Friedel vinylation

Rctrosynthetic path a corresponds to Pd-catalysed exo-trig cyclization of o-halo-JV-allylanilines. Path b involves the endo-trig cyclization of o-halo-JV-vinyl anilines. Path c is a structurally similar cyclization which can be effected photochemically in the absence of an o-substituent. Retrosynthetic path d involves intramolecular Friedel-Crafts oxyalkylation followed by aromatiz-ation. 

Alkenyl halides such as vinyl chloride (H2C=CHC1) do not form carbocations on treatment with aluminum chloride and so cannot be used m Friedel-Crafts reactions Thus the industrial preparation of styrene from benzene and ethylene does not involve vinyl chloride but proceeds by way of ethylbenzene... 

Vinylic halides and aryl halides do not form carbocations under conditions of the Friedel-Crafts reaction and so cannot be used in place of an alkyl halide or an acyl halide... 

Highly unstable vinyl cations, generated in situ from vinyl triflates have also been arylated (the triflate group is replaced by the aromatics) to give vinyl aromatics under Friedel-Crafts conditions (28). 

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

Various vinyl chloride adducts can be formed under acid-catalyzed Friedel-Crafts conditions. Vinyl chloride can add tertiary alkyl haHdes (54). It can... 

Another important use of BCl is as a Ftiedel-Crafts catalyst ia various polymerisation, alkylation, and acylation reactions, and ia other organic syntheses (see Friedel-Crafts reaction). Examples include conversion of cyclophosphasenes to polymers (81,82) polymerisation of olefins such as ethylene (75,83—88) graft polymerisation of vinyl chloride and isobutylene (89) stereospecific polymerisation of propylene (90) copolymerisation of isobutylene and styrene (91,92), and other unsaturated aromatics with maleic anhydride (93) polymerisation of norhornene (94), butadiene (95) preparation of electrically conducting epoxy resins (96), and polymers containing B and N (97) and selective demethylation of methoxy groups ortho to OH groups (98). 

It is not possible to polymerise vinyl ethers by free-radical-initiated methods but, as with isobutylene polymers, it is possible to make polymers using Friedel-Crafts type catalysts. 

These will not react with benzene under Friedel-Crafts conditions Vinylic halide Aryl halide... 

Despite its utility, the Friedel-Crafts alkylation has several limitations. For one thing, only alkyl halides can be used. Aromatic (atyl) halides and vinylic halides do not react because aryl and vinylic carbocations are too high in energy to form under Friedel-Crafts conditions. 

Thus, a polyester with the ring in the backbone can be prepared by the Friedel-Crafts polyalkylation of vinyl 2-furoate. BF3 Et20 gave similar results at room temperature in methylene chloride. Structure 30 was arrived at by spectroscopy and by alcoholysis of the polymers. 

This section will describe the Friedel-Crafts alkylation reactions of aromatic hydrocarbons with alkenylchlorosilanes containing short chain alkenyl groups such as allyl and vinyl. The reaction will be discussed in terms of the substituent effect on silicon and the arene rings. 

The acyclic oligoynes 23-26 can be cyclized under Friedel-Crafts conditions, i. e. by treatment with AICI3 in CS2, which presumably proceeds via the intermediate tertiary propargylic 27 and )3-silyl-substituted vinylic carbocations of type 28 (Scheme 5). 

In principle, polymers equivalent to those obtained from vinyl and divinyl monomers may be synthesized by this method. The product in the above example possesses the same chain structure as polyethylene. The polymerization process, notwithstanding the likelihood of a metal alkyl intermediate, should conform satisfactorily to stepwise condensation. However, the product, and those obtained by Friedel-Crafts condensation as well, lack the recurrent functional groups which generally characterize condensation polymers. 

To be really satisfactory, a Friedel-Crafts alkylation requires one relatively stable secondary or tertiary carbocation to be formed from the alkyl halide by interaction with the Lewis acid, i.e. cases where there is not going to be any chance of rearrangement. Note also that we are unable to generate carboca-tions from an aryl halide - aryl cations (also vinyl cations, see Section 8.1.3) are unfavourable - so that we cannot nse the Friedel-Crafts reaction to join aromatic gronps. There is also one further difficulty, as we shall see below. This is the fact that introduction of an alkyl substitnent on to an aromatic ring activates the ring towards fnrther electrophilic substitution. The result is that the initial product from Friedel-Crafts alkylations is more reactive than the... 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

Kim N, Sudol ED, Friedel P, El-Aasser MS (2003) Poly (vinyl alcohol) stabilization of acrylic emulsion polymers using the miniemulsion approach. Macromolecules 36 5573-5579... 

Alkynes are readily acylated with acid chlorides under Friedel-Crafts conditions to form, in most cases, fram-p-chlorovinyl ketones through the corresponding vinyl cation intermediate [Eq. (8.16)]. The first study in 1935 reported low yields.11 Later in acylations with acyl triflates, p-keto vinyl triflates were obtained in satisfactory yields.123 When aroyl derivatives are used, the intermediate can undergo cyclization to form indenones. Chlorovinyl ketones formed from terminal alkynes may also react further losing hydrogen chloride to yield conjugated acetylenic ketones 11,13... 

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