REACTIONS OF CARBENES WITH OXETANE AND WITH OXETANE METHANOL MIXTURES

Ethoxycarbonylcarbene, bis(methoxycarbonyl)carbene, phenylcarbene (17a), diphenylcarbene (17b), fluorenylidene (17c), 2-furylcarbene (31 a), 2-furyl(phenyl)carbene (31 b), 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; 31 b does not attack oxetane but rearranges exclusively with formation of 26. The ylides undergo Stevens rearrangement to give tetrahydrofurans (C --> D) and alpha',beta-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.