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A Neutral PCNHCP Co(I)-Me Pincer Complex as a Catalyst for N-Allylic Isomerization with a Broad Substrate Scope

Sakthi Raje, Tofayel Sheikh Mohammad, Graham de Ruiter*

*Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Technion City, 3200008 Haifa, Israel, Email: grahamtechnion.ac.il

S. Raje, T. S. Mohammad, G. de Ruiter, J. Org. Chem., 2024, 89, 4319-4325.

DOI: 10.1021/acs.joc.3c02349


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Abstract

A neutral PCNHCP cobalt(I) pincer complex catalyzed an efficient selective isomerization of N-allylic organic compounds, including amines, amides, and imines via a π-allyl mechanism. The isomerization occurs readily at 80-90 蚓 and is compatible with a broad range of functional groups. The in situ formed enamines could additionally be used for a one-pot inverse-electron-demand Diels-Alder reaction.


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Details

​The article discusses the development of a neutral PCNHCP cobalt(I) pincer complex as an efficient catalyst for the isomerization of N-allylic substrates, including amines, amides, and imines. This process offers a sustainable and atom-economical route to synthesize internal alkenes, particularly focusing on N-allylic compounds, which have been underrepresented in literature. The isomerization occurs at 80-90蚓 and is compatible with a wide variety of functional groups. The resulting enamines can be used in a one-pot inverse-electron-demand Diels-Alder reaction to produce diversely substituted heterobiaryls. The study highlights the catalyst's broad substrate scope and excellent stereoselectivity, demonstrating its utility in organic synthesis. The methodology represents a significant advancement as it provides a universal strategy for the isomerization of various N-allylic substrates using a single catalyst. The research also explores the potential for multiple bond isomerization and the synthesis of 1,3-azadienes, which are valuable in organic chemistry. The findings suggest that the Co(I)-Me complex is a versatile and efficient catalyst, paving the way for further developments in the isomerization of more complex alkenes.


General procedure for selective isomerization of N-allylamines

Inside the glovebox, an ovendried J-Young tube was charged with substrate (0.15 mmol) and catalyst Co-Me in toluene-d8 (50 無 to 125 無, 0.003 mmol (2 mol%) to 0.0075 mmol (5 mol%), from a 0.06 M stock solution prepared in toluene-d8) was added. To the reaction mixture was added an additional amount of toluene-d8 to make a total volume 400 無. The J-Young tube was sealed and immersed in a pre-heated oil bath at 80, 90 or 100 ˚C and the progress of the reaction was monitored by 1H NMR spectroscopy. After completion of the reaction, the J-Young tube was transferred to the glovebox and the crude reaction mixture was filtered through short alumina (basic) plug to remove the cobalt catalyst. The alumina was washed with additional toluene-d8 (400 無) to collect all the organic products. For heteroatom containing products, CD3CN (300 無) was used. The 1H NMR spectrum was taken after the addition of 1,3,5-trimethoxybenzene (2.9 mg, 0.017 mmol, 0.34M stock solution in toluene-d8) as an internal standard. The yield and E:Z ratio of the products were determined by combination of 1H and 13C NMR spectroscopy. Isolation of the resulting enamines by column chromatography leads to decomposition of the product.

General procedure for selective isomerization of N-allylamides

Inside the glovebox, an ovendried J-Young tube was charged with substrate (0.15 mmol) and catalyst Co-Me in toluene-d8 (75 痞 to 125 無, 0.0045 mmol (3 mol%) to 0.0075 mmol (5 mol%), from a 0.06 M stock solution prepared in toluene-d8) was added. To the reaction mixture was added an additional amount of toluene-d8 to make a total volume of 400 無. The J-Young tube was sealed and immersed in a pre-heated oil bath at 90 ˚C or 100 ˚C, and the progress of the reaction was monitored by 1H NMR spectroscopy. After completion of the reaction, the mixture was exposed to air and subsequently filtered through a short alumina (neutral) plug to remove the cobalt catalyst. The alumina was washed with an ethyl acetate/hexane (2 mL 50:50) mixture to collect all the organic products. The resulted mixture was evaporated to dryness then the resulted product was re-dissolved in 400 無 of CDCl3 solvent. The yield and E:Z ratio of the products were determined by combination of 1H and 13C NMR spectroscopy. The enamides were isolated by column chromatography using SiO2.


Key Words

enamines, isomerizations


ID: J42-Y2024