bromination product of anisaldehyde ?

[psyloxy]

Is there something in 4-MeO-benzaldehyde that would prevent the successful formation of 4-MeO-3,5-diBr-benzaldehyde with Br2 ? The MeO group directs ortho/para, para is blocked by the -COH group, which itself directs meta, so everything *should* really go in the right position. But browsing thru Beilstein, 4-MeO-3,5-diBr-benzaldehyde is never made from anisaldehyde.Even more puzzling is the fact that the analogous reaction with 4-HO-benzaldehyde works like a charm[1]:

Yield   95.5% 
Reagent Br2 
Solvent MeOH 
Time    1h 
Temp    ambient

Is it because after the monobromination of anisaldehyde the 3-Br-4-MeO-BA is too deactivated or what ?? I would like to say I don't understand chemistry anymore, but I never understood the basics anyway.

[1]Manchand, Percy S.; Belica, Peter S.; Wong, Harry S.; SYNCAV; Synth.Commun.; EN; 20; 17; 1990; 2659-2666

--psyloxy--

[Rhodium]

Is it because after the monobromination of anisaldehyde the 3-Br-4-MeO-BA is too deactivated or what?

Yes, exactly. After the first bromination, you have one strongly deactivating group (-CHO) and one weakly deactivating group (-Br), while you have only one activating group (-OCH₃), this making the whole molecule too deactivated.

Hydroxy is significantly more activating than methoxy, so in the case of 3-Br-4-OH-BA, the molecule is just enough activated to be brominated a second time, while 3-Br-4-MeO-BA is not.

[moo]

How about using a Lewis base to catalyze the second bromination? Something along the lines of the synthesis of 2,5-dibromobenzaldehyde

Post 421565

(Rhodium: "Chinese Translation needed", Novel Discourse) from benzaldehyde. By the way, I searched the literature for syntheses of 2,5-dibromo-BA similar to the chinese ref, with no luck.