Oxidative cleavage of p-methoxybenzyl ethers with methyl(trifluoromethyl)dioxirane

[reaction: see text] The ability of methyl(trifluoromethyl)dioxirane to cleave p-methoxylbenzyl ethers oxidatively in the presence of various additional functional groups has been investigated. These reactions, performed in aqueous acetonitrile, transform a reasonably robust aryl substituent into a dienyl aldehydo ester. The originally generated E,Z-isomer undergoes slow conversion to the more stable E,E-form at 20 degrees C.     

Oxidation of peptides by methyl(trifluoromethyl)dioxirane: the protecting group matters

Representative Boc-protected and acetyl-protected peptide methyl esters bearing alkyl side chains undergo effective oxidation using methyl(trifluoromethyl)dioxirane (1b) under mild conditions. We observe a protecting group dependency in the chemoselectivity displayed by the dioxirane 1b. N-Hydroxylation occurs in the case of the Boc-protected peptides, and side chain hydroxylation takes place in the case of acetyl-protected peptides. Both are attractive transformations since they yield derivatized peptides that serve as valuable synthons.     

Reactions at interfaces: oxygenation of n-butyl ligands anchored on silica surfaces with methyl(trifluoromethyl)dioxirane

The oxygenation of n-butyl and n-butoxy chains bonded to silica with methyl(trifluoromethyl)dioxirane (1) revealed the ability of the silica matrix to release electron density toward the reacting C(2)-H σ-bond through the Si-C(1) and Si-O(1) σ-bonds connecting the alkyl chain to the surface (silicon β-effect). The silica surface impedes neither the alkyl chain adopting the conformation required for the silicon β-effect nor dioxirane 1 approaching the reactive C(2) methylene group. Reaction regioselectivity is insensitive to changes in the solvation of the reacting system, the location of organic ligands on the silica surface, and the H-bonding character of the silica surface. Reaction rates are faster for those organic ligands either within the silica pores or bonded to hydrophilic silica surfaces, which evidence the enhanced molecular dynamics of confined dioxirane 1 and the impact of surface phenomena on the reaction kinetics. The oxygenation of n-butyl and n-butoxy chains carrying trimethylsilyl, trimethoxysilyl, and tert-butyl groups with dioxirane 1 under homogeneous conditions confirms the electronic effects of the silyl substituents and the consequences of steric hindrance on the reaction rate and regioselectivity. Orthosilicic acid esters react preferentially at the methylene group adjacent to the oxygen atom in clear contrast with the reactivity of the carboxylic or sulfonic acid alkyl esters, which efficiently protect this position toward oxidation with 1.     

The effect of substituents on the structure of dioxirane

Ab initio calculations have been performed for methyl- and fluorine-substituted dioxirane to analyze the influence of the substituents on the molecular structure and the peroxy bond. Fluorine substitution increases the O-O bond length and shortens the C-O bond, while methyl substitution does not introduce significant changes. These results are discussed using a molecular orbital interaction diagram. Both substituents affect the basicity of the molecule, and it is concluded that the peroxy bond is stabilized on methyl substitution since the methyl group acts as an electron donor, while fluorine substitution destabilizes the peroxy bond.     

Epoxidation of chiral camphor N-enoylpyrazolidinones with methyl(trifluoromethyl)dioxirane and urea hydrogen peroxide/acid anhydride: reversal of stereoselectivity

Both diastereomeric isomers of epoxides with high optical purity are obtained when camphor N-methacryloylpyrazolidinone (1a) and N-tigloylpyrazolidinone (1b) are treated with a urea hydrogen peroxide/TFAA and methyl(trifluoromethyl)dioxirane, respectively.     

Mechanism of singlet oxygen generation during catalytic decomposition of methyl(trifluoromethyl)dioxirane by chloride ions

The mechanism of the chloride ion-induced catalytic decomposition of methyl(trifluoromethyl)dioxirane in trifluoroacetone was studied at the MP4//MP2/6-31+G(d) level of theory. The solvated chloride ion interacts with dioxirane to form an ion-dipole pair, which is transformed into the key intermediate ClO—C(Me)(CF₃)—O⁻ acting as a chain carrier in the catalytic decomposition of dioxirane. The generation of singlet oxygen occurs during the transformations of this intermediate on the singlet potential energy surface.     

Influence of substituents on the strength of aryl C-H...anion hydrogen bonds

[structure: see text]When electron-withdrawing substituents are present, aryl C-H groups become powerful hydrogen bond donors, forming stronger complexes than obtained with conventional O-H and N-H groups.     

