1,4‐Hydrosilylation of Pyridine by Ruthenium Catalyst: A New Reaction and Mechanism

Abstract

homogeneous catalysis · hydrosilylation · pyridine · rutheniumThe hydrosilylation of unsaturated molecules is recognized as the most common means of introducing a silicon-containing functionality to organic substrates. Complexes of various transition metals are employed as homogeneous catalysts for the hydrosilylation of C=C and C=O bonds.[1] Reports on the hydrosilylation of C=N bonds, however, were even much less common than those on the reaction of C=O bonds until recently. Over the past 15 years, complexes of various metals, including early-, late-, and post-transition metals (including Ti, [2] Re, [3] Ru, [4] Rh, [5] Ir, [6] Cu, [7] and Zn [8] ), have been reported to catalyze the addition of SiÀH groups of primary or secondary organosilanes or of polymethylhydrosiloxanes (PMHS) to imines. Since no hydrogenation of imines, giving the corresponding amines, takes place under mild conditions, the hydrosilylation of prochiral imines and the subsequent hydrogenolysis of the N À Si bond formed provide an alternative for the conversion of prochiral imines into optically active amines.Nikonov et al. of Brock University chose a cationic ruthenium complex, [Cp(iPr 3 P)Ru(NCMe) 2 ][B(C 6 F 5 ) 4 ] (1; Cp = cyclopentadienyl), as the catalyst for the hydrosilylation of ketones and nitriles in their recent studies.[9] This catalyst converts aliphatic and aromatic nitriles into the corresponding N-silylimines with high chemoselectivity or into N,Ndisilylamines formed by double hydrosilylation, depending on the reaction conditions. This Highlights describes the latest report from this research group on catalysis by the complex in the hydrosilylation of pyridine. [10] Pyridine and its derivatives having chloro and methyl substituents at 3-and 5-positions undergo hydrosilylation with HSiMe 2 Ph in the presence of catalytic amounts of 1 (2-5 mol % of the substrate) to yield the N-silyl-4-hydropyridine derivatives, as shown in Equation (1). The reaction is complete within several hours at room temperature.[Cp 2 TiMe 2 ] catalyzes the hydrosilylation of pyridine, which is the sole precedent using a homogeneous catalyst. [11] The reaction, however, requires higher amounts of the titanium catalyst (10 mol % of the substrate) and heating the reaction mixture at 80 8C. The 1,2-addition of the Si À H group in the hydrosilylation was confirmed by deuteriumlabeling experiments. The actual products are tetrahydropyridine derivatives which result from the concurrent hydrogenation of the pyridine ring. Thus, hydrosilylation using the half-sandwich ruthenium complex as the catalyst differs in chemoselectivity from the reaction catalyzed by the titanium complex and proceeds under milder conditions.The experimental results in this recent article may encourage creative applications by the readers who are interested in the reaction mechanism. Scheme 1 summarizes the reactions of N-silyl-4-hydropyridine (2) catalyzed by the complex 1. PhCN reacts with 2 in the presence of the catalyst 1 leading to the formation of N-silylphenylimine and the re...

Calcium Hydride Catalyzed Highly 1,2‐Selective Pyridine Hydrosilylation

“…A more challenging alternative application is the hydrosilylation of pyridines 8–11. 14 A tentative general catalytic cycle, based on earlier reported investigations with a titanium hydride catalyst,11 is shown in Scheme .…”

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…For example, ruthenium η 2 -H-SiR3 complexes catalyze challenging pyridine and nitrile hydrosilations in good yield and with high selectivity for a single product. [75,78] Even more interesting pyridine transformations were achieved with an electrophilic ruthenium complex possessing a cooperative basic site (a thiolate ligand). This latter complex catalyzes pyridine C-H silylations by a mechanism involving ionic hydrosilation/retrohydrosilation processes in addition to the C-H functionalization.…”

Elektrophile Aktivierung von Silicium‐Wasserstoff‐ Bindungen in katalytischen Hydrosilierungen

“…Weitere Arbeiten von Nikonov und Mitarbeitern bewiesen die Nützlichkeit von 33 in der anspruchsvollen 1,4‐Hydrosilierung von Pyridinen . Die Autoren verwendeten anschließend einen ähnlichen Katalysator für Hydrosilierungen von Phenanthrolin, Chinolin, Acridin und 1,3,5‐Triazin .…”

“…Darüber hinaus wurde für einige elektrophile Katalysatoren berichtet, dass sie Reaktivitäten und/oder Selektivitäten herbeiführen, die nicht mit anderen Katalysatoren erzielt werden können. Beispielsweise katalysieren Ruthenium‐η 2 ‐H‐SiR 3 ‐Komplexe anspruchsvolle Pyridin‐ und Nitril‐Hydrosilierungen in guter Ausbeute und mit hoher Selektivität für ein einzelnes Produkt . Sogar noch interessantere Pyridin‐Umwandlungen wurden mit einem elektrophilen Rutheniumkomplex mit einer kooperierenden Basenfunktion (einem Thiolat‐Liganden) erreicht.…”