Carbene durch thermische Cycloeliminierung

Abstract

Das zunehmende Interesse an Carbenen verlangt Methoden, mit denen man diese Zwischenstufen in unimolekularen Zerfallsprozessengegebenenfalls in der Gasphaseohne storende Nebenprodukte erzeugen kann. Dazu haben sich neben dem Zerfatl von Diazoverbindungen thermische Cycloeliminierungen aus Carbo-und Heterocyclen ungerader Gliederzahl bewahrt. I n diesem Fortschrittsbericht werden Probleme, strukturelle Voraussetzungen und Anwendungsbreite derartiger Cycloeliminierungsreaktionen diskutiert. Am Beispiel der anderweitig nur sehr schlecht zuganglichen Dialkoxycarbene lassen sich die Vorteile der Carbenerzeugung durch thermische Cycloeliminierung aufzeigen.Angew. Chem. / 83. Jahrg. 1971 Nr. 16

New precursors for arylcarbenes. Photocycloelimination reactions of cyclic carbonates

“…8a, Ri = Ph; R2 = R4 = H; R:l = CH;i (syn) b, R, = Ph; R4 = R4 = H; R:, = CHS (anti) c, R = Ph; R, -R, = H; R4 = CR, (trans) PhCHO + CO, (6) PhCH2OCH:l + PhCHO + C02 (7) 9 Statistically corrected for the number of hydrogen atoms of each type. * Limits of error in each case represent standard deviations obtained by multiple integration of several chromatograms.…”

“…Pays-Bas, 87, 824 (1968)] have Investigated the Ionic chlorine substitution reaction which occurs with 4a,b. We observed that chlorination of 2 In carbon tetrachloride produced benzene (Identified by the NMR singlet at 6 7.26) in a mole ratio equal to the dlchlorocyclohexenes. Benzene would be obtained by HCI elimination from the allylic substitution product of cyclohexadiene (2), 5-chloro-1,3-cyclohexadlene (benzene hydrochloride).…”

“…In each of four identical, fused quartz tubes was placed 3.30 X 10~2 mmol of the substrate to be studied. The specific amounts employed were as follows: 7.94 mg of meso-hydrobenzoin carbonate (lb), 7.93 mg of dZ-hydrobenzoin carbonate (lc), 6.48 mg of trans-2,3-diphenyloxirane (10b), and 3.90 mg of phenyldiazomethane. A 10-ml aliquot of a 1.0 X 10~5 M solution of amylbenzene in rc-pentane (99%) was transferred to each tube by means of a 10-ml volumetric pipet.…”

Zwitterion from a Cyclopropane with Geminal Donor and Acceptor Groups

“…Dimethoxycarbene (DMC) is nucleophilic, , and it reacts with electrophilic alkenes bearing four electron-withdrawing groups 3 by an unknown mechanism (Scheme ). A stepwise cycloaddition mechanism has been proposed, but electron transfer followed by bond formation are among other possibilities.…”