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, the 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 coupling here. It’s all a question of balance. As you yourself have pushed 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 every 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 hasn’t been 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 do it well, and there have been reports of first row also, I believe. It’s very much in the sensitivity to side reactions, air, water, etc…
owner-chemistry+per-ola.norrby==astrazeneca.com $#at#$ ccl.net
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On Behalf Of Sebastian Kozuch seb.kozuch]^[gmail.com
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.
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