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Organocatalytic Enantioselective Photoreactions.

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Highlights
DOI: 10.1002/anie.200503908
Photochemistry
Organocatalytic Enantioselective Photoreactions
Pablo Wessig*
Keywords:
asymmetric catalysis и enantioselectivity и
photochemistry и photoinduced electron transfer и
sensitizers
In the area of enantioselective reactions
tremendous progress has been made in
the last decades, mainly because of the
development of efficient transition-metal catalysts.[1] These transition-metalcatalyzed, enantioselective reactions
will certainly continue to play a central
role in synthetic organic chemistry in the
future. However, the last years have
seen an increasing trend to the use of
metal-free catalysts, so-called organocatalysts.[2] The reasons are the often
high costs of transition metals and the
problems that their residues, mainly in
pharmaceutical products, can cause. The
35-year-old Hajos?Parrish?Eder?Sauer?Wiechert reaction,[3] an asymmetric
Robinson anellation catalyzed by the
natural amino acid proline, is a classic
example of organocatalysis.
If one considers the extensive
knowledge concerning enantioselective
catalysis in thermal organochemical reactions,[4] it is astonishing that so little is
known about catalytic enantioselective
photochemical reactions. Upon closer
look the possible reasons become clear.
The reaction rate in the electronic
ground state is determined by the activation barrier, the height of which can
be controlled by catalysts. Photochemical reactions often proceed by extremely fast elementary steps in the excited
state. The macroscopically observed
rate is mostly controlled by the addition
of the ?reagent light?, that is, by the
proton flow, and by the degree of ?non-
[*] Priv.-Doz. Dr. P. Wessig
Institut f)r Chemie
Humboldt-Universit.t zu Berlin
Brook-Taylor-Strasse 2
12489 Berlin (Germany)
Fax: (+ 49) 30-2093-7450
E-mail: pablo.wessig@chemie.hu-berlin.de
2168
productive? chemical and physical channels of deactivation, which are expressed in terms of the quantum yield.
Thus, it could be considered hopeless to
try to catalyze photochemical reactions
in the conventional sense, to say nothing
of enantioselective reactions. Recently
significant success has been achieved in
this area, which is highlighted here. A
range of approaches should also be
mentioned that have been published
lately, for example photoreactions in
zeolites doped with chiral-auxiliary reagents and photoreactions in crystals,
but they are not the main subject of this
article.[5]
The known organocatalytic enantioselective photoreactions (OCEP)[6] are
based on two different approaches. In
the first, the photochemical excitation
and the enantioselective key step are
decoupled (Scheme 1). The prerequisite
Scheme 1.
is that reactants A and B do not react
with each other (or if they do, only very
slowly) in either the ground state or in
the excited state. One of the reactants
(B) is, through sensitization (S) if necessary, converted into the excited state B*,
while the other reactant (A) forms a
(not necessarily covalent) complex A?K
with the chiral catalyst K. This complex
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
now reacts with B* because of its
changed electronic properties to give
the complex B?A?K, which dissociates
into the product A?B, releases K, and
completes the catalytic cycle.
This concept is illustrated in research conducted by Cord8va et al.
which is based on the fact that photochemically (sensitized) generated singlet oxygen (B*) reacts with an enamine
(A?K) but not with the carbonyl compound (A) from which it is formed. In
the present case aldehydes[7a] and ketones[7b] were treated with singlet oxygen in the presence of a natural amino
acid (Scheme 2). The carbonyl compound 1 and the amino acid 2 reversibly
form enamine 3, which reacts highly
stereoselectively to give the hydroperoxides 4. Subsequent hydrolysis (4!5)
regenerates the organocatalyst, and the
hydroperoxides 5 are transformed spontaneously into the a-hydroxycarbonyl
compounds 6 (which in the case of an
aldehyde are reduced to the corresponding 1,2-diols). Starting from the aldehydes an enantiomeric excess of up to
66 % ee and from ketones even up to
72 % ee could be achieved. Remarkably,
the amino acid catalyst with the shortest
side chain, alanine, provided the best
selectivities. The relevance of this process in the prebiotic enantioselective
formation of a-hydroxycarbonyl compounds was discussed but should be
regarded critically. Furthermore, some
mechanistic details of this reaction still
remain vague: up to now the hydroperoxycarbonyl compounds 5 have not
been detected but only postulated.
The second approach differs from
the first mainly in the tight coupling of
the energy transfer to the substrate and
the
enantioselective
key
step
(Scheme 3). The central role is played
Angew. Chem. Int. Ed. 2006, 45, 2168 ? 2171
Angewandte
Chemie
Scheme 2.
