Tin Catalyst Research Articles

Bimetallic PtSn/C catalysts promoted by ceria: Application in the nonoxidative dehydrogenation of isobutane

A series of bimetallic platinum–tin catalysts supported on CeO 2/C were prepared and tested in the nonoxidative catalytic dehydrogenation of isobutane. Pt–Sn/CeO 2/C catalysts with varying Sn content (Sn/Pt = 0, 0.25, 0.5, and 0.75) were prepared using a co-impregnation method and chlorinated metal precursors. They were characterized by XPS and by CO and ethylene adsorption microcalorimetry at room temperature after in situ reduction treatments. It was found that the presence of cerium in bimetallic catalysts inhibited reduction of tin species, whereas tin facilitated reduction of cerium(IV) to cerium(III). Reduction at 773 K caused a strong decrease in CO saturation coverage for all catalysts studied. For the bimetallic catalysts, a strong interaction between tin and platinum produced significant changes in the initial heat of CO adsorption. The presence of cerium caused a strong decrease in the heat of ethylene adsorption on platinum in catalysts reduced at high temperature. The monometallic Pt catalyst undergoes a fast deactivation with time on stream in the isobutane dehydrogenation reaction. The addition of tin strongly suppresses the deactivation process. It was found that cerium-containing catalysts showed higher isobutene yields compared with their corresponding cerium-free counterparts. The bimetallic catalyst with Sn/Pt = 0.5 showed the best isobutene yield, and it was studied further. This optimized catalyst demonstrated good deactivation behaviour (comparable with the best catalysts reported in the literature). Furthermore, oxidation treatment at a mild temperature (573 K) allowed recovery of nearly all of its catalytic properties.

Interplay of Structure and Reactivity in a Most Unusual Furan Diels-Alder Reaction

Difluorinated alkenoate ethyl 3,3-difluoro-2-(N,N-diethylcarbamoyloxy)-2-propenoate reacts rapidly and in high yield with furan and a range of substituted furans in the presence of a tin(IV) catalyst. Non-fluorinated congener 2-(N,N-diethylcarbamoyloxy)-2-propenoate fails to react at all under the same conditions. These reactions have been explored using density functional theory (DFT) calculations. They reveal a highly polar transition state, which is stabilized by the Lewis acid catalyst SnCl(4) and by polar solvents. In the presence of both catalyst and solvent, a two-step reaction is predicted, corresponding to the stepwise formation of the two new carbon-carbon bonds via transition states which have similar energies in all cases. Our experimental observations of the lack of reaction of the non-fluorinated dienophile, the stereochemical outcomes, and the rate acceleration accompanying furan methylation are all well predicted by our calculations. The calculated free energy barriers generally correlate well with measured reaction rates, supporting a reaction mechanism in which zwitterionic character is developed strongly. An in situ ring opening reaction of exo-cycloadduct ethyl exo-2-(N,N-diethylcarbamoyloxy)-3,3-difluoro-7-oxabicyclo[2.2.1]hept-5-enyl-2-endo-carboxylate, which results in the formation of cyclic carbonate ethyl 4,4-difluoro-5-hydroxy-2-oxo-5,7a-dihydro-4H-benzo[1,3]dioxole-3a-carboxylate by a Curtin-Hammett mechanism, has also been examined. Substantial steric opposition to Lewis acid binding prevents carbonate formation from 2-substituted furans.

Thermal stability of foamed polysiloxane rubbers: Headspace analysis using solid phase microextraction and analysis of solvent extractable material using conventional GC–MS

