Allyl group

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Structure of the allyl group Allyl.svg
Structure of the allyl group

In organic chemistry, an allyl group is a substituent with the structural formula −CH2−HC=CH2. It consists of a methylene bridge (−CH2) attached to a vinyl group (−CH=CH2). [1] [2] The name is derived from the scientific name for garlic, Allium sativum. In 1844, Theodor Wertheim isolated an allyl derivative from garlic oil and named it "Schwefelallyl". [3] [4] The term allyl applies to many compounds related to H2C=CH−CH2, some of which are of practical or of everyday importance, for example, allyl chloride.

Contents

Allylation is any chemical reaction that adds an allyl group to a substrate. [1]

Nomenclature

The free radical pathway for the first phase of the oxidative rancidification of fats Lipid peroxidation.svg
The free radical pathway for the first phase of the oxidative rancidification of fats

A site adjacent to the unsaturated carbon atom is called the allylic position or allylic site. A group attached at this site is sometimes described as allylic. Thus, CH2=CHCH2OH "has an allylic hydroxyl group". Allylic C−H bonds are about 15% weaker than the C−H bonds in ordinary sp3 carbon centers and are thus more reactive.

Benzylic and allylic are related in terms of structure, bond strength, and reactivity. Other reactions that tend to occur with allylic compounds are allylic oxidations, ene reactions, and the Tsuji–Trost reaction. Benzylic groups are related to allyl groups; both show enhanced reactivity.

Pentadienyl group

A CH2 group connected to two vinyl groups is said to be doubly allylic. The bond dissociation energy of C−H bonds on a doubly allylic centre is about 10% less than the bond dissociation energy of a C−H bond that is allylic. The weakened C−H bonds reflect the high stability of the resulting pentadienyl radicals. Compounds containing the C=C−CH2−C=C linkages, e.g. linoleic acid derivatives, are prone to autoxidation, which can lead to polymerization or form semisolids. This reactivity pattern is fundamental to the film-forming behavior of the "drying oils", which are components of oil paints and varnishes.

A representative triglyceride found in linseed oil features groups with both doubly allylic
CH2 sites (linoleic acid and alpha-linolenic acid) and a singly allylic site (oleic acid) Triglyceride unsaturated Structural Formulae V.1.png
A representative triglyceride found in linseed oil features groups with both doubly allylic CH2 sites (linoleic acid and alpha-linolenic acid) and a singly allylic site (oleic acid)

Homoallylic

The term homoallylic refers to the position on a carbon skeleton next to an allylic position. In but-3-enyl chloride CH2=CHCH2CH2Cl, the chloride is homoallylic because it is bonded to the homoallylic site.

The allylic, homoallylic and doubly allylic sites are highlighted in red Allyl,homo,doubly.png
The allylic, homoallylic and doubly allylic sites are highlighted in red

Bonding

The allyl group is widely encountered in organic chemistry. [1] Allylic radicals, anions, and cations are often discussed as intermediates in reactions. All feature three contiguous sp²-hybridized carbon centers and all derive stability from resonance. [5] Each species can be presented by two resonance structures with the charge or unpaired electron distributed at both 1,3 positions.

Resonance structure of the allyl anion. The cation is identical, but carries an opposite-sign charge. Allyl anion.svg
Resonance structure of the allyl anion. The cation is identical, but carries an opposite-sign charge.

In terms of MO theory, the MO diagram has three molecular orbitals: the first one bonding, the second one non-bonding, and the higher energy orbital is antibonding. [2]

MO diagram for allyl p orbitals. In the radical (shown), the intermediate Ps2 orbital is singly occupied; in the cation, unoccupied; and in the anion, full. AllylMO.png
MO diagram for allyl π orbitals. In the radical (shown), the intermediate Ψ2 orbital is singly occupied; in the cation, unoccupied; and in the anion, full.

[7]

Reactions and applications

This heightened reactivity of allylic groups has many practical consequences. The sulfur vulcanization or various rubbers exploits the conversion of allylic CH2 groups into CH−Sx−CH crosslinks. Similarly drying oils such as linseed oil crosslink via oxygenation of allylic (or doubly allylic) sites. This crosslinking underpins the properties of paints and the spoilage of foods by rancidification.

The industrial production of acrylonitrile by ammoxidation of propene exploits the easy oxidation of the allylic C−H centers:

An estimated 800,000 tonnes (1997) of allyl chloride is produced by the chlorination of propylene:

It is the precursor to allyl alcohol and epichlorohydrin.

Allylation

Allylation is the attachment of an allyl group to a substrate, usually another organic compound. Classically, allylation involves the reaction of a carbanion with allyl chloride. Alternatives include carbonyl allylation with allylmetallic reagents, such as allyltrimethylsilane, [8] [9] [10] or the iridium-catalyzed Krische allylation.

Allylation can be effected also by conjugate addition: the addition of an allyl group to the beta-position of an enone. The Hosomi-Sakurai reaction is a common method for conjugate allylation. [11]

insert a caption here Carbonyl Allylation Scheme 4.png
insert a caption here

Oxidation

Allylic C-H bonds are susceptible to oxidation. [12] One commercial application of allylic oxidation is the synthesis of nootkatone, the fragrance of grapefruit, from valencene, a more abundantly available sesquiterpenoid: [13]

The conversion of valencene to nootkatone is an example of allylic oxidation. ValenceneToNootkatone.svg
The conversion of valencene to nootkatone is an example of allylic oxidation.

