Rieke’s activated zinc

General procedure for the eyelization involving Rieke s activated zinc (Scheme 7-39)... 

The zinc-mediated Reformatsky reaction is one of the classical methods for carbon-carbon bond formation. To date, various main group metals and transition metals have been used for this reaction. Rieke s activated indium powder mediates readily the coupling of ethyl a-bromoacetate and a variety of carbonyl compounds yielding /3-hydroxy esters in good yields (Scheme 87).3 Later, commercially available indium powder has been found to be equally effective for the indium-based Reformatsky reaction in THF.28 This indium Reformatsky reaction is accelerated by ultrasound irradiation (Scheme 88).322,323 Indium(i) iodide also mediates the Reformatsky reaction of aldehydes and ketones to give /3-hydroxy esters, presumably via organoindium(m) diiodide (Scheme 89).27... 

Lithium halides (bromide or Iodide) may well modify the Lewis character of the zinc atom, probably via a zincate species [53], and prevent the efficient coordination of the zinc atom to the double bond, coordination which is required for the carbocyclization. Thus, in the Rieke method, it is essential to wash the active zinc thoroughly since the lithium naphthalenide reduction of zinc bromide also generates lithium bromide, which is detrimental to the success of the reaction. Indeed, the insertion of Rieke s zinc in the presence of LiBr leads to the linear organozinc iodide but not to the cyclic product [52]. 

Rieke, R. D., Uhm, S. J. Activated metais. Xi. improved procedure forthe preparation of 3-hydroxy esters using activated zinc. Synthesis 1975,452453. 

The lithium reduction of zinc (II) cyanide using naphthalene or biphenyl as a catalytic electron carrier yields a more reactive form of Rieke zinc. This new form of Rieke zinc is able to undergo direct oxidative addition to alkyl chlorides under mild conditions and tolerates the presence of nitriles and bulky tertiary amides [17]. Table 3.5 shows representative reactions of alkyl zinc chloride reagents with benzoyl chloride. The activation of the zinc surfeice could originate by the adsorption of the Lewis base cyanide ion on the metal surface. The adsorbed cyanide ion can affect the metal s reactivity in two possible ways. One possible mode of activation would be the reduction of the metal s work function in the vicinity of the adsorbed cyanide ion, and the second could be that the cyanide ion is acting as a conduction path for the transfer of the metal s electrons to the alkyl chloride. One or both processes could account for the observed enhanced chemical reactivity. 

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