Ethylene oxide stereochemistry

Two further illustrations, one from the sugar1 8 and the other from the steroid field,1 8 complete this presentation of the reactions of ethylene oxides with alkylmercaptam. As Been in Eqe. (660) and (661). attack follows the genera] sense indicated above with respect to direction and stereochemistry. 

In this bicyclic case the palladium and methoxyl groups are trans to each other 1X>. A cis stereochemistry would have been expected on the basis of the ethylene oxidation mechanism. Trans-addition, however, is unusually favorable in the bicyclic examples. Although addition to the exo positions is generally strongly preferred, it cannot occur here if the favorable chelating effect of the second double bond is to be obtained. As a result, only the solvent methanol can attack from the exo side. The endo cis adduct has not been prepared and it conceivably could rearrange to the trans isomer even if it were formed initially. Clearly, more work needs to be done on the stereochemistry of the addition reactions. 

It is essential, for successful retention of stereochemistry, to form the organolithiums by halogen-metal exchange, and not by reductive lithiation. While transmetallation of cis- and trans-24 with BuLi, quenching the organolithium 25 with ethylene oxide, gives complete retention of stereochemistry in 26. Owing to the intervention of radicals, reductive lithiation leads to considerable (but not total) epimerisation ... 

However, the stereochemistry as a diagnostic tool to distinguish between Si>(l and Si 2 mechanisms has recently been questioned because racemization in the S( l process may not take place if a contact ion pair is involved and the anion stericatly hinders the approach of the nucleophile from either ade of the carbenium ion " Since the agreement between the threo/erythro ratio in the polymer and the ds trans ratio in the monomer feed (e. g. 7 3 in both cases in the polymerization of ethylene oxide described above) is adequate, this uncertainty can be dismissed . ... 

Vinyl copper derivatives such as 157 do not react with epoxides but transformation of the vinyl copper into a cuprate by the addition of pentynyl lithium gives a cuprate that preferentially transfers the vinyl group (the less stable anion is transferred from copper) to ethylene oxide to give the homoallylic alcohol39 -159. Note that 157 has the opposite stereochemistry to 149. 

Water is also involved as a substrate in the Wacker- Hoechst acetaldehyde process based on a partial, selectie oxidation of ethylene [16]. According to Eq. (10), it is necessary to form the new C—O bond starting from ethylene (trans-stereochemistry), while the oxygen of Eq. (11) regenerates the catalyst (Pd° —> Pd2+), but does not oxidize the ethylene as suggested by the net Eq. (12). Metal attachment of ethylene is the prerequisite to make it accessible to nucleophilic attack by water (cf. Section 6.4.2). 

Thionylaniline reacted with ethylene oxide at low temperatures to afford the oxathiazolidine (127), which was converted into a piperazine (128) at 160°C. The same workers also studied the stereochemistry of the products formed in a similar reaction using propylene oxide. A similar... 

Nucleophilic bimolecular ring-opening of ethylene oxide by the hydride anion has been investigated theoretically. Molecular orbital calculations of the interaction energy (a combination of coulomb, exchange, delocalization, and polarization interaction terms) were carried out. The kinetics and stereochemistry of base-catalysed polymerization of epoxides have been studied using optically active epoxide monomers. 

Thermolysis of 16e,f in either solution or gas phase (150-350 °C) gave deuteriated ethylenes (i.e. 40e from 16e and 41f from 16f) with about 95% retention of stereochemis-try ". Similarly, pyrolysis of the stereoisomeric 2,3-diphenylthiirane oxides 16g,h proceeded smoothly to yield stilbenes and sulfur monoxide in more than 70% yield . The extrusion of SO from the trans-isomer proceeds almost stereospecifically, while that from the cis-isomer occurs with complete loss of stereochemistry. This indicates the intervention of a stepwise mechanism, and not a symmetry-allowed nonlinear chelatropic reaction . Based on the fact that all attempts to trap the intermediate with 1,3-dipolarophiles were in vain, whereas a 1 1 adduct was obtained in good yield (about 60%) with the carbon radical scavenger di-p-anisyl thioketone, a mechanistic scheme as depicted in equation 10 has been proposed . Although the radical intermediates are capable of internal rotation about the carbon-carbon bond, for the 2,3-diphenyl case (i.e. 16g,h), the rotation would be... 

The complex 100 is calculated to be more stable than the separated aldehyde and carbonyl oxide by 9 kcal mol-1 and the formation of complex 100 from ethylene and ozone is endothermic by only 3.1 kcal mol-1 <1991CPL(187) 491 >. Cycloaddition then leads to the secondary ozonide 101 (1,2,4-trioxolane). subsequent study of the stereochemistry of ozonation reactions using the AMI method provides further support for the modified Criegee mechanism C1997JOC2757, CHEC-III(6.06.2)193>. 

Demonstration of cis addition with acetylene is unequivocal, in that the stereochemistry of the resulting bis(dihaloboryl)ethylene has been shown directly by spectroscopic evidence (90) and by conversion of the initially produced cis isomer to the trans compound by photochemical isomerization (27). Stereochemical inferences in the olefin addition reactions are based on the assumption that hydrolysis and oxidation of the bis(boryl) compounds to the corresponding diols occurs with retention of configuration, as is the case in closely related systems 19). The NMR spectrum of the addition product of B2CI4 with 1,3-cyclohexadiene is also consistent with cis addition (120). 

The stereochemistry of epoxide formation from ethylenes-tE was investigated also by taking the IR spectra of dideuterioethylene oxides prepared from gem-C2H2D2, from... 

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