Organozinc reagents addition reactions

Henry reactions 317-20 hydrosilylations 333 organozinc reagents, addition to ketones 156-80 PrMgCl to cyclohexenone with crown thioether ligands 100-1 see also specific ZnEt2 and ZnR2 addition reactions... 

Copper-catalyzed Enantioselective Conjugate Addition Reactions of Organozinc Reagents... 

The addition usually takes place from the sterically less hindered side of the alkene. The stereochemical course of the addition can be controlled by suitably positioned oxygen center that can coordinate to the organozinc reagent. For example the reaction with 4-hydroxycyclopentene 6 as substrate exclusively yields the c -3-hydroxybicyclo [3.1.0] hexane 7 ... 

Even if hundreds of chiral catalysts have been developed to promote the enantioselective addition of alkylzinc reagents to aldehydes with enantioselectivities over 90% ee, the addition of organozinc reagents to aldehydes is not a solved problem. For example, only very few studies on the addition of vinyl groups or acetylides and even arylzinc reagents to aldehydes have been published, in spite of the fact that the products of these reactions, chiral allylic, propargylic and aryl alcohols, are valuable chiral building blocks. 

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. 

Addition of Organozinc Reagents to Aldehydes, Ketones, and a-Ketoesters 2.06.16.2.1 Chiral ligands used in addition reactions of diorganozincs with aldehydes... 

One of the important new directions in the study of addition reactions of organozinc compounds to aldehydes is the use of ionic liquids. Usually, application of these compounds in reactions with common organometallic reagents has a serious problem ionic solvents are usually reactive toward them, particularly Grignard and organolithium derivatives. It has been recently reported that carbonyl compounds react with allylzinc bromide formed in situ from allyl bromide and zinc in the ionic liquid 3-butyl-l-methylimidazolium tetrafluoroborate, [bmim][BF4].285 Another important finding is that the more reactive ZnEt2 alkylates aldehydes in a number of ionic liquids at room temperature.286 The best yields (up to 96%) were obtained in A-butylpyridinium tetrafluoroborate, [bpy][BF4] (Scheme 107). 

Overall, green solvents appear to be a valuable alternative to conventional solvents in addition reactions of diverse organozinc reagents to aldehydes. 

Feringa and co-workers described the tandem addition-aldol cyclization protocol leading to the formation of 6,6-, 6,7-, and 6,8-annulated bicyclic systems (Scheme 68).39 Using Cu(n)-29 as catalyst and functionalized organozinc reagents as nucleophiles, the conjugate addition reaction followed by aldol cyclization can offer highly enantioselec-tive annulation products (up to 98% ee). This method can be used in the synthesis of carbocyclic compounds, such as steroids, terpenes, and other natural products. 

Feringa, B. L. Naasz, R. Imbos, R. Arnold, L. A. Copper-catalyzed Enantioselective Conjugate Addition Reactions of Organozinc Reagents. In Modem Organocopper Chemistry Krause, N. Ed. Wiley-VCH GmbH Weinheim, 2002 Chapter 7, pp 224—258. 

---