Organoboron compounds, reaction mechanisms addition reactions

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

Fig. 16.18. Representative mechanism of the Pd-catalyzed C,C coupling of an organoboron compound. The elementary steps, discussed in the text, are (1) complexation, (2) oxidative addition, (3) transmetalation of the alkenylboron compound to afford an alkenylpalladium compound, (4) reductive elimination, and (5) dissociation of the coupled product from the metal. - Note Regarding the arrangement of the ligands around the metal center of the individual intermediates and the details of the transmetalation the present mechanistic analysis is less complete than the mechanistic analysis of other Pd-catalyzed C,C couplings, namely the Stille coupling (Figure 16.27) or Heck reaction (Figure 16.35, part II), which have been investigated in great detail.

In addition, Wipf and co-workers104 have used silver(i)-catalyzed addition of zirconocenes to 3,4,6-tri-O-benzyl-D-glucal epoxide 93 for the stereoselective synthesis of a-C-glucosyl compounds 95 and 96 following a similar mechanism as in the reaction with organoaluminium and organoboron reagents (Scheme 32). 

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