16240-40-7Relevant articles and documents
Enantioselective synthesis of epoxides by α-deprotonation - Electrophile trapping of achiral epoxides
Hodgson, David M.,Buxton, Timothy J.,Cameron, Iain D.,Gras, Emmanuel,Kirton, Eirene H.M.
, p. 4293 - 4301 (2003)
Enantioselective α-deprotonation of achiral epoxides 1, 21, and 26 using organolithiums in the presence of (-)-sparteine 2 and subsequent electrophile trapping gives access to enantioenriched trisubstituted epoxides 9-17, 22, 23, 27 and 28 (in up to 86% ee).
Effect of a trimethylsilyl moiety on the nucleophilic character of the C=C bond: A comparative kinetic investigation of the epoxidation of substituted and unsubstituted cycloalkenes
Patil,Nagendrappa
, p. 1099 - 1102 (2001)
The rates of epoxidation of twelve cycloalkenes (6-17) with MCPBA were determined at four temperatures (298, 303, 308 and 313 K). All of them were found to follow second-order kinetics. The silylated cycloalkenes (10-13) react faster than the corresponding unsubstituted cycloalkenes (6-9), but slower than the corresponding methyl cycloalkenes (14-17). Thus, for epoxidation, the silyl moiety is a deactivating group relative to an alkyl group, while it is activating in comparison with hydrogen. When the homologous series 6-9, 10-13 and 14-17 are considered, the order of the rates of epoxidation in each series seems to follow the order of strain energy of its members, i.e., the rate decreases in the sequence: 5-membered > 8-membered > 7-membered > 6-membered. The rate data obtained at four temperatures were employed to calculate ΔH?, ΔS? and ΔG?, based on which a common reaction mechanism is proposed.
Chiral epoxides by desymmetrizing deprotonation of meso-epoxides
Hodgson, David M.,Gras, Emmanuel
, p. 2376 - 2378 (2007/10/03)
Simple meso-epoxides can be asymmetrically functionalized: Ligand-assisted direct hydrogen-lithium exchange allows the generation of destabilized oxiranyl lithium species and their subsequent trapping by a wide array of electrophiles (see scheme; E = grou
Unusual temperature dependence in the cis/trans-oxetane formation discloses competitive syn versus anti attack for the Paterno-Buechi reaction of triplet-excited ketones with cis- and trans-cylooctenes. Conformational control of diastereoselectivity in the cyclization and cleavage of preoxetane diradicals
Adam, Waldemar,Stegmann, Veit R.
, p. 3600 - 3607 (2007/10/03)
Toluene-d8 solutions of cis- and trans-cyclooctene (cis- and trans-1a) as well as (Z)- and (E)-1-methylcyclooctene (cis- and trans-1b) have been irradiated at temperatures between -95 and +110 °C in the presence of benzophenone (BP) to afford mixtures of the cis- and trans-configured oxetanes 2a,b and the regioisomeric 2b′. Correspondingly, benzoquinone (BQ) gave with cis- and trans-1a the cycloadducts cis- and trans-3a. The cis/trans diastereomeric ratios of the [2 + 2]-cycloadducts 2 and 3 display a strong temperature dependence; with cis- and trans-1a or cis-1b as starting materials, the diastereoselectivity of the oxetane formation is high at low temperature, under preservation of the initial cyclooctene configuration. With increasing temperature, the cis diastereoselectivity decreases continuously for the cis-cyclooctenes; in the case of the cis-1a, the diastereoselectivity is even switched to trans (cis/trans ca. 20:80) at very high temperatures. For the strained trans-1a, the trans-oxetanes are strongly preferred over the entire temperature range, with only minor leakage (up to 10%) to the cis-oxetanes at very high temperatures. Oxetane formation is accompanied by nonthermal trans-to-cis isomerization of the cyclooctene. The methyl-substituted trans-1b constitutes an exceptional substrate; it displays cis diastereoselectivity in the [2 + 2] photocycloaddition at low temperatures for both regioisomers 2b and 2b′, and the trans selectivity increases at moderate temperature (cis/trans = 4:96), to decrease again at high temperature, especially for the minor regioisomer 2b′. This complex temperature behavior of the cis/trans diastereoselectivity may be rationalized in terms of the triplet-diradical mechanism of the Paterno-Buechi reaction. We propose that the cyclooctene may be competitively attacked by the triplet-excited ketone from the higher (syn) or the less (anti) substituted side; such syn and anti trajectories have hitherto not been considered. To account for the unusual temperature behavior in the diastereoselectivity of the present [2 + 2] photocycloaddition, we suggest that temperature-dependent conformational changes of the resulting triplet preoxetane diradicals compete with their cyclization to the cis/trans-oxetane diastereomers and retro cleavage to the cis-cyclooctene.
