Is cyanogen chloride (fluoride) a source of C⩸N(+)? More mechanistic insights.

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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.

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.

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5 responses to “Is cyanogen chloride (fluoride) a source of C⩸N(+)? More mechanistic insights.”

December 8, 2020

Mike Casey

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.

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1. December 9, 2020
  
  look@s
  
  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 spectroscopy. 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.
  
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2. December 9, 2020
  
  look@as
  
  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
  
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  1. December 9, 2020
    
    Henry Rzepa
    
    Here is an energy profile in which cyanogen chloride is replaced by acetyl chloride, one of the examples noted above. The barrier corresponds to a very facile thermal reaction. The mechanism is as with cyanogen chloride, in that the departure of the chloride from the acyl group to become AlCl₄^– is synchronous with the formation of the benzene C-C bond to the acyl group. Clearly such a synchronous route is very viable. To exclude them, other mechanisms would have to be computed and compared in energy.
    
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December 8, 2020

Henry Rzepa

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.

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