Relative reactivity of peracids versus dioxiranes (DMDO and TFDO) in the epoxidation of alkenes. A combined experimental and theoretical analysis

Mechanistic Details of the Sharpless Epoxidation of Allylic Alcohols—A Combined URVA and Local Mode Study

In this work, we investigated the catalytic effects of a Sharpless dimeric titanium (IV)–tartrate–diester catalyst on the epoxidation of allylalcohol with methyl–hydroperoxide considering four different orientations of the reacting species coordinated at the titanium atom (reactions R1–R4) as well as a model for the non-catalyzed reaction (reaction R0). As major analysis tools, we applied the URVA (Unified Reaction Valley Approach) and LMA (Local Mode Analysis), both being based on vibrational spectroscopy and complemented by a QTAIM analysis of the electron density calculated at the DFT level of theory. The energetics of each reaction were recalculated at the DLPNO-CCSD(T) level of theory. The URVA curvature profiles identified the important chemical events of all five reactions as peroxide OO bond cleavage taking place before the TS (i.e., accounting for the energy barrier) and epoxide CO bond formation together with rehybridization of the carbon atoms of the targeted CC double bond after the TS. The energy decomposition into reaction phase contribution phases showed that the major effect of the catalyst is the weakening of the OO bond to be broken and replacement of OH bond breakage in the non-catalyzed reaction by an energetically more favorable TiO bond breakage. LMA performed at all stationary points rounded up the investigation (i) quantifying OO bond weakening of the oxidizing peroxide upon coordination at the metal atom, (ii) showing that a more synchronous formation of the new CO epoxide bonds correlates with smaller bond strength differences between these bonds, and (iii) elucidating the different roles of the three TiO bonds formed between catalyst and reactants and their interplay as orchestrated by the Sharpless catalyst. We hope that this article will inspire the computational community to use URVA complemented with LMA in the future as an efficient mechanistic tool for the optimization and fine-tuning of current Sharpless catalysts and for the design new of catalysts for epoxidation reactions.

A molecular electron density theory study of the mechanism, chemo- and stereoselectivity of the epoxidation reaction of R-carvone with peracetic acid

The epoxidation reaction of R-carvone 8 with peracetic acid 9 has been studied within the molecular electron density theory at the B3LYP/6-311(d,p) computational level. The chemo- and stereoisomeric reaction paths involving the two C-C double bonds of R-carvone 8 have been studied. DFT calculations account for the high chemoselectivity involving the C-C double bond of the isopropenyl group and the low diastereoselectivity, in complete agreement with the experimental outcomes. The Baeyer-Villiger reaction involving the carbonyl group of R-carvone 8 has also been analysed. A bonding evolution theory analysis of the epoxidation reaction shows the complexity of the bonding changes taking place along this reaction. Formation of the oxirane ring takes place asynchronously at the end of the reaction by attack of anionic oxygen on the two carbons of the isopropenyl C-C double bond.

Vanadyl β-tetrabromoporphyrin: synthesis, crystal structure and its use as an efficient and selective catalyst for olefin epoxidation in aqueous medium

We hereby report the synthesis, characterization and catalytic applications in the epoxidation of alkenes by a vanadyl porphyrin having bulky bromo substituents at the β-positions viz. vanandyltetrabromotetraphenylporphyrin (1). The synthesized porphyrin was characterized by various spectroscopic techniques like UV-visible, FT-IR, EPR, MALDI-TOF mass spectrometry and single crystal X-ray analysis. Porphyrin 1 has a nonplanar structure as indicated by its X-ray structure, DFT and electrochemical studies. 1 was analyzed for its catalytic application in the epoxidation of various alkenes. The catalytic reactions were carried out in CH₃CN/H₂O mixture in 3 : 1 (v/v) ratio. 1 displayed good efficiency in terms of mild reaction conditions, lower reaction temperature and minimal catalyst amount consumption. 1 exhibited excellent selectivity, high conversion efficiency and huge TOF (7600–9800 h⁻¹) in a significantly low reaction time of 0.5 h. Catalyst 1 was regenerated at the end of various catalytic cycles making it reusable and industrially important.

