Boronic acid speciation in Suzuki-Miyaura cross–coupling

Abstract

Since its discovery in 1979, the Suzuki-Miyaura (SM) reaction has become one of the most widely utilised tools for carbon-carbon bond formation. The palladium catalysed coupling of an organoboron and organohalide compounds proceeds through a three-stage mechanism of oxidative addition, transmetalation and reductive elimination. The transmetalation of boronic acids to a palladium(II) complex has been widely studied. However, very little is known about the transmetalation of boronic esters, which are commonly used as an alternative to unstable boronic acids. Whether these species undergo direct transmetalation or prior hydrolysis to the boronic acid under SM conditions remains unknown. This research aimed to elucidate the mechanism of this cross-coupling process. Initial results under typical SM conditions created a biphasic reaction, promoted by the inorganic base and solvent composition, and showed that the boronic esters and corresponding boronic acid couple at the same absolute rate. This is thought to be a consequence of the formation of a biphasic mixture, rendering phase transfer the turnover-limiting step. The conditions were thus adapted to maintain a monophasic system using an organic soluble base, 2-tert-butyl-1,1,3,3-tetramethylguanidine, enabling the focus to be transmetalation as the turnover-limiting step. These new conditions show a significant difference in both reaction rate and induction period when using a boronic ester compared to the corresponding boronic acid. The use of guanidine was also shown to have an interesting effect on the boronic acid/ester species by 19F and 11B NMR. Further studies found the use of guanidine to create a boronate species, with this species being an aryl trihydroxyboronate or the hydroxyl“ate”-complex of the boronic ester, depending on the presence of diol in the system. Formation of a boronate species was found to be crucial for efficient cross-coupling. When testing weaker bases, unable to form a boronate species, poor SM cross-coupling conversion was found using the newly developed phosphine-free guanidine conditions, showing the importance of the boronate species under these conditions. The results suggest that depending on the strength of base used, the pathway of transmetalation pathway can be switched, between the boronate pathway and the oxo-palladium pathway, under the specific conditions developed