The schematic representation of a chemical reaction mechanism is often drawn using a palette of arrows connecting or annotating the various molecular structures involved. These can be selected from a chemical arrows palette, taken for this purpose from the commonly used structure drawing program Chemdraw.

The Swern oxidation[cite]10.1016/0040-4020(78)80197-5[/cite] is a class of “activated” dimethyl sulfoxide (DMSO) reaction in which the active species is a chlorodimethylsulfonium chloride salt.

In another post, a discussion arose about whether it might be possible to trap cyclopropenylidene to form a small molecule with a large dipole moment. Doing so assumes that cyclopropenylidene has a sufficiently long lifetime to so react, before it does so with itself to e.g. dimerise.

I occasionally spot an old blog that emerges, if only briefly, as “trending”. In this instance, only the second blog I ever wrote here, way back in 2009 as a follow up to this article.[cite]10.1021/ed084p1535[/cite] With something of that age, its always worth revisiting to see if any aspect needs updating or expanding, given the uptick in interest.

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.[cite]10.1002/anie.201805511[/cite] 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.

With the current global lockdown, and students along with everyone else staying at home, I have noticed some old posts of mine are getting more attention than normal.

Earlier, I explored the choreography or “timing”, of what might be described as the curly arrows for a typical taught reaction mechanism, the 1,4-addition of a nucleophile to an unsaturated carbonyl compound (scheme 1). I am now going to explore the consequences of changing one of the actors by adding the nucleophile to an unsaturated imine rather than carbonyl compound (scheme 2).

A reaction can be thought of as molecular dancers performing moves. A choreographer is needed to organise the performance into the ballet that is a reaction mechanism. Here I explore another facet of the Michael addition of a nucleophile to a conjugated carbonyl compound.

In the previous post, I looked at the mechanism for 1,4-nucleophilic addition to an activated alkene (the Michael reaction). The model nucleophile was malonaldehyde after deprotonation and the model electrophile was acrolein (prop-2-enal), with the rate determining transition state being carbon-carbon bond formation between the two, accompanied by proton transfer to the oxygen of the acrolein.

Students learning organic chemistry are often asked in examinations and tutorials to devise the mechanisms (as represented by curly arrows) for the core corpus of important reactions, with the purpose of learning skills that allow them to go on to improvise mechanisms for new reactions.