Styrenes reaction with arenes

Alkyl radical addition reactions to styrene chromium tricarbonyl can be accomplished using alkyl halides (10 equiv) and (TMSlsSiH (5 equiv) in the presence of AIBN in refluxing benzene, for 18 h (Reaction 66). " These reactions are believed to proceed through intermediates in which the unpaired electron is interacting with the adjacent arene chromium tricarbonyl moiety since the analogous reaction with styrene affords only traces of addition products. 

Another important class of dienes which react readily with ADC compounds are the vinyl arenes. However, the reaction of styrene with ADC compounds does not give simple tetrahydrocinnolines. Styrene and DEAZD give a 1 2 adduct formed by the initial Diels-Alder adduct reacting rapidly in an ene reaction with more DEAZD (Eq. 18).181 Substituted styrenes react... 

The first systematic investigations of the catalytic Friedel-Crafts-type reaction with alcohols and olefines were performed by Yamamoto and colleagues. After reporting the development of a Pd-catalyzed method for the allylation of different naphthol derivatives [24], the authors used Mo(CO)g for the Friedel-Crafts-type alkylation of electron-rich arenes with allyl acetates [25], The same molybdenum catalyst was additionally used for a Friedel-Crafts-type alkylation of arenes using 1-phenylethanol and styrene as alkylating reagents [26], However, Mo(CO)g is toxic and must be handled under strictly inert conditions. Thus, more stable Lewis acids were necessary. 

Mukai et al. (36,37) applied the chiral tricarbonyl(r -arene)chromium(0)-derived nitrone 24b in 1,3-dipolar cycloadditions with various alkenes, such as styrene 25 (Scheme 12.11). The analogous nonmetallic nitrone 24a was used in a reference reaction with 25, giving the isoxazohdine 26a with an endo/exo ratio of 82 18. By the apphcation of nitrone 24b in the 1,3-dipolar cycloaddition with 25, the endo/exo-selectivity changed significantly to give exo-26b as the only observable product. The tricarbonylchromium moiety effectively shielded one face of the nitrone, leading to high diastereofacial selectivity. The product exo- 26b was obtained with 96-98% de. 

Very recently, the reaction of styrene 3,4-epoxide (245) with ethyl mercaptan has been reported.148 A mixture of 2-, 3-, and 4-ethylthiostyrenes is formed in the ratio of 1 9 7 along with small amounts (18%) of 4-vinylphenol. These results can be explained as nucleophilic attack on the intact arene oxide and the reaction of the zwitterion formed by the spontaneous reaction. 

Scheme 6.15 Reaction of styrene and 4-chlorostyrene with various (hetero)arenes GC conversion of styrene/GC product yield (regioselectivity). Reaction conditions 0.5 mmol styrene/ 4-chlorostyrene, 10mol% FeCl3, 5mLarene, 80°C, 4h (a)...

The reaction of styrenes with (difluoroiodo)arenes always leads to rearranged products, i.e. geminal rather than vicinal difluorides.1,1-Diphenylethene, for example, reacts with (difluoroiodo)benzene to yield 1,1-difluoro-1,2-diphenylethane (24), not the expected 1,2-difluoro-l,l-diphenylethane. Examples of reactions with (difluoroiodo)arenes are found in refs 109 and 156-159. 

Even superheated (to 260 °C) or supercritical (to 400 °C) water was employed in the Heck reaction with several catalyst precursors and aryl halides with styrene. However, all conversions show large amounts of side products and the yields were in the 5-30% range, indicating radical intermediates and by-products from decomposition of the arene starting material [15]. 

The failure of tertiary (AI,IV-dimethylaminoalkyl)arenes and stilbenes to undergo intramolecular addition may reflect structural differences between inter- vs. intramolecular exciplexes. Polar solvents are generally required for the observation of in-termolecular addition reactions of tertiary amine exciplexes. Equilibration between solvent-separated and radical ion pairs may be necessary in order to achieve an appropriate reaction trajectory for a-C-H proton transfer. In the case of intramolecular exciplexes with short chain linkers, electron transfer in polar solvents may occur in extended geometries which are inappropriate for proton transfer and chain folding may not compete effectively with exciplex decay. The exceptions to these generalizations, benzene and styrene, form more localized anion radicals which undergo both inter- and intramolecular reactions with tertiary amine cation radicals in nonpolar solvents. 

Further studies by Bargon and co-workers used PHIP to obtain evidence for coordination of styrene hydrogenation products to rhodium through the arene group." Upon reaction of styrene and p-Wz with [Rh(cod)(dppbu)]BF4 as catalyst (dppbu= l,4- A(diphenylphosphino)butane), polarized resonances for the ethyl resonances of the ethylbenzene product appeared in the NMR spectrum in addition to resonances further upfield with the same coupling constants. The fact that the former resonances were downfield relative to those of free ethylbenzene led to the suggestion that the ethylbenzene product remained bound to the Rh center immediately after it was produced (Scheme 9). (The iridium analogs of these complexes are thermodynamically stable and had been isolated and... 

The first waste-free vinylation of arenes under C—H activation is as old as the Mizoroki-Heck reaction itself already in 1967, Moritani and Fujiwara [6] revealed a stoichiometric reaction of styrene-palladium(II) chloride dimers with benzene in the presence of acetic acid to give stilbenes in a modest 24% yield. During this process, the palladium(II) precursor is reduced to palladium(O), so that the key to closing the catalytic cycle was to add an efficient reoxidation step to regenerate an active palladium(II) species. One year later, the same group presented a first approach, substoichiometric in palladium. 

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