09-24-03 18:57
No 460870
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--
(Chief Bee)
09-25-03 05:51
No 460975
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.
(Hive Bee)
09-25-03 09:43
No 461012
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.
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