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

Abstract

The conjugated bis-guanidinate-supported zinc hydride [{LZnH} 2 ; L = {(ArHN) (ArN)−C�N−C�(NAr) (NHAr); Ar = 2,6-Et 2 -C 6 H 3 }] (I)-catalyzed highly demanding exclusive 1,2-regioselective hydroboration and hydrosilylation of Nheteroarenes is demonstrated with excellent yields. This protocol is compatible with many pyridines and N-heteroarene derivatives, including electron-donating and -withdrawing substituents. Catalytic intermediates, such as [(LZnH) (4-methylpyridine)] IIA, [(L′ZnH) (4-methylpyridine) IIA′, where L′ = CH{(CMe) (2,6-Et 2 C 6 H 3 N)} 2 )], LZn(1,2-DhiQ) (isoquinoline) III, [L′Zn(1,2-DhiQ) (isoquinoline)] III′, and LZn(1,2-(3-MeDHQ)) (3-methylquinoline) V, were isolated and thoroughly characterized by NMR, HRMS, and IR analyses. Furthermore, X-ray single-crystal diffraction studies confirmed the molecular structures of compounds IIA′, III, and III′. The NMR data proved that the intermediate III or III′ reacted with HBpin and gave a selective 1,2addition hydroborated product. Stoichiometric experiments suggest that V and III independently reacted with silane, yielding selective 1,2-addition of mono-and bis-hydrosilylated products, respectively. Based on the isolation of intermediates and a series of stoichiometric experiments, plausible catalytic cycles were established. Furthermore, the intermolecular chemoselective hydroboration reaction over other reducible functionalities was studied.

Regioselective 1,2-Hydrosilylation of N-Heterocycles Catalyzed by a Ruthenium Olefin Metathesis Catalyst

“…Quantitative yields of 3a–3c were obtained when increasing the amount of substrate to 0.5 mmol. Compared to the currently reported catalysts, ,,,− M2 exhibited higher efficiency in the 1,2-hydrosilylation. Notably, when a quinoline derivative substituted with a strong electron-donating group (−OMe) was used, an 83% yield of 3e was obtained after 6 h, while prolonging the time to 18 h achieved complete conversion, indicating advantageous catalyst lifetime properties.…”

“…Unexpectedly, when the hydrosilylation reaction was performed using 1,5-naphthyridine as a starting material, a single 1,2-hydrosilylation product 4g was obtained in complete conversion. We targeted the double-hydrosilylation product by prolonging the reaction time or increasing the catalyst loading, but the single N-sila-functionalized compound 4g remained the main obtained product after 48 h. We suspect that the formation of the single-hydrosilylation product 4g , broking the overall aromaticity of the 1,5-naphthyridine, leads to a higher resonance stability of the single aromatic ring and is then less prone to a subsequent reduction reaction . Replacing Et 2 SiH 2 by Ph 2 SiH 2 or PhSiH 3 provided 4h – 4i in only 46–71% yields at 60 °C and requires longer reaction times.…”

“…We targeted the double-hydrosilylation product by prolonging the reaction time or increasing the catalyst loading, but the single N-silafunctionalized compound 4g remained the main obtained product after 48 h. We suspect that the formation of the singlehydrosilylation product 4g, broking the overall aromaticity of the 1,5-naphthyridine, leads to a higher resonance stability of the single aromatic ring and is then less prone to a subsequent reduction reaction. 24 Replacing Et 2 SiH 2 by Ph 2 SiH 2 or PhSiH 3 provided 4h−4i in only 46−71% yields at 60 °C and requires longer reaction times. 2-Methyl-benzimidazole and benzothiazole can also undergo hydrosilylation, leading to 41−90% conversion of 4j and 4k.…”

“…The former complex (M2) is particularly attractive due to its facile preparation and relatively low cost. 11 Encouraged by the above results, the use of Ph 2 SiH 2 , as a challenging hydrogen source, 18a, 22,24 was examined in combination with M2 in the reduction reaction. In addition, from a more practical perspective, diphenylsilane appears to be the optimal silane candidate because of its low price and the stability of the resulting intermediates.…”

A N‐Heterocyclic Carbene‐Supported Zinc Catalyst for the 1,2‐Regioselective Hydroboration of N‐Heteroarenes

“…substrate) is doubled. The product NMR chemical shifts were compared with the literature [16,21c, 37–38] …”