [N-
(2-nitrophenylsulfonyl)imino]phenyliodinane
(NsN=IPh) to provide a-amino ketones in high
yields with high levels of enantioselectivity and
could be used up to 20 times as the catalyst readily
withstood stirring in the presence of the solid reac-
tant.
of [Rh2ACTHNGUTERNNUG
(S-PTTL)4] (1a)[19–21] (Figure 1), which was
achieved by preparation of a dirhodium(II) complex-
containing monomer 2a followed by copolymerization
with styrene (5a) and 1,6-bis(4-vinylbenzyloxy)hexane
(6) as a flexible cross-linker (Scheme 1).[22–25] The
polymer-supported complex 7a with no unreacted
linkers or free ligands[26] catalyzed asymmetric C H
À
insertions with high enantioselectivities similar to
those found with the homogeneous catalyst 1a and
could be used for up to 100 sequential applications
with a low leaching level.[22] Very recently, a similar
method proved to be beneficial for the immobiliza-
tion of a chiral chlorinated dirhodium(II) complex.[27]
The immobilized catalyst 7b prepared by copolymeri-
zation of dirhodium(II) complex-containing monomer
2b with 2-(trifluoromethyl)styrene (5b) and cross-
linker 6 (Scheme 1) was successfully applied to inter-
molecular carbonyl ylide cycloaddition reactions
Keywords: asymmetric catalysis; heterogeneous cat-
alysis; immobilization; nitrenes; rhodium
Chiral dirhodium(II) carboxylate and carboxamidate under continuous flow conditions, where high yields
complexes are exceptional catalysts for a wide range as well as high levels of enantioselectivity (up to 99%
of enantioselective metal carbene transformations ee) and turnover number (up to 11,700) were ach-
with diazocarbonyl compounds, including cyclopropa- ieved.[27] Given that catalysts 7a and 7b were remarka-
À
nation, C H insertion, and rearrangement or cycload- bly effective catalysts for metal carbene transforma-
dition via ylide generation.[1] Recently, they have also tions, we were intrigued by the applicability of a poly-
been recognized as effective catalysts in metal nitrene mer-supported dirhodium(II) complex to metal
À
reactions such as C H amination and olefin aziridina- nitrene reactions. In this context, we previously
tion.[2–8] Although such reactions frequently proceed demonstrated that dirhodium(II) tetrakis[N-tetra-
in high yield and with high levels of asymmetric in- fluorophthaloyl-(S)-tert-leucinate],
duction, their practical applications in pharmaceutical
(S-TFPTTL)4] (1c),[28] the fluorinated analogue of
ACHUTGTNERN[NUG Rh2ACHTUNGTRENNUGN
production are hampered by the high costs of both 1a, is an effective catalyst for the enantioselective
rhodium[9] and chiral ligands as well as the difficulty amination of silyl enol ethers derived from acyclic ke-
in catalyst recovery and recycling. These drawbacks tones or enones with [N-(2-nitrophenylsulfonyl)imi-
have stimulated a variety of efforts toward immobili- no]phenyliodinane (NsN=IPh), in which high levels of
zation of chiral dirhodium(II) complexes.[10–16] While asymmetric induction (up to 95% ee) were achieved
considerable advances have been made in selected when the reaction was conducted in CH2Cl2 at
catalytic asymmetric carbene processes, to the best of À408C.[28b] Toward this goal, however, a problem
arose since NsN=IPh[29] is practically insoluble in
Adv. Synth. Catal. 2012, 354, 1 – 8
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Tadashi Oohara et al.
UPDATES
Figure 1. Chiral dirhodium(II) catalysts. PTTL=N-phthaloyl-tert-leucinate, TCPTTL=N-tetrachlorophthaloyl-tert-leucinate,
TFPTTL=N-tetrafluorophthaloyl-tert-leucinate.
Scheme 1. Immobilization of chiral dirhodium(II) complexes.
2
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Adv. Synth. Catal. 2012, 354, 1 – 8

A Polymer-Supported Chiral Fluorinated Dirhodium(II) Complex for Asymmetric Amination
most solvents[2c] including CH2Cl2, the optimal solvent ous reference catalyst 4a (76% ee, entry 3). Although
for this transformation.[28b,30] Therefore, a major chal- the ee value was unsatisfactory, this result indicates
lenge was the development of polymer-supported dir- that the present method of immobilization had very
hodium(II) catalysts that could readily withstand the little effect on the chiral environment around the im-
dynamic contact with solid reactants. Herein, we mobilized catalyst.[36,37] At this point, we turned our
report the immobilization of a chiral fluorinated dir- attention to the performance of dirhodium(II) com-
hodium(II) complex and its use for enantioselective plex 4b, a catalyst characterized by substitution of flu-
amination of silyl enol ethers.
