2-Methylglutaronitrile

2-Methylglutaronitrile is the organic compound with the formula NCCH2CH2CH(CH3)CN. This dinitrile is obtained in the large-scale synthesis of adiponitrile. It is a colorless liquid with an unpleasant odor. It is the starting compound for the vitamin nicotinamide and for the diester dimethyl-2-methylglutarate and the ester amide methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate, which are promoted as green solvents. 2-Methylglutaronitrile is chiral but is mainly encountered as the racemate. It is also used to make Dytek A.

Occurrence and production
2-Methylglutaronitrile is a by-product of the production of adiponitrile, the precursor of hexamethylenediamine and adipic acid as building blocks for nylon 66.

Starting from 1,3-butadiene or a butadiene-rich C4-section (> 40% by volume) from a naphtha steamcracker in the first stage a mixture of pentenenitriles is obtained through hydrocyanation (using as catalyst Ni0-phosphine [PR3] or phosphite or phosphonite [P(OR)2R] ). The mixture contains mainly trans-3-pentenenitrile in addition to the isomers 2-methyl-2-butenenitrile, 4-pentenenitrile and 2-pentenenitrile.


 * Pentennitrile durch Hydrocyanierung von Butadien.svg

The mixture of monoolefinic C5 mononitriles is isomerized to 3- and 4-pentenenitrile with a hydrocyanation catalyst and a Lewis acid (such as ZnCl2). In the third step, the mixture is reacted with hydrogen cyanide to give a mixture of dinitriles which contains in addition to 2-methylglutaronitrile also adiponitrile and 2-ethylbutanedinitrile.


 * Dinitrile aus Pentennitrilen.svg

2-Methylglutaronitrile can be separated by fractional distillation.

The 2-methylglutaronitrile-rich fraction has hitherto been combusted as an undesired by-product of adiponitrile production, having the typical composition of about 86 wt% 2-methylglutaronitrile, 11 wt% 2-succinonitrile and 3 wt% adiponitrile.

Applications
2-methylglutaronitrile can be converted to 3-methylpyridine (β-picoline) by partial hydrogenation.


 * 2-MGN zu 3-Methylpyiridin.svg

In addition to 3-methylpyridine, 3-methylpiperidine is obtained as a by-product from which further 3-methylpyridine can be obtained by dehydrogenation.

Ammonoxidation of 3-methylpyridine on transition metal contacts yields 3-cyanopyridine (nicotinonitrile) in yields of 95%.


 * 3-Methylpyridin zu Nicotinsäureamid.svg

Hydrogenation of a solution of 2-methylglutaronitrile in ethanol in the presence of Raney cobalt at 15 bar and 100 °C yields 2-methylpentane-1,5-diamine.


 * 2-MGN zu 2-Methylpentan-1,5-diamin.svg

2-Methylpentanediamine can be converted to 3-methylpiperidine at 300 to 400 °C on a zeolite contact and then dehydrated on a palladium contact to 3-methylpyridine, which can be converted via nicotinonitrile into nicotinamide.

The racemic diamine can also be used for the preparation of specific polyamides and after reaction with phosgene to form 2-methylpentane diisocyanate as a reaction component in polyurethanes. Nitrilases regioselectively hydrolyze the ω-nitrile group in α, ω-dinitriles without detectable amide intermediate directly to the carboxyl group. 4-cyanopentanoic acid is formed in high yield.


 * 2-MGN zu 4-Cyanopentansäure.svg

The ammonium salt of 4-cyanopentanoic acid can be converted by catalytic hydrogenation in the presence of methylamine in 1,5-dimethyl-2-piperidone, an environmentally compatible solvent.


 * 1,5-Dimethyl-2-piperidon-Synthese.svg

The hydrolysis of both nitrile groups of 2-methylglutaronitrile with e.g. 20% sodium hydroxide solution at 50 °C and subsequent acidification produces 2-methylglutaric acid.


 * 2-MGN zu 2-Methylglutarsäure.svg

Starting from 2-methylglutaronitrile the hydrolysis to 2-methylglutaric acid can also be accomplished via the 2-methylglutarimide obtained by heating a 2-methylglutaronitrile/water mixture to 275 °C in the presence of a titanium dioxide catalyst in yields of 94%.


 * 2-MGN zu 2-MGN-Imid.svg

Hydrolysis in the alkaline provides 2-methyl glutaric acid.

The reaction of 2-methylglutarimide with e.g. methanol (methanolysis) produces the diester dimethyl-2-methylglutarate in the presence of titanium dioxide or lanthanum oxide. It was commercialized as an environmentally friendly aprotic dipolar solvent under the name Rhodiasolv IRIS with the typical composition 87-89% dimethyl-2-methylglutarate, 9-11% dimethyl 2-ethylbutanedioate and 1-2% dimethyl hexanedioate as a substitute for acetone, dichloromethane, N-methylpyrrolidone and the like.


 * 2-MGN-Imid zu Diester.svg

The ester mixture is very similar to so-called dibasic esters, which are commercially available as FlexiSolv DBE esters.

The diester can be selectively converted into a mixture of 1- or 5-substituted methyl ester amides with dimethylamine in methanol/sodium methoxide, which is used under the name Rhodiasolv Polarclean as formulation auxiliaries for crop protection preparations. The resulting ester amides are readily biodegradable and good solvents for a variety of different plant protection agents (such as insecticides or fungicides), also compared to the frequently used N-methylpyrrolidone, cyclohexanone or isophorone.

Other esteramides are derived, e. g. from 2-methylglutaronitrile which, after alkaline hydrolysis, is converted into 2-methylglutaric acid, cyclized with acetic anhydride to give 2-methylglutaric anhydride, reacted with dimethylamine to form the monoamide, reacted to an acid chloride with thionyl chloride and formed to an ester with more hydrophobic alcohols (like butanols or cyclohexanol).