Cytochrome P-450 model compound catalyzed selective hydroxylation of C-H bonds: dramatic solvent effect

Selective hydroxylation of cyclohexane and cyclohexene by t-BuOOH in presence of F2oTPPFe(III)Cl as the catalyst has been achieved at room temparature in high yields.     

Chemistry of trifluoromethyl compounds : I. NMR evidence for bis(trifluoromethyl)zinc and methyl(trifluoromethyl)zinc

Bis(trifluoromethyl)zinc and methyl(trifluoromethyl)zinc have been identified by ¹⁹F and ^1H NMR methods. The compounds were formed in the following reactions: (1) dimethylzinc and bis(trifluoromethyl)mercury and (2) dimethylzinc and bis(trifluoromethyl)cadmium.     

The effect of solvent on the homo-propagation rate coefficients of styrene and methyl methacrylate

The free radical propagation rate coefficients of both Methyl Methacrylate (MMA) and Styrene (STY) have been measured using Pulsed-Laser Polymerization. The effect of solvents on the propagation rate coefficient, k_p, is reported for several solvents, namely, bromobenzene, chlorobenzene, dimethyl sulphoxide, diethyl malonate, diethyl phthalate, benzonitrile, and benzyl alcohol, at 26.5°C. This preliminary data indicated that benzyl alcohol (BzA) had a large effect on the MMA propagation reaction. As earlier work indicated that N-methyl pyrrolidinone (NMP) would also have a large effect on the k_p of MMA, Arrhenius parameters were evaluated for both MMA and STY at two different concentrations of monomer in BzA and NMP. BzA had a significant effect (at 95% confidence) increasing both the activation energy (E_a) and the preexponential factor (A) for MMA and STY. In NMP, a similar trend is observed for MMA polymerization; however, while a solvent effect on STY was observed, the effect on E_a and A was too small to discern with confidence. A series of additional experiments was performed to evaluate the influence of camphorsulfonic acid (CSA) as an additive in STY polymerization. There was no effect of CSA on k_p, confirming that the strong effect CSA has on “living” radical polymerization of styrene does not originate from complexation leading to an accelerated propagation step but rather by altering the ratio of active-to-dormant chains in the reaction. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2311–2321, 1997     

Thiyl radicals abstract hydrogen atoms from the (alpha)C-H bonds in model peptides: absolute rate constants and effect of amino acid structure

Thiyl radicals are important intermediates in biological oxidative stress and enzymatic reactions, for example, the ribonucleotide reductases. On the basis of the homolytic bond dissociation energies (BDEs) only, the (alpha)C-H bonds of peptides and proteins would present suitable targets for hydrogen abstraction by thiyl radicals. However, additional parameters such as polar and conformational effects may control such hydrogen-transfer processes. To evaluate the potential of thiyl radicals for hydrogen abstraction from (alpha)C-H bonds, we provide the first absolute rate constants for these reactions with model peptides. Thiyl radicals react with (alpha)C-H bonds with rate constants between 1.7 x 10(3) M(-1) s(-1) (N-acetylproline amide) and 4 x 10(5) M(-1) s(-1) (sarcosine anhydride). However, the correlation of rate constants with BDEs is poor. Rather, these reactions may be controlled by conformation and dynamic flexibility around the (alpha)C-H bonds.     

Activation of C-H and C-Br bonds in cyclopalladation reactions of Schiff base ligands: Influence of the benzylidene ring substituents

Reaction of Pd(AcO)₂ with the Schiff base ligands 2-Br-4,5-(OCH₂O)C₆H₂C(H)N(Cy) (a) and 4,5-(OCH₂CH₂)C₆H₃C(H)N(Cy) (b) leads to the cyclometallated compounds [Pd{2-Br-4,5-(OCH₂O)C₆HC(H)N(Cy)-C6,N}(μ-O₂CMe)]₂ (1a) and [Pd{4,5-(OCH₂CH₂)C₆H₂C(H)N(Cy)-C6,N}(μ-O₂CMe)]₂ (1b), respectively, via C-H activation. Treatment of a with Pd₂(dba)₃ gave [Pd{4,5-(OCH₂O)C₆H₂C(H)N(Cy)-C2,N}(μ-Br)]₂ (6a), via C-Br activation. The metathesis reaction of 1a and 1b with aqueous sodium chloride gave the corresponding cyclopalladated dimers with bridging chloride ligands, 2a and 2b, respectively. Treatment of the halogen-bridged compounds with tertiary tri- and diphosphines in the appropriate molar ratio gave the mono and dinuclear compounds 3a-5a, 7a-9a and 3b-5b. The structure of compounds 3a, 4a, 5a, 8a, 2b, 3b and 5b has been determined by X-ray diffraction analysis.     