In our previous work,[27] the immobilization of Gratifyingly, the reaction using catalyst 4b afforded
[Rh2A
(S-TCPTTL)4] (1b),[7,31–33] the chlorinated ana- 10a in 91% yield with 91% ee (entry 4). This promis-
orine atoms for all phthalimido hydrogen atoms.
A
CHTUNGTRENNUNG
logue of 1a, was unsuccessful simply because nucleo- ing result had important implications for the design of
philic substitution of tetrachlorophthalic acid with hy- an immobilized catalyst based on copolymerization of
droxide ion failed to give the 4-hydroxy-3,5,6-trichlor- monomer 2d that incorporates N-4-[6-(4-vinylbenzyl-
ophthalic acid required for the preparation of a re-
ACHTUNGTRENoNUNG xy)hexyl]oxy-3,5,6-trifluorophthaloyl-(S)-tert-leucine
placeable ligand 3a. Conveniently, however, the im- (3b) as one of the bridging ligands.
mobilized catalyst 7b emerged as a surrogate for the
Dirhodium(II) complex-containing monomer 2d
unfinished goal (Scheme 1).[27] Thus, our initial study was synthesized as shown in Scheme 2. 4-Benzyloxy-
focused on the immobilization of a mixed dirhodiu- 3,5,6-trifluorophthalic anhydride (14) was prepared in
m(II) complex 2c,[34] one bridging ligand of which is five steps from tetrafluorophthalic acid (11) via 4-hy-
N-4-[6-(4-vinylbenzyloxy)hexyl]oxyphthaloyl-(S)-tert-
droxy-3,5,6-trifluorophthalic acid[38] in 71% yield.
leucine. AIBN-initiated and acacia gum-stabilized sus- Condensation of (S)-tert-leucine with 14 proceeded
pension copolymerization of monomer 2c with sty- without racemization to form a carboxylic acid,
rene (5a) and cross-linker 6 afforded a polymer-sup- which, upon treatment with N,N’-diisopropyl-O-tert-
ported complex 7c in 70% yield (Scheme 1).
The immobilized complex 7c was examined for its Hydrogenolysis of the benzyl ether followed by O-
catalytic performance in the amination of silyl enol alkylation with 6-(4-vinylbenzyloxy)hexanol under
butylisourea, afforded tert-butyl ester 15 in 75% yield.
AHCTUNGTRENNUNG
ether 8a (Z/E=96:4) derived from phenylacetone Mitsunobu conditions and subsequent exposure to
with NsN=IPh (9) (Table 1).[28b] The reaction in the formic acid provided the targeted ligand 3b in 89%
presence of 5 mol% of 7c proceeded in CH2Cl2 at yield. Treatment of [Rh2ACHTUNTRGNEUNG(S-TFPTTL)4] (1c) with 3b in
À208C for 20 h and, after separation of 7c from the refluxing chlorobenzene gave an equilibrium mixture
reaction mixture and subsequent treatment of the of dirhodium(II) complexes 2d and 1c, which were
liquid phase with 90% aqueous trifluoroacetic acid separable by gel permeation chromatography. The de-
(TFA), gave a-amino ketone 10a in 94% yield with sired complex 2d was isolated in 44% yield; 1c was
79% ee (entry 2).[35] The enantioselectivity was sub- recovered in 53% yield. After a two-cycle sequence
stantially lower than that observed with [Rh
2ACHTUNGTREN(NGNU S- of ligand-exchange reactions of recovered 1c and 3b,
TFPTTL)4] (1c) (94% ee, entry 1) but was found to complex 2d was obtained in 60% combined yield.
be comparable to that obtained with the homogene-
Copolymerization of 2d with styrene (5a) and 6 as
a cross-linker gave a polymer-supported fluorinated
dirhodium(II) complex 17 in 83% yield (Scheme 3). It
is well known that the mechanical properties of the
resin strongly depend on a homogeneous composition
of the copolymer.[39] The chemical composition of 17
determined by elemental analysis and inductively cou-
pled plasma atomic emission spectroscopy (ICP-AES)
analysis was very close to the original monomer com-
position, indicating similar reactivity of the comono-
mers 2d, 5a and 6. Furthermore, evidence for a uni-
form dispersion of dirhodium(II) complex in the poly-
mer matrix was obtained from surface analysis of
polymer 17 using scanning electron microscopy with
an X-ray micro-analyzer (SEM-XMA).
