[macman104]

Just wondering people's opinions on which of the following is a faster Michael Acceptor.  I reasoned, that the ester has more pulling power, and will react faster.  My professor argues that for the same electronic reasons that a ketone is more reactive that an ester (more carbonyl like), it is a faster michael acceptor.

Thoughts?

EDIT:  If it helps, consider it separately under basic and acidic catalysis.

[spirochete]

I have a semester of advanced organic under my belt now where we considered a lot of situations like this.  I definitely agree with your prof that the ester version is less reactive.  The additional oxygen donates electron density through resonance.  It is electron withdrawing by induction but resonance almost always beats out induction.

It's true you can't actually draw a resonance structure where the oxygen pushes a negative charge onto the beta carbon that gets attacked, but it still pushes electron density toward it.

[nj_bartel]

I'm at a lower level of chem, but I think this is a good demonstration of this - You can see this in ¹³C nmr spectra, where the carbonyl carbon of an ester is further upfield than the carbonyl carbon of an aldehyde or ketone.

[azmanam]

It's all about the size of the pi* orbital.  the larger the pi* orbital, the easier (and faster) nucleophiles will add to it.  The conjugation in the ketone is uninterrupted and there is a larger (delta)positive on the beta carbon - thus a larger pi* orbital.

In the ester case, the conjugation is interrupted by the ester oxygen.  The ester oxygen has its own resonance thing going on with the carbonyl oxygen.  That diminishes the conjugation with the alpha and beta carbons.  Smaller (delta)positive.  Smaller pi* orbital.  Slower reaction with nucleophile.

(nice structures btw

[azmanam]

re: nj

If you go to this website and "agree the disclaimer," then search for SDBS nos 3298 and 2825 you can see this graphically.  They aren't exactly the same (one has the extra beta methyl group, one doesn't), but you can clearly see BOTH the ketone carbonyl carbon and the ketone beta carbon are further downfield (more deshielded, less electron density) than the ester version.

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng

[macman104]

(nice structures btw

Heh, thanks .

Ok, so what about in the case of an acid catalyzed michael addition.  I found a paper:

http://dx.doi.org/10.1021/jo982521z

Where acid catalyzed addition of esters was the quickest, I've cut and paste some pictures of the paper together to show the table they talked about.

Notes for the superscripts in the picture below:

^aUnless otherwise noted, 1.2 equiv of the Michael acceptor and 0.3 equiv of TfOH were used under solvent-free conditions.
^bIsolated yield based on the starting Michael donor. Yields in parentheses are recovery of the Michael donor.
^eSolvent (CH₂Cl₂) was used, and in total 2.7−3.3 equiv of 2b was added.
^fSolvent (CH₃CN) was used, and in total 1.5 equiv of 2c was added.
^gA 1:1 diastereomeric mixture.

[azmanam]

the only thing that jumps out at me is the ester run was solvent free and the ketone run was in DCM.  I'm not on campus, so I haven't read the paper.  Maybe something will be more clear if I read the paper.  The C13 shifts between the beta carbons is very small for those 2 acceptors (128 vs 130 ppm), so maybe the electronics just aren't very different between those two.