Investigation of Nitrile Hydration Chemistry by Two Transition Metal Hydroxide Complexes: Mn–OH and Ni–OH Nitrile Insertion Chemistry

Abstract

Herein we describe the synthesis of a series of nickel complexes, including the formation of [( iPr PN H P)Ni-(PMe 3 )][BPh 4 ] ( iPr PN H P = HN(CH 2 CH 2 (PiPr 2 )) 2 ). The ability of this phosphine complex to perform the 1,2-addition of H 2 O to produce the Ni−OH species [( iPr PN H P)NiOH]-[BPh 4 ] has been investigated. The nucleophilicity of the hydroxide moiety of both [( iPr PN H P)NiOH][BPh 4 ] and the previously reported ( iPr PN H P)MnOH(CO) 2 was investigated through the hydration of aryl and alkyl nitriles, leading to the formation of a number of metal carboxamide (RC(O)NH − ) bonds. This reactivity generated complexes with the general structures of [( iPr PN H P)Ni(NHC(O)R)][BPh 4 ] for nickel and ( iPr PN H P)Mn(NHC(O)R)(CO) 2 for manganese. Under catalytic conditions, the hydration of nitriles using nickel complexes yielded only a single turnover. However, ( iPr PN H P)MnOH(CO) 2 produced several turnovers, and the reaction conditions were probed for optimization.

Hydration of Nitriles Enabled by PNP−manganese Pincer Catalyst

“…By the addition of H 2 O to Mn‐A and stirred at room temperature for 1 h in THF led to the signal shift from 91.0 ppm to 72.5 ppm in 31 P NMR spectrum (Figure S2). It might be caused by formation of hydroxide complex Mn‐B [21,24] . The treatment of the above reaction mixture with benzonitrile, the signal at 72.5 ppm was disappeared, and a new peak appeared at 78.4 ppm (Figure S3).…”

“…However, based on the 31 P NMR spectra analyses and previous reports, a plausible catalytic cycle for the MACHO−Mn catalysed hydration of nitriles was proposed (Scheme 4). [21,23e,24b,25] The catalytically active amido complex Mn‐A was initially generated from the reaction of precatalyst with KO t −Bu, followed by 1,2‐addition of H 2 O to afford manganese hydroxide species Mn‐B . Subsequent nucleophilic reaction with nitrile furnished intermediate Mn‐C .…”

“…Boncella/Tondreau et al . elegantly investigated the hydration of nitrile with stoichiometric amount of pincer−Mn complexes [21] . However, only limited examples on pincer manganese catalyzed transformation of nitriles to amides have been reported by Milstein group until recently [22] .…”

Recent Advances in Chemical Modifications of Nitriles

“…Other transition metal catalysts such as Ir, [29] Mn, [30] Rh [31] complexes have been widely applied in the hydration reactions of nitriles. During the reaction process, the metal centre was mostly used to coordinate with the cyanogen group to activate the C�N triple bond, and the desired amide product was finally provided through the process of intermolecular or intramolecular nucleophilic attack.…”

Catalytic Conversion of Nitriles by Metal Pincer Complexes

“…Boncella, Tondreau and co-worker described another approach for the hydration of nitriles using non-innocent ligand-based pincer complexes [92]. They synthesised and characterised 5 Ni PNP pincer complexes of which 50 was studied extensively together with the known Mn-OH pincer complex 49.…”