Haloarene triflate aryl ester

The direct preparation of arylboronic esters from aryl halides or triflates now allows a one-pot, two-step procedure for the synthesis of unsymmetrical biaryls. The synthesis of biaryls was readily carried out in the same flask when the first coupling of aryl halide with diboron was followed by the next reaction with another halide or triflate [240]. The protocol offered a direct and efficient method for the synthesis of the boronic ester in the solid-phase that hitherto met with little success using classical methodology (Scheme 44). The solid-phase boronate (106, 107) [243] was quantitatively obtained by treating a polymer-bound haloarene with the diboron. The subsequent coupling with haloarenes furnished various biaryls. The robot synthesis or the parallel synthesis on the surface of resin is the topic of further accounts of the research. 

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. 

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