Ruthenium-Catalyzed Regioselective 1,4-Hydroboration of Pyridines

Abstract

Simple ruthenium precursor [Ru(p-cymene)Cl2]2 1 catalyzed regioselective 1,4-dearomatization of pyridine derivatives using pinacolborane is reported. Two catalytic intermediates, [Ru(p-cymene)Cl2Py] 2 and [Ru(p-cymene)Cl2(P(Cy)3)] 3, involved in this process are identified, independently synthesized, characterized, and further used directly as effective catalysts; two more catalytic intermediates [Ru(p-cymene)Cl2(Py)(P(Cy)3)] 4 and [Ru(p-cymene)(H)Cl(Py)(P(Cy)3)] 5 are identified in solution. Complex 5 is the active catalytic intermediate. An intramolecular selective 1,5-hydride transfer in 5 leading to the regioselective 1,4-hydroboration of pyridine compounds is proposed.

Regioselective 1,4-hydroboration of pyridines catalyzed by an acid-initiated boronium cation

“…Note and references † 1 H and 11 B NMR spectroscopy of the reaction mixture containing only HBpin and NH 4 BPh 4 in acetonitrile show decomposition of the system at 90 °C over a 24 hour period, giving moderate amounts of C 6 H 6 , NH 3 , and possibly B 2 pin 3 . Decomposition is not observed in the presence of pyridine.…”

“…Decomposition is not observed in the presence of pyridine. ‡ On a few occasions minor amounts of pyridine-BH 3 adduct is observed by 11…”

“…Additionally, the groups of Okuda and Gunanathan have used Mg(II) hydridotriphenylborate and Ru(II) catalysts, respectively, for the selective 1,4hydroboration of pyridine. 10,11 Here, we present an efficient and practical metal-free catalytic method for the highly regioselective 1,4hydroboration of pyridines using only pyridines, pinacolborane, and low loadings of an inexpensive and commercially available ammonium salt initiator (NH 4 BPh 4 ). This method relies on the formation of a boronium cation through the reaction between pyridine-activated HBpin and NH 4 BPh 4 , releasing H 2 and NH 3 .…”

“…In this mechanism, coordination of pyridine to HBpin in acetonitrile increases the hydridicity of the borane, allowing it to react with NH 4 + , generating H 2 and NH 3 as well as the pyridine-stabilized boronium-BPh 4 salt (Figure 1, initiation). Both H 2 and the boronium cation are observed by 1 H and 11 B NMR, respectively (the boronium species exhibits a characteristic resonance at δ = 7.1 ppm in the 11 B NMR spectrum). 9 The formation of the boronium cation and H 2 does not occur in the absence of pyridine, suggesting that pyridine donation is crucial for the activation of the B-H bond (Supporting information, Figures S4 and S5).…”

Catalytic Dearomatization of N‐Heteroarenes with Silicon and Boron Compounds

“…Recently,G unanathan and co-workers developed ac atalytic procedure for the 1,4-selective hydroboration of pyridines using [Ru(p-cymene)Cl 2 ] 2 15 as ac atalyst (Scheme 36). [50] This study represents the first example of at ransition-metal-catalyzed 1,4-hydroboration of pyridines. Areaction of 15 with 2equiv of PCy 3 at 25 8 8Cled to the clean formation of the phosphine adduct Ru(p-cymene)Cl 2 (PCy 3 ) (15 a).…”