Alkoxy groups, nucleophilic 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. 

Alkoxy-2,l-benzisoxazole-4,7-diones undergo ready nucleophilic displacement of the 3-alkoxy substituent, yielding 3-alkylamino and 3-dialkylamino derivatives with primary and secondary amines, respectively (67TL4313). In this instance the 4-carbonyl group apparently provides an activating effect. 

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. 

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. 

Alkyl and aryl azides are prepared by the nucleophilic displacement by azide ion on halide, sulfate, phenyldiazonium, hydroxyl, nitrate, iodoxy, alkoxy, and tosylate groups [6]. Sodium azide is the most useful and practical reagent. The use of silver azide is not necessary in most cases. Some examples from the literature [8-33] employing these methods are shown in Table I. 

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>. 

An alkoxy group at C-l of isochroman (711) is displaced by nucleophiles such as phenols or alcohols and a variety of interesting compounds have thus been obtained (Scheme 39) (80CPB2967). 

Nucleophilic displacements on the sp2 carbons at C-3 or C-5, in contrast to electrophilic reactions, are plentiful. Chlorine, for example, may be displaced by amino, hydroxy, or alkoxy groups173 (Eq. 63). The ether group in 84 may in turn be displaced by hydroxy or amino (as shown) groups. 

The mechanism for amide formation proceeds via attack by the ammonia molecule, which acts as a nucleophile, on the carboxyl carbon of the acid chloride or ester. The alkoxide ion that forms assists with the displacement of the chloride ion or alkoxy group. 

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