The thermal stability of a condensation cured polysiloxane rubber (RTV5370) has been investigated using both solid phase microextraction (SPME) for headspace analysis of volatiles and conventional gas chromatography–mass spectrometry (GC–MS) for the assessment of toluene extractable fraction. In each case, the chemical composition and distribution of dimethylsiloxane cyclic species were assessed. Of particular interest is how the levels of the species change with age and the likely degradation processes that contribute to their production. Such studies are important as they help identify potential compatibility issues relating to the use of this material. SPME studies show typically that headspace volatile species consist primarily of a range of cyclic polydimethylsiloxanes (D 3–D 8, where D refers to the dimethylsiloxane unit and the subscript refers to the number of Si–O bonds making up the cyclic). 2-Ethylhexanoic acid, a by-product from hydrolysis of the tin catalyst used in the synthesis of the rubber, was also detected in the headspace. The toluene extractable fractions typically showed a wide range of cyclic polydimethylsiloxane (D 4–D 18) species, as well as 2-ethylhexanoic acid and a number of phthalate-based plasticisers. Our studies on thermally aged samples show significant increases in the octamethylcyclotetrasiloxane (D 4) cyclic both within the polymer matrix and in the headspace. These effects have been attributed to thermally activated depolymerisation reactions leading to the production of octamethylcyclotetrasiloxane (D 4) cyclic monomer. Quantification studies, to establish the likely levels of silicone cyclics present in the polymer matrix, were conducted by calibration of the gas chromatography column using silicone standards. The concentration of octamethyltetrasiloxane (D 4) and dodecamethylcyclohexasiloxane (D 6) in pristine, thermally aged and real-time aged samples (from field trials) were all found to be within a few parts per million concentration range.

Tin and tin–titanium as catalyst components for reversible hydrogen storage of sodium aluminium hydride

This paper is concerned with the effects of adding tin and/or titanium dopant to sodium aluminium hydride for both dehydrogenation and re-hydrogenation reactions during their reversible storage of molecular hydrogen. Temperature programmed decomposition (TPD) measurements show that the dehydrogenation kinetics of NaAlH 4 are significantly enhanced upon doping the material with 2 mol% of tributyltin hydride, Sn(Bu) 3H but the tin catalyst dopant is shown to be inferior than titanium. On the other hand, in this preliminary work, a significant synergetic catalytic effect is clearly revealed in material co-doped with both titanium and tin catalysts which shows the highest reversible rates of dehydrogenation and re-hydrogenation (after their hydrogen depletion). The re-hydrogenation rates of depleted Sn/Ti/NaAlH 4 evaluated at both 9.5 and 140 bars hydrogen are also found to be favourable compared to the Ti/NaAlH 4, which clearly suggest the importance of the catalyst choice. Basing on these results some mechanistic insights for the catalytic reversible dehydrogenation and re-hydrogenation processes of Sn/Ti/NaAlH 4 are therefore made.

Towards developing surface eroding poly( α-hydroxy acids)

We have prepared a library of biodegradable polyesters derived from poly( α-hydroxy acids) (PHAs) that appear to primarily exhibit surface erosion behavior. This was achieved by increasing the hydrophobicity of the polymers in two distinct steps, namely: macromer formation and a coupling step. In the first step, macromerdiols (MDs) with varying lipophilicities were prepared by polymerization of l-lactide or mixture of l-lactide and glycolide (3/1 by mole) to various lengths ( n = 1 0 , 20, 30, and 40) using alkanediols of increasing C-chain length (C6, C8, and C12) as initiators in the presence of Tin(II) catalyst. In the second step, the macromer diols were linked together with diacid dichlorides of varying C-chain lengths (C6, C8, C10, and C12) to yield polyesters ranging in molecular weight ( M w) from 20 to 130 KDa and polydispersity of 1.5–6. These polyesters exhibited different thermal behavior from pure PHAs that can be tuned by changing the initiator core, the lactide/glycolide chain length, and diacid dichloride type. In addition, all these polymers showed solubility in tetrahydrofuran unlike poly( l-lactic acid) (PLLA) and poly(lactide- co-glycolide) (PLGA). In contrast to PLLA and PLGA, the degradation behavior of these novel polyesters exhibited linear profiles consistent with a surface erosion behavior. Release studies using Congo red as a model drug from microspheres prepared from these polyesters showed linear release profiles with correlation constants of least-square fits approaching a value of unity. Degradable polyesters with tunable thermal and degradation behavior may find applications in drug delivery and tissue engineering, where control over these parameters is critical to ensure predictable outcomes.

