Methyleneimine and relatives

Methyleneimine, also known as methanimine, is the simplest imine compound, H₂C=NH. Much research has been done regarding this compound, the majority of which seems to focus on spectra, geometry and physical constants. Furthermore, the vast majority of references that I have found are purely theoretical (quantum chemistry), and very few have practical experiments documented therein, which is what I am interested in.

I first became interested in methyleneimine while brainstorming ways to add a carbon with an amine moiety to a functional group. I thought a Grignard (or Barbier) reaction would be a very convenient way to accomplish this. For example, in theory one could synthesize propylamine from ethyl magnesium bromide as such:

EtMgBr + H₂C=NH ---> EtCH₂NH₃

I found little on the forum regarding methyleneimine, besides passing mention as an intermediate in the synthesis of methylamine from formaldehyde and an ammonium salt or degradation of hexamine. These reactions happen in aqueous solution, and furthermore, the methyleneimine is a transient species which quickly polymerizes to form cyclotrimethylenetriamine (hexahydro-1,3,5-triazine) among other compounds. In aqueous formaldehyde/ammonia solution, hexamine forms eventually if formaldehyde is in excess¹.

Since my idea requires anhydrous conditions (for Grignard, at least), dry methyleneimine would be required. It is a gas at room temperature and polymerizes when condensed at liquid nitrogen or dry ice temperatures². I figured the most obvious way to prepare it would be via condensation (chemical, not physical) of formaldehyde and ammonia gas in a 1 to 1 ratio, likely over a catalyst and/or desiccant. I am so far unable to find any references for such a procedure, however.

The main and seemingly most reliable method of production I have found (for spectrometry purposes) is the decomposition of methyl azide, by photolysis^3,4. Methyleneimine is also formed in pyrolysis of methylamine or photolysis of diazomethane (all in inert gas matrices)⁴. These methods are not terribly attractive; Methylamine pyrolysis gives a mix of products including hydrogen cyanide, methane, ammonia, and glycinonitrile (aminoacteonitrile). Methyl azide photolysis appears to be a cleaner reaction, but azides are toxic and sensitive explosive compounds.

I found another method used to do some actual experimental chemistry, which relies on the decomposition of aminoacetonitrile. The aminoacetonitrile is vaporized under vacuum (0.1 mbar) and lead through a quartz tub heated to 600^oC. Metheneimine and hydrogen cyanide are formed, which are then lead through a KOH tube to remove the hydrogen cyanide. The metheneimine gas is then lead into a U tube immersed in liquid nitrogen. “Methanimine was thus obtained in a 73% yield. It can be kept indefinitely at the liquid nitrogen temperature but only some hours at –78 °C (the temperature of dry ice)⁵.“
Google also came up with a mere partial reference to a synthesis of a heterocycle using methyleneimine, but now I can’t find it…

The methods above will be out of my reach for some time due to money and space issues, which leads me to an inquiry. What about synthetic equivalents? Hexamethylenetriamine has come to mind since 1,3,5-trioxane (cyclic formaldehyde trimer) can behave like or generate formaldehyde. I can’t find much information on it though. Does anyone know if hexamethylenetriamine is stable in pure (dry) form? The only thing I can find is suppliers, who sell it in aqueous solution.

Hopefully some others will find this seemingly “simple” compound as interesting as I do. Please chime in if you know something I couldn’t find or think of, especially alternative methods of preparation or synthetic equivalents.

¹ J. Am. Chem. Soc., 1948, 70 (11), pp 3659–3664
DOI: 10.1021/ja01191a034 The Reaction Between Formaldehyde and Ammonia

² J. Phys. Chem., 1961, 65 (12), pp 2139–2143 DOI: 10.1021/j100829a006 Low-Pressure, Fast Flow Pyrolysis of Methylamines

³ J. Phys. Chem. A, 2004, 108 (20), pp 4433–4439 DOI: 10.1021/jp037938+

⁴J. Phys. Chem., 1978, 82 (4), pp 391–393 DOI: 10.1021/j100493a005 Methyleneimine

⁵ A&A 535, A47 (2011) DOI:10.1051/0004-6361/201117602 Experimental investigation of aminoacetonitrile formation through the Strecker synthesis in astrophysical-like conditions: reactivity of methanimine (CH2NH), ammonia (NH3), hydrogen cyanide (HCN)

The first reference is already on sciencemadness, for some reason the link doesn't work when I copy it. I have PDFs of references 2 and 4, I will upload them if a mod says that I am allowed to do so (don't want to break copyright law). I link the fifth one because it is a free article.