Nucleophilic catalysts preferential

Asymmetric allylic substitutions are widely applied in organic synthesis, using various metal complexes, chiral ligands, nucleophiles and allyl systems [39]. Although Pd is often the metal of choice, this is not the case for monosubstituted allylic substrates, where most Pd catalysts predominantly produce the achiral linear product. In contrast. Mo, W and Ir catalysts preferentially give rise to the desired branched products and, in recent years, a number of very effective Ir catalysts for various substrates have been developed [40]. Since, to the best of our... 

Less activated substrates such as uorohaloben2enes also undergo nucleophilic displacement and thereby permit entry to other useful compounds. Bromine is preferentially displaced in -bromofluoroben2ene [460-00-4] by hydroxyl ion under the following conditions calcium hydroxide, water, cuprous oxide catalyst, 250°C, 3.46 MPa (500 psi), to give -fluorophenol [371-41-5] in 79% yield (162,163). This product is a key precursor to sorbinil, an en2yme inhibitor (aldose reductase). 

The interfacial mechanism provides an acceptable explanation for the effect of the more lipophilic quaternary ammonium salts, such as tetra-n-butylammonium salts, Aliquat 336 and Adogen 464, on the majority of base-initiated nucleophilic substitution reactions which require the initial deprotonation of the substrate. Subsequent to the interfacial deprotonation of the methylene system, for example the soft quaternary ammonium cation preferentially forms a stable ion-pair with the soft carbanion, rather than with the hard hydroxide anion (Scheme 1.8). Strong evidence for the competing interface mechanism comes from the observation that, even in the absence of a catalyst, phenylacetonitrile is alkylated under two-phase conditions using concentrated sodium hydroxide [51],... 

The oxidation of sulfides to sulfoxides (1 eq. of oxidant) and sulfones (2 eq. of oxidant) is possible in the absence of a catalyst by employing the perhydrate prepared from hexafluoroacetone or 2-hydroperoxy-l,l,l-trifluoropropan-2-ol as reported by Ganeshpure and Adam (Scheme 99 f°. The reaction is highly chemoselective and sulfoxidation occurs in the presence of double bonds and amine functions, which were not oxidized. With one equivalent of the a-hydroxyhydroperoxide, diphenyl sulfide was selectively transformed to the sulfoxide in quantitative yield and with two equivalents of oxidant the corresponding sulfone was quantitatively obtained. 2-Hydroperoxy-l,l,l-fluoropropan-2-ol as an electrophilic oxidant oxidizes thianthrene-5-oxide almost exclusively to the corresponding cw-disulfoxide, although low conversions were observed (15%) (Scheme 99). Deprotonation of this oxidant with sodium carbonate in methanol leads to a peroxo anion, which is a nucleophilic oxidant and oxidizes thianthrene-5-oxide preferentially to the sulfone. 

S-Allyl dithiocarbonates on treatment with Pd° catalyst expel COS and serve as precursors to RS-nucleophiles. Alkyl-substituted precursors show preferential attack at the more substituted allyl terminus, while aryl-substituted precursors give exclusive attack at the less substituted allyl terminus (equation 286).222... 

Selective formation of cw-monochlorocyclopropanes in the absence of base or in the presence of monoamines has been explained by preferential homolytic cleavage of a less hindered C-Cl linkage to form the more stable surface intermediate Ic rather than the less stable It under the circumstances that the substrate can be strongly adsorbed. On the other hand, in the presence of an alkali hydroxide or a diamine, which may be adsorbed strongly to the catalyst, it is probable that the substrate is adsorbed weakly and formation of the more stable fran.s-monochlorocyclopropanes increases with an increased contribution of a nucleophilic reaction (Scheme 13.17).211... 

The classical Perrier rearrangements of acetyl glycals lead to the introduction of a nucleophilic group at C-1 (preferentially a-anomers) and the migration of the double bond to C-2 in the presence of acid catalysts (O Scheme 20) [24]. In some particular conditions (high temperature or stronger acid), 3-substituted isomeric products can be formed (O Scheme 20) [25]. 

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