Synthesis and Responsive Properties of Dibenzoylmethane Metallobiomaterials

ABSTRACTIncorporation of ligands and metals into polymers by controlled methods yields multifunctional responsive materials. Dibenzoylmethane (dbm) is a beta-diketone ligand that binds metal and metalloid ions such as boron, iron or europium in a bidentate fashion producing complexes with one or more dbm chelates. Modification of dbm with biocompatible poly(lactic acid) (PLA) allows materials processing and leads to responsive metallobiomaterials. This is accomplished by modification of dbm with hydroxyl groups (e.g. dbmOH), that serve as initiators for lactide polymerization. Improved control is noted when dbm is protected as iron or boron complexes, namely Fe(dbmOH)3 or BF2(dbmOH). Furthermore, the iron center in iron tris(dbmOH) also functions as a catalyst for the polymerization; no tin catalyst is required. Redorange iron-centered three arm star polymers, Fe tris(dbmPLA), are obtained. Acid sensitivity of these materials provides a method for dbm ligand dissociation, demetalation and production of dbmPLA macroligands for coordination to other metals such as Eu. Currently, dbmPLA and its polymeric iron complexes are being fabricated as nanoparticles. Light emitting B and Eu dbm complexes are also under investigation. Progress in the synthesis, characterization and nanoscale fabrication of dbm ligand and metal based polymeric biomaterials is reported.

Wlasciwosci mechaniczne usieciowanych polimetylowodorosiloksanami kauczukow silikonowych zawierajacych krzemionke i weglan wapnia

Silicone vulcanizates were obtained by crosslinking of poly(dimethyl-siloxane-α,ω--diols) (Polastosil, Table 1) with poly[(methyl, phenyl) hydrosiloxanes] (PMHS, PMFHS) of different functionality (Table 2) in the presenceof tin catalyst: dibutyltin dilaurate (DBTL). PMHS and PMFHS of linear and branched random structures: MD 2 2 D H 1 0 M, MD 6 6 D H 1 0 M, MD4o(T P h M) 1 0 (T P h OH) 1 0 D H 1 0 M, MD 4 2 (T P h OH) 5 D H 1 0 M, MD 4 2 (T P h OH) 1 0 D H 1 0 M and MD42(T P h M)5(T P h OH) 5 D H 1 0 M (where: M = Me 3 SiO 0 . 5 , D = Me 2 SiO, D H = MeHSiO, T P h OH = Oo.5(C6H5)Si(OH)O 0 . 5 , T P h M = O 0 . 5 (C 6 H 5 )Si(OSi Me 3 O 0 . 5 ) were synthesized by hydrolytic polycondensation of appropriate chlorosilanes. Chemical structure and tacticity of PMHS were analyzed using NMR spectroscopy (Fig. 1-4). The content of Si-H groups was determined by iodometric titration (according to Westermark-Schowen procedure). Elastomer and rubber blends, without fillers or containing them (Zeosil 45, Sipernat 310 or precipitated calcium carbonate), were prepared by addition of stoichiometric amounts of crosslinking agent and catalyst in order to obtain 2-3 mm thick layers. Their gelation times, measured at room temperature, increased with a decrease in Si-H bonds content in crosslinking agent (Fig. 5). Tensile strength (R r ) and relative elongation at break (e r ) were measured for solid (non-porous) samples (Fig. 6-10). Vulcanizates prepared of Polastosil M-2000 filled with: Sipernat 310 (for 10 wt. %: R r = 1.30-0.62 MPa, e r = 170-310 %), calcium carbonate (for 10-20 wt. %: R r = 0.62-0.83 MPa, e r = 150-180 %), and mixtures of Zeosil 45 with CaCO 3 showed the best mechanical properties.

Effects of a biodegradable polymer synthesized with inorganic tin on the chondrogenesis of human articular chondrocytes

Recent study has shown that biodegradable polymers are attractive candidates for chondrocyte fixation and further transplantation in cartilage tissue engineering. Poly (glycolic acid) (PGA), a polymer of glycolic acid, is widely used in orthopedic applications as a biodegradable polymer. Organotin, lead, antimony, and zinc are catalysts commonly used in synthesizing PGA. Here, we investigated the biocompatibility of PGA, synthesized with and without inorganic tin as a catalyst in chondrogenesis of human articular chondrocytes in a micromass culture system. Significant enhancement of chondrocyte proliferation and expression of the collagen type II protein gene were observed in cultures treated with PGA synthesized with a tin catalyst. However, aggrecan gene expression was very similar to the control culture. Amount of collagen type II protein was also increased in the same group of cultured chondrocytes. In contrast, PGA without a catalyst caused overall inhibition of chondrogenesis. Despite several positive findings, extensive investigations are essential for the feasibility of this PGA(Sn) in future clinical practice.

