A new approach to osmium-catalyzed asymmetric dihydroxylation and aminohydroxylation of olefins.

Full text of DOI:10.1002/1521-3773(20020201)41:3<472::AID-ANIE472>3.0.CO;2-7

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[6] K. F. M. G. J. Scholle, W. S. Veeman, J. Phys. Chem. 1985, 89, 1850
1852.
[7] a) P. Sarv, C. Fernandez, J.-P. Amoureux, K. Keskinen, J. Phys. Chem.
1996, 100, 19223 19226; b) P. P. Man, Phys. Rev. B 1998, 58, 2764
2782.
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Chem. 1981, 85, 2238 2243; b) H. Lermer, M. Draeger, J. Steffin,
K. K. Unger, Zeolites 1985, 5, 131 134.
[9] E. Lippmaa, A. Samoson, M. M‰gi, J. Am. Chem. Soc. 1986, 108,
1730 1735.
exist between zeolites synthesized in hydroxide and fluoride
media in this respect. Further study is necessary to determine
the underlying mechanisms of this intriguing behavior. How-
ever, the overall NMR results presented here strongly suggest
that the substitution of Al atoms into the ZSM-5 framework
during the crystallization process may be kinetically, rather
than energetically, controlled. This led us to believe that the
substitution patterns of other heteroatoms (e.g., B, Ga, Ti,
etc.) in the ZSM-5 framework would also be nonrandom,
although they may differ in manner from Al substitution. In
fact, recent neutron powder diffraction studies on TS-1, the
titanosilicate analogue of ZSM-5, showed that Ti atoms are
not uniformly distributed over the 12 ZSM-5 T-sites.[13]
Finally, it is of interest whether our concept can be applied
to other important high-silica materials with multiple T-sites
such as zeolites b and MCM-22.[1]
[10] K. A. Valiyev, M. M. Zripov, Zh. Strukt. Khim. 1966, 7, 494 502.
[11] A. Medek, J. S. Harwood, L. Frydman, J. Am. Chem. Soc. 1995, 117,
12779 12787.
[12] We also considered the possibility that differences in the CQ value for
the specific Al sites in our ZSM-5 samples could produce significant
changes in the intensity ratio of the respective 27Al MAS NMR peaks
in Figure 1. We calculated possible intensity variations for four ZSM-5
samples with Si/Al ˆ 11.9 300 using the reported crystallographic
data, according to a method available in the literature (for example,
see D. Massiot, C. Bessada, J. P. Coutures, F. Taulelle, J. Magn. Reson.
1990, 90, 231 242). The extent of intensity variation was less than
3.5% in all four cases, and no correlation with the Si/Al ratio of the
ZSM-5 samples could be observed. Thus, differences in the CQ value
for the specific Al sites have little effect on the intensity ratio of the
respective two 27Al MAS NMR peaks.
Experimental Section
Six ZSM-5 zeolites with different Si/Al ratios were synthesized by using
NH4F (‡98%, Aldrich) and TPABr (98%, Aldrich) in fluoride media to
minimize the possible influence of connectivity defects on the distribution
of Al in the ZSM-5 framework, according to procedures described
elsewhere.[14] The Si/Al ratio in the final product was varied by adjusting
the amount of Al(NO3)3 ¥ 9H2O (99%, Wako) that was added to the
synthesis mixture as Al source. Amorphous silica (Aerosil 200, Degussa)
was used as Si source. All ZSM-5 zeolites prepared here were phase-pure
and highly crystalline (powder XRD, Rigaku Miniflex, CuKa radiation).
The Si/Al ratios of these six as-made samples were analyzed in the
Analytical Laboratory of KIST. The absence of any detectable line at d ꢀ 0
in the 27Al MAS NMR spectra (Figure 1) indicates that all the Al atoms in
these samples are isomorphously incorporated into the T-sites of the ZSM-
5 framework. Also, thermogravimetric and differential thermal analyses
(TA Instruments SDT 2960) reveals that they contain 3.8 4.3 TPA cations
per unit cell.
