{"id":22445,"date":"2020-06-10T16:53:51","date_gmt":"2020-06-10T15:53:51","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=22445"},"modified":"2020-06-15T15:27:16","modified_gmt":"2020-06-15T14:27:16","slug":"curly-arrows-in-the-21st-century-proton-coupled-electron-transfers","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=22445","title":{"rendered":"Curly arrows in the 21st Century. Proton-coupled electron transfers."},"content":{"rendered":"
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One of the most fascinating and important articles dealing with curly arrows I have seen is that by Klein and Knizia on the topic of C-H bond activations using an iron catalyst.[1]<\/a><\/span> These are so-called high spin systems with unpaired electrons and the mechanism of C-H activation involves both double headed (two electron) and fish-hook (single electron) movement. Here I focus on a specific type of reaction, the concerted proton-coupled-electron transfer or cPCET, as illustrated below. These sorts of reactions happen also to be of considerable biological importance, including e.g.<\/em> the mechanism of photosynthesis and many other important transformations.<\/p>\n

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A hydrogen atom comprises a proton and one electron. The question is whether the proton and the electron travel together as a true hydrogen atom when the hydrogen relocates, or do they each take their separate way, as in the PCET reaction shown above. I will discuss the arrows shown briefly first.<\/p>\n

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  1. The blue arrow originates at an oxygen lone pair, donating two electrons into a O-H bond.\u00a0<\/li>\n
  2. The hydrogen starts off with two electrons in a C-H bond. In a pure proton transfer, it would lose both these electrons to the carbon (as an acid) and the proton would travel to the oxygen (as the base, which receives the proton). In fact these two C-H electrons go off in different directions.\n