Engineering catalysts for enantioselective addition of diethylzinc to aldehydes with racemic amino alcohols: Nonlinear effects in asymmetric deactivation of racemic catalysts

Article abstract of DOI:10.1002/1521-3773(20010202)40:3<544::AID-ANIE544>3.0.CO;2-8

Correct additions make a difference: Asymmetric deactivation and asymmetric amplification concepts coupled with a high-throughput screening technique provided a successful strategy for designing a highly enantioselective catalytic system by simple combination of a racemic amino alcohol (rac-DB) and a nonracemic additive (AA). The example in the scheme shows the conversion of 1 into 2 with up to 92.7% ee.

Full text of DOI:10.1002/1521-3773(20010202)40:3<544::AID-ANIE544>3.0.CO;2-8

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electrospray ions. A voltage of 2500 V was applied to the electrospray
needle, which was held 1 ± 3 cm away from the inlet to the vacuum.
Engineering Catalysts for Enantioselective
Addition of Diethylzinc to Aldehydes with
Racemic Amino Alcohols: Nonlinear Effects in
Asymmetric Deactivation of Racemic
Catalysts**
Received: September 1, 2000 [Z15749]
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[2] G. Siuzdak, Mass Spectrometry for Biotechnology, Academic Press,
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Jiang Long and Kuiling Ding*
ªAsymmetric amplificationº or the ªpositive nonlinear
effectº ((‡)-NLE) is a very attractive phenomenon in
catalytic asymmetric processes, since it gives enantioselectiv-
ities which are improved with respect to the expectations
based on the ee value of the auxiliary.[1±7] Therefore, because
of asymmetric amplification, high-enantiopurity chiral ligands
need not necessarily be applied to achieve high enantiopur-
ities of product. Nevertheless one has to make a partial
resolution of the chiral ligands from their racemic forms.
Interestingly, asymmetric catalysis with racemic catalysts has
been successfully achieved by ªchiral poisoningº[8] or ªasym-
metric activationº[9] strategy.
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After the first report on the nonlinear effect in asymmetric
catalysis by Kagan and co-workers,[2] the early experiments
with strong asymmetric amplification were demonstrated by
Oguni et al. in 1988 and by Noyori et al. in 1989 for
enantioselective addition of diethylzinc to aldehydes.[3] In
this type of alkylation, the asymmetric amplification is well
recognized to be a consequence of an in situ increase in the ee
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[7b]
ment by Noyori and co-workers
of the systems involving
the dynamic monomer± dimer equilibria where the mono-
mers are active species is complementary to the MLn models
of Girard and Kagan.[1b] In principle, if racemic ligands are
used alone, the reaction will definitely give racemic product.
The addition of an alternative nonracemic additive (which
should be cheap and easily obtainable) to the racemic catalyst
system may enantioselectively generate a new species of
dinuclear zinc complex with one enantiomer of racemic ligand
through ªnon-self-recognitionº[7a] to release the opposite
enantiomer of catalyst for asymmetric catalysis. To exemplify
this strategy, we choose Oguni et al.ꢁs racemic amino alcohols
to carry out asymmetric catalysis by adding nonracemic
additives. In this contribution, we report our results on the
first example of highly enantioselective addition of diethyl-
zinc to aldehydes with the catalysis of racemic amino alcohols
in the presence of chiral additives.
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[*] Prof. Dr. K. Ding, Dr. J. Long
Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (China)
Fax : (‡86)21-6416-6128
E-mail: kding@pub.sioc.ac.cn
[**] Financial support from the National Natural Science Foundation of
China, the Chinese Academy of Sciences, the Major Basic Research
Development Program of China (Grant no. G2000077506), and the
Science and Technology Commission of Shanghai Municipality is
gratefully acknowledged.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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We began this work by investigating the effect
of various random chiral additives, such as amino
acids, tartaric acid, diols, diamine, diimine, and
simple amino alcohols, on the enantioselectivity
of ethylation of benzaldehyde (1a) with dieth-
ylzinc catalyzed by racemic DB1. In some cases,
we could really see the influences of the chiral
additives on the asymmetric induction of the
reaction. For example, (R)-1,1'-bi-2-naphthol (BI-
NOL) alone catalyzed the reaction to give the R
product (8.2% ee), but the addition of (R)-
BINOL to the reaction system catalyzed by
racemic DB1 resulted in the formation of S
product (8.5% ee). Similarly, the reaction pro-
moted by (S)-2-N,N-dimethylamino-2'-hydroxy-
1,1'-binaphthyl (DM-NOBIN) yielded S product
(89% ee) and the addition of (S)-DM-NOBIN to
the racemic DB1 system gave R product (38%
ee). These data could be interpreted by a phe-
nomenon where a catalytically inert heterochiral
dimeric zinc complex may be formed between the
chiral additive and one of the enantiomers of
racemic amino alcohol through non-self-recogni-
Figure 1. High-throughput evaluation of the catalyst library generated from racemic
amino alcohols (DB1 ± DB5) and chiral additives (AA1 ± AA13) for enantioselective
addition of diethylzinc to benzaldehyde.
