[kanonsviel]

Or it's just a simple aldol reaction?
As far as I can remember, an enolate ion can serve as a Michael donor only if it's a beta-dicarbonyl compound or beta-keto nitrile, nitro compund.
but this reaction seems so suspicious as to remind me nothing but a Michael reaction...  

[mehc]

As you said, Michael reaction requires enolate stabilized by two electron withdrawing groups (just as beta dicarbonyl compound etc).

In case of one electron withdrawing group, Michael reaction can only occur
1) if enolate is derived from nitroalkane

or

2) electrophile is acrylonitrile (little possibility).

[Dan]

Is this condition sufficient to undergo the Michael reaction? Or it's just a simple aldol reaction?

Both....

This is a named reaction, but have a go applying the principles of Aldol and Michael chemistry and see what you get first.

As you said, Michael reaction requires enolate stabilized by two electron withdrawing groups (just as beta dicarbonyl compound etc).

In this case there is a possible enolate conjugated with the aromatic system, this is sufficient stabilisation. 

[discodermolide]

Robinson annulation?

[Dan]

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Never mind then....

[kanonsviel]

I see...so it can be stabilized by resonance with the benzene ring, I was under the impression that the benzene ring is somewhat electron-donating (inductively??).
thanks for your gentle explanations

[discodermolide]

I see...so it can be stabilized by resonance with the benzene ring, I was under the impression that the benzene ring is somewhat electron-donating (inductively??).
thanks for your gentle explanations

Eh, not really, read up on the mechanism of the named reaction

[kanonsviel]

Eh, not really, read up on the mechanism of the named reaction

Hum...it's weird. I referred to both English text book and  Japanese site, it seems that the benzene ring is not the decisive factor that enabled the initial Michael reaction, for the Michael donor in the original Michael reaction was simply a normal cycloketone. And both of them didn't explain why the normal ketone is sufficient to serve as a Michael donor, nay there was some contradictions in the reaction conditions they showed.


this is the original reaction, it's under 0℃


while the Japanese site says it should be heated under a basic condition

[Doc Oc]

Those substrates aren't the same as the one you showed in your first post.  In the compound you showed there is a distinct difference in the stability of the two possible enolates, and that will significantly change the product that you get in the end.

[kanonsviel]

Those substrates aren't the same as the one you showed in your first post.  In the compound you showed there is a distinct difference in the stability of the two possible enolates, and that will significantly change the product that you get in the end.

well...I do understand that it would give different products beginning with different reactants in this reaction (eg, use an enamine to reverse the result)
But, my interest however, was how the initiative Michael reaction managed to occur despite the general demanding for a Michael donor.
Was that because the NaNH2 is too basic as to be able to force it to serve as a donor? Does LDA do the same thing?

[Doc Oc]

I think you're confused about the order of operations in this reaction.  ANY a,b-unsaturated carbonyl compound can undergo Michael addition, regardless of whether you're talking ketone, aldehyde, amide, etc.  But you don't throw everything into the flask at the same time and hope it works, that's why the reactions are laid out the way they are.

For example, in the Robinson/Rapson paper, you see the cyclohexanone is first mixed with base (NaNH2 in this case), then mixed with the a,b-unsaturated ketone second.  There is a distinct separation in steps, they don't all go in at the same time.  That's why you're able to get the products that you do.

The two examples you posted don't contradict, they have to do with using different alpha carbons.  With cyclohexanone the alpha carbons are the same so it doesn't matter which alpha carbon reacts in the Michael addition.  But with 2-methyl-cyclohexanone, the alpha carbons are different so there are elements of control that are used to favor one particular alpha carbon participating over the other in the Michael addition (this is known as thermodynamic vs kinetic control).

[kanonsviel]

I see...so the general rule for being a Michael donor can be applied only if the reaction is carried out in one-step?

[discodermolide]

I see...so the general rule for being a Michael donor can be applied only if the reaction is carried out in one-step?

Nope, you require an alpha-beta unsaturated carbonyl compound and a nucleophile. Alpha-beta unsaturated sulfones will also undergo a Michael Reaction. Anything which can stabilise the intermediate "enolate" will do it. You can trap the intermediate enolate as a TMS ether if you wish.

[kanonsviel]

I got it...it seems that I have jumped to the wrong conclusion, for a long time I have been considering that the Michael donor must be a 1,3-dicarbonyl compound. I find that the text book did say that these conditions represent the "best" Michael reaction...yet I have taken it for a necessary condition for such a long time
thanks for your answers.

[orgopete]

This is how I have thought of the Michael addition. The 1,4-addition is in competition with 1,2-addition. Groups that add electron density to the beta-carbon will retard 1,4-addition and groups that withdraw electrons will enhance 1,2-addition. The best reactions seem as though an equilibrium may exist with formation of either addition product, though protonation of an enolate will be less reversible than protonation of a 1,2-addition (ketone pK_a ca. 22, tert alcohol pK_a ca. 18).

If the nucleophile has a pKa of less than 18, then you may expect the 1,2-addition to reverse and if 1,4-addition occurs, it will be more stabile. Because 1,3-dicarbonyl compounds are more acidic, they will favor reversal of 1,2-additions and equilibrate to the 1,4-addition products.

You may wish to look at a range or Michael addition reactions to determine if my analysis is correct (or even useful). Aldehydes may not be good groups unless rather less effective nucleophiles are used, amines, mercaptans, halides. Beta substitution is not common as they generally add electron density are retard 1,4-addition. By that analysis, the Robinson reaction is surprising as it almost does everything wrong and still succeeds. A number of proton transfer steps must occur and I might have thought the alternate enolization reactions that could occur would make this reaction messy. I could make a number of arguments why it should succeed, but that would be making a house of cards. However, I might argue that in the original post, just as the CH₂-group is activated by the carbonyl and phenyl (as an electron withdrawing group), then the Robinson phenyl ring may also serve as an electron withdrawing group to enable 1,4-addition.