Iron Tris(dibenzoylmethane)-Centered Polylactide Stars: Multiple Roles for the Metal Complex in Lactide Ring-Opening Polymerization

Poly(lactic acid) (PLA)-supported dibenzoylmethane (dbm) and corresponding metal complexes have potential applications as biomaterials and catalysts. Using hydroxyl-functionalized dbm (i.e., dbmOH) as the initiator and Sn(oct)2 as the catalyst, lactide ring-opening polymerizations were unexpectedly slow (6 h), and PDIs broadened when molecular weights >10 000 were targeted. Because interactions between the dbm ligand and Sn(oct)2 may be responsible for the diminished catalyst activity and molecular weight control, iron(III) was employed as a protecting group to form Fe(dbmOH)3. Shorter reaction times (10 min) were noted with this trifunctional metalloinitiator and Sn(oct)2, and higher molecular weights were achievable. Moreover, it was discovered that Fe(dbmOH)3 serves not only as an initiator but also as a catalyst and activating group for the polymerization. Even without the tin catalyst, iron-centered polymers with low PDIs (<1.1) were obtained within 10 min ( approximately 70% monomer conversion). The resulting Fe(dbmPLA)3 stars were demetalated by acid treatment to generate dbmPLA for subsequent coordination to other metals. To explore the scope of iron beta-diketonate complexes as catalysts, additional studies were performed with dbmOH and benzyl alcohol initiators using either iron tris(dbm) or iron tris(acac) (acac = acetylacetonate) as the catalyst. Here too, PLA products were obtained, and the iron catalysts were readily separated by treatment with aqueous HCl.

Use of formic acid as reducing agent for application in catalytic reduction of nitrate in water

The reduction of nitrate in nitrogen using bimetallic palladium tin catalysts and hydrogen is an interesting process for water treatment. The aim of the present study is to use formic acid (FA) as a reducing agent and a pH buffer in order to substitute the mixture of hydrogen and carbon dioxide. The catalytic performances of a palladium tin catalyst supported on silica were evaluated in the presence of FA, as a function of the initial acid concentration and of the gas phase (N 2, CO 2, or H 2). Results were compared to those obtained with hydrogen in the presence of carbon dioxide. Similar mechanisms seem to explain the identical catalytic performances observed with these two reducing agents.

Degradation studies of crosslinked polyethylene. I Aged in air

Silane crosslinking of polyethylene, carried out by grafting an organofunctional silane (vinyltrimethoxysilane) onto polyethylene and subsequent moisture crosslinking in hot water using a tin catalyst, has been widely used in industrial applications because of its advantages in terms of low cost and easy processing. This study focused on the degradation processes that occurred in the material after air ageing in an oven; temperatures ranged from 90 to 220°C, while ageing times ranged from 2 to 500 h. Significant structural changes were observed according to the different ageing conditions (below and above the melting region of the material), since the carbonyl group concentration increased substantially during ageing above the melting region and the silicone-containing groups were also affected by the degradation. These structural changes affected the mechanical and thermal properties of the material, which was annealed at lower ageing temperatures (up to 155°C) and highly degraded at higher temperatures, when C–C crosslinks formed. Experimental design software was used in order to optimise the number of experimental trials and to model the results obtained; its analysis contributed to the interpretation of the results.

Degradation studies of crosslinked polyethylene. II Aged in water

Silane crosslinking of polyethylene was carried out by grafting an organofunctional silane (vinyltrimethoxysilane) onto polyethylene and by subsequent moisture crosslinking in hot water using a tin catalyst. The degradation processes, which occurred in the material after water ageing in an autoclave were studied; ageing temperatures were 90 to 190°C and ageing times were 2 to 500 h. Significant changes in the chemical structure of the material were observed by FT IR, as carbonyl group concentration increased and different structures formed in the region of absorbance of groups containing silicone; the structural changes significantly affected the mechanical properties as shown by the tensile data. A chemical analysis of the extracts in chloroform of water-aged samples carried out by using FT IR, LIMA and GPC techniques and some optical microscopy evidence suggested that the mechanism of degradation in water is different from the one in air, as during water ageing the antioxidants are washed away by water and hydrolytic oxidation also occurs. ECHIP experimental design software was used in order to optimise the number of experimental trials and to model the results obtained.

