organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, carboboration is an addition of both a

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

and a

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

moiety to certain carbon-containing double and triple bonds, such as

alkenes In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

alkynes \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and no ...

, and

allenes In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms (, where R is H or some organyl group). Allenes are classified as cumulated dienes. The parent compound o ...

. In the synthesis of

organic compounds Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

, this

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

is used to install a new carbon-carbon bond and carbon-boron bond. The product of carboboration reactions are organoborane compounds which prove to be useful in

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

, containing both a new carbon group and a boron handle for further functionalization. This carbon-boron bond allows for

organoboron chemistry Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes. Or ...

, which facilitates a wide variety of chemical transformations such as

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

and the

Suzuki Reaction The Suzuki reaction or Suzuki coupling is an organic reaction that uses a palladium complex catalyst to cross-couple a boronic acid to an organohalide. It was first published in 1979 by Akira Suzuki, and he shared the 2010 Nobel Prize in Chemi ...

. The carbon-boron bond can be transformed into a variety of functional groups and moieties, making it highly useful in pharmaceutical chemistry and organic synthesis. Carboboration was developed soon after the advent and widespread use of

hydroboration In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon (, , , and ). This chemical reaction is useful in the organic synthesis of organic compounds. Hydroboration ...

. Carboboration is often facilitated via catalysis, often employing

transition metals In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

, and usually involves an activated alkene or alkyne. The two most well-documented categories of carboboration are 1,1 and 1,2 carboboration, which differ in the

regioselectivity In organic chemistry, regioselectivity is the preference of chemical bonding or breaking in one direction over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a str ...

of the incoming carbon group.

1,1 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 A 1,2-rearrangement or 1,2-migration or 1,2-shift or Whitmore 1,2-shift is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over l ...

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. can be

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

germanium Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...

tin Tin is a chemical element; it has symbol Sn () and atomic number 50. A silvery-colored metal, tin is soft enough to be cut with little force, and a bar of tin can be bent by hand with little effort. When bent, a bar of tin makes a sound, the ...

, or

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

compounds with various substituents or ligands. and BR₂ are typically cis to one another in the Wrackmeyer reaction, with some exceptions. The_Wrackmeyer_Reaction

Mechanism

Wrackmeyer-type 1,1 carboboration is proposed to go through a zwitterionic intermediate, and this intermediate has been isolated and characterized in some cases. However, the mechanism can be highly substrate and reagent dependent. In a

borane Borane is an inorganic compound with the chemical formula . Because it tends to dimerize or form adducts, borane is very rarely observed. It normally dimerizes to diborane in the absence of other chemicals. It can be observed directly as a c ...

, the compound typically adopts a

trigonal planar molecular geometry In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands a ...

, making the boron atom an electrophilic center. The substituents can affect the strength of the borane as a

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...

. Boranes which are stronger Lewis acids are better electrophiles and therefore better able to facilitate carboboration. Boranes can be optimized to work on less activated substrates. Tris(pentafluorophenyl)borane (C₆F₅)₃is a strongly Lewis acidic borane which functions well in 1,1 carboborations with both activated and unactivated substrates, and it allows for the reaction to be facilitated with more mild conditions. An activated substrate such as an alkene or alkyne has an

electron-withdrawing group An electron-withdrawing group (EWG) is a Functional group, group or atom that has the ability to draw electron density toward itself and away from other adjacent atoms. This electron density transfer is often achieved by resonance or inductive effe ...

directly attached to a carbon within the double or triple bond. Transition metal catalysts have been utilized to develop enantioselective 1,1 carboborations on unactivated alkenes. These reactions go through a

catalytic cycle In chemistry, a catalytic cycle is a multistep reaction mechanism that involves a catalyst. The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, bioinorganic chemistry, materials s ...

which may or may not go through a zwitterionic intermediate. Zwitterionic_intermediate_in_Wrackmeyer_reaction

Examples

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 Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

, and

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

, which are often coupled with an organoborane or a boron source with an

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively Electric charge, charged, have an ...

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

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. However, the Cu- and Ni-catalyzed reactions can proceed through similar mechanisms. These two mechanisms mainly differ in the

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

of the active catalyst and how the carbon group is delivered to the substrate: whether the C–C bond is formed via migratory insertion from the catalyst ( inner sphere) or attack by an external nucleophile ( outer sphere). Wacker-type carboborations, catalyzed by Pd^II, are much rarer than Heck-type. The first example of a Wacker-type 1,2 carboboration was reported by the Engle group in 2019. Despite the common trend of utilizing transition metals, transition metal-free processes have also been developed, such as utilizing boronic acids or light-mediated radical initiation. 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.

Examples

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

{{reflist Organic chemistry Carbon