MCQ
In the reaction sequence
- A
- ✓
- C
- D
✓
Answer
Correct option: B.
b
$(b)$ Copper $(I)$ salts have a remarkable effect on organometallic reagents. Grignard reagents add directly to the carbonyl group of $\alpha , \beta$ -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 $C = C$ electrophilic 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.
$(b)$ Copper $(I)$ salts have a remarkable effect on organometallic reagents. Grignard reagents add directly to the carbonyl group of $\alpha , \beta$ -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 $C = C$ electrophilic 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.
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