The 27Al MAS NMR spectra were recorded on a Bruker DSX 400 NMR
spectrometer that operates at a 27Al frequency of 104.27MHz and a
spinning rate of 13 kHz. The spectra were obtained by acquisition of 300
400 pulse transients, which was repeated with a p/12 rad pulse length of
0.8 ms and a recycle delay of 1 s. The 27Al chemical shifts are referenced to a
1n [Al(H2O)6]3‡ solution. The 2D 27Al 3Q MAS NMR spectra were
recorded on a Varian INOVA 400 NMR spectrometer by the two-pulse z-
filtered procedure with rotor synchronization[15] at a spinning rate of
10 kHz, with an excitation pulse of 6 ms and a conversion pulse of 1.8 ms for
an rf field strength of 84 kHz. For each t1 384 or 768 scans were
accumulated, and t1 was incremented 64 times.
[13] a) C. A. Hijar, R. M. Jacubinas, J. Eckert, N. J. Henson, P. J. Hay, K. C.
Ott, J. Phys. Chem. B 2000, 104, 12157 12164; b) C. Lamberti, S.
Bordiga, A. Zecchina, G. Artioli, G. Marra, G. Spano, J. Am. Chem.
Soc. 2001, 123, 2204 2212.
[14] a) E. M. Flanigen, R. L. Patton, US 4073865, 1978 [Chem. Abstr.
1978, 88, 138653p]; b) J. L. Guth, H. Kessler, R. Wey, Stud. Surf. Sci.
Catal. 1986, 28, 121 128.
[15] J.-P. Amoureux, C. Fernandez, S. Steuernagel, J. Magn. Reson. A 1996,
123, 116 118.
A New Approach to Osmium-Catalyzed
Asymmetric Dihydroxylation and
Aminohydroxylation of Olefins**
Malin A. Andersson, Robert Epple, Valery V. Fokin,*
and K. Barry Sharpless*
Osmium-catalyzed asymmetric dihydroxylation (AD) of
olefins using cinchona alkaloid derived ligands is known for its
exceptional scope and reliability across nearly the entire
Received: June 26, 2001
Revised: November 6, 2001 [Z17357]
[*] Prof. V. V. Fokin, Prof. K. B. Sharpless, Dr. M. A. Andersson,
Dr. R. Epple
Department of Chemistry
and The Skaggs Institute for Chemical Biology, BCC-315
The Scripps Research Institute
10550 N. Torrey Pines Rd.
[1] International Zeolite Association, Structure Commission, http://
www.iza-structure.org.
[2] D. H. Olson, N. Khosrovani, A. W. Peters, B. H. Toby, J. Phys. Chem.
B 2000, 104, 4844 4848.
[3] a) J. G. Fripiat, F. Berger-Andre, J.-M. Andre, E. G. Derouane,
Zeolites 1983, 3, 306 310; b) E. G. Derouane, J. G. Fripiat, Zeolites
1985, 5, 165 172; c) A. E. Alvarado-Swaisgood, M. K. Barr, P. J. Hay,
A. Redondo, J. Phys. Chem. 1991, 95, 10031 10036; d) A. Redondo,
P. J. Hay, J. Phys. Chem. 1993, 97, 11754 11761; e) G. J. Kramer,
R. A. van Santen, J. Am. Chem. Soc. 1993, 115, 2887 2897; f) G.
Ricciardi, J. M. Newsam, J. Phys. Chem. B 1997, 101, 9943 9950.
[4] R. M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press,
London, 1982.
La Jolla, CA 92037(USA)
Fax : (‡1)858-784-7562
E-mail: fokin@scripps.edu, sharples@scripps.edu
[**] We thank the National Institute of General Medical Sciences,
National Institutes of Health (GM 28384), National Science Founda-
tion (CHE-9985553), and the W. M. Keck Foundation for financial
support. M.A. is grateful for postdoctoral fellowships from the Bengt
Lundqvist Memorial Foundation, the Swedish Institute, and the
Swedish Research Council for Engineering Sciences. We also thank
Prof. M. G. Finn and Dr. Luke Green for many helpful discussions,
and Frank I. Garcia (Hilltop High school, San Diego) for help with the
synthesis of ligands.
[5] C. F. Fyfe, G. C. Gobbi, J. Klinowski, J. M. Thomas, S. Ramdas, Nature
1982, 296, 530 533.