We have synthesized β-tetrabromo-meso-tetraphenylporphyrinatooxidovanadium(iv) (VOTPPBr₄) which possesses high thermal stability and nonplanar macrocyclic core. Further, it was utilized for selective epoxidation of olefins in good yields with very high TOF numbers.

In situ epoxide generation by dimethyldioxirane oxidation and the use of epichlorohydrin in the flow synthesis of a library of β-amino alcohols

The flow coupling of epichlorohydrin with substituted phenols, while efficient, limits the nature of the epoxide available for the development of focused libraries of β-amino alcohols. This limitation was encountered in the production of analogues of 1-(4-nitrophenoxy)-3-((2-((4-(trifluoromethyl)pyrimidin-2-yl)amino)ethyl)amino)propan-2-ol 1, a potential antibiotic lead. The in situ (flow) generation of dimethyldoxirane (DMDO) and subsequent flow olefin epoxidation abrogates this limitation and afforded facile access to structurally diverse β-amino alcohols. Analogues of 1 were readily accessed either via (i) a flow/microwave hybrid approach, or (ii) a sequential flow approach. Key steps were the in situ generation of DMDO, with olefin epoxidation in typically good yields and a flow-mediated ring opening aminolysis to form an expanded library of β-amino alcohols 1 and 10a-18g, resulting in modest (11a, 21%) to excellent (12g, 80%) yields. Alternatively flow coupling of epichlorohydrin with phenols 4a-4m (22%-89%) and a Bi(OTf)₃ catalysed microwave ring opening with amines afforded a select range of β-amino alcohols, but with lower levels of aminolysis regiocontrol than the sequential flow approach.

Synthesis of Natural O-Linked Carba-Disaccharides, (+)- and (-)-Pericosine E, and Their Analogues as α-Glucosidase Inhibitors

Pericosine E (6), a metabolite of Periconia byssoides OUPS-N133 was originally isolated from the sea hare Aplysia kurodai, which exists as an enantiomeric mixture in nature. The enantiospecific syntheses of both enantiomers of Periconia byssoides OUPS-N133 has been achieved, along with six stereoisomers, using a common simple synthetic strategy. For these efficient syntheses, highly regio- and steroselective processes for the preparation of bromohydrin and anti-epoxide intermediates were applied. In order to access the unique O-linked carbadisaccharide structure, coupling of chlorohydrin as a donor and anti-epoxide as an acceptor was achieved using catalytic BF₃·Et₂O. Most of the synthesized compounds exhibited selectively significant inhibitory activity against α-glycosidase derived from yeast. The strongest analog showed almost 50 times the activity of the positive control, deoxynojirimycin.

Computational investigation into the gas-phase ozonolysis of the conjugated monoterpene α-phellandrene

Reaction with ozone is a major atmospheric sink for α-phellandrene, a monoterpene found in both indoor and outdoor environments, however experimental literature concerning the reaction is scarce. In this study, high-level G4(MP2) quantum chemical calculations are used to theoretically characterise the reaction of ozone with both double bonds in α-phellandrene for the first time. Results show that addition of ozone to the least substituted double bond in the conjugated system is preferred. Following addition, thermal and chemically activated unimolecular reactions, including the so-called hydroperoxide and ester or 'hot' acid channels, and internal cyclisation reactions, are characterised to major first generation products. Conjugation present in α-phellandrene allows two favourable Criegee intermediate reaction pathways to proceed that have not previously been considered in the literature; namely a 1,6-allyl resonance stabilised hydrogen shift and intramolecular dioxirane isomerisation to an epoxide. These channels are expected to play an important role alongside conventional routes in the ozonolysis of a-phellandrene. Computational characterisation of the potential energy surface thus provides insight into this previously unstudied system, and will aid future mechanism development and experimental interpretation involving α-phellandrene and structurally similar species, to which the results are expected to extend.

