# What are the byproducts in a Suzuki reaction?

I am looking precisely at a coupling of a trifluoroborate with an aryl bromide. I know the mechanism of the Suzuki reaction, yet I cannot find any mention of the side products or degradation products that one could obtain if the coupling fails. Maybe those products are obvious for those who know well how palladium reacts, but this is not my case. I tried to find a book that would give me a feel for what might happen and could find anything close enough.

I'm expecting a protodeboration as a side reaction, yet I can't find any source nor mechanism. Could anyone shed some light on the subject?

• I believe homocoupling of the aryl bromide is a minor byproduct. There is enough literature on the reaction, but the information may be tucked away in the supporting info Jul 24 '17 at 11:58

It is postulated that the trifluroborates are not the transmetallating specis, rather than to be the source of slow hydrolysis to yield boronic acids:

(source 1, slide 41; citing Molander and Biolatto, 2003 (Ref.1))

which then enter the catalytic cycle:

(preview in Kürti & Czakó, page 448 bottom (Ref.2)

Organic Reactions (vide infra) explicitly states that complete hydrolysis of the trifluoroborates is necessary to allow successful transmetallation.

In addition, the fluoride anion is said to interact in the following way with the palladium catalyst:

(source 2)

References:

1. Gary A. Molander, Betina Biolatto, "Palladium-Catalyzed Suzuki−Miyaura Cross-Coupling Reactions of Potassium Aryl- and Heteroaryltrifluoroborates," J. Org. Chem. 2003, 68(11), 4302-4314 (https://doi.org/10.1021/jo0342368).
2. László Kürti, Barbara Czakó, In Strategic Applications of Named Reactions in Organic Synthesis: Background and Detailed Mechanisms; Forwarded by E. J. Corey, Introduction by K. C. Nicolaou; Elsevier Academic Press: Burlington, MA, 2005 (ISBN-13: 978-0-12-429785-2).
• Thanks, but I had already found these mechanisms. I'm wondering more precisely about the mechanisms that don't give the desired product and the factors influencing them. Jul 23 '17 at 16:12
• Oh. Sorry, I misunderstood "byproduct" as "what is naturally formed beside the product of normal coupling" rather than "product(s) obtained if the intended coupling reaction fails". Let us see suggestions by others. Jul 23 '17 at 16:18
• I clarified the question. Thanks anyways, this is an excellent summary of the general mechanism with sources. Jul 23 '17 at 17:57

Buttonwood has given an excellent answer based on mechanism and inorganic byproducts. Since OP has indicated that the anticipation is about unintended products, I have done a little search and found some help in Ref.1, Its abstract states that:

The outcome of the Suzuki–Miyaura cross‐coupling for the direct competition reaction between two boronic acids was evaluated under routine synthesis conditions. The reaction selectivity was found to depend on the amount of the base used, with fewer bases favoring the reactivity of the boronic acid with lower $$\mathrm{p}K_\mathrm{a}$$ (stronger acid). The dependence of the reaction selectivity on base stoichiometry was found to increase with the increase in the difference in the $$\mathrm{p}K_\mathrm{a}$$ values of the competing boronic acids. These results confirm a relationship between acid–base chemistry and the Suzuki–Miyaura reaction catalytic cycle. Moreover, the results indicate that under these specific conditions, the most reactive organoboron species toward transmetalation is the borate anion $$\ce{R-B(OH)3−}$$ instead of the neutral boronic acid $$\ce{R-B(OH)2}$$. Hence, the main role of the base in the reaction mechanism is to increase the reactivity of the boronic acid toward the $$\ce{Pd}$$–halide complex by converting it into the respective organoborate. In addition, boric acid, an important reaction byproduct, affects the selectivity in the Suzuki reaction because its gradual formation in the reaction medium disturbs the acid–base equilibrium.

Following scheme would show the outcome:

Reference:

1. Carlos F. R. A. C. Lima, Ana S. M. C. Rodrigues, Vera L. M. Silva, Artur M. S. Silva, Luís M. N. B. F. Santos, "Role of the Base and Control of Selectivity in the Suzuki–Miyaura Cross‐Coupling Reaction," ChemCatChem 2014, 6(5), 1291-1302 (https://doi.org/10.1002/cctc.201301080).