Amination of Di- and Polyhydroxy Compounds to Acyclic Amines

There are few reports of successful one-step synthesis of primary diamines, and the examples are limited to amines with a special structure. Amination of 1,4-cy-clohexanediol in supercritical ammonia (135 bar) over a Co-Fe catalyst alforded 67 % 1,4-diaminocyclohexane [21]. Excess ammonia, as both supercritical solvent and reactant, and short contact time in the continuous fixed-bed reactor favored the desired reactions. In the best example the cumulative selectivity for the diamine and the intermediate amino alcohol was 97 % at 76 % conversion. Recycling of the unreacted diol and amino alcohol can provide an alternative to the eurrent process, the hydrogenation of pnra-phenylenediamine. The high seleetivity was because of the rigid structure and the relative positions of OH functionality in the substrate. For comparison, amination of 1,4-butanediol under similar conditions yielded pyiTolidine as the major product 1,4-diaminobutane was barely detectable. When 1,3-cyclohexanediol was aminated with the same catalyst in the continuous system, the yield of 1,3-diaminoeyclohexane dropped below 5%, mainly because elimination of water led to undesired monofunctional products via a,/9-unsaturated alcohol, ketone, and/or amine intermediates [22]. 

The importance of these side-reactions was corroborated by results from the Ni-catalyzed amination of 1,3-propanediols differently substituted at the C2-position 

Conversion of dihydroxy compounds to diamines requires the repetition of all reaction steps (dehydrogenation, addition, elimination, hydrogenation). Selectivity is much higher when diols are transferred only to amino alcohols or amino alcohols to diamines. This difference is exemplified by the reaction of 1,6-hexanediol with di-methylamine over CU/AI2O3 [25]. Over 90% selectivity for the intermediate N,N-dimethyl-6-amino-l-hexanol was achieved at 180 °C in a continuous fixed-bed reactor. To complete the amination of the second OH group the reactor temperature had to be raised to 230 °C and the highest selectivity for diamine was only 65 %. 

Transformation of different polyoxyalkylene diols and triols with ammonia to the corresponding primary amines is of great practical importance. The results have been disclosed almost exclusively in the patent literature a summary can be found in a previous review [26], The catalyst of choice is Raney Ni or supported Ni, modified with selectivity and stability promoters such as Fe, Co, Cr, or Mo. The use of a non-porous or mesoporous support is essential to enable access of the bulky reactants to the active sites. Scientific evaluation of these processes is, unfortunately, difficult on the basis of the information available. At best, the conversion is estimated from the amount of water formed and the ratio of primary to secondary amines is determined. On the basis of these data the yields of primary amines are excellent, reaching 85-90%. It seems that selectivity is barely affected by the conversion, in contrast with general observation in the amination of short-chain alcohols and diols [3,4,14]. 

Reaction of dihydroxyacetone with ammonia and hydrogen is a good illustration of the different reactivities of hydroxyl and carbonyl groups during amination over metal catalysts. When the reaction is performed at a relatively mild temperature ( 100 °C, 100 bar) in liquid ammonia, Raney Ni affords 2-amino-1,3-propanediol in 99 % yield [28]. Under these conditions activation of hydroxyl groups is negligible. Similarly, the carbonyl group of an aldose or ketose reacts with ammo- 

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