Substitution, nucleophilic heterogeneous

Asymmetric nucleophilic allylic substitution has rarely been studied in its heterogeneous version, probably because of the difficulties encoimtered in properly stabilizing and recycling Pd(0) species. Nevertheless, some promising examples have been pubhshed. Lemaire et al. [143] studied the activity and enantioselectivity of various chiral C2-diamines for the asymmetric Pd-catalyzed transformation of various allyl acetates. The structures tested are represented in Scheme 58. 

Nucleophilic substitution reactions, and others involving attack by nucleophilic species on fully and partially saturated systems, form a very heterogeneous set of reactions, from which we select only a few representative examples to illustrate special points. For the most part, the reactions involved are of the kind familiar from aliphatic chemistry. 

Stabilized carbon nucleophiles (e.g. from 3-diketones, 3-keto esters, malonate esters, etc.) can be aryl-ated by substitution for chloride on the arene in (Fe(arene)Cp] cation complexes.72-78 81 A base is necessary and two heterogeneous systems are favored potassium carbonate in DMF or potassium fluoride prepared on Celite-545. As usual in the [FeCp]+ system, detachment of the substituted arene requires somewhat extreme conditions, usually pyrolytic sublimation at 200 C.46 An example is given in equation (27). 

Many steroid secondary alcohols are resistant to acid-catalysed dehydration, but can be converted into olefins by elimination reactions of their sulphonate esters, usually the toluene 3 -sulphonate ( tosylate ) or methanesulphonate ( mesylate ). These elimination reactions always accompany attempted nucleophilic substitution of the sulphonate (p. 41), and can be made to predominate by appropriate choice of reaction conditions. Activated alkaline alumina has been used to prepare A - and AH-olehns from the sulphonates of 3 - [131] and i2a-alcohols [132]. In the latter case the naphthalene-i-sulphonate was most effective, and the methanesulphonate distinctly less so, suggesting that steric acceleration occurs with the more bulky naphthalenesulphonate [ 132c]. This view is strengthened by the enhanced efficiency of elimination of a i2a-sulphonate in the presence of a 17a-methyl group, which increases the compression at C(i2) [132b]. The mechanism and stereochemical features of these heterogeneous reactions have not been described in detail. 

Nucleophilic aromatic substitutions are a type of reactions frequently applied in the synthesis of chemical intermediates and fine chemicals. In general, these processes are performed in the liquid phase, batchwise, with dissolved copper salts as catalysts [1]. It is of interest to investigate the possibilities of heterogeneous catalysis, as a more convenient catalyst recycle can thus be achieved. 

A much better overlap is provided by the direct attack on the covalent bond, thus favoring frontside attack. Depending on the heterogeneity of the molecular orbital (HOMO), one may envision various possible cases, from a central attack on the covalent bond all the way to a backside attack. The latter rarely occurs in electrophilic reactions of saturated carbon atoms. In fact, a frontside attack as a possible interpretation of the formation of dicyclopropyl-bromonium ion under superacidic conditions (SbF5-SO2ClF, -78°C) was suggested. Other examples of frontside nucleophilic substitution are also known. ... 

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