Imperial College London

Henry Rzepa

Consider the four reactions. The first two are taught in introductory
organic chemistry as (a) a [proton
transfer, ]{style="color: #0000ff;"}often abbreviated
[PT,]{style="color: #3366ff;"} from X to B (a base) and (b) a [hydride
transfer]{style="color: #0000ff;"} from X to A (an acid). The third
example is taught as a hydrogen atom transfer or
[HAT]{style="color: #0000ff;"} from X to (in this example) O. Recently
an article has appeared\[cite\]10.1002/anie.201805511\[/cite\] citing an
example of a fourth fundamental type (d), which is given the acronym
[cPCET [which I will expand
later.]{style="color: #000000;"}]{style="color: #3366ff;"} Here I
explore this last type a bit further, in the context that X-H bond
activations are currently a very active area of research.  
[![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/07/XH-page001.svg){.aligncenter
.size-large .wp-image-19809 decoding="async"
width="450"}](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/07/XH-page001.svg)

To help understand these four types, I have colour-coded the electron
pair constituting the X-H covalent bond in
[red]{style="color: #ff0000;"}.

1.  In mechanism (a), this electron pair stays with X, thus liberating a
    proton which is captured by the base.
2.  The hydride transfer (b) is so-called because in fact this electron
    pair travels together with the proton, hence the term hydride or
    H**[^--^]{style="color: #ff0000;"}**.
3.  Hydrogen atom transfers as in (c) in effect transfer both a proton
    and one electron to another atom (oxygen in the example above),
    leaving behind one electron on X. The electron and the proton are
    said to travel together as a "true" hydrogen atom.
4.  The fourth mechanism (d) is fundamentally different from (c) in that
    whilst the electron and the proton travel in concert (at the same
    time), they do not travel together. In this example the proton
    travels to the oxygen, whilst the electron travels to the iron
    centre, in the process reducing its oxidation state. This mode is
    now called a *concerted proton-coupled electron transfer*, or
    **[cPCET]{style="color: #3366ff;"}** as above.

The tool employed to distinguish between mechanisms (c) and (d) is the
IBO or intrinsic bond orbital localisation
scheme.\[cite\]10.1021/ct400687b\[/cite\] One practical advantage of
such a scheme over better known localisation methods such as NBO
(Natural bond orbitals) is that IBOs can be made to transform smoothly
during the course of a reaction, as followed by say an IRC (Intrinsic
reaction coordinate). NBOs may instead show discontinuous behaviour
along a reaction IRC. Klein and Knizia have located transition states
for examples of both (c) and (d) above and studied the IBOs along such
IRCs. The two IBO reaction transformations are very different, as
illustrated below (used, with permission, from the article itself). For
the HAT type (X=C above), an α-spin IBO morphs from a C-H bond into a
H-O bond, whilst the β-spin counterpart morphs from being located on the
C-H bond into a carbon-centered radical. For the cPCET mode, the α-spin
IBO morphs from C-H to a C-centered radical, but the β-spin region grows
onto an iron d-orbital. It is in fact even more complex than the diagram
above implies, since some reorganisation of the O-Fe region occurs and
the H...:O region is still anti-bonding at the transition state.

![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/07/024-1024x350.jpg){.aligncenter
.size-large .wp-image-19821 loading="lazy" decoding="async" width="450"
height="154"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/07/024-1024x350.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/07/024-300x103.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2018/07/024-768x262.jpg 768w"
sizes="(max-width: 450px) 100vw, 450px"}

We can see from this that mechanistic reaction analysis is starting to
track the "curly arrows" we conventionally use to represent reactions in
some detail, as well as informing us about the relative detailing timing
of the various curly arrows used. Of course this latter aspect cannot be
easily represented by conventional curly arrows. It seems timely to
revisit the vast corpus of organic and organometallic "curly arrow
pushing" to starting adding such information!

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Messaggi di Rogue Scholar

A Theoretical Method for Distinguishing X‐H Bond Activation Mechanisms.