Communications
selectivity increases with decreasing temperature in favor of
the other enantiomer. This behavior can be explained by the
compensation of entropic and enthalpic influences. At low
temperatures, the R-configurated reactants 5 always lead to
the products with R configuration at C-b, which is consistent
with our prediction (see Scheme 2).
Received: April 29, 2005
Revised: July 1, 2005
Published online: September 19, 2005
Keywords: asymmetric synthesis · chirality transfer · cyclization ·
.
photochemistry · vibrational circular dichroism
[1] Selected reviews and books concerning asymmetric photochem-
istry: a) Memory effect of chirality: H. Zhao, D. C. Hsu, P. R.
Carlier, Synthesis 2005, 1 – 16; b) “Chiral photochemistry”:
Molecular and Supramolecular Photochemistry,Vol. 11 (Eds.:
Y. Inoue, V. Ramamurthy), Dekker, New York, 2004;
c) Asymmetric photochemistry and photochirogenesis: A. G.
Griesbeck, U. J. Meierhenrich, Angew. Chem. 2002, 114, 3279 –
3286; Angew. Chem. Int. Ed. 2002, 41, 3147 – 3154; d) “Oxetane
formation: stereocontrol”: A. G. Griesbeck, S. Bondock in
Organic Photochemistry and Photobiology, 2nd ed. (Eds.:
W. M. Horspool, F. Lenci), CRC, Boca Raton, FL, 2003,
pp. 59/1 – 59/19; e) “Enantioselective photocycloaddition reac-
tions in solution”: B. Grosch, T. Bach in Organic Photochemistry
and Photobiology, 2nd ed. (Eds.: W. M. Horspool, F. Lenci),
CRC, Boca Raton, FL, 2003, pp. 61/1 – 61/14; f) “Induced
diastereoselectivity in photodecarboxylation reactions”: K.
Pitchumani, D. Madhavan in Organic Photochemistry and
Photobiology, 2nd ed. (Eds.: W. M. Horspool, F. Lenci), CRC,
Boca Raton, FL, 2003, pp. 65/1 – 65/14; g) Stereoselective
intermolecular [2+2] photocycloadditions: T. Bach, Synthesis
1998, 683 – 703; h) Y. Inoue, Chem. Rev. 1992, 92, 741 – 770; i) H.
Rau, Chem. Rev. 1983, 83, 535 – 547.
[2] a) “Chirality transfer via sigmatropic rearrangements”: R. K.
Hill in Asymmetric Synthesis,Vol. 3 (Ed.: J. D. Morrison),
Academic Press, San Diego, 1984, pp. 503 – 572; b) [3,3]
Sigmatropic rearrangements: U. Nubbemeyer, Synthesis 2003,
961 – 1008; c) [3,3] Sigmatropic rearrangements: H. Frauenrath
in Methods Of Organic Chemistry (Houben Weyl),Vol. E21 ,
Thieme, Stuttgart, 1995, pp. 3301 – 3756; d) [2,3] Sigmatropic
rearrangements: J. Kallmerten in Methods Of Organic Chemis-
try (Houben Weyl),Vol. E21 , Thieme, Stuttgart, 1995, pp. 3757 –
3809; e) Wittig rearrangement: T. Nakai, K. Mikami in Organic
Reactions,Vol. 46 , Wiley, New York, 1994, pp. 105 – 209;
f) Copper-mediated allylic substitution: B. Breit, P. Demel, C.
Studte, Angew. Chem. 2004, 116, 3874 – 3877; Angew. Chem. Int.
Ed. 2004, 43, 3786 – 3789, and references therein.
[3] A search for the keyword “1,2-chirality transfer” provided only
two references: a) W. Smadja, S. Czernecki, G. Ville, C.
Georgoulis, Organometallics 1987, 6, 166 – 169; b) P. Compain,
J. GorØ, J.-M. Vatle, Tetrahedron 1996, 52, 6647 – 6664.
[4] a) P. Wessig, O. Mühling, Angew. Chem. 2001, 113, 1099 – 1101;
Angew. Chem. Int. Ed. 2001, 40, 1064 – 1065; b) P. Wessig, O.
Mühling, Helv. Chim. Acta 2003, 86, 865 – 893.
[5] a) P. Wessig, J. Schwarz, U. Lindemann, M. C. Holthausen,
Synthesis 2001, 1258 – 1262; b) P. Wessig, C. Glombitza, G.
Müller, J. Teubner, J. Org. Chem. 2004, 69, 7582– 7591; c) P.
Wessig, G. Müller, A. Kühn, R. Herre, H. Blumenthal, S.
Troelenberg, Synthesis 2005, 1445 – 1454.
[6] a) R. G. W. Norrish, M. E. S. Appleyard, J. Chem. Soc. 1934, 874;
b) N. C. Yang, D.-D. H. Yang, J. Am. Chem. Soc. 1958, 80, 2913;
c) P. J. Wagner, Acc. Chem. Res. 1971, 4, 168; d) P. J. Wagner,
Figure 1. Eyring plots for the irradiation of 12a. A=(R)-13a, B=(S)-
13a. Inset: DDH° values in kcalmolꢀ1, DDS° values in calmolꢀ1 Kꢀ1
T0 =DDH°/DDS° in 8C.
,
unimolecular reaction like 12!13 is based on a difference in
the free enthalpy of activation DDG°, which in turn is the sum
of an enthalpic and an entropic term (DDG° =
DDH°ꢀTDDS°, Gibbs–Helmholtz equation). Thus, the
stereoselectivity of such a reaction is always a function of
temperature if the entropies of activation of the two channels
of selectivity are different. A special situation occurs if the
differences of the enthalpies of activation DDH° and the
entropies of activation DDS° have the same sign. In this case a
temperature T0 = DDH°/DDS° exists, at which the selectivity
is inverted.
We observed exactly this behavior when we irradiated
compound 12a (Figure 1). For the reaction in dichlorome-
thane as well as in methanol, DDH° and DDS° have the same
sign, so that in both cases a temperature T0 can be calculated,
at which 13a is formed as a racemic mixture and below which
the formation of the other enantiomer is favored ((R)-13a,
Scheme 3). While for methanol this temperature lies at an
experimentally impractical region (ꢀ1048C), for dichloro-
methane it is ꢀ238C. The selectivity for the formation of 13 at
room temperature is determined by the entropies of activa-
tion, which compensate the enthalpic influence. Only below
T0 do the differences in the enthalpies of activation determine
the selectivity.[16]
In summary, we have described a novel concept for a 1,2-
chirality transfer. The mechanistic basis is the preferred
colinear arrangement of a leaving group and the p system of a
photochemically excited carbonyl group, which can be
explained by stereoelectronic effects.[4b] We were able to
demonstrate this concept by means of the photochemical
cyclization of four ketones 12a–d to the bicyclic compounds
13a–d. The configuration of the preferred enantiomer at
room temperature has been determined easily for the photo-
lytic products 13a,b,d by a combination of quantum chemical
calculations and VCD spectroscopy. Because both the differ-
ences of the entropies of activation and the enthalpies of
activation are positive, the stereoselectivity increases at
temperatures above T0. On the other hand, at temperatures
below T0 the ratio of the enantiomers is inverted, and
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 6778 –6781