Electrophilic Addition to Alkenes: The Relation between Reactivity and Enthalpy of Hydrogenation: Regioselectivity is Determined by the Stability of the Two Conceivable Products

Although electrophilic addition to alkenes has been well studied, some secrets still remain. Halogenations, hydrohalogenations, halohydrin formations, hydrations, epoxidations, other oxidations, carbene additions, and ozonolyses are investigated to elucidate the relation of alkene reactivities with their enthalpies of hydrogenation (ΔHhyd ). For addition of electrophiles to unconjugated hydrocarbon alkenes, ln(k) is a linear function of ΔHhyd , where k is the rate constant. Linear correlation coefficients are about 0.98 or greater. None of the many previously proposed correlations of ln(k) with the properties of alkenes or with linear free-energy relationships match the generality and accuracy of the simple linear relationship found herein. A notable exception is acid-catalyzed hydration in water or in solvents stabilizing relatively stable carbocation intermediates (e.g., tertiary, benzylic, or allylic). (13) C NMR chemical shifts of the two alkene carbons also predict regioselectivity. These effects have not been noted previously and are operative in general, including addition to heteroatom-substituted alkenes.

Synthesis of marine natural product (-)-pericosine E

The first synthesis of (-)-pericosine E (6), a metabolite of the Periconia byssoides OUPS-N133 isolated originally from the sea hare Aplysia kurodai, has been achieved. Efficient and regioselective synthetic procedures for the synthesis of key intermediates, anti- and syn-epoxides 9 and 10, were developed using an anti-epoxidation of diene 12 with TFDO and a bromohydrination of 12 with NBS in CH(3)CN/H(2)O (2:3), respectively. In addition, comparison of the specific optical rotations between synthetic 6 and natural 6 elucidated that the naturally preferred enantiomer of pericosine E had the same absolute configuration as (-)-6 synthesized from chlorohydrin (-)-8 and anti-epoxide (+)-9.

Asymmetric organocatalytic epoxidations: reactions, scope, mechanisms, and applications

Chiral epoxides serve as versatile building blocks in the synthesis of complex organic frameworks. The high strain imposed by the three-membered ring system makes epoxides prone to a variety of nucleophilic ring-opening reactions. Since the development of the Sharpless epoxidation, there have been many important contributions and advances in this area. With the rapid development of the field of asymmetric organocatalysis, a wide range of organocatalysts is now able to catalyze the epoxidation of broad class of unsaturated carbonyl compounds. In this Minireview, recent progress in the development of organocatalytic asymmetric epoxidation methods, the proposed mechanisms of these reactions and their applications as intermediates is reported.

Inverse solvent effects in the heterogeneous and homogeneous epoxidation of cis-2-heptene with [2-percarboxyethyl]-functionalized silica and meta-chloroperbenzoic acid

The organized solvent layer on the solid surface determines the reaction rate in the heterogeneous epoxidation of cis-2-heptene.

Stereoelectronic factors in the stereoselective epoxidation of glycals and 4-deoxypentenosides

Glycals and 4-deoxypentenosides (4-DPs), unsaturated pyranosides with similar structures and reactivity profiles, can exhibit a high degree of stereoselectivity upon epoxidation with dimethyldioxirane (DMDO). In most cases, the glycals and their corresponding 4-DP isosteres share the same facioselectivity, implying that the pyran substituents are largely responsible for the stereodirecting effect. Fully substituted dihydropyrans are subject to a "majority rule", in which the epoxidation is directed toward the face opposite to two of the three groups. Removing one of the substituents has a variable effect on the epoxidation outcome, depending on its position and also on the relative stereochemistry of the remaining two groups. Overall, we observe that the greatest loss in facioselectivity for glycals and 4-DPs is caused by removal of the C3 oxygen, followed by the C5/anomeric substituent, and least of all by the C4/C2 oxygen. DFT calculations based on polarized-π frontier molecular orbital (PPFMO) theory support a stereoelectronic role for the oxygen substituents in 4-DP facioselectivity, but less clearly so in the case of glycals. We conclude that the anomeric oxygen in 4-DPs contributes toward a stereoelectronic bias in facioselectivity whereas the C5 alkoxymethyl in glycals imparts a steric bias, which at times can compete with the stereodirecting effects from the other oxygen substituents.

