B(C₆F₅)₃-Promoted hydrogenations of N-heterocycles with ammonia borane

Abstract

A transition-metal-free method for the B(CF)-promoted hydrogenations of N-heterocycles using ammonia borane under mild reaction conditions has been developed. The reaction affords a broad range of hydrogenated products in moderate to good yields. The enantioselective versions for the corresponding products were also investigated via our approach, showing good feasibility.

B(C6F5)3‐Catalyzed Deoxygenative Reduction of Amides to Amines with Ammonia Borane

“…Based on the above observations and previous reports, [14] a plausible reaction mechanism is proposed (Scheme 5). The present reaction proceeds with the generation of a zwitterion species A from B(C 6 F 5 ) 3 and AB.…”

“…Varying the amount of BF 3 · OEt 2 did not lead to any improvement (see SI). Attempts to decrease the catalyst loading, the temperature or the amount of AB led to diminished yields of 2 a (Table 1, entries [14][15][16]. Some other boron catalysts, such as (5-bromobenzo[b]thiophen-2yl)boronic acid, [9b] BPh 3…”

“…The same group also developed B(C 6 F 5 ) 3 ‐catalyzed selective reduction of pyridines to piperidines with AB . More recently, the group of Shi described the selective reduction of N ‐heteroarenes with AB under the catalysis of B(C 6 F 5 ) 3 . The broad substrate scope, high functional group tolerance and mild reaction conditions observed in these studies led us to envisage that AB, if generally applicable, would have the potential to serve as a promising reducing agent for transforming amides into the corresponding amines.…”

Tris(pentafluorophenyl)borane-Catalyzed Reactions Using Silanes

“…Substrates that were deemed electronically symmetric were found to rely on the steric hindrance of the groups that were found to dictate the regiochemistry of the reduction. An additional metal-free method for the BCF-promoted reactivity of heterocycles was reported by Shi and coworkers [40]. They found that BCF was catalytically competent to promote the reduction of various N -heterocycles.…”

FLP‐Catalyzed Transfer Hydrogenation of Silyl Enol Ethers

“…[1] Representing an area of particular interest, FLPs can be employed as catalysts in metal free hydrogenation processes. [2] Dihydrogen is typically employed as the reductant in such processes,h owever, recent advances have shown that amines, [3] cyclohexadienes, [4] ammonia borane, [5] and Hantzsch esters [6] can be employed as dihydrogen surrogates in B(C 6 F 5 ) 3 -catalyzed transfer hydrogenation. Systems employing an additional Lewis base,r endering it an FLPtype process,have been developed by Du and co-workers for the enantioselective transfer hydrogenation of ketimines and quinoxalines.…”