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Basically, I've recently been thinking about using HSAB theory to rationalize the concept of michael donors and michael acceptors. Essentially, if we have a deprotonated diketone (with a lone pair on the alpha position carbon) or the Gilman reagent, it attacks the carbon in a vinylic ketone in a 1,4 addition.

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However, if we used a Grignard reagent, than it attacks the carbonyl carbon in a 1,2 addition.

How can we use HSAB theory to justify this? I've figured out that the carbonyl carbon's LUMO is the pi-star orbital while the vinylic carbon center's LUMO is also the pi-star orbital except with the carbon bonded to another carbon, thus it should be higher in energy than a pi star orbital bonded with an oxygen.

I've also figured that the carbon nucleophile center on a Michael donor (such as that of the diketone) should be "softer" than the nucleophile center on a Grignard reagent's carbanion because the charge is less concentrated due to resonance and thus the HOMO of the Michael donor is not as high as the HOMO of a Grignard reagent.

However, I do not know where to go from here. Can someone help me what I'm doing wrong?

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  • $\begingroup$ Your train of thought seems reasonable and I think it does lead to the expected conclusions. The soft nucleophiles (e.g. diketone enolate) adds to the soft Michael acceptor in 1,4 manner. The hard nucleophiles (e.g. RMgX) adds to the carbonyl group in direct fashion. $\endgroup$ – Tan Yong Boon Dec 13 '18 at 8:00

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