Insights into the formation and isomerization of the benzene metabolite muconaldehyde and related molecules: comparison of computational and experimental studies of simple, benzo-annelated, and bridged 2,3-epoxyoxepins

2,8-Dioxabicyclo[5.1.0]octa-3,5-diene ("2,3-epoxyoxepin") has been postulated as an intermediate in ring-opening metabolism of benzene. Density functional theory (B3LYP/6-31G*) is employed to study the activation and reaction energies for ring-opening isomerization of 2,3-epoxyoxepin, its 4,5-benzo derivative, and its 3,6-hexamethylene derivative. The results are compared with published experimental data. The markedly different fates of these three molecules suggest a means for testing the postulated metabolic pathway.

Iminohydantoin lesion induced in DNA by peracids and other epoxidizing oxidants

The oxidation of guanine to 5-carboxamido-5-formamido-2-iminohydantoin (2-Ih) is shown to be a major transformation in the oxidation of the single-stranded DNA 5-mer d(TTGTT) by m-chloroperbenzoic acid (m-CPBA) and dimethyldioxirane (DMDO) as a model for peracid oxidants and in the oxidation of the 5-base pair duplex d[(TTGTT).(AACAA)] with DMDO. 2-Ih has not been reported as an oxidative lesion at the level of single/double-stranded DNA or at the nucleoside/nucleotide level. The lesion is stable to DNA digestion and chromatographic purification, suggesting that 2-Ih may be a stable biomarker in vivo. The oxidation products have been structurally characterized and the reaction mechanism has been probed by oxidation of the monomeric species dGuo, dGMP, and dGTP. DMDO selectively oxidizes the guanine moiety of dGuo, dGMP, and dGTP to 2-Ih, and both peracetic and m-chloroperbenzoic acids exhibit the same selectivity. The presence of the glycosidic bond results in the stereoselective induction of an asymmetric center at the spiro carbon to give a mixture of diastereomers, with each diastereomer in equilibrium with a minor conformer through rotation about the formamido C-N bond. Labeling studies with [(18)O(2)]-m-CPBA and H(2)(18)O to determine the source of the added oxygen atoms have established initial epoxidation of the guanine 4-5 bond with pyrimidine ring contraction by an acyl 1,2-migration of guanine carbonyl C6 to form a transient dehydrodeoxyspiroiminodihydantoin followed by hydrolytic ring-opening of the imidazolone ring. Consistent with the proposed mechanism, no 8-oxoguanine was detected as a product of the oxidations of the oligonucleotides or monomeric species mediated by DMDO or the peracids. The 2-Ih base thus appears to be a pathway-specific lesion generated by peracids and possibly other epoxidizing agents and holds promise as a potential biomarker.

Quantitative DFT modeling of the enantiomeric excess for dioxirane-catalyzed epoxidations

Herein we report the first fully quantum mechanical study of enantioselectivity for a large data set. We show that transition state modeling at the UB3LYP-DFT/6-31G* level of theory can accurately model enantioselectivity for various dioxirane-catalyzed asymmetric epoxidations. All the synthetically useful high selectivities are successfully "predicted" by this method. Our results hint at the utility of this method to further model other asymmetric reactions and facilitate the discovery process for the experimental organic chemist. Our work suggests the possibility of using computational methods not simply to explain organic phenomena, but also to predict them quantitatively.

