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Asymmetric Autocatalysis with Amplification of Chirality.

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HIGHLIGHTS
Asymmetric Autocatalysis with Amplification of Chirality**
Carsten Bolm," Frank Bienewald, and Andreas Seger
Asymmetric metal catalysis has been intensively studied in
recent years, and some efficient catalytic methods for the synthesis of enantiomerically pure compounds have been developed.['] With suitable metal/ligand combinations excellent
enantioselectivities and high conversion rates have frequently
been achieved. Normally the catalyst must remain unaffected by
the continually formed new product in order to achieve a constant, high stereoselectivity. How does it behave, however, when
the product itself is a catalyst and, moreover, catalyzes its own
asymmetric synthesis? Wynberg recognized very early the great
potential of such "asymmetric autocatalysis" for synthesis and
as early as 1989 had formulated the challenge that asymmetric
autocatalysis could constitute the next generation of asymmetric synthesis.['] In spite of great efforts the discovery of the first
autocatalytic system operating with high enantioselectivity has
only recently been made.[31Two important contributions from
Soai et al.[3.41take pride of place here, but before introducing
them we will describe some other fundamental studies.
Seebach, Dunitz et al. first drew attention to the fact that in
stereoselective reactions with organometallic reagents differing
diastereo- and enantioselectivities could arise because of the
formation of various mixed complexes during the course of
product f o r m a t i ~ n . [ ~This
, ~ I led Alberts and Wynberg to investigate the asymmetric additions of organometallic carbon nucleophiles (e.g. ethyllithium) to benzaldehyde (Scheme l ) , and
they showed that the stereochemical course of both stoichiometric and catalyzed reactions were influenced by metal-containing
product molecules (here lithium alcoholates) .['I The product
itself is not a catalyst but nevertheless ensures that the newly
formed product is optically active. Alberts and Wynberg coined
the term "enantioselective autoinduction" for this effect.
PhCHO
1. (+)-PhC*D(OLi)Et / EtLi
2. H30+
*
PhC*H(OH)Et
(+ PhC*D(OH)Et)
Scheme 1 Asymmetric addition ofethyllithium to benzaldehyde [7]. Chirality centers are identified by a slar
[*I
Proll Dr C. Bolm,"' Dr. F. Btenewald. Dip1 -Chem. A. Seger"'
Fachbereich Chemte der Universitit
Hans-Meerwcin-Strasse. D-35032 Marburg (Germany)
[ - ] New Address. Institut fur Organische Chemie der RWTH
Prolrssor-Ptrlet-Strasse I. D-52074 Aachen (Germany)
Fax. Int. code +(241)8888-391
e-mail carsten.bolm(n rwth-adchen.de
['*I
We wish to thank the DFG (Graduiertenkolleg). BASF AG. and the Volkswagen Stiftung for stipends and financial support.
Danda et al. established that the product powerfully influenced the catalyst in the metal-free catalyzed asymmetric hydrocyanation of 3-phenoxybenzaldehyde 1 to ( 9 - 2 by the cyclic
dipeptide (R,R)-3.[81In the presence of the product the reactionaccelerating effect of the cyclic dipeptide was enhanced. (R,R)-3
alone showed little catalytic activity at first, and with increasing
reaction time the enantiomeric excess of ( S ) - 2 increased. If a
small amount of (S)-2was added to the reaction mixture at the
start the product was formed with an almost constant, high ee
value. Here (S)-2 itself is not a catalyst and only the interaction
between the correct product enantiomer and the cyclic dipeptide
leads to better catalysis. In addition asymmetric amplification
occurred only in the presence of the product.['] Thus, 2.2 mol O/O
(R,R)-3 with 2 % ee at 4 3 % conversion yields the cyanohydrin
(S)-2with 81.6% ee when 8.8 mol% (S)-2with 92 YOee is added
at the start of the reaction. If (S)-2 is not added at the start, the
conversion and enantiomeric excess are 4 % and 3.4%, respectively. It is also interesting that in this asymmetric amplification
the absolute configuration of the product is not determined by
the configuration of the catalyst but by that of the added
cyanohydrin.
In the examples quoted the product influences stereoselectivity o r has a positive effect on an existing catalysis."'] Things
become especially interesting, however, when the chiral product
itself is a catalyst for its own formation from achiral precursors.
