Organozinc bromide

SCHEME 12. Iodine catalyzed formation of organozinc bromides... 

The organozinc bromide prepared from ethyl 2-bromoacrylate in the presence of naphthalene can be used for a coupling reaction with a variety of aromatic iodides in quasi-quantitative yields. However, the use of Pd[P(Tol-o)3]2Cl2 as catalyst is required for reactions carried out in the presence of aromatic bromides non substituted or substituted by electron-donating or electron-withdrawing groups. In this case, reactions are performed under reflux in THF. 

REFORMATSKY REACTION. Dating back to 1887, this reaction depends on interaction between a carbonyl compound, an a-halo ester, and activated zinc in the presence of anhydrous ether or ether-benzene, followed by hydrolysis. The halogen component for example ethyl bromoacetate, combines with zinc bo form an organozinc bromide that adds to the carbonyl group of the second component to give a complex readily hydrolyzed to carbinol. The reaction... 

Secondary and tertiary alkyl bromides react with active zinc at ambient temperature. However, the reaction time for the oxidative addition can be shortened by heating the reaction mixture at reflux in THF. Methyl 3-bromo-butyrate gives the organozinc bromide intermediate upon reaction with active zinc and reacts with 2-cyclohexen-l-one to afford the 1,4-addition product (Protocol 6, Scheme 2.3).14... 

Fig. 16.26. Pd(0)-catalyzed atkenylations of the organozinc bromides A and B and the organozinc chloride G with the iodoalkene C and the bro-moalkenes D and E, respectively, as an efficient method for the preparation of trans-configured C=C double bonds.

The organozinc bromide 23ZnBr undergoes a regiospecific 5-exo-dig cyclization to give 24, which was characterized by hydrolysis or iodinolysis to give 25 or 26, respectively as a single isomer [20]. 

Hydrolysis of the reaction mixture afforded 63 in 80% yield as a single cis diastereomer, and the formation of cyclic organozinc bromide was checked by iodinolysis, by coupling reactions with vinyl iodides in the presence of a catalytic amount of Pd(PPh3)4, or by reaction with allyl halides after transmetallation of the organozinc bromide into an organocopper reagent [60] (Scheme 7-53). 

Cure and Gaudemar5 report that yields in the Reformatsky reaction are generally improved if the reaction is carried out in two steps. First, the a-bromo ester (1) is converted into an organozinc bromide (2) by reaction with zinc in pure and dry dimethoxymethane.6 This derivative is apparently formed in almost quantitative yield. The aldehyde or ketone is then added at 0°. The /3-hydroxy ester is obtained in about 60-90% yield. 

The stereoselectivity of the reaction of 1 -alkynylzinc bromides 130 (R == heptyl or Ph) with various chiral a-benzyloxy aldehydes 131 (R = Me, /-Pr or PhCH20CH2) to yield the alcohols 132 depends on the nature of the substituents R and R Regiospecific propargylation with acylsilanes is exemplified by the reaction of the organozinc bromide 133 (from 2-octynyl bromide and zinc dust in THF) with benzoyltrimethylsilane, followed by desilylation, to yield only the alcohol 134 . 

Allylic, benzylic and propargylic bromides will also react with zinc to give the corresponding organozinc bromides. Reaction with both C—C and C—N multiple bonds has already been shown to provide a potentially useful route to some Olefinic compounds [135]. However, the reaction with terminal acetylenes produces mixtures of mono- and bis-addition products (27) and (28) in low yields [136] (Scheme 57). 

Organozinc bromides are formed as intermediates in the R ormatsky reaction in which a zinc derivative of an a-bromo-ester reacts with an aldehyde or ketone ... 

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