Hydrolytic deboronation

Even if there is no great steric hindrance, the reaction under aqueous conditions is often accompanied by undesirable competitive hydrolytic deboronation [79]. Kinetic studies [80] on the reaction of substituted arylboronic acids indicated that electron-withdrawing substituents accelerate the deboronation. Although there is no great difference between meta-and para-substituted phenylboronic acids, substituents at the ortho position may have a... 

The fact that stronger bases speed up both the coupling and the hydrolytic deboronation means that the choice of base is not always obvious. Although aqueous carbonate is the one most commonly used, stronger alkoxide bases appear to yield the best results for stericaUy hindered boronic acids. For base-sensitive substrates, bicarbonate can be used successfully when THF is the organic cosolvent. However, this base is less successful when employed with mostly nonaqueous reaction conditions. On the other hand, fluoride, an even weaker base, appears to work well with both aqueous and nonaqueous protocols. ... 

Sterically hindered or// o-disubstituted arylboronic acids such as mesitylboronic acid (9) or arylboronic acids with electron-withdrawing substituents (10) do not provide satisfactory results due to the steric hindrance and competitive hydrolytic deboronation. Consequently, we reinvest ated the coupling reactions of sterically hindered arylboronic aids having two or//io-substituents or functional groups which accelerate hydrolytic deboronation. It was discovered that this difficulty can be overcome by using suitable bases and solvent systems examples are shown in eqs 8 and 9 (II). This procedure gives an excellent method for the synthesis of biaryls. 

A heavily functionalized atropisomeric biphenyl derivative (designed for use as liquid crystal dopant) has been recently synthesized, although in low yield, with Suzuki coupling as the key step [40], The coupling reaction is complicated by rapid hydrolytic deboronation of the sterically crowded electron-deficient boronate ester 55. Rigorously anhydrous conditions are required to avoid the deboronation step (Scheme 3.24). 

---