472
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Angew. Chem. Int. Ed. 2002, 41, No. 3

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range of olefin types and substitution patterns on both
laboratory and industrial scales.[1] Its sister reaction, the
asymmetric aminohydroxylation (AA), is the aza-analogue of
the parent transformation and represents a powerful, direct
route to vicinal amino alcohols.[2] After its discovery in 1996, a
great deal of effort has been devoted to make it as reliable,
versatile, and convenient to use as its dihydroxylation
counterpart. Although the AA process has already found
substantial use in organic synthesis, it still lacks the generality
and reliability of the AD process. In many cases, problems
arise that appear to be related to some combination of the
following issues: 1) selectivities (chemo-, regio-, and enan-
tio-), 2) substrate scope, and 3) catalyst activity. Of these,
chemoselectivity is the most serious, with up to 70% of
corresponding vicinal diol being produced in unfavorable
cases.
A uniquely helpful mechanistic insight about these os-
mium(viii)-catalyzed systems arose during our process im-
provement endeavors on the AD–namely, there are two
catalytic cycles producing the diol (Scheme 1).[3] Under
homogeneous conditions, where the reoxidant has constant
access to all the catalytic intermediates, the turnover is locked
into the second cycle, simply because hydrolysis steps (h1 and
h2) are much slower than the redox steps (r1, r2, and r3). As a
consequence, the osmium(vi) bis(glycolate) (II) is the resting
form of the catalyst.
In the last few years, we have discovered that certain classes
of olefins give extraordinary results when subjected to the
standard reaction conditions for either the osmium-catalyzed
dihydroxylation or aminohydroxylation process.[4, 6] In the
absence of the alkaloid ligand, and even with very low catalyst
loadings, these special olefins undergo rapid and nearly
quantitative conversion to the expected products, vicinal diols
or amino alcohols. This is in sharp contrast to other olefins,
whose turnover is crucially dependent on the ligand-accel-
eration effect.[5] Having recently found that unsaturated
carboxylic acid[6] substrates constitute an extreme example,
even within this ™special∫ reactivity zone, we redoubled
efforts to get at the mechanistic underpinnings which account
for such phenomenally vigorous catalysis.
With all the ™special∫ substrates only racemic products are
formed, even when an enormous excess of the chiral ligand is
added. This and other available evidence[7] strongly suggest
that these olefins turnover almost exclusively in the second
catalytic cycle, in which osmium(vi) bis(glycolate) (II) (or the
bis(azaglycolate) in the case of aminohydroxylation) is the
most stable intermediate–it is so stable that it is the only
detectable osmium complex present under steady-state con-
ditions. According to our current mechanistic hypothesis, the
À
À
resident carboxylate groups ( COO ) in this complex facili-
tate the rate-determining step, hydrolysis, thereby accounting
for the dramatically increased reactivity of these substrates.
Although we had traditionally sought to avoid the second
cycle at all costs, deleterious as it is to enantioselectivity, the
enticing possibilities it offers for a new way to control
osmium(viii) catalysis have been clear since the time of its
discovery in 1982. Although early attempts to obtain enan-
tioselectivity with second-cycle ligands failed, the recent
enormous jump in effectiveness of the second-cycle systems[6]
convinced us that their inherent advantages must be exploited
to develop new catalytic processes. To tame the second cycle,
one needs to design a ligand that a) is chiral and capable of
controlling stereochemistry in the olefin oxidation step r3;
b) aids in the hydrolytic release (h2) of the diol from the initial
OsVI product complex (II); and c) is not itself hydrolytically
removed from the osmium coordination sphere.[8]
Herein, we describe the first ligands found to induce
asymmetry in the osmium-catalyzed dihydroxylation and
aminohydroxylation proceeding in the second catalytic cycle.
As a simple model for screening the ligand candidates shown
in Table 1, we have chosen the dihydroxylation of styrene
under the Upjohn conditions.[9] Incidentally, overoxidation of
the product diol, a fairly common side reaction in the Upjohn
dihydroxylation, was not observed.
Even tartaric acid, although needed in quantity of
25 mol%, showed asymmetric induction. Finding ligands with
a higher affinity for osmium was an obvious
requirement to reduce the amount of
OH
R
ligand.