Photocatalytic Oxygenation of Selected Cycloalkenes in Aqueous Solutions Induced by Water-Soluble Metal Porphyrin Complexes
Hennig, Horst,Behling, Joerdis,Meusinger, Reinhard,Weber, Lutz
, p. 229 - 234 (2007/10/02)
Water-soluble manganese(III) as well as iron(III) porphyrinates are introduced as light-sensitive precursor compounds for the photocatalytic activation of dioxygen in aqueous solutions.It is shown that in the presence of α-pinene (4) and the further cycloalkenes 11-13 photocatalytic oxygenation reactions occur.The dependence of the selectivity of the oxygen transfer to the olefin on both the presence of water and the variation of the substrate-to-catalyst ratio is discussed.The catalyst may be conveniently separated from the substrates/products by using aqueous solvent systems. - Key Words: Photooxygenation / Dioxygen activation / Metallporphyrins / Epoxidation
Influence of strain on chemical reactivity. Relative reactivity of torsionally distorted double bonds in MCPBA epoxidations
Shea, Kenneth J.,Kim, Jang-Seob
, p. 3044 - 3051 (2007/10/02)
The second-order reaction rates were measured for the MCPBA epoxidation in CH2Cl2 for a series of cyclic olefins including bridgehead olefms and trans-cycloalkenes. As expected, strained bridgehead alkenes and trans-cycloalkenes showed faster reaction rates than nonstrained cis-cycloalkenes. The MM-2 steric energies of alkenes, alkanes, and their corresponding epoxides were calculated to evaluate the strain energy released in each reaction (ΔSE). Plots of log krel vs olefin strain did not show a good correlation. However, the plot of log krel vs ΔSE (which is defined as the steric energy difference between olefin and the corresponding epoxide) showed a good correlation for each set of di- and trisubstituted olefins. This result suggests that ΔSE directly reflects strain energy relief in the transition state. From the slope for the plot log krel vs ΔSE, it was thought that approximately 42% of strain (ΔSE) was released in the transition state for the MCPBA epoxidation. Also, trialkyl-subtituted alkenes were found to be about 50 times more reactive than dialkyl-substituted alkenes in cases where the strain energy relief (ΔSE) is the same. The reaction rate is also plotted versus ionization potential of the olefin, assuming that the major orbital interaction lies between the LUMO of the peracid and the HOMO of the olefin. Although, in some cases, a rough correlation of the reaction rate with the ionization potential of the olefin exists, the frontier orbital interaction is not viewed as the dominant factor since conjugated alkenes, which have higher HOMO energies than simple olefins, are not more reactive in MCPBA epoxidation.
Studies on the Autoxidation of Some Monocyclic Olefins
Blau, K.,Mueller, U.,Pritzkow, W.,Schmidt-Renner, W.,Sedshaw, Z.
, p. 915 - 932 (2007/10/02)
The autoxidations of cyclopentene, cyclohexene, cycloheptene, cyclooctene, cycloocta-1,5-diene, 1-methylcyclopentene, 1-methylcyclohexene, 1-methylcycloheptene, 1-methylcyclooctene, methylene cyclopentene, and methylene cyclohexene with pure oxygen under normal pressure were studied.The epoxides formed were determined gaschromatographically.In most cases also the products of allylic oxidation were analyzed and their structures elucidated after reduction to the corresponding allyl alcohols.The portions of high boiling or polymeric products which could not be detected gaschromatographically and also the real yields of epoxides were determined by balance experiments in the presence of inert internal standards.
