Reactivity of the Bis(dihydrogen) Complex [RuH 2 ( η 2 -H 2 ) 2 (PCy 3 ) 2 ] toward N-Heteroaromatic Compounds. Regioselective Hydrogenation of Acridine to 1,2,3,4,5,6,7,8-Octahydroacridine

The reaction of pyridine (Py), pyrrole (Pyr), or acridine with the bis(dihydrogen) complex [RuH 2 ( η 2 -H 2 ) 2 (PCy 3 ) 2 ] ( 1 ) produces compounds containing heteroaromatic ([RuH 2 ( η 2 -H 2 )( η 1 ( N )-C 5 H 5 N)(PCy 3 ) 2 ] ( 2 ), [RuH( η 5 -C 4 H 4 N)(PCy 3 ) 2 ]·Pyr ( 3 )) or aromatic rings ([RuH 2 ( η 4 -C 13 H 9 N)(PCy 3 ) 2 ] ( 5 )) coordinated in η 1 (N) ( 2 ), η 5 (N,C) ( 3 ), or η 4 (C,C) ( 5 ) modes for Py, Pyr, and acridine, respectively. Complex 3 has been characterized by X-ray crystallography. Its protonation by HBF 4 affords the cationic dihydride complex [RuH 2 ( η 5 -C 4 H 4 N)(PCy 3 ) 2 ][BF 4 ] ( 4 ). The coordinated Py ligand in 2 and acridine in 5 can readily be displaced by dihydrogen, with regeneration of 1 . Regioselective hydrogenation of representative polynuclear heteroaromatic nitrogen compounds is achieved in the presence of 1 under mild reaction conditions (80 °C, 3 bar of H 2 ). Quinoline (Q) and isoquinoline (iQ) are hydrogenated to 5,6,7,8-tetrahydro derivatives, while acridine is quickly reduced to 1,2,3,4-tetrahydroacridine followed by much slower saturation to 1,2,3,4,5,6,7,8-octahydroacridine (8H-Acr), the nitrogen-containing aromatic ring remaining intact. 8H-Acr has been isolated in analytically pure form and characterized by 1 H and 13 C NMR as well as by X-ray crystallography. 5 is also active for catalytic acridine hydrogenation and can be regarded as an intermediate in the catalytic cycle. Saturation of a five-membered indole ring proceeds much slower than hydrogenation of six-membered aromatic rings in Q and iQ. Pyridine, pyrrole, and 7,8-benzoquinoline are not hydrogenated under the applied reaction conditions, as a result of the formation of new stable complexes.