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

Abstract

Reactions of Carbenes with Oxetane and with OxetaneJMethanol MixturesEthoxycarbonylcarbene, bis(methoxycarbonyl)carbene, phenylcarbene (17a), diphenylcarbene (l?b), fluorenylidene ( I~c ) , 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 a'$-elimination, leading to ally1 ethers (C + E). With oxetane/rnethanol 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.Carbene reagieren mit den nichtbindenden Elektronenpaaren von Heteroaromatcn zu Yliden'). Wahrend die Bildung von Stickstoff-, Phosphor-und Schwefel-Yliden gut untersucht ist und auch stabile Vertreter dieser Verbindungsklassen dargestellt werden konnten, weiB man uber Sauerstoff-Ylide relativ wenig. Aus Alkylethern entstehen Ylide, die nicht umlagern, sondern durch Spuren von Wasser oder Alkoholen protoniert und dann nucleophil gespalten werden'). Formale C -0-Einschiebung erfolgt bei Benzylethern und Allylethern4); fur letztere weist Allyl-Inversion auf eine 2,3-sigmatrope Ylid-Umlagerung hin. Arylcarbene mit geeigneten Seitenketten zeigen intramolekulare Einschiebung in 0-Benzyl-und O-Allyl-Bind~ngen~). Tetrahydrofuran reagiert mit Methylen nur in der Gasphase, nicht aber in Losung unter Ringerweiterung6'. Dagegen beobachteten wir bei der Belichtung von Diazomethan in Oxetan (1) ca. 40% C -0-Einschiebung'). Weitgehende Racemisierung bei der Ringerweiterung von (S)-ZMethyloxetan (2) zu 3-Methyltetrahydrofuran (3) sprach fur einen radikalischen Mechanismus dcr Stevens-Umlagerung8).In dieser Arbeit berichten wir iiber die Reaktionen weiterer Carbene mit Oxetan und Oxetan/Methanol-Gemischen. Der Zusatz von Methanol dient zum Abfangen in-3 ( 2 7 % )termediarer Ylide, gibt aber auch Einblick in das Verhalten der Carbene gegenuber 0 -H-Bindungen. Alkox ycarbon ylcarbeneBei

Photochemical reactions of diazodihydronaphthalenonesThese compounds are often referred to as naphthoquinone diazides in the literature. in cyclic ethers

“…Photolysis of 1b in THF produced similar spirocyclic compounds 3Ј-methyl-4Ј-oxo-1Ј,4Ј-dihydronaphthalene-1Ј-spiro-2pyrane (3bЈ), 2-methyl-4-(tetrahydrofuran-2-yl)naphthol (4bЈ) and 5b, but no polyether-bridged 2-methylnaphthalene (2b analogues) was obtained [eqn. (3)]. Structures of these compounds were characterized by spectroscopic data including 2D NMR, and in the case of 2b, also confirmed by single crystal X-ray analysis (Fig.…”

100 Years of the Wolff Rearrangement

“…Org a) The reaction of carbenes with alcohols to give ethers, [95] a very fast process, [96] was used extensively to distinguish stepwise and concerted Wolff rearrangements (Section 3.2.). The OϪH insertion of carbonyl carbenes is thought to involve electrophilic attack at the oxygen atom of ROH (98 Ǟ 99), [95,97] followed by or associated with proton transfer to the carbonyl oxygen (99 Ǟ 100, Scheme 18). For a series of carbenes with R 2 ϭ aryl, the enols 100 were identified by time-resolved spectroscopy.…”

“…[119,121] Ring strain promotes [1,2] alkyl shifts of the oxonium ylides (Ǟ 127, X ϭ O), as illustrated by the ring expansion of oxetane leading to 129. [97,122] Ketones undergo CϪH insertion as well as formation of carbonyl ylides 131, the latter giving rise to enol ethers and acetals (Scheme 21). [123] With aliphatic tertiary amines, CϪH insertion prevails over ylide formation.…”

Configuration‐Dependent Ring Opening of Silyloxiranes: Synthesis of Functionalized Alkenes or Tetrahydrofurans

“…This has precedent in similar oxetane ring-enlargement reactions. [22,23] Both mechanisms account for product formation and the diastereoselectivity of the reaction resulting from stereospecific oxirane ring opening. However, the latter mechanism implies that cis-8 and trans-silyloxiranes 13 and 15 react with 23 -by quite different mechanisms and so may be less probable (Scheme 6).…”