Abstract
A simple and efficient system for the hydration and α-deuteration of nitriles to form amides, α-deuterated nitriles, and α-deuterated amides catalyzed by a single pincer complex of the earth-abundant manganese capable of metal–ligand cooperation is reported. The reaction is selective and tolerates a wide range of functional groups, giving the corresponding amides in moderate to good yields. Changing the solvent from tert-butanol to toluene and using D2O results in formation of α-deuterated nitriles in high selectivity. Moreover, α-deuterated amides can be obtained in one step directly from nitriles and D2O in THF. Preliminary mechanistic studies suggest the transformations contributing toward activation of the nitriles via a metal–ligand cooperative pathway, generating the manganese ketimido and enamido pincer complexes as the key intermediates for further transformations.
Highlights
A simple and efficient system for the hydration and α-deuteration of nitriles to form amides, α-deuterated nitriles, and α-deuterated amides catalyzed by a single pincer complex of the earth-abundant manganese capable of metal−ligand cooperation is reported
Recent investigations have shown that Metal−ligand cooperation (MLC) of dearomatized lutidine-based pincer complexes can provide a new approach to the activation of double and triple bonds via simultaneos substrate binding to the ligand and metal center.[10]
We have observed that rhenium and manganese pincer complexes catalyze the addition of nitriles to Michael acceptors, as well as oxa- and aza-Michael addition reactions, in which the catalytic activity is based on the nitrile bound to the complex via C−C and M−N bond formation of the C N group, which we termed “Template Catalysis”, (Scheme 1a).[11]
Summary
AReaction conditions: 1a (1.0 mmol), H2O (5.0 mmol, 5.0 equiv), Mn-1 (0.01 mmol, 1 mol %), and tBuOH (1.0 mL) at 90 °C for 24 h. bYields were determined by GC with biphenyl as an internal standard. cIsolated yield in parentheses. The observed ability of Mn-1 to activate benzonitrile differs from the reactivity of dearomatized Ru PNP complexes with nitrile substrates under ambient temperature.[13] Catalyst Mn-1 readily reacts with benzyl nitrile 1a, bearing an α-methylene group, to form the manganese enamido complex Mn-6 (Scheme 3, b).11b This intermediate was detected as the major metal species at the beginning of hydration and α-deuteration reactions at room temperature. By switching the solvent to THF, we realized the sequential α-deuteration and hydration processes to synthesize α-deuterated amides in a one-step process for the first time These processes involve the reversible Mn−N and C−C bond formation between the substrate and the dearomatized manganese pincer complex through MLC.
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