Benzyne, radical cations

This general mechanistic scheme readily explains a number of experimental observations. For instance it is very clear why such ester shifts only ever take place between vicinal carbons [1], as it is only this arrangement that permits the formation of an alkene radical cation as intermediate. Intermolecular ester shifts are excluded for the same reason. Rearrangements of o-(acyloxy)aryl radicals (Scheme 7) [13, 14] and their vinyl counterparts would require the intermediacy of very high energy benzyne radical cations, as such no examples are known. Failed migrations between two secondary radicals (Scheme 8) may now be seen as being due not so... 

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. 

It is clear from the recorded CID data that aniline radical cations are produced in the protonation of aniline, followed by loss of a hydrogen atom. The distonic species are differentiated by a significant signal at m/z 76 for benzyne ions. 

On pyrolysis chlorobenzene gives phenyl radicals and a small amount of benzyne. ) The phenyl cation is the most intense fragment in the mass... 

One of the simplest routes to benzyne starts with aniline which is converted to benzenediazonium ion. Benzyne is liberated when this ion is deprotonated by acetate base in solvents such as benzene, Eq. (20). Detracting from this approach, however, are competing reactions in which phenyl radicals and phenyl cations are formed. Presently it is possible to minimize the importance of radical chain side reactions and thereby to favor benzyne formation by adding a radical trapping agent such as 1,1-diphenylethylene. 8S ... 

There are many other reactions that possibly involve aryne intermediates. While some are of mechanistic curiosities, some have been studied in detail, although none are generally synthetically useful. Irradiation of 1,2-diiodobenzene can lead to ortho-benzyne, probably via an aryl radical intermediate resulting from cleavage of the weak C-1 bond (Scheme 7.19). Aryl cations, formed by the decomposition of diazonium salts are also possible intermediates to ortho-benzynes provided that a large ortho-substituent is present, loss of a proton to give an aryne becomes competitive with the normal nudeophihc addition to the cation. 

To the more usual homolytic fragmentation of aryl halides (from the excited state or from the radical anion, the well known SrnI reaction, for a recent example see the arylation of aromatics), the heterolytic version of the reaction which produces phenyl cations has more recently joined. A theroretic study on the photodissociation of fluorinated iodobenzenes has been published. The perfluoroallgrlation of various alkenes has been obtained by irradiation in the presence of iodoperfluorobutane. The formation of phenyl cations is exemplified in many arylation reactions and, in the case of o-chlorostannane, also a benzyne has been reported. In the field of polymer chemistry, iodonium salts are model cationic photoinitiators. In particular the truxene-acridine/diphenyl iodonium salt/9-vinylcarbazole combination is able to promote the ringopening polymerization of an epoxide, whereas the truxene AD/allq l halide/amine system is very efficient in initiating the radical photopolymerization of an acrylate. ... 

Synthesis and Properties.—The two most general methods for preparing sulphonium ylides continue to be a-deprotonation of a sulphonium salt and the reaction of a sulphide (or disulphide with a carbene. A new development involves the reaction of the thianthrene or phenoxathiin cation radical with a dicarbonyl compound, e.g. ethyl benzoylacetate, to give (4). An infrequently used but useful route to sulphonium ylides involves reaction of sulphides e.g. dithia[3,3]cyclophanes with benzyne. A detailed description of a preparation of the sulphonium salt precursor to Trost s diphenylsulphonium cyclopropylide has appeared. The selectivity of ylide formation in the reaction of cyclic and acyclic sulphides with carbenes has been examined and compared with the much... 

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