Since
N-sulfonyl-1,2-hydroxy-
H2O
R
amines (vicinal hydroxysulfonamides) have
much higher binding constants for osmium
than analogous 1,2-diols, a number of N-
toluenesulfonyl derivatives of a,b-hydroxy-
amino acids were screened resulting in the
improvement of the enantiomeric excess
(ee) to 42%. The ee values remained
constant throughout the course of reactions
and, more importantly, as little as 1.5
2 mol% ligand was sufficient to attain the
ceiling enantioselectivity (Figure 1).[10]
Simple structure activity studies have
revealed that a free carboxylate group
appears to be an essential component of a
successful ligand. Thus, only racemic diol
was obtained when the methyl ester of 1
was used as a ligand. Location of the HO
and TsNH groups was found to play an
R
R
O
R
OH
O
O
O
Os
r3
h1
O
R
I
O
O
R
R
NMM
O
O
First
Cycle
O
O
Os
Second
Cycle
Os
O
O
O
r2
R
R
NMO
II
r1
R
O
O
O
h2
Os
R
O
H2O
R
III
OH
R
R
R
OH
Scheme 1. The two catalytic cycles in the osmium-catalyzed dihydroxylation of olefins using
4-methylmorpholine-N-oxide (NMO) as reoxidant.
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Table 1. Dihydroxylation of styrene with novel ligands.[a]
effect on the stereochemical outcome of the reaction. How-
ever, the ligand should preferentially contain an N-sulfonyl
moiety, as its replacement with an amide (as in 5, see Table 1,
entry 7) or a carbamate resulted in very low to no enantio-
selectivity.
Most of the ligands discussed above can be readily prepared
in enantiomerically enriched form by using previously devel-
oped catalytic olefin transformations.[2] Furthermore, some
hydroxyamino acids are commercially available compounds,
and can be easily converted to their corresponding N-sulfonyl
derivatives.[11] For example, N-(p-toluenesulfonyl)threonine,
(2S,3R)-6 (see Table 1, entry 8) has been found to be
particularly effective for dihydroxylation of cinnamate esters,
with 70% ee realized in one case (Table 2).[12]
Entry
Ligand
Ligand
[mol%]
Conversion
[%]
ee
[%] (abs. conf.)[b]
HNTs
S
COOH
1
2
5
2
> 99
> 99
25 (R)
25 (R)
R
OH
1
OH
S
COOH
R
3
4
5
5
> 99
> 99
7( R)
HNTs
2
HNTs
R
COOH
R
29 (R)
OH
Table 2. Asymmetric dihydroxylation of cinnamates using new ligands.
ligand, 2 mol%
3
OH
O
O
OsO4, 0.2 mol%
NMO, 1.1 equiv
HNTs
OR
OR
S
COOH
OH
R
X
5
6
5
2
> 99
> 99
42 (R)
42 (R)
X
tBuOH/H2O (1:1)
OH
pH 5, RT, 0.5 M
7-10a
7-10b
Olefin
X
R
ee [%]
ee [%]
4
(ligand 4)
(ligand 6)
O
7
8
9
10
H
H
H
Me
Et
iPr
Me
48
50
53
40
51
48
44
70
NH
S
7
5
> 99
5 (R)
COOH
R
NO2
OH
5
OH
In initial studies on the effects of these new second-cycle
ligands on the related osmium-catalyzed aminohydroxylation
process, we found that both styrene and methyl cinnamate
were converted to the corresponding hydroxysulfonamides in
high yields with ee values ranging from 25 to 55% (Table 3).
Very importantly (and in stark contrast to the traditional AA
with the cinchona alkaloid derived ligands), no diol formation
was detected, and the products were free of osmium
contamination.
R
COOH
S
8
5
> 99
25 (S)
HNTs
6
[a] All reactions were performed on 1 mmol scale at 0.5m concentration in
tBuOH/H2O (1:1) with 1.1 equiv NMO and 0.2 mol% of OsO4. The progress
was monitored by GC, and ee values were determined by HPLC (Chiralcel OB,
10% iPrOH/Hexane). [b] The absolute configuration of styrene diol was
assigned by comparison with authentic samples.
In summary, a new way of controlling osmium-catalyzed
oxidations is offered by a powerful class of ligands, which
enforce second-cycle turnover by never leaving the catalytic
center. Such processes offer many variables for optimization
(in fact, considerably more than the AD offered when it was
at the same stage of development) and present an opportunity
for a quantum jump in the utility of the osmium-catalyzed
oxidation processes. If these second-cycle processes can be
optimized, and there are many obvious variables to explore,
they should surpass, and thence replace, the existing AD and
AA, which as first-cycle processes are dependent on external
chiral ligands.
