Organozincs diethyl zinc

An unusual syn addition to epoxides occurs when 1,3-diene monoepoxides are treated with organozinc reagents. Thus, the cyclic vinyl epoxide 72 was converted to the cis-ethyl-cyclohexenol 75 with diethyl zinc in methylene chloride and trifluoroacetic acid. The syn addition is believed to derive from an initial coordination of the oxiranyl oxygen to the organozinc compound, which then delivers the alkyl group to the same face. This transfer is facilitated by a relaxation of the sp3 hybridization brought about by the Lewis acidic zinc center and the allylic character of the incipient carbocation <020L905>. 

The first organometallic compound of any kind prepared was diethyl-zinc, (C2Hs) Zn and for many years organozinc compounds were important intermediates in organic preparations. Today, except for a few... 

Cahiez, Knochel, and co-workers have developed a mixed catalytic system consisting of MnBr2/GuGl and diethyl-zinc in iV,A -dimethylpropyleneurea (DMPU), which can be used for the stereocontrolled formation of tetrahydro-furan organozinc compounds from readily available unsaturated bromoacetals. The organozinc compounds are readily transmetalated with GuGN-2LiGl, and upon treatment with ethyl (o -bromomethyl)acrylate or ethyl propiolate homoallyl- and allyl-substituted bicyclic tetrahydrofurans are obtained in 71% and 63% yield (Scheme 70). 

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. 

By reaction of an a-halo ester 1 with zinc metal in an inert solvent such as diethyl ether, tetrahydrofuran or dioxane, an organozinc compound 2 is formed (a Grignard reagent-like species). Some of these organozinc compounds are quite stable even a structure elucidation by x-ray analysis is possible in certain cases ... 

Until fairly recently, little was known of the stmcmres and properties of the organozinc compounds occurring as intermediates in varions reactions. Interestingly, the complexforming ability of organozinc componnds had already been recognized very early. In 1858, Wanklyn reported the formation of the ionic sodium triethylzinc complex. One year later, Frankland observed that the formation of dimethylzinc from methyl iodide and zinc was accelerated by the addition of dimethyl ether or diethyl ether. It appeared that separation of the dimethylzinc from the ether was impossible, bnt it lasted nntil 1962 when it was established that dimethylzinc and dimethyl ether form a 1 1 complex in solntion, which is appreciably dissociated in the vaponr phase. ... 

The synthesis and stmctnral characterization of a series of spirocyclic organozincates containing two five- or six-membered metallacycles in which zinc is the central spiro atom, is shown in Scheme 1. Compound 8a was prepared via an elegant one-pot synthesis, starting from 1,5-dichloropentane, ZnCl2 and a lithium/sodinm alloy (1% sodinm) in diethyl ether as a solvent (eqnation 4 in Scheme 1). Snbseqnent treatment of a soln-tion of 8a with TMEDA afforded the corresponding TMEDA complex 8b of which the structnre in the solid state was unambiguously established by an X-ray crystal structnre determination. 

Bis(alk-2-enyl)zinc compounds 12, on addition to 3,3-dimethylcyclopropene, yield more than 90% of organozinc addition compounds 13 that can, by acidolysis, be converted into substituted cyclopropanes. Depending on the substitution pattern of the starting material, the reaction can be seen to proceed with allylic inversion. In contrast to addition reactions with unstrained double bonds, the reaction proceeds smoothly at — 20 to — 5 °C. Diethyl ether or tetrahy-drofuran can be employed as solvent and the cA-addition products are formed stereoselectively and in high yield. 

A series of addition reactions use the more nucleophilic cerium(III)5 " - or zinc(II) derivatives prepared in situ. Thus, ihe organozinc reagent derived from diethyl l-bromo-1,1-difluoromethylphosphonate reacts smoothly with cx-bromoacrylic acid in THF at room temperature in the presence of catalytic quantities of Cul to give the 1,4-addition product in 33% yield. After several transformations, this is converted into diethyl 3-(hydroxycarbonyl)-3-amino-l,l-difluoro-propylphosphonate, an analogue of phosphoserine (Scheme 3.84). °" ... 

Pebal 1861. A further six years passed before Pebal [5] discovered the general method for the preparation of organozinc compounds. Instead of sealed tubes and by using diethyl ether as the solvent alkylzinc reagents could be synthesized in ordinary laboratory glassware. He demonstrated this for the reaction of iodoethane with zinc which had first been activated by reaction with sulfuric acid. 

The only examples of electrophilic amination of organozinc compounds described in the literature correspond to the reaction of an excess of diethyl/ diisopropyl zinc with several chloroamines [61]. The reaction afforded moderate to low yields of substitution products, often as a mixture of amines [62,63]. On the other hand, electrophilic amination using dialkyl azodicarboxylates has been reported for other nucleophilic species, including lithium enolates and silyl enol ethers [64-70]. 

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