Chemoselective Silylative Reduction of Conjugated Nitriles under Metal‐Free Catalytic Conditions: β‐Silyl Amines and Enamines

Abstract

The B(C 6 F 5 ) 3 -catalyzed silylative reduction of conjugated nitriles has been developed to affords ynthetically valuable b-silyl amines.T he reaction is chemoselective and proceeds under mild conditions.M echanistic elucidation indicates that it proceeds by rapid double hydrosilylation of the conjugated nitrile to an enamine intermediate which is subsequently reduced to the b-silyl amine,thus forming anew C(sp 3 )ÀSi bond. Based on this mechanistic understanding, ap reparative route to enamines was also established using bulky silanes.

B(C6F5)3‐Catalyzed sp3 C—Si Bond Forming Consecutive Reactions

“…Based on the above observations and a previous report,[14a] a reaction pathway for the B(C 6 F 5 ) 3 ‐mediated silylative reduction of α,β‐unsaturated aldimines was proposed (Scheme ). Initially, B(C 6 F 5 ) 3 was assumed to form an adduct 4a with an aldimine substrate, which is a resting species in the overall processes .…”

“…Based on this background and precedence on such consecutive silylative reduction for N ‐heteroarenes, Park and Chang group next turned their attention to non‐cyclic N ‐containing conjugated systems for C(sp 3 )—Si bond forming consecutive hydrosilylation reactions [14a]. At the outset, cinnamonitrile as a model substrate was subjected to reaction conditions similar to those applied for the silylative reduction of quinolines [5 mol% B(C 6 F 5 ) 3 , 4 equiv Ph 2 SiH 2 at 25 °C].…”

“…Having the mechanistic insights involving the enamine intermediacy, Park and Chang group next conceived a one‐pot synthetic route towards β‐silylated secondary amines from conjugated imines that can be in situ generated through a simple condensation reaction of α,β‐unsaturated aldehydes with primary amines [14b]. This work represents the first example of reductive amination accompanied by sp 3 C—Si bond formation (Scheme ).…”

“…While numerous catalytic systems for the derivatization of furans have been well documented, the ring‐opening processes of furans leading to a range of synthetic intermediates have also received great attention . Based on these backgrounds and our precedence regarding consecutive silylative reductions, Park and Chang envisaged the silylative reduction cascades of furans via the B(C 6 F 5 ) 3 catalysis (Scheme ) . As a preliminary test, a reaction of 2‐methylfuran with one equivalent of PhMe 2 SiH was conducted in the presence of B(C 6 F 5 ) 3 (5 mol%) in CH 2 Cl 2 to give an unprecedented α‐silyloxy‐3‐pentenylsilane with an ( Z )‐alkenyl geometry in 50% yield with the unreacted furan substrate (~50%).…”

“…Despite of remarkable advances in the metal‐catalyzed hydrosilylation, there had been no example of the consecutive silylative reduction of conjugated systems such as pyridines until we communicated for the first time the B(C 6 F 6 ) 3 ‐catalyzed silylative reduction of quinolines leading to unprecedented tetrahydroquinolines bearing a sp 3 C—Si bond β to the nitrogen atom . Very recently, the scope of this borane catalysis has been successfully expanded from N ‐conjugated systems to unactivated piperidines and furans to provide a new class of silylated products with high chemo‐, regio‐, and stereoselectivities. In 2015, Oestreich published a review regarding electron‐deficient borane‐catalyzed reductive transformations with hydrosilanes and hydrogen as a reducing agent, while our group reported a review on the catalytic dearomatization of N ‐heteroarenes with silicon and boron compounds, where few examples of the consecutive silylative reduction were presented.…”