Styrene hydrocarboalkoxylation
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 10, October, 2001
1843
occurs less regioselectively (S < 80%) than that in the
case of the neutral HPdClL2 complex (S ≈ 100%).
A weighty contribution to the differences between
the mechanisms discussed is made by the ionic nature of
the catalytically active complexes with weakly bound
acido ligands. Since the dielectric constant of the medium
central atom in palladium phosphine complexes, the
hydride mechanism of the carbonylation catalysis of
unsaturated hydrocarbons is retained. However, in this
case, due to the ionic character of catalytically active
intermediates with the weakly bound acido ligand, the
mechanism somewhat changes, which influences the
process kinetics.
1
ε is less than 5.7 (in the region of [BuOH] ≤ 5 mol L ),15
the majority of these complexes form, apparently, ion
pairs with the TsO counter-ion.16 In this system, spe-
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 98-03-
32035).
cific solvation of the anion by n-butanol is possible.
Therefore, it can be assumed that solvent-separated ion
pairs are mainly formed. The coordinatively unsaturated
PdII cationic complexes contained in these ion pairs
appear to be more reactive than neutral intermediates;
this is the reason for the increase in the catalytic activity
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1
1
(from 16 h
for complex 4 to 78 h
for complex 3
under conditions of run 2). The ionization accelerates
butanolysis of the acyl complexes; as a consequence,
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ln(r) = 3.22 + 8.11/ε,
(9)
whose pattern is similar to that of the dependence
describing the effect of the dielectric constant on the
dissociation constants of electrolytes.17 However, the
basicity of the solvent might also play a role. Thus when
dioxane is replaced by equally non-polar but less basic
toluene, the catalyst activity increases by almost an
order of magnitude.7
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Thus, the results of this study demonstrate that on
weakening of the coordination of the acido ligand to the
Received March 5, 2001;
in revised form May 11, 2001