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Regioselectivity and Stereochemistry of Hydroboration

Regioselectivity and Stereochemistry of Hydroboration

A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.

Hydroboration-Oxidation of Alkenes

Hydroboration-Oxidation of Alkenes

In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...

Radical Substitution: Allylic Bromination

Radical Substitution: Allylic Bromination

In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide...

Regioselectivity of Electrophilic Additions-Peroxide Effect

Regioselectivity of Electrophilic Additions-Peroxide Effect

In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.

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Sterically Unprotected Nucleophilic Boron Cluster Reagents.

Xin Mu¹, Jonathan C Axtell¹, Nicholas A Bernier¹

¹Department of Chemistry and Biochemistry, University of California, Los Angeles, 609 Charles E. Young Drive East, Los Angeles, CA 90095, USA.

Chem

|April 16, 2020

Summary

Researchers developed a new method using the closo-hexaborate cluster anion to create nucleophilic boron reagents. This breakthrough enables the formation of boron-carbon bonds for diverse applications in synthetic chemistry.

Keywords:

Nucleophilic boron alkyl bromides alkyl pseudo halides boron-heteroatom bonds borylation closo-hexaborate cluster cluster deconstruction main group electrophiles stereoinversion

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Published on: March 25, 2017

Area of Science:

Synthetic Chemistry
Organoboron Chemistry
Materials Science

Background:

Modern synthetic chemistry relies on controlling carbon reactivity (electrophilic/nucleophilic).
Achieving a similar reactivity spectrum for boron reagents is challenging.
Readily accessible, unprotected boron nucleophiles are not yet established.

Purpose of the Study:

To demonstrate the nucleophilic substitution capability of the closo-hexaborate cluster anion.
To enable the formation of boron-carbon bonds using a stable boron cluster.
To provide a pathway to versatile tricoordinate boron species.

Main Methods:

Utilizing the bench-stable closo-hexaborate cluster anion.
Reacting the cluster with various organic and main group electrophiles.
Characterizing the resulting boron-carbon bonded molecules.

Main Results:

The closo-hexaborate cluster anion successfully acts as a nucleophile.
A wide range of organic and main group electrophiles were substituted.
New molecules featuring B-C bonds were synthesized.
These molecules can be transformed into useful tricoordinate boron species.

Conclusions:

The closo-hexaborate cluster anion is a viable precursor for nucleophilic boron reagents.
This work expands the synthetic utility of boron clusters in organic synthesis.
The developed method facilitates access to valuable tricoordinate boron compounds.