Organozinc reagents reaction with carbonyl compounds

Several papers are concerned with the threo-erythro stereoselectivity of the reaction of allylic organozinc reagents with carbonyl compounds. The addition (involving allylic rearrangement) of crotylzinc derivatives to various aldehydes occurs stereoselectively, and the relative amount of tAreo-alcohol increases with increasing steric demand of the group R tert-Bu, S4% f-Pr, 707o n-Bt, 46%) and in the sequence of metals Mg < Zn < Cd 3, 7). The temperature or the polarity of the solvent... 

Reductive coupling of polyhaloalkanes with carbonyl compounds in the presence of Zn is used in the synthesis of various halogen-substituted alkenes. For example, the reaction of benzaldehyde with the organozinc reagent 4.101 gives the trifluoromethyl-substituted alkene 4.102 (Scheme 4.50). 

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). 

Recently, even organozinc carbenoids have been used for the preparation of vinylcyclo-propanes. These species are formed upon reaction of zinc with an aldehyde or ketone in the presence of 1,2-bis(dichlorodimethylsilyl)ethane. A synthetic procedure that avoids competitive carbonyl coupling has been developed the carbonyl compound is added to a refluxing mixture of the reagents, namely amalgamated zinc, the silyl chloride and two equivalents of the alkene. 

Alternatives which are more useful for /3-hydroxy carbonyl compounds are organometallic reagents. Organozinc reagents (67) react with aldehydes and ketones but not with esters so they can be made from a-halo esters. This, the Reformatsky reaction, has the advantage that the -halo esters are easily made (see Chapter 7). 

A "green chemistry" alternative to the Grignard reaction was introduced in Experiment 36. This was the preparation of an organozinc reagent that was then allowed to react with a carbonyl compound. The reaction results in the formation of a new... 

The Reformatsky reaction is one of the representative reactions of zinc being used as a catalysts [12,35-38,41,64—74]. The Reformatsky reaction is that reaction with a-haloesters, carbonyl compounds and zinc that produces )5-hydroxyester as shown in eq. (5.20). The reaction of a-haloesters with zinc yields organozinc compounds (XZnCH2COOR) which are called the Reformatsky reagents. These Reformatsky reagents are the organozinc compounds which are considered to be the intermediates of the Reformatsky reaction. 

Addition to Carbonyl Compounds. (TMSM)2Zn can be used for the preparation of a wide range of mixed organozinc compounds, in which the (trimethylsilyl)methyl (TMSM) moiety acts as a non-transferable group.As a result only a slight excess of the mixed zinc reagent is required in the reaction with aldehydes, a process which affords secondary alcohols in good yields (eq... 

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