This area of asymmetric autocatalysis was hardly developed for
a long time, though in 1953 Frank had already formulated a
mathematical model that indicated that "spontaneous asymmetric synthesis", as he called the process, is quite possible.["]
As a natural property of life this could be of fundamental importance in the genesis of asymmetry in nature."' - 14] In the scenar-
HIGHLIGHTS
io described by Frank two achiral substances, A and B. react to
form optically active products (R)-C and (S)-C,each of which
can catalyze its own synthesis (Scheme 2a). This corresponds to
the working of conventional autocatalysis. If one imagines that
the two enantiomeric catalysts mutually reduce (or destroy)
their effects (Scheme 2b), the system would show flip-flop
(R)-C (cat.)
/
tR1-C
------L
(S)-C
y
lTCHo+
ZniPr2
(S)-8I (S)-9 (cat.) t
R
N-Qe)zP
R
A + B
( S ) - c (cat.)
b)
(Rj-C
+ (S)-C
-
.
[(Rj-C (S)-C]
(inactive)
Scheme 2. a ) Model for "spontaneous asymmetric synthesis" according to Frank
[ I 11. b) Deactivation oi'the catalytically a c h e chiral product.
switching properties: even a small statistical ffuctuation (for
example. if the reaction catalyzed by (R)-C were preferred for a
short time) would have the effect of reducing the catalytic activity of (S)-C and new (R)-Cwould be formed ever more quickly.
In such an "aggressive" system a trace of a chiral substance
would be enough to ensure its own production in large quantities by autocatalysis. The inductor does not even need to be
enantiomerically pure since the model implies the principle of
asymmetric amplification. In his article Frank pleaded for the
development and testing of simple autocatalytic systems.[' * I The
first successful reactions with a "chiral autocatalyst" were reported by Soai et al. in 1990.['51They found that the pyridinyl
alcohol 6 catalyzed its own formation from pyridine-3-carbaldehyde (4) and diisopropylzinc via the isopropylzinc alcoholate 5.
With 20 mol% (-)-6(86%rr), workupafforded (-)-6in67%
yield with an ee of 35%.
OZniPr
As if this were not enough, Soai et al.14]also used this system
to demonstrate for the first time the asymmetric amplification
during autocatalysis that is inherent in the Frank model. Thus,
20 mol% of (S)-9b
100,
with an er of only 2 a/'
1
gave, with autocatalysis, (S)-9bwith an ee
of 10%. In further
reaction cycles the
eel%
enantiomeric excess
rose from 10 through
57 to 81 and finally to
0
1
2
3
4
88% (Fig. 1). There
nwas a 942-foid inFig. 1. Increase in the enantiomeric excess of
(S)-9bwith the number of reaction cycles n.
crease in the amount
of product after four
cycles. The asymmetric amplification shown by this simple selfreplicating system behaved, in fact, as predicted by the simple,
theoretical Frank model.
As the mechanism of the reaction and possible intermediates
in solution are still unknown, the phenomenon described here
cannot yet be used for the targeted development of further reactions. It is, nevertheless, clear that complex formation with the
organometallic reagents is essential to autocatalysis with asymmetric amplification. For this reason a deeper understanding of
the behavior of chiral compounds during the formation of complexes would be very useful for the design and discovery of new
autocataIytic processes.
German cersion.
4
PH
5
Aii,qw
Keywords: asymmetric amplification
autocatalysis
Ciioii. 1996, IOK. 1767 1769
- asymmetric catalysis .
[ I ] a ) R . Noyori. A.\i.iiwi(Jjric C'<iiui>,.\i.\i i i Orguiiic Siiitiwsi.\. Wilry. New York.
1994. b) C(mi/i.iic A\,i~iiiiii<,iri~. S ~ . i i ~ / i ~ ~(Ed..
. s i s J 0.jima). VCH. Weinheim.
1993
Similarly, in other enantioselective additions of organozinc
compounds asymmetric autocatalyses were also demonstrated,['61though they always gave a product with a much lower re
value than that of the catalyst used. Only at the end of 1995 was
an important breakthrough a ~ h i e v e d ' ~ . when
'~
Soai et al.
showed that the presence of 20 mol% of the pyrimidyl alcohol
(S)-9a (94.8 O h ee) in the reaction between the corresponding
aldehyde 7a and diisopropylzinc led to the formation of (S)-9a
in 4 8 % yield with an ee of 95.7%. The product underwent
automultiplication rc~itliouta significant change in the enantiomeric excess.