Thermal and Morphological Properties of Silicone‐Polyurethane‐Epoxy Intercrosslinked Matrix Materials

Three different types of epoxy resins based on an unmodified epoxy (diglycidyl ether of bisphenol‐A), polyurethane (PU) modified epoxy and silicone modified polyurethane‐epoxy matrices were developed. Epoxy resin was modified with PU prepolymer and hydroxyl terminated poly (dimethyl siloxane), also referred to as ‘silicones’ using a silane crosslinking agent and tin catalyst. Aromatic polyamine adduct (A), diethylenetriamine (B) and polyamidoamine (C) were used as epoxy curatives. The formation of intercrosslinked network was confirmed using Fourier transform‐infrared (FT‐IR) spectra and viscosity data. Thermo gravimetric analysis revealed reduced thermal stability for PU (10 wt%) modified epoxy, whereas the incorporation of silicone (10 wt%) into PU modified epoxy matrix improved their thermal properties. The morphology of silicone modified PU‐epoxy was also studied by scanning electron microscopy (SEM).

Effect of the support on tin distribution in Pd–Sn/Al2O3 and Pd–Sn/SiO2 catalysts for application in water denitration

The reduction of nitrate to nitrogen is a key process for nitrate removal in water. Among others bimetallic, palladium tin catalysts are well suited for this purpose. In the present study Pd–Sn catalysts are used and characterized in the mentioned process. Alumina and silica supported catalysts were prepared by deposition of tin onto palladium particles by using controlled surface reaction. The accessibility of palladium and the amount of reducible tin are tested by O2/H2/O2 titration. The structural characterization is based on 119Sn Mössbauer spectroscopy, TEM, XRD, and catalytic performances are tested in nitrate and nitrite reductions at room temperature. Remarkably, the properties of Pd–Sn/Al2O3 and Pd–Sn/SiO2 are different, although they were prepared according to the same procedure. Namely, the silica supported catalyst is more stable and selective, but is less active than the alumina supported one. This could be explained by the difference of (i) the interaction between the two metals and (ii) the affinity of tin with the support depending on the support used. The characterization results are interpreted with respect to the catalytic behavior of the materials.

Tin(II) Ethyl Hexanoate Catalyzed Precipitation Polymerization of ε-Caprolactone in Supercritical Carbon Dioxide

The synthesis of biodegradable polyesters for biomedical applications in supercritical dioxide has been the subject of a number of recent publications. However, systematic studies of the mechanism of polymerization in this novel medium are rare. In this work, we present the first systematic study of the ring-opening polymerization of e-caprolactone catalyzed by the industrially important catalyst tin(II) ethyl hexanoate. We report on the effects of varying reagent concentrations and conditions on the products obtained after polymerization in scCO2 and compare this with the results obtained in conventional reaction media.

Cinnamoyl esters of lesquerella and castor oil: Novel sunscreen active ingredients

AbstractLesquerella and castor oils were esterified with cinnamic acid (CA) and 4‐methoxycinnamic acid (MCA). Esterification of the hydroxy oils reached 85% completion with CA and 50% conversion with MCA. The hydroxy oils were esterified at 200°C under a nitrogen atmosphere within a sealed system. Unreacted CA and MCA were removed from the reaction mixtures by sublimation at 100°C under vacuum. The resultant methoxycinnamic oils possessed a broader, more blue‐shifted UV absorbance, 250 to 345 nm with a λmax of 305 nm, compared with the cinnamic oils, which absorbed from 260 to 315 nm, λmax of 270 nm. The methoxycinnamic oils provide better UV‐B absorption and thus are better candidates to be used as sunscreen active ingredients. Esterifications of the hydroxy oils with MCA at 200°C resulted in conversion of 40% of the MCA to undesirable by‐products. Esterifications with MCA performed at 175°C in the presence of a tin catalyst resulted in similar percent conversions to product without degradation of MCA. Esterifications of lesquerella oil with MCA at 175°C resulted in higher conversions, 43%, than analogous esterifications with castor oil, 29%. The hydroxyl groups of the lesquerella and castor oils provide their excellent emolliency, lubricity, and noncomedogenicity in skin and personal‐care products. Therefore, reactions that convert only 50% of the available hydroxyl groups of the lesquerella oil to cinnamoyl‐esters are preferred.

