Paths for substitution

Reactivity toward nucleophiles and comparison with other electrophilic centers 152 Paths for nucleophilic substitution of sulfonyl derivatives 156 Direct substitution at sulfonyl sulfur stereochemistry 157 Direct substitution at sulfonyl sulfur stepwise or concerted 158 The elimination-addition path for substitution of alkanesulfonyl derivatives 166 Homolytic decomposition of a-disulfones 172 10 Concluding remarks 173 Acknowledgement 174 References 174... 

THE ELIMINATION-ADDITION PATH FOR SUBSTITUTION OF ALKANESULFONYL DERIVATIVES... 

The assumption of a definite location for the negative charge on the j8-carbon requires that 59a and 60a both be on the lowest energy path for substitution and isomerization. That this location should be formed anti derives from orbital symmetry, i.e. microscopic reversibility suggests that anti entry of one halide and syw-departure of the other are improbable. We deduce, therefore, that the substitutions in (142) require at least three elementary steps, e.g. cis-1 - 59a -> 60b -v cis-2 isomerizations with exchange require at least three steps, e.g. cis-l 59a —> 59b - trans-2 isomeriza-... 

Figure 4.3 The electron flow paths for substitution at a tetrahedral carbon.

The much studied coordination of metal ions by di- and triketone ligands continues to generate interest as the following spectrophotometric SF studies demonstrate. The keto tautomer of bidentate 4,4,4-trifluoro-l-(2-thienyl)butane-1,3-dione exists in equilibrium with its enol tautomer which, in its protonated and anionic forms, provides two paths for substitution on Ni(II) that are characterized by rate constants equal to 11.3 and 1294 dm mol" s" in aqueous solution (/ = 0.2 mol dm", Na2S04) at 298.2 These results are at variance with those previously reported and reasons for this are discussed. The substitution of Co(II)... 

Phosphate esters have a variety of mechanistic paths for hydrolysis. Both C-O and P-0 cleavage are possible depending on the situation. A phosphate monoanion is a reasonable leaving group for nucleophilic substitution at carbon and so 8 2 or SnI reactions of neutral phosphate esters are well known. PO cleavage can occur by associative (by way of a pentacoordinate intermediate), dissociative (by way of a metaphosphate species), or concerted (avoiding both of these intermediates) mechanisms. 

The [Ni°(CDT)] product complex 8b is formed via reductive elimination under ring closure starting from the dodecatrienediyl-Ni11 complex. The formation of the several isomers of CDT occurs via competing paths for reductive elimination that involves different stereoisomers. Displacement of the cyclotrimer product in subsequent consecutive substitution steps with butadiene, which is supposed to take place without a significant barrier, regenerates the [Ni°( butadiene) J active catalyst thus completing the catalytic cycle. 

The subscript s denotes an isentropic path for ideal nozzle flow. For ideal gas with Pok = constant, substitution of this isentropic expansion law into Eq. (23-98) yields the following critical pressure ratio PJP and critical flow rate Gc ... 

As with the ci ,cis-l-decalol, tmna,cis-l-decalol, with its hydroxyl group locked in an axial position, presents the catalyst with two paths for tmws-diaxial elimination. In agreement with the previous observation, the production of the more substituted 1,9-octalin was favored over the [Pg.64]

One path for HIV infection may involve damaged (or healthy) vaginal epithelia [41,42] through cell surface adhesion [40]. Recently it has been reported that the major HIV envelope protein is held intact through hydrophobic residues [43]. For these and other reasons, we sought to combine hydrophobe substituted cationic polymers, particularly DCEs with active anionic polymers, with or without N9 present. 

Figure 2. Bimolecular displacement mechanism for substitution reactions of square planar complexes. ka is the rate constant for the solvent path and ky is the rate constant for the direct reagent path.

Under copper catalyzed conditions azoles (i.e. imidazoles) couple not only with aryl halides but also with arylboronic acids. The reaction, run in the presence of oxygen, follows a unique path (for details see Chapter 2.5.). From the synthetic point of view, the arylation of imidazole proceeds in good yield, although the regioselectivity in the arylation of 4-substituted imidazoles is only moderate (6.70.),102... 

A similar dual-path fragmentation is also observed for substituted methylenebicyclo (2.2.0) hexenes, eq. 36 (104) ... 

Figure 10.11 offers the general reaction network for the SCWO of cresol. This network shows three parallel paths for the oxidation of cresol by SCW. Three reaction intermediates are a hydroxybenzaldehyde via oxidation of the methyl substitute ring-opening products and phenol via demethylation. The end products are COz and HzO. The relative importance of the parallel pathways depends on the specific cresol isomer being oxidized. Figure 10.11 shows that phenol and hydroxybenzaldehydes are key organic intermediates in the reaction network, so the reaction network should also include the reaction paths for these compounds. Two parallel primary paths produce... 

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