= Solution
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 \b[<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 $\mathrm{C(D)(OEt)(o\!-\!tolyl)}$. Formally Cr(0) becomes Cr(II). <Reductive elimination> between those two <carbon> ligands forms the new aryl–$\mathrm{C(D)(OEt)(o\!-\!tolyl)}$ 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.<Azacycle-directed formal aromatic C-H insertion with a chromium Fischer carbene by Xing-Qi Yao and colleagues>{ref}
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-3-q38-chromium-migration.png]
{title=Directed oxidative deuterium migration followed by carbon-carbon reductive elimination}
Back to article page
