Editing Ylide

{{Short description|Organic compound}}

An '''ylide''' ({{IPAc-en|ˈ|ɪ|l|ai|d}})<ref>{{Cite web|url=https://encyclopedia2.thefreedictionary.com/ylide|title=ylide|via=The Free Dictionary}}</ref> or '''ylid''' ({{IPAc-en|ˈ|ɪ|l|ɪ|d}}) is a [[electric charge|neutral]] [[Dipole|dipolar]] [[molecule]] containing a formally negatively charged [[atom]] (usually a [[carbanion]]) directly attached to a [[heteroatom]] with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons. The result can be viewed as a structure in which two adjacent atoms are connected by both a [[covalent bond|covalent]] and an [[ionic bond]]; normally written X<sup>+</sup>–Y<sup>−</sup>. Ylides are thus 1,2-[[dipolar compound]]s, and a subclass of [[zwitterions]].<ref>{{GoldBookRef|title=ylides|file=Y06728}}</ref> They appear in [[organic chemistry]] as [[reagent]]s or [[reactive intermediate]]s.<ref>{{cite book | author = McMurry, John | title = Organic Chemistry, 7th Ed. | publisher = Thomson Brooks/Cole| pages = 720–722 | year = 2008 | isbn = 978-0-495-11258-7}}</ref>

The class name "ylide" for the compound should not be confused with the [[suffix]] "-ylide".

== Resonance structures ==
Many ylides may be depicted by a [[Multiple bond|multiply bonded]] form in a [[resonance structure]], known as the ylene form, while the actual structure lies in between both forms:{{cn|date=December 2019}}
:[[Image:Ph3P=CH2-ylid-phosphorane-resonance-2D.png|260px|Wittig reagent resonance structures]]
The actual bonding picture of these types of ylides is strictly zwitterionic (the structure on the right) with the strong Coulombic attraction between the "onium" atom and the adjacent carbon accounting for the reduced bond length.  Consequently, the carbon anion is trigonal pyramidal.{{Fact|date=February 2021}}

==Phosphonium ylides==
:[[Image:Ph3P=CH2-from-xtal-1989-3D-balls.png|thumb|right|240px|Structure of [[methylenetriphenylphosphorane]]]]

Phosphonium ylides are used in the [[Wittig reaction]], a method used to convert [[ketones]] and especially [[aldehyde]]s to alkenes. The positive charge in these [[Wittig reaction#Wittig reagents|Wittig reagents]] is carried by a [[phosphorus]] atom with three [[phenyl]] substituents and a bond to a [[carbanion]].  Ylides can be 'stabilised' or 'non-stabilised'. A phosphonium ylide can be prepared rather straightforwardly. Typically, [[triphenylphosphine]] is allowed to react with an [[alkyl halide]] in a mechanism analogous to that of an [[SN2 reaction|S<sub>N</sub>2 reaction]]. This [[quaternization]] forms an alkyltriphenyl[[phosphonium]] salt, which can be isolated or treated in situ with a strong base (in this case, [[butyllithium]]) to form the ylide.
:[[File:Ph3PCH2prep.png|440px]]

Due to the S<sub>N</sub>2 mechanism, a less sterically hindered alkyl halide reacts more favorably with triphenylphosphine than an alkyl halide with significant steric hindrance (such as [[tert-butyl bromide]]). Because of this, there will typically be one synthetic route in a synthesis involving such compounds that is more favorable than another.

Phosphorus ylides are important reagents in organic chemistry, especially in the synthesis of naturally occurring products with
biological and pharmacological activities. Much of the interest in the coordination properties of a-keto stabilized phosphorus
ylides stems from their coordination versatility due to the presence of different functional groups in their molecular structure.

