Diboronic cross-coupling with aryl halides triflates

Several new synthetic methods of arylboronates, such as pinacol ester, have been developed. A convenient preparative method of arylboronates 4 is Pd-catalyzed cross-coupling of aryl halides and triflates with bis(pinacolato)diboron (3), which can be handled easily in air. The best ligand for the coupling is DPPF [12]. In addition, one-pot biaryl synthesis can be carried out most conveniently without isolation of the boronate 4 to provide 5 [13], Ligandless Pd(OAc)2 is active for... 

The Pd-catalyzed cross-coupling reaction of the pinacol ester of diboronic acid with aryl halides and triflates gives a direct procedure for various functionalized arylboronic esters (Table 6), which are useful for highly efficient and selective carbon-carbon bond formation by Pd catalysts (Sect. III.2.2). The combination of PdCl2(dppf) with KOAc (and dppf) is effective in the carbon-boron bond formation. KOAc is essential not only to accelerate the reaction but also to prevent the formation of biaryl by-products. 

The Pd-catalyzed cross-coupling reaction of the pinacol ester of diboronic acid with aryl halides and triflates gives a direct procedure for various functionalized arylboronic esters (Table which are useful for highly efficient and selective carbon-carbon bond... 

In another publication, the Miyaura group described the cross-coupling of aryl triflates to arylboronic esters as well as the synthesis of unsymmetrical biaryls using a modified protocol. In both cases, the authors used KOAc as base since stronger bases such as K2CO3 afforded symmetrical biaryls (see also 4.18) as by-products. The proposed mechanism of this reaction is depicted in Scheme 5-166. Oxidative addition of the catalyst to haloarene followed by displacement of the halide leads to the corresponding pentacoordinated palladium(II) species. Transmetalation with bis(pinacolato)diboron followed by reductive elimination yields the arylboronic ester. 

Ready availability of arylboronic esters from aryl halides or triflates (Eqs. 13 and 14) now offers a one-pot, two-step procedure for synthesizing unsymmetrical biaryls. The cross-coupling reaction of bis(pinaco-lato)diboron with triflate in dioxane is followed by a subsequent coupling with another triflate in the presence of K3PO4 to furnish an unsymmetrical biaryl from two triflates (Eq. 63). The synthesis from two different... 

---