Ethylene receptor antagonists: strained alkenes are necessary but not sufficient

Plants use ethylene as a hormone to control many physiological processes. Ethylene perception involves its binding to an unusual copper-containing, membrane-bound receptor. Inhibitors of ethylene action are valuable to study signaling and may have practical use in horticulture. Past investigation of alkene ligands for this receptor has identified strain as the key factor in antagonism of ethylene binding and action, consistent with known trends in metal-alkene complex stability. However, in this work, this principle could not be extended to other alkenes, prompting development of the proposal that a ring-opening reaction accounts for the unusual potency of cyclopropene ethylene antagonists. Another factor augmenting the affinity of alkenes for the copper binding site is pyramidalization, as in trans-cycloalkenes. The enantiomeric selectivity in the binding of one such alkene to the ethylene receptor demonstrates its protein-composed asymmetric environment.

The effect of carbonyl substitution on the strain energy of small ring compounds and their six-member ring reference compounds

High level ab initio calculations have been applied to the estimation of ring strain energies (SE) of a series of three- and six-member ring compounds. The SE of cyclohexane has been estimated to be 2.2 kcal/mol at the CBS-APNO level of theory. The SE of cyclopropane has been increased to 28.6 kcal/mol after correction for the one-half of the SE of cyclohexane. The SEs of a series of carbonyl-containing three-member ring compounds have been estimated at the CBS-Q level by their combination with cyclopropane to produce a six-member ring reference compound. The SEs of cyclopropanone (5), the simplest alpha-lactone (6) [oxiranone], and alpha-lactam (7) [aziridinone] have been predicted to be 49, 47, and 55 kcal/mol, respectively, after correction for the SE of the corresponding six-member ring reference compound. The SEs of cyclohexanone, delta-valerolactone, and delta-valerolactam have been estimated to be 4.3, 11.3, and 5.1 kcal/mol, respectively. Marked increases in the SE of silacyclopropane and siladioxirane have been established, while significant decreases in the SEs of phosphorus, sulfur, dioxa- and diaza-containing three-member ring compounds were observed. The ring strain energies of the hydrocarbons (but not heterocycles) exhibit a strong correlation with their C-H bond dissociation energies.

Generation of mono- and bis-dioxiranes from 2,3-butanedione

Biacetyl reacts with oxone to give bis-dioxirane [3,3'-dimethyl-3,3'-bidioxirane, 3B] and mono-dioxirane [1-(3-methyl-dioxiran-3-yl)ethanone, 3A)]. Bis-dioxirane 3B is formed when two oxygens are incorporated into biacetyl, while mono-dioxirane 3A incorporated only one. A greater stability is observed in 3B compared to 3A, which is attributed to an alpha-dioxiranyl (anomeric) effect in the former. In contrast, 3A suffers from a destabilizing pi-electron withdrawing effect from the adjacent carbonyl group.

Highly enantioselective CH oxidation of vic-Diols with Shi's oxazolidinone dioxiranes

[reaction: see text] Through an analogical study of the transition states of CH oxidation and asymmetric epoxidation of terminal alkenes, the first dioxirane-mediated catalytic highly enantioselective CH oxidation method was realized with Shi's oxazolidinone ketone derivatives. Very good enantioselectivity (up to 92% ee) may be obtained for both asymmetrization of meso vic-diols and kinetic resolution of racemic vic-diols.

Peroxy acid epoxidation of acyclic allylic alcohols. Competition between s-trans and s-cis peroxy acid conformers

