Elsevier

Inorganica Chimica Acta

Volume 361, Issue 11, 27 July 2008, Pages 3230-3236
Inorganica Chimica Acta

Platinum(II) diphosphine complexes as catalysts for the Baeyer–Villiger oxidation of ketones: Is it possible to increase the concentration of the active species?

Dedicated to Professor Robert J. Angelici in recognition of his many outstanding contributions to inorganic chemistry.
https://doi.org/10.1016/j.ica.2007.10.042Get rights and content

Abstract

The synthesis and characterization of new bis-aquo platinum(II) complexes of the type [Pt(H2O)(P–P)][OTf]2 (OTf = triflate anion), in which the diphosphine P–P is a series of 1,n-bis-diphenyphosphinoalkanes (1ad, with n = 1–4), 1,2-bis-(di-n-fluorophenylphosphino)ethanes (2ac, with n = 2, 4–5) and 1,2-bis-dialkylphosphinoethanes (3ae), where the alkyl substituents at phosphorus have been systematically changed (dmpe) (3a), (depe) (3b), (dippe) (3c), (dcype) (3d) are reported. These complexes were used as catalysts in the Baeyer–Villiger oxidation of 2-methylcyclohexanone, 2-methylcyclopentanone and cyclobutanone with 35% hydrogen peroxide as an environmentally friendly oxidant. The reactions were performed at 25 °C in a chlorinated solvent/H2O two-phase system. All the investigated catalysts performed better than the corresponding dimeric complexes of general formula [Pt(μ-OH)(P–P)]2[BF4]2 as a consequence of the positive effect imparted by the triflate counter-anion on catalysts speciation.

Graphical abstract

The synthesis and characterization of new bis-aquo platinum(II) complexes of the type [Pt(H2O)(P–P)][OTf]2 (OTf = triflate anion, P–P = various aryl and alkyl diphosphines), is reported. These complexes were used as catalysts in the Baeyer–Villiger oxidation of 2-methylcyclohexanone, 2-methylcyclopentanone and cyclobutanone with 35% hydrogen peroxide as environmentally friendly oxidant. All the investigated catalysts performed better than the corresponding dimeric complexes of general formula [Pt(μ-OH)(P–P)]2[BF4]2 as a consequence of the positive effect imparted by the triflate counter-anion on catalysts speciation.

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Introduction

Since their application as catalysts in the Baeyer–Villiger oxidation of ketones in the early 1990s [1] cationic diphosphine Platinum(II) complexes have been studied in depth with the aim of developing more stable and more active species. Complexes of general formula [Pt(CF3)(P–P)(solv)]+ [2] showing good conversions in the oxidation of cyclic ketones under mild experimental conditions (rt with 35% H2O2 as oxidant) represented the first unambiguous example of transition metal catalysis in this synthetically interesting oxidation reaction. Later on, the class of hydroxo-bridged dimeric complexes [Pt(μ-OH)(PP)]22+ proved to be active in the same type of reaction, including its asymmetric version [3], and, using the 1,4-bis-(diphenylphosphino)butane (dppb) as the chelating ligand, even in the oxidation of some acyclic ketones [4].

Kinetic studies performed on the latter class of complexes indicated that the most likely active species is a coordinatively unsaturated monomeric species derived from bridging –OH ligand splitting (Scheme 1) [4], in which the Lewis acidity of the metal center is crucial to activate the ketone substrate towards nucleophilic attack by the oxidant. The influence of the Lewis acidity of the metal center was systematically investigated by synthesizing a series of complexes containing differently fluorinated tetra-aryl diphosphines [5] and electron-poor P–P ligand 1,2-bis-(dimethoxyphosphino)ethane [6], while the effect of the steric hindrance of the ancillary diphosphine ligand was studied on a series of tetra-alkyl diphosphines [7]. A clear correlation between the steric/electronic properties of the ligand and the catalytic activity of the complex was observed [5], [7].

One major problem with these catalysts is their limited lifetime under the strongly oxidizing reaction conditions. In fact, during catalytic experiments the diphosphine ligand starts being oxidized in a parallel side reaction until the catalyst is completely destroyed. Nevertheless, T.O.N. up to 130 was observed with the electron-poor fluorinated catalysts at low catalyst concentration [5]. An important issue to be addressed to improve activity is to increase the concentration of the active monomeric species according to equilibrium 1. The latter is very little shifted to the right. For example, an NMR analysis of the working catalytic mixture when using the highly reactive [Pt(μ-OH)(dppb)]2[BF4]2 complex shows only the presence of the starting complex. A possible answer to this issue might be the use of a complex that is already in the monomeric form but with characteristics similar to the μ-OH counterparts.

The above point is addressed in the present paper where we report our more recent studies in the BV oxidation of cyclic ketones with hydrogen peroxide using a series of [Pt(OH2)2(P–P)](OTf)2 complexes (OTf = triflate anion), in which the diphosphine is a series of 1,n-bis-diphenyphosphinoalkanes (1ad, with n = 1–4), 1,2-bis-(di-n-fluorophenylphosphino)ethanes (2ac, with n = 2, 4–5) and 1,2-bis-dialkylphosphinoethane, where the alkyl substituents at phosphorus have been systematically changed (3ad) (Chart 1). In principle, this formulation should allow to check the effect of a larger concentration of monomeric species on the catalytic activity in the Baeyer–Villiger oxidation of cyclic ketones with H2O2. Recently some of us explored the use of monomeric bis-triflate Pt(II) chiral complexes in the catalytic BV dissymetrization of meso cyclic ketones and in the kinetic resolution of cyclobutanones with interesting results [8].

Section snippets

General procedures and materials

All synthetic work was carried out with the exclusion of atmospheric oxygen under a dinitrogen atmosphere using standard Schlenck techniques. Solvents were dried and purified according to standard methods. Substrates were purified by passing through neutral alumina and stored in the dark at low temperature. Hydrogen peroxide (35% Fluka) and AgOTf (Aldrich) were commercial products and used without purification. The complexes [PtCl2(dppm)] and [PtCl2(dppp)] [9], [PtCl2(2Fdppe)] [10], [PtCl2

Synthesis and characterization of complexes

The design of complexes 1, 2 and 3 is based on the idea exposed in Section 1. It is known that the use of the complexes of the type [PtCl2(P–P)] and AgOTf as dechlorinating agent in hydrate media leads to the corresponding bis-aquo complexes [13], [14]. Their coordination similarities to the previously reported active μ-hydroxo complexes (a diphosphine and two O-donor ligands) as well as their ease of synthesis prompted us to consider their use as a possible simpler way to obtain the desired

Concluding remarks

The present work complements a series of investigations carried out on the catalytic Baeyer–Villiger oxidation of cyclic ketones with hydrogen peroxide as an environmentally friendly oxidant mediated by Pt(II) catalysts. After disclosing the electronic [5], [6] and steric effects [7] imparted by the ligand on the catalytic activity, we observed substantial increase in catalytic activity as well as productivity employing monomeric Pt(II) complexes of general formula [Pt(H2O)2(P–P)](OTf)2with

Acknowledgements

The authors thank MiUR, the Universitá Ca’ Foscari di Venezia and Università degli Studi di Padova for financial support. G.S. thanks Johnson-Matthey for the loan of platinum.

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