Table 1. Rh(II)-catalyzed enantioselective amination of silyl
enol ether 8a with 9.[a]
Entry Rh(II) mol% Time [h] Yield [%][b] ee [%][c]
1
2
3
4
1c
7c
4a
4b
2
5
2
2
3.5
20
5
93
94
94
91
94
79
76
91
Using 5 mol% of the immobilized catalyst 17, we
were gratified to find that the reaction of silyl enol
ether 8a with NsN=IPh (9) at À208C afforded a-
amino ketone 10a in 94% yield with high asymmetric
induction (92% ee), though catalyst 17 displayed low
4
[a]
All reactions were performed at À208C on a 0.1-mmol
scale (0.1M) with 1.1 equiv. of 8a.
Isolated yield.
[b]
[c]
Determined by HPLC.
reactivity relative to [Rh2ACTHUNGTRENNU(G S-TFPTTL)4] (1c) (Table 2,
Adv. Synth. Catal. 2012, 354, 1 – 8
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Tadashi Oohara et al.
UPDATES
Scheme 2. Preparation of dirhodium(II)-complex-containing monomer 2d.
Scheme 3. Immobilization of chiral dirhodium(II) complex.
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A Polymer-Supported Chiral Fluorinated Dirhodium(II) Complex for Asymmetric Amination
Table 2. Rh(II)-catalyzed enantioselective amination of silyl
enol ethers 8 with 9.[a]
Entry
8
Rh(II) Time [h]
Product
Yield [%][b] ee [%][c]
1[d]
2[e]
3[d]
4[e]
5[d]
6[e]
8a 1c
8a 17
8b 1c
8b 17
8c 1c
8c 17
3.5
20
1
18
3
10a 93
94
92
91
88
82
80
10a 94
10b 93
10b 91
10c 87
10c 88
Figure 2. Photographs of the Rh(II) catalyst 17 (A) immedi-
ately after addition of NsN=IPh (9); (B) after 72 h of stir-
ring (Table 3, entry 6).
30
[a]
All reactions were performed at À208C on a 0.1-mmol
scale (0.1M) with 8.
Isolated yield.
the potency of its recovery and reuse (Table 3). After
the amination of silyl enol ether 8a with NsN=IPh (9)
was complete, the liquid phase containing the product
was easily separated from the solid catalyst 17 with
a Pasteur pipette. The leaching level of rhodium atom
was examined by ICP-MS. The liquid phase of the
[b]
[c]
[d]
[e]
Determined by HPLC.
2 mol% of [Rh
2ACHTUNGTRENNUNG(S-TFPTTL)4] (1c) was used.
5 mol% of 17 was used.
entries 1 and 2). Furthermore, the reaction of benzyl- first cycle contained only 10 ppm rhodium, which cor-
substituted silyl enol ethers 8b and 8c with catalyst 17 responds to 0.04% of the initial catalyst charge.
afforded the respective a-amino ketones 10b and 10c, Indeed, the immobilized catalyst 17 could be used 20
a key intermediate in asymmetric formal synthesis of times with virtually no drop in product yield or enan-
(À)-metazocine,[28b] in high yields and good to high tioselectivity, although reaction times were gradually
enantioselectivities as found with 1c (entries 3–6).[40]
prolonged with each subsequent recycling step (en-
Since the catalyst 17 displayed adequate activity tries 2–6). The polymer-supported complex 17 is
and enantioselectivity, attention was next turned to formed in beads that readily withstand stirring in the
presence of practically insoluble solid reactant 9
(Figure 2). It is documented that the cross-link can
Table 3. Enantioselective amination of silyl enol ether 8a
have a profound effect on mechanical strength.[41] The
with 9 catalyzed by immobilized Rh(II) complex 17.[a]
mechanical stability of 17 can be attributed to its ho-
mogeneous composition and the choice of 1,6-bis(4-
vinylbenzyloxy)hexane (6) as a flexible cross-linker.
The polymer-supported complex 17 was also found
to be an effective catalyst for enantioselective amina-
tion of silyl enol ether 18 (Z/E= >99:1) derived from
propiophenone with NsN=IPh (9) at À208C
(Table 4).[28b] The catalysis with 17 afforded a-amino
Entry
Cycle
Time [h]
Yield [%][b]
ee [%][c]
ketone 19 in virtually the same product yield and
enantioselectivity as found with the [Rh2ACHTUNGTRENNUNG(S-
1
2
3
4
5
6
1
2
5
10
15
20
20
20
24
30
48
72
94
93
93
94
94
95
92
92
90
90
90
91
TFPTTL)4] (1c) (entries 1 and 2).[42] The robust cata-
lyst 17 could be recycled 20 times without compromis-
ing the product yield or enantioselectivity (entries 3–
7).
