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45.30.3.1.2 Method 2: Rhodium-Catalyzed Addition of Diazo Compounds

DOI: 10.1055/sos-SD-045-01426

Koo, S.Science of Synthesis, (2010451405.

Thermal or photochemical decomposition of ethyl diazoacetate in benzene produces transient (ethoxycarbonyl)carbene, which undergoes addition to benzene to give the unstable norcaradiene intermediate 195 that is in equilibrium with the more stable cycloheptatriene tautomer 196 (Scheme 43).[‌187‌] The kinetic nonconjugated isomer 196 undergoes thermally or photochemically induced sigmatropic rearrangement to three conjugated cycloheptatriene tautomers. The thermal or photochemical Buchner reactions are thus characterized by the formation of inseparable mixtures of cycloheptatrienyl esters. This problem can be solved by the use of an electron-poor rhodium(II) carboxylate catalyst at room temperature for the generation of (ethoxycarbonyl)carbene.[‌188‌] The kinetic product, ethyl cyclohepta-2,4,6-triene-1-carboxylate (196) can be obtained quantitatively with rhodi­um(II) trifluoroacetate in a large excess of benzene. The rhodium(II)-catalyzed decomposition of α-diazoamide 197 leads to a transient metalcarbenoid which may undergo two different intramolecular reactions: either addition to the aromatic ring to afford cycloheptapyrrolone 198 or insertion into the benzylic CH bond to give β-lactam 199 (Scheme 43).[‌189‌] The chemoselectivity can be controlled by the ligand in the rhodium(II) catalyst. Thus, the perfluorobutanamide ligand favors attack on the aromatic ring to give cycloheptapyrrolone 198 in 70% yield together with some cis-β-lactam 199 (12% yield).

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