5926
J . Org. Chem. 2001, 66, 5926-5928
carboxylate group.4 Mechanistic evidence has been col-
A Din u clea r Str on tiu m (II) Com p lex a s
lected that whereas one of the metal ions serves as a
binding unit for the carboxylate anchoring group, the
other binds to ethoxide and activates its addition to the
ester carbonyl, as depicted in I.
Su bstr a te-Selective Ca ta lyst of Ester
Clea va ge
Roberta Cacciapaglia,* Stefano Di Stefano, and
Luigi Mandolini*
Dipartimento di Chimica and Centro CNR Meccanismi di
Reazione, Universita` La Sapienza, Box 34 - Roma 62,
00185 Roma, Italy
roberta.cacciapaglia@uniroma1.it
Received March 26, 2001
In tr od u ction
In their attempts to mimic the modes of action of
hydrolytic enzymes that make use of two metal centers
in their active sites, several workers have developed
dinuclear metal complexes in which the two metal centers
act synergetically in a more or less efficient fashion.
Frequently used metals are transition metals (e.g., ZnII,
CuII, CoIII)1 and lanthanides (III).2
Less common are examples involving block 1s metal
ions. The couples ZnII-SrII and ZnII-BaII have been used
by Canary et al.3 in the construction of heteronuclear
complexes that catalyze the hydrolysis of an activated
phosphodiester with moderate synergism between the
metal centers. In our ongoing studies of the catalytic
properties of block 1s metal ions, we have recently
reported that the bis-barium (II) complex of the dinucle-
ating ligand 1 catalyzes with turnover the cleavage of
esters and activated amides endowed with a distal
In a series of exploratory experiments aimed at defin-
ing the range of action of our dinuclear catalysts, we
subjected the bicarboxylate ester 6 to basic ethanolysis
in the presence of the bis-strontium (II) complex of 1.5
We found that a bimetallic catalyst concentration as low
as 30 µM increases the rate of cleavage of 6 by a
remarkable 5700-fold, but only by 9.5-fold the rate of
cleavage of the monocarboxylate ester 4. To shed light
into the origin of the widely different behaviors of the
two esters and to find an explanation for the surprisingly
high selectivity toward 6, we have carried out an exten-
sive kinetic investigation of the catalyzed ethanolysis of
esters 3-6. The results of such an investigation are
reported herein.
* To whom correspondence should be addressed.
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B.; Chin, J . Acc. Chem. Res. 1999, 32, 485. (g) Murthy, N. N.; Mahroof-
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Resu lts a n d Discu ssion
The kinetics of the ethoxide-induced ethanolyses of
substrates 3-6 were investigated at 25 °C on very dilute
substrate solutions (15 µM) in the presence of excess
EtONMe4 (1.00 mM). The 1‚[Sr]2 - catalyzed reactions
were carried out at two different catalyst concentrations,
namely, 30 and 100 µM, respectively. To assess the
degree of synergism of the two metal centers in 1‚[Sr]2,
a complete set of kinetic experiments was carried out also
in the presence of the mononuclear catalyst 2‚[Sr] at
concentrations of 60 and 200 µM, respectively. Clean
first-order kinetics were observed in all cases. The results
are summarized in Table 1.
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Angew. Chem., Int. Ed. Engl. 1999, 38, 348.
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(5) Slightly lower rate-enhancements were observed in the presence
of the bis-barium (II) complex.
10.1021/jo015648o CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/01/2001