In summary, we have accomplished the immobiliza-
tion of a chiral fluorinated dirhodium(II) complex by
copolymerization of a dirhodium(II) complex-contain-
ing monomer 2d with styrene (5a) and 1,6-bis(4-vinyl-
benzyloxy)hexane (6) as a flexible cross-linker. The
polymer-supported fluorinated dirhodium(II) complex
[a]
All reactions were performed at À208C on a 0.1-mmol
scale (0.1M) with 1.1 equiv. of 8a in the presence of
5 mol% of Rh(II) catalyst 17.
Isolated yield.
Determined by HPLC.
[b]
[c]
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Tadashi Oohara et al.
UPDATES
Table 4. Enantioselective amination of silyl enol ether 18
with 9 catalyzed by immobilized Rh(II) complex 17.[a]
for 1.5 h at room temperature. The mixture was partitioned
between EtOAc (15 mL) and pH 7.0 phosphate buffer
(3 mL). The separated organic layer was washed with water
(2ꢁ3 mL) and brine (2ꢁ3 mL), and dried over anhydrous
Na2SO4. Filtration and evaporation furnished the crude
product, which was purified by column chromatography on
silica gel (2:1 hexane/EtOAc) to provide 10a as a colorless
viscous oil; yield: 31.4 mg (94%). The spectroscopic data
were consistent with the previously reported data; [a]2D1:
À328 (c 1.02, CHCl3) for 92% ee [lit.[28b] [a]D20: À340 (c 0.92,
CHCl3) for R 95% ee of 10a]. The enantiomeric excess of
10a was determined to be 92% by HPLC [Chiralpak AD-H
column (3:1 hexane/i-PrOH, 1.0 mLminÀ1)]: tR =13.3 min
for major enantiomer, tR =14.9 min for minor enantiomer.
Entry Rh(II) Cycle Time [h] Yield [%][b] ee [%][c]
1[d]
2
3
4
5
1c
17
17
17
17
17
17
–
1
2
5
10
15
20
3
95
92
91
92
93
91
92
91
90
91
91
90
90
90
10
10
12
14
18
22
6
7
Acknowledgements
[a]
This research was supported by a Grant-in Aid for Scientific
Research on Innovative Areas “Organic Synthesis Based on
Reaction Integration” (No. 2105) from the Ministry of Edu-
cation, Culture, Sports, Science and Technology, Japan. We
thank S. Oka and M. Kiuchi from the Center for Instrumen-
tal Analysis at Hokkaido University for technical assistance
with mass spectrometry and elemental analysis.
All reactions were performed at À208C on a 0.2-mmol
scale (0.1M) with 1.1 equiv. of 18 in the presence of
5 mol% of Rh(II) catalyst 17.
Isolated yield.
Determined by HPLC.
[b]
[c]
[d]
2 mol% of [Rh2ACHTUNGTRENNUNG(S-TFPTTL)4] (1c) was used.
17 catalyzed the amination of silyl enol ethers to give
a-amino ketones in high yields with high levels of References
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Experimental Section
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A reaction vessel was equipped with a stirring bar and
charged with polymer-supported dirhodium(II) complex 17
(19.8 mg, 5 mol%), silyl enol ether 8a (27.1 mg, 0.11 mmol,
Z:E=96:4) and CH2Cl2 (1 mL), and allowed to swell at
room temperature for 0.5 h. NsN=IPh (9) (40.4 mg,
0.10 mmol) was added in one portion to the mixture at
À208C. After 20 h of stirring at this temperature, the organ-
ic layer was separated from the solid catalyst 17 with a Pas-
teur pipette. The remaining resin was washed with an addi-
tional CH2Cl2 (1 mL), and the organic layer was combined
with the crude reaction mixture. The catalyst 17 was reused
without further treatments. The reaction mixture was treat-
ed with 90% aqueous TFA (ca. 0.1 mL) followed by stirring
6
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7

Tadashi Oohara et al.
UPDATES
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À
strated in enantioselective C H aminations (up to 94%
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[34] Very recently, Fox and co-workers developed dirhodiu-
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wheel complex was shown to catalyze asymmetric cy-
À
clopropanation, cyclopropenation, and indole C H
functionalization with a-alkyl-a-diazo esters in yields
and enantioselectivities that are comparable or superior
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NsNH2 was examined. However, the amination reac-
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solubility of NsNH2 in CH2Cl2. For instance, the reac-
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[Rh2ACHTUNGTRENNUNG(S-TFPTTL)4] (1c) at 08C required 16 h for full
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8
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 1 – 8