psygn (Stranger)
02-12-03 11:49
No 407131
      in situ MeNH2 from MeNH2.HCl/Et3N  Bookmark   

In ../rhodium/chemistry /redamin.titanium.html, there is a general procedure for the reductive alkylation of ketones using MeNH2.HCl, Et3N, Ti(iPrO)4 and NaBH4:

10 mmol of Ketone, titanium(IV)isopropoxide (5.9 ml, 20 mmol), methylamine hydrochloride (1.35 g, 20 mmol) and triethylamine (2.79 ml, 20 mmol) in absolute Ethanole (15 ml) was stirred in a capped flask at room temp. for 8-10 h. Sodium borohydride (0.57 g, 15 mmol) was added and the mixture stirred for additional 7-8 h at room temp. The reaction was quenched by pouring into aq. ammonia (30 ml, 2M), resulting inorganic precipitate filtered off, organic layer separated, A/B extraction, drying. Yield of primary amine after isolation purification for acetophenone was 86 % of N-methyl-phenethylamine.

As I understand it after reading this document, the MeNH2.HCl and Et3N react to form anhydr. MeNH2 and Et3NCl in situ. The imine is then formed between the ketone and MeNH2, and the produced water is scavenged by the Ti(iPrO)4. The imine is then reduced to the amine w/NaBH4.

Is my interpretation correct? Can the MeNH2.HCl/Et3N method for in situ MeNH2 production be used generally to produce anhydr. MeNH2 in situ? Why is it not used in the standard NaBH4/MeNH2 reduction without Ti(iPrO)4? Or in bromosafrole amination? Or N-methylacetaldimine production?

Have I got the wrong end of the stick?

Thanks laugh
(Chief Bee)
02-12-03 13:57
No 407169
      Triethylamine/MeNH2.HCl = anhydrous methylamine  Bookmark   

Yes, your interpretation is correct. The Ti(iPrO)4 also helps the reductive amination by coordinating to the formed imine and keeping everything "in place" until it has been reduced.

The reason the MeNH2.HCl/Et3N system isn't more often used to replace anhydrous methylamine freebase is probably because of the added cost, and that careful vacuum distillation is absolutely required to separate the triethylamine from the product freebase, as they are both carried through an acid/base extraction. I think there is a meth synthesis using this system on my page though.