Xylylene intermediate, zinc

When zinc and a,a -dibromo-o-xylene are irradiated with ultrasonic waves at room temperature, synthetically useful quantities of the reactive intermediate, o xylylene, are generated which can be treated in situ with activated olefins to give good yields of cycloaddition products(30). Chew and Ferrier used this methodolgy to generate a-xylylene for the synthesis of optically pure functionalized hexahydroanthracenes(31). The reaction with lithium takes a different course(19). Rather than generate the -xylylene intermediate, ionic species are produced. The two fates of a, a -dibromo-o-xylene are presented in the scheme below ... 

Although 1, -elimination of o xylene polyhalides with sodium iodide, zinc, copper, or iron metal is a fundamental method for the formation of o-xylylene intermediates, it is difficult to carry out the reaction under mild conditions such as at room temperature. o Trimethylsilylmethyl)-benzyltrimethylammonium halides were devised for this purpose and were shown to generate the o-xylylene at room temperature. However, we have successfully generated o-xylylene at room temperature by the reaction of a, 0,-dibromo o-xylene with metallic nickel(51). The Diels-Alder reaction of... 

Ultrasonic irradiation of a mixture of zinc and a,a -dibromo-orthoxylene in dioxane results in dehalogenation and the generation of a xylylene intermediate (9) which readily adds to any dienophiles present in the reaction mixture e. g. with maleic anhydride or methyl propenoate to afford high yields of (10) and (11) respectively [95]. In the absence of dienophile the product is mainly polymer with a trace of (12) (Scheme 3.16). The work was performed in a cleaning bath at 25 °C on 10 mmol scale using 23 mmol zinc under N2. There was no reaction in the absence of ultrasound. 

One of the more fundamental methods for generation of the o-xylylene intermediate is 1,4-elimination from o-xylene derivatives. o-Xylene di-, tri-, or tetrabromides undergo intramolecular 1,4-dehalogenation by means of sodium iodide [102], lithium [103], zinc [104], copper [105], iron [106], and chromium [107]. Hoffman degradation of (o-methylbenzyl)trimethylammonium hydroxides [108], as well as the more recently developed fluoride ion-induced elimination from [o-[a-(trimethylsilyl)alkyl]benzyl]trimethylammonium halides [109], have also been effectively utilized in the generation of 2. 

Ultrasound has been exploited in the zinc mediated generation of a number of reactive intermediates. Most of these are only normally attainable in very low yield as a result of the forcing conditions necessary for their generation. On the other hand Han and Boudjouk showed that ortho-xylylene (35) was readily available from o-dibromoxylene. Trapping the intermediate tetraene with methyl vinyl ketone gave an 87 % yield of the product (36) arising from Diels-Alder reaction with the exocyclic diene [154] (Scheme 72). 

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