i) Bromo-substrate with electronic withdrawing group and boronic acid-substrate with electronic donating group
OR ii) electron-rich bromo-substrate and electron-deficient boronic acid-substrate.
Also please let me know which one will give better yield: boronic acid or boronic-pinacolate.
Use the Bromo substrate with the electron withdrawing group. The electron withdrawing group facilitates the rate limiting oxidative addition step.
Comparatively speaking the boronic acids are quite robust in their reactivity.
That is true, as EWG facilitate the oxidative addition. If the bromo derivative is electron rich, then is there any effect possible on the reaction rate by using either boronic acid or B-Pin.
It can vary considerably, I have had good experience using the boronic acids. The most robust conditions that I have found are in the following paper which activates the system with KF, and uses bulky tBu3P ligands with Pd2(dba)3.
J. Am. Chem. Soc. 2000, 122, 4020-4028
Suzuki-Miyaura cross-coupling reaction :
PdCl2(PPh3)2 (caution: toxic; irritant; incompatible with oxidants, moisture; 42 mg; 0.06 mmol) and Na2CO3 (140 mg; 1.32 mmol) were added to a degassed solution of 2,5-dibromo-3,4-dinitrothiophene (0.602 mmol) and boronic acid/ester (1.32 mmol) in THF/H2O (20 mL; 4:1) under an argon atmosphere. The mixture was stirred under Ar at 65 oC for 3-4 h, diluted with H2O (30 mL) and extracted with CH2Cl2 (2 x 30 mL). The combined organic extracts were dried (Na2SO4) and concentrated in vacuo. Subsequent column chromatography afforded pure products. use this procedure
As said before, the EWG is better to be on the aryl-Br to facilitate OA step. But also, if you use a boronic acid with EWG, they tend to proto-deborylate in the presence of aq base. For example, 2,6-difluorophenyl boronic acid have an half-life of about 30 minutes in THF/0.5M K3PO4 at room temperature.
As for which is better between boronic acid and pinacol ester, they are in most cases comparable.
The coupling can be performed in the absence of water with boronic acid, which can be useful if you have hydrolyzable groups. The acid can form trimers more readily though and these are much less reactive in standard Suzuki-Miyaura Coupling (SMC).
Degassing your reaction is key...
i)The presence of electron withdrawing group in arty halie & electron donating group in aryl boranic acic will enhance the reaction rate of Suzuki coupling reaction.ii)Cyclic dioxy boaryl borane expected to react faster than the diol
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