[2] ti. Wynherg, < . i i i i i i i ( , 1989. 43. 150. b) J. . M u ( . r o i i i o / . S r r . CIr~~iii.
4 1989. 26.
1033
[3] T Shihata. H. Morioka. T. Hayase, K . ChoJi. K . Saoi. J A n i . Ciic,in. Soc. 1996.
118. 471
[4] K Soai. T. Shihata. H Morioka. K . ChoJi, Xariirr 1995, 378. 767.
[5] D Seebach. R. Aimtutz. J. D. Dunitz. H i k C h i . Acrtr 1981. 64. 2622.
[6] Review articles: a ) D Secbach. Pro(.. Roheri A. W?/(./iFoinid. Coiif. Cliein. Rcs.
1984. 27. Y3: b) .Aiig<,ii. C'hriii 1988, 100. 1685. APIRCII'. C l i ~ i i/ i l l . G I . Eiigl
1988.27. 1624
[7] a ) A. H Alberts, H. Wynberp. J Ani. Claw. Soc 1989. 111. 7265: h ) J Clieiii.
.SIK C'hnir. C ' o i i i ~ i i i ~1990.
i~.
453
[XI a) H Danda. H. Nishikawa. K Otakn. J. Or,?. C h i . 1991.56. 6740: b) mechanistic interpreiniion' Y. Shvo. M. Gal. Y. Becker. A Elgzvv1. E~irdwrirniiLcir.
1996. 7. 91 1
191 21) R. Noyoi-I, M K m m u r a . Aiige~i.. Clici?i. 1991. 103, 34, . 4 i r p . C / K ~1111.
.
Ed €ii~yi. 1991.30.49: b) D. Guillaneux. S.-H. Zhao. 0. Samuel. D. Rainford.
H. B. K;ipai1. J Ail?. C/WV. Sot. 1994, 116. 9430; c ) C. Balm in ,4(/~([/7(~d
~ ~ \ 1 ' 1 1 1 1 1 1 < ' 1 ! ' 1 ( ~. S j ~ i i I / i ~(Ed.:
w~
G. R. Stephenson), Blackie. Glasgow, 1996, p 9.
HIGHLIGHTS
[lo] Optically iictive products can also be formed with racemic metal catalysts. With
:I chiral additive one of the enantiomeric forms of the catalyst is "poisoned" in
\itti. a ) K Maruoka. H Yamamoto.J. Af77 Clim? SJC
1989. I l l . 789: b) 1. M.
Brown. P. 1. Maddox. C'hii-dif?, 1991. 3. 345: c) J. W. Faller. J. Parr. J ,4177.
c'h~wi Sor 1993, 1 / 5 . 804, d ) J. W Faller. M . Tokunaga. 7hrahnlrofi Lrtr
1993. 34. 73s'). e ) J. W. Faller. D. W. I Sams. X . Liu. J A m c'limi .So(. 1996.
118. 1217
[ I I] F. C. Frank. Birwhfi7i. ~kIp/ii..\.Acro 1953. 11. 459.
(121 a ) _I L. Bada. .Vurwr, 1995, 374, 594: b) W. A. Bonner. To/>.S r e r e o h i i i . 1988.
18. I : c ) W 1. Metring. Nrtriire 1987. 32Y. 71 2 : d ) P. Decker. N d i r . Climi. Zdi.
L ~ 1975.
J 23. 167; e) S. Mason. Chew Sw.Rei. 1988. 17. 347: f l W. A. Bonncr. (Yioni /id 1992. 640; g) S. Mason. N L I ~ Y
1985.
P 314. 400.
[I31 Noiia\yiiiiiictric molecular replication and autocntalyses: a ) L. E. Orgel. Nor m ' 1992. 3iX. 203: h) E. A. Wintner. M. M Conn. J. Rebek. Jr.. Ace. C/iwi.
R m . 1994. 2'. 198. c) .I A m . Chrw7.S o . . 1994. 116. 8877: d ) G von Kiedrowski.
