Benzynes substituted

Effect of ortho substituents on patterns of benzyne substitution. 

The Elimination-Addition Mechanism of Nucleophilic Aromatic Substitution Benzyne... 

THE ELIMINATION-ADDITION MECHANISM OF NUCLEOPHILIC AROMATIC SUBSTITUTION BENZYNE... 

Although nucleophilic aromatic substitution by the elimination-addition mecha nism IS most commonly seen with very strong amide bases it also occurs with bases such as hydroxide ion at high temperatures A labeling study revealed that hydroly SIS of chlorobenzene proceeds by way of a benzyne intermediate... 

Nucleophilic aromatic substitution can also occur by an elimination-addition mechanism This pathway is followed when the nucleophile is an exceptionally strong base such as amide ion m the form of sodium amide (NaNH2) or potassium amide (KNH2) Benzyne and related arynes are intermediates m nucleophilic aromatic substitutions that pro ceed by the elimination-addition mechanism... 

The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

The benzyne adducts prepared from A -methylpyrrole (and A -methylisoindole) are deaminated conveniently by dichlorocarbene generated under phase-transfer conditions (81JOC1025 to give a convenient route to substituted naphthalenes (134) (and anthracenes) (Scheme 49). 

Addition of nucleophiles sueh as ammonia or alcohols or their conjugate bases to benzynes takes place very rapidly. These nueleophilie additions are believed to involve eapture of the nueleophile, followed by protonation to give the substituted benzene. ... 

Tetrafluorobenzyne, generally generated by the treatment of pentafluoro-benzene with butyllithium at -78 °C in ether in the presence of the substrate diene, is a versatile dienophile [9, 103, 104], In an interesting study of the use of substituted benzynes to synthesize isoindoles, tetrafluorobenzyne, 4-fluorobenzyne, and 4-(tntIuoromethyl)benzyne were shown to react in moderate yields with A7-(trimethylsiIyl)pyrroles, with the adducts being easily converted to the respective fluorinated isoindoles [705] (equation 87). 

According to Roberts et al. the direction of addition of ammonia to 3-substituted benzynes might be predicted by considering the amide ion to add so as to provide the most favorable location of the negative charge with respect to the inductive effect of the orienting substituent. Thus, ammonia adds to 3-methoxybenzyne (39) producing chiefly n-aminoanisole (40). 

Halobenzenes undergo nucleophilic aromatic substitution through either of two mechanisms. If the halobenzene has a strongly electron-withdrawing substituent in the ortho or para position, substitution occurs by addition of a nucleophile to the ring, followed by elimination of halide from the intermediate anion. If the halobenzene is not activated by an electron-withdrawing substituent, substitution can occur by elimination of HX to give a benzyne, followed by addition of a nucleophile. 

The reaction of pyridazine 1-oxide or 3- and/or 6-substituted pyridazine 1-oxides with benzyne gives 1 -benzoxepins 2 in variable yield. As byproducts, the respective 3-(2-hydroxyphenyl)pyrid-azines 3 can be isolated in 0-20% yield.88... 

In the context of this section it is important that Ruchardt and Tan (1970 a) found that (solid) benzenediazonium fluoroborate gave benzyne adducts with potassium acetate in the presence of aryne trapping agents such as tetracyclone or anthracene. This is, however, not the case if water is present (Cadogan, 1971). As a consequence of these observations, Cadogan et al. (1971) simplified the formation of arynes from diazonium ions by converting aniline or its substitution products into arynes in a... 

Unactivated aryl halides can be converted to amines by the use of NaNH2, NaNHR, or NaNR2. With these reagents, the benzyne mechanism generally operates, so cine substitution is often found. Ring closure has been effected by this type of reaction,for example. 

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