Inhibiting amidine formation during hydrogenation of organo nitriles

This invention relates to a process for reducing amidine formation during reduction of organonitriles. The process comprises including a boron compound in the reaction medium in sufficient amount to complex the amidine compound as it is formed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improvement in a process for hydrogenating 
activated organonitriles to form the corresponding amines. 
2. Description of the Prior Art 
The hydrogenation of nitriles to form the corresponding amine is well-known 
in the prior art. Generally, the hydrogenation is carried out at 
temperatures of from 50.degree. to 200.degree. C., at pressures of from 
atmospheric to 3000 psig and in the presence of a hydrogenation catalyst, 
e.g. Raney nickel or other metal. Activated organonitriles, i.e. those in 
which the nitrile group has been activated by another group, generally an 
electron withdrawing group alpha or beta positioned to the nitrile group, 
have been difficult to reduce. One of the problems in the hydrogenation of 
these activated organonitriles is that hydrogenation results in the 
formation of compounds having an amidine linkage. Amidines not only are 
difficult to reduce to an amine structure, but their presence can 
influence the hydrogenation reaction and create problems downstream from 
the reaction, particularly in the separation. 
Representative articles relating to the hydrogenation of nitriles include: 
U.S. Pat. No. 3,369,002 discloses a process for producing compounds having 
amidine linkages therein and are produced in equilibrium by the reaction 
of a nitrile and ammonia. 
U.S. Pat. No. 2,049,582 discloses a process for producing amidines by the 
reaction of a nitrile and an alkali amide. In the discussion of the prior 
art, the patentee noted that it was difficult to produce amidines in large 
quantities via the ammonolysis or ammonation of nitriles. 
U.S. Pat. No. 2,252,723 discloses a process for producing halogen-alkyl 
amidines by reacting a haloamidoether with an amine or ammonia. 
U.S. Pat. No. 3,733,325 discloses a process for producing 
aminoethylpiperizine by the catalytic hydrogenation of 
nitrilotriacetonitrile. Hydrogenation catalysts consisting of nickel or 
cobalt and optionally containing chromium and copper components are well 
suited for effecting hydrogenation at temperatures of 
75.degree.-200.degree. C. and pressures of 1,000-3,000 psig. 
U.S. Pat. No. 3,117,162 discloses a process for producing amines, e.g., 
propylamine, by the reduction of aliphatic nitriles, the reduction being 
in the presence of hydrogenation catalysts. Rhodium and platinum on carbon 
are suggested as hydrogenation catalysts with the hydrogenation being 
conducted in the liquid phase. 
SUMMARY OF THE INVENTION 
This invention relates to an improvement in a process for forming amines 
via the catalytic hydrogenation of an activated organo nitrile having from 
2 to 16 carbon atoms, the nitrile being sufficiently active for forming 
by-product compounds having amidine linkages therein during the 
hydrogenation reaction. The improvement for reducing or inhibiting the 
formation of compounds having amidine linkages resides in complexing the 
compounds having the amidine linkages as they are formed with a boron 
compound during the hydrogenation reaction. In this regard, an effective 
proportion of a boron compound capable of complexing the amidine is added 
to the reaction medium prior to hydrogenation. 
There are several advantages associated with this process, and these 
advantages include: 
an ability to effect hydrogenation of various activated organonitriles, 
which normally result in the production of by-product compounds having 
amidine linkages therein, to produce amines in good yield; and 
an ability to reduce the formation of compounds having amidine linkages 
therein and thereby reduce problems in separating the amine product from 
by-product. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Amidine formation can be a problem where an amine or ammonia is in contact 
with an activated nitrile, the amidine linkage being represented by the 
formula: HN.dbd.C--NH Compounds having the amidine linkage are extremely 
rare as a by-product in those processes where the nitrile is relatively 
inactive, e.g. propionitrile. On the other hand, as the nitrile group 
becomes activated, e.g. via the withdrawal of electrons from the nitrile 
group, the reactant nitrile can react with the product amine to produce 
the amidine linkage in substantial amounts. This can be shown by the 
reaction of methoxyethylamine and methoxyacetonitrile to yield an amidine 
of the formula: 
##STR1## 
Activated organonitriles suited for the present process are those activated 
aliphatic or cyclic having from 3-16 carbon atoms in the structure. The 
nitriles typically are represented by the formula: 
EQU R--(X)--C--CN 
where X is alpha or beta positioned to the nitrile group. Activation of the 
nitrile occurs by the presence of an electron withdrawing group on the 
alpha or beta carbon atom. X in the formula, for example, can represent an 
ether oxygen, an amine group, a nitro group, a nitrile group, a halogen 
atom, a sulfone group, a sulfamide group, a sulfur atom in the form of a 
sulfido group, a sulfoxide group, a O.dbd.C--O-- group, an amide group, 
and a ketone carbonyl group 
##STR2## 
The remaining R can be virtually any carbon structure either linear or 
cyclic. specific examples of suited compounds include methoxyacetonitrile, 
methoxypropionitrile, iminodiacetonitrile, chloropropionitrile, 
aminoacetonitrile, N,N-dimethylaminoacetonitrile, nitrotriacetonitrile, 
bis(cyanomethyl)ether, and malonitrile. 
