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

The mechanism of the chiral phosphoric acid catalyzed Baeyer–Villiger (B–V) reaction of cyclobutanones with hydrogen peroxide was investigated by using a combination of experimental and theoretical methods. Of the two pathways that have been proposed for the present reaction, the pathway involving a peroxyphosphate intermediate is not viable. The reaction progress kinetic analysis indicates that the reaction is partially inhibited by the γ‐lactone product. Initial rate measurements suggest that the reaction follows Michaelis–Menten‐type kinetics consistent with a bifunctional mechanism in which the catalyst is actively involved in both carbonyl addition and the subsequent rearrangement steps through hydrogen‐bonding interactions with the reactants or the intermediate. High‐level quantum chemical calculations strongly support a two‐step concerted mechanism in which the phosphoric acid activates the reactants or the intermediate in a synergistic manner through partial proton transfer. The catalyst simultaneously acts as a general acid, by increasing the electrophilicity of the carbonyl carbon, increases the nucleophilicity of hydrogen peroxide as a Lewis base in the addition step, and facilitates the dissociation of the OH group from the Criegee intermediate in the rearrangement step. The overall reaction is highly exothermic, and the rearrangement of the Criegee intermediate is the rate‐determining step. The observed reactivity of this catalytic B–V reaction also results, in part, from the ring strain in cyclobutanones. The sense of chiral induction is rationalized by the analysis of the relative energies of the competing diastereomeric transition states, in which the steric repulsion between the 3‐substituent of the cyclobutanone and the 3‐ and 3′‐substituents of the catalyst, as well as the entropy and solvent effects, are found to be critically important.     

A quantum chemical study of the generation of a potential prebiotic compound, cyanoacetaldehyde, and related sulfur containing species

Cyanoacetaldehyde (NCCH 2CHO), which may have played a role in the prebiotic formation of the pyrimidine bases cytosine and uracil, is formed in water solutions by addition of water to cyanoacetylene (HCC-CN), a compound that exists in interstellar space, in comets, and planetary atmospheres. A gas-phase model of the uncatalyzed addition of water to cyanoacetylene is explored by ab initio calculations at the MP2/6-311++G** level of theory. A reaction path consisting of several steps was found in these calculations, but the activation energy of the first step is relatively high, which makes it unlikely that cyanoacetaldehyde is formed in an uncatalyzed reaction. Similar calculations were also performed for the uncatalyzed reaction of water to protonated cyanoacetylene (HCCCNH (+)), a component of the interstellar medium, forming protonated cyanoacetaldehyde (HNCCH 2CHO (+)), but a high activation energy was found for this reaction as well. Moreover, the corresponding addition reactions of hydrogen sulfide (H 2S) to HCCCN, as well as to HCCCNH (+), have been explored with similar results.     

Cross-coupling reaction of alpha-chloroketones and organotin enolates catalyzed by zinc halides for synthesis of gamma-diketones

The reaction of tin enolates 1 with alpha-chloro- or bromoketones 2 gave gamma-diketones (1,4-diketones) 3 catalyzed by zinc halides. In contrast to the exclusive formation of 1,4-diketones 3 under catalytic conditions, uncatalyzed reaction of 1 with 2 gave aldol-type products 4 through carbonyl attack. NMR study indicates that the catalyzed reaction includes precondensation between tin enolates and alpha-haloketones providing an aldol-type species and their rearrangement of the oxoalkyl group with leaving halogen to produce 1,4-diketones. The catalyst, zinc halides, plays an important role in each step. The carbonyl attack for precondensation is accelerated by the catalyst as Lewis acid and the intermediate zincate promotes the rearrangement by releasing oxygen and bonding with halogen. Various types of tin enolates and alpha-chloro- and bromoketones were applied to the zinc-catalyzed cross-coupling. On the other hand, the allylic halides, which have no carbonyl moiety, were inert to the zinc-catalyzed coupling with tin enolates. The copper halides showed high catalytic activity for the coupling between tin enolates 1 and organic halides 7 to give gamma,delta-unsaturated ketones 8 and/or 9. The reaction with even chlorides proceeded effectively by the catalytic system.     

