[115-11-7] · C4H8 · Isobutene · (MW 56.12)
(carboxy and hydroxy protection; photochemical cycloaddition with enones; ene reaction with enophiles; Friedel-Crafts and other alkylations; reacts with carbenes; acid-catalyzed cycloadditions)
Alternate Name: isobutylene.
Physical Data: mp -140.4 °C; bp -6.9 °C; vapor density 1.947.
Solubility: insol H2O; sol THF, pet ether, benzene.
Form Supplied in: gas (lecture bottle); widely available.
Handling, Storage, and Precautions: flammable gas; store and use with adequate ventilation. Do not store with oxidizing materials and compounds that add across double bonds.
Isobutene has been widely used in synthesis to convert carboxylic acids into corresponding t-butyl esters.1,2 t-Butyl esters of aliphatic acids (eq 1),3 aromatic carboxylic acids (eq 2),4 and N-protected amino acids (eq 3)5 have been prepared. The hindered t-butyl esters are stable to saponification but can be cleaved by acid-catalyzed hydrolysis, with liberation of isobutene.
In the presence of acid catalysts, isobutene reacts to convert a variety of alcohols and phenols to their corresponding t-butyl ethers. t-Butyl ethers are stable to most reagents but are readily cleaved by strong acids. Propargyl alcohols (eq 4),6 steroidal alcohols (eq 5),7 and phenols (eq 6)8 have been protected. The t-butyl group has also been used to protect the hydroxy group of valinol (eq 7),9 serine derivatives (eq 8),10 and tyrosine.11
Isobutene has been widely used in intermolecular [2 + 2] photocycloaddition reactions with enones.11 The weakly polarized isobutene is often used to study the regioselectivity of the photocycloaddition. Cyclohexenones (eq 9),12 cyclopentenones (eq 10),13 and functionalized enones (eq 11)14 undergo cycloaddition with isobutene. Paterno-Büchi photoadditions with isobutene have produced various oxetanes (eq 12).15
In the presence of Lewis acids, isobutene adds to various enophiles to yield ene adducts.16 Enophiles such as alkoxyaldehyde (eq 13),17 dialkyl aminoaldehyde (eq 14),18 haloaldehyde (eq 15),19 and vinyl sulfoxides (eq 16)20 have been utilized. Chiral organoaluminum (eq 17)21 and organotitanium reagents (eq 18)22 have been reported to give high levels of asymmetric induction in the ene reaction of isobutene with activated aldehydes.
A variety of t-butyl-substituted aromatics have been prepared with isobutene in the presence of acids (eqs 19-21).23-25 Due to steric factors, the regiochemistry is enhanced. Isobutene can be alkylated by benzylic and allylic halides in the presence of Zinc Chloride (eqs 22-24).26,27
Isobutene reacts readily with carbenes and metallocarbenes to form substituted cyclopropanes (eqs 25-27).28 Reaction of Ethyl Diazoacetate with isobutene in the presence of a chiral copper catalyst affords the cyclopropane in high optical purity (eq 28).29 Isobutene has also been used to form titanocyclobutanes that are more stable forms of the Tebbe reagent (see m-Chlorobis(cyclopentadienyl)(dimethylaluminum)-m-methylenetitanium) (eq 29).30
Isobutene has been used to form pyrylium salts (eqs 30 and 31),31 oxetanes by Boron Trifluoride-catalyzed [2 + 2] cycloaddition (eq 32),32 and 3,4-dihydropyrans by Diels-Alder reactions (eq 33).33
Isobutene, as the carbenium ion source for Ritter reactions, produces N-t-butyl amides (eq 34).34 Isobutene undergoes highly regioselective cycloaddition with isocyanate to form azetidinones and with nitrones to yield isoxazolidines (eqs 35 and 36).35,36 Allyl thioethers are formed by the electrophilic addition of Benzenesulfenyl Chloride to isobutene (eq 37).37 Alkenylphosphonous dichlorides and alkenylthionophosphonic dichlorides are also formed by addition of PCl5 and P2S5 (eqs 38 and 39).38
Michael T. Crimmins & Agnes S. Kim-Meade
University of North Carolina at Chapel Hill, NC, USA