The role of hydrogen bonds in Baeyer-Villiger reactions

Various Baeyer-Villiger (B-V) oxidation reactions were examined by density functional theory calculations. Proton movements in transition states (TSs) of the two key steps, the nucleophilic addition of a peroxyacid molecule to a ketone (TS1) and the migration-cleavage of O-O (TS3), were discussed. A new TS of a hydrogen-bond rearrangement in the Criegee intermediate (TS2) was found. The hydrogen-bond directionality requires a trimer of the peroxyacid molecules at the nucleophilic addition of a peroxyacid molecule to a ketone TS (TS1). At the migration-cleavage of O-O TS (TS3), also three peroxyacid molecules are needed. Elementary processes of the B-V reaction were determined by the use of the (acetone and (H-CO-OOH)n, n=3) system. The geometries of the nucleophilic addition of a peroxyacid molecule to a ketone TS (TS1) and the migration-cleavage of O-O TS (TS3) in the trimer (n=3) participating are nearly insensitive to the substituent on the peroxyacid. The directionality is satisfied in those geometries. The migration-cleavage of O-O TS (TS3) was found to be rate-determining in reactions, [Me2C=O+(H-CO-OOH)3], [Me2C=O+(F3C-CO-OOH)3], and [Me2C=O+(MCPBA)3]. In contrast, the nucleophilic addition of a peroxyacid molecule to a ketone (TS1) is rate-determining in the reaction, [Ph(Me)C=O+(H-CO-OOH)3].     

Theoretical and experimental studies on the Baeyer-Villiger oxidation of ketones and the effect of alpha-halo substituents

The Baeyer-Villiger reactions of acetone and 3-pentanone, including their fluorinated and chlorinated derivatives, with performic acid have been studied by ab initio and DFT calculations. Results are compared with experimental findings for the Baeyer-Villiger oxidation of aliphatic fluoro and chloroketones. According to theoretical results, the first transition state is rate-determining for all substrates even in the presence of acid catalyst. Although the introduction of acid into the reaction pathway leads to a dramatic decrease in the activation energy for the first transition state (TS), once entropy is included in the calculations, the enthalpic gain is lost. Of all substrates examined, pentanone reacts with performic acid via the lowest energy transition state. The second transition state is also lowest for pentanone, illustrating the accelerating effect of the additional alkyl group. Interestingly, there is only a small energetic difference in the transition states leading to migration of the fluorinated substituent versus the alkyl substituent in fluoropentanone and fluoroacetone. These differences match remarkably well with the experimentally obtained ratios of oxidation at the fluorinated and nonfluorinated carbons in a series of aliphatic ketones (calculated, 0.3 kcal/mol, observed, 0.5 kcal/mol), which are reported herein. The migration of the chlorinated substituent is significantly more difficult than that of the alkyl, with a difference in the second transition state of approximately 2.6 kcal/mol.     

Remote substituent effects in the Baeyer-Villiger oxidation. I. through-bond γ substituent effect on the regioselectivity

Regioselectivity of the Baeyer-Villiger oxidation of 8-oxabicyclo[3.2.1] octan-3-one derivatives is markedly affected by the electronic nature of substituents at position γ to the carbonyl function.     

The Baeyer-Villiger oxidation of ketones with Oxone in the presence of ionic liquids as solvents

Cyclic and linear ketones were readily oxidised with Oxone^® at 40 °C in ionic liquids as solvents and short times (2.5-20 h), affording their corresponding lactones and esters in high yields (65-95%). Both, aprotic and protic ionic liquids were used. The best conversion of ketones and the highest yields of products were obtained with 1-buty-3-methylimidazolium tetrafluoroborate and 1-methylimidazolium acetate as solvents. These ionic liquids were also efficiently recycled in the Baeyer-Villiger reaction without significant loss of activity. Several factors, such as the partial solubility of KHSO₅ in the ionic liquid, its viscosity and the presence of a proton in protic ionic liquids, have an influence on the course of the reaction.     

A nonenzymatic acid/peracid catalytic cycle for the Baeyer-Villiger oxidation

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 H2O2 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.     

