The full pathway
The undesired product forms in three connected steps.
- Exchange. Aryl bromide and nBuLi equilibrate to give aryllithium and nBuBr. The equilibrium leaves some nBuBr in the mixture.
- Carbene formation. The aryllithium attacks the metal carbonyl, giving an acylate, which is alkylated or protonated at oxygen to give the oxy-carbene.
- Butyl attack. The carbene carbon is electrophilic, and the residual nBuBr delivers a butyl group to it. The bromide leaves, and the butyl group ends up bonded to the carbene carbon.
The product of step 3 is the undesired butyl–carbene adduct.
Why this is a side product and not the main product
The desired product comes from the carbene reacting with whatever electrophile or reagent you intended. The butyl–carbene adduct is a side product because it forms from an unintended nucleophile that happens to be present: the nBuBr left over from the exchange.
What controls how much adduct forms
The amount of adduct depends on how much nBuBr is left after the exchange and on how long the carbene is exposed to it. If the exchange leaves more nBuBr, or if the carbene sits in the mixture for a long time, more adduct forms. That is why the timing of the carbene-forming step relative to the exchange matters.
The key takeaway
The undesired product is not caused by the aryllithium or by the metal carbonyl alone. It is caused by the combination of residual nBuBr and an electrophilic carbene carbon. Any strategy that removes the nBuBr, or that prevents the carbene from seeing it, will suppress the adduct.