Metal mediated coupling with allylic halides

A two-atom carbocycle enlargement is accomplished by Barbier-type reaction in water (Scheme 8.26) [43]. The coupling reaction of a-keto esters with allyl, propargyl, and allenyl halides using indium metal in aqueous solvents afforded a-hydroxy- y,(5-unsaturated esters (Scheme 8.27) [44]. Indium-mediated allylation of 1,2-diones affords a-hydroxy keto compounds. In some cinnamylation reactions the corresponding a-coupling products are obtained (Scheme 8.28) [45]. 

Similarly, homocoupling and coupling of two different allyl moieties (cross coupling) can be efficiently run with Ni or Pd complexes, the reaction proceeding by jr-allyl type intermediates formation through the oxidative addition of the allyl halide to the metal centre. A mixture of isomers is usually obtained as a result of the allylic resonance, but the reaction is however of interest for the preparation of macrocycles the selectivity being in such cases mediated by geometrical and conformational factors. 

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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