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Intramolecular Hydroamination/Cyclisation of Aminoallenes Mediated by a Neutral Zirconocene Catalyst: A Computational Mechanistic Study
Author(s) -
Tobisch Sven
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200601287
Subject(s) - hydroamination , protonolysis , chemistry , moiety , intramolecular force , metallacycle , protonation , catalytic cycle , stereochemistry , reaction mechanism , catalysis , medicinal chemistry , organic chemistry , ion , x ray crystallography , physics , diffraction , optics
The complete catalytic cycle for the intramolecular hydroamination/cyclisation (IHC) of 4,5‐hexadien‐1‐ylamine ( 1 ) by a prototypical [ZrCp 2 Me 2 ] precatalyst ( 2 ) has been scrutinized by employing a reliable DFT method. The present study conducted by means of a detailed computational characterisation of structural and energetic aspects of alternative pathways for all of the relevant elementary steps complements the mechanistic insights revealed from experimental results. The operative mechanism entails an initial transformation of precatalyst 2 into the thermodynamically prevalent, but dormant, bis(amido)–Zr compound in the presence of aminoallene 1 . This complex undergoes a reversible, rate‐determining α‐elimination of 1 to form the imidoallene–Zr complex. The substrate‐free form, which contains a chelating imidoallene functionality, is the catalytically active species and is rapidly transformed into azazirconacyclobutane intermediates through a [2+2] cycloaddition reaction. This highly facile process does not proceed regioselectively because the alternative pathways for the formation of five‐ and six‐membered azacycles have comparable probabilities. Degradation of cyclobutane intermediates by following the most feasible pathway occurs through protonolysis of the metallacycle moiety and subsequent proton transfer from the ZrNHR moiety onto the azacycle. The five‐membered allylamine is generated through protonation at carbon atom C 6 followed by α‐hydrogen elimination, whereas protonolysis of the cyclobutane moiety at the ZrN bond followed by proton transfer onto carbon atom C 5 is the dominant route for the six‐membered product. Of the two consecutive proton transfer steps, the second one determines the overall kinetics of the entire protonation sequence. This process is predicted to be substantially slower than the cycloaddition reaction. The factors that regulate the composition of the cycloamine products have been elucidated.