In the synthesis of some fine chemicals, selenium dioxide is used to convert alkenes to allylic alcohols: [14]

R2C=CR'-CHR"2 + [O] → R2C=CR'-C(OH)R"2

where R, R', R" may be alkyl or aryl substituents.

From the industrial perspective, oxidation of benzylic C-H bonds are conducted on a particularly large scale, e.g. production of terephthalic acid, benzoic acid, and cumene hydroperoxide. [15]

Allyl compounds

Many substituents can be attached to the allyl group to give stable compounds. Commercially important allyl compounds include:

See also

Related Research Articles

<span class="mw-page-title-main">Alkene</span> Hydrocarbon compound containing one or more C=C bonds

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

<span class="mw-page-title-main">Carboxylic acid</span> Organic compound containing a –C(=O)OH group

In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group attached to an R-group. The general formula of a carboxylic acid is often written as R−COOH or R−CO2H, sometimes as R−C(O)OH with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.

<span class="mw-page-title-main">Vinyl group</span> Chemical group (–CH=CH₂)

In organic chemistry, a vinyl group is a functional group with the formula −CH=CH2. It is the ethylene molecule with one fewer hydrogen atom. The name is also used for any compound containing that group, namely R−CH=CH2 where R is any other group of atoms.

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<span class="mw-page-title-main">Benzyl group</span> Chemical group (–CH₂–C₆H₅)

In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure R−CH2−C6H5. Benzyl features a benzene ring attached to a methylene group group.

Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond. Palladium is also prominent in carbon-carbon coupling reactions, as demonstrated in tandem reactions.

<span class="mw-page-title-main">Organotin chemistry</span> Branch of organic chemistry

Organotin chemistry is the scientific study of the synthesis and properties of organotin compounds or stannanes, which are organometallic compounds containing tin–carbon bonds. The first organotin compound was diethyltin diiodide, discovered by Edward Frankland in 1849. The area grew rapidly in the 1900s, especially after the discovery of the Grignard reagents, which are useful for producing Sn–C bonds. The area remains rich with many applications in industry and continuing activity in the research laboratory.

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<span class="mw-page-title-main">Organosilicon chemistry</span> Organometallic compound containing carbon–silicon bonds

Organosilicon chemistry is the study of organometallic compounds containing carbon–silicon bonds, to which they are called organosilicon compounds. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, flammable, hydrophobic, and stable to air. Silicon carbide is an inorganic compound.

<span class="mw-page-title-main">Organozinc chemistry</span>

Organozinc chemistry is the study of the physical properties, synthesis, and reactions of organozinc compounds, which are organometallic compounds that contain carbon (C) to zinc (Zn) chemical bonds.

<span class="mw-page-title-main">Group 2 organometallic chemistry</span>

Group 2 organometallic chemistry refers to the chemistry of compounds containing carbon bonded to any group 2 element. By far the most common group 2 organometallic compounds are the magnesium-containing Grignard reagents which are widely used in organic chemistry. Other organometallic group 2 compounds are rare and are typically limited to academic interests.

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Organobromine chemistry is the study of the synthesis and properties of organobromine compounds, also called organobromides, which are organic compounds that contain carbon bonded to bromine. The most pervasive is the naturally produced bromomethane.

The Tsuji–Trost reaction is a palladium-catalysed substitution reaction involving a substrate that contains a leaving group in an allylic position. The palladium catalyst first coordinates with the allyl group and then undergoes oxidative addition, forming the π-allyl complex. This allyl complex can then be attacked by a nucleophile, resulting in the substituted product.

<span class="mw-page-title-main">Cyclopentadienyliron dicarbonyl dimer</span> Chemical compound

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2, often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp2Fe2(CO)4 is insoluble in but stable toward water. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).

The Kharasch–Sosnovsky reaction is a method that involves using a copper or cobalt salt as a catalyst to oxidize olefins at the allylic position, subsequently condensing a peroxy ester or a peroxide resulting in the formation of allylic benzoates or alcohols via radical oxidation. This method is noteworthy for being the first allylic functionalization to utilize first-row transition metals and has found numerous applications in chemical and total synthesis. Chiral ligands can be used to render the reaction asymmetric, constructing chiral C–O bonds via C–H bond activation. This is notable as asymmetric addition to allylic groups tends to be difficult due to the transition state being highly symmetric. The reaction is named after Morris S. Kharasch and George Sosnovsky who first reported it in 1958. This method is noteworthy for being the first allylic functionalization to utilize first-row transition metals and has found numerous applications in chemical and total synthesis.

In organic chemistry, carbonyl allylation describes methods for adding an allyl anion to an aldehyde or ketone to produce a homoallylic alcohol. The carbonyl allylation was first reported in 1876 by Alexander Zaitsev and employed an allylzinc reagent.

<span class="mw-page-title-main">Transition-metal allyl complex</span>

Transition-metal allyl complexes are coordination complexes with allyl and its derivatives as ligands. Allyl is the radical with the connectivity CH2CHCH2, although as a ligand it is usually viewed as an allyl anion CH2=CH−CH2, which is usually described as two equivalent resonance structures.

<span class="mw-page-title-main">Krische allylation</span>

The Krische allylation involves the enantioselective iridium-catalyzed addition of an allyl group to an aldehyde or an alcohol, resulting in the formation of a secondary homoallylic alcohol. The mechanism of the Krische allylation involves primary alcohol dehydrogenation or, when using aldehyde reactants, hydrogen transfer from 2-propanol. Unlike other allylation methods, the Krische allylation avoids the use of preformed allyl metal reagents and enables the direct conversion of primary alcohols to secondary homoallylic alcohols.

References

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