Organozinc compounds preparation

SCHEME 3. Functionalized organozinc compounds prepared by oxidative addition... 

Yus, M., Gomis, J. Negishi cross-coupling with functionalized organozinc compounds prepared by lithium-zinc transmetallation. 

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. 

Lack of cooling during preparation of the Simmons-Smith organozinc reagent caused the reaction to erupt. The possibly pyrophoric nature of organozinc compounds and the presence of ether presents a severe fire hazard [1]. An alternative, safer method of activating the zinc for the reaction involves use of ultrasonic irradiation rather than the copper-zinc couple [2]. 

Because of the enormous volume of information regarding the use of organozinc derivatives in organic chemistry, this survey is focused only on recent major trends in this area. Valuable information about preparation methods of major types of organozinc compounds and their reactions can be found in two recent books246 247 and reviews. There have been a number of reviews devoted to specific applications of organozinc reagents... 

RCu(CN)ZnI.u These new copper reagents are prepared by reaction of primary or secondary iodides with zinc that has been activated with 1,2-dibromoethane and chlorotrimethylsilane. The resulting organozinc compounds are then allowed to react with the THF-soluble CuCN-2LiCl (equation I). Because of the mild conditions, these new reagents can be prepared from iodides containing keto, ester, and nitrile groups. 

Some synthetically important allenylmetallics, such as allenylzinc and allenylin-dium reagents, are prepared from allenylpalladium intermediates. These reactions are discussed in appropriate sections of this chapter. This section covers the reactions of allenylpalladium compounds without further transmetallation. Allenylpalladium complexes can be prepared from propargylic halides, acetates, carbonates, mesylates, alcohols and certain alkynes [83-87], The allenylpalladium compound prepared from 3-chloro-3-methyl-l-butyne has been isolated and characterized spectroscopically (Eq. 9.106) [83], It was found to couple with organozinc chlorides to produce homologated allenes quantitatively (Eq. 9.107). 

More attractive copper-catalyzed (mediated) transformations of allenes into alkynes were reported by Caporusso and co-workers [27f, 73-75], Allenes 142 were converted into alkynes 143 by treatment with stoichiometric amounts of a cuprate species, as exemplified in Scheme 14.35. The problem of regioselective formation of either alkyne 143 or allene 144 was solved by the proper choice of the organometallic species. Preferential formation of alkynes 143 could be achieved employing cuprates such as R3Cu(CN)ZnCl-LiCl, which are prepared from organozinc compounds. On the other hand, reactions of organomagnesium derived cuprates (R3CuBr)Mg-LiBr mostly provided allenes 144 as major components. 

Functionalized organozinc halides are best prepared by direct insertion of zinc dust into alkyl iodides. The insertion reaction is usually performed by addition of a concentrated solution (approx. 3 M) of the alkyl iodide in THF to a suspension of zinc dust activated with a few mol% of 1,2-dibromoethane and MeaSiCl [7]. Primary alkyl iodides react at 40 °C under these conditions, whereas secondary alkyl iodides undergo the zinc insertion process even at room temperature, while allylic bromides and benzylic bromides react under still milder conditions (0 °C to 10 °C). The amount of Wurtz homocoupling products is usually limited, but increases with increased electron density in benzylic or allylic moieties [45]. A range of poly-functional organozinc compounds, such as 69-72, can be prepared under these conditions (Scheme 2.23) [41]. 

The photochemistry of true organomagnesium compounds remains almost completely unexplored. A literature search in preparation of this work found only a few scattered examples of photochemical studies, mostly in relation to Grignard reactions and 1,3-diketonate chelates Similar to the situation with organozinc compounds magnesium tetrapyrrole chelates, i.e. magnesium porphyrins 1, 5,10,15,20-tetraazaporphyrins (por-phyrazines) 2 and phthalocyanines 3 have found more interest. This is primarily related... 

The stereocontrolled (at the C—Zn center) preparation and reactions of organozinc compounds are, however, an active field of research. Diastereomeric cyclic secondary organozinc halides were prepared as mixtures of isomers that appear to be configurationally stable under the experimental reaction conditions used, although the distribution of products can be electrophile-dependent Other cyclic and acyclic diastereomerically... 

The synthesis of functionalized zinc organometallics can be accomplished with a variety of methods that have been developed in recent years. The intrinsic moderate reactivity of organozinc reagents can be dramatically increased by the use of the appropriate transition metal catalyst or Lewis acid. Furthermore, the low ionic character of the carbon-zinc bond allows the preparation of a variety of chiral zinc organometallics with synthetically useful configurational stability. These properties make organozinc compounds ideal inteimediates for the synthesis of complex and polyfunctionalized organic molecules. 

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