Natural Science Forum / Chemistry / Organic Synthesis / November 2005

Phe-MgCl is preferable to Phe-MgBr for lots of application because the
grignard from bromobenzene has lots of biphenyl sideproduct in it -
which may not be fun to remove, depending on the polarity and other
properties of your product.

I found Phe-MgCl in THF (commercial from Aldrich) to be a much better
alternative. I have never made it myself but I remember that you need
reflux in THF (ether will not work in this case) that is perfectly
anhydrous and preferably oxygen-free. Do the usual sodium
dust/benzophenone still distillation to get the good-grade THF.

There is a Mg activation that works realy well: take excess (several
equivs of theoretical amount) of Mg turnings. Add a large egg-shaped
stirbar in a large round flask, flush it with Ar, add few drops of neat
bromine and turn the speed to maximum to have the dry turnings flying
around in the brownish vapors of bromine. Let to stirr overnight, then
add your dry solvent (THF) through canula.

With such pre-treated Mg Grignard usualy starts with almost no
induction period. Argon is required, it does not work well with
nitrogen because of the black magnesium nitride forming upon reaction
of freshly-crushed Mg surface with N2. Bromine is there to coat and
temporarily protect the new surface with bromide (it dissolves upon
addition of the solvent). Iodine does not work nearly as well as
bromine in this case.

reaction of chloro benzene is difficult with Mg, because the Cl group
is in resonance with the benzene ring. Hence partial sp2 character is
present.

The yield may not be good, but you are requested to check it from a
more reliable source.

Also Mg is added in presence of non polar solvent such as ether, hence
your saying that using THF as a solvent is correct.

I hope the information was helpful .