ques.[9f] Figure 1 shows the details of the screening results. It is
obvious that the chiral additives with large steric hindrance at
the hydroxy-funtionalized carbon, AA8 ± AA13, were more
influential on the enantioselectivity of the reaction than the
simple amino alcohols, AA1 ± AA7. Particularly AA10,
AA12, and AA13 showed significant synergetic effects on
the enantioselectivity of the reaction. For example, with only
AA13 (5 mol%) as the chiral inducer, (R)-1-phenylpropanol
was obtained with 15.6% ee. However, the addition of
racemic DB1 or DB2 (10 mol%) to the AA13-catalyzed
reaction system resulted in the formation of the S product in
65.8% and 70.4% ee, respectively.
The reactions catalyzed by the better combinations, AA12/
DB1, AA12/DB2, AA13/DB1, and AA13/DB2, were further
optimized in parallel by varying the molar ratio of racemic DB
to chiral AA and decreasing the reaction temperature to
208C and 408C. We were pleased to find that (S)-1-
phenylpropanol could be obtained with up to 92.7% ee and in
>95% yield under the catalysis of AA13/DB2 (ratio: 1/2;
5 mol% of AA13) at 408C. Catalyst combi-
tion; as a result the zinc complex of racemic amino alcohol is
enantioselectively enriched and catalyzes the reaction to give
the nonracemic product. From the primary random screening,
we found amino alcohols to be one type of effective chiral
additive candidates for this purpose (Scheme 1).
High-throughput screening (HTS) is essential for tuning a
variety of modifications in lead optimization[10] and has been
found to be an effective technique for finding the most
efficient catalysts for asymmetric reactions as well.[11] With the
lead result mentioned above, a library of optically active
amino alcohols (AA1 ± AA13; Scheme 1) was then created by
parallel synthesis from amino acids, and a family of racemic
amino alcohols (DB1 ± DB5), which have been reported to
show significant asymmetric amplification,[3a] was also
formed. The combined use of 10 mol% of the racemic amino
alcohols (DB1 ± DB5) and half an equivalent amount
(5 mol%) of optically active additives (AA1 ± AA13) in the
presence of diethylzinc formed a chiral catalyst library of 65
members, which were then evaluated with HPLC-CD techni-
nations AA12/DB1, AA12/DB2, AA13/DB1,
and AA13/DB2 also proved to be effective for
the ethylation of a variety of aldehydes under
the optimized conditions (Table 1). To the best
of our knowledge, this work represents the
first example of highly enantioselective addi-
tion of diethylzinc to benzaldehyde with the
catalysis of racemic amino alcohols in the
presence of chiral additives.
In an effort to elucidate the mechanism for
the asymmetric induction, the quantitative
effect of chiral additive on the reaction rate
was investigated by rapid quenching of the
reaction and HPLC analysis with 1-phenyl-
ethanol as internal standard (Table 2). It was
found that both (R)- and (S)-DB1 homochiral
Scheme 1. Racemic (DB) and nonracemic (AA) ligands employed for creating the chiral
catalyst library.
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Table 1. Enantioselectivities for the ethylation of aldehydes with catalysis by racemic DB in the presence of optically active AA.[a] Parallel screening of
matched substrate/catalyst pairs.
Entry
R
ee values [%]
AA12/DB1
AA12/DB2
AA13/DB1
AA13/DB2
1
2
3
4
5
6
7
8
9
10
11
12
13[c]
phenyl (a)
p-chlorophenyl (b)
m-tolyl (c)
p-anisyl (d)
trans-styryl (e)
o-anisyl (f)
ferrocenyl (g)
p-dimethylamino-phenyl (h)
a-naphthyl (i)
p-tolyl (j)
m-anisyl (k)
p-bromophenyl (l)
trans-propenyl (m)
86.0
69.9
85.7
87.5
67.7
69.8
81.9
23.6
69.9
74.7
79.2
76.6
78.3
86.1
82.3
81.0
90.3
82.3
76.7
90.6
55.6
72.2
80.0
80.7
73.6
84.4
92.7
84.6
90.1
90.0
69.4
74.5
91.7
36.2
69.3
82.4
76.9
82.7
80.7
90.6
92.1
87.3
91.4
69.4
86.3
72.9
81.0[b]
86.3
80.0
76.4
79.5
84.4
[a] All of the reactions were carried out at 408C in CH2Cl2/hexane mixed solvent for 48 h with catalysis by racemic DB1 (10 mol%) and optically active AA
(5 mol%) unless otherwise stated. The conversions of the aldehydes were >90%. All the ee values were determined by HPLC or GC on chiral columns.
