Computational study of the mechanism and selectivity of ruthenium-catalyzed hydroamidations of terminal alkynes

Density functional theory calculations were performed to elucidate the mechanism of the ruthenium-catalyzed hydroamidation of terminal alkynes, a powerful and sustainable method for the stereoselective synthesis of enamides. The results provide an explanation for the puzzling experimental finding th...

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Bibliographic Details
Published in:Chemical science (Cambridge) 2015-04, Vol.6 (4), p.2532-2552
Main Authors: Maity, Bholanath, Gooßen, Lukas J, Koley, Debasis
Format: Article
Language:English
Subjects:
Alkynes
Catalysis
Catalysts
Computation
Density functional theory
Ligands
Mathematical models
Stereoselectivity
Terminals
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Summary:Density functional theory calculations were performed to elucidate the mechanism of the ruthenium-catalyzed hydroamidation of terminal alkynes, a powerful and sustainable method for the stereoselective synthesis of enamides. The results provide an explanation for the puzzling experimental finding that with tri- -butylphosphine (P(Bu) ) as the ligand, the -configured enamides are obtained, whereas the stereoselectivity is inverted in favor of the -configured enamides with (dicyclohexylphosphino)methane (dcypm) ligands. Using the addition of pyrrolidinone to 1-hexyne as a model reaction, various pathways were investigated, among which a catalytic cycle turned out to be most advantageous for both ligand systems that consists of: (a) oxidative addition, (b) alkyne coordination, (c) alkyne insertion (d) vinyl-vinylidene rearrangement, (e) nucleophilic transfer and finally (f) reductive elimination. The stereoselectivity of the reaction is decided in the nucleophilic transfer step. For the P( Bu) ligand, the butyl moiety is oriented to the incoming 2-pyrolidinyl unit during the nucleophilic transfer step, whereas for the dcypm ligand, steric repulsion between the butyl and cyclohexyl groups turns it into a orientation. Overall, the formation of -configured product is favorable by 4.8 kcal mol (Δ SDL) for the catalytic cycle computed with P(Bu) as ancillary ligand, whereas for the catalytic cycle computed with dcypm ligands, the -product is favored by 7.0 kcal mol (Δ SDL). These calculations are in excellent agreement with experimental findings.
ISSN:2041-6520
2041-6539
DOI:10.1039/c4sc03906h