Tuan5583 - 19-5-2008 at 02:54
Hi all,
I'm currently working on the synthesis of benzil derivates by the Friedel Crafts acylation method, using Oxalyl chloride as reagent, AlCl3 as catalyst
and CS2 as solvent (described bellow). But unfortunately, the reaction did not run.
Ar-H + (COCl)2 + AlCl3 (cat.) + CS2 (solvent) => Ar-C(=O)-C(=O)-Ar
With ArH : 1,2-Dialkoxy-Substituted Benzenes.
Is there someone having experience with this reaction and can give me some advices, please?
Thank everybody
Tuan
Nicodem - 19-5-2008 at 03:56
What makes you believe oxalyl chloride behaves like that in Friedel-Crafts acylation? What is the reference for your reaction?
Oxalyl chloride behaves as expected in its reaction with AlCl3, meaning the intermediate ClCOCO<sup>+</sup> carbocation fragmentates into
ClCO<sup>+</sup> and CO. Thus the resulting species from the reaction between AlCl3 and oxalyl chloride is phosgene. There is at least one
paper where this fragmentation is used for the preparation of benzoic acids from (alkyl)benzene(s), oxalyl chloride and AlCl3, but as far as I know
you can not avoid the fragmentation of instable carbocations during Friedel-Crafts reactions and obtain something like you propose.
Tuan5583 - 19-5-2008 at 08:29
Hello
I used this reference J. Org. Chem. 1994,59, 635-638 but I couldn't obtain the benzil as described ...
Nicodem - 19-5-2008 at 09:39
Interesting in that they managed to prevent chlorocarbonylation simply by using CS2 as solvent. This gives hopes that similar acylations where
fragmentations occur, could actually work differently in CS2. For example the Friedel-Crafts with alkyl chloroformates which otherwise give
alkylbenzenes instead of alkyl benzoates (now that would be quite a useful reaction). (The referenced paper is attached.)
As far as your problem, without properly presenting it, how do you think anybody can give you any answer or advice? You did not even bother telling us
what you did, how many spots the TLC shows, what did you recover from the reaction, what have you done differently, and so on... So I'm moving this to
Beginnings section where vague questions are acceptable.
The reaction is obviously quite messy. Just check what they did to purify the products: extraction, flash, recrystallization and column
chromatography!
Attachment: Synthesis of Alkyl- and Alkoxy-Substituted Benzils.pdf (548kB)
This file has been downloaded 1159 times
Sauron - 20-5-2008 at 01:55
Aldrich has a tech data sheet for oxalyl chloride with many reactions shown and references, available from their website free of charge. I will dig up
the url and post it here, possibly this reaction may be mentioned.
http://www.sigmaaldrich.com/aldrich/bulletin/al_techbull_al1...
The reaction of anisole with oxalyl chloride and AlCl3 in CS2 is indeed shown, as I remembered, the prodict is 4,4'-dimethoxybenzil, which can be
oxidized to p-methoxybenzoic acid with H2O2.
See reference 8
F.C.Mitter and H.Muherjee, J.Ind.Chem.Soc. 16, 393 (1939)
[Edited on 20-5-2008 by Sauron]