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Some of my teachers were debating about whether a double bond in conjugation with a ketone is reduced or not by $\ce{NaBH4}.$ One of my teachers presented the evidence from Clayden which had an example where a cyclic ketone was reduced alongside the conjugated double bond.

However, there was no example for a straight chain compound. Also, the book said that steric hindrance may affect the reduction process and only favour the reduction of ketone.

I want to know that is there any authentic source which has an example of reduction of a conjugated double bond and ketone via $\ce{NaBH4}.$

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Scifinder shows lots of reactions such as you describe. Often (but not always) copper is used to help favour the 1,4-addition (alkene reduction) process.

scifinder screenshot

The simplest example I can see is the reduction of 3-penten-2-one[[2]] - when treated with NaBH4 the majority (89%) is reduced at the ketone only, while 11% of the doubly-reduced product is formed:

enone reduction

Obviously different substrates would give different product ratios, but certainly this is a plausible (and precedented) reaction.

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  • $\begingroup$ Thanks a lot. But why does it have such a low yield in the example you gave. I don't see much steric hindrance. Do you know the reason? $\endgroup$ – user78585 Jun 25 '19 at 16:12
  • $\begingroup$ Related question: chemistry.stackexchange.com/questions/87138/… As a rule, NaBH4 reduces 1,2. $\endgroup$ – user55119 Jun 25 '19 at 16:49

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