Transmetallation boron-zinc exchange

An example for the use of the boron-zinc exchange reaction for copper-mediated SN2 -substitutions of allylic electrophiles is the hydroboration of nitroolefin 130 with diethylborane, followed by successive transmetallation of the borane 131 with diethylzinc and CuCN-2LiCl, and final trapping with allyl bromide to give the product 133 with 83% yield over four steps (Scheme 34).34,34a This transformation again demonstrates the tolerance of the method towards functional groups and acidic hydrogen atoms. 

As mentioned in Section 9.12.2.1.1, the boron-zinc exchange can be performed stereoselectively if diisopropyl-zinc instead of diethylzinc is used. For example, hydroboration of the chiral, racemic endocyclic olefin 134 with diethylzinc, followed by twofold transmetallation and electrophilic capture of the resulting copper intermediate with allyl bromide was used for the highly diastereoselective formation of the stereotriad in product 136 (Scheme 35).35,35a 103 QorreSp0nding enantioselective transformations were carried out with chiral boranes and catalytic amounts of copper salts (see Section 9.12.2.2.2).36... 

Preparation of alkenylzinc reagents by boron-zinc transmetalation from stable organoboronic esters and dialkylzinc is hampered by the instability of the evolved alkenylzinc mixtures of the two reagents turned black and the reaction of the evolved alkenylzinc on remaining pinacol alkenylboronic ester was speculated on the basis of the structure of by-products. Conversely, this boron to zinc exchange can lead to good results if the evolved alkenylzinc is trapped in situ by an electrophile. The reaction of an alkenylboronic ester... 

The hydroboration of allylic silanes proceeds with high diastereoselectivity as demonstrated by Fleming and Lawrence.87 It is difficult to use the newly formed carbon-boron bond for making new carbon-carbon bonds due to its moderate reactivity. However, the B/Zn exchange converts the unreactive carbon-boron bond to a reactive carbon-zinc bond, as in compound 24. A further transmetallation with the THF soluble salt CuCN-2LiCl provides copper reagents, which can be allylated, alkynylated, or acylated (Scheme 6). 

The most general route to organolead tricarboxylates, such as aryllead, vinyllead or alkynyllead tricarboxylates, is the metal-metal exchange which can involve mercury-lead, thallium-lead, silicon-lead, tin-lead, boron-lead or zinc-lead transmetallation. 

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