The Hive > Novel Discourse

Arylboronic acids for Suzuki Couplings

(1/2) > >>

Arylboronic acids [Ar-B(OH)2] are very useful precursors used in Pd-catalyzed Suzuki Couplings, where almost any organic halide can be coupled with an arylboronic acid, one example being that allyl bromide would give allylbenzenes.

General procedure for arylboronic acid synthesis1

Magnesium turnings (30 mmol) were placed in a round-bottomed flask and then flame-dried under N2. Aryl bromide (30 mmol) dissolved in THF (20 mL) was added with an addition funnel to the flask slowly. The reaction mixture was gently refluxed for 3 to 4 h. After cooling, the Grignard reagent was transferred to a solution of (CH3O)3B in THF (10 mL) at -78°C and stirred overnight while warming up to RT slowly. After acidification with 10% HCl (10 mL), the product was extracted into ether (3x100 mL) and dried (sodium sulfate). The solvent was then removed under reduced pressure, and the products was precipitated by hexane with further recrystallization from water.

p-Tolylboronic acid: white needles were obtained in 81% yield; mp 243-244°C (lit.2 242-243°C).

p-Methoxyphenylboronic acid: White needles were obtained in 72% yield; mp 197-198°C (lit.2 202-205°C). 

Arylboronic Acid References:

[1]: Tetrahedron 51(11), 3129 (1995)
[2]: J. Am. Chem. Soc. 53, 711 (1931)

Suzuki Coupling

In this reaction is a bromo compound R-Br coupled to a boronic acid R'-B(OH)2 with Pd-catalyst. In the first step the bromo compound is added oxidative at Pd(0) and the bomide is metathetically substituted by an hydroxy ion. After trans-metallation and secession of tetrahydroxyborate the Pd(II) with both organic moieties is obtained. The Pd is reductively eliminated and the residues coupled to R-R'.

Suzuki Coupling References:

why not just use the protected aminoalkylhalide to directly produce amphetamine or meth?

These catalysts are recomended by the catalyst manufacturers;

Bedford's acetate
Bedford's chloride

Polymer supported Pd(PPh3)4 is used in the industry today because it has the advantages of a heterogenous catalysts (easy removal by filtration) combined with the properties of the homogenous catalyst (better utilisisation of the metal, elimination of in-pore diffusion, easy exotherm control).

why not just use the protected aminoalkylhalide to directly produce amphetamine or meth?

That is of course possible, I didn't mention any such halide specifically, as I am not 100% sure exactly which protecting groups are compatible with Suzuki Chemistry. Also, allylbromide given as example in my post has a history of giving very high yields in coupling reactions, while other more complex halides may have unpredictable yields.

Good point on the yields comment.  However, what kind of yields could be acheived overall through each route?  Do you have any more references for this type of coupling?


[0] Message Index

[#] Next page

Go to full version