C O M M U N I C A T I O N S
Scheme 1. Plausible Mechanism for the Catalytic Tishchenko
a
Reaction Mediated by Cp*2ThMe2
by both the chloride and the methyl groups which hinder the
approaching of an aldehyde to the metal-alkoxide bond when
disposed in the ortho position (See Figure 1 and Scheme 1), and
(2) the electrostatic interaction between the metal and the chloride
which enhances the approach of the aldehyde to the metal center
and the activity.
To our knowledge, this is the first example of a catalytic coupling
process of aldehydes mediated by actinide complexes. Unexpectedly
the reaction proceeds via an actinide-alkoxo bond activation, which
was believed to be a dead end for actinide complexes in terms of
catalysis. We believe the findings here presented will inspire this
new uprising field. New catalytic processes involving the activation
of actinide-oxygen bonds are underway.
a
We use R* instead of PhCH(CH3)O- or PhCH2O- for clarity.
Acknowledgment. This research was supported by the Israel
Science Foundation Administered by the Israel Academy of Science
and Humanities under Contract 83/01-1.
Figure 1. Illustration showing the higher steric interference between the
two methyls on the ortho positions (blue and light blue atoms, right
molecule) compared to para postition (left). Only one benzylalkoxo
substitution is shown for clarity.
Supporting Information Available: Experimental section including
1
13
the synthesis and H and C NMR analysis. This material is available
free of charge via the Internet at http://pubs.acs.org.
benzaldehyde yield the 2-phenetylbenzoate (9) (step 1-3 in Scheme
References
1), demonstrating that an aldehyde is able to insert into complex 1
(
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producing the active alkoxo species.
Based on the kinetic and thermodynamic data a plausible
mechanism for the Tishchenko reaction is presented in Scheme 1.
In the first step of the reaction, the precatalyst 1 reacts with 2
equiv of the aldehyde to give the alkoxo complex 7, via a four-
center transition state. The first step is thermodynamically favor-
able because of the oxophilic nature of thorium (∆Hcalcd ) -68
(
(
(
(
(
(
5) Haskel, A.; Straub, T.; Dash, A. K.; Eisen, M. S. J. Am. Chem. Soc. 1999,
1
21, 3014-3024.
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kcal/mol).2
0,21
Chem. Soc. 1999, 121, 3025-3034.
A second insertion of an aldehyde into the thor-
7) Stubbert, B. D.; Stern, C. L.; Marks, T. J. Organometallics 2003, 22,
ium-alkoxide bond produces complex 8. The following metathesis
of complex 8 with an additional aldehyde releases the ester 9
producing the active catalytic species 10. The catalytic insertion
of an aldehyde into a thorium-alkoxo bond takes place in step 4
to give complex 11, and its hydride transfer reaction (step 5, rate
determining step) with an additional aldehyde via a plausible six-
centered chairlike transition state produces the ester 3 and regener-
ates the active complex 10.
4836-4838.
(8) Wang, J.; Dash, A. K.; Kapon, M.; Berthet, J.-C.; Ephritikhine, M.; Eisen,
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(
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(
10) Dash, A. K.; Gourevich, I.; Wang, J. Q.; Wang, J.; Kapon, M.; Eisen, M.
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(
(
(
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It is possible to envision a â-hydrogen elimination in steps 3
and 5 producing the same results; however, the kinetic data does
not support the â-hydrogen elimination mechanism. In addition,
from a thermodynamic point of view, we calculated the enthalpy
of the reaction for a â-hydrogen elimination and found it to be
(
(
14) Seki, T.; Nakajo, T.; Onaka, M. Chem. Lett. 2006, 35, 824-829.
15) Burgstein, M. R.; Berberich, H.; Roesky, P. W. Chem. Eur. J. 2001, 7,
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17) Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 6149-6167.
1
(
20,21
energetically higher as compared to the six-center mechanism
(18) Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 4253-4271.
(19) O’Hagan, D.; Gross, R. J. M.; Meddour, A.; Courtieu, J. J. Am. Chem.
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(+6 and -47 kcal/mol, respectively, eq 4-6). Therefore, we suggest
that if operative, the â-hydrogen elimination is not the main
termination pathway.
The effect of the substitution on the phenyl ring can be explained
by considering two parallel effects: (1) the steric “obstacle” created
(
20) Simoes, J. A. M.; Beauchamp, J. L. Chem. ReV. 1990, 90, 629-688.
(21) McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33,
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