Carbocationic rearrangement of pivaloyl cation and protonated pivalaldehyde in superacid medium: A novel solution equivalent of the McLafferty rearrangement

Both pivaloyl cation in the presence of hydride donors and protonated pivalaldehyde in superacid media (both aprotic and protic) rearrange to protonated methyl isopropyl ketone involving gitionic dicationic intermediates. In our earlier studies we have found that the rearrangement of pivaladehyde to methyl isopropyl ketone occurs quantitatively in the presence of various superacidic media such as anhydrous HF, triflic acid, boron trifluoride-2,2,2-trifluoroethanol complex (BF(3).2CF(3)CH(2)OH) etc. Our present study with environmentally more benign and stable amine:HF complexes, namely pyridinium poly(hydrogen fluoride) (PPHF) (5), poly(4-vinylpyridinium) poly(hydrogen fluoride) (6), and poly(ethyleniminium) poly(hydrogen fluoride) (PEIHF) (7) shows that these modified HF equivalents can carry sufficient amount of immobilized HF and provide ample acidity for complete isomerization of pivalaldehyde to methyl isopropyl ketone. Calculations on protioformyl, acetyl and pivaloyl dications at the B3LYP/6-311 ++ G(d,p) and CCSD(T)/6-311 ++ G(d,p)//B3LYP/6-311 ++ G(d,p) levels have been performed to compare the nature of protosolvation of formyl, acetyl, pivaloyl cations and protonated pivaladehyde in superacid media. These studies further suggest protosolvation of protonated pivalaldehyde leading to gitionic dications at high acidities resulting in the carbocatioinic rearrangement. The reported carbocationic rearrangement under superacidic activation represents a novel solution chemistry equivalent of the well known gas-phase McLafferty rearrangement.     

Theoretical study of the role of solvent H2O in neopentyl and pinacol rearrangements

The neopentyl and the pinacol rearrangements as examples of Wagner-Meerwein rearrangements were investigated by the use of DFT calculations. As the first reaction, a model of neopentyl chloride (1b) and (H2O)12 was employed. In the reaction, the patterns of C--Cl scission, methyl migration, and C--OH formation were analyzed. The calculations have shown that the 2-methyl-2-butanol (6) is formed in two steps with the transient intermediate, neopentyl alcohol (3). The first step is the nucleophilic substitution reaction and is the rate-determining one. The second step is the dual migration of methyl and OH2 groups. The primary and tertiary carbocations were calculated to be absent in the neopentyl rearrangement starting from the hydrolysis. As the second reaction, the pinacol rearrangement of two substrates 2,3-dimethyl-2,3-butanediol (7) and 2,3-diphenyl-2,3-butanediol (12) was investigated. Acidic aqueous solvent was modeled by H3O+ and 12H2O. The reaction paths were promoted by a hydrogen-bond circuit of H3O+(H2O)2 and were determined as completely concerted processes. Protonated species and carbocations as intermediates also do not intervene during the pinacol rearrangement. Active functions of proton relays along the hydrogen bonds in the two rearrangements were demonstrated.     

Nukleophile Reaktionen an 2-Phenylindolenin-3-onen

Nucleophilic Reactions on 2-Phenylindolenin-3-ones Nucleophilic reagents such as 4-nitrophenylhydrazine, malonic acid derivatives, and hydrogen peroxide react with 2-phenylindolenin-3-ones giving, in the first step, addition products to the (d) N?C(2) bond. This addition can be reversible, but in most cases new rearrangement products are formed. The structure of the new compounds were elucidated by ¹H- and ¹³C-NMR spectroscopy.     

Determination of hydroxyl radical photoproduction rates in natural waters.

The photochemical formation rates of hydroxyl radicals (OH radicals) in river water and seawater were determined by a simple, rapid and sensitive benzene probe method, in which phenol formed by the reaction between benzene and photochemically-generated OH radicals was analyzed by on-line preconcentration HPLC. The OH radical formation rates from well-known OH radical sources, such as nitrate, nitrite and hydrogen peroxide, were in good agreement with those reported previously. River water samples containing high concentrations of nitrate and nitrite were found to show high OH radical formation rates. Ten to 80% of the OH radical formation in river water and seawater was due to the photolysis of nitrate and nitrite, but OH radical formation from hydrogen peroxide was negligible. The OH radical formation from unknown sources other than nitrate, nitrite and hydrogen peroxide was strongly correlated to the amount of fluorescent matter.     

