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Cycloaddition Reactions: Overview

Cycloaddition Reactions: Overview

Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

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.

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.

Cycloaddition Reactions: MO Requirements for Thermal Activation

Cycloaddition Reactions: MO Requirements for Thermal Activation

Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.

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.

Preparation of Alcohols via Addition Reactions

Preparation of Alcohols via Addition Reactions

Overview
The acid-catalyzed addition of water to the double bond of alkenes is a large-scale industrial method used to synthesize low-molecular-weight alcohols. An acidic atmosphere is required to allow the hydrogen in the water molecule to act as an electrophile and attack the double bond in an alkene. The addition of a proton to the double bond creates a carbocation intermediate. The proton preferentially bonds to the less substituted end of the double bond to create a more stable carbocation...

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Rethinking Borole Cycloaddition Reactivity.

Felix Lindl¹, Xueying Guo², Ivo Krummenacher¹

¹Institute for Inorganic Chemistry and Institute for Sustainable Chemistry & Catalysis with Boron (ICB), Julius-Maximilians-Universität Würzburg, Am Hubland, 97074, Würzburg, Germany.

Chemistry (Weinheim an Der Bergstrasse, Germany)

|May 19, 2021

Summary

This summary is machine-generated.

Seven-membered borepins are not stable intermediates in reactions with alkynes. Instead, bicyclic boranorbornadienes form fluorescent tricyclic boracyclohexenes, challenging established pericyclic rearrangement mechanisms.

Keywords:

Boron computational chemistry isomer isomerization pericyclic reaction

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Published on: February 7, 2019

Area of Science:

Organoboron chemistry
Organic synthesis
Materials science

Background:

Boroles are versatile compounds with applications in synthesis and materials.
Their reactivity is linked to cyclic conjugated dienes, participating in cycloadditions like the Diels-Alder reaction.
Established mechanisms describe reactions of boroles with alkynes yielding boranorbornadienes, borepins, and boracyclohexenes.

Purpose of the Study:

To investigate the mechanistic pathway of reactions between boroles and alkynes.
To challenge the long-held mechanistic picture of pericyclic rearrangements in these systems.
To clarify the role of borepins in the formation of boracyclohexenes.

Main Methods:

Thermal reactions of boroles with alkynes.
Spectroscopic analysis of reaction products.
Density Functional Theory (DFT) calculations for mechanistic insights.

Main Results:

Seven-membered borepins exist in a dynamic equilibrium with bicyclic boranorbornadienes, not as isolable products.
Heating leads to the formation of fluorescent tricyclic boracyclohexenes from isomeric mixtures.
DFT calculations indicate tricyclic boracyclohexenes form from boranorbornadienes, not borepins.

Conclusions:

The established mechanistic picture of pericyclic rearrangements in borole-alkyne reactions requires revision.
Borepins are not intermediates in the formation of tricyclic boracyclohexenes.
Boranorbornadienes are the direct precursors to the stable, fluorescent tricyclic boracyclohexenes.