Figure 1. Dependence of enantioselectivity on ligand concentration in
dihydroxylation of styrene with 1 as a ligand.
Experimental Section
important role as well. For example, the phenylisoserine-
based ligand 1 afforded a higher ee value than its regioisomer
2. The absolute configuration of the diol product appears to be
determined by the stereochemistry of the a-carbon atom of
the ligand (see Table 1, entries 1, 4, and 8). Additional
investigations have shown that modification of the substitu-
ents on the sulfonamide group (R-SO2NH-) has only a minor
Typical dihydroxylation procedure as exemplified for methyl 4-nitrocinna-
mate: Methyl 4-nitrocinnamate (207mg, 1 mmol) and N-(4-toluenesulfon-
yl)-(l)-threonine (13.6 mg, 5 mol%)[11] were dissolved in a tBuOH/H2O
mixture (1:1, 6 mL). NMO (50 wt% in water, 228 mL, 1.1 mmol) and OsO4
(0.1m in acetonitrile, 20 mL, 0.002 mmol) were added successively. The pH
was adjusted to 5 by addition of 2n H2SO4 (150 mL), and the reaction
mixture was stirred vigorously for 24 h, at which time the pH was adjusted
474
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Table 3. Asymmetric aminohydroxylation of olefins using ligands 4 and 6.[a]
ligand, 2-5 mol%
HNTs
OH
R
K2OsO2(OH)4, 1 mol%
R
R
TsN(Na)Cl · 3H2O, 1 equiv
+
OH
HNTs
tBuOH/H2O (1:1)
pH 8, RT, 0.5 M
11
12
13
R
H
Ligand (mol%)
Conversion
[%]
ratio
12:13
ee of 12 [%]
(absolute conf.)[b]
ee of 13 [%]
(absolute conf.)[b]
4 (2)
4 (5)
6 (2)
6 (5)
4 (2)
4 (5)
89
86
80
75
85
93
1:2
1:2
1:2
1:2
1:2
1:2
45 (S)
48 (S)
30 (S)
47 (S)
52 (2S,3R)
59 (2S,3R)
55 (R)
55 (R)
25 (R)
38 (R)
24 (2R,3S)
25 (2R,3S)
COOCH3
[a] The reactions were typically run for 20 h and were monitored by HPLC at 228 nm. The regioisomers were separated by reversed-phase preparative liquid
chromatography and the ee values were determined for each regioisomer separately (R ˆ H, 12: Chiralcel OG (30% iPrOH in hexane), 13: Chiralpak AS
(15% iPrOH in hexane); R ˆ COOCH3, 12: Chiralcel OG (30% iPrOH in hexane), 13: Chiralpak AD (20% iPrOH in hexane)). [b] The absolute
configuration of the products was assigned by comparison with authentic samples.
to
5
again. After an additional 24 h (ˆ98% conversion by liquid
[4] a) A. E. Rubin, K. B. Sharpless, Angew. Chem. 1997, 109, 2751 2754;
Angew. Chem. Int. Ed. Engl. 1997, 36, 2637 2640; b) W. Pringle, K. B.
Sharpless, Tetrahedron Lett. 1999, 40, 5150 5154.
[5] For a review of ligand-accelerated catalysis, see: D. J. Berrisford, C.
Bolm, K. B. Sharpless, Angew. Chem. 1995, 107, 1159 1171; Angew.
Chem. Int. Ed. Engl. 1996, 35, 451 454.
chromatography), methyl (2R,3S)-(‡)-2,3-dihydroxy-3-(p-nitrophenyl)-
propionate[13] was obtained in 70% ee (HPLC: Chiralcel OG, 20%
iPrOH/hexane). The reaction time can be reduced to about 24 h by
maintaining constant pH using a pH-stat. A 10 mmol scale reaction,
performed under these conditions, afforded product as white solid in 75%
yield (1.8 g) and 70% ee. Recrystallization from ethanol produced needle-
shaped crystals in 57% yield and 81% ee.
[6] V. V. Fokin, K. B. Sharpless, Angew. Chem. 2001, 113, 3563 3565;
Angew. Chem. Int. Ed. 2001, 40, 3455 3457.