[Reaction: see text]. RB3LYP calculations, reported here, indicate that peroxy acid s-cis conformer is more stable than its s-trans counterpart, in agreement with experimental data. Difference in stability is the highest in the gas phase, but it falls considerably on going from the gas phase to moderately polar solvent. In the case of peroxy formic acid, the enthalpy (free energy) difference is about 3.4 (2.5) kcal/mol, respectively, in the gas phase but decreases to 1.2 (0.6) kcal/mol in dichloromethane solution. Introduction of an alkyl or aryl substituent on the peroxy acid, that is, on passing to peroxy acetic, peroxy benzoic (PBA), and m-chloroperoxy benzoic acid (MCPBA), adds a further significant (1.0-1.5 kcal/mol) favor to the s-cis isomer. RB3LYP/6-31+G(2d,p) calculations on the epoxidation of 2-propenol with peroxy formic and peroxy benzoic acids, respectively, suggest that the less stable peroxy acid s-trans conformer can compete with the more stable s-cis form in epoxidation reaction of these substrates. Transition structures arising from s-trans peroxy acids ("trans" TSs) retain both the well-established, for "cis" TS, perpendicular orientation of the O-H peroxy acid bond relative to the C=C bond and the one-step oxirane ring formation. These TSs collapse to the final epoxide via a 1,2-H shift at variance with the 1,4-H transfer of the classical Bartlett's "cis" mechanism. The "trans" reaction pathways have a higher barrier in the gas phase than the "cis" reaction channels, but in moderately polar solvents they become competitive. In fact, the "trans" TSs are always significantly more stabilized than their "cis" counterparts by solvation effects. Calculations also suggest that going from peroxy formic to peroxy benzoic acid should slightly disfavor the "trans" route relative to the "cis" one, reflecting, in an attenuated way, the decrease in the peroxy acid s-trans/s-cis conformer ratio. The predicted behavior for MCPBA parallels that of PBA acid.

The Role of Asynchronous Bond Formation in the Diastereoselective Epoxidation of Cyclic Enol Ethers: A Density Functional Theory Study

Density functional theory (DFT) (Becke3LYP functional and the D95 basis set) was used to study the influence of substitution on the dimethyldioxirane epoxidation reaction of six- and seven-membered cyclic enol ethers. In agreement with our previously reported experimental results, the calculations predict that substitution on the cyclic enol ether influences the level of diastereoselectivity. Apparent only from the calculations is that the degree of synchronicity in the transition state is important in the diastereoselectivity.

Predicting Facile Epoxidation of the Diamond (100) Surface by Dioxiranes and Subsequent Ring-Opening Reactions with Nucleophiles

By means of density functional theory coupled with effective cluster models, we have theoretically predicted the viability of epoxidation of the diamond (100) surface by organic dioxiranes. In addition, subsequent ring-opening reactions of the as-formed epoxide surface species with some nucleophiles, including water, ammonia, and alcohol, have also been explored. The facile epoxidation of diamond (100) by dioxiranes presents a new alternative for oxidation of the diamond (100) surface. More importantly, the as-formed epoxide-like surface species would be a useful springboard for further functionalizations of the diamond surface given the well-known abundant chemistry of organic epoxides. Therefore, this approach provides another new route to chemical functionalization of the diamond surface, which is potentially useful for leading to the improvement of diamond behavior and constructing new hybrid diamond-based materials for wide potential applications in many fields. In perspective, implications for other theoretical work are also discussed.

New paradigms for the peroxy acid epoxidation of CC double bonds: the role of the peroxy acid s-trans conformer and of the 1,2-H transfer in the epoxidation of cyclic allylic alcohols

RB3LYP calculations, on reaction of performic acid with cyclic allylic alcohols, demonstrate that the less stable s-trans conformer of peroxy acids can be involved in epoxidations of C=C bonds. Transition structures (TSs) arising from s-trans performic acid retain some of the well-established characteristics of the TSs of the s-cis isomer such as the perpendicular orientation of the O-H peroxy acid bond relative to the C=C bond and a one-step oxirane ring formation. These TSs are very asynchronous but collapse directly (without formation of any intermediate) to the final epoxide-peroxy acid complex via a 1,2-H shift. Thus, our findings challenge the traditional mechanism of peroxy acid epoxidation of C=C bonds by demonstrating that the involvement of the s-trans isomer opens an alternative one-step reaction channel characterized by a 1,2-H transfer. This novel reaction pathway can even overcome, in the case of the reaction of cyclic allylic alcohols in moderately polar solvents (e.g., in dichloromethane), the classical Bartlett's mechanism that is based on the s-cis peroxy acid form and that features a 1,4-H shift. However, the latter mechanism remains strongly favored for the epoxidation of normal alkenes.

