Nucleophilic aromatic substitution coupling

The general approaches for the synthesis of poly(arylene ether)s include electrophilic aromatic substitution, nucleophilic aromatic substitution, and metal-catalyzed coupling reactions. Poly(arylene ether sulfone)s and poly(arylene ether ketone)s have quite similar structures and properties, and the synthesis approaches are quite similar in many respects. However, most of the poly(arylene ether sul-fone)s are amorphous while some of the poly(arylene ether)s are semicrystalline, which requires different reaction conditions and approaches to the synthesis of these two polymer families in many cases. In the following sections, the methods for the synthesis of these two families will be reviewed. 

Scheme 6.20 Sequential one-pot nucleophilic aromatic substitution and Suzuki cross-coupling reactions.

Over the past decade, literally dozens of new AB2-type monomers have been reported leading to an enormously diverse array of hyperbranched structures. Some general types include poly(phenylenes) obtained by Suzuki-coupling [54, 55], poly(phenylacetylenes prepared by Heck-reaction [58], polycarbosilanes, polycarbosiloxanes [59], and polysiloxysilanes by hydrosilylation [60], poly(ether ketones) by nucleophilic aromatic substitution [61] and polyesters [62] or polyethers by polycondensations [63] or by ring opening [64]. 

Kita and Tohma found that exposure of p-substituted phenol ethers to [bis(tri-fluoroacetoxy)iodo]benzene 12 in the presence of some nucleophiles in polar, less nucleophilic solvents results in direct nucleophilic aromatic substitution [Eq. (84)] [156]. Involvement of a single-electron transfer (SET) from phenol ethers to A3-iodane 12 generating arene cation radicals was suggested by the detailed UV-vis and ESR studies. SET was involved in the oxidative biaryl coupling of phenol ethers by 12 in the presence of BF3-Et20 [157]. 

Substituted 1,2,3-triazole 1-oxides 448 have been reported to undergo electrophilic and nucleophilic aromatic substitution and are subject to debromination, proton-metal exchange, and halogen-metal exchange followed by electrophilic addition. Transmetallation and cross-coupling have not been described. 3-Substituted 1,2,3-triazole 1-oxides 448 can be proton-ated or alkylated at the O-atom and they can be deoxygenated and deal-kylated. The individual reactions are described in Section 4.2.7.1-4.2.7.14. 

Multicomponent Synthesis of Annelated Thiopyranones by Coupling-Addition-Nucleophilic Aromatic Substitution Sequence... 

Scheme 45 Coupling-addition-nucleophilic aromatic substitution three-component synthesis of 4//-thiochromen-4-ones 82, 4/f-thiopyrano[2,3-i)]pyridin-4-ones 83, 2-chloro-4/f-thieno[2,3-b] thiopyran-4-ones 84, or 7//-benzo-[b]thieno[3,2-i)]thiopyran-7-ones 85...

Scheme 46 Mechanistic rationalization of the coupling-addition-nucleophilic aromatic substitution sequence to annelated 4//-thiochromen-4-ones 82-85...

Mechanistically, the one-pot transformation can be rationalized by a sequence of chemoselective coupling of ort/to-halogenated (hetero)aromatic acid chlorides 81 and electron rich terminal alkynes 4, followed by nucleophilic addition of the sulfide ion to the a,p-unsaturated system 86 to furnish the anionic Michael adduct 87, and finally an intramolecular nucleophilic aromatic substitution in the sense of an addition-elimination process concludes the sequence (Scheme 46). 

An aryl halide can also be coupled to an amine using metal catalysis. The reaction represents an alternative to the classical methods for the synthesis of aryl amines, such as reduction of nitro groups and nucleophilic aromatic substitution (see Chapter 8). 

N-arylimidazoles, important compounds in medicinal research, have been synthesized by nucleophilic aromatic substitution and Ulmann-type coupling. Aromatic substitution is, however, limited by the need for substrates activated by electron-withdrawing groups. The arylation of diazoles and triazoles, e.g. imidazole, by p-tolyllead triacetate compares very favorably with the Ullmann and related methods in that the conditions employed are much milder and the yields are usually excellent and reproducible (Scheme 13.43) [64]. 

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