Aminocarbenes, cycloaddition

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. 

A cycloaddition analogous to that in Eq. 12 also occurs with aminocarbene 9. Thus, addition of formaldehyde to the cocondensate of C - - NH3 generates serine 18 by the mechanism in Eq. 13. This reaction is the key step in the formation of serine when water is added to the C + NH3 reaction (with C + H2O producing the CH2=0). Addition of acetaldehyde to the C - - NH3 cocondensate results in threonine and allothreonine (Eq. 14). ... 

Electrophilic transition-metal-carbene complexes (Fischer carbene complexes) serve as formal carbene transfer reagents in reactions with alkenes to give functionalized cyclopropanes. This reaction behavior is well documented for alkoxycarbene complexes of elements of group In contrast, aminocarbene complexes exhibit a different reactivity over a wide range of conditions and [2 + 1] cycloadditions to alkenes represent exception. 

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). 

Cyclization and cycloaddition. Aminocarbene complexes behave like amino-ketenes under photochemical conditions. With the participation of a benzene ring, cyclization leads to aminonaphthols, whereas the amino ketenes can also be trapped with imines to form )3-lactams. Intramolecular capture of analogous ketenes by an aldehyde leads to /S-lactones." The intermolecular process gives much inferior results. 

The subsequent steps of the reaction are too fast in order to allow further kinetic investigation. A previously proposed 16-electron chromacyclobutene intermediate arising from a formal [2-i-2]-cycloaddition of the alkyne ligand aross the metal-carbene bond was later discarded as a result of theoretical studies which support a direct insertion of the alkyne into the metal-carbene bond to generate an 18-electron valence-isomer, the rj -vinylcarbene complex D. [48] A related species III has been isolated from the reaction of an aminocarbene complex. [46] Subsequent insertion of a carbonyl ligand leads to an rj -yinylketene complex E, of which structural analogues as enaminoketene complex IV have been synthesized. 

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). 

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