# How Dihalogens Catalyze Michael Addition Reactions. - NCBI - piectron?

#### jim mcnamara

Mentor
Summary
Michael addition is found to be caused by Pauli exclusion. Question
How Dihalogens Catalyze Michael Addition Reactions. - NCBI

I do not know what the definition of 'piectron' is. In the abstract of the paper, I assume it is a misspell. Which makes me wonder?

I learned about Michael Addition in an Organic Chemistry class (loooong ago)- the use of halogens to catalyze many reactions. This paper is supposed to assert that there is a quantum level effect in how the catalytic molecule facilitates the reaction.

A really interesting idea. I do not know if it can be extended more generally.

I just get the heeby-jeebies when some mistakes like that in a peer reviewed journal. Makes you wonder what else got missed. And yes, I am the second worst typist here on PF. So I am well acquainted with stupid typographic errors.

#### TeethWhitener

Gold Member
I do not know what the definition of 'piectron' is. In the abstract of the paper, I assume it is a misspell.
It's a typo on Pubmed. In the actual abstract at the Angewandte website, it's clearly $\pi$-electron.
This paper is supposed to assert that there is a quantum level effect in how the catalytic molecule facilitates the reaction.
I suppose it's interesting, and Angewandte is a reputable journal. But it wasn't particularly surprising to me at first blush. I think the "Pauli repulsion" is more marketing than anything else. What they've done is theoretical calculations on the aza-Michael addition catalyzed by the halogens $X_2$. They break their energy calculations into three main pieces, one of which is the Pauli repulsion. When I tried to follow the reference trail to find exactly how they calculated this, it took me down a rabbit hole that I got tired of. As far as I can tell, the Pauli repulsion they refer to is--at least in part--related to the Hartree-Fock exchange integral (the three main integrals that one calculates in HF theory to get energies are the Coulomb (electrostatic), the orbital overlap, and the exchange (Pauli) integrals).

The Michael addition is trotted out every once in a while as an example of frontier molecular orbital theory (NB--I'm not convinced this approach has advantages beyond a simple nucleophilic addition picture), where the activation energy is roughly inversely proportional to the overlap between the HOMO of one molecule and the LUMO of the other (the Diels-Alder reaction is the canonical example of this effect). The current paper asserts that the catalytic effect of the halogens on the Michael addition has less to do with the orbital overlap and more to do with reduced repulsion between specific filled orbitals on the reagent molecules.

The reason it doesn't surprise me much is that they're basically saying that the halogen polarizes the acceptor such that electron density at the reaction center decreases--exactly the kind of thing you'd expect to speed up a nucleophilic addition. I suppose the really interesting part of the paper is that the energy change here is dominated by the exchange integral, not the electrostatic integral. This also explains why iodine is a more effective catalyst than fluorine, as the electrostatic contribution is larger in fluorine than iodine, but the exchange contribution is larger in iodine.

Anyway, that's my takeaway from a first readthrough. Take it with a grain of salt.

#### Ygggdrasil

Gold Member
I just get the heeby-jeebies when some mistakes like that in a peer reviewed journal. Makes you wonder what else got missed. And yes, I am the second worst typist here on PF. So I am well acquainted with stupid typographic errors.
Note that the paper is an accepted article, not the final article. Accepted articles have passed peer review but have not yet been copy edited by the journals. The final article will have undergone additional editing by the journal and authors to hopefully catch typos like that and other issues. The following disclaimer is present on the page for the article (https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201903196): "Accepted, unedited articles published online and citable. The final edited and typeset version of record will appear in the future."

#### jim mcnamara

Mentor
Thanks. Saw the article on NIH archive. Still an interesting paper.

"How Dihalogens Catalyze Michael Addition Reactions. - NCBI - piectron?"

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