Theoretical study of reaction pathways for the rhodium phosphine-catalysed borylation of C-H bonds with pinacolborane

The reaction mechanism of the rhodium-phosphine catalysed borylation of methyl-substituted arenes using pinacolborane (HBpin) has been investigated theoretically using DFT calculations at the B3PW91 level. Factors affecting selectivity for benzylic vs. aromatic C-H bond activation have been examined. It was found that [Rh(PR3)2(H)] is the active species which oxidatively adds the C-H bond leading to an eta3-benzyl complex which is the key to determining the unusual benzylic regioselectivity observed experimentally for this catalyst system. Subsequent reaction with HBpin leads to a [Rh(PR3)2(eta3-benzyl)(H)(Bpin)] complex from which B-C reductive elimination provides product and regenerates the catalyst. The electrophilic nature of the boryl ligand assists in the reductive elimination process. In contrast to Ir(L)2(boryl)3-based catalysts, for which Ir(III)-Ir(V) cycles have been proposed, the Rh(I)-Rh(III) cycle is operating with the system addressed herein.     

Insight into the photoexcitation effect on the catalytic activation of H2 and C-H bonds on TiO2(110) surface

Semiconductor photocatalysis holds great promise for breaking the inert chemical bonds under mild condition; however, the photoexcitation-induced modulation mechanism has not been well understood at the atomic level. Herein, by performing the DFT+U calculations, we quantitatively compare H₂ activation on rutile TiO₂(110) under thermo- versus photo-catalytic condition. It is found that H₂ dissociation prefers to occur via the heterolytic cleavage mode in thermocatalysis, but changes to the homolytic cleavage mode and gets evidently promoted in the presence of photoexcited hole (h⁺). The origin can be ascribed to the generation of highly oxidative lattice O-radical (O_br^??) with a localized unoccupied O-2p state. More importantly, we identify that this photo-induced promotion effect can be practicable to another kind of important chemical bond, i.e., C–H bond in light hydrocarbons including alkane, alkene and aromatics; an exception is the C(sp¹)-H in alkyne (HCCH), which encounters inhibition effect from photoexcitation. By quantitative analysis, the origins behind these results are attributed to the interplay between two factors: C-H bond energy (E_bond) and the acidity. Owing to the relatively high E_bond and acidity, it favors the C(sp¹)-H bond to proceed with the heterolytic cleavage mode in both thermo- and photo-catalysis, and the photoexcited O_br^?? is adverse to receiving the transferred proton. By contrast, for the other hydrocarbons with moderate/low E_bond, the O_br^?? would enable to change their activation mode to a more favored homolytic one and evidently decrease the C–H activation barrier. This work may provide a general picture for understanding the photocatalytic R–H (R = H, C) bond activation over the semiconductor catalyst.     

Activation of sp3 C-H bonds with cobalt(I): catalytic synthesis of enamines

C-H bond activation has been extensively studied with (Cp*)M(L)n (M = Ir, Rh), but cobalt, the third member of this triad, has not previously been shown to activate sp3 C-H bonds. Further, practical functionalization of the metal alkyl products of oxidative addition has not been fully explored. Toward these ends, we have developed catalytic dehydrogenation of alkyl amines with a Co(I) catalyst. Amine substrates are protected with vinyl silanes, followed by catalytic transfer hydrogenation, to yield a broad range of stable protected enamines and 1,2-diheteroatom-substituted alkenes, including several unprecedented heterocycles. (Cp*)Co(VTMS)2 catalyzes transfer hydrogenation under surprisingly mild conditions with high chemo-, regio-, and diastereoselectivity, while tolerating additional functionality.     

Rhodium(III)-catalyzed intermolecular direct amination of aromatic C-H bonds with N-chloroamines

A Rh(III)-catalyzed direct aromatic C-H amination is achieved using N-chloroamines as a reagent. Furthermore, we also developed a one-pot amination protocol involving in situ chlorination of the secondary amines. The catalytic amination operates at mild conditions with excellent functional group tolerance and regioselectivity.