Pedro C. Vasquez

Dimethyldioxirane: mechanism of benzaldehyde oxidation

Abstract Under inert atmosphere, dimethyldioxirane 1 converts benzaldehydes solely to the corresponding acids; the reaction is insensitive to electronic effects; yields are limited by the competing decomposition of 1 to acetol; O2 trapping of free-radical intermediates is observed.

Oxygen-atom transfer reagents: new, reactive α-azohydroperoxides

Abstract 3,4,4-Trimethyl-4,5-dihydro-5-hydroperoxy-3,5-diaryl-3H-pyrazoles, synthesized by autoxidation of the corresponding 3,4-dihydro-2H-pyrazoles, are a new type of cyclic α-azohydroperoxide which is of high reactivity in oxygen-atom transfer chemistry.

Reaction of tetramethyl-1,2-dioxetane with phosphines: deuterium isotope effects

Avec les phosphines tertiaires, il y a formation de tetramethyl-4,4,5,5 dioxaphospholannes-1,3,2 trisubstitues en 2,2,2 par insertion concertee de la phosphine dans la liaison peroxy du dioxetanne

17O-enriched α-azohydroperoxides: 17O NMR spectroscopy, 17O-labeling reagents

Abstract 17 O-enriched α-azohydroperoxides, prepared by autoxidation, are efficient 17 O-labeling reagents; 17 O NMR (CD 3 CN) of 2 showed broad signals at δ 254 and 204 PPM; the solvent dependence of the 17 O chemical shifts and the kinetics of ionic oxidations are interrelated.

Thermolysis of hexasubstituted-4,5-dihydro-3H-pyrazoles: Kinetics and activation parameters

A kinetics study of the thermolysis of a series of hexasubstituted-4,5-dihydro-3H-pyrazoles (pyrazolines 1a: 3,3,4,4-tetramethyl-5-phenyl-5-acetoxy; 1b: cis-3,5-diphenyl-3,3,4-trimethyl-5-acetoxy; 1c: cis-3,5-diphenyl-3,4,4-trimethyl-5-methoxy; 1d: 3,3,5-triphenyl-4,4-dimethyl-5-acetoxy), which produced the corresponding hexasubstituted cyclopropanes 2a–d in quantitative yields was carried out. The first order rate constants (k1) for thermal decomposition and activation parameters were determined. The relative reactivity series was found to be 1d >> 1b ∼ 1c > 1a. The activation parameters for thermolysis were found to be: for 1a ΔH‡ = 39.8 kcal/mol, ΔS‡ = 14 eu, k150° = 6.8 × 10−5 s−1; for 1b ΔH‡ = 33.5 kcal/mol, ΔS ‡ = 0.2 eu, k150° = 1.7 × 10−4s−1; for 1c ΔH‡ = 32.7 kcal/mol, ΔS‡ = −1.8 eu, k150° = 1.2 × 10−4s−1; for 1d ΔH‡ = 30.1 kcal/mol, ΔS‡ = −1.6 eu, k150° = 8.8 × 10−3s−1. The effect of variation of C3 substituents on the activation parameters for thermolysis paralleled the trend reported for acyclic analogs. The results are consistent with the formation of a (singlet) 1,3-diradical intermediate with subsequent closure to yield the cyclopropanes. The mechanism of diradical formation appears to involve N2-C3 bond cleavage as the rate determining step rather than simultaneous two bond scission.

Epoxidation of 2,3-dimethyl-2-butene by substituted benzylazobenzene α-hydroperoxides in C6D6

The reaction of substituted benzylazobenzene α-hydroperoxides, 2a–f, with 2,3-dimethyl-2-butene in C6D6 at 34° produced tetramethyloxirane in ∼90% yield; was of the first order in ROOH and in alkene; and exhibited an excellent LFER vs. sigma values (ρ=1.1 ± 0.1).

