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. 

 

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.