Utilization of Ce[N(SiMe3)2]3 as a Catalyst for the Hydroboration of Organic Substrates: Scope and Mechanistic Aspect

Abstract

The homoleptic lanthanide-amide complexes (Ln[N(SiMe3)2]n (n = 2, 3), (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Ho, Yb and Lu) were introduced in 1973 by Bradley and co-workers. However, their catalytic potential is not well exploited in organic synthesis. It is important to mention that homoleptic lanthanide amides (Ln = Y, La, Sm) were revealed as the highly efficient catalysts for deoxygenation of 2° and 3° amides with pinacolborane (HBpin). In addition, they can also hydroborate aldehydes, ketones, and esters. The ease of access and easy handling of lanthanide amides make them potential candidates as catalysts for the organic transformations. Therefore, we envisioned to use Ce[N(SiMe3)2]3 as a catalyst for the hydroboration of unsaturated substrates. First, Ce[N(SiMe3)2]3 was explored for the deoxygenative reduction of amides to amines, which is a promising organic transformation in synthetic chemistry. Previously, Marks and co-workers introduced the deoxygenative reduction of secondary and tertiary amides to amines using La[N(SiMe3)2]3 as a catalyst but could not reduce the more challenging primary amides. Herein, we successfully unveil the deoxygenative reduction of primary and secondary amides to their corresponding borylated amines under mild reaction conditions using the commercially available Ce[N(SiMe3)2]3 as a catalyst. Kinetic and experimental studies are performed to understand the mechanism of the hydroboration reaction with Ce[N(SiMe3)2]3. Furthermore, we have demonstrated Ce[N(SiMe3)2]3 as an efficient, highly active and highly selective homogenous catalyst for the rapid reduction of ester and epoxide groups using HBpin. Kinetic analysis and control experiments were performed to get more insight into the mechanistic aspects. We also found that Ce[N(SiMe3)2]3 behaves as a highly selective and active catalyst for the hydroboration of CO2, which is extremely useful for the conversion of CO2 to value added products. Finally, we have disclosed the hydroboration of unsaturated bonds e.g., C=C, C≡C, C≡N, C=N and performed a broad substrate scope to show the utility of Ce[N(SiMe3)2]3 as an efficient and robust catalyst.