Development of a rapid crosslinking preceramic polymer system

A low-viscosity, fast crosslinking preceramic polymer system was developed as a base for liquid state processing. The system consists of a water-crosslinkable silicone polymer, a latent water source for in situ water generation, and a tin catalyst. While the silicone polymer and the water source can be mixed in any proportion, the catalyst must be added separately to achieve crosslinking at room temperature within a short time. By pyrolysis in inert atmosphere at 1000 °C the system was shown to have a high ceramic yield of ∼54 wt.%. The base system is compatible with alkanes, which makes it suitable for viscosity control when the system is filler-loaded for tailoring of properties. Due to its low viscosity one possible use of the system is in inkjet printing. Further since the crosslinking is rapid it can also be used in the layer by layer manufacturing of ceramic parts.

Hyperbranched Polyesters Based on Adipic Acid and Glycerol

AbstractSummary: Hyperbranched polyesters with controlled molecular weights and properties have been prepared by an A2 + B3 approach by reacting glycerol and adipic acid without any solvents in the presence of tin catalysts. The hyperbranched polyesters have been evaluated by size exclusion chromatography (SEC) analysis and NMR spectroscopy in order to determine molecular weights and degrees of branching.Idealized structure of the hyperbranched polyesters synthesized here from adipic acid and glycerol in the presence of tin catalysts.imageIdealized structure of the hyperbranched polyesters synthesized here from adipic acid and glycerol in the presence of tin catalysts.

Studies on the synthesis and thermal properties of copoly(L‐lactic acid/glycolic acid) by direct melt polycondensation

AbstractA two‐step direct copolymerization process of L‐lactic acid (L‐LA)/glycolic acid (GA) was developed. The first step was to produce an oligomer of L‐LA/GA and then the oligomer was polymerized with binary catalyst tin chloride dihydrate/p‐toluenesulfonic acid. In this way, the copoly(L‐LA/GA) (PLGA), without any organic solvent, was synthesized directly. The thermal properties and solubility in chloroform of PLGA were studied by DSC and NMR. The results showed that the melting point of PLGA decreases with increasing mole fraction of GA units in copolymer. In addition, the melting point of polymer also decreased with increasing degree of racemization of polymer. The solubility of PLGA in chloroform decreased with the increase of the average lengths of the glycolic acid units. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2163–2168, 2004

Computational Study of Tin-Catalyzed Baeyer−Villiger Reaction Pathways Using Hydrogen Peroxide as Oxidant

Density functional theory has been used to study several possible reaction mechanisms for tin-catalyzed Baeyer−Villiger oxidation of the model ketone acetone with hydrogen peroxide. The tin catalyst has been modeled with unconstrained single coordination sphere clusters using a B3LYP/ECP methodology. The Baeyer−Villiger reaction mechanism involves two principal steps: (1) addition of the hydrogen peroxide oxidant and the ketone substrate to form the Criegee intermediate and (2) Baeyer−Villiger rearrangement of the Criegee intermediate to yield the ester product. The Gibbs activation barriers for the addition and rearrangement steps in the noncatalyzed mechanism are 39.8 and 41.7 kcal/mol, respectively. In the absence of solvent coordination, tin activates hydrogen peroxide to produce a tin hydroperoxo (SnOOH) intermediate with a Gibbs activation barrier of 15.4 kcal/mol. Water molecules coordinating the tin active site facilitate proton transfer and lower the Gibbs activation barrier for tin hydroperoxo intermediate formation by 3−7 kcal/mol. The tin-catalyzed Baeyer−Villiger mechanism proceeds through a Criegee intermediate that contains a five-membered chelate ring with the tin center. This intermediate can be generated via two degenerate reaction pathways, one of which involves participation of the tin hydroperoxo intermediate. The Gibbs activation barriers for chelated Criegee intermediate formation are less than 14 kcal/mol. Rearrangement of the chelated Criegee intermediate is the rate-determining step for the tin-catalyzed Baeyer−Villiger mechanism. The Gibbs activation barrier for rearrangement is 24.1 kcal/mol when tin possesses no ligands and 21.4 kcal/mol when a single water ligand coordinates tin. The Lewis acidic tin center assists departure of the hydroxyl leaving group in the transition state for Baeyer−Villiger rearrangement. Increased branching of the migrating α carbon in the ketone substrate reduces the activation barrier for the rate-determining rearrangement step by promoting nucleophilic attack of the peroxo bond.