Zinc-mediated reductive elimination

In 1979, Bernet and Vasella reported the preparation of aldehydes via a zinc-mediated reductive elimination of haloglycosides [15], In this pioneering work, activated zinc and acetic acid were added to a solution of methyl bromoglucoside 1 in ethanol and the solution was stirred at reflux to give aldehyde 2 (Scheme 3.1). 

Alternative methods for the deprotection of the allyl group at the anomeric position include Pd(PPh3)4/AcOH [402], in which the reaction proceeds by the formation of a 7r-allyl complex, or PdCl2/CuCl/02 followed by photolysis in the presence of triethylamine [403]. Per-fluoroalkylation with perfluoroalkyl iodide under sodium dithionite and sodium bicarbonate followed by elimination in the presence of zinc powder and ammonium chloride has also been disclosed as an efficient procedure for deprotection of the anomeric allyl group of carbohydrates (O Scheme 73) [404]. The reaction goes through the intermediacy of a radical addition of a perfluoroalkyl iodide to the double bond followed by Zn-mediated reductive -elimination. 

Ultrasound can give useful improvements in the yield, reaction time, and convenience of organometallic reactions, and these facets of the technique are demonstrated well in its application to the Reformatsky reaction. Yields are improved by up to almost 100 % over the conventional method, and are nerally superior even to those of the trimethyl borate method. Reaction times are reduced by a factor of 5-10, and the need to prepare zinc powder by the active-metal reduction of anhydrous zinc chloride is eliminated. This technique has also been applied to the zinc-mediated dimerization of a,a -dibromo -xylene, although only low yields are obtained, and to the Pd-catalysed zinc coupling of allyl or aryl halides with perfluoroalkyl iodides. ... 

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