Copper is less electropositive, so it produces a less-charged (or "softer") carbanion. And we know soft nucleophiles are a good way to encourage conjugate addition instead of direct attack. Example:
Another method to produce an organocopper is reacting it with two equivalents of alkyl lithium:
Which also has a soft carbanion. These are unstable and must be used immediately, this is why they are kept at low temperatures.
Reacting these happens much better by adding Me3SiCl:
Though its mechanism is not fully understood.
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