Dehydrogenative Coupling of 4-Substituted Pyridines Catalyzed by a Trinuclear Complex of Ruthenium and Cobalt

Abstract

The dehydrogenative coupling of 4-substituted pyridines catalyzed by a heterometallic trinuclear complex composed of Ru and Co, (Cp*Ru)2(Cp*Co)­(μ-H)3(μ3-H) (1, Cp* = η5-C5Me5), was investigated. When the pyridine substrate contains an electron-donating group at the 4-position, complex 1 showed a high catalytic activity compared to di- and triruthenium complexes (Cp*Ru)2(μ-H)4 (4) and (Cp*Ru)3(μ-H)3(μ3-H)2 (5). The catalytic activity of 1 was also remarkably higher than the congeners of other group 9 metals, Ru2Rh (2) and Ru2Ir analogues (3). The distinctive reactivity of 1 was attributed to a paramagnetic intermediate, (Cp*Ru)2{(dmbpy)­Co}­(μ-H)­(μ3-H)2 (12, dmbpy = 4,4′-dimethyl-2,2′-bipyridine), which was formed by the reaction of 1 with 4-picoline accompanied by the dissociation of the Cp* at the Co atom. The reaction of 12 with unsubstituted pyridine resulted in the elimination of 4,4′-dimethyl-2,2′-bipyridine, indicating that the Co atom in 12 acts as a dissociation site. In contrast to the reaction of 1 with 4-picoline, the reaction of 2 and 3 with 4-picoline afforded the corresponding μ3-pyridyl complexes (Cp*Ru)2(Cp*M)­(μ-H)3­(μ3-η2(||)-C5H3NCH3) (15, M = Rh; 16, M = Ir). 4-(Trifluoromethyl)­pyridine was not dimerized by 1; however, a similar μ3-pyridyl complex, (Cp*Ru)2(Cp*Co)­(μ-H)3­(μ3-η2(||)-C5H3NCF3) (13), was obtained. The stability of the μ3-pyridyl complex is probably one of the reasons for the low catalytic activity of 2 and 3 in the coupling reaction.

Homogeneous Catalysis by Organometallic Polynuclear Clusters

“…The conversion difference of approximately 20% (in toluene) is also not addressed in detail, [19]. [160]. Compared to the all ruthenium cluster 45, a significant increase in TOF (3h) (from 0.1 h -1 to 0.3 h -1 ) was achieved by substituting one Ru-atom for Co. Interestingly, poor to no reactivity was observed for 128Rh and 128Ir, respectively.…”

Geometric and electronic effects on the performance of a bifunctional Ru2P catalyst in the hydrogenation and acceptorless dehydrogenation of N-heteroarenes

“…In particular, 2,2'-biquinoxaline was formed upon hydrogenation of quinoxaline in the presence of Ru2P/AC, except for the product 1,2,3,4-tetrahydro-quinoxline (Table 3). This out- come may be attributed to the dehydrogenative dimerization of quinoxaline, which is the most straightforward and attractive route [49][50][51][52][53]. The tetrahydro-quinoxaline product (4a) was obtained in 9.1% yield under 10 bar H2 atmosphere at 60 °C using the Ru2P/AC catalyst, while 5a was detected in 6.1% yield.…”

Recent Advances in the Synthesis of 2,2′-Bipyridines and Their Derivatives

“…However, at the moment, this methodology has only started to emerge as an alternative to the existing cross-coupling methods employing prefunctionalized compounds. Several attempts at the homocoupling of 4-alkylpyridines were reported that used palladium on charcoal (10 mol%) in the presence of MnO 2 (1 equiv) 32 or trinuclear Ru-Co clusters as catalysts, 33 though the scope was narrow and the reactions required 4 days or longer to achieve practical yields.…”