OXIDATION OF ACETALS BY DIMETHYLDIOXIRANE

Kinetic data (k2s, LFER and activation parameters) for the oxidation of a series of acetals by dimethyldioxirane to the corresponding esters in dried acetone are reported; the results are consistent with either a Η-atom abstraction or direct insertion mechanism. Introduction Dioxiranes are powerful, versatile oxidants for the mild oxidation of a great variety of organic substrates. Dimethyldioxirane is often the reagent of choice rather than the more reactive methyl(trifluoromethyl)dioxirane because of its inexpensive, facile preparation. Dimethyldioxirane has been shown to be useful in epoxidation, heteroatom oxidation and C-H oxidation. Specifically, extensive CH bond oxidation studies have been carried out on saturated hydrocarbons, phenyl-substituted hydrocarbons, aldehydes, alcohols and ethers. Several reports have appeared on the reactions of acetals with either dimethyldioxirane or methyl(trifluoromethyl)dioxirane. We report here a kinetic study of the oxidation of a series of acetals by dimethyldioxirane in dried acetone. Results The reaction of dimethyldioxirane 1 [isolated; -0.1 Μ in dried acetone] with substituted benzaldehyde dimethyl acetals (2a-c), 2-aryl-1,3-dioxolanes (3a-g), 2-methyl-1,3-dioxolane (4) and 2phenyl-1,3-dioxane (5) produced the corresponding esters in good to excellent yields (reaction 1). A threefold excess of 1 was employed to achieve efficient conversion. Product studies in the dark under inert X p-R1 R Ο ς Η O R t k2 acetone Ο Λ ο + II R C O R i (1) 1 2 Rj = Rj = Me 3,4 R1( Rj = .CH2CH25 R1( Rj = -CH2CH2CH2· (N2) atmosphere, yielded results essentially identical to those obtained when carried out under normal conditions (Table 1). The yields for oxidation of 2a and 3c are essentially identical to those previously

The reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadiene: monoepoxidation kinetics and computational modeling

Abstract The reaction of dimethyldioxirane (1) with excess 1,3-cyclohexadiene (2a) and 1,3-cyclooctadiene (2b) in dried acetone yielded the corresponding monoepoxides in excellent yield. Second-order rate constants for monoepoxidation were determined using UV methodology. The k2 value for the monoepoxidation of 1,3-cyclohexadiene was found to be 1.14±0.06 m-1 s-1, whereas that for the monoepoxidation of 1,3-cycloctadiene was 0.31±0.03 m-1 s-1. Basic density functional calculations at the B3LYP/6-31G level were employed to model the monoepoxidations. As expected, the calculations were consistent with a concerted, electrophilic process with a spiro-transition state. As found for the epoxidation of simple alkenes, the calculated transition-state geometry showed a slight asynchronous tilt of the dioxirane plane relative to that of the remaining alkene portion of the diene and a slight tilt back from the face of the diene. Relative reactivities (relative k2 values) were determined using the difference in the calculated electronic activation energies and were consistent with the experimental relative k2 values without the need to correct for the medium (solvent). Reactivity differences for epoxidation can be quickly predicted by this approach as long as there are reasonable structural similarities between the substrates.

Synthesis and chemistry of structurally unique hexasubstituted pyrazolines

Abstract A review focused on our contributions to the synthesis and chemistry of hexasubstituted pyrazolines is presented. The development of a synthetic route to pentasubstituted 2H-pyrazoles 1 provides the key starting materials that are used in the synthesis of several unique series of highly substituted pyrazolines 2–6. Thermolysis of pyrazolines 2–4 allows the facile synthesis of hexasubstituted cyclopropanes. Autoxidation of pentasubstituted 2H-pyrazoles 1 in acetone produces a series of hydroperoxy substituted pyrazolines 5 which are effective oxygen-atom transfer reagents. The thermal decomposition of 5 produces β-keto radicals, the transformation of which in the presence of oxygen provides a good route for the synthesis of 3-hydroxy-1,2-dioxolanes. The reaction of tosyl chloride with a pentasubstituted 2H-pyrazole yields a chloro-substituted pyrazoline 6 rather than the expected N-tosyl product. Thermolysis of 6 yields products, the structure of which is consistent with the formation of an unstable intermediate chloro-substituted cyclopropane.

EPOXIDATION OF cis/trans-ENOL ESTERS BY DIMETHYLDIOXIRANE: KINETICS

Abstract. The epoxidation of a series of cis/trans-enol esters with R groups of varying steric bulk by dimethyldioxirane in dried acetone at 23 °C produced the corresponding ris/ira/«-epoxides as the sole observable products. The kinetics study showed kcis / ktrans ratios of 1.6 to 2.5. The k2 values for the ciscompounds were essentially identical while those for the ?raws-compounds showed a consistent decrease with increasing steric bulk.