Re: ORGLIST: bromination of substituted pyridine - correction

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From: Gabriel Tojo (qogatojo$##$usc.es)
Date: Thu Jul 27 2006 - 16:44:21 EDT

Hello, Philip,

Polar solvents promote electrophylic brominations. NBS is a good
brominating agent of aromatic compounds, specially in polar
solutions. That is by, radical brominations are done by default in
CCl4, which is a very apolar solvent that hinders electrophylic
brominations.

I would try the traditional conditions for radical bromination that
have been well-tested in thousands of compounds, before embarking in
some fancy new method. Time will tell us if the procedure from Green
Chemistry is reliable in a broad category of compounds.

Sometimes benzylic bromitions work better with 1,3-dirbomo-5,5-
dimetylhydantoin, rather than NBS (J.Org.Chem. 2054 (1997))

Good luck!

Gabriel

> Hello everybody, i am sorry but i forgot to mention that i am using
> 0.1 equivalent of AIBN as the free radical initiator. So the
> conditions for the reaction are:
> 1.1eqNBS/0.1eq AIBN/112C/ 10mins/microwave, reaction run in ethyl
> acetate.
> [ref, Green chemistry, 2003, 5, 470-474]
> [Original Query - i am trying to brominate the methyl group of 2-
> piperazine-6-methyl pyridine-4- carboxylic acid but i am always
> getting ring subsitution and not benzylic bromination]
> Philip.
>
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Gabriel Tojo

Department of Organic Chemistry
Faculty of Chemistry
University of Santiago de Compostela (SPAIN)
tel: 981-581524 ext. 14244
qogatojo$##$usc.es


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