In the reaction sequence :

Options

A.
B.
C.
D.

Answer

B

Explanation

Copper (1) salts have a remarkable effect on organometallic reagents. Grignard reagents add directly to the carbonyl group of a, B-unsaturated aldehydes and ketones to give allylic alcohols : you have seen several examples of this, and you can now explain it by saying that the hard Grignard reagent prefers to attack the harder C=O rather than the softer C=C ectrophilic centre. Here is a further example - the addition of MeMgBr to a cyclic ketone to give an allylic alcohol, plus, as it happens, some of a diene that arises from this alcohol by loss of water (dehydration). Below this example is the same reaction to which a very small amount (just 0.01 equivalents, that is 1%) of copper(I) chloride has been added. The effect of the copper is dramatic : it makes the Grignard reagent undergo conjugate addition, with only a trace of the diene.

Organocopper reagents undergo conjugate addition:
The copper works by transmetallating the Grignard reagent to give an organocopper reagent. Organocoppers are softer than Grignard reagents, and add in a conjugate fashion to the softer C=C double bond. Once the organocopper had added, the copper salt is available to transmetallate some more Grignard, and only a catalytic amount is required.