CCL: Explanation on specific metal reactivity
- From: JPD-workstation_home <djukic**unistra.fr>
- Subject: CCL: Explanation on specific metal reactivity
- Date: Mon, 4 May 2020 20:26:37 +0200
You are right.
However, you will find out readily that some issues are
unanswered/unsolved still today.
One example is for instance the reactivity of M-H bonds in transfers
organic substrates. What is being actually transfered and
a lot not only on the metal, its oxidation state, the ligand retinue
perhaps also on "environmental factors".
It is indeed easy to draw a simplified picture of the earth from the
moon, things get tougher when one gets on the surface.
This potential review you are mentioning should be a critical one made
from the "surface", and should confront
experimental evidence (for the
more documented cases) to theory.
Le 04/05/2020 à 15:18, Sebastian Kozuch seb.kozuch[a]gmail.com a
Sent to CCL by: Sebastian Kozuch [seb.kozuch[a]gmail.com] Hi Per-Ola,
This was actually useful. I guess that a thorough discussion like
this, considering the difficulties in obtaining such information,
should be written in a review or perspective article. So here is a
challenge for the organometallic community ;)
On 4/5/20 9:55 AM, Norrby, Per-Ola Per-Ola.Norrby=astrazeneca.com wrote:
I guess most of your answers here will point out that the question is
wrong. There are plenty of metals that do each of the tasks you have
pointed out. I’d say the least specific one is hydrogenation,
worlds largest asymmetric catalytic process uses Rh, not Ir. But I
guess you want to find out why some metals seem ideally balanced and
thus have gained high popularity. I’ll just address cross
here. It’s all a question of balance. As you yourself have
earlier, each of these cases are catalytic cycles, and for an
efficient catalytic coupling cycle you need to have two states that
are easily accessible, with similar energies. Additionally, for
robustness, you only want these states, not any others, to be
accessible. Furthermore, organic reactions can diverge if you start
forming radicals (even though that reactivity can be tamed, with
effort). If you want to avoid radicals, go for something that has
only closed shell states. This would exclude first row transition
metals, each of which have easily accessible open shell states. But
to reiterate, sometimes we want to pay the effort to fine-tune the
reactivity of these metals and limit the side paths. You can do much
more efficient coupling with Fe, Ni, or even Cu than with Pd, but due
to the presence of additional oxidations states, each system must be
optimized, you don’t have “off-the-shelf”
solutions tat always work.
Staying in the second or third row, the second row is generally
favored by price. Then, why Pd? Obviously it has two easily
accessible oxidation states, 0 and +2. Other states can be reached,
but only under forcing or bimetallic conditions. The energies of
these two states happen to be in a range suitable for the needed
reactions, mostly because aryl halides are very common. If we had
focused on diazonium salts instead, you’d find that other metals
would become optimal, but for safety reasons, we will not. What
alternatives are there? Rh is used, but if you go to closed shell, it
has to utilize the +1,+3 cycle. The cost of reaching +3 seems to be
high for simple aryl halides, and the high charge of +3 would make it
sensitive, reducing robustness. How about Ru? Both 0 and +4 seems to
require too much energy, and the odd numbers would be open shell. A
bimetallic +2,+3 should be possible, but stabilizing a multimetallic
catalyst in only the bimetallic state seems hard and not overly
robust (there are solutions, for specific problems). I could continue
like this, but it would be very long, you’d have to explain for
metal exactly why it has the wrong balance, and for every metal, you
could probably use those arguments to come up with the exception,
where that metal too would be a coupling catalyst. Even more so for
hydrogenation; I don’t think there is a single metal that
used to hydrogenate something.
Metathesis is harder to elucidate, there you need the perfect balance
between single and double bonds to metal at a single oxidation state.
I don’t know enough of the details, but Ru, Mo, and Os seem to
well, and there have been reports of first row also, I believe.
very much in the sensitivity to side reactions, air, water, etc…
Behalf Of *Sebastian Kozuch seb.kozuch]^[gmail.com
*Sent:* den 2 maj 2020 16:19
*To:* Norrby, Per-Ola <Per-Ola.Norrby*_*astrazeneca.com>
*Subject:* CCL: Explanation on specific metal reactivity
Sent to CCL by: Sebastian Kozuch [seb.kozuch^gmail.com]
For all the computational organometallists out there:
Why Ir and not other metals catalyses hydrogenation?
Why Pd for cross-coupling?
Why Ru for metathesis?
I cannot find any specific explanation that connects a metal to its
reaction. Tons of mechanistic studies and reviews, without
explanations on why this metal and only this metal for this reactions.
Is this info hidden or unknown? Any good paper or book?
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