A-bromostyrene

The first genuine example of a solvolytic generation of vinyl cations was the pioneering work of Grob and co-workers (121) on substituted a-bromostyrenes... 

It is evident that the results of Grob on the a-bromostyrene system, 145, are most consistent with path D i.e., a unimolecular ionization and formation of an intermediate vinyl cation. Further evidence is provided by the very large effect of substituents upon the solvolysis rate, with the p-amino compound, for example, reacting some 10 times faster than the parent bromostyrene. The log... 

The Br/Mg-exchange reactions on alkenyl bromides are very sluggish. This problem was overcome by the use of 3-Bu2Mg LiCP . For example, Grignard reagent 66 is obtained by reaction of a-bromostyrene (67) with the complex 40 for 1 h at 25 °C. Quenching with benzaldehyde gives the allylic alcohol 68 in 93% yield (equation 41). 

Use of Alkylzincs and 2-Bromo-l,3-dienes, 2-Bromo-l,3-enynes, or a-Bromostyrenes... 

The double conjugate addition of sulfide to the dienones 540 leads to 3-aryltetrahydrothiopyran-4-ones. The dienones are obtained from the reaction of electron-rich a-bromostyrenes with ot,(3-unsaturated aldehydes. When the latter is 3-phenylpropenal, a diastereomeric mixture resulted in which the trans-isovaet was predominant (Scheme 223) <2006EJ04044>. 

The most simple a-aryl substituted vinyl halide studied, a-bromostyrene, flash photolytically yields the enol of acetophenone in aqueous solution, which has been used to measure rates of ketonization302,303. With a,/2-diarylvinyl halides, loss of the /2 (vinylic) proton from the intermediate vinyl cation is competitive with nucleophilic trapping Irradiation of compounds such as 96 in methanol gives about equal amounts of tolan and... 

Charge a two-necked round-bottomed flask (25 mL) equipped with a magnetic stirrer bar and fitted with a condenser and nitrogen inlet, with a-bromostyrene (0.26 mL, 0.366 g, 2 mmol), anilinium phosphinate 3572 (0.382 g, 2.4 mmol), anhydrous triethylamine (0.84 mL, 0.61 g, 6.0 mmol), palladium(ll) acetate (9.0 mg, 0.04 mmol), dppp (19.8 mg, 0.048 mmol), and benzene (10 mL). 

The dibromo-acid yields a dihy droxy-acid when treated with water alone, but with aqueous sodium carbonate it gives a-bromostyrene ... 

One of great synthetic applications of electrochemical fixation of carbon dioxide is synthesis of 2-arylpropanoic acids, nonsteroidal anti-inflammatory drugs (NSAlDs), and their derivatives. Electrochemical carboxylations of benzyl halides [1, 3, 8-13], aryl methyl ketones [14, 15], and a-bromostyrenes [16] are reported to be successfully applied to the synthesis of several NSAIDs, such as ibuprofen and naproxen, and their precursors and derivatives (Scheme 4). 

An interesting example of aromatic unimolecular solvolysis is the interaction of/ ara-substituted a-bromostyrenes with 80% ethanol to yield the corresponding acetophenones and small amounts of the phenylacetylenes . A two-step l-iSl mechanism (reaction 18)... 

The corresponding ( )-alkenyl halides (eq 51) as well as a-bromostyrenes are also readily accessible from the TMSA-ethynylated products. 

The Br/Mg exchange reaction on alkenyl bromides is usually slow. Using more reactive 5ec-Bu2Mg LiCl allows to overcome this difficulty. Thus, the styrylmagnesium reagent 45 can be obtained by reaction of a-bromostyrene (46) with this complex at 25 °C... 

Reaction of hindered trialkylsilyl ethers with Ph3PBr2 in CH2CI2 at rt affords primary and secondary alkyl bromides in excellent yield (70-94%) the reaction is valuable in the p-lactam field. The reaction rate is increased by addition of a catalytic amount of Zinc Bromide. Silylated enol ethers, such as trimethylsilyl(l-phenylvinyloxy)silane, provide vinyl bromides such as a-bromostyrene with Ph3PBr2 in refluxing CCL... 

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