Ruthenium(II)-Catalyzed Regioselective 1,2-Hydrosilylation of N-Heteroarenes and Tetrel Bonding Mechanism

Abstract

An efficient regioselective dearomatization of N-heteroarenes using a ruthenium precatalyst [Ru-(p-cymene)(PCy 3 )Cl 2 ] 1 is achieved. Reactions were performed under mild and neat conditions. A wide variety of N-heteroarenes undergo the addition of silanes in the presence of precatalyst 1, leading to exclusive N-silyl-1,2-dihydroheteroarene products. This catalytic method displays a broad substrate scope; quinolines, isoquinolines, benzimidazoles, quinoxalines, pyrazines, pyrimidines, and pyridines undergo highly selective 1,2-dearomatization. Both electron-donating and electronwithdrawing substituents on N-heteroaromatics are well tolerated in this protocol. Mechanistic studies indicate the presence of [Ru-(p-cymene) (PCy 3 )HCl] 4 in the reaction mixture, which may be the resting state of the catalyst. The complete catalytic cycle as revealed from density functional theory (DFT) studies show that the product formation is governed by N → Si tetrel bonding. Initially, PCy 3 dissociates from 1, and further reaction of [(p-cymene)RuCl 2 ] 20 with silane generates the catalytically active intermediate [(p-cymene)RuHCl] 7. Heteroarene coordinates with 7, and subsequent dearomative 1,3-hydride transfer to the C2 position of the heteroaryl ligand generates an amide-ligated intermediate in which the reaction of silane occurs through a tetrel bonding and provides a selective pathway for 1,2-addition. DFT studies also revealed that ruthenium-catalyzed 1,4-hydroboration of pyridines is a facile process with a free energy barrier of 3.2 kcal/mol, whereas a pathway for the 1,2-hydroboration product is not observed due to the steric effects exerted by methyl groups on pinacolborane (HBpin) and p-cymene. Notably, enabled by the amine−amide inter-conversion of the coordinated heteroarene ligand, the +2 oxidation state of ruthenium intermediates remains unchanged throughout the catalytic cycle.

Noncovalent Interaction- and Steric Effect-Controlled Regiodivergent Selectivity in Dimeric Manganese-Catalyzed Hydroarylation of Internal Alkynes: A Computational Study

“…While in the unfavorable 2,1-insertion transition state TS5-2 , such two interactions disappear and two distinct noncovalent interactions (CH···O and OH···π) are found. Such a kind of O···C noncovalent interaction between the hydroxyl oxygen and carbonyl carbon has been observed in previous research, which is found to promote reaction selectivity …”

“…Further examining the structures of the preferable transition states, TS5-1 (Figure S4), a newly identified O···C tetrel bonding between alkynyl hydroxyl oxygen and carbonyl carbon is found. Tetrel bonding is regarded as a kind of powerful noncovalent interaction between electron donors and the group 14 elements of the periodic table, which is a significantly important interaction in metal complex catalysis . The O···C distance is calculated to be 2.96 Å.…”

Intermediates, Isolation and Mechanistic Insights into Zinc Hydride-Catalyzed 1,2-Regioselective Hydrofunctionalization of N-Heteroarenes

“…The metal-catalyzed hydrosilylation of N-heteroarene with phenylsilane gave the mono- and bis-1,2-hydrosilylation derivatives. Unfortunately, none of the previous reports − revealed the mechanism for the formation of bis-1,2-hydrosilylation derivatives.…”

“…As far as 1,2-selective hydrosilylation of Nheteroarenes is concerned, only a few reports are known, including transition-metal and main-group metal catalysts. 18−22 The research groups of Chang, 18 Gunanathan, 19 Harder, 20 Nikonov, 21 and Yao 22 independently reported Ir, Ru, Ca, and Zn metal-catalyzed 1,2-selective hydrosilylation of N-heteroarenes, respectively (Scheme 1b).…”

N-Silylamines in catalysis: synthesis and reactivity

“…Expanding to N -heterocycles, the groups of Suresh and Guananathan reported the ruthenium catalysed 1,2-hydrosilylation of N -heteroarenes (Scheme 24), using [Ru-( p -cymene)(PCy 3 )Cl 2 ] as the precatalyst. 70 In contrast to previous hydrosilylations of N -heteroarenes with calcium, 71 iridium 72 and zinc, 73 this is exclusively regioselective. The N -heteroarene substrate scope included quinolines, isoquinolines, benzimidazoles, quinoxalines, acridines, pyridines, pyrazines and pyrimidines.…”