Baeyer-Villiger oxidation of ketones with hydrogen peroxide catalyzed by Sn-palygorskite

Palygorskite-supported Sn complexes were prepared by a simple procedure. Cyclic ketones and acyclic ketones were oxidized by hydrogen peroxide in a reaction catalyzed by palygorskite-supported Sn complexes, affording corresponding lactones or esters with selectivity for the product of 90-100%. The catalysts can be recycled for several times without significant decline in catalytic activity.     

Carbon-oxygen bond formation via organometallic Baeyer-Villiger transformations: a computational study on the impact of metal identity

Metal-mediated formation of C-O bonds is an important transformation that can occur by a variety of mechanisms. Recent studies suggest that oxygen-atom insertion into metal-hydrocarbyl bonds in a reaction that resembles the Baeyer-Villiger transformation is a viable process. In an effort to identify promising new systems, this study is designed to assess the impact of metal identity on such O-atom insertions for the reaction [(bpy)(x)M(Me)(OOH)](n) → [(bpy)(x)M(OMe)(OH)](n) (x = 1 or 2; bpy = 2,2'-bipyridyl; n is varied to maintain the d-electron count at d(6) or d(8)). Six d(8)-square-planar complexes (M = Pt(II), Pd(II), Ni(II), Ir(I), Rh(I), and Co(I)) and eight d(6)-octahedral systems (M = Ir(III), Rh(III), Co(III), Fe(II) Ru(II), Os(II), Mn(I), and Tc(I)) are studied. Using density functional theory calculations, the structures and energies of ground-state and transition-state species are elucidated. This study shows clear trends in calculated ΔG(++)'s for the O-atom insertions. The organometallic Baeyer-Villiger insertions are favored by lower coordination numbers (x = 1 versus x = 2), earlier transition metals, and first-row (3d) transition metals.     

Identification of the product of toxoflavin lyase: degradation via a Baeyer-Villiger oxidation

Toxoflavin (an azapteridine) is degraded to a single product by toxoflavin lyase (TflA) in a reaction dependent on reductant, Mn(II), and oxygen. The isolated product was fully characterized by NMR and MS and was identified as a triazine in which the pyrimidine ring was oxidatively degraded. A mechanism for toxoflavin degradation based on the identification of the enzymatic product and the recently determined crystal structure of toxoflavin lyase is proposed.     

Exploring the uranyl organometallic chemistry: from single to double uranium-carbon bonds

Uranyl organometallic complexes featuring uranium(VI)-carbon single and double bonds have been obtained from uranyl UO(2)X(2) precursors by avoiding reduction of the metal center. X-ray diffraction and density functional theory analyses of these complexes showed that the U-C and U=C bonds are polarized toward the nucleophilic carbon.     

Gas chromatography of organometallic compounds of group IVB with heteroelement-heteroelement bonds

Russian Chemical Bulletin -     

Palladium-catalyzed, asymmetric Baeyer-Villiger oxidation of prochiral cyclobutanones with PHOX ligands

Described in this report is a general method for the conversion of prochiral 3-substituted cyclobutanones to enantioenriched γ-lactones through a palladium-catalyzed Baeyer-Villiger oxidation using phosphinooxazoline ligands in up to 99% yield and 81% ee. Lactones of enantiopurity ≥93% could be obtained through a single recrystallization step. Importantly, 3,3-disubtituted cyclobutanones produced enantioenriched lactones containing a β-quaternary center.     

Baeyer-Villiger oxidation of ketones with hydrogen peroxide catalyzed by Sn-aniline complex

Sn-aniline complex was prepared by a simple procedure.Cyclic and acyclic ketones were oxidized into lactones or esters with very high selectivity and yield with 30% hydrogen peroxide in the presence of Sn-aniline complex.     

Metal hydroxides as catalysts for the Baeyer-Villiger oxidation of cyclohexanone with hydrogen peroxide

This paper reports the results obtained in the Baeyer-Villiger oxidation of cyclohexanone with a hydrogen peroxide/benzonitrile mixture as oxidant in the presence of synthetic metal hydroxides or their calcined products as catalysts. The metal hydroxides were obtained by coprecipitation. The best ε-caprolactone conversion results were provided by magnesium hydroxide.