ACS Catalysis
Letter
conclusion that the optical yield is defined via the difference in
REFERENCES
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−1
stabilities of TS12 and TS13, 1.7 kcal mol . This calculated
value correlates closely to the experimentally determined value
of 1.74 kcal mol− (vide supra). Given the judicious modeling
of a complex system, such a level of agreement demonstrates
the virtue of DFT computations in predicting the sense and
(
1) (a) Catalytic Asymmetric Synthesis, 3rd ed.; Ojima, I., Ed.; Wiley-
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23
level of enantioselection.
The TS’s for the reductive elimination from the mono-
hydride 27 was computed to be too high in energy (TS20, 12.5
(
2) Halpern, J. Science 1982, 217, 401−407. (b) Halpern, J. In
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−1
kcal mol ), but it can easily rearrange into 31 (TS21, −3.2 kcal
1
(
(
985; Vol. 5, pp 41−69.
−1
mol ), which readily eliminates the product via TS22, −6.5
3) Brown, J. M. Chem. Soc. Rev. 1993, 22, 25−45.
−
1
kcal mol ). Similarly in the β-S-pathway, the reductive
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(
Scheme 5). Similar rearrangements of monohydride inter-
10,13,14
mediates were previously described.
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(
1
(
2
(
754.
In conclusion, we have discovered an example of a catalytic
asymmetric hydrogenation reaction in which activation of H2
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followed by the dissociation of the carbon−carbon double
bond. This means that experimental observation of a relatively
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fast reaction of H with some catalyst−substrate complex does
2
not necessarily mean that the stereoselection takes place at this
stage of the catalytic cycle. Therefore, the early experiments of
(
2
−4
Halpern and Brown can be considered as a useful illustration
of different reactivity of diastereomeric intermediates on one of
the stages in a catalytic cycle. However, whether or not the
induction of chirality really happened on the stage that has been
directly monitored must be studied separately.
2
(
Of course, in any catalytic asymmetric reaction, the
stereoinduction must proceed via the competition of R- and
S-pathways, i.e., via faster transformation of some intermediate
compared to the similar transformation of its diastereomer.
However, unfortunately this competition is not necessarily
directly observed. Neither can simple qualitative structural
considerations help in a proper understanding of the intrinsic
mechanisms of stereoselection. On the other hand, with the
development of computational chemistry, a careful computa-
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available.
(
(
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We are currently studying the mechanisms of the asymmetric
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(
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ASSOCIATED CONTENT
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Supporting Information
Experimental procedures, NMR charts, computed
energies, Cartesian coordinates of all computed
2
(
Spannenberg, A.; Fischer, C.; Buschmann, H.; Heller, D. Angew. Chem.,
Int. Ed. 2005, 44, 1184.
AUTHOR INFORMATION
Notes
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(
23) For recent discussion of the state-of-art computational
*
modelling of enantioselective catalysts, see Armstrong, A.; Boto, R.
A.; Dingwell, P.; Contreras-Garcia, J.; Harvey, M. J.; Mason, N. J.;
Rzepa, H. S. Chem. Sci. 2014, 5, 2057−2071.
*
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The authors are grateful to Ms. Yumi Horiuchi, Nippon
Chemical Industrial Co., Ltd. for the measurement of
enantiomeric excesses of the hydrogenation products.
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ACS Catal. 2015, 5, 2911−2915