Ethyl magnesiation reaction

Scheme 1 Mechanism of the zirconocene-catalyzed ethyl magnesiation reaction...

Scheme 3 Enantioselectivity within the ethyl magnesiation reaction induced by 12 or 14...

The zirconocene catalysts described above are very oxophilic, which provides several synthetically useful transformations. Oxygen substitution at the al-lylic or homoallylic position of an olefin substrate allows for excellent regio-and diastereocontrol in the ethyl magnesiation reactions of a-olefins and dienes [21]. When 29 is substituted with a hydroxyl group (29a), syn 30a is favored over anti in a 95 5 ratio, while substitution with OCH3 (29b) reversed the diastereoselectivity to 11 89 (Eq. 6). Use of THF in place of diethyl ether as the reaction solvent for the reaction of 29a lowered the overall diastereo-... 

The excellent direct magnesiating properties of Turbo-Hauser reagent A extends beyond the confines of aromatic substrates (though these are by far the most frequently encountered substrates in such transformations). For example, the ether-functionalised ester, ethyl 2-ethoxyacrylate, can be selectively magnesiated in a reaction favoured by the stabilising effect of the adjacent carbonyl group, with work-up of the reaction mixture affording a non-aromatic cyclic lactone in an excellent yield (Fig. 16) [39]. 

Although a chlorine-magnesium exchange is a very slow reaction, the presence of four chlorine atoms in tetrachlorothiophene (154) accelerates this exchange (25 °C, 2h) leading to the magnesiated heterocycle 155 that reacts with ethyl cyanoformate providing the thienylester 156 in 78% yield (Scheme 4.32) [45b]. 

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