Diphosphine−Palladium and −Platinum Complexes as Catalysts for the Baeyer−Villiger Oxidation of Ketones:  Effect of the Diphosphine, Oxidation of Acyclic Ketones, and Mechanistic Studies

Abstract

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.

1,2‐Bis‐N‐[2′‐(diphenylphosphanyl)benzoyl]diaminobenzene, a New Chelating Ligand with Versatile Coordination Properties

“…[2] Pd II and Pt II complexes are used in organic synthesis reactions such as DielsϪAlder cycloadditions, [3] BaeyerϪVilliger oxidation of ketones, [4] hydrogenolysis [5] or Sonogashira cross-coupling. [6] Electron-rich ligands have been used to increase the electron density at the metal center in order to facilitate the oxidative addition step in a catalytic cycle.…”

Transition Metal Catalysis in the Baeyer-Villiger Oxidation of Ketones

“…derivatives has been employed by several authors for the preparation of homologous compounds containing a variety of diphosphanes. Interestingly, the ability of 8 to promote the Baeyer ± Villiger oxidation with hydrogen peroxide is dependent on the size of the diphosphane ± platinum ring, [31] as was demonstrated for a series of homologous complexes on the substrate methylcyclohexanone (Scheme 10). The most active was also reactive towards a series of acyclic ketones, [31] thus representing the first example of transition metal catalysis (Scheme 11).…”

“…Interestingly, the ability of 8 to promote the Baeyer ± Villiger oxidation with hydrogen peroxide is dependent on the size of the diphosphane ± platinum ring, [31] as was demonstrated for a series of homologous complexes on the substrate methylcyclohexanone (Scheme 10). The most active was also reactive towards a series of acyclic ketones, [31] thus representing the first example of transition metal catalysis (Scheme 11). The reactivity observed indicates the scale of migratory aptitude tert-alkyl b vinyl b s-alkyl b phenyl b n-alkyl and parallels the reactivity known with peracids as oxidants (see above).…”

The Pt(ii)-catalyzed Baeyer–Villiger oxidation of cyclohexanone with H2O2 in ionic liquids

“…1), since it was shown that, the larger the P-Pt-P angle, the more effective the catalyst. 58 Actually, the inert atmosphere is not strictly necessary as far as the Baeyer-Villiger oxidation is concerned, but is desirable for prolonging the catalyst life especially in view of a possible recycle. Cyclohexanone was chosen as the substrate, because its reaction in the organic solvent gave poor yields due to extensive formation of 1,19-dihydroxy dicyclohexylperoxide, 59 as a parallel undesired reaction promoted by the acidity of commercial H 2 O 2 solutions.…”

“…Additionally, when in selected experiments carried out at room temperature conversion of the substrate has been quantitatively calculated, it has been found that it never exceeds 35% which compares well with literature data. 58 Finally, the effect of the catalyst amount, other conditions being equal, was investigated. The results are collected in Table 5.…”