Abstract
Reaction of 8-amino-5,6,7,8-tetrahydroquinoline with RuCl2(PPh3)3 at room temperature affords the ruthenium(ii) chelate (8-NH2-C9H10N)RuCl2(PPh3)2 (E), in which the two triphenylphosphine ligands are disposed mutually cis. By contrast, when the reaction is performed at reflux ligand oxidation/dehydrogenation occurs along with cis-trans reorganization of the triphenylphosphines to form the 8-imino-5,6,7-trihydroquinoline-ruthenium(ii) complex, (8-NH-C9H9N)RuCl2(PPh3)2 (F). Complex F can also be obtained in higher yield by heating a solution of E alone to reflux. Comparison of their molecular structures highlights the superior binding properties of the bidentate imine ligand in F over its amine-containing counterpart in E. Both complexes are highly effective in the transfer hydrogenation of a wide range of alkyl-, aryl- and cycloalkyl-containing ketones affording their corresponding secondary alcohols with loadings of as low as 0.1 mol%. Significantly, F can deliver excellent conversions even in bench quality 2-propanol in reaction vessels open to the air, whereas the catalytic efficiency of E is diminished by the presence of air but only operates efficiently under inert conditions.
Highlights
Besides the significant breakthrough made by Noyori,[3] some recent developments in metal-mediated transfer hydrogenation[4,5,6,7] have been achieved through ligand modification of Ru-1b,4i-n,6 and Oscatalysts,1b,4b,c,5e and in particular those based on phosphine and amino-containing alkylpyridines by the groups of Baratta,[4] Yu6 and Mezzetti[7]
In the first instance we focused on synthesizing (8-NH2C9H10N)RuCl2(PPh3)[2] (E) by reacting 8-amino-5,6,7,8tetrahydroquinoline[9] with RuCl2(PPh3)3.10 Pleasingly, with dichloromethane as the solvent and under ambient conditions, E was isolated in 85% yield after one hour (Scheme 1)
To explore the potential of the 8-amine-containing E and imine F to serve as catalysts for the transfer hydrogenation of ketones, E was used in the first instance to allow an optimization of the conditions (Table 1)
Summary
The hydrogenation of unsaturated carbonyl and imine substrates has attracted considerable attention with regard to developing more sustainable, efficient and environmentally friendly processes.[1,2] Besides the significant breakthrough made by Noyori,[3] some recent developments in metal-mediated transfer hydrogenation[4,5,6,7] have been achieved through ligand modification of Ru-1b,4i-n,6 and Oscatalysts,1b,4b,c,5e and in particular those based on phosphine and amino-containing alkylpyridines by the groups of Baratta,[4] Yu6 and Mezzetti[7] (see A – D in Chart 1). To explore the potential of the 8-amine-containing E and imine F to serve as catalysts for the transfer hydrogenation of ketones, E was used in the first instance to allow an optimization of the conditions (Table 1). It was found that the combination of RuCl2(PPh3)[2] and 8-amino-5,6,7,8tetrahydroquinoline as catalyst exhibited only a modest conversion (44%, entry 10, Table 1).
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