Calcium Mediated C−H Silylation of Aromatic Heterocycles with Hydrosilanes

Abstract

Treatment of mononuclear calcium hydride complex [(TpAd,iPr)Ca(H)(THP)] (1) (TpAd,iPr=hydrotris(3‐adamantyl‐5‐isopropyl‐pyrazolyl)borate, THP=tetrahydropyran) with 2‐methylthiophene, 2‐methylfuran, and 1‐methyl‐1H‐indole in THF/hexane solution led to the formation of calcium thiophenyl (2), furanyl (3), and indolyl (4) complexes, via sp2 C−H bond activation. The reaction of complex 1 with 2‐methylpyridine and quinoline afforded calcium benzyl pyridinyl (5) and 1,2‐dihydroquinolide (6) complexes, through sp3 C−H bond activation and hydride insertion reaction, respectively. Under mild conditions, the catalytic regioselective sp2 C−H silylation of a series of aromatic heterocycles with secondary hydrosilane was achieved by the use of complex 1. This protocol offers a straightforward method for the synthesis of silylated heteroaromatic compounds without a hydrogen acceptor and free of transition metal.

Research Progress in Transition-Metal-Free C—Si Bond Formation

“…他们分离 并表征了催化反应中间体茴香醚基钙配合物结构, 证明 底物烷氧基与钙中心的配位在该反应中起了重要作用, 并 根 据 动 力 学 研 究 绘 制 了 可 能 的 机 理 循 环 (Scheme 38B). 2022 年, 他们 [73] 还进一步将该催化体系拓展到杂 芳环的 α 位脱氢硅基化反应(Scheme 38C), 为合成含有 叔硅烷单元的(杂)芳基化合物提供了一种新的途径.…”

Organocalcium Hydride-Catalyzed Intramolecular C(sp3)–H Annulation of Functionalized 2,6-Lutidines

“…This is because the reaction of Ca–H species with an unsaturated substrate has a high propensity to undergo hydride transfer rather than deprotonation . In recent years, Harder, Cheng, and our group have shown that molecular calcium hydrides/alkyls also enabled catalytic C–H alumination, silylation, and alkylation of a diverse array of substrates with high selectivities.…”

Tandem Synthesis of Linear Hydridopolycarbosilanes and Postfunctionalization by a Calcium Catalyst

“…Unwanted oxidation/reduction pathways typically with transition metals can be avoided as the metal oxidation state remains intact during the reaction. Due to the nontoxic, environmentally benign, earth-abundant, and biocompatible nature, calcium-based efficient catalysts have been actively pursued by many scientists as potential alternatives to expensive, toxic, and rare metal complexes. − The hydrosilylation of hydrosilanes and olefins catalyzed by calcium complexes has been well studied during the past decade. ,, However, calcium-mediated catalytic bis-hydrosilylation of dienes through step-growth polyaddition to afford high-molecular-weight hydridopolycarbosilanes remains unreported, as far as we are aware. More recently, our group found that scorpionate-supported calcium benzyl complex [(Tp Ad, i Pr )­Ca­( p -CH 2 –C 6 H 4 -Me)­(THP)] (Tp Ad, i Pr = hydrotris­(3-adamantyl-5-isopropyl-pyrazolyl)­borate, THP = tetrahydropyran) ( 1 ) can serve as a powerful catalyst precursor for the catalytic hydrogenation of alkenes, ortho -regioselective dehydrogenative silylation of aromatic ethers, and aromatic heterocycles. , During these transformations, a mononuclear calcium hydrido complex was proposed as the key intermediate .…”

“…More recently, our group found that scorpionate-supported calcium benzyl complex [(Tp Ad,iPr )Ca-(p-CH 2 −C 6 H 4 -Me)(THP)] (Tp Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate, THP = tetrahydropyran) (1) can serve as a powerful catalyst precursor for the catalytic hydrogenation of alkenes, 56 ortho-regioselective dehydrogenative silylation of aromatic ethers, and aromatic heterocycles. 54,55 During these transformations, a mononuclear calcium hydrido complex was proposed as the key intermediate. 50 These findings have inspired our interest in the copolymerization of dienes and bis-hydrosilane by a calcium complex.…”