==Non-symmetric phosphorus ylides==
The a-keto stabilized ylides derived from bisphosphines like [[dppe]], [[dppm]], etc., viz., [Ph<sub>2</sub>PCH<sub>2</sub>PPh<sub>2</sub>]C(H)C(O)R and [Ph<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>]C(H)C(O)R (R = Me, Ph or OMe) constitute an important class of hybrid ligands containing both [[phosphine]] and ylide functionalities, and can exist in ylidic and enolate forms. These ligands can therefore be engaged in different kinds of bonding with metal ions like [[palladium]] and [[platinum]].<ref>{{cite journal |first1=Seyyed Javad |last1=Sabouncheia |first2=Mohsen |last2=Ahmadi |first3=Zahra |last3=Nasri |first4=Esmaeil |last4=Shams |first5=Sadegh |last5=Salehzadeh |first6=Yasin |last6=Gholiee |first7=Roya |last7=Karamian |first8=Mostafa |last8=Asadbegy |first9=Sepideh |last9=Samiee |title=Synthesis, characterization, thermal, electrochemical, and DFT studies of mononuclear cyclopalladated complexes containing bidentate phosphine ligands and their biological evaluation as antioxidant and antibacterial agents |journal=[[Comptes Rendus Chimie]] |volume=16 |issue=2 |year=2013 |pages=159–175 |doi=10.1016/j.crci.2012.10.006|url=https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2012.10.006/ }}</ref>

==Other types==

===Based on sulfur===
Other common ylides include '''sulfonium ylides''' and '''sulfoxonium ylides'''; for instance, the [[Corey-Chaykovsky reagent]] used in the preparation of [[epoxide]]s or in the [[Stevens rearrangement]].

===Based on oxygen===
'''Carbonyl ylides''' (RR'C=O<sup>+</sup>C<sup>−</sup>RR') can form by ring-opening of [[epoxide]]s or by reaction of [[carbonyls]] with electrophilic [[carbenes]],<ref>{{cite journal |first1=Albert |last1=Padwa |title=Catalytic Decomposition of Diazo Compounds as a Method for Generating Carbonyl-Ylide Dipoles |journal=[[Helvetica Chimica Acta]] |year=2005 |volume=88 |issue=6 |pages=1357–1374 |doi=10.1002/hlca.200590109}}</ref> which are usually prepared from [[diazo]] compounds. '''Oxonium ylides''' (RR'-O<sup>+</sup>-C<sup>−</sup>R'R) are formed by the reaction of [[ethers]] with electrophilic [[carbenes]].

===Based on nitrogen===
Certain [[nitrogen]]-based ylides also exist such as '''[[azomethine ylide]]s''' with the general structure:
:[[File:Azomethine ylide 2.svg|150px]]

These compounds can be envisioned as [[iminium]] cations placed next to a [[carbanion]]. The [[substituent]]s R<sub>1</sub>, R<sub>2</sub> are [[electron withdrawing group]]s. These ylides can be generated by condensation of an α-[[amino acid]] and an [[aldehyde]] or by thermal ring opening reaction of certain N-substituted [[aziridines]]. 

The further-unsaturated [[nitrile ylide]]s are known almost exclusively as unstable intermediates.  

A rather exotic family of dinitrogen-based ylides are the [[isodiazene]]s (R<sup>1</sup>R<sup>2</sup>N<sup>+</sup>=N<sup>–</sup>), which generally decompose by extrusion of dinitrogen.

[[Stable carbene]]s also have a ylidic resonance contributor, ''e.g.'':
:[[Image:Stable-carbene-resonance-2D.png|300px]]

===Other===
'''Halonium ylides''' can be prepared from allyl halides and metal [[carbenoid]]s. After a [2,3]-rearrangement, a homoallylhalide is obtained.

The active form of [[Tebbe's reagent]] is often considered a titanium ylide. Like the Wittig reagent, it is able to replace the oxygen atom on carbonyl groups with a methylene group. Compared with the Wittig reagent, it has more functional group tolerance.

==Reactions==
An important ylide reaction is of course the [[Wittig reaction]] (for phosphorus) but there are more.

===Dipolar cycloadditions===
Some ylides are [[1,3-dipole]]s and interact in [[1,3-dipolar cycloaddition]]s. For instance an azomethine ylide is a dipole in the [[Prato reaction]] with [[fullerene]]s.