Reactivity of fullerene epoxide: preparation of fullerene-fused thiirane, tetrahydrothiazolidin-2-one, and 1,3-dioxolane

The epoxide moiety in the fullerene-mixed peroxide C60(O)(OOtBu)4 1 reacts readily with aryl isocyanates ArNCS (Ar = Ph, Naph) to form both the thiirane derivative C60(S)(OOtBu)4 and fullerene-fused tetrahydrothiazolidin-2-one. The reaction of 1 with trimethylsilyl isothiocyanate TMSNCS yields the isothiocyanate derivative C60(NCS)(OH)(OOtBu)4, the isothiocyanate and hydroxyl moieties of which could be converted to a fullerene-fused tetrahydrothiazolidin-2-one ring with alumina quantitatively. Treating 1 with BF3.Et2O yields the fullerene-fused [1,3,2]-dioxoborolane derivative C60(O2BOH)(OOtBu)4. In the presence of aldehyde or acetone, BF3.Et2O catalyzes the conversion of epoxide to fullerene-fused 1,3-dioxolane derivatives. The products are characterized by spectroscopic data. Two of the compounds are also characterized by single-crystal X-ray analysis.     

Sequential Norrish Type II Photoelimination and Intramolecular Aldol Cyclization of α‐Diketones: Synthesis of Polyhydroxylated Cyclopentitols by Ring Contraction of Hexopyranose Carbohydrate Derivatives

The excitation of the innermost carbonyl of nono‐2,3‐diulose derivatives by irradiation with visible‐light initiates a sequential Norrish type II photoelimination and aldol cyclization process that finally gives polyfunctionalized cyclopentitols. The rearrangement has been confirmed by the isolation of stable acyclic photoenol intermediates that can be independently cyclized by a thermal 5‐(enolexo)‐exo‐trig uncatalyzed aldol reaction with high diastereoselectivity. In this last step, the large deuterium kinetic isotope effect found for the 1,5‐hydrogen atom transfer seems to indicate that the aldol reaction runs through a concerted pericyclic mechanism. Owing to the ready availability of pyranose sugars of various configurations, this protocol has been used to study the influence of pyranose ring‐substituents on the diastereoselectivity of the aldol cyclization reaction. In contrast with other pyranose ring contraction methodologies no transition‐metal reagents are needed and the sequential rearrangement occurs simply by using visible light and moderate heating (0 to 60 °C).     

Initial oxidation and hydroxylation of the Ge(100)-2 x 1 surface by water and hydrogen peroxide

We use density functional theory to investigate the surface chemistry of initial oxidation and hydroxylation of the Ge(100)-2 x 1 surface by water and hydrogen peroxide. Comparison of the reaction of water on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces shows that the kinetics of oxidation of the Ge(100)-2 x 1 surface with water is slower. Our calculations also show that oxidation products on the Ge(100)-2 x 1 surface are less thermodynamically stable than on Si. We also investigate two competing dissociation reactions of H2O2 on the Ge(100)-2 x 1 surface. We find that dissociative adsorption via cleavage of the OH bond is less exothermic than OO dissociation. Furthermore, interdimer OO dissociation has a lower activation barrier than interdimer or intradimer OH dissociation, although interdimer dissociation products are found to be less stable compared than those formed from intradimer dissociation reactions. Finally, we find that the oxidation products formed from hydrogen peroxide are more stable than those formed from water.     

Kinetic studies on the reaction of N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetate chromium(III) with hydrogen peroxide

The rate of reaction of Cr(III)L (L=N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetate) with hydrogen peroxide was studied in aqueous media and was found to yield Cr(VI) over the temperature range of 25–41 °C. The reaction was followed spectrophotometrically at 396 nm under pseudo-first order conditions with hydrogen peroxide in a large excess. The reaction follows first-order kinetics in Cr(III). The dependence of the rate constant on hydrogen peroxide concentration is attributed to the formation of an intermediate between the monohydroxy chromium(III) complex and hydrogen peroxide which decomposes in the rate determining step. At high hydrogen peroxide concentration, the order is changed from first to zero order.The values of the intramolecular electron transfer rate constant, the formation constant of the intermediate complex and the activation parameters were calculated.     

