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I'm curious as to the role of molecular oxygen in Suzuki homocoupling reactions, as discussed here.

In a standard Suzuki cross-coupling reaction, molecular oxygen is not required. However, this paper claims that the homocoupling reaction will not proceed in an Argon environment. Is there a (relatively) straight forward justification for this that I am missing?

This is the reaction mechanism from the paper:

mechanism

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  • $\begingroup$ They are not using a base either here, are they? Typically, Suzuki-Miyaura couplings (with arylhalides as the reaction partners) are performed in the presence of base to "activate" the arylboronic acid. $\endgroup$ Feb 24 '14 at 12:26
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I don't remember a direct palladation of arylboronic acids or their derivatives in any of the couplings. Typically, $\ce{Pd(0)}$ inserts into the $\ce{C-Hal}$ bond of arylhalides and is thus oxidized to $\ce{Pd(II)}$. Ouite often, $\ce{Hal}$ is then displaced with $\ce{OR}$ by the action of a base. Think in $\ce{KOt-Bu}$ often used in the Kumada coupling.

In the homo coupling that you described here, there is - for obvious reasons - no $\ce{Ar-Hal}$ for the oxiative addition step. Without $\ce{O2}$ for the oxidative activation of $\ce{Pd(Ph3P)2}$, the mixture will thus remain inactive.

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