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Chemo- and Regioselective Catalytic Reduction of N‑Heterocycles by Silane

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journal contribution
posted on 26.08.2013, 00:00 by Sun-Hwa Lee, Dmitry V. Gutsulyak, Georgii I. Nikonov
The ruthenium complex [Cp­(iPr3P)­Ru­(NCCH3)2]+ (1) catalyzes the regioselective hydrosilylation of pyridines to 1,4-dihydropyridines. Substitution in the 3- and 5-positions is tolerated, whereas pyridines with substituents in the 2-, 4-, and 6-positions are not reduced. Reduction of functionalized pyridines having keto and ester substituents results in a mixture of products. N-Silyl-1,4-dihydropyridine reacts with ketones and aldehydes to give products of N–Si addition across the CO bond. Hydrosilylation of pyridine in acetone results quantitatively in the addition product PhMe2SiO–CMe2–NC5H6, which decomposes in hexane to give the parent dihydropyridine HNC5H6. The phenanthroline complex [Cp­(phen)­Ru­(NCCH3)2]+ (10) catalyzes regioselective 1,4-reduction of phenanthroline by a 3–4-fold excess of silane/water or silane/alcohol mixtures. The Cp* analogue [Cp*­(phen)­Ru­(NCCH3)2]+ (9) catalyzes 1,4-regioselective monohydrosilylation of phenanthroline, quinoline, acridine, and 1,3,5-triazine and the 1,2-reduction of isoquinoline. In contrast, 2-substituted phenanthroline, pyrazine, 2-ethylpyridine, 2,6-lutidine, 2,4-lutidine, and pyrimidine are not reduced under these conditions by either of the catalysts studied.