Trans pathway

The second C-C bond forming step (step C), while occurring after the first irreversible ee determining step (step B), can affect the observed enantioselective outcome of the reaction. If the radical intermediate collapses without rotation (k3 Ict, k5 ke), then the observed ee would be determined by the first C-C bond forming step (ki vs. k2), that is the facial selectivity (Scheme 1.4.6). However, if rotation is allowed followed by collapse, then the rate of both trans pathways (Ic and k ) will proportionally effect the observed ee of the cis epoxide (ks vs. ks). Should bond rotation be permissible, the diastereomeric nature of the radical intermediates 9a and 9b renders the distinct possibility of different observed ee s for frany-epoxides and dy-epoxides. 

In the case of 1,2-disubstituted alkenes, the nonstereospecificity of the epoxidation reaction results in formation of diastereomeric epoxides. In contrast, for terminal alkenes the trans pathway results in partitioning to enantiomers. Thus, diminished enantiose-lectivity observed in the epoxidation of terminal alkenes such as styrene (50-70% ee) relative to sterically similar cis-disuhstituted alkenes can be attributed to enantiomeric leakage due to the trans pathway. Suppression of this pathway has not been accomplished successfully, and synthetically useful enantioselectivities with terminal alkenes have not yet been achieved using the chiral (salen)Mn systems. 

With ferrocene as quencher and triphenylene as sensitizer the cis trans isomerization of several 4-nitrostilbenes has been examined [201], From linear plots of ([t]/[c])sens versus [Q], larger slope/intercept ratios were found as compared to direct excitation conditions (Table 8). A cis -> trans pathway partly bypassing the triplet state accounts for this difference (Section VI). Plots of ([t]/[c])scns as a function of the azulene or ferrocene concentrations are shown in Figure 10 for three stilbenes. 

Pathway for Cis -> Trans Photoisomerization. The cis - trans pathway is more difficult to analyze because of the problems in selective observation of fluorescence from m-stilbene at ambient temperatures [5, 81, 240, 241]. Therefore, more sophisticated techniques have to be applied. Sumitani et al. [315] determined the time interval for appearance of the fluorescence from t after pulsed excitation of c/s-stilbene This delay is only a few picoseconds. Since the cis - trans photoisomerization does not occur via excited states of DHP to a considerable extent (as postulated for bromostilbenes [105]) and since the rate constant for this pathway is larger than 1011 s an intersystem crossing step (involving 3c or 3p ) is not likely. Furthermore, no triplet intermediate has been observed in the nanosecond timescale by direct flash excitation of the cis form in solution [315] or in rigid glasses at — 196°C [96,114] in contrast to the results with frans-stilbene (Table 16b). This suggests that cis -> trans photoisomerization occurs via Eq. (15) ... 

Nucleophilic agents that favor reaction by the trans pathway are potassium acetate in acetic acid, which leads to formation of trans-1,2-diol diacetates, and lithium chloride and tetrabutylammonium bromide, which afford the corresponding trons-1,2-chlorohydrin or bromohydrin acetates (20). In reactions by the trans pathway, the dioxolanylium ring is opened by rearside attack by the nucleophile, so that inversion of configuration at the reaction center takes place consequently, a trans product is obtained from a cis-diol precursor. 

At low temperatures, and by a rapid reaction that prevents a reverse rearrangement, the D-ido salt 62 reacts in acetonitrile with sodium borohydride by the cis pathway, to give the ethylidene derivative 66, and with lithium bromide by the trans pathway, to give the 6-bromo-6-deoxy-D-idose derivative 68. Under similar conditions, treatment with lithium azide did not provide a 6-azido-6-deoxy-D-idose derivative. 

Reductive elimination through the s-trans pathway retains C2 symmetry and has to overcome the activation barrier of 29.4 kcal/mol. On the other hand, initial complex (lA Init) and transition state (1A TS) for s-cis pathway belong to Cj point group and the activation energy is 4.9 kcal/mol higher than for the former (Table 2). In the latter process, the product buta- 1,3-diene will be released in the s-gauche (C2) form. Reductive elimination reactions are exothermic by... 

Energy difference between the initial bis-o-vinyl complexes l Init and lA Init is only 0.6 kcal/mol, with the former being more stable. The relative order of the transition states (1 TS and 1A TS) reflects the stability of corresponding buta- 1,3-diene isomers in a free form. Thus, s-cis transition states are expected to lie higher in energy and we will continue the study following the s-trans pathway only. 

