Benzyne 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. 

Species such as 5 and 6 are called benzynes (sometimes dehydrobenzenes), or more generally, arynes, and the mechanism is known as the benzyne mechanism. Benzynes are very reactive. Neither benzyne nor any other aryne has yet been isolated under ordinary conditions, but benzyne has been isolated in an argon matrix at 8 where its IR spectrum could be observed. In addition, benzynes can be trapped for example, they undergo the Diels-Alder reaction (see 15-58). It should be noted that the extra pair of electrons does not affect the aromaticity. The... 

To explain the iodo result, it has been proposed that besides the benzyne mechanism, this free radical mechanism is also operating here ... 

Benzyne Mechanism. Two factors affect the position of the incoming group, the first being the direction in which the aryne forms. When there are groups ortho or para to the leaving group, there is no choice ... 

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. 

One other possible short cut is to disconnect the amino group, hoping to add it by a benzyne mechanism from easily made (17). In fact, this reaction gives a mixture of three isomers of (15) in which (15) does not predominate. ... 

Direct high-temperature microwave-assisted aminations of a variety of aryl halides [257] or aryl triflates [258] under transition metal-free conditions have also been reported, probably involving a benzyne mechanism. 

Elimination-addition (benzyne mechanism) could occur. 

With very strong bases, such as amide ion, NHj, unactivated aryl halides undergo substitution by an elimination-addition (benzyne) mechanism. 

Problem 11.28 How do the following observations support the benzyne mechanism (a) Compounds lacking ortho H s, such as 2,6-dimethylchlorobenzene, do not react, (t) 2,6-Dideuterobromobenzene reacts more slowly than bromobenzene. (c) o-Bromoanisole, o-CHjOC H Br, reacts with NaNH /NH, to form m-CHjOC H.NHj. 

Problem 11.29 How is it that the nucleophilic substitution of nitrochlorobenzenes is by addition and elimination, whereas a benzyne mechanism is found with chlorobenzene M... 

Thc benzyne mechanism for this reaction is also supported by NC labeling experiments Bottini Roberts/. Am Chem. Soc. 1957 79, 1458 Dalman Neumann J. Am. Chem. Soc. 1968, 90, 1601. 

Unactivated aryl iodides undergo the conversion Arl — ArCHj when treated with tris(diethylamino)sulfonium difluorotrimethylsilicate and a palladium catalyst.131 A number of methods, all catalyzed by palladium complexes, have been used to prepare unsymmetrical biaryls (see also 3-16). In these methods, aryl bromides or iodides are coupled with aryl Grignard reagents,152 with arylboronic acids ArB(OH)2,153 with aryltin compounds Ar-SnR3,154 and with arylmercury compounds.155 Unsymmetrical binaphthyls were synthesized by photochemically stimulated reaction of naphthyl iodides with naphthoxide ions in an SrnI reaction.156 Grignard reagents also couple with aryl halides without a palladium catalyst, by the benzyne mechanism.157 OS VI, 916 65, 108 66, 67. 

Exercise 14-19 The intervention of benzyne in the amination of chlorobenzene, bromobenzene, and iodobenzene with sodium amide in liquid ammonia originally was demonstrated by J. D. Roberts using 14C-labeled halobenzenes. Show explicitly how the use of a chlorobenzene-14C label could differentiate between amination by addition-elimination (Section 14-6B) versus amination by elimination-addition (benzyne mechanism). 

However, arylation with such systems will occur with strong bases by the benzyne mechanism (Sections 14-6C and 23-8). 

---