[b] The reaction was carried out at 08C. [c] The reaction was carried out at 208C.
Table 2. The quantitative effect of chiral additive AA13 on the rate of reaction, with 1a as the substrate and DB1 as the catalyst.[a]
Entry
Config. of DB1
DB1 [mol%]
AA13 [mol%]
Yield [%][b]
ee value [%][c]
Config. of 2a[d]
1
2
3
4
5
6
S
S
R
Æ
±
Æ
5
5
5
5
0
0
5
5
0
5
5
16.0
6.0
1.6
1.6
0.7
5.7
95.0
91.1
66.2
0
15.9
65.8
S
S
R
±
R
S
10
[a] The reaction was carried at 08C and quenched with an aqueous solution of NH4Cl after 5 min. [b] The yield was determined by HPLC with
1-phenylethanol as the internal standard. [c] Determined by HPLC on a Chiralcel OD column. [d] Determined from optical rotation measurements and
HPLC retention times on a Chiralcel OD column in comparison with authentic samples.
catalysts were deactivated by AA13 (entry 1 compared with
entries 2 and 3, respectively), which indicated some interac-
tions between them. However the levels of deactivation for
the R and S catalysts by AA13 were quite different. The
reaction catalyzed by (S)-DB1/AA13 is 3.8 times as fast as
that catalyzed by (R)-DB1/AA13 (entries 2 and 3, respective-
ly). (Æ)-DB1 and AA13 on their own were found to be
catalytically much less reactive (entries 4 and 5). However,
use of (Æ)-DB1 combined with half an equivalent of AA13
yielded the product 3.6 and 8.1 times as fast as the reactions
with the separate component ligands, respectively (entry 6
versus entries 4 and 5). Obviously, such synergetic effects are
due to the interaction differences between the AA13 ± zinc
complex and the complexes of (S)-DB1 and (R)-DB1 with
zinc.
The nonlinear effect in the present catalytic system was also
disclosed by an investigation of the influence of molar ratios
between AA13 and (R)-DB1 or (S)-DB1 on the enantiose-
lectivity of the reaction.[12] The different behavior of the
nonlinear effects in the AA13/(R)-DB1 and AA13/(S)-DB1
systems again supported the presence of non-self-recognition
between AA13 and DB and also indicated the differences in
their non-self interactions.
In conclusion, we have shown the first examples of a highly
enantioselective addition of diethylzinc to aldehydes with the
catalysis of racemic amino alcohols in the presence of chiral
additives, through an asymmetric deactivation strategy with a
combinatorial approach. The mechanism of asymmetric
induction and the nonlinear effect in chiral deactivation were
also studied. This work demonstrated that clever combined
use of asymmetric deactivation strategies and combinatorial
chemistry approaches will provide a powerful tool for design-
ing and finding new, practical, and efficient asymmetric
catalytic systems.
Received: August 29, 2000 [Z15725]
[1] For comprehensive reviews on NLEs in asymmetric catalysis, see:
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(Ed.: S. E. Denmark), Wiley, New York, 1999, pp. 257 ± 296; b) C.
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London, 1996, pp. 9 ± 26; d) D. Heller, H.-J. Drexler, C. Fischer, H.
Buschmann, W. Baumann, B. Heller, Angew. Chem. 2000, 112, 505 ±
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[2] For first reports on NLEs in asymmetric catalysis, see: C. Puchot, O.
Samuel, E. Dunach, S. Zhao, C. Agami, H. B. Kagan, J. Am. Chem.
Soc. 1986, 108, 2353 ± 2357.
SrN and SrN2: Diazenides by Synthesis under
High N2-Pressure**
[3] For the early reports on (‡)-NLE, see: a) N. Oguni, Y. Matsuda, T.
Kaneko, J. Am. Chem. Soc. 1988, 110, 7877 ± 7878; b) M. Kitamura, S.
Okada, S. Suga, R. Noyori, J. Am. Chem. Soc. 1989, 111, 4028 ± 4036.
[4] For other examples with significant (‡)-NLEs (amplification index
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Oderaotoshi, S. Sakaguchi, H. Yamamoto, J. Tanaka, E. Wada, D. P.