Scheme 3.
by a chiral complexing reagent SK,
which at the same time acts as a
sensitizer and transfers the energy to
the substrate. The prerequisite is that A
and B (which can be linked) are photochemically inactive or not very active.
After the excitation of SK, a complex
with A and B is formed, in which the
excitation energy is transferred to the
substrate; this can also be coupled with a
photoinduced electron transfer (PET).
The enantioselective key step then occurs, and SK is released again
(Scheme 3, the excitation can alternatively proceed in the preformed complex).
The important points of this approach are high facial differentiation in
the complex SK?AиииB and the exclusion
of intermolecular sensitization.[8] The
complexation can be achieved by directed hydrogen bonds. In this context, Bach
and his group have been examining an
interesting class of complexing reagents
for potential applications in enantioselective photoreactions. These reagents
are derived from KempBs triacid (7),
which can be converted in a few steps
into molecules with unique architecture.
Angew. Chem. Int. Ed. 2006, 45, 2168 ? 2171
The most impressive structural feature
of compounds like 8 and 9 is the aryl
moiety, which shields one face of the
lactam ring like a rigid wall. If the
substrate of a photochemical reaction
also contains a lactam moiety, it can
bind to a receptor like 8 through two
hydrogen bonds and ideally can be
attacked only from one side (Scheme 4).
Scheme 4.
In the last years Bach et al. have
applied this concept successfully to a
range of photochemical reactions.[9?13]
Despite impressive success, this approach had the disadvantage of requiring at least equimolar amounts of the
complexing reagent; that is, it is not
catalytic.
Most recently Bach and his co-workers achieved a breakthrough.[14] In contrast to the previously applied photochemically ?passive? complexing reagents such as 8, a reagent with an
?integrated? photoelectron acceptor (9)
is used. In the presence of only 0.3 equiv
of 9 (pyrrolidinylethyl)quinolone 10 cyclizes to form the spiro compound 15 in
64 % yield and with a selectivity of
70 % ee (Scheme 5). The key step of this
reaction probably is a photoinduced
electron transfer (PET) in complex 11
from the nitrogen atom of the tertiary
amine to the excited benzophenone
chromophore in 9 with formation of
the radical anion/radical cation pair 12.
This is followed by a proton transfer and
formation of the radical pair 13. A
crucial point seems to be the existence
of this radical pair in the triplet state,[15a]
which prevents the radical centers from
recombining through bond formation.
An attack of the pyrrolidin-2-yl radical
moiety on the CC double bond of the
quinolone proceeds preferably from the
left side and the product complex 14 is
formed by hydrogen back-transfer. The
secret of success is probably the presence of a highly acidic OH group[15b] in
complex 13, which could stabilize 13
significantly relative to 11 and 14 by an
additional hydrogen bond. The decrease
in the enantiomeric excess at a lower
catalyst concentration is ascribed to
radical-chain processes[16] and insufficient complexation.
In the absence of face differentiation
or complexation, the enantiomeric excess decreases, in keeping with the
concept shown in Scheme 3, and no
multiplication of chirality occurs. In
one example, Krische et al.[17] employed
receptor 16, which contains a benzophenone moiety as a triplet sensitizer.
Compound 16 is capable of binding
quinolone 17 through hydrogen bonds
with formation of complex 18 (lg Ka =
2.5 0.2). After photochemical excitation the benzophenone moiety sensitizes
the quinolone and the tricycle 20 is
formed as a result of [2+2] cycloaddition. Receptor 16 can be applied in
substoichiometric amounts (25 mol %),
which proves a certain catalytic activity.
However, the maximum achievable enantiomeric excess amounts to only
22 % ee (with 1 equiv 16, 19 % ee with
0.25 equiv; Scheme 6). The yield of the
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2169
Highlights
catalysts are used. Hopefully, based on
the reported results, many more organocatalytic enantioselective photoreactions will be discovered in the next
years.
Published online: February 24, 2006
Scheme 5.
reaction was not noted, and the absolute
configuration of the preferred enantiomer was not determined.
In summary, it can be stated that
photochemical reactions can indeed be
carried out enantioselectively, if suitable
Scheme 6.
2170
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[1] Outstanding developments in this area
have been honored with the Nobel Prize
in Chemistry in 2001: a) R. Noyori,
Angew. Chem. 2002, 114, 2108; Angew.
Chem. Int. Ed. 2002, 41, 2008; b) W. S.
Knowles, Angew. Chem. 2002, 114, 2096;
Angew. Chem. Int. Ed. 2002, 41, 1998;
c) K. B. Sharpless, Angew. Chem. 2002,
114, 2126; Angew. Chem. Int. Ed. 2002,
41, 2024.
