A Nonenzymatic Acid/Peracid Catalytic Cycle for the Baeyer−Villiger Oxidation

Abstract

The combined action of carbodiimide and hydrogen peroxide upon exposure to carboxylic acid catalysts serves to generate transient peracids that can be engaged in the Baeyer-Villiger rearrangement of ketones to lactones. Up to 35 turnovers of the catalytic cycle may be achieved. The conditions are especially useful in the context of reactive cyclohexanones, and allow the use of H 2 O 2 as the terminal oxidant. A singular example of a chiral catalyst demonstrates, in principle, that enantioselective catalysis will be possible with this strategy for catalyst turnover.In spite of the fact that the oxidative conversion of ketones/aldehydes to esters via the Baeyer-Villiger reaction (BV) has been known for more than one hundred years, 1 there remain innumerable optimizations of this venerable reaction yet to be developed. One major challenge in the field has been the development of robust catalytic cycles that allow for efficient and rapid catalyst turnover. Moreover, highly enantioselective variants of the BV reaction are almost exclusively performed by enzymatic systems, 2 which by their nature, exhibit substrate specificity. Nonenzymatic catalysts for this goal have been demonstrated with organometallic reagents 3 as well as with potential mimics of biological "Baeyer- 6 We now wish to establish that the same catalysis concept is portable to the oxidation of ketones (e.g., 2, Scheme 2). This strategy may represent a fundamentally new catalytic cycle for the BV reaction. We targeted carboxyl activation through the combined action of diisopropyl carbodiimide (DIC), and 4-dimethylamino pyridine (DMAP). Capture of intermediate 3 with H 2 O 2 as the terminal oxidant leads to the formation of aspartate peracid (4, Scheme 2). The classical BV reaction then leads to product esters, and regeneration of 2 for re-entry into the catalytic cycle.Initially, exposure of 4-t-butyl-cyclohexanone (eq 1, 7) to 10 mol% of 2 activated by DIC/H 2 O 2 /DMAP resulted in a low yield of lactone 8 (13%; eq 1). We attributed the low yield to reaction of the aspartic peracid with excess DIC to give inactive diacyl peroxide 6 (k 1 ; Scheme 2). 7 This process was already observed in our epoxidation study, and the introduction of a nucleophilic co-catalyst (DMAP or N-methylimidazole; NMI) enabled perhydrolysis of the diacyl peroxide and regeneration of the active peracid (i.e., 6 to 4 in Scheme 2). In the context of the BV, a nucleophilic additive likewise facilitates diacyl peroxide perhydrolysis. 8 Furthermore, the low yield of lactone suggested that the rate of the reaction of peracid with ketone (k 2 , Scheme 2) was slow in comparison to the rate of peracid reaction with DIC (k 1 , Scheme 2). Thus, to compensate, we sought to maintain a low concentration of DIC during the reaction through slow addition of DIC. Indeed, addition of DIC at a rate of 0.13 equiv/hr, in the presence of 0.1 equiv of 2, delivers 8 in 76% yield (eq 1).(1) Having adapted 2 for catalytic BV oxidation, we wondered next if use of more acidic carboxylic acids woul...

Baeyer–Villiger oxidation under Payne epoxidation conditions

“…[10][11][12][13][14][15][16][17] Other metals used for the activation of hydrogen peroxide in the BV oxidation include Se, 18 Re, 19 Li and Ca borates, 20 Pt, [21][22][23] Pd, 24 Si polyoxometalates, 25 and Zr. 26 Metal-free methods also exist and make use of enzymes, 27 ionic liquids, 28 carboxylic acids, 29 phosphoric acids, 30,31 and bisflavins 32 to activate the peroxide. Of these methods, a few have been shown to be effective for enantioselective BV oxidations, which include metals such as Pt, Pd or Zr with chiral ligands, 21,24,26 pro-chiral carboxylic acids, 29 phosphoric acids, 30,31 and bisflavins.…”

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

“…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. Complementary to metal catalysis, several chiral organocatalysts based on organoseleniums, [40] flavins, [15] amino acids, [41] and organophosphoric acids [42] have also been demonstrated to be a useful and green alternative for the conversion. With regards to mechanistic aspects of the catalytic B-V reactions, Strukul and co-workers have investigated some Pt II and Pd II complexes of achiral diphosphines using a combination of kinetic and spectroscopic techniques.…”

Asymmetric Oxidations and Related Reactions

“…Peracid is a standard reagent for the nonasymmetric Baeyer -Villiger reaction and it is utilized for a wide range of ketones irrespective of the ring size. The authors applied the peracid/carboxylic acid shuttle, which has been previously applied to asymmetric epoxidation of olefi ns [78] , and achieved to render the reaction catalytic (Scheme 11.72 ) [132] . Aspartate -derived oligopeptide 61 catalytically promotes the oxidation of ketones to yield lactones with moderate but promising enantioselectivity.…”