The oxy-carbene carbon is the electrophilic site that determines whether an undesired adduct forms. Its reactivity is governed by two factors:
Electronic factor: The carbene carbon is electron-poor because the metal–carbonyl fragment withdraws electron density and the heteroatom substituent donates through resonance. This makes it a good electrophile for nucleophilic attack.
Steric factor: The carbene carbon is flanked by the metal center and its ancillary carbonyl ligands. Only nucleophiles small enough to fit into the available space can form the new C–C bond.
Putting these together:
- n-Butyl carbanion: electronically nucleophilic and sterically small. It attacks the carbene carbon readily, forming the butyl–carbene adduct. This is the undesired product.
- tert-Butyl carbanion: electronically nucleophilic but sterically bulky. It cannot reach the carbene carbon, so no tert-butyl–carbene adduct forms.
This is the core selectivity argument. The carbene does not distinguish n-butyl from tert-butyl by charge or basicity; it distinguishes them by size. The same steric filter that blocks tert-butyl also explains why tBuLi can be used in excess without generating the butyl–carbene side product that plagues nBuLi.