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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.

[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.

[3,3] Sigmatropic Rearrangement of Allyl Vinyl Ethers: Claisen Rearrangement

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The Claisen rearrangement is a [3,3] sigmatropic rearrangement of allyl vinyl ethers to unsaturated carbonyl compounds. The rearrangement is a concerted pericyclic reaction proceeding via a chair-like transition state.

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.

Preparation of Diols and Pinacol Rearrangement

Preparation of Diols and Pinacol Rearrangement

Compounds bearing two hydroxyl groups are known as diols. When the hydroxyl groups are located on adjacent carbon atoms, the diols are called vicinal diols or glycols. Under acidic conditions, vicinal diols undergo a specific reaction called pinacol rearrangement.
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α-Alkylation of Ketones via Enolate Ions

α-Alkylation of Ketones via Enolate Ions

Ketones with α protons are deprotonated by strong bases like lithium diisopropylamide (LDA) to form enolate ions. The anion is stabilized by resonance, and its hybrid structure exhibits negative charges on the carbonyl oxygen and the α carbon. This ambident nucleophile can attack an electrophile via two possible sites: the carbonyl oxygen, known as O-attack, or the α carbon, known as C-attack. The nucleophilic attack via the carbanionic site is preferred. This is due to the strong interaction...

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A Protocol for Safe Lithiation Reactions Using Organolithium Reagents

A Protocol for Safe Lithiation Reactions Using Organolithium Reagents

Published on: November 12, 2016

Rearrangement reactions of lithiated oxiranes.

B Ramu Ramachandran¹, Shannon Waithe, Lawrence M Pratt

¹College of Engineering & Science, Louisiana Tech University , Ruston, Louisiana 71272, United States.

The Journal of Organic Chemistry

|October 2, 2013

Summary

This study computationally investigates oxirane reactions with lithium dialkylamides, revealing both β-elimination and α-lithiation mechanisms can occur simultaneously. Ketones primarily form via α-substitution, while dienes result from LiOH elimination.

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Published on: September 8, 2013

Area of Science:

Organic Chemistry
Computational Chemistry

Background:

Oxirane rearrangement reactions are initiated by lithium dialkylamides.
Previous studies proposed β-elimination or α-lithiation as exclusive mechanisms.
Products include allyl alcohols, dienes, and ketones.

Purpose of the Study:

To computationally investigate oxirane rearrangement reactions.
To determine the operational mechanisms and product formation pathways.
To elucidate the roles of β-elimination and α-lithiation.

Main Methods:

First computational study of oxirane rearrangement reactions.
Analysis of reaction mechanisms initiated by lithium dialkylamides.
Investigation of potential pathways like β-elimination and α-lithiation.

Main Results:

Both β-elimination and α-lithiation mechanisms can operate simultaneously.
Allyl alcohols from β-elimination are unlikely to form ketones via 1,3-hydrogen transfer.
Ketones are likely formed through oxirane ring opening after α-substitution.
Diene products result from LiOH elimination from lithiated allyl alcohol.
Low activation barriers explain exclusive allyl alcohol formation in specific cases.

Conclusions:

Computational findings align with experimental product distributions.
The study clarifies the mechanistic pathways in oxirane rearrangements.
Provides insights into the formation of allyl alcohols, ketones, and dienes.