[2] a) Special Issue ?Asymmetric Organocatalysis?, Acc. Chem. Res. 2004, 37(8);
b) P. I. Dalko, L. Moisan, Angew. Chem.
2004, 116, 5248; Angew. Chem. Int. Ed.
2004, 43, 5138; c) S. Jayasree, B. List,
Org. Biomol. Chem. 2005, 3, 719.
[3] a) Z. G. Hajos, D. R. Parrish, J. Org.
Chem. 1974, 39, 1615; b) U. Eder, G.
Sauer, R. Wiechert, Angew. Chem. 1971,
83, 492; Angew. Chem. Int. Ed. 1971, 10,
496.
[4] A CA search with the key word ?catalytic enantioselective? yielded 124 hits
for the year 2004.
[5] Chiral Photochemistry. Molecular and
Supramolecular Photochemistry, Vol. 11
(Eds.: Y. Inoue, V. Ramamurthy), Marcel Dekker, New York, 2004.
[6] ?Organocatalytic enantioselective? in
this case means reactions in which the
catalyst is an organic, metal-free substance bearing the stereochemical information.
[7] a) A. C8rdova, H. SundOn, M. Engqvist,
I. Ibrahem, J. Casas, J. Am. Chem. Soc.
2004, 126, 8914; b) H. SundOn, M.
Engqvist, J. Casas, I. Ibrahem, A. C8rdova, Angew. Chem. 2004, 116, 6694;
Angew. Chem. Int. Ed. 2004, 43, 6532.
[8] Earlier reports by Kim and Schuster [a?
c] and by Inoue et al. [d?f] about asymmetric cycloadditions with chiral sensitizers only gave very low yields and
enantiomeric excesses: a) N. Akbulut,
D. Hartsough, J. I. Kim, G. B. Schuster,
J. Org. Chem. 1989, 54, 2549; b) J. I.
Kim, G. B. Schuster, J. Am. Chem. Soc.
1990, 112, 9635; c) J. I. Kim, G. B. Schuster, J. Am. Chem. Soc. 1992, 114, 9309;
d) Y. Inoue, T. Okano, N. Yamasaki, A.
Tai, J. Photochem. Photobiol. A 1992, 66,
61; e) S. Asaoka, M. Ooi, P. Jiang, T.
Wada, Y. Inoue J. Chem. Soc. Perkin
Trans. 2 2000, 77; f) Y. Inoue, N. Sugahara, T. Wada, Pure Appl. Chem. 2001,
73, 475.
Angew. Chem. Int. Ed. 2006, 45, 2168 ? 2171
Angewandte
Chemie
[9] [2+2] Cycloadditions: a) T. Bach, H.
Bergmann, K. Harms, J. Am. Chem.
Soc. 1999, 121, 10 650; b) T. Bach, H.
Bergmann, J. Am. Chem. Soc. 2000, 122,
11 525; c) T. Bach, H. Bergmann, B.
Grosch, K. Harms, J. Am. Chem. Soc.
2002, 124, 7982.
[10] [4+4] Cycloadditions: T. Bach, H. Bergmann, K. Harms, Org. Lett. 2001, 3, 601.
[11] Norrish?Yang reactions: a) T. Bach, T.
Aechter, B. NeumPller, Chem. Commun. 2001, 607; b) T. Bach, T. Aechter,
B. NeumPller, Chem. Eur. J. 2002, 8,
2464.
Angew. Chem. Int. Ed. 2006, 45, 2168 ? 2171
[12] 6p Photocyclizations: T. Bach, B.
Grosch, T. Strassner, E. Herdtweck, J.
Org. Chem. 2003, 68, 1107.
[13] Diels?Alder reactions of photochemically generated dienols: a) B. Grosch,
C. N. Orlebar, Y. Inoue, E. Herdtweck,
W. Massa, T. Bach, Angew. Chem. 2003,
115, 3822; Angew. Chem. Int. Ed. 2003,
42, 3693; b) B. Grosch, C. N. Orlebar, E.
Herdtweck, M. Kaneda, T. Wada, Y.
Inoue, T. Bach, Chem. Eur. J. 2004, 10,
2179.
[14] A. Bauer, F. WestkQmper, S. Grimme, T.
Bach, Nature 2005, 436, 1139.
[15] a) After photochemical excitation benzophenone undergoes a very efficient
intersystem crossing to the triplet state;
b) the acidity of a-hydroxy-substituted
radicals is comparable to that of carboxylic acids.
[16] S. Bertrand, N. Hoffmann, J.-P. Pete,
Eur. J. Org. Chem. 2000, 2227.
[17] D. F. Kauble, V. Lynch, M. J. Krische, J.
Org. Chem. 2003, 68, 15.
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