A variety of Pd and Pt complexes of the type
[(P−P)M(μ-OH)]2
2+ (M = Pd, Pt;
P−P = a
series of tetraphenyldiphosphines) were tested in the Baeyer−Villiger
oxidation of ketones
with hydrogen peroxide. The effect of the diphosphine−metal ring
size on the catalytic
activity indicates that the larger the ring, the better the catalyst
and that, in general, Pt
complexes are superior. The complex modified with P−P = dppb
is the most active catalyst
and allows for the first time the oxidation of a series of acyclic
ketones. The corresponding
migratory aptitude series is in full agreement with the one known for
the stoichiometric
organic reaction employing peracids as oxidants. A test of the
reactivity of different peroxidic
oxidants (H2O2, t-BuOOH,
KHSO5, carbamide peroxide) shows that hydrogen peroxide
is
the most effective. A kinetic study of the oxidation of
2-methylcyclohexanone with [(dppb)Pt(μ-OH)]2
2+ as the catalyst shows
typical half-order dependence on the catalyst concentration, suggesting that the hydroxy dimer opens up to form the
catalytically active species.
The reaction is first order in ketone and hydrogen peroxide and is
independent of the acidity
of the system. The reaction is suggested to proceed via a
quasi-peroxymetallacyclic
intermediate and bears strong similarities to the stoichiometric
organic reaction.
The synthesis and characterization of a number of chiral complexes of the type [(P−P)Pt(μ-OH)]2
2+ (P−P = (R)-binap, (S,S)-diop, (R,R)-pyrphos, (R,R)-norphos, (R,R)-Me-duphos,
(S,S)-bppm) are reported. These are used for the enantioselective Baeyer−Villiger oxidation
of substituted meso-cyclohexanones using 35% hydrogen peroxide as oxidant. The reaction
is performed at 0 °C with moderate yields, showing in some cases ee higher than 50%. Best
catalysts are those containing atropoisomeric ligands possessing C
2 symmetry and/or capable
of making seven-membered chelate rings with the metal. The results represent the first
example of dissymmetrization of achiral cyclohexanones obtained via the Baeyer−Villiger
oxidation using a transition metal catalyst.
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