Chromium aminocarbenes, cycloaddition

Formal intramolecular [4+1]-cycloaddition of chromium aminocarbenes (16) has provided a nice entry to iV-heteropolycyclic compounds." The reaction occurs via a [2+2]-cycloaddition between the carbene and the internal olefin yielding an allylchromium species which equilibrates with the metallacyclohexane that finally provides the polycycle after reductive coupling (Scheme 2). 

The Dotz benzannulation reaction, based on the alkyne cycloaddition to chromium carbene complexes, is the most important application of Fischer carbene complexes. Among the various Fischer carbene complexes, alkoxy and aminocarbene complexes of chromium undergo a novel inter- and intramolecular tandem alkyne insertion/ carbene annotation sequence to give 9H-carbazoles and nf/-benzo[fl]carbazoles. 

Pyrrol-1-ylcarbene complexes 18 effect the cyclopropanation of electron-deficient alkenes in good to very good yield. The C-H insertion products usually observed with aminocarbene complexes are only formed in low yields. Molybdenum and tungsten complexes are more favorable for cyelopropanation reactions than the analogous chromium complexes. The diaste-reoelectivity of the cycloaddition, however, was only moderate cisjtrans ratio of 1 1.6 to 3.4). 

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