[insertwittyname]

in what grignard reagent will be formed? the one with Br or with Cl? I'm guessing that since a radical formation is involved and Br radical is more stable, the Grignard reagent with Br should be preferentially formed. Would this be correct?

[Hunter2]

I think there will be not much difference. Both halogenes create Grignard.

[Altered State]

I think there will be not much difference. Both halogenes create Grignard.

+1, you will need to use other starting material or substitute one of these halogens by a non-reactive group with Mg

[discodermolide]

This compound,

forms the Grignard selectively at the bromine's position.
You have the meta compound so I think there will be a good chance that the Br will exchange.

[OrgXemProf]

Selective LiCl mediated direct insertion of Mg into the C-Br bond in m-bromochlorobenzene has been reported.

See Entry #3, Table 1, page 19 in: Fabian Michel Piller, LiCl-Mediated Direct Insertion of Magnesium Into Aryl, Heteroaryl and Benzylic Halides. Regio- and Chemoselective Synthesis of 5-Membered Ring Heterocycles. PhD dissertation, Ludwig-Maximilians-Universität München, 2010. The dissertation (pdf file) can be found online c/o http://edoc.ub.uni-muenchen.de/11412/2/Piller_Fabian_Michel.pdf

Ba-da BING, ba-da BOOM.