Mesitylboronic acids

Although steric hindrance of aryl halides is not a major factor in the formation of substituted biaryls, low yields are obtained when ortho-disubstituted arylboronic acids are used. For example, the reaction with mesitylboronic acid proceeds only slowly because of steric hindrance during transmetalation to the palladium(II) complex. 

The reaction of mesitylboronic acid with iodobenzene at 80 °C in the presence of Pd(PPh3)4 and various bases has been reported [78]. The results are summarized in Table 2. 

Tab. 2. Reaction of mesitylboronic acid with iodobenzene under different conditions.

For such sterically hindered boronic acids, an alternative procedure using the esters of bor-onic acids and an anhydrous base has been developed. Thus, the coupling can readily be achieved by employing the trimethylene glycol ester of mesitylboronic acid and Cs2C03 or K3PO4 in DMF at 100 °C, whereby the coupling product is obtained quantitatively (Eq. (49)) [78]. 

Mesitylboronic acid was used in reactions with group 13 metal alkyls. The expected B-O-M species was obtained only for M = Ga, according to the reaction in Equation 3. This is possibly due to the properties of Me3Ga, in which only one methyl group is used in the protonolysis reaction. 

Diaryl ketones.Pd-catalyzed cross-coupling of aryl halides with arylboronic acids under 1 atmosphere of CO provides these products. With highly hindered ArB(OH>2 such as mesitylboronic acid, good yields of the ketones are obtained only with Arl (not ArBr). 

Method L is conducted with thallium hydroxide as the base and Pd(PPh3)4 as catalyst under very mild reaction conditions to afford a wide variety of biaryls, including highly encumbered, in fair yields [16]. The biaryl 189 was prepared from mesitylboronic acid (283) and iodobenzene in almost quantitative yield. Scheme 23. Whereas tri-ort/io-substituted biphenyls were obtained by this method in respective yields, synthesis of tetra-ort/io-substituted analogues failed [16], Among base-sensitive groups, only an ester function has survived thallium hydroxide-mediated reactions at room temperature. Concerning the extreme toxicity of thallium compounds, the method is applicable only in the small-scale labo-synthesis. 

In a flask equipped with a Dean-Stark apparatus and a condenser, mesitylboronic acid-2,2-dimethyl-1,3-propanediol ester (360, 87 mg, 0.37 mmol, 1.1 eq.), methyl 3-iodo-4-methoxybenzoate (361, 100 mg, 0.34 mmol), Pd(PPh3)4 (26 mg, 0.022 mmol, 6... 

Synthesis of arylboronic acids via Grignard reaction of sluggishly reactive aryl halides Preparation of mesitylboronic acid (283) 1143]... 

Table 2. Effects of Base on the Coupling of Mesitylboronic Acid with...

Aqueous TlOH solution, which produces highly insoluble salts similar to the barium base, completes the coupling reaction of mesitylboronic acid with iodobenzene within 30 min at room temperature (Table 2 and the procedure in the text)." However, this excellent base for mesitylboronic acid, unfortunately, does not work well for other less substituted arylboronic acids. Aryl- and 1-alkenylboronic acids transmetallate to thallium salts. 

Pd(PPhj)4 (0.02 mmol) and mesitylboronic acid (1.6 mmol) are dissolved in DME (6 mL) under argon. Iodobenzene (1 mmol) and aqueous TlOH (IM, 2 mmol) are added successively, and then the mixture is stirred for... 

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. 

There are, however, some cases in which the aqueous technique can lead to the formation of side-products, as e.g. in arylation of 3-bromochromone by sterically hindered mesitylboronic acid, which is complicated by protonolysis [37] when the reaction is carried out in aqueous solvent in the presence of Na2C03. This reaction can be made selective if it is run without water with TI2CO3 as a solid base ... 

The use of aqueous TlOH often allows one to lower the temperature, even in the case of cross-coupling with sterically hindered reagents. Thus, the reactions of aryl iodides with mesitylboronic acid can be performed at room temperature in dimethylacetamide with 10% aqueous TlOH giving a high yield of cross-coupling product [39] ... 

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