A Neutral Ru^II Hydride Complex for the Regio‐ and Chemoselective Reduction of N‐Silylpyridinium Ions

Abstract

A detailed experimental analysis of the 1,4-selective reduction of pyridine with hydrosilanes catalyzed by a coordinatively unsaturated Ru thiolate complex is reported. The previously suggested intermediates, N-silylpyridinium ions and a neutral Ru hydride, have been independently synthesized and do indeed participate in the catalytic cycle. The resting state is not the cationic Ru complex initially used as the catalyst but its pyridine-coordinated congener. All Ru complexes, including the one resulting from hydrosilane activation, are in equilibrium with each other. The N-silylated 1,4-dihydropyridine together with the cationic Ru complex convert back into the corresponding N-silylpyridinium ion and the neutral Ru hydride (retro-hydrosilylation), followed by further backward reaction into the hydrosilane and the pyridine adduct of the cationic complex. These steps prove the overall reversibility of the transformation.

Isolable Silicon‐Based Polycations with Lewis Superacidity

“…[29] Furthermore, the catalytic cycle for the hydrosilylation of pyridines to 1-silyl-4-hydropyridines is supposed to involve hydride transfer to N-silyl-pyridinium cations. [30] We attribute the activation of the terpy-backbone to the lower spatial accessibility of the silicon center in 6 3+ . Finally, the applicability of the silicon cations in the catalytic HDF of AdF with Et 3 SiH was demonstrated with a catalyst loading of 10 mol % ( Table 3).…”

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

“…In contrast, in situ generation of the silylium ion in the presence of the σ-donor (116 → 117 + , Scheme 34, right) circumvents the transient formation of a highly reactive silylium ion intermediate in form of the less electrophilic silylonium ion, thereby providing access to onium ions with heteroleptic silyl groups. 263 This strategy also allows the use of counteranions (e.g., BAr F 4 − , 38,41 [Al(OC(CF 3 ) 3 ) 4 ] − or [FAl(OC(CF 3 ) 3 ) 3 ] − 75,76 ) and solvents (e.g., CH 2 Cl 2 38,41 ) that are decomposed by the "naked" silylium ion. Moreover, the silylated onium ion can be prepared by chloride abstraction from the corresponding chlorosilane using the sodium 38 or silver salt 299 of a WCA.…”

Activation of Si–H and B–H bonds by Lewis acidic transition metals and p-block elements: same, but different

“…On the other hand, quinolines and pyridines have been used as starting materials in metal-catalysed reactions with ruthenium complexes at room temperature. 98,99 However, at variance with B(C 6 F 5 ) 3 , the catalytic reaction stops after the first step (1,4-addition) and no subsequent reactivity was observed leading to formation of C–Si bonds. Nevertheless, pyridines do react in the presence of Oestreich's ruthenium catalyst 19 leading to C3-silylated pyridines under heating (Scheme 45).…”

“…100 A close inspection of the reaction mechanism revealed that generation of the C3-silylated pyridines takes place in a three-step sequence involving a first reduction of pyridine, at room temperature, to the N -silylated 1,4-dihydropyridine. 98,100 The second step requires heating at 80 °C and involves the formal transfer of a silylium cation to the nucleophilic C3 carbon atom of the 1,4-dihydropyridine, followed by a deprotonation process mediated by the ruthenium hydride intermediate. The 1,3-disilylated 1,4-dihydropyridine thus formed undergoes rearomatization, yielding the C3-silylated pyridine.…”