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. Explanations of how to apply the individual arrows are not always easy to find however! Circled in red are the ones to be discussed here, although most carry fascinating and often subtle meanings!‡
Archive for the ‘reaction mechanism’ Category
The “double-headed” curly arrow as used in mechanistic representations.
Tuesday, August 29th, 2023Pre-mechanism for the Swern Oxidation: formation of chlorodimethylsulfonium chloride.
Friday, August 25th, 2023The 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. The mechanism of this transformation as shown in e.g. Wikipedia is illustrated below.‡ However, an interesting and important aspect of chemistry is not apparent in this schematic mechanism and to rectify this, a full computed mechanism is laid out below, for which the FAIR data has a DOI: 10.14469/hpc/13151
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Gaseous carbon: The energetics of two forms of tetracarbon, C4 and a challenge!
Tuesday, November 29th, 2022The topic of dicarbon, C2, has been discussed here for a few years now. It undoubtedly would be a gas! This aspect of the species came to the fore recently[cite]10.1039/D2CP01214F[/cite] when further experiments on a potential chemical precursor of dicarbon, the zwitterion X(+)-C≡C(-), showed that different variants of X(+), such as not only X=PhI(+), but also e.g. X=dibenzothiophenium(+) appeared to generate a gaseous species, which could be trapped as “C2” in a solvent-free connected flask experiment.
Nitroaryls- A less-toxic alternative reagent for ozonolysis: modelling the final step to form carbonyls.
Saturday, October 8th, 2022Sometimes you come across a reaction which is so simple in concept that you wonder why it took so long to be accomplished in practice. In this case, replacing toxic ozone O3 as used to fragment an alkene into two carbonyl compounds (“ozonolysis”) by a relatively non-toxic simple nitro-group based reagent, ArNO2 in which the central atom of ozone is substituted by an N-aryl group. As reported by Derek Lowe, two groups have published[cite]10.1021/jacs.2c05648[/cite], [cite]10.1038/s41586-022-05211-0[/cite] details of such a reaction (Ar = 4-cyano or 3-CF3,5-NO2). But there are (at least) two tricks; the first is to use photo-excitation using purple LEDs (390nm light) to activate the nitro group. The second is to establish the best aryl substituents to use for achieving maximum yields of the carbonyl compounds and the best conditions for achieving the cyclo-reversion reaction, shown below as TS1. That step requires heating the cyclo-adduct up to ~80° in (aqueous) acetonitrile for anywhere between 1-48 hours. Here I take a computational look at that last step, the premise being that if such a model is available for this mechanism, it could in principle be used to optimise the conditions for the process.
A new type of bispericyclic reaction: Cyclopropanone + butadiene.
Friday, September 30th, 2022The term bispericyclic reaction was famously coined by Caramella et al in 2002[cite]10.1021/ja016622h[/cite] to describe the unusual features of the apparently innocuous dimerisation of cyclopentadiene. It shows features of two paths for different pericyclic reactions, comprising a 2+4 cycloaddition in the early stages, but evolving into a (degenerate) pair of [3,3] sigmatropic reactions in the latter stages. Houk (who also uses the term ambimodal) has in recent years extended the number of examples of such pericyclic sequences to trispericyclic[cite]10.1021/jacs.8b12674[/cite] (see here) and even an ambimodel tetrapericyclic reaction, as reported at the recent WATOC event. Here I show an example of a new type of bispericyclic reaction, comprising a 2+4 cycloaddition combined with a electrocyclic ring opening.
Unexpected Isomerization of Oxetane-Carboxylic Acids – an alternative autocatalytic mechanism evaluated.
Wednesday, August 17th, 2022Previously, I looked at autocatalytic mechanisms where the carboxyl group of an oxetane-carboxylic acid could catalyse its transformation to a lactone, finding that a chain of two such groups were required to achieve the result. Here I look at an alternative mode where the oxetane-carboxylate itself acts as the transfer chain, via a H-bonded dimer shown below.
Unexpected Isomerization of Oxetane-Carboxylic Acids – substrate design.
Sunday, August 14th, 2022Having established a viable model for the unexpected isomerism of oxetane carboxylic acids to lactones[cite]10.1021/acs.orglett.2c01402[/cite], and taken a look at a variation in the proton transfer catalyst needed to accomplish the transformation, I now investigate the substrate itself.
Unexpected Isomerization of Oxetane-Carboxylic Acids – catalyst design.
Saturday, August 13th, 2022Previously, a mechanism with a reasonable predicted energy was modelled for the isomerisation of an oxetane carboxylic acid to a lactone by using two further molecules of acid to transfer the proton and in the process encouraging an Sn2 reaction with inversion to open the oxetane ring. 
Unexpected Isomerization of Oxetane-Carboxylic Acids – a first look at the mechanism
Sunday, August 7th, 2022Derek Lowe’s blog has a recent post entitled A Downside to Oxetane Acids which picks up on a recent article[cite]10.1021/acs.orglett.2c01402[/cite] describing how these acids are unexpectedly unstable, isomerising to a lactone at a significant rate without the apparent need for any catalyst. This is important because these types of compound occur frequently in the medicinal chemistry literature.
Dioxane tetraketone – an ACS molecule of the week with a mystery.
Wednesday, June 22nd, 2022I have long been fascinated by polymers of either carbon dioxide,† or carbon monoxide, or combinations of both. One such molecule, referred to as dioxane tetraketone when it was featured on the ACS molecule-of-the-week site and also known as the anhydride of oxalic acid, or more formally 1,4-dioxane-2,3,5,6-tetraone, has been speculated upon for more than a century.[cite]10.1002/cber.19080410335[/cite]