State-to-state dynamics of the Cl + CH3OH --> HCl + CH2OH reaction

Molecular chlorine, methanol, and helium are co-expanded into a vacuum chamber using a custom designed "late-mixing" nozzle. The title reaction is initiated by photolysis of Cl2 at 355 nm, which generates monoenergetic Cl atoms that react with CH3OH at a collision energy of 1960 +/- 170 cm(-1) (0.24 +/- 0.02 eV). Rovibrational state distributions of the nascent HCl products are obtained via 2 + 1 resonance enhanced multiphoton ionization, center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the CH3OH co-products is deduced by measuring the spatial anisotropy of the HCl products. The majority (84 +/- 7%) of the HCl reaction products are formed in HCl(v = 0) with an average rotational energy of [Erot] = 390 +/- 70 cm(-1). The remaining 16 +/- 7% are formed in HCl(v = 1) and have an average rotational energy of [Erot] = 190 +/- 30 cm(-1). The HCl(v = 1) products are primarily forward scattered, and they are formed in coincidence with CH2OH products that have little internal energy. In contrast, the HCl(v = 0) products are formed in coincidence with CH2OH products that have significant internal energy. These results indicate that two or more different mechanisms are responsible for the dynamics in the Cl + CH3OH reaction. We suggest that (1) the HCl(v = 1) products are formed primarily from collisions at high impact parameter via a stripping mechanism in which the CH2OH co-products act as spectators, and (2) the HCl(v = 0) products are formed from collisions over a wide range of impact parameters, resulting in both a stripping mechanism and a rebound mechanism in which the CH2OH co-products are active participants. In all cases, the reaction of fast Cl atoms with CH3OH is with the hydrogen atoms on the methyl group, not the hydrogen on the hydroxyl group.     

Catalytic properties of the polyoxometalate [Ti2(OH)2As2W19O67(H2O)]8? in selective oxidations with hydrogen peroxide

The catalytic properties of the sandwich polyoxometalate [Ti₂(OH)₂As₂W₁₉O₆₇(H₂O)]⁸⁻, which contains two (B-α-As^IIIW₉O₃₃) fragments linked together by a “belt” consisting of one octahedral WO(H₂O)⁴⁺ and two square-pyramidal Ti(OH)³⁺ groups, have been investigated in the selective liquid-phase oxidation of organic compounds by aqueous hydrogen peroxide. The polyoxometalate shows high catalytic activity and selectivity in the oxidation of alkenes, alcohols, diols, and thioethers. The composition of the reaction products indicates that hydrogen peroxide is activated via a heterolytic mechanism.     

The monooxidation of ethylene with hydrogen peroxide on the per-FTPhPFe<Superscript>3+</Superscript>OH/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> biomimetic

The gas-phase monooxidation of ethylene by hydrogen peroxide on a biomimetic heterogeneous catalyst (per-FTPhPFe³⁺OH/Al₂O₃) was studied under comparatively mild conditions. The biomimetic oxidation of ethylene with hydrogen peroxide was shown to be coherently synchronized with the decomposition of H₂O₂. Depending on reaction medium conditions, one of two desired products was formed, either ethanol or acetaldehyde. The kinetics and probable mechanism of ethylene transformation were studied.     

Impact of water on the OH + HOCl reaction

The effect of a single water molecule on the OH + HOCl reaction has been investigated. The naked reaction, the reaction without water, has two elementary reaction paths, depending on how the hydroxyl radical approaches the HOCl molecule. In both cases, the reaction begins with the formation of prereactive hydrogen bond complexes before the abstraction of the hydrogen by the hydroxyl radical. When water is added, the products of the reaction do not change, and the reaction becomes quite complex yielding six different reaction paths. Interestingly, a geometrical rearrangement occurs in the prereactive hydrogen bonded region, which prepares the HOCl moiety to react with the hydroxyl radical. The rate constant for the reaction without water is computed to be 2.2 × 10(-13) cm(3) molecule(-1) s(-1) at room temperature, which is in good agreement with experimental values. The reaction between ClOH···H(2)O and OH is estimated to be slower than the naked reaction by 4-5 orders of magnitude. Although, the reaction between ClOH and the H(2)O···HO complex is also predicted to be slower, it is up to 2.2 times faster than the naked reaction at altitudes below 6 km. Another intriguing finding of this work is an interesting three-body interchange reaction that can occur, that is HOCl + HO···H(2)O → HOCl···H(2)O + OH.     

