57338-07-5Relevant academic research and scientific papers
Chiral epoxides by desymmetrizing deprotonation of meso-epoxides
Hodgson, David M.,Gras, Emmanuel
, p. 2376 - 2378 (2002)
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
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 (2007/10/03)
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).
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.
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.
