2,3 -Biquinoline oxidations

Oppenauer-type oxidation of secondary alcohols can be a convenient procedure for obtaining the corresponding carbonyl compounds. It was found recently [19], that Ir(I)- and Rh(I)-complexes of 2,2 -biquinoline-4,4 -dicarboxylic acid dipotassium salt (BQC) efficiently catalyze the oxidation of secondary alcohols with acetone in water/acetone 2/1 mixtures (Scheme 8.5). The reaction proceeds in the presence of Na2C03 and affords medium to excellent yields of the isolated ketones. The process is much faster in largely aqueous solutions, such as above, than in wet organic solvents in acetone, containing only 0.5 % water, low yields were observed (15 % vs. 76 % in case of cyclohexanol). 

Water-soluble catalysts for Oppenauer-type oxidation of alcohols can be achieved by adding functionalized salts of classical ligands such as dipotassium 2,2 -biquinoline-4,4 -dicarboxylate (BQC) to acetone-water mixtures. In this way, the catalyst system [ Ir(ix-Cl)(cod) 2]/BQC is highly efficient for the selective oxidation of a wide range of alcohols such as benzylic. 

The electrochemical properties of another series of Cu(I) complexes, based on substituted bipyridine and quinoline derivatives, have been also investigated68 (Fig. 17.31). To stabilize the Cu(I) oxidation state of Cu(I) polypyridine complexes, electron-withdrawing substitutents like esters have been considered. The same effect was also obtained with pyridyl-quinoline and biquinoline complexes, thanks to the increased 7i-accepting properties of the quinoline condensed aromatic ring. 

Pyridine-type N-oxides (Fig. 7.2) represent another, no less-successful class of catalysts for the allylation reaction. Thus, Nakajima first demonstrated that the axially chiral biquinoline N,N -bisoxide 17 can indeed catalyze the allylation... 

Annealing two benzene rings to the bpy framework leads either to biquinoline or isomeric Ao-biquinoline. Their respective complexes differ significantly in their redox behavior [74, 153, 227, 228] The first reduction of [Ru(biq)3] + is by 0.77 V more positive than that of [Ru(i-biq)3] +, in accordance with the trend in the reduction potentials of the free ligands. The difference between the oxidation and reduction potential for [Ru(biq)3] + is unusually small, 2.20 V, relative to [Ru(i-biq)3] +, 2.63 V, and most other Ru-polypyridines. 

It has recently been discovered that2-(l-methylindol-3-yl)quinoline is formed in a good yield on reacting quinoline with Al-methyUndole in the presence of hydrogen chloride in dioxane. Two equivalents of quinoline substrate (quinolinium hydrochloride + quinoline) are necessary for this reaction, since one equivalent of quinoline (or the NH-protonated quinolinium salt) acts as oxidant to abstract hydrogen atoms from the intermediate o -adduct, thus giving the Sn product and a mixture of 1,2,3,4-tetrahydroquinoline and l, 2, 3, 4 -tetrahydro-2,6 -biquinoline (Scheme 30) [118]. 

In the reaction of quinoline W-oxide with 1-phenylbenzimidazole, 2,2 -biquinoline derivative was isolated as a side product. In order to examine the reaction pathway, quinoline W-oxide was allowed to react without 1-phenylbenzimidazole. Quinoline A-oxide was treated with TMAF and dimethylaminosilane at room temperature, and 2,2-biquinoline W-oxide was obtained in 40% yield. On the other hand, the same reaction at 120 °C gave deoxygenated 2,2-biquinoline, which was obtained in 70%... 

Enantioselective Michael additions of various esters of 1-oxoindan-2-carboxylic acid to methyl vinyl ketone and acrolein were investigated employing the first example of a combination of Sc(OTf)3 and a chiral biquinoline N,N -oxide [122]. 

---