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Carbene addition to an aromatic bond gives a cyclopropane-containing intermediate that can open to a seven-membered cycloheptatriene.

A metal carbene with substantial electrophilic character at carbon, commonly stabilized by a heteroatom substituent and attached to a low-oxidation-state metal carrying pi-acceptor ligands.

A carbene centre in a nitrogen-containing heterocycle, often used as a strong sigma-donor ligand. Steric substitution on nitrogen controls access to a coordinated metal.

CO first dissociates from the 18-electron Fischer carbene. Pyridyl nitrogen of C then coordinates at the vacant chromium site, positioning an ortho C-D bond of its phenyl group beside the carbene unit.

The appropriate pathway is oxidative hydrogen migration: chromium assists C-D cleavage while D transfers to the electrophilic carbene carbon. The resulting cyclometalated intermediate has both an aryl-Cr bond and an alkyl-Cr bond, the latter to $\mathrm{C}(\mathrm{D})(\mathrm{OEt})(\mathrm{o}-\mathrm{tolyl})$. Formally Cr(0) becomes Cr(II). Reductive elimination between those two carbon ligands forms the new aryl–$\mathrm{C}(\mathrm{D})(\mathrm{OEt})(\mathrm{o}-\mathrm{tolyl})$ bond and returns chromium to low oxidation state.

This accounts for all three controls. Extra CO suppresses the coordinative vacancy. Biphenyl E lacks the pyridyl-N directing group and cannot organize the necessary C-H activation geometry. The ortho deuterium moves onto the former carbene carbon rather than being lost to solvent: four D remain on the original phenyl ring and the fifth becomes the benzylic C-D bond.

This is not adequately represented as electrophilic aromatic substitution followed by proton loss, which would discard the diagnostic D. The primary mechanistic study distinguishes oxidative hydrogen migration/reductive elimination from that classical alternative.^[ref]

A Fischer carbene complex typically has a low-oxidation-state, relatively electron-rich middle/late transition metal with strong pi-acceptor ligands such as CO. The carbene is treated as a neutral two-electron donor; metal-to-carbene back-donation contributes to its bond, but its carbon remains electrophilic. An OR or NR${}_2$ substituent stabilizes that carbon by heteroatom donation. Here the metal is Cr(0), $d^6$, in an 18-electron pentacarbonyl complex.

A Schrock carbene complex usually contains an early metal in a high oxidation state, often $d^0$, with electron-donating ligands rather than a shell of pi-accepting carbonyls. Its alkylidene has substantial dianionic character in ionic counting, and its carbon is correspondingly nucleophilic. Alkyl substituents are common; heteroatom stabilization of an electrophilic carbon is not the defining feature. These are bonding/electronic limiting descriptions rather than a rule based only on the drawn metal-carbon double bond.

With an N-trityl substituent, the carbene attacks a nearby trityl phenyl ring. Addition across an arene bond gives a norcaradiene-like cyclopropane, followed by electrocyclic opening to the seven-membered cycloheptatriene: this is a Buchner ring expansion. Intramolecular trapping explains the fused product rather than an intermolecular carbene reaction.

With N-tert-butyl, there is no pendant arene. Instead the carbene inserts into a C-H bond of one tert-butyl methyl group. The new bond between the carbene carbon and that methyl carbon closes the five-membered N-containing ring; H is transferred to the carbene carbon in the same insertion. The remaining two methyl groups stay on the carbon attached to N.

The alcohol first attacks CDI, forming an activated O-carbonylimidazole derivative. Its departure gives the resonance-stabilized propargyl/allenyl cationThe ortho amino nitrogen attacks the central carbon of the allenyl system, closing the five-membered indole ring. Electron redistribution and loss of the N-H proton establish the indole aromatic system while leaving a carbene at the original distal, phenyl-bearing alkyne carbon.

For atom tracking, the original carbinol carbon becomes indole C3, the proximal alkyne carbon becomes C2, and the distal $\mathrm{C}(\mathrm{Ph})$ becomes the exocyclic carbene. Attack at the distal alkyne carbon would make a six-membered ring and cannot give the displayed skeleton. This carbene-based rearrangement has a primary experimental precedent.^[ref]

An alkylidene complex with substantial nucleophilic/dianionic carbene character, commonly involving an early high-oxidation-state metal and strongly donating ancillary ligands.