I have a Moral Issue with C-H Bond Activation

Today’s group meeting literature discussion was an interesting one. One of my group members discussed about the synthesis, characterization and reactivity of (TMC)Cr(O2) : a Chromium(III) Superoxo complex. It’s a very important  and interesting compound, and it indeed activates pretty high energy C-H bonds of 1,4-cyclohexadiene (CHD: 78 kCal/mol), 9,10-dihydroanthacene (DHA: 77 kCal/mol) and Xanthene (75.5 kCal/mol)..

It’s all good, because the bond dissociation energies (BDE) of above mentioned compounds are in a good high range. OK,.. setting how awesome the Cr(III) superoxo complex and its C-H activation are, I was not content with the term “bond activation” due to one reason.

All of the above compounds are aromatically unsaturated compounds waiting to become aromatic, and whenever there is a chance and enough activation, it loses H and become aromatic. If any C-H bond activating “agent” is to be considered as a good C-H bond activating compound, is it suitable to utilize the instability of a compound to achieve that target ? It’s like poking into the eyes of a baby about to cry, he is looking for a reason to cry, and you trigger it.

This is the question I had, and indeed our awesomely outspoken group contributed with their ideas, and my boss also poked our (and his) brains well…

One conclusion we made was to extrapolate the rate constants of the reactions Vs the bond activation energy of different compounds (which give a straight line) and extrapolate it to figure out if a particular compound is capable of activating C-H bonds of CH4 or C6H6 etc. Which is reasonable, but again the reaction conditions (some are unstable at higher temps etc) prevent such a study to go to conclusion.

So,.. What do you think ? Do you think actual bond activation should be free of added factors ?

Can we find something that doesn’t trigger other processes ? and what would be the best benchmark for the lower and/upper limits of C-H bond activation.. ?

It might sound somewhat confusing, but I really would like to see your ideas in this regard to learn something….

Let’s talk…..

Ref : Jaeheung Cho et. al (JACS 2010, ASAP)



3 thoughts on “I have a Moral Issue with C-H Bond Activation

  1. hey piyal, hope all is well. my first comment is that jae admits in the last paragraph of the article that the substrates are already activated, in a sense, for reasons you already mentioned. the term “c-h bond activation” is probably not even the most accurate phrase for it because the activation of the bond is sort implied in the BDE. when i read these papers, to evaluate how noteworthy the c-h activation is, i tend to look at the substrates.

    i guess i’m wondering what some of these “added factors” you allude to are? i mean, what’s your plan to do C-H activation if you’re not using a metal complex “on the edge of existence”? there’s so much more than just a BDE here – you can’t overoxidize (nobody cares about converting methane to a carboxylic acid, for example), you have selectivity factors to consider for three carbons or more, etc.

  2. Steve-O,
    The added factor I am concerned about is “Attaining Aromaticity” by the subjective molecules…
    If you activate a molecule that doesn’t have such a driving force, I can say it’s “C-H bond activation” But, my issue is, can we consider that “Attaining Aromaticity by C-H bond activation” as a true C-H bond activation, because it’s driven by the transformation from non-aromatic compound to an aromatic compound…

    Yes, I am more inclined to assess the Quality of the C-H bond…

    Anyway, thanks for the intriguing reply…

  3. i guess to clarify my point, compare benzene and 1,4-cyclohexadiene: BDEs are 112 vs. 78 kcal/mol. the difference is that one is aromatic and the other isn’t. everybody knows that 1,4-cyclohexadiene is sort of “cheating.” again, to me, the extent of “activation” is in the BDE.

    honestly, jae probably wanted to try wimpy substrates in order to show some reactivity for JACS. and hey, it worked!

    c-h activation of alkanes is really hard because once you get a complex that’s suitably strong enough to do the oxidation, ligand decomposition almost always ensues.

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