Hydrogenation of polyenes

Hydrogenation of a polyene to a monoene using a ruthenium catalyst that is promoted with a sterically-hindered tertiary amine.

FIELD OF THE INVENTION 
This invention relates to the hydrogenation of a polyene to produce a 
monoene using a ruthenium catalyst that is promoted by a 
sterically-hindered tertiary amine or a sterically-hindered pyridine. 
BACKGROUND OF THE INVENTION 
The hydrogenation of polyenes to produce monoenes using ruthenium complex 
catalysts is disclosed in Fahey's U.S. Pat. No. 3,925,494. Fahey's U.S. 
Pat. No. 3,804,914, in Example II shows hydrogenation of polyenes using a 
ruthenium complex catalyst and excess triphenylphosphine. Fahey discloses 
in Journal of Organic Chemistry, Vol. 38, No. 1, 1973, pages 80-87 that 
Lewis Bases, including diethylamine were tested in the hydrogenation of 
cyclododecatriene. No bases were found to be more effective at enhancing 
selectivity than triphenylphosphine. No other effects were noted. 
The rate of hydrogenation of a particular polyene using a ruthenium 
catalyst is taught to be increased by adding certain specific amines in an 
article titled "Selective Homogeneous Hydrogenation of 3-Oxo-1,4-diene 
Steroids. II. Effects of Basic Additives and para Substituents on the 
Hydrogenation with Dichlorotris(triphenylphosphine) ruthenium", by 
Nishimura et al, appearing in Bulletin of the Chemical Society of Japan, 
Vol. 46, 279-283 (1973). 
The process disclosed by the Fahey patents suffers from the difficulty that 
there is an induction period of considerable length between the time the 
ingredients of the reaction mixture are combined under reaction 
conditions, and the time that the mixture actually begins to react at a 
significant rate. 
An object of the present invention is to shorten this induction period. 
SUMMARY OF THE INVENTION 
The present invention is a process for the hydrogenation of a polyene to a 
monoene which comprises forming a reaction mixture containing the polyene, 
hydrogen, a ruthenium-ligand complex hydrogenation catalyst, free 
triarylphosphine, and a sterically-hindered tertiary amine of the formula 
NR.sub.3 where R is alkyl or aryl or a sterically-hindered pyridine having 
an alkyl or aryl substitutent in the 2 and/or 6 position, and reacting the 
mixture under hydrogenation conditions. The presence of the 
sterically-hindered tertiary amine or pyridine promotes the reaction, 
i.e., it reduces the induction period. Preferred sterically-hindered 
tertiary amines have cone angles of greater than 200 degrees. The term 
"cone angle" is used in accordance with the Seligson et al. article, J. 
Am. Chem. Soc., 1991, Vol. 113, No. 7, page 2520. Cone angle describes the 
amount of space taken up by a particular amine ligand on the metal i.e., 
ruthenium atom.