J Helbing. B. Wlotzka. S. Jordan. M. Mathen. T. Achilles. D. Severs. A.
Terfort. B C. Kahrs. , V d i i - Clirwi. E d ? .Lrih 1992. 411. j ? X . c ) T. Achilles. G.
von Kiedrowski. A t i p i i . . Climi. 1993. 105. 1225: A~,FPII U i m 7 . h i . €r/. Eiigl.
1993. 32. 1 198.
[14] Formation of homochiral crystals froin solutions of opticallq inacti\e comi,
uiid
pounds: a ) J Jacques. A. Collet. S H. Wilen. € n ~ i f i t i ~ f i wRocrwiorn.
Rr.so/iitroni. Wiley. New York. 1981: b) D. K. Kondcpudi. R. J. Kaufmman. N .
Singh. Scicwe 1990. ZW,975: c ) J. M. McBrtde. R. L. ('nrter. A n , p . C l i m i
1991. 103. 298: A i i p . . C l i e m / f i r €d. €rig/. 1991. 311. 3 . 3 . and references
therein.
[15] K. Soai. S. Niwa. H. Hori, J. Chrvii. Soc C'hein Coiiiiniiii 1990. 983
[16] a ) K . Soai. T. Hayase. C. Shimada. K Isobe. Prrnhetli-oii A . s i ~ f i i i i ~ e /1994.
i ~ i ~ 5.
789: b) K Soai. T. Hayase, K Takai. ihid. 1995. 6. 637: c ) C Bolm. G. Schlingloff. K . Harms. Clirni. Ber. 1992. 125. 1191 : d ) S. Li. Y. Jmnp. A Mi. G Ymp.
J CIi~n7.So.. Perkin E-ms I 1993. 885.
Organocyanide Acceptor Molecules as Novel Ligands
Kim R. Dunbar"
Pioneering research carried out at Dupont in the 1950s and
1960s established a wealth of interesting chemistry for conjugated organic molecules with cyanide functionalities.['] Forty years
later, research involving organocyanide molecules continues to
flourish, owing to their promise as precursors for moleculebased materials. Among the organocyanide materials demonstrated to exhibit unusual properties is a class of ionic materials
that consists of paramagnetic transition metal metallocene
cations and radical anions of tetracyanoethylene (TCNE,
Scheme la).['' The compounds [M(Cp),*][TCNEJ (M =
Mn, Fe) crystallize in columns of donors and acceptors,
TCNE
DM-DCNQI
A primary motivation for coassembling metal centers
and organic radicals in this manner is to achieve new pathways
for electronic coupling through p,-d, overlap in addition to the
usual p, overlap of the organic acceptors. With the proper energy match of metal and organic orbitals it may be possible to
achieve an interplay between superexchange and charge-transport pathways, perhaps leading to a synergistic state wherein
superconductivity and ferromagnetism coexist. At the very least
i t appears that this strategy holds promise for the design of
highly conducting organometallic polymers, as evidenced by the
work of Hunig and co-workers, who synthesized a new family of
organic acceptors known as dicyanoquinodiimines (DCNQIs,
Scheme 1). These DCNQIs form crystalline network solids with
copper that exhibit extraordinarily high conductivities that persist to the lowest temperature^.^^] The structures of [Cu(2,5Me,-DCNQI)], (Fig. 1. 2,5-Me2-DCNQI = DM-DCNQI)
TCNQ
Scheme 1. Important organocyanide acceptor molecules.
D + A - D + A - . (D' = [M(Cp*),]+; A - = TCNE-) and are remarkable in that they order ferromagnetically at Curie temperatures 7; of 4.8 K (Fe) and 8.8 K (Mn). These results are quite
surprising in the context of classical magnets, considering that
the materials are not three-dimensional and that the properties
are based on spins of organic molecules.
Solids containing transition metals cations n-bonded to the
nitrile groups of polycyano anions are an entirely different category of organocyanide materials than the ionic, stacked sys[*I Prof K R DunbaiDepartment of Chemistry and
The Center lbr Fundamental Materials Research
Michigan State University
East Lansing. MI 48824 (USA)
Fax, Int. code t(S17)353-1793
e-mail . dunbarro cemvax.cem rnsu.edu
Fig. 1. Pluto representation of a portion of the extended framework structure of
[Cu(DM-DCNQI),]
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