Reductive elimination through the s-cis pathway (see Fig. 4) all the other calculations were done for the s-trans pathway. 

Microbial Electrosynthesis, Fig. 2 (a) Electron trans- pathway via multiheme proteins in Shewanella oneidensis fer pathway via multiheme proteins and conductive pili in (for further details, see text)... 

As the reaction of 1 is not catalyzed by THF, thus, rejecting the trans pathway for elimination process. The reaction, indeed, involves the cis pathway and relatively exposed boron atom of 9-BBN readily facilitates elimination (Eq. 32.5). 

Accumulating evidence makes it increasingly clear that there is no single dominant Wittig transition state geometry and, therefore, no simple scheme to explain cis/trans selec-tivities. The conventional betaine pathway may not occur at all, the stabilized ylides, e,g., PhsP—CH —C02Et, can be ( )- or (Z)-selective, depending on the solvent and substrate (E. Vedejs, 1988 A, B, 1990). 

Thermal decomposition of unsubstituted 3,4,5,6-tetrahydropyridazine at 439 °C in the gas phase proceeds 55% via tetramethylene and 45% via a stereospecific alkene forming pathway. The thermal decomposition of labelled c/s-3,4,5,6-tetrahydropyridazine-3,4- f2 affords cfs-ethylene-l,2- f2, trans-ethylene-l,2-if2, c/s-cyclobutane-l,2- f2 and trans-cyclo-butane-1,2- /2 (Scheme 57) (79JA3663, 80JA3863). 

FIGURE 24.23 )3-Oxidation of unsaturated fatty acids. In the case of oleoyl-CoA, three /3-oxidation cycles produce three molecules of acetyl-CoA and leave m-AAdodecenoyl-CoA. Rearrangement of enoyl-CoA isomerase gives the tran.s-A species, which then proceeds normally through the /3-oxidation pathway. 

Polyunsaturated fatty acids pose a slightly more complicated situation for the cell. Consider, for example, the case of linoleic acid shown in Figure 24.24. As with oleic acid, /3-oxidation proceeds through three cycles, and enoyl-CoA isomerase converts the cA-A double bond to a trans-b double bond to permit one more round of /3-oxidation. What results this time, however, is a cA-A enoyl-CoA, which is converted normally by acyl-CoA dehydrogenase to a trans-b, cis-b species. This, however, is a poor substrate for the enoyl-CoA hydratase. This problem is solved by 2,4-dienoyl-CoA reductase, the product of which depends on the organism. The mammalian form of this enzyme produces a trans-b enoyl product, as shown in Figure 24.24, which can be converted by an enoyl-CoA isomerase to the trans-b enoyl-CoA, which can then proceed normally through the /3-oxidation pathway. Escherichia coli possesses a... 

FIGURE 24.24 The oxidation pathway for polyunsaturated fatty adds, illustrated for linoleic add. Three cycles of /3-oxidation on linoleoyl-CoA yield the cis-A, d.s-A intermediate, which is converted to a tran.s-A, cis-A intermediate. An additional round of /S-oxi-dation gives d.s-A enoyl-CoA, which is oxidized to the trans-A, d.s-A species by acyl-CoA dehydrogenase. The subsequent action of 2,4-dienoyl-CoA reductase yields the trans-A product, which is converted by enoyl-CoA isomerase to the tran.s-A form. Normal /S-oxida-tion then produces five molecules of acetyl-CoA. 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

Metalated epoxides can react with organometallics to give olefins after elimination of dimetal oxide, a process often referred to as reductive alkylation (Path B, Scheme 5.2). Crandall and Lin first described this reaction in their seminal paper in 1967 treatment of tert-butyloxirane 106 with 3 equiv. of tert-butyllithium, for example, gave trans-di-tert-butylethylene 110 in 64% yield (Scheme 5.23), Stating that this reaction should have some synthetic potential , [36] they proposed a reaction pathway in which tert-butyllithium reacted with a-lithiooxycarbene 108 to generate dianion 109 and thence olefin 110 upon elimination of dilithium oxide. The epoxide has, in effect, acted as a vinyl cation equivalent. 

Different substrate geometries can even result in alternate reaction pathways operating. The reactions between trans-a, (3-epoxytrimethylsilane 115 and organo-metals (metal = Li, Ce, or La) give predominantly trans-alkene 116 in high yields (Scheme 5.25) [38]. In contrast, treatment of cis-115 with some of the same organo-metals produces (Z)-vinylsilanes. The use of a bulkier substituent on silicon (e. g.,... 

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