[7] Both osmium(vi) bis(glycolates) and bis(azaglycolates) have been
isolated and characterized. See: H.-T. Chang, Ph.D. dissertation, The
Scripps Research Institute, 1996; Diss. Abstr. Int. B 1997, 57(12), 7528.
[8] This restriction is not really so severe–it only has to dominate the
catalysis. This can be achieved with a mobile ligand too (e.g., ligand-
accelerated catalysis or simply the equilibrium favoring the desired
osmium complex in the olefin oxidation step).
[9] V. Van Rheenen, R. C. Kelly, P. Y. Cha, Tetrahedron Lett. 1976, 1973.
[10] To ensure that the maximum possible ee value was afforded by each
ligand, 5 mol% of ligand was routinely used. We now know from
different examples that 1.5 2.0 mol% is sufficient. The finding that
the dependence of the enantioselectivity on the amount of the ligand
shows saturation behavior points to the fact that the process in its
present form operates at the maximum ee afforded by the ligand.
[11] M. Brenner, K. R¸fenacht, E. Sailer, Helv. Chim. Acta 1951, 36, 2102
2106.
Typical aminohydroxylation procedure as exemplified for styrene: (2R,3S)-
N-(4-Toluenesulfonyl)-4-nitroisophenylserine (190 mg, 0.5 mmol) and so-
dium bicarbonate (8.4 mg, 1 mmol) were dissolved in tBuOH/H2O (1:1,
20 mL). Styrene (1.040 mg, 10 mmol), Chloramine-T trihydrate (2.870 mg,
10 mmol), and K2OsO2(OH)4 (36 mg, 0.1 mmol) were then added succes-
sively. The reaction mixture was stirred at room temperature for 20 h, at
which point HPLC analysis indicated 90% conversion. Sodium sulfite
(100 mg) was added, and the mixture was stirred for an additional hour. It
was then extracted (ethyl acetate, 3 Â 25 mL), dried, and concentrated to
yield amorphous solid. Flash chromatography purification afforded a
mixture of regioisomers 12:13 (29:71, determined by NMR) as a white
crystalline product (2.5 g, 86%). Regioisomers were separated by prepa-
rative HPLC (CH3CN/H2O, 30:70, 0.1% trifluoroacetic acid (TFA), YMC
C18 column, 100 mg scale). Enantiomeric excess was determined by chiral
HPLC and the absolute configuration was established by comparing optical
rotation with authentic samples. (S)-12, 51%, (Chiralcel-OG, iPrOH/
hexane, 30:70, 1.5 mLminÀ1) and (R)-13 , 54% (Chiralcel-AS, iPrOH/
hexane, 30:70, 1.5 mLminÀ1).
[12] Acidification of the reaction mixture with sulfuric or acetic acid to a
value of about
5 considerably accelerates the reaction without
jeopardizing the enantioselectivity.
[13] J. A. Denis, A. Correa, A. E. Greene J. Org. Chem. 1990, 55, 1957.
Received: August 6, 2001
Revised: November 26, 2001 [Z17673]
[1] a) H. C. Kolb, M. VanNieuwhenze, K. B. Sharpless, Chem. Rev. 1994,
94, 2483 2547; b) H. C. Kolb, K. B. Sharpless in Transition Metals in
Organic Synthesis, Vol 2 (Eds.: M. Beller, C. Bolm), Wiley-VCH,
Weinheim, 1998, pp. 219 242.
[2] a) G. Li, H.-T. Chang, K. B. Sharpless, Angew. Chem. 1996, 108, 449
452; Angew. Chem. Int. Ed. Engl. 1996, 35, 451 454; b) H. C. Kolb,
K. B. Sharpless in Transition Metals for Fine Chemicals and Organic
Synthesis, Vol. 2 (Eds.: M. Beller, C. Bolm), Wiley-VCH, Weinheim,
1998, pp. 243 260; c) G. Schlingloff, K. B. Sharpless in Asymmetric
Oxidation Reactions: A Practical Approach (Ed.: T. Katsuki), Oxford
University Press, Oxford, in press.
¬
[3] J. S. M. Wai, I. Marko, J. S. Svendsen, M. G. Finn, E. N. Jacobsen, K. B.
Sharpless, J. Am. Chem. Soc. 1989, 111, 1123.
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