Hydrosilylative Reduction of Tertiary Amides to Amines Catalyzed by N‐(Phosphinoaryl)anilido Complexes of Iron and Cobalt

Abstract

The synthesis and structural characterization of low‐coordinate Fe(II) and Co(II) complexes supported by the monoanionic P,N‐ligand N‐(2‐dicyclohexylphosphinophenyl)‐2,6‐diisopropylanilide are described. A three‐coordinate (P,N)Fe‐hexamethyldisilazide complex (2), and four‐coordinate (P,N)Fe‐ (3‐Fe) and (P,N)Co‐alkyl (3‐Co) complexes were evaluated as pre‐catalysts for the hydrosilylative reduction of amides with PhSiH3 (5 mol % pre‐catalyst, 1 equiv. PhSiH3, 80 °C, 1–24 h). The Fe complex 2 proved to be more broadly effective for the reduction of a variety of tertiary amide substrates, and was shown to mediate the reduction of N,N‐dibenzylbenzamide at a loading of 1 mol %, to achieve near quantitative formation of tribenzylamine in 1 h (80 °C). Complex 2 also proved effective for the hydrosilylation of tertiary amides under ambient conditions (5 mol % Fe, 24 h), which is a unique example of room temperature amide hydrosilylation mediated by an Fe catalyst without the need for photochemical activation. Given the widespread use of amide reduction protocols in synthesis, the development of efficient Fe‐based catalysts that operate under mild conditions is an important target.

Hydrosilylative reduction of secondary amides to amines catalyzed by geometry-constrained NNN-cobalt complexes

“…In particular, a variety of noble metal catalysts, including Ru, 12 Rh, 13 Ir, 14 and Pt, 15 have proven to be effective for this transformation. Very recently, the replacement of noble metal catalysts with non-precious, abundant and environmentally benign 3d metal catalysts (including Fe, 16 Co, 16 i ,17 Ni, 18 Mn, 19 Zn, 20 Cu 21 ) for the hydrosilylative reduction of amides has also made considerable progress. However, in the reported protocols of hydrosilylative reductions catalyzed with non-precious metals, tertiary amides typically serve as ideal substrates, whereas secondary amides are rather problematic.…”

Meet the Women of Catalysis

“…These efforts emphasize a ligand design approach for the preparation of transition metal complexes that exhibit new and/or improved reactivity properties by virtue of their unique construction, with the goal of utilizing such complexes as catalysts for new chemical transformations. Current work in the Turculet group includes the study of PSiP bis(phosphino)silyl pincer ligation, as well as mono‐anionic, bidentate P,N ligation, especially in the context of first‐row transition metal catalysis …”