Epoxidation of unfunctionalized olefins by Mn(salen) catalyst using organic peracids as oxygen source: a theoretical study

The mechanism and origin of asymmetric induction in the Mn(III)(salen)-catalyzed epoxidation by peracetic acid have been elucidated by the density functional [Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional (B3LYP)] method in two different regimes: with and without an axial ligand. The acylperoxo complexes of Mn(II,III,IV) in cisON cisNO and trans geometrical configurations cannot compete with the catalyst-free Prilezhaev epoxidation. Instead, oxo species perform epoxidation following the O-O bond cleavage in the acylperoxo complexes. The epoxidation may proceed in a concerted and/or radical-mediated stepwise manner. The actual mechanism of the epoxidation depends on the electronic and oxidation state of the oxo species and the nature of the axial ligand. The olefin can approach the reactive MnO fragment of both cis and trans-l-isomers of the plain oxo species along multiple distinct directions: native approaches. The native approaches are used to rationalize the inversion of the absolute configuration of the product epoxide due to the axial ligand.

Model Studies onp-Hydroxybenzoate Hydroxylase. The Catalytic Role of Arg-214 and Tyr-201 in the Hydroxylation Step

A model C-(4a)-flavinhydroperoxide (FlHOOH) is described that contains the tricyclic isoalloxazine moiety, the C-4a-hydroperoxide functionality, and a beta-hydroxyethyl group to model the effect of the 2'-OH group of the ribityl side chain of native FADHOOH. The electronic structures of this reduced flavin (H(3)()Fl(red)()), its N1 anion (H(2)()Fl(red)()(-)()), oxidized flavin (HFl(ox)()), and FlHOOH have been fully optimized at the B3LYP/ 6-31+G(d,p) level of theory. This model C-4a-flavinhydroperoxide is used to describe the transition state for the key step in the paradigm aromatic hydroxylase, p-hydroxybenzoate hydroxylase (PHBH): the oxidation of p-hydroxybenzoate (p-OHB). The Tyrosine-201 residue in PHBH is modeled by phenol, and Arginine-214 is modeled by guanidine. Electrophilic aromatic substitution proceeds by an S(N)2-like attack of the aromatic sextet of p-OHB phenolate anion on the distal oxygen of FlHOOH 3. The transition structure for oxygen atom transfer is fully optimized [B3LYP/6-31+G(d,p)] and has a classical activation barrier of 24.9 kcal/mol. These data suggest that the role of the Tyr-201 is to orient the p-OHB substrate and to properly align it for the oxygen transfer step. Although the negatively charged phenolate oxygen does activate the C-3 carbon of p-OHB phenolate anion toward oxidation relative to ortho oxidation of the carboxylate anion, it appears that H-bonding the Tyr-201 residue to this phenolic oxygen stabilizes both the ground state (GS) and the transition state (TS) approximately equally and therefore plays only a minor role, if any, in lowering the activation barrier. Complexation of p-OHB with guanidine has only a modest effect upon the oxidation barriers. When the complex is in the form of a salt-bridge (10a), the barrier is only slightly reduced. When the TSs are placed in THF solvent (COSMO) with full geometry optimization, salt-bridge TS-A is slightly favored (DeltaDeltaE() = 2.3 kcal/mol).

Sidewall epoxidation of single-walled carbon nanotubes: a theoretical prediction

[reaction: see text] By means of a two-layered ONIOM approach, we predict that sidewall epoxidation of single-walled carbon nanotubes (SWNTs) with dioxiranes is viable. The SWNT epoxides thus produced could be precursors for further chemical modification of SWNTs, given the abundant and well-established chemistry of organic epoxides. This opens the door for routine chemical manipulation of SWNTs.