Relative Reactivity of Peracids versus Dioxiranes (DMDO and TFDO) in the Epoxidation of Alkenes. A Combined Experimental and Theoretical Analysis

Abstract

Comparative analysis of the calculated gas-phase activation barriers (DeltaE++) for the epoxidation of ethylene with dimethyldioxirane (DMDO) and peroxyformic acid (PFA) [15.2 and 16.4 kcal/mol at QCISD(T)// QCISD/6-31+G(d,p)] and E-2-butene [14.3 and 13.2 kcal/mol at QCISD(T)/6-31G(d)//B3LYP/6-311+G(3df,2p)] suggests similar oxygen atom donor capacities for both oxidants. Competition experiments in CH(2)Cl(2) solvent reveal that DMDO reacts with cyclohexene much faster than peracetic acid/acetic acid under scrupulously dried conditions. The rate of DMDO epoxidation is catalyzed by acetic acid with a reduction in the classical activation barrier of 8 kcal/mol. In many cases, the observed increase in the rate for DMDO epoxidation in solution may be attributed to well-established solvent and hydrogen-bonding effects. This predicted epoxidative reactivity for DMDO is not consistent with what has generally been presumed for a highly strained cyclic peroxide. The strain energy (SE) of DMDO has been reassessed and its moderated value (about 11 kcal/mol) is now more consistent with its inherent gas-phase reactivity toward alkenes in the epoxidation reaction. The unusual thermodynamic stability of DMDO is largely a consequence of the combined geminal dimethyl- and dioxa-substitution effects and unusually strong C-H and C-CH(3) bonds. Methyl(trifluoromethyl)dioxirane (TFDO) exhibits much lower calculated activation barriers than DMDO in the epoxidation reaction (the average DeltaDeltaE++ values are about 7.5 kcal/mol). The rate increase relative to DMDO of approximately 10(5), while consistent with the higher strain energy for TFDO (SE approximately 19 kcal/mol) is attributed largely to the inductive effect of the CF(3) group. We have also examined the effect of alkene strain on the rate of epoxidation with PFA. The epoxidation barriers are only slightly higher for the strained alkenes cyclopropene (DeltaE++ = 14.5 kcal/mol) and cyclobutene (DeltaE++ = 13.7 kcal/mol) than for cyclopentene (DeltaE++ = 12.1 kcal/mol), reflecting the fact there is little relief of strain in the transition state. Alkenes strained by twist or pi-bond torsion do exhibit much lower activation barriers.

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

“…For the reference, the barrier of the bimolecular catalyst-free Prilezhaev epoxidation by peracetic acid is calculated to be 16.4 kcal͞mol (Ϸ17 kcal͞mol in ref. 50). Thus, below we disregard the pathways of the Mn(salen)-catalyzed epoxidation, which depend on bimolecular elementary steps with barriers Ͼ17 kcal͞mol.…”

Episulfidation of trans‐Cyclooctene with an 1,2,4‐Oxadithiolane

“…After evaporation of the solvent (40°C/200 mbar), the known products 4, 5, 7, 9 were isolated as pure compounds by silica-gel chromatography. Additionally, a mixture (67.6 mg) of 6 and 8 was isolated, of which the known cycloadduct 6 [22,23] was identified by its characteristic NMR signals. Benzophenone and thiobenzophenone were identified in the crude reaction mixture, the former by comparison with a commercial sample, the latter by comparison with a sample obtained from an independent synthesis.…”

Enantioselective Epoxidation of Electrophilic Olefins by Using Glycosyl Hydroperoxides

“…[12][13][14][15] Despite of spectacular achievements of catalytic asymmetric epoxidation of allylic alcohols by Sharpless [16] or Z alkenes by Jacobsen, [17] the main issue concerns the stepwise [18] versus concerted [16] addition of oxygen to a double bond. On the other hand, the epoxidation of a,b-unsaturated ketones with peroxides under basic conditions has been known for decades to proceed in a stepwise manner.…”

“…To compare the results of epoxidation by a-and b-anomers, the anomeric hydroperoxides (5 and 6), derived from 3,4,6-tri-O-benzyl-2-deoxy-glucose, were separated and used independently. To compare our stereoselectivity results with those reported in other laboratories, four electrophilic olefins were selected: the quinone 12, chalcone (13), (E)-1,2-dibenzoyl ethylene (14) and (E)-iso-butyryl-phenyl, ethylene (15). The aim of this investigation was to find not only the optimal conditions, but also to suggest a plausible rationale of the stereochemical pathway of the epoxidation.…”

“…[5] The particular aim of our investigation was to find a sugar, which, after oxidation of the anomeric center, would Abstract: Anomeric hydroperoxides derived from 3,4,6-tri-O-benzyl-galactose and glucose were used for enantioselective epoxidation of naphthoquinone (12), chalcone (13), (E)-1,2-dibenzoyl ethylene (14) and (E)-iso-butyryl-phenyl ethylene (15). In the presence of sodium hydroxide, the epoxidations showed exceptional high asymmetric induction.…”