Carben‐Reaktionen mit Oxetan und mit Oxetan/Methanol‐Gemischen

Reactions of Carbenes with Oxetane and with Oxetane/Methanol Mixtures Ethoxycarbonylcarbene, bis(methoxycarbonyl)carbene, phenylcarbene ( 17a ), diphenylcarbene ( 17b ), fluorenylidene ( 17c ), 2‐furylcarbene ( 31a ), 2‐furyl(phenyl)carbene ( 31b ), 4‐oxo‐2,5‐cyclohexadienylidene ( 40 ), and 4 , 4 ‐dimethyl‐2,5‐cyclohexadienylidene ( 53 ) were generated by photolysis of the appropriate diazo compounds. With neat oxetane, most of these carbenes react by competitive C – H insertion ( B → A , Scheme 1 ) and ylide formation ( B → C ). 31a and 40 do not insert into C – H bonds; 31b does not attack oxetane but rearranges exclusively with formation of 26 . The ylides undergo Stevens rearrangement to give tetrahydrofurans ( C → D ) and α´,β‐elimination, leading to allyl ethers ( C → E ). With oxetane/methanol mixtures, the intervention of oxonium ions ( H ) is indicated by the formation of 1,3‐dialkoxypropanes ( I ). The oxonium ions arise either by protonation of the ylides ( C → H ) or by protonation of the carbenes ( B → G ), followed by electrophilic attack of the carbocations ( G ) at oxetane ( G → H ). The former route is followed by the alkoxycarbonylcarbenes and by 40 ; the ylides derived from the remaining carbenes do not react with methanol, owing to their rapid Stevens rearrangements. Protonation of the carbenes 17b, 31 , and 53 is clearly indicated by their product ratios and, for 31 , by the formation of isomeric ethers ( 33 , 36 ). The more electrophilic carbenes discriminate but slightly between oxetane and methanol while the more nucleophilic carbenes ( 17b , 31 , 53 ) prefer the protic methanol strongly over the aprotic oxetane.