Organozinc reactions activated zinc

An organozinc compound that occupies a special niche in organic synthesis is iodo-methylzinc iodide (ICH2ZnI). It is prepared by the reaction of zinc-copper couple [Zn(Cu), zinc that has had its surface activated with a little copper] with diiodomethane in diethyl ether. 

Under a high electrolysis current intensity, the zinc(II) ions electroformed in the active zinc vicinity prevent the chemical reaction from becoming the major process. In an undivided cell and under a high electrolysis current intensity, the anodic scoring of zinc is negligible. In this case, the organozinc compounds are produced after reaction of the electrolytic zinc with CF3Br (Scheme 7). 

REFORMATSKY REACTION. Dating back to 1887, this reaction depends on interaction between a carbonyl compound, an a-halo ester, and activated zinc in the presence of anhydrous ether or ether-benzene, followed by hydrolysis. The halogen component for example ethyl bromoacetate, combines with zinc bo form an organozinc bromide that adds to the carbonyl group of the second component to give a complex readily hydrolyzed to carbinol. The reaction... 

Secondary and tertiary alkyl bromides react with active zinc at ambient temperature. However, the reaction time for the oxidative addition can be shortened by heating the reaction mixture at reflux in THF. Methyl 3-bromo-butyrate gives the organozinc bromide intermediate upon reaction with active zinc and reacts with 2-cyclohexen-l-one to afford the 1,4-addition product (Protocol 6, Scheme 2.3).14... 

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]. 

As shown in Scheme 3.5, 5-(l,3-dioxolan-2-yl)-2-furanylzinc bromide 1 was easily prepared as expected by the direct insertion of active zinc to 2-(5-bro-mofuran-2-yl)-l,3-dioxolane under mild conditions. To confirm the formation of the corresponding organozinc reagent, an aliquot of the reaction mixture was quenched with iodine and analyzed by GC and GC-MS. Both analyses... 

The first attempt to synthesize 3-pyriylzinc bromide from the direct reaction of active zinc and 3-bromopyridine in THF at rt and refluxing temperature resulted in low conversion (70%) to the organozinc reagent. Almost the same result was obtained from an extended reaction time (reflux/24h). However, a dramatic improvement in the oxidative addition of active zinc has been achieved by adding 10-20 mol% of lithium chloride to the reaction mixture. More than 99% conversion of 3-bromopyridine to 3-pyridylzinc bromide was... 

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