A recent article reports, amongst other topics, a computationally modelled reaction involving the capture of molecular hydrogen using a substituted borane (X=N, Y=C).[1] The mechanism involves an initial equilibrium between React and Int1, followed by capture of the hydrogen by Int1 to form a 5-coordinate borane intermediate (Int2 below, as per Figure 11).‡ This was followed by assistance from a proximate basic nitrogen to complete the hydrogen capture via a TS involving H-H cleavage. The forward free energy barrier to capture was ~11 kcal/mol and ~4 kcal/mol in the reverse direction (relative to the species labelled Int1), both suitably low for reversible hydrogen capture. Here I explore a simple variation to this fascinating reaction.∞
This variation involves transposing X and Y such that Y=N+ and X=C– to form a carbon ylide such that X=C becomes much more nucleophilic than the original nitrogen nucleophile. An animation of the full IRC† (intrinsic reaction coordinate computed at ωB97XD/cc-pvtz; FAIR data doi: 10.14469/hpc/2704) is shown below.
The profile shows that the reaction is concerted between the species labelled React and Prod; no sign of Int1 and Int2!
The evolution of the dipole moment along the IRC shows very non-linear behaviour (such plots are rarely shown in most published IRC analyses; they should be!), ending of course with the ionic zwitterion that is the imminium borohydride Prod. Indeed the entire reaction coordinate is an unusually vivid example of a non-least motion path!
This simple atom transposition has given us a very instructive exercise in reaction paths, by-passing entirely both Int1 and Int2 (making them hidden intermediates), and converting React → Prod into a concerted reaction. It would be great to probe this convoluted journey using reaction dynamics!
∞Archived as DOI: 10.14469/hpc/3096
‡ Such a species can be seen as a hidden intermediate in the mechanism of reduction of a carboxylic acid by diborane.
†None were shown in the original study.[1]
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As one might infer from the IRC above, the transition state normal vibrational mode has a very unusual form. Given all the bond breaking and making that goes on in this reaction, this mode is in fact almost a pure bond rotation (See DOI: 10.14469/hpc/2705).