An allyl group is a substituent with the structural formula H2C=CH-CH2R, where R is the rest of the molecule. It consists of a methylene bridge (-CH2-) attached to a vinyl group (-CH=CH2). The name is derived from the Latin word for garlic, Allium sativum. In 1844, Theodor Wertheim isolated an allyl derivative from garlic oil and named it "Schwefelallyl". The term allyl applies to many compounds related to H2C=CH-CH2, which includes thousands of different chemical compounds, some of which being of practical or everyday importance.
Nomenclature and bonding
Allyl is a widely used term in organic chemistry. The unpaired electron is delocalized. Allylic radicals, anions, and cations are often discussed as intermediates in reactions. All feature three contiguous sp²-hybridized carbon centers.
A site on the saturated 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. This heightened reactivity has many practical consequences. The industrial production of acrylonitrile by ammoxidation of propene exploits the easy oxidation of the allylic C-H centers.
Unsaturated fats spoil by rancidification involving attack at allylic C-H centers.
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.
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.
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 occupies a homoallylic position.
Allylation is the process of adding an allyl group to a substrate. Typically allylation refers to the addition of an allyl anion equivalent to an organic electrophile: A typical allylation of an aldehyde (RCHO) is represented by the following two-step process that begins with allylation followed by hydrolysis of the intermediate:
- RCHO + CH2=CHCH2M → CH2=CHCH2RCH(OM)
- CH2=CHCH2RCH(OM) + H2O → CH2=CHCH2RCH(OH) + MOH
Many substituents can be attached to the allyl group to give stable compounds.
Allyl alcohol, the parent of allylic alcohols, has the structure H2C=CH-CH2OH. Another example of a simple allyl compound is allyl chloride. Substituted versions of the parent allyl group, such as the trans-but-2-en-1-yl or crotyl group (CH3CH=CH-CH2-), may also be referred to as allylic groups. Many allylic compounds are lachrymatory.
Dimethylallyl pyrophosphate is an allylic compound that is central in the biosynthesis of terpenes, a large family of naturally occurring organic compounds. Isopentenyl pyrophosphate, is a homoallylic isomer of the dimethylallyl compound. It also is the precursor to many natural products, including natural rubber.
Metal allyl complexes
The allyl ligand is commonly encountered in organometallic chemistry. Most commonly, allyl ligands bind to metals via all three carbon centers, the so-called η3-binding mode. Examples beyond those discussed below include Ir(η3-allyl)3 and (η3-allyl)Mn(CO)4. Some complexes with η1-allyl ligands are also known, e.g., CpFe(CO)2(η1-allyl) where only the methylene group is attached to the Fe centre. Such compounds often convert to the η3-allyl derivatives by dissociation of a ligand:
- CpFe(CO)2(η1-allyl) → CpFe(CO)(η3-allyl) + CO
Allyl complexes are usually generated by oxidative addition of allylic halides to low-valent metal complexes. This route affords pi-allyl nickel compounds, such as (allyl)2Ni2Cl2:
- 2 Ni(CO)4 + 2 ClCH2CH=CH2 → Ni2(μ-Cl)2(η3-C3H5)2 + 8 CO
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