Computational characterization of the role of the base in the Suzuki-Miyaura cross-coupling reaction

The role of the base in the transmetalation step of the Suzuki-Miyaura cross-coupling reaction is analyzed computationally by means of DFT calculations with the Becke3LYP functional. The model system studied consists of Pd(CH=CH2)(PH3)2Br as the starting catalyst complex, CH2=CHB(OH)2 as the organoboronic acid, and OH- as the base. The two main mechanistic proposals, consisting of the base attacking first either the palladium complex or the organoboronic acid, are evaluated through geometry optimization of the corresponding intermediates and transition states. Supplementary calculations are carried out on the uncatalyzed reaction and on a process where the starting complex is Pd(CH=CH2)(PH3)2(OH). These calculations, considered together with available experimental data, strongly suggest that the main mechanism of transmetalation in the catalytic cycle starts with the reaction of the base and the organoboronic acid.     

Theoretical study of the reactions CF3CH2OCHF2 + OH/Cl and its product radicals and parent ether(CH3CH2OCH3) with OH

A dual-level direct dynamic method is employed to study the reaction mechanisms of CF3CH2OCHF2 (HFE-245fa2; HFE-245mf) with the OH radicals and Cl atoms. Two hydrogen abstraction channels and two displacement processes are found for each reaction. For further study, the reaction mechanisms of its products (CF3CH2OCF2 and CF3CHOCHF2) and parent ether CH3CH2OCH3 with OH radical are investigated theoretically. The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at the B3LYP/6-311G(d,p) level. The energetic information along the MEPs is further refined at the G3(MP2) level of theory. For reactions CF3CH2OCHF2 + OH/Cl, the calculation indicates that the hydrogen abstraction from --CH2-- group is the dominant reaction channel, and the displacement processes may be negligible because of the high barriers. The standard enthalpies of formation for the reactant CF3CH2OCHF2, and two products CF3CH2OCHF2 and CF3CHOCHF2 are evaluated via group-balanced isodesmic reactions. The rate constants of reactions CF3CH2OCHF2 + OH/Cl and CH3CH2OCH3 + OH are estimated by using the variational transition state theory over a wide range of temperature (200-2000 K). The agreement between the theoretical and experimental rate constants is good in the measured temperature range. From the comparison between the rate constants of the reactions CF3CH2OCHF2 and CH3CH2OCH3 with OH, it is shown that the fluorine substitution decreases the reactivity of the C--H bond.     

具有酰胺键的离子负载双(三氟乙酰氧基)碘苯对乙酰苯胺类化合物的乙酰氧基化反应

以氯乙酰氯、N-乙基对碘苯胺为原料,经酰化反应合成了N-乙基-N-(4-碘苯基)氯乙酰胺,接着与1-甲基咪唑反应,随后在水溶液中和四氟硼酸钠发生阴离子交换,合成了四氟硼酸1-甲基-3-(N-乙基-4-碘苯氨基甲酰甲基)咪唑鎓盐,该四氟硼酸盐在过氧化氢/三氟乙酸酐体系中氧化得到一种新的离子负载的双(三氟乙酰氧基)碘苯试剂四氟硼酸(1-甲基-3-{N-乙基-4-[双(三氟乙酰氧基)碘]苯氨基甲酰甲基}咪唑鎓盐)。所合成的碘苯试剂没有吸湿性,在空气中长期放置不变质。以这种新试剂为氧化剂,室温下乙酰苯胺及其衍生物与乙酸区域选择性地发生乙酰氧基化反应,产率较高。该离子负载型碘苯试剂可回收后再生,而再生试剂用于乙酰氧基化反应时活性几乎保持不变。     