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[5] For a recent comprehensive review on autocatalysis with asymmetric
amplification, see: K. Soai, T. Shibata, I.
Gudrun Auffermann, Yurii Prots, and Rüdiger Kniep*
In the binary system Sr± N, only the existence of Sr2N is
certain.[1] This compound crystallizes in the CdCl2 structure
(layers of octahedra, Figure 1 left). Reports on a binary phase
with the stoichiometry ªSrNº[2] were corrected afterwards as
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2
2
Figure 1. Crystal structures of Sr2N (left), SrN ˆ^ (Sr2‡)4[N3 ]2[N2 ] (center), and SrN2 ˆ^ Sr2‡[N2
]
(right). The top and bottom boundaries of the figures are represented by layers of Sr6/3 octahedra
(polyhedral representation), occupied by [N3 ] (Sr2N, SrN) or [N2 ] (SrN2), respectively. Ball-and-stick
2
2
representations between the polyhedral layers: Sr2‡, red; [N3 ], light green; [N2 ], dark green. The
[8] For examples of chiral poisoning, see:
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a) N. W. Alcock, J. M. Brown, P. J. Mad-
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Recently, we carried out high-pressure experiments for the
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modified high-pressure equipment, which was originally
constructed by Bronger and Auffermann[4] for the syntheses
of extremely air- and moisture-sensitive metal hydrides and
hydridometalates.
Using Sr2N (blue-black powder with metallic luster) as the
starting material (reaction temperature 920 K, reaction time
72 h), we obtained single-phase SrN (black-gray powder)
under an N2 pressure of 400 bar and single-phase SrN2 (brown
powder) under an N2 pressure of 5500 bar.[5] No impurities of
the phases were detected by X-ray and neutron diffraction
investigations at ambient pressure[6] nor by chemical analy-
sis.[12] The contents of carbon, hydrogen, and oxygen were
below the detection limits.
Am. Chem. Soc. 1989, 111, 789 ± 790; c) J. W. Faller, D. W. Sams, X.
Liu, J. Am. Chem. Soc. 1993, 115, 804 ± 805; d) J. W. Faller, D. W. Sam,
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Y. Matsumoto, M. Ueki, R. Angelaud, Angew. Chem. 2000, 112,
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examples of asymmetric activation, see: c) K. Mikami, S. Matsukawa,
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Mikami, T. Korenaga, M. Terada, R. Noyori, J. Am. Chem. Soc. 1998,
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1597 ± 1601; h) K. Mikami, T. Korenaga, T. Ohkuma, R. Noyori,
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3707 ± 3710.
The crystal structures of SrN and SrN2 were solved by a
combination of X-ray and neutron diffraction experiments on
air- and moisture-sensitive microcrystalline powders.[6] The
neutron diffraction diagrams (observed, calculated, and
difference profile) are given in Figure 2. The crystal structures
[10] High Throughput Screening (Ed.: J. P. Devlin), Marcel Dekker, New
York, 1997.
[11] For comprehensive reviews on combinatorial catalysis, see: a) B.
Jandeleit, D. J. Schaefer, T. S. Powers, H. W. Turner, W. H. Weinberg,
Angew. Chem. 1999, 111, 2648 ± 2689; Angew. Chem. Int. Ed. 1999, 38,
2494 ± 2532; b) K. D. Shimizu, M. L. Snapper, A. H. Hoveyda, Chem.
Eur. J. 1998, 4, 1885 ± 1889; c) M. B. Francis, T. F. Jamison, E. N.
Jacobsen, Curr. Opin. Chem. Biol. 1998, 2, 422 ± 428; d) M. T. Reetz,
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[*] Prof. Dr. R. Kniep, Dr. G. Auffermann, Dr. Yu. Prots
Max-Planck-Institut für Chemische Physik fester Stoffe,
Nöthnitzer Strasse 40, 01187 Dresden (Germany)
Fax : (‡49)351-46463002
[12] Experimental data, including synthetic details and 1HNMR spectro-
scopic data for racemic and nonracemic ligands and full data for the
evaluation of the catalyst library, as well as the graphical representa-
tion for investigation of the nonlinear effect in the present catalytic
system, have been included in the Supporting Information.
E-mail: Kniep@cpfs.mpg.de
[**] We thank Dr. D. Többens for support in carrying out the neutron
diffraction experiments, the Hahn-Meitner-Institut for giving us access
to their high-resolution powder diffractometer E9, B. Bayer, S. Müller,
and Dipl.-Chem. U. Schmidt for assistance with the chemical analyses,
and Dr. R. Cardoso and S. Hückmann for recording the X-ray data.
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