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Author: Subject: Versatile "new" enamine synthesis
chemrox
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[*] posted on 13-5-2014 at 12:33
Versatile "new" enamine synthesis


New ca 1967- A Versatile New Enamine Synthesis
WILLIAM A . WHITE AND HAROLD WEINGARTE, JOCX Notes, 9/7/66

Has anyone tried this synthesis using TiCl4 as a catalyst and H2O scavenger for making enamines from carbonyl and amine? It appears attractive because of the rapidity and superior water scavenging. If you haven't the paper PM me and I'll post it in refs. Basically:
2RCH2C=OR' + 6HNR2" + TiCl4 -> 2CRCH=C[NR"]R + 4R2"NH2Cl + TiO2

One unknown is the applicability to aromatic amines such as aniline. Someone who knows more about the reactivity of TiCl4 might address this before I experiment. As always, I have searched for the topic and not found it. If it's been discussed I apologize.




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plante1999
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[*] posted on 13-5-2014 at 13:10


I'm not a great organic chemist, but from what I know of titanium, I can say titanium tetrachloride is EXTREMELY reactive, and will convert to the dioxide at the first opportunity.



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AvBaeyer
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[*] posted on 13-5-2014 at 13:29


I am very familiar with use of TiCl4 in the preparation of otherwise difficult to form imines/enamines. It works well when it is needed although it is not generally a first choice if other routes are available.

There is one reference that I have regarding TiCl4 and N-methylaniline which says that these reagents form a stable complex. See: Tet. Letters, p3519, (1979). If you can locate it, it may be helpful in tracking down related references which may be useful.

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[*] posted on 14-5-2014 at 00:11


Guillaume Bélanger ,* Michaël Doré, Frédéric Ménard , and Véronique Darsigny. Highly Chemoselective Formation of Aldehyde Enamines under Very Mild Reaction Conditions. J. Org. Chem., 2006, 71 (19), pp 7481–7484.

This article is quite interesting if you want to make the enamine from an aldehyde. The method described is easy, cheap and simple.
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Dr.Bob
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[*] posted on 14-5-2014 at 07:30


The TiCl4 method works great, but it also makes a lot of fine TiO2, which is not very easy to clean up. If you cannot make the imine via other methods, this will work, but is overkill for most imines. I'm pretty sure that it will work with aromatics, but they often form imines just fine without much work. I just did a reductive amination of an aniline with benzaldehyde to make a N-Bz aniline, and all I did was add 1 eq BnCHO to 1 eq Ar-NH2 in MeOH, add a drop of AcOH, then stir a while and then rotovap and high vacuum. I got a nice solid that by TLC showed some BnCHO, some anline, but mostly a spot in between, presumed to be the imine. That was then treated with NaCNBH3 in DCE to provide a modest yield of the BnNH-Ar. Trying to do the reaction in one step with Ar-NH2, BnCHO, and NaCNBH3 in MeOH provided a mix of some SM, very little desired product, and mostly dibenzylated aniline. So that inidicates that the secondary amine is as or more reactive than the primary aniline, and that I likely used too much benzaldehyde, due to the SM not being completely pure as well as me being careless in measuring a small amount of BnCHO.
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[*] posted on 14-5-2014 at 10:19


One relatively simple method of making imines, particularly N-aryl imines, is to throw an isocyanate at your carbonyl compound. It eliminates out CO<sub>2</sub>.

I think this works with ketones, but I know it works with aldehydes - benzaldehyde is for sure a go.

(Isocyanates aren't exactly OTC, but TiCl<sub>4</sub> isn't either.)




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chemrox
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[*] posted on 23-5-2014 at 14:57


Well.. I have TiCl4 which is a plus. 4 day reaction times are a bother using conventional methods. Won't the TiO2 stay in the aqueous phase on workup? Or drop out as a solid?

[Edited on 23-5-2014 by chemrox]




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