Diastereoselective addition of gamma-substituted allylic nucleophiles to ketones: highly stereoselective synthesis of tertiary homoallylic alcohols using an allylic tributylstannane/stannous chloride system

The diastereoselective addition of gamma-substituted allylic nucleophiles to ketones has been accomplished to give tertiary homoallylic alcohols. The reaction of tributylcinnamyltin 1a with simple ketones 2 in the presence of stannous chloride (SnCl(2)) gave the tertiary homoallylic alcohols 3, which include the anti form (based on Ph and OH), with high diastereoselectivity. In the reaction course, transmetalation of tributylcinnamyltin 1a with SnCl(2) proceeds to form an active nucleophile which is tentatively considered to be a cinnamyltin(II) species. A cyclic transition state A is favorable because the chlorinated tin(II) center is highly capable of accepting ligands. The other diastereomers (syn form) 4 were obtained in the reaction of tributylcinnamyltin 1a with ketones 2 by the use of BF(3) x OEt(2) instead of SnCl(2). This reaction proceeds through an acyclic transition state in which BF(3) acts as a Lewis acid for activation of ketones. When 3-tributylstannylcyclohexene 1b or 3-tributylstannylcyclopentene 1c was used with SnCl(2), high diastereoselective formation of the corresponding homoallylic alcohols 6 which have the syn form (based on ring chain and OH) was observed. The selectivity was also explained by the cyclic transition state B. When tributylcrotyltin 1d or 1e was used, the stereochemistry of the products depends on the additives (SnCl(2) or BF(3) x OEt(2)), substituents of ketones, and reaction temperature. It is interesting that those additives compensate for each other in terms of diastereoselective alkylation. The alkylation of alpha-alkoxy, aryloxy, or hydroxyketones 16 was achieved in extremely high selectivity using an allylic tributyltin 1a-c/SnCl(2) system. The chelation by carbonyl and beta-oxygens provides a rigid transition state (E or F) for selective reactions. It is noted that the hydroxyketone can be used without protection in this reaction system. The relative stereochemistry of the produced tertiary homoallylic alcohols was determined on the basis of X-ray analyses.     

Dioxaborirane: a highly reactive peroxide that is the likely intermediate in borate catalysed electrophilic reactions of hydrogen peroxide in alkaline aqueous solution

This paper reports on a kinetic and theoretical study into the borate mediated reaction of dimethyl sulfide with hydrogen peroxide in both acid and alkaline conditions. At high pH, whilst the kinetic data is consistent with the catalytic species being monoperoxoborate, formed from the rapid equilibrium between hydrogen peroxide and boric acid, DFT calculations show that this species is in fact less reactive than hydrogen peroxide, requiring us to seek an alternative catalytic mechanism. DFT provides an important insight for this, showing that although boric acid and peroxoboric acid are primarily Lewis acids, they can exhibit a small degree of Br?nsted acidity, allowing, respectively, the B(O)(OH)(2)(-) and HOOB(OH)(O)(-) anions to exist in small concentrations. Whilst the peroxoborate anion, HOOB(OH)(O)(-), is predicted to have only marginal catalytic activity, its tautomer, dioxaborirane, (HO)(2)BO(2)(-), a three membered cyclic peroxide, has a very low activation barrier of 2.8 kcal/mol. Hence, even though dioxaborirane is likely to be present in very low concentrations, it is still sufficiently reactive for overall rate enhancements to be observed for this system. This is the first literature report of this species. The observed low selectivity observed for borate catalysed reactions of hydrogen peroxide with a range of substituted phenyl methyl sulfides in our previous study (D. M. Davies, M. E. Deary, K. Quill and R. A. Smith, Chem.-Eur. J. 2005, 11, 3552-3558) is further evidence in favour of a highly reactive catalytic species. At low pH, kinetic data shows that borate catalyses the reaction between hydrogen peroxide and dimethyl sulfide; this is supported by DFT calculations that predict peroxoboric acid to be an effective catalytic intermediate, with an energy barrier of 7.4 kcal mol(-1) compared to 10.1 kcal mol(-1) for the uncatalysed system. Nevertheless, the overall contribution of this pathway is small because of the unfavourable equilibrium between hydrogen peroxide and boric acid to form peroxoboric acid.