Carboboration

1,1 carboboration delivers both the carbon-carbon bond and the carbon-boron bond to the same carbon in the substrate. It requires a 1,2-migration of a substituent from one carbon to the other in the double bond. The Wrackmeyer reaction is typically credited as being the pioneering example of 1,1 carboboration and utilizes a metal migrating group to help facilitate the transformation. However, there are several modern examples of carboboration with a variety of migrating groups. The Wrackmeyer reaction involves 1,1 carboboration of a 1-alkynylmetal compound to yield alkenylborane compounds. [M] can be silicon, germanium, tin, or lead compounds with various substituents or ligands. [M] and BR2 are typically cis to one another in the Wrackmeyer reaction, with some exceptions. However, the mechanism can be highly substrate and reagent dependent. In a borane, the compound typically adopts a trigonal planar molecular geometry, making the boron atom an electrophilic center. The substituents can affect the strength of the borane as a Lewis acid. Transition metal catalysts have been utilized to develop enantioselective 1,1 carboborations on unactivated alkenes. These reactions go through a catalytic cycle which may or may not go through a zwitterionic intermediate. == 1,2 Carboboration ==

1,2 carboboration delivers the carbon-carbon bond and the carbon-boron bond to adjacent carbons in the substrate. It is typically facilitated by transition metal catalysis, but transition-metal-free 1,2 carboborations have been developed and continue to be of interest to synthetic chemists. The benefit of utilizing transition metals is that the reactions can often have enantioselective control based on the ligands used on the metal complex. Common metals used are palladium, nickel, and copper, which are often coupled with an organoborane or a boron source with an electrophile or nucleophile. Mechanism The mechanism of carboboration depends highly on the substrate and reagents utilized in the reaction. Shown below are examples of two types of Pd-catalyzed alkene 1,2 carboborations, Heck-type and the Wacker-type. Despite the common trend of utilizing transition metals, transition metal-free processes have also been developed, such as utilizing boronic acids These reactions usually lead to the boron substituent being at the terminus or less substituted side of the substrate, but anti-carborborations have also been developed which produce reverse regioselectivity. Much work has also been done to render 1,2 carboboration enantioselective using various ligands on transition metal catalysts. == 1,n Carboboration ==

A nickel-catalyzed 1,n arylboration was developed in 2019 by Yin and coworkers and remains the only example of a chain-walking arylboration. This was accomplished via a nitrogen-based ligand and a three-component coupling. The general scheme plus proposed mechanism is shown. == References ==