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