Secondary organozinc reagents

Selective cross-coupling of secondary organozinc reagents... 

Enantioselective preparation of the secondary organozinc reagent 309 and its reaction with MesSnC " ... 

Harada T, Kotani Y, Katsuhira T, Oku A (1991) Novel method for generation of secondary organozinc reagent application to tandem carbon-carbon bond formation reaction of... 

A conjugate addition of l,l-his(iodozincio)ethane to a,y3-unsaturated ketone arises the interest for diastereoselectivities. As shown in Scheme 8.35, its diaste-reoselective 1,4-additions of l,l-bis(iodozincio)ethane were performed to give a secondary organozinc reagent with diastereomercally enriched form [40]. 

Organozinc reagents have been used in conjunction with a-bromovinylboranes in a tandem route to Z-trisubstituted allylic alcohols. After preparation of the vinylborane, reaction with diethylzinc effects migration of a boron substituent with inversion of configuration and exchange of zinc for boron.176 Addition of an aldehyde then gives the allylic alcohol. The reaction is applicable to formaldehyde alkyl and aryl aldehydes and to methyl, primary, and secondary boranes. 

The benzotriazole derivatives 111, obtained from benzotriazole, ethyl glyoxylate and secondary amines (diethylamine, pyrrolidine, piperidine or morpholine), furnish the amino esters 112 by the action of organozinc reagents R2ZnX (R2 = Me, Bu, PhCH2 or Ph)120. 

Crucially, this allows organozinc reagents to be prepared from less reactive aryl bromides and secondary or tertiary alkyl bromides. Alternatively, organozinc iodides can be prepared by means of a palladium(0)-catalyzed reaction between alkyl iodides and Et2Zn (Scheme 2.25) [53-56]. 

Aryl and alkyl organozincate reagents, generated in situ by reaction of Grignard reagents and sub-stoichiometric amounts of ZnCl2, cross-couple smoothly in refluxing THF with functionalized aryl and alkenyl as well as primary and secondary alkyl chlorides in the presence of Pd(dppf)Cl2. ... 

In early 1990 we began to work on catalyzed carboligations using organozinc reagents [1]. The reactions commonly involved diethylzinc 2, which was added enantioselectively to aldehydes 1 affording secondary alcohols 3. Initially, bipyridine 4 [2-6], pyridine 5 [3], and sulfoximines 6 [7] were applied as catalysts, which in some cases led to enantioselectivities greater than 95% ee [8, 9]. 

The high chemoselectivity of organozinc reagents also allowed the use of the functionalized secondary a-acetoxy alkyl bromide 50 which was converted to 34a (equation 19)36, a product previously obtained by the cyclization of a 5-ethylenic primary iodide bearing the acetoxy group at the allylic position (equation 13). 

One of the first reports dealing with the carbozincation of alkenes by allylic organozinc reagents was the addition of allylzinc bromide to the homoallylic amine 118 which occurred in refluxing THF. A secondary organozinc 119 was regioselectively produced and provided the amine 120 after hydrolysis. A small amount of aminoalcohol 121, resulting from oxidation of 119, was also isolated when the reaction mixture was exposed to air (equation 52)80,81. 

Recently, three research groups independently disclosed that benzotriazoles 18 attached through various linkers to a polymeric support react with aldehydes and amines to form polymer-anchored Mannich-type adducts 19 (Scheme 7) [29]. These intermediates are cleaved under reducing conditions and in the presence of organomagnesium or organozinc reagents to provide libraries of secondary and tertiary amines in moderate yield (11-65%) and with acceptable purity (13->99%) [29]. 

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