New Chiral Complexes of Platinum(II) as Catalysts for the Enantioselective Baeyer−Villiger Oxidation of Ketones with Hydrogen Peroxide:  Dissymmetrization of meso-Cyclohexanones

Abstract

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.

Mechanistic Investigation of Chiral Phosphoric Acid Catalyzed Asymmetric Baeyer–Villiger Reaction of 3‐Substituted Cyclobutanones with H2O2 as the Oxidant

“…[29,30] Chemical catalysts offer an attractive alternative, especially reactions using environmentally benign oxidants such as H 2 O 2 , but they are still limited in number and are often less enantioselective than their biological counterparts. [5] Since the first reports on metal-catalyzed asymmetric B-V reactions in 1994, [31] various chiral metal catalysts based on Cu II , [32] Pt II , [33] Ti IV , [34] Co III , [35] Zr IV , [36] Mg II , [37] Al III , [38] and Pd II [39] have been developed in recent years for this transformation. However, so far, very few of them could attain enantioselectivity at a level comparable to enzymes, and wide substrate generality has not yet been achieved in each case.…”

Oxidation of Carbonyl Compounds

“…The BV transformations with chiral metal complexes are based on either transition metals such as Cu [37b, 40,41], Pt [42,43], Co [44], Pd [45], Zr [46], Hf [47] or nontransition metals such as Mg [48] and Al [49]. For example, Ko covsk y and coworkers have developed a series of new terpene-derived pyridinephosphine ligands, whose complexes with Pd(II) have been proven to catalyze the BV oxidation.…”

Catalytic Asymmetric Baeyer–Villiger Oxidation in Water by Using PtII Catalysts and Hydrogen Peroxide: Supramolecular Control of Enantioselectivity

“…At present, high conversions together with high enantioselectivities can be achieved either with chiral enantiopure organocatalysts [6] or with chiral metal complexes. The latter are based on either transition metals such as Co, [7] Cu, [4,8] Pd, [9] Pt, [5,10] Zr, [11] Hf, [12] or non-transition metals such as Mg [13] and Al. [14] These complexes are all active towards meso or chiral cyclobutanone substrates, which are intrinsically much more reactive than larger cyclic ketones.…”

“…[14] These complexes are all active towards meso or chiral cyclobutanone substrates, which are intrinsically much more reactive than larger cyclic ketones. Oxidation of the latter was successful only with Pt II [2,5,10,15] or Cu II [16] catalysts, with moderate to good ee values. Among the synthetic catalysts used for cyclohexanones, the organocatalyst developed by Peris and Miller is noteworthy.…”

“…[15h] Pt II complexes that have chiral enantiopure ligands gave good enantioselectivities both for five-and six-membered ring ketones in the desymmetrization [10,24] and the kinetic resolution [5,25] of cyclic ketones in organic solvents.…”