enol ether
{{Short description|Class of chemical compounds}}
File:Enamine-2D-skeletal.svgs are chemically related to enol ethers.]]
In organic chemistry an enol ether is an alkene with an alkoxy substituent.{{cite book | author1 = Jonathan Clayden | author-link = Jonathan Clayden | last2 = Greeves | first2 = Nick | author3 = Stuart Warren | author3-link = Stuart Warren | title = Organic Chemistry | edition = 2nd | publisher = Oxford University Press | date = 2012 | isbn = 978-0-19-927029-3 | page = 295}} The general structure is R2C=CR-OR where R = H, alkyl or aryl. A common subfamily of enol ethers are vinyl ethers, with the formula ROCH=CH2. Important enol ethers include the reagent 3,4-dihydropyran and the monomers methyl vinyl ether and ethyl vinyl ether.
Reactions and uses
Akin to enamines, enol ethers are electron-rich alkenes by virtue of the electron-donation from the heteroatom via pi-bonding. Enol ethers have oxonium ion character. By virtue of their bonding situation, enol ethers display distinctive reactivity. In comparison with simple alkenes, enol ethers exhibit enhanced susceptibility to attack by electrophiles such as Bronsted acids. Similarly, they undergo inverse demand Diels-Alder reactions.{{cite journal|journal=EEROS|title=Ethyl Vinyl Ether|author=Percy S. Manchand|year=2001|doi=10.1002/047084289X.re125|isbn=0-471-93623-5}}
The reactivity of enol ethers is highly dependent on the presence of substituents alpha to oxygen. The vinyl ethers are susceptible to polymerization to give polyvinyl ethers.{{Ullmann|title=Poly(Vinyl Ethers)|author=Gerd Schröder|year=2012|doi=10.1002/14356007.a22_011}} They also react readily with thiols in the thiol-ene reaction to form thioethers. This makes enol ether-functionalized monomers ideal for polymerization with thiol-based monomers to form thiol-ene networks.{{cite journal |last1=Hoyle |first1=Charles E. |last2=Bowman |first2=Christopher N. |title=Thiol–Ene Click Chemistry |journal=Angewandte Chemie International Edition |year=2010 |volume=49 |issue=9 |doi=10.1002/anie.200903924 |pages=1540–1573}}
Some vinyl ethers find some use as inhalation anesthetics. Enol ethers bearing α substituents do not polymerize readily. They are mainly of academic interest, e.g. as intermediates in the synthesis of more complex molecules.
The acid-catalyzed addition of hydrogen peroxide to vinyl ethers gives the hydroperoxide:{{cite journal |doi=10.1021/ja01638a012|title=Organic Peroxides. XIX. α-Hydroperoxyethers and Related Peroxides|year=1954|last1=Milas|first1=Nicholas A.|last2=Peeler|first2=Robert L.|last3=Mageli|first3=Orville L.|journal=Journal of the American Chemical Society|volume=76|issue=9|pages=2322–2325}}
:C2H5OCH=CH2 + H2O2 → C2H5OCH(OOH)CH3
Nazi Germany used vinyl ether mixtures as rocket propellants during WWII, because their hypergolic combustion with a mixture of nitric and sulfuric acids is relatively insensitive to temperature.{{cite book |isbn = 978-0-8135-9918-2 |title = Ignition!: An Informal History of Liquid Rocket Propellants |last1 = Clark |first1 = John Drury |author-link=John Drury Clark |date = 23 May 2018 |publisher = Rutgers University Press |url=https://books.google.com/books?id=BdU4DwAAQBAJ&q=vinyl%20ether |pages=302 |oclc=281664}}{{rp|16}}
