Sensitized Redox Photochemistry - And Some Opinions

by: Derek Lowe

Redox photochemistry marches on – that’s what I take away from this new paper from a group at Regensburg, which uses sensitized photoredox as a new variation. In this case, the light is harvested by a ruthenium complex (as in many other photoredox system), but the energy is then transferred to a polycyclic hydrocarbon, which does the actual redox reaction with the substrate. This allows access to much higher redox potentials than having everything done by one species, and potentially more reactions can be discovered out in this range.

The tricky part is that the transfer of that energy needs to be fast relative to other possible pathways in the system, but using anthracene or pyrene (for example) seems to do the trick, fortunately. The luminescence of the Ru catalyst is quenched more quickly than it can do electron-transfer with a tertiary amine, which is the order that you want, and the authors demonstrate a range of aryl halides reacting with N-methylpyrrole as a trapping agent to form new C-C bonds in good yields. Even aryl chlorides and aryl triflates are shown to react under these conditions. On the other side of the reaction, photo-Arbuzov reactions are also shown, with a trialkylphosphite as the trap (and those of you who have smelled them will welcome the news of another way to get rid of the awful things).

I would guess that this system will be applicable to a whole range of reactions, and I look forward to seeing what gets developed in this area. But with that in mind, I wanted to bring up another topic which has been on my mind. It seems that every time photoredox chemistry gets mentioned here, there are a number of comments on the work that can only be described as “hostile”. These vary, from accusations that the reactions were known before, or are only minor variations on what’s been published, to talk of how such things have no practical applications in the real world and are only fit for high-profile CV padding, etc. I don’t think I’ve written on any of the newer photochemistry work without these things showing up, and I’ve always been surprised at the vehemence out there.

I would have thought (mainly because this is my own view!) that any new fast way to make bonds would be of interest to synthetic and/or medicinal chemists. So what’s the problem? I haven’t encountered anything like these attitudes at the bench, I have to say. My own colleagues have, over the last few years, enthusiastically adopted photoredox chemistry, recognizing its potential for chemical diversity and ease of synthesis. Talking with friends in other organizations, I hear the same sort of thing. This makes me wonder if these comments are not reflecting the opinions of the broader community of organic chemistry, but just represent a vocal minority. I have no way to judge this other than what I’ve seen and heard myself, of course, but that’s what I have to conclude, for now.

So those readers who are not redox photochemistry fans, if you would take the time to say just what it is that sets you off about the field, I’d be glad to see it. And those of you who just quietly try out these reactions and use them to make new molecules, I wouldn’t mind seeing you put in a quick vote on your side as well.

Disclosure: None.