I asked the question in my previous post. A computational mechanism revealed that AlCl3 or its dimer Al2Cl6 could catalyse a concerted 1,1-substitution reaction at the carbon of Cl-C≡N, with benzene displacing chloride which is in turn captured by the Al. Unfortunately the calculated barrier for this simple process was too high for a reaction apparently occuring at ~room temperatures. Comments on the post suggested using either a second AlCl3 or a proton to activate the carbon of the C≡N group by coordination on to nitrogen. A second suggestion was to involve di-cationic electrophiles. Here I report the result of implementing the N-coordinated model below.
Click on image for 3D model
The free energy barrier ΔG‡298 is 20.8 kcal/mol (FAIR Data DOI: 10.14469/hpc/7584), which corresponds to a facile reaction at room temperatures. There does not seem to be any need to invoke super-reactive di-cationic electrophiles in this instance. This is yet another illustration that computational modelling nowadays is good enough to flag unviable mechanisms, and hence to instigate a search for a better model.
Is a two-step addition-elimination mechanism involving ClCN coordinated to one AlCl3 via nitrogen possible? As a corollary, which is preferred, bonding to AlCl3 via Cl or via N?
A related question – has the mechanism by which the well-studied acyl chloride-AlCl3 complexes (O-Al bond) give acylium ions with loss of Cl4Al- been studied? That step is glossed over in teaching the Friedel-Crafts acylaion mechanism.
I once checked a few references starting from “March’s Advanced Organic Chemistry”, and one (from 1996) I found interesting is https://doi.org/10.1021/ja9624331 and (more recent) https://doi.org/10.1039/C3OB27094G
Both represent experimental studies with kinetics by specstroscopy. The introduction to the articles, and the respective chapter in the March book (11-17) are worth reading regarding your question.
All kinds of variations with electrophiles including free acylium cation (as SbF6 salt), O-complexed acid chlorides, Cl-complexed acid chloride, O,Cl-bis-complexed acid chloride have been considered and may be operative under specific conditions.
Maybe even better as authoritative review: A. Molnar, G. A. Olah, G. K. S. Prakash, Chapter 8 (“Acylation”) in “Hydrocarbon Chemistry”, 3d ed 2018, J. Wiley and Sons. https://doi.org/10.1002/9781119390541.ch8
The mechanism shown above with ClCN coordinated to one AlCl3 is significantly lower in energy than the one where AlCl3 is coordinated to another AlCl3.
I have not looked to see if the complex between an acyl chloride and AlCl3 shows synchronous or stepwise behaviour with benzene. It may well have been done by someone though.