Preparation
Vinyl ethers can be prepared from alcohols by iridium-catalyzed transesterification of vinyl esters, especially the widely available vinyl acetate:{{cite journal|title=Iridium-catalyzed Synthesis of Vinyl Ethers from Alcohols and Vinyl Acetate |author=Tomotaka Hirabayashi |author2=Satoshi Sakaguchi |author3=Yasutaka Ishii |journal=Org. Synth.|year=2005|volume=82|page=55|doi=10.15227/orgsyn.082.0055|doi-access=free}}
:ROH + CH2=CHOAc → ROCH=CH2 + HOAc
Vinyl ethers can be prepared by reaction of acetylene and alcohols in presence of a base.{{Ullmann|title=Vinyl Ethers |author=Ernst Hofmann |author2=Hans‐Joachim Klimisch |author3=René Backes |author4=Regina Vogelsang |author5=Lothar Franz |author6=Robert Feuerhake |year=2011|doi=10.1002/14356007.a27_435.pub2}}
Although enol ethers can be considered the ether of the corresponding enolates, they are not prepared by alkylation of enolates. Some enol ethers are prepared from saturated ethers by elimination reactions.{{cite journal|title=Alkenes Via Hofmann Elimination: Use of Ion-exchange Resin for Preparation of Quaternary Ammonium Hydroxides: Diphenylmethyl Vinyl Ether|author1=Carl Kaiser |author2=Joseph Weinstock |journal=Org. Synth.|year=1976|volume=55|page=3|doi=10.15227/orgsyn.055.0003}}
File:Ethyl vinylether.svg is a potent anesthetic.]]
Occurrence in nature
A prominent enol ether is phosphoenol pyruvate.{{cite journal |doi=10.1016/s1074-5521(96)90282-3 |title=The versatility of phosphoenolpyruvate and its vinyl ether products in biosynthesis |date=1996 |last1=Walsh |first1=C.T. |last2=Benson |first2=T.E. |last3=Kim |first3=D.H. |last4=Lees |first4=W.J. |journal=Chemistry & Biology |volume=3 |issue=2 |pages=83–91 |pmid=8807832 |doi-access=free }}
The enzyme chorismate mutase catalyzes the Claisen rearrangement of the enol ether called chorismate to prephenate, an intermediate in the biosynthesis of phenylalanine and tyrosine.{{cite journal|author=Ganem, B. |journal=Angew. Chem. Int. Ed. Engl.|year=1996|volume=35|pages= 936–945|doi=10.1002/anie.199609361|title=The Mechanism of the Claisen Rearrangement: Déjà Vu All over Again|issue=9}}
Image:Chorismate Mutase Scheme.png
Batyl alcohol and related glycyl ethers are susceptible to dehydrogenation catalyzed unsaturases to give the vinyl ethers called plasmalogens:{{cite journal |doi=10.1002/(SICI)1098-1128(199801)18:1<43::AID-MED3>3.0.CO;2-S |title=Glyceryl-Ether Monooxygenase [EC 1.14.16.5]. A Microsomal Enzyme of Ether Lipid Metabolism |date=1998 |last1=Taguchi |first1=Hiroyasu |last2=Armarego |first2=Wilfred L. F. |journal=Medicinal Research Reviews |volume=18 |issue=1 |pages=43–89 |pmid=9436181 |s2cid=432376 }}
:{{chem2|HOCH2CH(OH)CH2OC18H37 + [O] -> HOCH2CH(OH)CH2OCH\dCHC16H35 + H2O}}
File:Strigolactones general chemical structure.pngs, a family of plant hormones.{{cite journal | vauthors = Umehara M, Cao M, Akiyama K, Akatsu T, Seto Y, Hanada A, Li W, Takeda-Kamiya N, Morimoto Y, Yamaguchi S | display-authors = 6 | title = Structural Requirements of Strigolactones for Shoot Branching Inhibition in Rice and Arabidopsis | journal = Plant & Cell Physiology | volume = 56 | issue = 6 | pages = 1059–72 | date = June 2015 | pmid = 25713176 | doi = 10.1093/pcp/pcv028 | doi-access = free }}]]
