Alkoxy groups, displacement

Delignification Chemistty. The chemical mechanism of sulfite delignification is not fully understood. However, the chemistry of model compounds has been studied extensively, and attempts have been made to correlate the results with observations on the rates and conditions of delignification (61). The initial reaction is sulfonation of the aUphatic side chain, which occurs almost exclusively at the a-carbon by a nucleophilic substitution. The substitution displaces either a hydroxy or alkoxy group ... 

O- Alkylation is comparable to A/-alkylation, but since the sodium salts are water-soluble it is most convenient to treat the phenol or naphthol in aqueous caustic solution with dimethyl sulfate or diethyl sulfate. These are comparatively expensive reagents, and therefore, alkoxy groups are introduced at a prior stage by a nucleophilic displacement reaction whenever possible. 

Nucleophilic displacement of the alkoxy group of the readily obtainable 2-alkoxy-3//-azepines is a popular route to 2-substituted 3/Lazepines,65,81,124 although in some cases77,228 the reaction is slow (20 h). 

Another very mild procedure involves reaction of the alcohol with the heterocyclic 2-chloro-3-ethylbenzoxazolium cation.28 The alcohol adds to the electrophilic heterocyclic ring, displacing chloride. The alkoxy group is thereby activated toward a nucleophilic substitution that forms a stable product, 3-ethylbenzoxazolinone. 

Alkoxy groups can be displaced from boron by alkyl- or aryllithium reagents. The reaction of diisopropoxy boranes with an organolithium reagent, for example, provides good yields of unsymmetrically disubstituted isopropoxyboranes.3... 

In the hope of gaining further experimental insight into the mechanism of nucleophilic displacement at sulfur, the alkaline hydrolysis of chiral dialkoxysulfonium salts was investigated (162). At least two interesting features of these model systems for nucleo-phihc substitution studies should be accentuated both alkoxy groups are of comparable leaving group ability, and there are essentially two identical tetrahedral faces to be attacked by the nucleophile. 

The displacement of both alkoxy groups at sulfur in 239 with predominant inversion of configuration can be explained by the simultaneous formation of two different intermediates (or transition states) 242 and 243, which undergo decomposition before pseudorotation. In this context, it is interesting to note that the formation and decomposition of only one such sulfurane intermediate would give two sulfinate products with inverted and retained configuration at sulfur. 

The displacement of alkoxy groups from polynitroarylenes has been used for the indirect synthesis of some highly nitrated polynitroarylenes. Holleman synthesized 2,3,4,6-tetranitrotoluene (57) by treating 3-methoxy-2,4,6-trinitrotoluene (132) with ammonia in methanol, followed by oxidation of the resulting product (56) with peroxymonosulfuric acid. 

Dibenzothiophene 5-oxide can be 0-alkylated the alkoxy group in the product can be displaced by nucleophiles (Scheme 189) however, l-methoxy-2,3-dibromobenzo[6]thio-phenium perchlorate regenerates the sulfoxide on treatment with piperidine (72JCS(Pl)899>. 

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