===Dehydrocoupling with silanes===
In the presence of the [[group 3 element|group 3]] [[homoleptic]] [[catalyst]] Y[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>3</sub>, triphenylphosphonium methylide can be coupled with [[phenylsilane]].<ref>{{cite journal |first1=Adi E. |last1=Nako |first2=Andrew J. P. |last2=White |first3=Mark R. |last3=Crimmin |title=A metal–amide dependent, catalytic C–H functionalisation of triphenylphosphonium methylide |journal=[[Chemical Science]] | year=2013 | volume=4 |issue=2 |pages=691–695 | doi=10.1039/C2SC21123H|hdl=10044/1/15254 |url=http://spiral.imperial.ac.uk/bitstream/10044/1/15254/2/Chemical%20Science_4_2_2013.pdf |hdl-access=free }}</ref> This reaction produces H<sub>2</sub> gas as a byproduct, and forms a silyl-stabilised ylide.

:[[Image:ChemSci TOC.png|400px|Yttrium catalysed dehydrocoupling of triphenylphosphonium methylide and phenylsilane]]

===Sigmatropic rearrangements===
Many ylides react in [[sigmatropic reaction]]s.<ref>{{cite journal |first1=J. B. |last1=Sweeney | title=Sigmatropic rearrangements of 'onium' ylides | journal=[[Chemical Society Reviews]] |year=2009 |volume=38 |issue=4 |pages=1027–1038 |doi=10.1039/b604828p |pmid=19421580}}</ref> The [[Sommelet-Hauser rearrangement]] is an example of a [2,3]-sigmatropic reaction. The [[Stevens rearrangement]] is a [1,2]-rearrangement.

A {{bracket|3,3}}-sigmatropic reaction has been observed in certain phosphonium ylides.<ref name=Ferguson>{{cite journal |last1=Ferguson |first1=Marcelle L. |last2=Senecal |first2=Todd D. |last3=Groendyke |first3=Todd M. |last4=Mapp |first4=Anna K. |year=2006 |title=<nowiki>[3,3]</nowiki>-Rearrangements of Phosphonium Ylides |journal=[[J. Am. Chem. Soc.]] |volume=128 |issue=14 |pages=4576–4577 |doi=10.1021/ja058746q |pmid=16594686}}</ref><ref>(i) Reaction of [[allyl alcohol]] with 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphorinane forms a [[phosphite ester]]. (ii) [[Metal carbene]] addition (from [[ethyl diazoacetate]] and [[ClFeTPP]]) forms an ylide. (iii) A rearrangement reaction (in blue) yields a [[phosphonate]].</ref>

:[[Image:Phosphoniumylide rearrangement.png|400px|Scheme 1. Phosphonium ylide rearrangement]]

===Allylic rearrangements===
Wittig reagents are found to react as nucleophiles in [[Allylic rearrangement|S<sub>N</sub>2' substitution]]:<ref>{{cite journal | title = Facile SN2' Coupling Reactions of Wittig Reagents with Dimethyl Bromomethylfumarate: Synthesis of Enes, Dienes, and Related Natural Products | first1=Ramesh M. |last1=Patel |first2=Narshinha P. |last2=Argade | journal = [[J. Org. Chem.]] | year = 2007 | volume = 72 | issue = 13 | pages = 4900–4904 | doi = 10.1021/jo070728z | pmid = 17539690}}</ref>

:[[Image:WittigAllylicrearrangement.png|400px|Wittig reagent in allylic rearrangement. 8% ene product not depicted]]

The initial addition reaction is followed by an [[elimination reaction]].

==See also==

*[[1,3-dipole]]
*[[Betaine]]: a neutral molecule with an [[Onium compounds|onium]] cation and a negative charge
*[[Zwitterion]]: a neutral molecule with one or more pairs of positive and negative charges

==References==
{{reflist}}

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[[Category:Chemical nomenclature]]
[[Category:Functional groups]]