Catalytic activity of dihydride ruthenium complexes in the hydrogenation of nitrogen containing heterocycles

Abstract

The catalytic activity of the dihydride ruthenium complexes, RuH 2 (CO) 2 (P n Bu 3 ) 2 , RuH 2 (CO) 2 (PPh 3 ) 2 and RuH 2 (PPh 3 ) 4 , in the hydrogenation of nitrogen containing heterocycles has been tested by analyzing the influence of reaction parameters such as temperature, hydrogen pressure, catalyst concentration, on the rate and regioselectivity of the reaction.RuH 2 (PPh 3 ) 4 shows a better catalytic activity with an 86.7% conversion of quinoline after 24 h at 100°C under a hydrogen pressure of 25 bar, while RuH 2 (CO) 2 (PPh 3 ) 2 and RuH 2 (CO) 2 (P n Bu 3 ) 2 in the same conditions give a conversion of 37.1% and 35.6%, respectively. These results are confirmed by the reaction rate of the hydrogenation of quinoline, since the K c in the presence of RuH 2 (PPh 3 ) 4 (1.46 · 10 À5 s À1 ) is higher than others (6.37 · 10 À6 s À1 for RuH 2 (CO) 2 (PPh 3 ) 2 and 6.36 · 10 À6 s À1 for RuH 2 (CO) 2 (P n Bu 3 ) 2 ).Noteworthy is the selectivity of these catalytic systems in the hydrogenation of quinoline: in all tests the three catalysts lead to 1,2,3,4-tetrahydroquinoline as the major product, furthermore this compound is the only formed in the presence of RuH 2 (CO) 2 (PPh 3 ) 2 . The selectivity is affected by the presence of an acid (CH 3 COOH) or a base (N n Bu 3 ) in the reaction media.The complex RuH 2 (PPh 3 ) 4 is catalytically active, even if in a minor extent, in the hydrogenation of isoquinoline, pyridine and 2-methylpyridine.The basicity of the substrate and steric hindrance around the nitrogen atom show a great influence on the conversion. The results obtained suggest that the catalytic system activates a heterocyclic ring through the coordination of the heteroatom to the metal centre of the complexes.

Quinoline transfer hydrogenation by a rhodium bipyridine catalyst

“…These three complexes 4-6 are catalytically active in the hydrogenation of quinoline giving results comparable with those previously reported with other ruthenium systems like RuCl 2 (CO) 2 (PPh 3 ) 2 and Ru 4 H 4 (CO) 12 , but operating under milder conditions and with a lower catalyst/substrate ratio [6,7]. The complex 6 is also catalytically active in the hydrogenation of pyridine and methylpyridine although to a lesser extent than quinoline, confirming a strong influence of the bonding mode of nitrogen containing heterocycles to the metal center on the conversion and selectivity of these reactions 1 [5,6,8,9].…”

“…By a perusal of the results reported in this paper with those previously discussed for the hydrogenation of the same nitrogen containing heterocycles using Ru catalysts [6], we may observe that in the presence of H 2 the rhodium complex is more active than ruthenium compounds. The rhodium complex 1 in the presence of propan-2-ol as hydrogen source, gives lower conversions than ruthenium …”

“…Almost the same conversion (34.5%) was reached in the presence of helium (30.5% 1,2,3,4-THQ and 4.0% 5, 6,7, while in the presence of molecular hydrogen a conversion of 81.1% was obtained (73.8% 1,2,3,4-THQ, 1.1% 5,6,7,8-THQ and 6.2% DHQ). This considerable improvement of the conversion in the presence of hydrogen suggests that the catalyst is more efficient in the hydrogenation with molecular hydrogen than in the transfer reduction.…”

“…The hydrogenation of quinoline usually gives 1,2,3,4-tetrahydroquinoline (1,2,3,4-THQ) or 5, 6,7,6,7, in a first step and decahydroquinoline (DHQ) as the final product. The rhodium catalyst 1 was active in this reduction at 353 K with a conversion of 11.7% and a selectivity towards 1,2,3,4-THQ of 96.6%, 5,6,7,8-THQ was the other product.…”

“…Recently, some of us have reported a study on the hydrogenation of quinoline, and other nitrogen containing heterocycles, using the dihydride ruthenium complexes Ru-H 2 (CO) 2 (P n Bu 3 ) 2 (4), RuH 2 (CO) 2 (PPh 3 ) 2 (5) and RuH 2 -(PPh 3 ) 4 (6) [6]. These three complexes 4-6 are catalytically active in the hydrogenation of quinoline giving results comparable with those previously reported with other ruthenium systems like RuCl 2 (CO) 2 (PPh 3 ) 2 and Ru 4 H 4 (CO) 12 , but operating under milder conditions and with a lower catalyst/substrate ratio [6,7].…”