Catalytic hydrogenation of the nitrile is carried out in conventional 
manner, e.g. utilizing temperatures of about 50.degree.-200.degree. C. and 
pressures from about atmospheric to 3,000 psig. Such conditions permit the 
reaction to be carried out in the vapor or liquid phase. 
Often the hydrogenation of the nitrile to form the amine is carried out in 
the presence of a solvent preferably one that is inert to the reaction 
medium and one that does not poison the catalyst. Suitable solvents 
include benzene, acetic acid, butyl alcohol, hexand and the like and they 
may be present in various proportions, e.g. from about 1 to 99% by weight. 
Hydrogenation catalysts used for the catalytic reduction of the activated 
organonitriles can be any of those used in the art. Generally, these are 
metals, particularly the Group 6 or Group 8 metals and these catalysts 
generally include rhodium, palladium, ruthenium, nickel, cobalt, platinum, 
chromium and copper as a component. The metals generally are present upon 
a support such as carbon, alumina, alumina-silica, silica, kiesulguhr, 
calcium carbonate, barium sulfate, bentonite, and the like. The active 
metal generally is present in a proportion of from about 0.1 to 60% by 
weight, and generally 1 to 20%. 
The key to the invention is the complexing of the compound having amidine 
linkages therein early in the catalytic hydrogenation or reduction of the 
activated nitrile compound. Amidines are self-catalyzing and once formed 
tend to catalyze their formation, the amidine being formed by the reaction 
between the activated nitrile and amine product. By complexing the inital 
quantities of compound having amidine linkage therein, the effect of the 
amidine in self-catalyzing its own formation is minimized and therefore 
the presence of by-products having the amidine linkage is greatly reduced. 
In other words, by complexing a sufficient amount of the initial quanitity 
of amidine formed, the reduction of the nitrile to the amine can proceed 
rapidly with little by-product formation. 
Complexing of the by-product having the amidine linkage therein is 
accomplished via the incorporation of boron compound into the reduction 
reaction. Examples of boron compounds include boron salts of carboxylic 
acids having from 1-10 carbon atoms, e.g. boron acetate, boron propionate, 
boron butyrate; boron phosphates, e.g. boron triphosphate; alkyl borates, 
preferably C.sub.1-3 alkyl borates, e.g. tri-n-propyl borate, trimethyl 
borate, triethyl borate; boric acid and salts of these acids, e.g. boron 
oxide, boron halide, e.g. boron trifluoride and boron trifluoride and 
amino boron compounds, e.g. triethanolamine borate. 
The boron compound is included in the reduction reaction in an amount to 
provide at least an effective proportion of boron atom to complex the 
initial by-products containing the amidine linkages. An effective 
proportion is that portion where the amount of by-product containing 
amidine linkage is reduced as compared to a system where no complexing 
agent is used. Surprisingly then, a stoichiometric quantity of complexing 
agent based on orgnano nitrile or amidine capable of being formed is not 
required in the process. Because the rate of reduction of the nitrile to 
the amine is relatively rapid, and the initial catalytic quantities of 
amidine are complexed, there is insufficient time for the compounds having 
amidine linkages to be formed in substantial quantities. Typically, the 
boron proportions used for complexing the by-products containing amidine 
linkages are provided at levels from about 0.0005 to 1 gram atom boron per 
gram mole of activated nitrile reactant. Preferred ranges are from about 
0.01 to 0.5 g atoms/g mole. Quantities of boron compound which provide 
more than about 1 weight part boron atom per mole nitrile do not provide 
significant advantages, at least in terms of inhibiting the formation of 
compounds having the amidine linkage therein. On the other hand, as the 
quantity of complexing agent is reduced, particularly toward the low end 
of the range specified, larger proportions of compounds having amidine 
linkages may be produced. 
If a liquid phase reduction of the activated organonitrile is used, and 
generally this is preferred the complexing agent should be dispersible in 
the liquid phase. In general, the organoboron compounds should be soluble 
in the reaction medium or the carrier solvent in which the reaction is 
carried out. Complexing of the amidine using a complexing agent which is 
homogeneous with the reaction medium reduced the formation of by-product 
amidine.