Acid Joke - Redux

Figure 1. Platinum-catalyzed hydroarylation - or is it? Taken from Ref. [1].


Figure 2 . Palladium-catalyzed dioxygenation. Taken from Ref. [2].

If you remember the ‘inside joke’* I told you last week, for which hydrochloric acid might have the potential to replace all the precious gold catalysts, then it has clearly hit the spot. The 2 articles I am showing you share exactly this concern. Both publications ask the same question: when we are talking about a metal catalyst (platinum and palladium salts in these cases) participating in alleged catalytic cycles, is it really something else acting as the catalyst instead?

That contender, in many circumstances, is a proton (H+) – which addresses the implication behind my joke. Researchers have long been concerned about the notion that it can be through the generation of a catalytic amount of acid, at some point in the catalytic cycle, that really acts as the true catalyst for the reaction.

Thus the first paper [1], for which a platinum catalyst is supposed to catalyze a hydroarylation reaction (Figure 1), is in fact catalyzed by the protic acid (H+) generated during the catalytic cycle. The second paper [2], which describes a dioxygenation using PhI(OAc)2 as oxidant and Palladium salt as a catalyst. The researchers discover that the generation of acid through the catalytic cycle are again the true ‘magic’ behind the catalysis itself. Upon careful mechanistic studies, that leads to their development of a triflic acid (a strong protic acid)- catalyzed dioxygenation.

For the take home message – it is best for me to quote directly from Professors Bergman and Tilley’s paper – ‘We recommend that thorough control experiments for
acid catalysis become standard protocol for all new reactions of
organometallic catalysts, even when no acid is present in the
starting materials.’ [1] This can indeed save research funding too!

References:

*The line I wrote on Facebook for the 'Acid Test' Article last week –’There is a joke that HCl can replace all the expensive gold catalysts in catalytic reactions, and then there are real Bronsted acid catalysts out there to set things right.’

1. Disambiguation of Metal and Brønsted Acid Catalyzed Pathways for
Hydroarylation with Platinum(II) Catalysts
Miriam A. Bowring, Robert G. Bergman, and T. Don Tilley
Organometallics
dx.doi.org/10.1021/om2000458

2. The Nature of the Catalytically Active Species in Olefin Dioxygenation with PhI(OAc)2: Metal or Proton?
Yan-Biao Kang and Lutz H. Gade
J. Am. Chem. Soc.
dx.doi.org/10.1021/ja110805b